TECHNICAL REFERENCE GUIDE
Safety valves
Contents The need for safety valves Types and definitions Some common terms Pressure relief devices Non-reclosing pressure relief devices Vacuum relief devices Changeover valves Design Materials of construction Pressure containing components Internal components Springs Method of operation Setting and sealing How to establish set pressure Pressure reducing valve proportional offset Shut-off margin Pressure level relationships for pressure relief valves (from API 520) How to set Who can set ? Sealing Seat tightness The valve The installation Sealing face refurbishment Testing Marking Installation Transport Mounting Discharge pipework Manifolds Changeover valves Reaction force when discharging Determination of reaction force Noise emission Determination of the noise level for compressible fluids Selection Type of disposal system Valve construction Operating characteristic Approval
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Sizing Two-phase flow Coefficient of discharge Overpressure Critical and subcritical flow Back pressure Coefficients and correction factors Sizing according to AD-Merkblatt A2, DIN 3320 and TRD421 Sizing according to ASME / API RP 520 Sizing according to BS 6759 Outflow function Y Pressure medium coefficient of steam c Table 4 Nozzle gas constant C relative to isentropic coefficient k Sizing according to ASME / API RP 520 Table 5 Nozzle gas constant Cg relative to isentropic coefficient k Sizing according to BS 6759 for gas and vapour Table 6 Superheat correction factor Ksh Sizing according to ASME / API RP 520 (Imperial) Table 7 Superheat correction factor Ksh Sizing according to BS 6759 (Metric) Gas and vapour constant back pressure correction factor Kb Sizing according to ASME / API RP 520 Back pressure correction factor for liquids Kw Sizing according to ASME / API RP 520 Viscosity correction factor Kv Sizing according to ASME / API RP 520 and BS 6759 Compressibility factor Appendix - 1 Properties of industrial gases Properties of industrial liquids Appendix - 2 Steam tables Appendix - 3 Terminology
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The need for safety valves A safety valve is required when the safe working pressure or maximum allowable working pressure of a system or pressure containing vessel could be exceeded. The primary purpose of a safety valve is therefore to protect life and property. The proper sizing, selection, manufacture, installation and subsequent maintenance are all essential for the safety valve to fulfill this function reliably. There are many standards, codes and regulations controlling the design and application of safety valves such that overpressure protection is assured. For steam boiler applications there are very specific requirements for safety valve performance demanded by national standards and insurance companies. Approval by an independent authority is usually mandatory. Safety valves can also be used for process type applications, where they may be needed to protect the plant or to prevent spoilage of the product being processed. For these applications corrosion resistance to process fluids and their containment by discharge lines (to prevent emission of hazardous media) are often important considerations. Relief valves are very similar but not always safety related and are commonly used on liquid systems, especially for lower capacities and thermal expansion duty. Pumped systems often use relief valves as a pressure overspill device. It is interesting to consider the many varied circumstances which can result in an overpressure situation. Common steam applications for safety valves are: Boiler overpressure protection. Downstream of pressure reducing valve stations. Process applications, however, can experience many other situations such as: An inadvertently closed or opened stop valve on a process vessel or other factor leading to an imbalance of fluid flowrate. A failure in a cooling system allowing vapour or fluid to expand (note that overpressure could be caused by liquid volume expansion or vapour pressure). Air or electrical power failure to control instrumentation. Transient pressure surges. Plant fires. Uncontrolled exothermic reactions in chemical plants. Ambient temperature change. The events listed above are all examples of where the release of pressure is required. 3
Types and definitions The terms 'safety valve' and 'safety relief valve' are commonly used as a generic term to describe many varieties of this type of product. A huge range of different valves meet many different applications and performance criteria demanded by the relevant associated standards. There are specific definitions for the terms above and different 'standards defined terms' for all the major types of valve given in most standards. There is a complication in that in some cases the same term has a slightly different definition or the same definition is referred to by different terminology in different standards. Broadly speaking, (in Europe) a valve referred to as a safety valve would, (in USA), be referred to as a safety relief valve or pressure relief valve. In Europe a valve referred to as a full lift safety valve would be known in USA as a safety valve.
Fig. 1 A typical ASME valve. 4
Some common terms
ASME I valve A safety relief valve conforming to the requirements of Section I of the ASME pressure vessel code for boiler applications which will be open within 3% overpressure and close within 4%. Will usually feature two blowdown rings. Identified by a National Board 'V' stamp. ASME VIII valve A safety relief valve conforming to the requirements of Section VIII of the ASME pressure vessel code for pressure vessel applications which will be open within 10% overpressure and close within 7%. Identified by a National Board 'UV' stamp. DIN 'Volhub' valve A DIN specification valve where a lift of D/4 (where D equals orifice diameter) is achieved within 5% overpressure. This is another term for a full lift valve. Semi-nozzle valve A valve where the seat is a separate ring fitted locally in the body. Full nozzle valve A valve where the inlet tract / seat is formed by a nozzle preventing the contained media contacting any part other than the nozzle or disc. Listed below are definitions as given in DIN 3320, ISO 4126 and ASME / ANSI PTC25.3.
Pressure relief devices
Safety valve (DIN / ISO) A valve which opens automatically to prevent a predetermined pressure being exceeded and which closes after decrease in pressure. Controlled safety valve (DIN) A controlled safety valve consists of the main valve and a control device. It also includes direct acting safety valves with supplementary loading in which, until set pressure is reached, an additional force increases the closing force. Direct loaded safety valve (DIN / ISO) A valve in which the opening force underneath the valve disc is opposed by a closing force such as a spring or weight.
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Safety valve (ASME) A pressure relief valve actuated by inlet static pressure and characterised by rapid opening or pop type action. Low-lift safety valve: the actual discharge area is determined by the position of the disk. Full-lift safety valve: the actual discharge area is not determined by the position of the disk. Full-bore safety valve: has no protrusions in the bore and wherein the valve disk lifts to an extent sufficient for the minimum area at any section at / or below the seat to become the controlling orifice.
Fig. 2 A typical DIN valve Standard safety valve (DIN) Following opening, reaches the degree of lift necessary for the mass flow to be discharged within a pressure rise of not more than 10%. (valve is characterised by a pop type action and is sometimes known as high lift). 6
Full lift safety valve (DIN) After commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20% of the total lift. Pressure relief valve (ASME) A spring loaded pressure relief device which is designed to open to relieve excess pressure and to reclose and prevent further flow of fluid after normal conditions have been restored. It is characterised by rapid opening pop action or by opening generally proportional to the increase in pressure over the opening pressure. It may be used for either compressible or incompressible fluids, depending on design, adjustment or application. Safety relief valve (ASME) A pressure relief valve characterised by rapid opening or pop type action, or by opening in proportion to the increase in pressure over the opening pressure depending on the application and may be used either for liquid or compressible fluid. Conventional: the spring housing is vented to the discharge side and hence operational characteristics are directly affected by changes of the back pressure on the valve. Balanced: incorporates a means of minimising the effect of back pressure on the operational characteristics. Relief valve (ASME) Has a gradual lift generally proportional to the increase in pressure over opening pressure. Primarily used for liquid service. Pilot operated pressure relief valve (ASME) The major relieving device is combined with and controlled by a self-actuated auxiliary pressure relief valve. Proportional safety valve (DIN) Opens more or less steadily in relation to the increase in pressure. Sudden opening within a 10% lift range will not occur without pressure increase. Following opening within a pressure increase of not more than 10% these safety valves achieve the lift necessary for the mass flow to be discharged. Diaphragm safety valve (DIN) A diaphragm safety valve is a direct loaded safety valve wherein linear moving and rotating elements and springs are protected against the effects of the fluid by a diaphragm. Bellows safety valve (DIN) A bellows safety valve is a direct loaded safety valve wherein linear moving and rotating elements and springs are protected against the effects of the fluid by a bellows. The bellows may be of such a design that it compensates for influences of back pressure. 7
Non-reclosing pressure relief devices
All the valve types listed previously are for valves which will automatically reclose after opening. There are also many different types of non-reclosing pressure relief devices which include; bursting / rupture discs, breaking / shear pin devices and fusible devices. The most common of these is the bursting disc which is sometimes used in conjunction with a conventional safety relief valve for process type applications when, installed on the inlet side, it can isolate the contained media from the safety valve under normal operating circumstances. Similarly, when installed on the outlet side it can serve to isolate a safety valve from corrosive media present in common discharge lines.
Fig. 3 Bursting disc installed on the inlet 8
Vacuum relief devices
These valves are designed sometimes as derivatives of conventional valve ranges and sometimes with a more compact and distinctive body style. In effect the action is 'reverse acting' where the disc is pulled off the seat against the action of the spring by the vacuum rather than pushed off by an excess of contained pressure against the preset spring force. This limits the degree of vacuum which can exist. Vacuum relief valve (ASME) A pressure relief device designed to admit fluid to prevent an excessive internal vacuum; it is designed to reclose and prevent further flow of fluid after normal conditions have been restored.
Changeover valves
Changeover valves permit two valves to be mounted side by side, with one in service and one isolated. This means that regular maintenance can be carried out without interruption to the system or vessel being protected. They are designed so that even when they are being operated, the pass area is never restricted. Changeover valves can also be used to connect safety valve outlets as well as inlets so that discharge pipework does not have to be duplicated. The action of both inlet and outlet changeover valves has to be limited and synchronised for safety reasons. This is usually by means of a chain drive system linking both hand wheels. It should be noted that it is a requirement of both API 520 and AD-Merkblatt A2 that pressure loss at the inlet of a safety valve when discharging should not exceed 3% of the set pressure. This requirement may need to be taken into consideration when specifying a changeover valve.
Fig. 4 Changeover valve 9
Design The basic spring loaded safety valve, referred to as 'standard' or 'conventional' provides a simple, reliable self-acting device to provide over pressure protection. The basic elements of the design consist of a right angle pattern valve body with the valve inlet connection or nozzle mounted on the pressure containing system. A disc is held against the nozzle by a spring (under normal working conditions) all of which are housed by an open or closed spring housing arrangement mounted on top of the body. The valve inlet design will normally be either a full nozzle type where the entire 'wetted' inlet tract is formed from one piece, or the semi-nozzle type where a seat ring is fitted into a body. The full nozzle type is typical of the process / high pressure types of valve since the contained fluid will only contact the nozzle and disc when the valve is closed. Some safety valves (quite often ASME type) use an adjustable ring around the seat or an additional adjustable ring around the disc known as blowdown rings. The position of these can be used to fine tune the over pressure and blowdown values. See Section 'Method of operation', page 14. The compression on the spring is adjustable to alter the pressure at which the disc is forced off the seat or nozzle, known as the set pressure. In terms of dimensional requirements such as centreline to face and orifice size these are normally not defined. Many competing products particularly of European origin therefore have differing dimensions and capacities for the same nominal size. An exception to this situation is found with steel ASME specification valves which invariably follow the recommendations of API 526 (which are not mandatory) where centreline to face dimensions and orifice sizes are listed. The orifice area series are referred to by a letter. It is common for valves with the same orifice letter to have several different sizes of inlet and outlet connection. This letter series is also referenced in other standards, for example BS 6759 part 3 which deals with valves for process type applications and NF E 29-414.
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Typical ASME valve
Typical DIN valve
Adjuster
Spring
Spring housing
Body Upper blowdown ring Disc Lower blowdown ring Seat
Fig. 5
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Materials of construction Except when discharging, the only parts of safety valves which are wetted by the process fluid are the inlet formed by either the seat ring and body or the full nozzle, and the disc. Since safety valves operate infrequently under normal conditions standard materials of construction (as detailed below) are satisfactory for most applications. Special materials are required for conditions such as: Low temperatures. Corrosive fluid. Slight contamination of discharged fluid is not permitted. When valve is discharging into a manifold into which corrosive media is discharged by another valve. Pressure containing components
The principal pressure containing components of safety valves are normally constructed from bronze, cast iron, SG iron or stainless steel. In the ASME markets sector, materials are required to conform to ASTM standards, and in the majority of European markets, DIN type materials are required although ASTM equivalents are sometimes accepted. Bronze is commonly used for small screwed valves for the low and medium sectors of the market fulfilling the requirements of steam, air and hot water duty. Cast iron is extensively used for ASME type valves (typically for pressures up to 250 psi) whereas SG iron is more commonly used in European markets, in some ranges in place of cast iron and sometimes as a higher pressure alternative. Higher pressure valves will use cast steel and process type valves will commonly use a cast steel body with an austenitic full nozzle type construction or for higher material specifications e.g. food, pharmaceutical or clean steam applications all Austenitic stainless steel construction (typically 316 type stainless steel). For extremely high pressures, pressure containing components may be forged or fully machined from solid.
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Internal components
For all safety valves it is extremely important that moving parts, particularly the spindle, guides etc. are made from materials that will not easily gall or corrode. Seats and discs should resist the effects of erosion and corrosion in service. It is common for hardened martensitic stainless steel to be used for seats and discs but for process applications austenitic stainless steel is commonly used, sometimes stellite faced for increased durability. For extremely corrosive fluids then nozzles, discs etc. are made from special alloys such as monel or hastelloy. Bellows are commonly made in austenitic stainless steel but special materials such as inconel are used where fluids are especially corrosive.
Springs
The spring is a critical element of the safety valve and must provide reliable performance within the required parameters and must also adhere to certain design guidelines. BS 6759 lists recommended materials but most other standards just insist on sound engineering practice. Standard safety valves will typically use carbon steel (only recommended for moderate temperatures) or chrome vanadium material. Tungsten steel is used for high temperature non-corrosive applications and stainless steel for corrosive or clean steam duty. For sour gas and high temperature corrosive applications, often special materials such as monel, hastelloy and inconel are used.
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Method of operation When a safety valve begins to lift the spring force will increase. This means that system pressure must increase if the valve is to continue to lift. The pressure increase necessary for the design lift to be achieved (enabling the safety valve to discharge its rated capacity) is known as the overpressure and would normally vary between 3% and 10% for compressible fluids depending on the application and related standard. This is a relatively small margin and can only be achieved if the valve has a disc arrangement specially designed to provide a rapid opening characteristic. Most safety valves therefore, have a secondary chamber formed by a shroud, skirt or hood around the outside diameter of the disc. In addition ASME type valves will normally incorporate either one or two adjustable rings, upper and lower, the latter known as blowdown rings. The volume thus contained within the shroud is known as the control chamber or huddling chamber. As the disc begins to lift fluid enters the control chamber exposing a larger area of the disc to system pressure. This causes an incremental change in force which overcompensates for the increase in spring force and causes the valve to open at a rapid rate. At the same time the direction of the fluid flow is reversed and this vectoring effect further enhances lift. These combined effects allow the valve to achieve its design lift within a relatively small percentage overpressure. For compressible fluids a major contributory factor is the rapid expansion as the fluid volume increases from a higher to a lower pressure area. For liquids, however, the effect is more proportional and allowable performance parameters are often larger, 25% overpressure is common. Because of the larger disc area now exposed, the valve will not shut until the system pressure is reduced to a point below the original set pressure. This pressure difference is known as blowdown and is normally expressed as a percentage of set pressure, for compressible fluids this is typically 10% and for incompressible 20%. The design of the control chamber must therefore provide not only a rapid opening but also a relatively small blowdown. It is also worth noting that the lift characteristic e.g. full lift versus high lift will give large variations in capacity for any given nominal size. This must be taken into account when replacing a high lift valve with a full lift since the outlet pipework may well be undersized.
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Fig. 6 How safety valves work 15
Setting and sealing How to establish set pressure
There are two fundamental considerations which must be taken into account when specifying a safety valve set pressure. 1. The set pressure must be low enough to ensure that the maximum allowable accumulated pressure of the boiler, vessel or system it is protecting is not exceeded. 2. The set pressure must be high enough to ensure that there is sufficient margin above the normal system operating pressure to allow the valve to close. However it must be no higher than the maximum allowable working pressure of the system. Unless operational considerations dictate otherwise, the safety valve set pressure should always be significantly above the system operating pressure with a margin allowed for blowdown. A safety valve set just above the normal operating pressure can lead to poor shut-off. When the system operating pressure and safety valve set pressure have to be as close as possible to one another, a 0.1 bar minimum margin between reseat pressure and system operating pressure is recommended to guarantee tight shut-off. In this case it is important to take into account any variations in the system operating pressure (often the result of the inherent proportional band of a pressure control) before adding the 0.1 bar margin.
Pressure reducing valve proportional offset
This refers to the difference between a pressure reducing valve (PRV) set pressure and actual controlled pressure, under varying load conditions. Proportional offset is directly related to the proportional band of a control system. For example, if set pressure of a self-acting PRV is adjusted under full load conditions, then the valve's proportional band must be added to the PRV set pressure. Some pilot operated PRV's for example have a maximum proportional band of only 0.2 bar. With a set pressure of 5.0 bar adjusted under full load, it would give 5.2 bar with no load. The same valve would exhibit a set pressure of 4.8 bar under full load, if set pressure of 5.0 bar is adjusted under no load. The amount and position of pressure control offset depends on the type of valve or pressure controller being used. Important 1. Determine the proportional band of the control valve from the manufacturer. 2. Check how the PRV / controller is to be commissioned.
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Shut-off margin
When the system operating pressure and a safety valve set pressure have to be as close as possible to one another, a 0.1 bar margin between blowdown and system pressure is recommended, to guarantee tight shut-off. The following example uses a 0.1 bar shut-off margin. Example: safety valve set pressure selection to be as close as possible to PRV working pressure. Parameters: PRV working pressure: 6.0 bar PRV proportional band: this example uses 0.3 bar operating above PRV working pressure. Blowdown (reseat differential): 10% 6.0 + 0.3 + 0.1 = 7.1 bar 0.9* *10% blowdown used here might differ, check with manufacturer. Answer: 7.1 bar is closest safety valve set pressure to PRV working pressure. This must not exceed the maximum allowable working pressure under any circumstances. Important Always attempt to set the safety valve pressure as high as praticable above the PRV pressure, without exceeding the maximum allowable working pressure. The maximum allowable accumulated pressure varies according to the standard to which the equipment is designed, but is usually at least 10% above the maximum allowable working pressure. This means that a valve with an overpressure of 10% can be set at the maximum allowable working pressure. However valves with larger overpressures must be set at a lower pressure such that the permitted 10% accumulation is never exceeded. For example a valve with a 25% overpressure requirement would need to be set at least 12% below the maximum allowable working pressure if the 10% accumulation allowance was not to be exceeded. See Figure 7, page 18, illustrating pressure level relationships for pressure relief valves. For cases where a conventional valve is subject to a constant superimposed back pressure then it should be noted that the set pressure will be equal to the cold differential set pressure (pressure set on test stand) plus the superimposed back pressure. 17
Pressure level relationships for pressure relief valves (from API 520)
Pressure vessel requirements
Vessel pressure
Maximum allowable accumulated pressure (fire exposure only)
121 120
Maximum allowable accumulated pressure for multiple valve installation (other than fire exposure)
Maximum allowable working pressure or design pressure (hydro test at 150)
Equal maximum normal operating pressure
Maximum relieving pressure for fire sizing
Maximum relieving pressure for process sizing
116 115
Percent of maximum allowable working pressure (gauge)
Maximum allowable accumulated pressure for single valve (other than fire exposure)
Typical characteristics of safety relief valves
Margin of safety due to orifice selection
Multiple valves Single valves
Maximum allowable set pressure for supplemental valves (fire exposure)
110
Overpressure (maximum) Maximum allowable set pressure for supplemetal valves (process)
105
Overpressure (typical) 100
Simmer (Typical)
Maximum allowable set pressure for single valve (average) Start to open Blowdown (typical)
95
Seat clamping force Reseat pressure for single valve (typical)
90
Standard leak test pressure Set pressure tolerance ±3%
85
Fig. 7 18
How to set
For most types of valve, air or gas setting is permissible (see Appendix 3, for cold differential set pressure and test pressure definitions). A specially constructed test stand is usually employed allowing easy and quick mounting of the safety valve for adjustment and subsequent locking and sealing of the valve at the required set pressure. The most important requirement in addition to the usual safety considerations is that instrument quality gauges are used and a regular calibration system is in place. All safety valve standards will specify a particular tolerance for the set pressure (which is typically around ±3%) and this must be observed. It is also important that the environment is clean, dust free and relatively quiet. The source of setting fluid can vary from a compressed gas cylinder to an intensifier and accumulator vessel running off an industrial compressed air main. In the latter case the air must be clean and oil and water free. It is worth noting that there is no requirement for any sort of capacity test. The test stand simply enables the required set pressure to be ascertained. Usually this point is established by listening for an audible 'hiss' as set point is reached. When making adjustment it is imperative for both metal seated and soft sealed valves that the disc is not allowed to turn on the seat or nozzle since this can easily cause damage and prevent a good shut-off being achieved. The stem should therefore be gripped whilst the adjuster is turned to prevent this happening. There is a fundamental difference in the allowable setting procedures for ASME I steam boiler valves in that to maintain the National Board approval to apply the 'V' stamp these valves must be set on steam on a rig capable not only of achieving the desired set pressure but also with sufficient capacity to demonstrate the popping point and reseat point. This must be done in accordance with an approved and controlled quality procedure. For ASME VIII valves ('UV' stamp) if the setter has a steam setting facility then these valves must also be set on steam, if not then air / gas setting is permissible. In the case of valves equipped with blowdown rings, very often the set positions will need to be established and locking pins sealed in accordance with the relevant manufacturer's recommendations. For liquid applications with ASME VIII valves liquid (usually water) must be used for setting purposes.
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Who can set ?
For a valve not claiming any particular standard and with no reference to a standard on the name-plate or supporting literature anybody can set given suitable equipment. For a valve which has been independently approved by a notified body to a specific standard then the setting and sealing operation is part of that approval. If the valve is set by a third party for the approval to be valid, the setter must be an approved agent of the manufacturer working in accordance with agreed quality procedures using equipment approved by the manufacturer or the notified body.
Sealing
To prevent unauthorised alteration or tampering, most standards require provision to be made for sealing the valve after setting. The most common method is to use sealing wire to secure the cap to the spring housing and housing to the body and is also used to lock blowdown adjuster ring pins into position. The wire is subsequently sealed with a lead seal which may bear the imprint of the setters trademark.
Lead seal
Fig. 9 Sealed cap showing lead seal 20
Seat tightness Seat tightness of safety valves is a subject often discussed between manufacturer and user. It is an important consideration since leakage can cause deterioration of the sealing faces (in extreme cases this may cause premature lifting) and continuous loss of system fluid. The valve
For a metal seated safety valve to provide an acceptable shut-off the sealing surfaces need to have a high degree of flatness with a very good surface finish, the disc must articulate on the stem and the stem guide must not cause any undue frictional effects. It should be noted that, unlike an ordinary stop valve, there is only a small difference in force between the system pressure acting on the disc and the spring force opposing it that provides the closing force. In addition, for a reasonable service life, the mating and sealing surfaces must have a high wear resistance. Typical figures required for an acceptable shut-off for a metal seated valve are 0.5 micron for surface finish and two optical lightbands for flatness. Resilient or elastomer seals incorporated into valve discs are often used for an improved shut-off where system conditions permit. It should be noted, however, that a soft seal is often more susceptible to damage than a metal seat.
The installation
Seat damage can often occur when a valve is first lifted as part of the general plant commissioning procedure since very often dirt and debris are present in the system. To ensure that foreign matter does not pass through the valve, the system should be first be flushed and the valve must be mounted where dirt, scale and debris cannot collect. It is also important on steam applications that the valve is installed such that condensate cannot collect on the upstream side of the disc since this can also lead to leakage. At the same time, it is essential that the downstream side is well drained so that downstream flooding, (which can also encourage leakage) cannot occur. See 'Installation', page 27.
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Fig. 10 Dirt between seat and disc Sometimes when leakage is experienced, it may be due to dirt sitting across the seating face and damage may not have occurred, further lifting (using the lever) can sometimes clear any dirt and scale and restore the shut-off. This problem can also occur during the periodic lifting demanded by insurance companies and routine maintenance programs. It can be seen that the vast majority of safety valve seat leakage problems occur after initial manufacture and test and are the result of damage (sometimes in transit and sometimes as a result of misuse / contamination when installed) or else poor installation. Sealing face refurbishment
Unlike most pipeline products it is important to note that the repair and refurbishment (even resetting) of safety valves is something that only authorised personnel should carry out, working with the approval of the manufacturer, and using information supplied by the manufacturer. Routinely supplied spare parts are typically springs, discs and nozzles, resilient seals and gaskets. Many valves have seat rings which are not removable and if it is possible these are sometimes reprofiled and relapped in the body. It is important that the size of seat orifice is maintained exactly in line with the original drawings since this can affect the effective area and, subsequently, the spring range.
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It is critical this is done properly since for metal seated valves machine lapping of both disc and seat / nozzle is required to achieve the extremely high standard of flatness and finish which is required to meet the shut-off requirements. It is completely unacceptable for the disc to be lapped directly onto the seat in situ since a groove will be created on the disc preventing a consistent shut-off after lifting. In the case of resilient seal valves usually the seal (which is normally an 'O' ring or disc) can be changed in the disc assembly. If Independent Authority Approval is to be maintained then it is mandatory that the repairer is acting as the Manufacturer's approved agent. For ASME approved valves the repairer must be independently approved by the National Board and is subsequently allowed to apply a 'VR' stamp to indicate a valve which has been repaired. Testing
Note that, most safety valve standards do not detail shut-off parameters. For those that do the requirements and recommended test procedure is usually similar and is generally based on the American Petroleum Industry Standard API 527 which is commonly used throughout the safety valve industry.
Note: The cover plate should be fitted with a suitable device to relieve body pressure in case of accidental popping of the valve. Tube 5/18" (7.9 mm) Wall 00 x 0.035" (0.89 mm)
½" (12.7 mm)
Fig. 11 Apparatus to test seat tightness with air 23
The procedure for valves set on air, in short, involves blocking all secondary leakage paths and whilst maintaining the valve at 90% of the set pressure on air, measuring the number of bubbles discharged from a 6 mm internal diameter tube connected to the outlet held 12.7 mm below the surface of water. For the majority of valves set below 1000 psi the acceptance criteria is 20 bubbles per minute. For valves which are set on steam or water, the leakage rate should be assessed using the corresponding setting media. For steam, there must be no visible leakage observed against a black background for 1 minute after a 3 minute stabilisation period and for water there is a small leakage allowance, dependent on the orifice area, of 10 ml per hour per inch of nominal inlet size. The above procedure can be time consuming, so it is quite common for manufacturers to employ a test using alternative methods, for example, accurate flow measuring equipment which is calibrated against the parameters set in API 527. Safety Note: Care must be taken when checking a safety valve leakage rate in case of unintentional lifting of the safety valve.
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Marking Safety valve standards are almost without exception very specific about the information which must be carried on the valve. Marking is mandatory on both the shell, usually cast or stamped, and the name-plate which must be securely attached to the valve. A general summary of the information required is listed below. Table 1 details the marking system required by TÜV and Table 2, page 26 details the fluid reference letters. On the shell: Size designation. Material designation of the shell. Manufacturer's name or trademark. Direction of flow arrow. On the identification plate: Set pressure (in bar g for European valves and psi g for ASME valves). Number of the relevant standard. (or relevant ASME stamp - Table 1). Manufacturers model type reference. Derated coefficient of discharge (Table 1) or certified capacity. Flow area. Lift and overpressure. Date of manufacture or reference number. National Board approved ASME stamps are applied as follows: V UV UD NV VR
ASME I approved safety relief valves ASME VIII approved safety relief valves ASME VIII approved rupture disc devices ASME III approved pressure relief valves authorised repairer of pressure relief valves
Table 1 Shows the marking system used for valves approved by TÜV to AD-Merkblatt A2, DIN 3320 and TRD 421 Marking system:
TÜV.
SV.
98.
XXX.
XX.
DGF.
0.XX.
X
TÜV symbol Safety valve Year of test Test number Minimum flow diameter do Fluid identification character (see page 26) Flow coefficient or flow Set pressure (bar g) 25
Fluid flow coefficient of discharge identification letters The Kdr or a w value can vary according to the relevant fluid and is either suffixed or prefixed by the identification letters shown in Table 2. Table 2 Fluid types defined as steam, gas or liquid. for a w for Kdr D (dampf) for steam S for steam G (gase) for gas G for gas F (flussigkeiten) for liquids L for liquids
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Installation Transport
Safety valves are precision pieces of safety equipment and despite their industrial appearance they are set to close tolerances and have accurately machined internal parts which can be susceptible to misalignment and damage if mishandled. Valves should be transported upright if possible and should never be carried or lifted by the easing lever. Protective plugs and flange protectors should not be removed before actual installation.
Mounting
Safety valves should always be installed with the bonnet vertically upwards and should be mounted such that no excessive static, dynamic or thermal loads can be transmitted to the valve particularly via either the upstream or discharge pipework. Safety valves should never be capable of being isolated (see Section on 'Changeover valves', page 30). The valve should be mounted as directly onto the vessel or system as possible and the cross section of the inlet connection should never be smaller than the cross section of the safety valve inlet. When discharging, the pressure loss in the supply line should never exceed 3% of the set pressure (There are many standards which give more detail on how to establish this e.g. AD-Merkbatt A2 or API RP 520 part II)
Discharge pipework
For steam and gases discharge pipework should rise and for liquids it should fall. Rising discharge pipework must be drained. Any horizontal runs should have a downward gradient of at least 1:100 away from the valve and be drained at any point the pipe rises. It is absolutely essential that no fluid can collect on the downstream side of a safety valve discharging to atmosphere. This will impair the performance of the valve and cause corrosion of the spring and internal parts. Many safety valves are provided with a body drain connection, if this is not used or not provided then a small bore drain should be fitted in close proximity to the valve outlet. Small bore drains should be piped to a safe place not straight onto the floor ! Discharge pipework should be short and direct wherever possible. For a conventional valve the discharge pipework should not generate back pressure of more than 10 - 15% (depending on standard used). Discharge pipework should never be smaller in cross sectional area than the valve outlet, but can be larger. This will reduce built-up back pressure and can also significantly reduce noise levels. It is permitted to use silencers as long as due consideration is given to the applicable back pressure limitations.
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Discharge pipework should be anchored independently with due consideration given to the reaction force generated when discharging. (See Section on 'Reaction force when discharging', page 31). A slip joint arrangement is sometimes used at the inlet to the riser pipe utilising a drip pan elbow which ensures that the outlet pipe is independently supported.
Fig. 12 Reaction bars
28
Manifolds
Manifolded discharge pipework is only routinely used for process type applications in association with bellows balanced valves where discharged fluid must be contained. It should be noted that if conventional valves are set at varying pressures then the action of one discharging may influence the set point and capacity of another, particularly on closed systems. For steam applications it is not recommended, but can be utilised if proper consideration is given to all aspects of the design and installation. Manifolds must be sized so that in worst case (i.e. when all manifolded valves are discharging) pipework is large enough to cope without generating unacceptable levels of back pressure. It is also recommended that discharge connections enter at an angle of no greater than 45° to the direction of flow. The manifold should ideally step up in volume as each valve outlet enters and be properly secured and drained where necessary.
Fig. 13 Manifold
29
Changeover valves
Changeover valves enable two safety valves to be mounted such that one is in service and the other is in stand-by mode, therefore allowing maintenance without shutting down the plant. Principally used within the petrochemical and gas industries or where plant needs to be run continuously. All of the installation guidelines previously mentioned will apply to the safety valve sets when used in conjunction with a changeover valve.
Fig. 14 Changeover valve
30
Reaction force when discharging When a safety valve discharges there will be a significant resultant force acting in opposition to the direction of discharge. It is very important that excessive loads are not imposed on the valve or inlet connection by the resulting forces generated in the discharge pipework. When an elbow is installed in the discharge system to direct fluid up into a vent pipe, the location of the elbow and any supports are an important consideration in the analysis of the bending moments. For larger sizes of valve, reaction bar anchor points are often provided on the body to enable the loads generated to be directly transmitted to a separate anchor point.
Fig. 15 Reaction bars
31
Determination of reaction force
The reaction force (F) includes the effects of both momentum and static pressure; thus, for any gas, vapor or steam:-
Ö(k +kT1) M + (AP)
Imperial units:
F = W 366
Metric units:
F = 129 W
Ö(k +kT1) M
+ 0.1 (AP)
Where: F = Reaction force at the point of discharge to atmosphere. measured in pounds / newtons. W = Flow of any gas or vapor. measured in pounds per hour / kilograms per second. k = Ratio of specific heats (Cp / Cv). Cp = Specific heat at constant pressure. Cv = Specific heat at constant volume. T = Absolute gas temperature. Measured in Rankine / Kelvin. M = Molecular weight of the process fluid. A = Area of the outlet at the point of discharge. Measured in ins² / mm². P = Static pressure within the outlet at the point of discharge. Measured in pounds per square inch gauge / bar gauge.
32
Noise emission Although discharge from safety valves should only occur infrequently the noise generated can often be significant. There are several ways to reduce noise levels, the simplest being to use larger diameter discharge pipe or lag the discharge pipe (the valve must NOT be lagged). It is also permissible for a silencer to be used in extreme cases but any back pressure generated must then be taken into account. Determination of the noise level for compressible fluids
Assuming a sonic flow nozzle discharge, an approximate value of the sound power level Lp in decibels at flange outlet is given by the following equations: Lp = 60 log10C + 10 log10 W1 – 50.5 Where: Lp = Sound power level in db(A). C = Speed of sound in gas in ft / sec = 233 k
= Isentropic coefficient.
Ö kTM
M = Molecular weight. W1 = Mass flow in lb/sec. T = Absolute gas temperature in °Rankine. The sound power level L at distance R is calculated from the sound power level Lp as follows: L = Lp – 10 log102 P R² + 10 Where: R = Distance from source in feet.
33
Selection There are several different factors which need to be taken into consideration before choosing any particular type of valve. For a non-critical application the most obvious one is price ! It should be noted that when making price comparisons, then capacity must be taken into consideration (indicated by the Kdr or a value) as well as nominal size, since there can be large variations between models with the same inlet connection but varying lift characteristics. Selection of a suitable valve will depend on the following criteria: Type of disposal system
Valves with an open bonnet can be used on steam, air or non-toxic gas where some discharge to atmosphere other than through discharge pipework is acceptable. A lifting lever is normally specified for these applications. For gas or liquid applications, where escape to atmosphere is not permitted, a closed bonnet must be specified. A closed / gas-tight cap or sealed lever is also required. For applications with a superimposed back pressure (i.e. common manifolds typically seen in the process industry) a balancing bellows construction is required.
Valve construction
Semi-nozzle (separate seat ring). Valves of this type of construction are used for non-toxic, non-corrosive type media at moderate pressures. Full nozzle. Valves of this type of construction are typically used in the process industry particularly for corrosive media or for extremely high pressures. Fluid will normally only contact the nozzle (inlet) and disc until discharge occurs. For corrosive fluids or high temperatures then special materials of construction may be required.
Operating characteristic
Approval
34
Performance requirements vary according to application and the valve must be selected accordingly. For steam boilers, a small overpressure value is required, usually 3% or 5%. For most other applications 10% overpressure is required and for some special applications, such as fire protection according to API 520, larger values such as 20% are allowed. For liquids, overpressures 10% or 25% are common, and blowdown values tend to be up to 20%. For many valve applications the customer will state the required code or standard for the construction and performance of the valve. This is usually accompanied by a requirement for approval by independent authority, to guarantee conformance with the required standard.
Sizing The safety valve should be capable of relieving the maximum possible capacity of the supply to the system or vessel it is protecting to ensure that the maximum allowable working pressure is never exceeded. This is very often more than the normal working capacity. For a vessel or process, factors such as failure of control equipment, fire, pump failure, uncontrolled chemical reaction, and vessel isolation need to be considered. For pressure control valves, the maximum capacity must be established, which can sometimes be higher than working capacities. If a bypass or parallel valve is installed then the safety valve should be capable of relieving the combined capacity of both valves. For many straightforward applications involving saturated steam, air or water, after the type and model range of the safety valve have been chosen, the manufacturer's published capacity charts (See Table 3) are quite sufficient to select the correct size safety valve, i.e. one whose capacity just exceeds the required capacity at the desired overpressure. Table 3 Typical saturated steam capacity chart Valve size
DN15
Set pressure bar g
DN20
DN25
DN32
DN40
DN50
DN65
Saturated steam discharge capacities in kg/h at 10% accumulation
0.5
29
61
107
183
266
-
-
00.75
33
72
126
216
313
466
782
1.0
39
83
144
249
360
536
901
1.5
48
104
182
314
455
677
1 136
2.0
59
126
220
379
549
817
1 372
3.0
79
169
296
509
738
1 099
1 844
4.0
99
212
371
639
926
1 380
2 315
5.0
119
256
448
770
1 115
1 661
2 787
6.0
140
299
523
899
1 304
1 941
3 258
7.0
159
342
599
1 030
1 492
2 223
3 730
8.0
180
385
674
1 160
1 681
2 503
4 201
9.0
200
429
751
1 291
1 879
2 784
4 673
10.0
220
472
825
1 420
2 058
3 065
5 144
11.0
240
515
903
1 551
2 247
3 346
5 616
12.0
260
558
978
1 680
2 436
3 628
6 087
13.0
280
601
1 055
1 811
2 624
3 909
6 559
14.0
300
645
1 130
1 941
2 813
-
-
16.0
342
732
1 279
2 201
3 190
-
-
18.0
382
818
-
-
-
-
-
20.0
422
905
-
-
-
-
-
35
Where sizing charts are not available or do not cater for the particular fluid or conditions (e.g. back pressure, high viscosity) then the minimum required orifice area will need to be calculated and a suitable valve with a larger orifice area chosen. Alternatively, having selected a particular valve with known orifice area, the maximum capacity can be calculated. The exact sizing formula varies depending on the standard used. Detailed below are sizing methods according to: AD-Merkblatt A2, DIN 3320, TRD 421. ASME /API RP 520. BS 6759 for steam, air / gases and liquids. Two-phase flow
When sizing safety valves for boiling liquids (e.g. hot water) consideration must be given to vaporisation (flashing) during discharge. It is assumed that the medium is in liquid state when the valve is closed and that, when the valve opens, part of the liquid vaporises due to the drop in pressure through the valve. The required flow area has to be calculated for the liquid and vapour components of the discharged fluid. This is referred to as 'two-phase flow'. The sum of these two areas is then used to select the appropriate orifice size from the chosen valve range. Many standards do not actually specify sizing formula for twophase flow and recommend that the valve manufacturer be contacted directly for advice in these instances. An example of sizing for two-phase flow using the AD-Merkblatt A2 is provided in the section headed - 'Sizing according to AD-Merkblatt A2, DIN 3320, TRD 421', page 39. For pressures in excess of 100 bar (1 500 psi) it is recommended that reference is made to the manufacturer or appropriate standard since additional correction factors are required.
Coefficient of discharge
All sizing formulae are based around a 'coefficient of discharge' which is a figure established as a result of actual testing. It is specific to any particular safety valve range and will be provided by the manufacturer. If the valve is independently approved it is known as the 'certified coefficient of discharge'. The coefficient of discharge is the actual measured capacity of the valve divided by the theoretical capacity of a nozzle of equivalent flow area averaged over the valve range. Often this figure is further derated by multiplying by 0.9 giving a derated coefficient of discharge.
36
This figure will be different depending on the fluid, steam and air often have similar or identical values but for liquids the value is a lot lower for any given valve. For DIN type methods the coefficient of discharge value (referred to as outflow coefficient) is represented by a. For BS and ASME it is referred to as 'Kd'. Overpressure
Before sizing, the appropriate overpressure of the valve must be established. It is not permitted to calculate capacity at a lower overpressure than that at which the coefficient of discharge was established, although it is permitted to use a higher overpressure. For steam applications ASME I requires a 3% overpressure. For steam, gas and vapour applications, ASME VIII and DIN type safety valves will have a 10% overpressure. For full lift (vollhub) DIN type valves, the design lift must be achieved at 5% overpressure but for sizing purposes an overpressure value of 10% may be used. For liquid applications, the overpressure is 10% according to AD-Merkblatt A2, DIN 3320, TRD 421 and ASME. It is quite common for a figure of 25% to be used for non-certified ASME valve sizing and this figure is used in many standards (including BS 6759).
Critical and subcritical flow
The flow of a gas or vapour through an orifice, such as the flow area of a safety valve, increases as the downstream pressure is decreased to the critical pressure, when critical flow is achieved. Further decrease in downstream pressure will not result in any further increase in flow. Critical flow occurs when:
(
)
Pb 2 (k /(k – 1)) P1 £ k + 1
Where: Pb = Back pressure (bar abs). P1 = Actual relieving pressure (bar abs). k = Isentropic coefficient at the relieving inlet conditions. As a guide, critical pressure is taken as 55% of accumulated inlet pressure in absolute terms. The formulas provided on the following pages are for critical flow. Non-critical flow can occur when sufficient back pressure exists and in certain circumstances a correction factor can be applied to allow for this.
37
Back pressure
Back pressure is the pressure existing at the outlet of a safety valve due to pressure in the discharge system. Back pressure may influence valve set pressure, alter reseat pressure, decrease discharge capacity and cause instability. There are two types of back pressure: 1. Built-up back pressure: pressure built-up in the discharge pipework by the action of the valve discharging. 2. Superimposed back pressure: static pressure existing at the valve outlet at the time the valve is required to operate. Back pressure will affect the operation of the valve depending on whether it is constant or variable, whether the valve is closed or relieving and whether the valve is conventional in its construction or has a balancing bellows. For a conventional valve subject to a constant superimposed back pressure, the set pressure is effectively changed by an amount equal to the back pressure. The set pressure is therefore increased by the value of the back pressure. The cold differential set pressure will therefore be different to the actual set pressure of the valve e.g. RISP - CBP = CDSP Where: RISP = Required installed set pressure. CBP
= Constant back pressure.
CDSP = Cold differential set pressure. For variable superimposed back pressure the effective set pressure could change as the back pressure varies. In this case, a conventional valve could not be used if the variation was more than 10 to 15% and a balanced valve (removing the effects of back pressure) would be required. Built-up back pressure as a result of a conventional safety valve discharging will rapidly reduce the force which holds the valve open and so must be limited to between 10 to 15% depending on which standard is referred to. Alternatively a balanced valve could be used. The sizing calculations in accordance with AD-Merkblatt A2, DIN 3320, TRD 421 using the outflow function Y (which can be either calculated or obtained from the graph) will encompass back pressure correction. For calculation in accordance with ASME / API RP 520 the graphs provided should be used. 38
Coefficients and correction factors
Various coefficients and correction factors are used in the sizing formulae for all methods. Graphs and look up charts are given where most data can be found. ASME sizing data is always based on imperial units and BS / DIN on metric units. For steam sizing, the DIN formula uses a pressure medium coefficient termed c which provides a conversion for noncompatible units and also allows for superheated steam. For BS and ASME sizing methods, a superheat correction factor is retrospectively applied if necessary. For gas or vapour sizing, the DIN type calculations also include an outflow function known as Y which takes into account back pressure and isentropic coefficient. ASME sizing applies back pressure correction factors where necessary. The examples shown on the following pages, provide various formulae for sizing safety valves and an explanation of each.
39
Sizing according to AD-Merkblatt A2, DIN 3320 and TRD 421
For steam: c
Ao =
Qm
x
aw
Po
x
Ao
Qm =
x
a w x Po c
For air and gases: Ao =
0.179 1 Y
x
aw x
x
Qm
Po
Ö
x
Qm
Qm =
Ao
x
Y
aw
x
x
Po
0.1791
M kT
Ö kTM
For liquids: 0.621 1
Ao =
aw
Ö
Qm =
Ao
r DP
x
aw Ö r DP
0.6211
Where: Ao = Minimum cross sectional flow area (mm²). Qm = Mass flow to be discharged (kg / h). Po = Absolute set pressure (bar abs). DP = Po - absolute back pressure (bar abs). T
= Inlet temperature in Kelvin (°C + 273).
r
= Density (kg / m³). (Appendix page 59).
M = Molecular weight (kg / kmol). (Appendix page 58). Z
= Compressibility factor. (Page 57).
aw =
40
Outflow coefficient. (Specific to valve, stated by manufacturer).
c
= Pressure medium coefficient. (Fig. 17, page 49).
Y
= Outflow function. (Fig. 16, page 48).
Examples: where Ao is the minimum cross sectional flow area of the valve (mm²) For steam: Medium: Saturated steam Discharge quantity: 2 500 kg / h Set pressure: 4 bar abs Back pressure: Atmosphere Outflow coefficient: 0.7 Pressure medium coefficient: 1.88 (from diagram) Ao =
1.88 x 2 500 = 1 678 mm² 0.7 x 4
For air and gases: Medium: Air Discharge quantity: 10 000 kg / h Set pressure: 8 bar abs Back pressure: Atmosphere Inlet temperature: 293 K Molecular weight: 16 Outflow coefficient: 0.7 Outflow function: 0.484 2 (from chart) Ao =
0.179 1 x 10 000 0.484 2 x 0.7 x 8 16 1 x 293
Ö
= 2 826 mm²
For liquids: Medium: Water Discharge quantity: 12 500 kg / h Set pressure: 6 bar abs Back pressure: 2 bar abs Outflow coefficient: 0.5 Ao =
0.621 1 0.5
12 500
x
Ö4
x
x
= 246 mm²
998
41
Two-phase flow example: The following information will be required: The discharge capacity. Fluid. Temperature. The valve set pressure. Back pressure. The back pressure will be 1 bar absolute if discharge is to atmosphere. To establish what proportion (n) of the discharge will be vapour: n =
h1 - h 2 r
Where: h1 = Enthalpy of fluid before valve. h2 = Enthalpy of fluid after valve. r
= Enthalpy of evaporation after valve.
For hot water, enthalpy values can be obtained from steam tables. It may be necessary to interpolate to find the appropriate enthalpy value for the elevated temperature. 'n' multiplied by the discharge capacity refers to that portion of the flow which is vapour, the remainder being that portion of the flow which remains in the liquid state. The area sizing calculation is then used as given in the previous section, to calculate the required area to discharge the vapour portion, and the required area to discharge the liquid portion. The sum of these areas is then used to establish the total required area of the valve.
42
For example: Medium:
Hot water
Temperature:
160°C
Discharge quantity:
3 900 kg / h
Set pressure:
10 bar g
Back pressure:
Atmosphere
Vapour part (from Steam tables, Appendix 2, page 60): h1 =
675 kJ / kg (interpolating to find value at 160°C)
h2 =
417 kJ / kg (value at atmospheric pressure - 1 bar abs)
r
= 2 258 kJ / kg (value at atmospheric pressure - 1 bar abs) 675 - 417 2 258
n =
= 0.114 3
Capacity discharge as vapour (steam): =
0.114 3 - 3 900 kg / h
= 446 kg / h
Liquid part: Capacity discharged as liquid: = 3 900 kg / h - 446 kg / h = 3 454 kg / h Calculated area for steam portion: c
Ao =
Qm x Po
x
aw 1.9
446 0.7 x 11
Ao =
x
= 110 mm²
Calculated area for water portion: 0.621 1
Ao =
Ao =
x
Qm
a w Ö r DP 0.621 1 0.55
Total calculated area:
x
Ö
3 454
= 41 mm²
10 x 907
110 + 41
= 151 mm²
A valve must be selected with a discharge area greater than 151 mm². 43
Sizing according to ASME / API RP 520
For steam:
●
W
A =
51.5
P1
x
x
Kd
A
x
x
Ksh
●
W =
51.5
P1
x
For air and gases: 1.175 A
●
V =
x
x
C
x
Kd
x
C
x
1.175
●
A =
●
x
Kd
Ksh
x
P1
x
P1
x
Kb Kb
x
Ö TZG
For liquids:
Q =
Kd
ÖVTZG
●
A =
x
A
38
x
Kd
x
x
38
x
Kd
Ö
Q G Kv x Kw
x
Kp
Kv
Kw
x
x
x
ÖG
ÖP - P K ÖP - P 1
p
2
1
2
Where: A = Required effective discharge area (square ins). W = Required flow through valve (pounds per hour). V = Required flow through valve (standard cubic feet per minute). Q = Flowrate (U.S. gallons per minute). P1 = Upstream relieving pressure (psi abs) ●
●
●
(set pressure + allowable overpressure + atmospheric pressure).
P2 = C = T = G = Z = Kd = Ksh= Kb = Kv = Kw =
44
Absolute back pressure (psi abs). Nozzle gas constant. (Table 4, page 50). Relieving temperature degrees Rankine (°F + 460). Specific gravity. (Appendix 1, page 58). Compressibility factor. (Page 57). Effective coefficient of discharge. Superheat correction factor. (Table 6, page 52). Back pressure correction factor for gas and vapour. (Page 54). Viscosity correction factor. (Page 56). Backpressure correction factor for liquids (bellows balanced valves only). (Page 56)
Examples: where 'A' is the required effective discharge area (ins²). For steam: Medium: Superheated steam Discharge quantity: 88 500 lb / h Relieving temperature: 750°F Upstream relieving pressure: 262 psi abs Coefficient of discharge: 0.995 Superheat correction factor: 0.844 (from table) A =
51.5
For air and gases:
x
88 500 0.955 x 262
x
0.844
= 8.14 in²
Medium: Methane
Discharge quantity: 12 000 scfm Upstream relieving pressure: 210 psi abs. Back pressure: Atmosphere Relieving temperature: 580 R Coefficient of discharge: 0.955 Specific gravity: 0.554 Nozzle gas constant: 348 A =
Ö
12 000 x 580 x 1 x 0.554 = 2.62 in² 1.175 x 348 x 0.955 x 210
For liquids: Medium: Light fuel oil Discharge quantity: 450 USgpm Relieving pressure: 176 psi g Back pressure: Atmosphere Specific gravity: 0.9 Coefficient discharge: 0.65 A =
450 x 38 x 0.65
Ö 0.9 Ö 176 x
= 1.3 in²
45
Sizing according to BS 6759
For steam: ●
Ao =
0.525
x
E P x Kdr
x
Q1 Kdr
●
x
Ksh
E = 0.525
x
P
x
A
x
A
x
Kdr
x
Cg
Kdr
x
x
Ksh
For air: ●
Ao =
0.193
●
Ö
P 288 T
x
Q1 = 0.193
x
P
Ö 288 T
For gases: ●
Ao =
P
x
Q2 Cg x Kdr
●
M Ö ZT
Q2 = P
x
A
Kdr
x
M Ö ZT
For liquids: ●
Ao =
Q2 1.61 x Kdr
●
Ö r DP
Q2 = 1.61
x
Kdr
x
A
Ö r DP
For hot water: ●
Ao =
Q3 0.329 x Kdr
●
x
P
Q3 = 0.329
x
Kdr
x
A
x
P
Where: Ao = Flow area (mm²). E = Mass flow of saturated steam to be discharged (kg / h). Q1 = Flow to be discharged (litres / second). Q2 = Mass flow to be discharged (kg / h). Q3 = Hot water capacity (kw). P = Absolute relieving pressure (bar abs) (set pressure + overpressure + atmospheric pressure). Cg = Nozzle gas constant. (Table 5, page 51). DP = P - absolute back pressure (bar abs). T = Inlet temperature in Kelvin (°C + 273). r = Density (kg / m³). (Page 59). M = Molecular weight (kg / kmol). (Appendix 1, page 58). Z = Compressibility factor. (Page 57). Kdr = Derated coefficient of discharge. (As stated by manufacturer). Ksh = Superheat correction factor. (Table 7, page 53). ●
●
●
●
46
Examples: where Ao is the required flow area (mm²) For steam: Medium: Saturated steam Discharge quantity: 2 500 lb / h Relieving pressure: 9.8 bar abs Back pressure: Atmosphere Coefficient of discharge: 0.7 Ao =
2 500 0.525 x 9.8
x
= 694 mm²
0.7
For gas: Medium: Air Discharge quantity: 8 000 l / s Upstream relieving pressure: 14.2 bar abs Back pressure: Atmosphere Inlet relieving temperature: 323 K Coefficient of discharge: 0.7 Ao =
8 000 0.193
x
14.2
x
Ö
288 323
= 3 091 mm²
For liquid: Medium: Water Discharge quantity: 15 000 kg / h Relieving pressure: 7.6 bar abs Back pressure: Atmosphere Density: 998 kg / m³ Coefficient of discharge: 0.5 Ao =
1.61
x
15 000 0.5 x 998
Ö
= 214 mm² x
7.6
47
Outflow function Y 0.6
0.5
k 1.8 1.6
Y max. 0.527 0.507
1.4
0.484
1.2
0.459
1.0
0.429
Outflow function Y
0.4
0.3
0.2
0.1
0 0.2
0.3
0.4
0.5 0.6 0.7 Pressure ratio pa / p
pa = absolute relieving pressure p = absolute back pressure Fig. 16 48
0.8
0.9
1.0
Pressure medium coefficient of steam c
700
2.8
600 2.6
Pressure medium coefficient
h x mm² x bar kg
500 2.4 400 2.2 300 2.0 200
saturated steam
1.8
1.6
1.4 1
2
3
4 5 6
8 10 20 40 60 80 100 Set pressure (bar absolute)
200
400
Fig. 17 49
Table 4 Nozzle gas constant C relative to isentropic coefficient k Sizing according to ASME / API RP 520 k
C
k
C
k
C
k
C
1.01
317
1.26
343
1.51
365
1.76
384
1.02
318
1.27
344
1.52
366
1.77
385
1.03
319
1.28
345
1.53
367
1.78
386
1.04
320
1.29
346
1.54
368
1.79
386
1.05
321
1.30
347
1.55
369
1.80
387
1.06
322
1.31
348
1.56
369
1.81
388
1.07
323
1.32
349
1.57
370
1.82
389
1.08
325
1.33
350
1.58
371
1.83
389
1.09
326
1.34
351
1.59
372
1.84
390
1.10
327
1.35
352
1.60
373
1.85
391
1.11
328
1.36
353
1.61
373
1.86
391
1.12
329
1.37
353
1.62
374
1.87
392
1.13
330
1.38
354
1.63
375
1.88
393
1.14
331
1.39
355
1.64
376
1.89
393
1.15
332
1.40
356
1.65
376
1.90
394
1.16
333
1.41
357
1.66
377
1.91
395
1.17
334
1.42
358
1.67
378
1.92
395
1.18
335
1.43
359
1.68
379
1.93
396
1.19
336
1.44
360
1.69
379
1.94
397
1.20
337
1.45
360
1.70
380
1.95
397
1.21
338
1.46
361
1.71
381
1.96
398
1.22
339
1.47
362
1.72
382
1.97
398
1.23
340
1.48
363
1.73
383
1.98
399
1.24
341
1.49
364
1.74
383
1.99
400
1.25
342
1.50
365
1.75
384
2.00
400
C = 520
Ök( k +2 1 )((k + 1) / (k – 1))
C = 315 for k £ 1
50
Table 5 Nozzle gas constant Cg relative to isentropic coefficient k Sizing according to BS 6759 for gas and vapour k
Cg
k
Cg
k
Cg
0.40
1.65
1.02
2.41
1.42
2.72
0.45
1.73
1.04
2.43
1.44
2.73
0.50
1.81
1.06
2.45
1.46
2.74
0.55
1.89
1.08
2.46
1.48
2.76
0.60
1.96
1.10
2.48
1.50
2.77
0.65
2.02
1.12
2.50
1.52
2.78
0.70
2.08
1.14
2.51
1.54
2.79
0.75
2.14
1.16
2.53
1.56
2.80
0.80
2.20
1.18
2.55
1.58
2.82
0.82
2.22
1.20
2.56
1.60
2.83
0.84
2.24
1.22
2.58
1.62
2.84
0.86
2.26
1.24
2.59
1.64
2.85
0.88
2.28
1.26
2.61
1.66
2.86
0.90
2.30
1.28
2.62
1.68
2.87
0.92
2.32
1.30
2.63
1.70
2.89
0.94
2.34
1.32
2.65
1.80
2.94
0.96
2.36
1.34
2.66
1.90
2.99
0.98
2.38
1.36
2.68
2.00
3.04
0.99
2.39
1.38
2.69
2.10
3.09
1.001
2.40
1.40
2.70
2.20
3.13
Cg = 3.948
Ök( k +2 1 )((k + 1) / (k – 1))
51
Table 6 Superheat correction factor Ksh Sizing according to ASME / API RP 520
52
IMPERIAL
Set pressure (psi g)
Temperature (°F) 300
400
500
600
700
800
900
1 000
1 100
1 200
15
1.00
0.98
0.93
0.88
0.84
0.80
0.77
0.74
0.72
0.70
20
1.00
0.98
0.93
0.88
0.84
0.80
0.77
0.74
0.72
0.70
40
1.00
0.99
0.93
0.88
0.84
0.81
0.77
0.74
0.72
0.70
60
1.00
0.99
0.93
0.88
0.84
0.81
0.77
0.75
0.72
0.70
80
1.00
0.99
0.93
0.88
0.84
0.81
0.77
0.75
0.72
0.70
100
1.00
0.99
0.94
0.89
0.84
0.81
0.77
0.75
0.72
0.70
120
1.00
0.99
0.94
0.89
0.84
0.81
0.78
0.75
0.72
0.70
140
1.00
0.99
0.94
0.89
0.85
0.81
0.78
0.75
0.72
0.70
160
1.00
0.99
0.94
0.89
0.85
0.81
0.78
0.75
0.72
0.70
180
1.00
0.99
0.94
0.89
0.85
0.81
0.78
0.75
0.72
0.70
200
1.00
0.99
0.95
0.89
0.85
0.81
0.78
0.75
0.72
0.70
220
1.00
0.99
0.95
0.89
0.85
0.81
0.78
0.75
0.72
0.70
240
1.00
0.95
0.90
0.85
0.81
0.78
0.75
0.72
0.70
260
1.00
0.95
0.90
0.85
0.81
0.78
0.75
0.72
0.70
280
1.00
0.96
0.90
0.85
0.81
0.78
0.75
0.72
0.70
300
1.00
0.96
0.90
0.85
0.81
0.78
0.75
0.72
0.70
350
1.00
0.96
0.90
0.86
0.82
0.78
0.75
0.72
0.70
400
1.00
0.96
0.91
0.86
0.82
0.78
0.75
0.72
0.70
500
1.00
0.96
0.92
0.86
0.82
0.78
0.75
0.73
0.70
600
1.00
0.97
0.92
0.87
0.82
0.79
0.75
0.73
0.70
800
1.00
0.95
0.88
0.83
0.79
0.76
0.73
0.70
1 000
1.00
0.96
0.89
0.84
0.78
0.76
0.73
0.71
1 250
1.00
0.97
0.91
0.85
0.80
0.77
0.74
0.71
1 500
1.00
1.00
0.93
0.86
0.81
0.77
0.74
0.71
Table 7 Superheat correction factor Ksh Sizing according to BS 6759
METRIC
Set pressure (bar g)
Temperature (°C) 300
400
500
600
700
800
900
1 000
1 100
1 200
2
1.00
0.99
0.94
0.89
0.86
0.82
0.79
0.76
0.74
0.72
3
1.00
0.99
0.94
0.89
0.86
0.82
0.79
0.76
0.74
0.72
4
1.00
0.99
0.94
0.90
0.86
0.82
0.79
0.76
0.74
0.72
5
1.00
0.99
0.94
0.90
0.86
0.82
0.79
0.76
0.74
0.72
6
0.99
0.94
0.90
0.86
0.82
0.79
0.76
0.74
0.72
7
0.99
0.95
0.90
0.86
0.82
0.79
0.77
0.74
0.72
8
1.00
0.95
0.90
0.86
0.82
0.79
0.77
0.74
0.72
9
1.00
0.95
0.90
0.86
0.83
0.79
0.77
0.74
0.72
10
1.00
0.95
0.90
0.86
0.83
0.79
0.77
0.74
0.72
11
1.00
0.95
0.90
0.86
0.83
0.79
0.77
0.74
0.72
12
1.00
0.95
0.90
0.86
0.83
0.79
0.77
0.74
0.72
13
1.00
0.96
0.91
0.86
0.83
0.80
0.77
0.74
0.72
14
1.00
0.96
0.91
0.86
0.83
0.80
0.77
0.74
0.72
16
1.00
0.96
0.91
0.87
0.83
0.80
0.77
0.74
0.72
18
0.96
0.91
0.87
0.83
0.80
0.77
0.74
0.72
20
0.97
0.91
0.87
0.83
0.80
0.77
0.74
0.72
24
0.98
0.92
0.87
0.84
0.80
0.77
0.74
0.72
28
0.99
0.92
0.87
0.84
0.80
0.77
0.75
0.72
34
0.99
0.93
0.88
0.84
0.80
0.77
0.75
0.72
40
1.00
0.94
0.89
0.84
0.81
0.78
0.75
0.72
56
0.96
0.90
0.86
0.81
0.78
0.75
0.73
70
0.98
0.92
0.86
0.82
0.79
0.76
0.73
85
1.00
0.93
0.87
0.83
0.79
0.76
0.73
100
1.00
0.93
0.88
0.84
0.80
0.76
0.74
53
Gas and vapour constant back pressure correction factor Kb Sizing according to ASME / API RP 520
Balanced bellows valves only: Percent of gauge back pressure = PB
100
x
Ps
C1 = Capacity with back pressure. C2 = Capacity with zero back pressure. PB = Back pressure (psi g). Ps = Set pressure (psi g). Kb = Back pressure correction factor for gas and vapour. The curves shown in Figure 18, are applicable to set pressures of 50 psi g and above. They are limited to back pressure below critical flow pressure for a given set pressure. For sub critical flowback pressures below 50 psi g the manufacturer must be consulted for values of Kb.
1.0
20% o
verpre
0.9
Kb =
C1 C2
10
%
ssure
ov
er pr e
0.8
ss
ur
e
0.7 0.6 0.5
0
5
10
15
20
25
30
Percent of gauge back pressure =
35
40
PB x 100 Ps
Fig. 18 Kb gas and vapour constant back pressure correction factor balanced bellows valves
54
45
50
Conventional valves only: Percent of absolute back pressure =
PB x 100 P1
C1 = Capacity with back pressure. C2 = Capacity with zero back pressure. PB = Back pressure (psi abs). P1 = Upstream relieving pressure (psi abs). Kb = Back pressure correction factor for gas and vapour.
1.0 0.8
Kb =
C1 C2
Kb Kb Kb Kb
0.6 0.4
1.1 1.3 1.5 1.7
0.2 0
0
10
20
30
40
50
60
Percent of absolute back pressure =
70
80
90
100
PB x 100 P1
Fig. 19 Kb gas and vapour constant back pressure correction factor conventional valves
55
Back pressure correction factor for liquids KW Sizing according to ASME / API RP 520
Bellows balanced valves only: 1.00 0.95 0.90 0.85
Kw
0.80 0.75 0.70 0.65 0.60 0.55 0.50
0
10 20 30 = Back pressure (psi g)
PB Ps = Set pressure
(psi g)
40 x
50
100
Fig. 20 Kw liquid back pressure correction factor balanced bellows valves Viscosity correction factor Kv Sizing according to ASME / API RP 520 and BS 6759
To make allowances for high viscous fluids, first establish the valve size assuming the fluid is non-viscous. Once a size has been selected, the Reynolds number for the valve is calculated, and this figure used to establish the correction factor from the chart. (Fig. 21). The valve size should be checked to make sure the original size chosen will accommodate flow after the viscous correction factor has been applied. If not, the exercise should be repeated using the next valve size up. To establish Reynolds number Re : Imperial units
Metric units ●
Re = 0.341 4 ●
qv Q v U A Ao ●
●
56
= = = = = =
x
qv ●
v ÖA
Re = 12 700
x
Volume flow to be discharged (gal / min). Volume flow to be discharged (m³ / h). Viscosity (m³ / sec). Viscosity (Saybolt universal seconds). Discharge area (ins²). Discharge area (mm²).
●
Q U Ö Ao
1.0 0.9 0.8 0.7 Kv
0.6 0.5 0.4 0.3 10
20
40
100 200
400
1 000 2 000
10 000
20 000
100 000
Reynolds number Re
Fig. 21 Kv viscosity correction factor Compressibility factor
For gases, the compressibility factor (Z) needs to be ascertained. This factor accounts for the deviation of the actual gas from the characteristic of a perfect gas. It is often recommended that Z = 1 is used where insufficient data is available. Z can be calculated using the following formula: Z = 10
5 x
P R
x x
V T
x
M
Where: P = Absolute inlet pressure. V = Specific volume of gas at STP conditions (m³/kg). (Appendix 1). M = Molecular weight. (Appendix 1). R = Universal gas constant (8 314 N m / kmol.k). T = Inlet temperature (K).
57
APPENDIX - 1 Properties of industrial gases For specific gravity (G) used in ASME gas sizing calculations, divide molecular mass by 28.96 (molecular mass of air). Chemical formula
Molecular mass (M) kg / kmol
Isentropic coefficient (k) at 1.013 bar and 0°C
Specific volume (V) m³ / kg at 1.013 bar and 0°C
C2H2
26.02
1.26
0.853
28.96
1.40
0.773
NH3
17.03
1.31
1.297
Ar
39.91
1.66
0.561
Benzene
C6H6
78.00
1.10
Butane - n
C4H10
58.08
1.11
Butylene
C4H8
56.10
1.20
76.00
1.21
Gas
Acetylene Air Ammonia Argon
Carbon disulphide Carbon dioxide
CO2
44.00
1.30
0.506
Carbon monoxide
CO
28.00
1.40
0.800
Chlorine
Cl2
70.91
1.35
0.311
84.00
1.08
Cyclohexane Dipenyl
C12H10
154.00
Ethane
C2H6
30.05
1.22
0.737
Ethylene
C2H4
28.03
1.25
0.794
Freon 12
Cf2Cl2
121.00
1.14
Helium
He
4.00
1.66
Hexane
C6H14
86.00
1.08
Hydrogen
H2
2.02
1.41
11.124
Hydrogen chloride
HCl
36.46
1.40
0.610
Hydrogen sulphide
H2S
34.08
1.32
0.651
CH(CH3)3
58.05
1.11
0.375
CH4
16.03
1.31
1.395
CH3Cl
50.48
1.28
0.434
19.00
1.27
N2
28.02
1.40
0.799
N2O
44.02
1.30
0.746
Oxygen
O2
32.00
1.40
0.700
Pentane
C5H12
72.00
1.09
0.451
Propane
C3H8
44.06
1.13
0.498
Sulphur dioxide
SO2
64.07
1.29
0.342
Steam
H2O
18.00
1.33
Isobutane Methane Methyl chloride Natural gas Nitrogen Nitrous oxide
58
0.370
Properties of industrial liquids For specific gravity (G) used in ASME liquid sizing calculations, divide density by 998 (density of water). Liquid Acetone
Chemical formula
Boiling point at 1.013 mbar and 0°C
CH2 . CO . CH3
56.0
791
Density (kg / m³)
Ammonia
NH3
- 33.4
609
Benzene
C6H6
80.0
879
Butalene
C4H8
- 6.3
600
Butane
C4H10
- 0.5
580
Carbon disulphide
CS2
46.0
1 260
Carbon tetrachloride
CCl4
76.7
1 594
20% caustic soda
NaOH
1 220
Crude oil
700 to 1 040
Diesel oil
175.0
880
78.0
789
Ethanol
C2H5OH
Freon 12
CF2Cl2
- 29.8
1 330
C2H4(OH)2
197.5
1 140
175.0
850
Glycol Light fuel oil Heavy fuel oil
220 to 350
950
Kerosene
150 to 300
740
Methanol
C3OH
65.0
792
Naphthalene
C10H8
218.0
1 145
Nitric acid
HNO3
86.0
1 560
Propane
C3H8
- 42.0
500
H2SO3
338.0
1 400
Toluene
C6H5 . CH3
111.0
867
Trichlorethylene
CHCl . CCl2
87.0
1 464
H2O
100.0
998
Sulphurous acid
Water
59
APPENDIX - 2 Steam tables Specific enthalpy Pressure bar
Temperature kPa
°C
Water (h f ) kJ / kg
Evaporation (h fg ) kJ / kg
30.0 50.0 75.0 95.0
69.10 81.33 91.78 98.20
289.23 340.49 384.39 411.43
2 2 2 2
0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 110.0 120.0 130.0 140.0 150.0 160.0 170.0 180.0 190.0 200.0 220.0 240.0 260.0 280.0 300.0 320.0 340.0 360.0 380.0 400.0 450.0 500.0 550.0 600.0 650.0 700.0 750.0 800.0 850.0 900.0 950.0 000.0 050.0 100.0 150.0 200.0 250.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 000.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0
100.00 102.66 105.10 107.39 109.55 111.61 113.56 115.40 117.14 118.80 120.42 121.96 123.46 124.90 126.28 127.62 128.89 130.13 131.37 132.54 133.69 135.88 138.01 140.00 141.92 143.75 145.46 147.20 148.84 150.44 151.96 155.55 158.92 162.08 165.04 167.83 170.50 173.02 175.43 177.75 179.97 182.10 184.13 186.05 188.02 189.82 191.68 193.43 195.10 198.35 201.45 204.38 207.17 209.90 212.47 214.96 217.35 219.65 221.85 224.02 226.12 228.15 230.14
419.04 430.2 440.8 450.4 459.7 468.3 476.4 484.1 491.6 498.9 505.6 512.2 518.7 524.6 530.5 536.1 541.6 547.1 552.3 557.3 562.2 571.7 580.7 589.2 597.4 605.3 612.9 620.0 627.1 634.0 640.7 656.3 670.9 684.6 697.5 709.7 721.4 732.5 743.1 753.3 763.0 772.5 781.6 790.1 798.8 807.1 815.1 822.9 830.4 845.1 859.0 872.3 885.0 897.2 909.0 920.3 931.3 941.9 952.2 962.2 972.1 981.6 990.7
2 257.0 2 250.2 2 243.4 2 237.2 2 231.3 2 225.6 2 220.4 2 215.4 2 210.5 2 205.6 2 201.1 2 197.0 2 192.8 2 188.7 2 184.8 2 181.0 2 177.3 2 173.7 2 170.1 2 166.7 2 163.3 2 156.9 2 150.7 2 144.7 2 139.0 2 133.4 2 128.1 2 122.9 2 117.8 2 112.9 2 108.1 2 096.7 2 086.0 2 075.7 2 066.0 2 056.8 2 047.7 2 039.2 2 030.9 2 022.9 2 015.1 2 007.5 2 000.1 1 993.0 1 986.0 1 979.1 1 972.5 1 965.4 1 959.6 1 947.1 1 935.0 1 923.4 1 912.1 1 901.3 1 890.5 1 880.2 1 870.1 1 860.1 1 850.4 1 840.9 1 831.4 1 822.2 1 813.3
Steam (h g ) kJ / kg
Specific volume steam m³/ kg
absolute 0.30 0.50 0.75 0.95 0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 1.10 1.20 1.30 1.40 1.50 1.60 1.70 1.80 1.90 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40 3.60 3.80 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00 12.50 13.00 14.00 15.00 16.00 17.00 18.00 19.00 20.00 21.00 22.00 23.00 24.00 25.00 26.00 27.00
60
gauge
1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2
336.1 305.4 278.6 261.8
2 2 2 2
625.3 645.9 663.0 673.2
5.229 3.240 2.217 1.777
2 676.0 2 680.2 2 684.2 2 687.6 2 691.0 2 693.9 2 696.8 2 699.5 2 702.1 2 704.5 2 706.7 2 709.2 2 711.5 2 713.3 2 715.3 2 717.1 2 718.9 2 720.8 2 722.4 2 724.0 2 725.5 2 728.6 2 731.4 2 733.9 2 736.4 2 738.7 2 741.0 2 742.9 2 744.9 2 746.9 2 748.8 2 753.0 2 756.9 2 760.3 2 763.5 2 766.5 2 769.1 2 771.7 2 774.0 2 776.2 2 778.1 2 780.0 2 781.7 2 783.3 2 784.8 2 786.3 2 787.6 2 788.8 2 790.0 2 792.2 2 794.0 2 795.7 2 797.1 2 798.5 2 799.5 2 800.5 2 801.4 2 802.0 2 802.6 2 803.1 2 803.5 2 803.8 2 804.0
1.673 1.533 1.414 1.312 1.225 1.149 1.088 1.024 0.971 0.923 0.881 0.841 0.806 0.773 0.743 0.714 0.689 0.665 0.643 0.622 0.603 0.568 0.536 0.509 0.483 0.461 0.440 0.422 0.405 0.389 0.374 0.342 0.315 0.292 0.272 0.255 0.240 0.227 0.215 0.204 0.194 0.185 0.177 0.171 0.163 0.157 0.151 0.148 0.141 0.132 0.124 0.117 0.110 0.105 0.100 0.099 0.090 0.086 0.083 0.079 0.076 0.074 0.071
4 6 8 2 7 8 0 4
Specific enthalpy Pressure bar 28.00 29.00 30.00 31.00 32.00 33.00 34.00 35.00 36.00 37.00 38.00 39.00 40.00 41.00 42.00 43.00 44.00 45.00 46.00 47.00 48.00 49.00 50.00 51.00 52.00 53.00 54.00 55.00 56.00 57.00 58.00 59.00 60.00 61.00 62.00 63.00 64.00 65.00 66.00 67.00 68.00 69.00 70.00 72.00 74.00 76.00 78.00 80.00 82.00 84.00 86.00 88.00 90.00 92.00 94.00 96.00 98.00 100.00 102.00 104.00 106.00 108.00 110.00 112.00 114.00 116.00 118.00 120.00
kPa 2 800.0 2 900.0 3 000.0 3 100.0 3 200.0 3 300.0 3 400.0 3 500.0 3 600.0 3 700.0 3 800.0 3 900.0 4 000.0 4 100.0 4 200.0 4 300.0 4 400.0 4 500.0 4 600.0 4 700.0 4 800.0 4 900.0 5 000.0 5 100.0 5 200.0 5 300.0 5 400.0 5 500.0 5 600.0 5 700.0 5 800.0 5 900.0 6 000.0 6 100.0 6 200.0 6 300.0 6 400.0 6 500.0 6 600.0 6 700.0 6 800.0 6 900.0 7 000.0 7 200.0 7 400.0 7 600.0 7 800.0 8 000.0 8 200.0 8 400.0 8 600.0 8 800.0 9 000.0 9 200.0 9 400.0 9 600.0 9 800.0 10 000.0 10 200.0 10 400.0 10 600.0 10 800.0 11 000.0 11 200.0 11 400.0 11 600.0 11 800.0 12 000.0
Temperature °C 232.05 233.93 235.78 237.55 239.28 240.97 242.63 244.26 245.86 247.42 248.95 250.42 251.94 253.34 254.74 256.12 257.50 258.82 260.13 261.43 262.73 264.00 265.26 266.45 267.67 268.84 270.02 271.20 272.33 273.45 274.55 275.65 276.73 277.80 278.85 279.89 280.92 281.95 282.95 283.95 284.93 285.90 286.85 288.75 290.60 292.41 294.20 295.96 297.66 299.35 301.00 302.61 304.20 305.77 307.24 308.83 310.32 311.79 313.24 314.67 316.08 317.46 318.83 320.17 321.50 322.81 324.10 325.38
Water (h f ) kJ / kg 999.7 1 008.6 1 017.0 1 025.6 1 033.9 1 041.9 1 049.7 1 057.7 1 065.7 1 072.9 1 080.3 1 087.4 1 094.6 1 101.6 1 108.6 1 115.4 1 122.1 1 228.7 1 135.3 1 142.2 1 148.1 1 154.5 1 160.8 1 166.6 1 172.6 1 178.7 1 184.6 1 190.5 1 196.3 1 202.1 1 207.8 1 213.4 1 218.9 1 224.5 1 230.0 1 235.4 1 240.8 1 246.1 1 251.4 1 256.7 1 261.9 1 267.0 1 272.1 1 282.3 1 292.3 1 302.3 1 311.9 1 321.5 1 330.9 1 340.3 1 349.6 1 358.8 1 367.8 1 376.8 1 385.7 1 394.5 1 403.2 1 411.9 1 420.5 1 429.0 1 437.5 1 445.9 1 454.3 1 462.6 1 470.8 1 479.0 1 487.2 1 495.4
Evaporation (h fg ) kJ / kg 1 804.4 1 795.6 1 787.0 1 778.5 1 770.0 1 761.8 1 753.8 1 745.5 1 737.2 1 729.5 1 721.6 1 714.1 1 706.3 1 698.3 1 691.2 1 683.7 1 676.2 1 668.9 1 666.6 1 654.4 1 647.1 1 639.9 1 632.8 1 626.9 1 619.0 1 612.0 1 605.1 1 598.2 1 591.3 1 584.5 1 577.7 1 571.0 1 564.4 1 557.6 1 550.9 1 544.3 1 537.3 1 531.2 1 524.7 1 518.1 1 511.6 1 501.1 1 498.7 1 485.8 1 473.0 1 460.2 1 447.6 1 435.0 1 422.5 1 410.0 1 397.6 1 385.2 1 372.7 1 360.3 1 348.0 1 335.7 1 323.3 1 310.9 1 298.7 1 286.3 1 274.0 1 261.7 1 249.3 1 237.0 1 224.6 1 212.2 1 199.8 1 187.3
Steam (h g ) kJ / kg 2 804.1 2 804.2 2 804.1 2 804.1 2 803.9 2 803.7 2 805.5 2 803.2 2 802.9 2 802.4 2 801.9 2 801.5 2 800.9 2 799.9 2 799.8 2 799.1 2 798.3 2 797.6 2 796.9 2 796.6 2 795.2 2 794.4 2 793.6 2 792.6 2 791.6 2 790.7 2 789.7 2 788.7 2 787.6 2 786.6 2 785.5 2 784.4 2 783.3 2 782.1 2 780.9 2 779.7 2 778.5 2 777.3 2 776.1 2 774.8 2 773.5 2 772.1 2 770.8 2 768.1 2 765.3 2 762.5 2 759.5 2 756.5 2 753.4 2 750.3 2 747.2 2 744.0 2 740.5 2 737.1 2 733.7 2 730.2 2 726.5 2 722.8 2 719.2 2 715.3 2 711.5 2 707.6 2 703.6 2 699.6 2 695.4 2 691.2 2 687.0 2 682.7
Specific volume steam m³ / kg 0.068 9 0.066 6 0.064 5 0.062 5 0.060 5 0.058 7 0.057 1 0.055 4 0.053 9 0.052 4 0.051 0 0.049 8 0.048 5 0.047 3 0.046 1 0.045 1 0.044 1 0.043 1 0.042 1 0.041 2 0.040 3 0.039 4 0.038 6 0.037 8 0.037 1 0.036 4 0.035 7 0.035 0 0.034 3 0.033 7 0.033 1 0.032 5 0.031 9 0.031 4 0.030 8 0.030 3 0.029 8 0.029 3 0.028 8 0.028 3 0.027 8 0.027 4 0.027 0 0.026 2 0.025 4 0.024 6 0.023 9 0.023 3 0.022 6 0.022 0 0.021 4 0.020 8 0.020 2 0.019 7 0.019 2 0.018 7 0.018 3 0.017 8 0.017 4 0.017 0 0.016 6 0.016 2 0.015 8 0.015 4 0.015 0 0.014 7 0.014 4 0.014 1
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APPENDIX - 3 Terminology The following definitions are taken from DIN 3320 but it should be noted that many of the terms and associated definitions used are universal and appear in many other standards. Where commonly used terms are not defined in DIN 3320 then ASME / ANSI PTC25.3 has been used as the source of reference. This list is not exhaustive and is intended as a guide only, it should not be used in place of the relevant current issue standard. Operating pressure (working pressure) is the gauge pressure existing at normal operating conditions within the system to be protected. Set pressure is the gauge pressure at which under operating conditions direct loaded safety valves commence to lift. Test pressure is the gauge pressure at which under test stand conditions (atmospheric back pressure) direct loaded safety valves commence to lift. Opening pressure is the gauge pressure at which the lift is sufficient to discharge the predetermined flowing capacity. It is equal to the set pressure plus opening pressure difference. Reseating pressure is the gauge pressure at which the direct loaded safety valve is reclosed. Built up back pressure is the gauge pressure built up at the outlet side by blowing. Superimposed back pressure is the gauge pressure on the outlet side of the closed valve. Back pressure is the gauge pressure built up on the outlet side during blowing (built up back pressure + superimposed back pressure). Accumulation is the increase in pressure over the maximum allowable working gauge pressure of the system to be protected. Opening pressure difference is the pressure rise over the set pressure necessary for a lift suitable to permit the predetermined flowing capacity. Reseating pressure difference is the difference between set pressure and reseating pressure.
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Functional pressure difference is the sum of opening pressure difference and reseating pressure difference. Operating pressure difference is the pressure difference between set pressure and operating pressure. Lift is the travel of the disc away from the closed position. Commencement of lift (opening) is the first measurable movement of the disc or the perception of discharge noise. Flow area is the cross sectional area upstream or downstream of the body seat calculated from the minimum diameter which is used to calculate the flow capacity without any deduction for obstructions. Flow diameter is the minimum geometrical diameter upstream or downstream of the body seat. Nominal size designation of a safety valve is the nominal size of the inlet. Theoretical flowing capacity is the calculated mass flow from an orifice having a cross sectional area equal to the flow area of the safety valve without regard to flow losses of the valve. Actual flowing capacity is the flowing capacity determined by measurement. Certified flowing capacity is actual flowing capacity reduced by 10%. Coefficient of discharge is the ratio of actual to the theoretical discharge capacity. Certified coefficient of discharge is the coefficient of discharge reduced by 10%. (also known to as derated coefficient of discharge).
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The following terms are not defined in DIN 3320 and are taken from ASME / ANSI PTC25.3 Blowdown (reseating pressure difference) - difference between actual popping pressure and actual reseating pressure expressed as a percentage of set pressure or in pressure units. Cold differential test pressure the pressure at which a valve is set on a test rig using a test fluid at ambient temperature. This test pressure includes corrections for service conditions e.g. back pressure or high temperatures. Flow rating pressure is the inlet static pressure at which the relieving capacity of a pressure relief device is measured. Leak test pressure is the specified inlet static pressure at which a quantitative seat leakage test is performed in accordance with a standard procedure. Measured relieving capacity is the relieving capacity of a pressure relief device measured at the flow rating pressure. Rated relieving capacity is that portion of the measured relieving capacity permitted by the applicable code or regulation to be used as a basis for the application of a pressure relieving device. Overpressure is a pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure. Popping pressure is the value of increasing static inlet pressure of a pressure relief valve at which there is a measurable lift, or at which the discharge becomes continuous as determined by seeing, feeling or hearing. Relieving pressure is set pressure plus overpressure. Simmer is the pressure zone between the set pressure and popping pressure. Maximum operating pressure is the maximum pressure expected during system operation. Maximum allowable working pressure (MAWP) is the maximum gauge pressure permissible at the top of a completed vessel in its operating position for a designated temperature. Maximum allowable accumulated pressure (MAAP) is the maximum allowable working pressure plus the accumulation as established by reference to the applicable codes for operating or fire contingencies. 64
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TR-GCM-25
CM Issue 1
