TR3004 Pressure Relief Devices

Governing document

Classification: Internal

Pressure Relief Devices

Project development (PD) Technical and professional requirement, TR3004, Final Ver. 2.01, valid from 2012-08-06 Owner:

Leader (TPD TEX FOT PT PSCM)

Validity area:

Corporate technical requirements/On- and offshore

Governing document: Pressure Relief Devices


1

Objective, target group and provision ............................................................................................................... 3 1.1 Objective .................................................................................................................................................... 3 1.2 Target group .............................................................................................................................................. 3 1.3 Provision .................................................................................................................................................... 3

2

Requirements ........................................................................................................................................................ 4 2.1 General Requirements............................................................................................................................... 4 2.2 Pressure Relief Valves (PSVs) .................................................................................................................. 5 2.3 Rupture Disk Devices ................................................................................................................................ 7 2.4 Required overpressure .............................................................................................................................. 7 2.5 Backpressure ............................................................................................................................................. 8 2.6 Sizing ......................................................................................................................................................... 8 2.7 Installation .................................................................................................................................................. 8 2.8 Testing and maintenance .......................................................................................................................... 9 2.9 Documentation ......................................................................................................................................... 10

3

Additional information ....................................................................................................................................... 11 3.1 Definitions and abbreviations .................................................................................................................. 11 3.2 Changes from previous version ............................................................................................................... 12 3.3 References ............................................................................................................................................... 13

Project development (PD), Technical and professional requirement, TR3004, Final Ver. 2.01, valid from 2012-08-06 Page 2 of 13 Validity area:



1

Objective, target group and provision

1.1

Objective


This TR provides requirements and guidance on selection, installation and maintenance requirements of relief devices, ref API RP 520 and API 2000/ISO28300 (Atmospheric and low-pressure storage tanks). Specific requirements regarding the maintenance work process is not covered in this document, reference is made to OM02.04, “Certify safety valves”. This TR contains additional requirements and interpretations of the international standards where necessary. This TR is a complement to:  

API RP 520 (normative for onshore plants and offshore platforms) API 2000 /ISO 28300 (normative for onshore plants and offshore platforms)

For requirements and guidance on general shut down functions and pressure protection of equipment and piping, reference is made to TR3001. For the design of disposal systems (flare, vent and drain), reference is made to TR3002. For requirements regarding mechanical design of relief devices reference is made to TR3014 “Safety Relief Valves and Rupture Discs”. For overall description and identification of Safety Systems and Barriers against major accidents, reference is made to TR1055 (offshore) and TR2237 (onshore). Where reference is made to specific sections in international standards the edition number is included to enable traceability, it is emphasised that the current editions of the standards always shall be used. 1.2

Target group

The target group for this document is personnel involved in project planning and execution and plant engineers. 1.3

Provision

The document is provided for in “Process Technology, Technical requirements and standards”, (TR3000).




2

Requirements

2.1

General Requirements


Pressure Relief Devices shall be designed according to the requirements in API RP 520 and relevant pressure equipment design codes (i.e code(s) used for design of equipment in the specific protected system). The configuration of pressure relief devices on pressure equipment shall be evaluated. If the protected system is production critical and PSV recertification can not be done during planned maintenance stops, e.g. water wash of gas compressor turbines, an (N+1) configuration shall be installed. For rupture disks the configuration shall be evaluated based on the protected system’s production criticality and expected margins between normal operational pressure and burst pressure. Factors that should be taken into account when evaluating the configuration related to valve size versus number of valves are:  An increased number of relief devices may increase the installation cost, maintenance load and leak sources (increased number of flanges)  An increased number of relief devices will reduce the consequence (resulting pressure) if one of the devices fail to open at set pressure In a multiple safety relief device installation, all orifices should be equal to reduce complexity/likelihood of operator error when switches between valves in operation are performed. If different orifices are chosen, e.g. due to debottlenecking/modifications, the valves with equal orifices shall be grouped and block valves for each sub-group shall comply with the requirements of securing valves in correct position, ref TR2315. When selecting appropriate relief devices for a system, the dynamic pressure response during a relief scenario shall be taken into account. In cases where pressure relief valves do not have sufficient fast opening characteristics for adequate protection rupture disks shall be installed, e.g. tube rupture in a shell and tube heat exchanger when the low pressure side can be liquid filled, ref. TR3001 4.6.6.4. For pressure surges resulting from liquid hammering effects due to valve opening/closure, creating pressure peaks above the maximum pressure allowed by the relevant pressure equipment code, the preferred solution is to eliminate such scenarios by reducing the travel speed of the valve. If this is not feasible; dynamic analysis of the system shall be performed to demonstrate that chosen relief devices give adequate protection.




2.2

Pressure Relief Valves (PSVs)

2.2.1

General


The total effective flow area of the orifice(s) selected shall exceed the required area only by an amount as limited by standard orifice sizes available, for sizing reference is made to section 2.6. To minimize operational problems due to relief valve leakage, spurious opening and/or late reseating, the following shall be considered; start-up and shutdown conditions, process upsets, anticipated ambient conditions, instrument response times, pressure surges due to quick closing valves, the valve’s blowdown characteristics, relief valve simmer and margins in relief valve set point. Additional consideration should be given to hazard and pollution associated with the release of the fluid. A larger pressure margin between the maximum expected operating pressure and relief valve set pressure may be appropriate for fluids which are toxic, corrosive, or exceptionally valuable, further guidance can be found in the ASME BPVC, Div. 1, Appendix M. In API RP 520 Part I (figure 15 in 8th edition), it is indicated that a typical maximum expected operating pressure should be 90 percent of the maximum allowable working pressure (gauge). In process systems with a process shutdown system allowing the PSV to be reseated after a relief, the maximum expected operating pressure may be set higher, but evaluations regarding relief valve leakage, spurious opening and late reseating, as described above, needs to be performed to minimize operational problems. The relief valve manufacturer should be consulted. If staggering is implemented on non-ASME equipment a larger margin between design pressure and maximum expected operating pressure is normally required. This since the last valve's set point plus required overpressure shall not exceed the MAAP for the equipment (typically 10% for non-ASME equipment). Reference is also given to section 2.4. A PSV designed for gas will have different opening and closing characteristics than a valve designed for liquid service. In some applications, a valve may be required to relieve a liquid, a gas or a multi-phase mixture depending on the condition causing the overpressure. In such applications, a valve designed for liquid service or one designed for both liquid and gas service is recommended to avoid excessive accumulation, reference is made to API RP 520, Part 1(section 4.2.1.2 in 8th edition) for further guidance. Balanced PSVs are less sensitive to backpressure, ref. section 2.5, and may also be used as a means to isolate the guide, spring, bonnet and other top works parts within the valve from the relieving fluid. This may be important if there is concern that the fluid will cause corrosive damage to these parts. Other types of PSVs than conventional spring-loaded, balanced spring-loaded and pilot operated shall be regarded as a safety instrumented system (SIS) and the requirements in TR1956 shall be complied with. 2.2.2

Pilot operated relief valves

Pilot operated relief valves should not be used due to risk of freezing and/or plugging and due to increased complexity of the device introducing additional failure modes. If significant benefits with respect to material handling and project cost are achieved, pilot operated valves may be used in dry and clean service. When pilot operated valves are used, a modulating characteristic is preferred to reduce possible valve instability. Project development (PD), Technical and professional requirement, TR3004, Final Ver. 2.01, valid from 2012-08-06 Page 5 of 13 Validity area:




Use of pilot operated valves in wet and/or dirty service shall be treated as a deviation from this technical requirement and shall not be implemented unless installation of conventional valves are impractical, i.e. technical and/or economically not feasible. The fluid shall be defined as wet if the water content is sufficient to cause blockage in the pilot discharge. For non-flowing pilots a limited amount of free water in the pilot discharge can be accepted due to low gas consumption. Dehydrated gas will normally contain below 20 mg water/Sm3 (1,25 lb water/MMSCFD). This is regarded as sufficiently low to avoid blocking for a non-flowing pilot if safety critical heat tracing is installed. For a flowing pilot the fluid shall be defined as wet if the water dew point is above the minimum temperature expected in the pilot during relief, including pilot discharge and main valve dome. All relevant operating temperatures in the relieving segment shall be considered, e.g. pressurization, start-up and pressurized stop. The fluid shall be defined as dirty if it is anticipated that the fluid contains particles, e.g. wellstream processing or fluids containing asphaltenes, sand, corrosion products, coke or catalyst. The fluid shall not be regarded as clean before the processing step downstream of the unit separating particles from the fluid, e.g. a inlet separator on a production facility shall be regarded as dirty service, the same applies for a second stage separator since there is no effective oil/particle separation in a separator, but a downstream gas scrubber may be regarded as clean. A thorough analysis of possible contaminants shall be performed before a system is regarded as clean. If installation of conventional relief devices is impractical, the design of the pilot operated relief system shall take the following into account when installed in wet and/or dirty service: 1. 2. 3. 4.

Non-flowing pilots shall be used The pilot gas consumption when modulating shall be known by testing. The pilot shall be qualified for wet service according to WR1622. Remote-sensing back to the top of a vessel with a continuous gas phase present. a. Sensing line shall be insulated and equipped with safety critical heat tracing, ref TR3001 section 11. b. Sensing line shall have a nominal diameter of minimum 2” and a vertical section from the vessel of minimum 1 meter (3ft). The sensing line shall have no pockets and shall slope towards the vessel. The length of tubing to connect to the pilot shall be minimized and slope shall be verified. c. The design shall ensure that liquid and/or particles will not enter the pilot: i. If liquid relief (no gas) is a credible scenario: The volume of the sensing line shall be sufficient to avoid liquid entering the pilot during relief, i.e. gas consumption of pilot during any credible liquid relief case shall be less than the total volume of the line. If detailed evaluations not are performed the gas consumption of 100 cycles can be used. ii. If liquid relief (no gas) is not a credible scenario: The cross-sectional area of the sensing line shall be sufficient to reduce the gas velocity to such a level that gasliquid and gas-solid separation is achieved in the sensing line. To enable sufficient separation the gas velocity in the sensing line must be below the critical particle and/or liquid droplet's terminal velocity. The flow will be laminar and Stokes law can be used. A particle diameter of maximum 100 micron should be used. Equation 7-6 in GPSA's "Engineering Data Book" can be used to calculate the terminal velocity.





5. Include a valve branch immediately upstream of the pilot to enable verification of an open impulse line back to the vessel, to be used in commissioning and after major maintenance/modifications.

2.3

Rupture Disk Devices

Rupture disk devices are non-reclosing pressure relief devices used to protect vessels, piping and other pressure containing components from excessive pressure and/or vacuum. Rupture disks are used in single and multiple relief device installations. They are also used as redundant pressure relief devices. With no moving parts, rupture disks are simple, reliable, and faster acting than other pressure relief devices. Rupture disks installed in liquid filled systems may generate large reaction forces in the relief system upon rupture. This effect needs to be taken into account when the relief system piping is designed. When rupture disks are applied, it is important to include margins with respect to minimum and maximum burst pressure. It is also important to remember that the rupture disk back pressure is of high importance when specifying the burst pressure. The temperature sensitivity of the rupture disk shall also be taken into account when specifying burst pressure and temperature. The guidance in API RP 520 Part I (section 4.3 in 8th edition), shall be used. A normally closed flare system operating at a pressure significantly above atmospheric pressure represents a special challenge. To compensate for the backpressure, the rupture disk may need to have a set point below the equipment design pressure. When the pressure in the flare system drops to atmospheric, the disk could unintentionally break. To compensate for this, double disks in series may be considered. The downstream rupture disk shall then rupture at a differential pressure defined from the maximum continuous operating back pressure in the flare system. When rupture disks are used in series with a PSV, upstream or downstream (ref API 520 Part II, section 4.6 in 5th edition), and when multiple rupture disk in series are used; a pressure transmitter with alarm and a corrective action shall as a minimum be installed in-between. In addition, a free-vent should be installed with periodic check of leakage. Since keeping the space between PSVs/rupture disks at atmospheric pressure is safety critical, the instrumentation and/or procedures shall be designed/implemented with corresponding safety integrity.

2.4

Required overpressure

In some cases the pressure relief device(s) on equipment will also protect downstream systems such as flexible risers and pipelines. To be able to fully utilize the capacity of an export/re-injection system, it will, in some cases, be beneficial to reduce the required overpressure for the relief valves below the normal value of 10 %. For pressure relief valves certified in accordance with ASME BPVC VIII (UV), the certified capacity is given at 10 % overpressure (UG-129). In ISO 4126 overpressure is defined as: " the value stated by the manufacturer but not exceeding 10 % of set pressure or 0,1 bar whichever is greater ;" This implies that pressure relief valves that do not require certification in accordance with ASME, i.e. not protecting pressure equipment (vessels or piping) designed in accordance with ASME, can be specified to have a lower required overpressure than 10 %.





The required overpressure of any pressure relief valve should be 10%. If less overpressure is required to enable an optimal process design the overpressure can be reduced, but shall not be less than 5 %. To enable lower overpressure than 10 % a pop-action valve is required. 2.5

Backpressure

Superimposed backpressure is defined as the static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate. The increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device opens is defined as built-up backpressure. Short tailpipes that vent directly to the atmosphere typically result in lower built-up backpressures than long discharge systems. However, choked flow can occur at the outlet of even short tailpipes vented directly to atmosphere, resulting in a high built-up backpressure. For this reason, the magnitude of the built-up backpressure shall be evaluated for all systems, regardless of the outlet piping configuration. For sizing of flare/vent lines reference is made to TR3002 section 2.3.4.1. A balanced PSV should be used where the built-up backpressure is too high for conventional PSVs or where the superimposed backpressure varies widely compared to the set pressure. Balanced valves can typically be applied where the total backpressure (superimposed plus built-up) does not exceed 50 % of the set pressure. The specific manufacturer shall be consulted concerning the backpressure limitation of a particular valve design. For further guidance related to backpressure limitations reference is made to API RP 520, Part 1 (section 5.3.3 in 8th edition). The lift in pilot-operated valves is not sensitive to back-pressure if the pilot discharge is vented to atmosphere. 2.6

Sizing

All pressure relief valves shall be sized in accordance with the information on the data sheet and the methods outlined in API RP 520, part I and part II. For initial sizing the effective orifice areas in API 526 can be used in combination with effective coefficients of discharge given in API RP 520. When a specific valve design is selected for an application, the rated capacity of that valve shall be determined using the actual orifice area and the rated coefficient of discharge stated by the manufacturer, and the equations presented in API RP 520. The actual capacity of the relief devices shall always be confirmed by calculations performed by the manufacturer. For two-phase or boiling fluids, the actual flow rate through a device can be many times higher than calculated if equilibrium is not achieved in the nozzle. This may result in unexpectedly high back pressures. PSVs with modulating characteristics may prevent this problem. When the increase in backpressure cannot be adequately predicted and may result in more than 10% built-up back pressure, a balanced or pilot operated valve (dry and clean service) shall be used. For rupture disks used as the primary relief device, the relieving capacity shall be determined as specified in API RP 520, part 1, section 5.11.1.3 in 8th edition (derating the relieving capacity to 90% percent). Alternatively 5.11.1.2 in 8th edition may be used when a rupture disk device discharges directly to the atmosphere, is installed within eight pipe diameters from the vessel nozzle entry, has a length of discharge not greater than five pipe diameters, and has nominal diameters of the inlet and outlet discharge piping equal to or greater than the nominal pipe size of the device. 2.7

Installation

Discharge piping from pressure relief devices shall be drained properly to prevent the accumulation of liquids on the downstream side of the pressure-relief device. The outlet piping to closed systems shall be Project development (PD), Technical and professional requirement, TR3004, Final Ver. 2.01, valid from 2012-08-06 Page 8 of 13 Validity area:




self-draining to a liquid disposal point, thereby eliminating the need for a physical drain, reference is made to TR3002 section 2.3.5. The opening through all pipe and fittings (including valves) downstream the PRD shall have at least the area of the pressure relief device outlet. Flow straighteners, silencers and other potential restrictions shall not be installed. Administrative controls shall be in place that will prohibit the inappropriate closing of isolation valves in pressure relief system piping, ref TR2315. These controls shall require that the opening and closing of the isolation valves shall be done by an authorized person and that the status of the isolation valves are documented at all times, e.g. by the use of logs or an electronic system. Periodic inspections of isolation valves located in pressure-relief system piping are required. The inspection shall verify the position of isolation valves and the condition of the locking or sealing device. The system manuals shall contain a list of all isolation valves located in pressure-relief system piping which could isolate pressure relief valves. Documentation of the required position and reason for the lock or seal shall be provided. When gate valves are used, stem failure and subsequent closing of the valve in the relief path shall be avoided by orienting the valve stem in such a position that the gate will not fall down and block the relief path after stem breakage, ref. API RP 520 Part II (Section 6.3.1 and 6.3.2 in 5th edition). If pilot-operated valves are equipped with a pilot supply filter (PSF) this shall be installed as close to the pilot as possible to reduce likelihood of blocking. 2.8

Testing and maintenance

For optimum performance, pressure-relief devices shall be serviced and maintained regularly. Details for the care and servicing of specific pressure-relief devices are provided in the manufacturer’s maintenance bulletins and in API RP 576. Pressure Relief Devices shall be located for easy access, removal, and replacement so that servicing can be properly performed. Sufficient working space shall be provided around the pressure-relief device. Isolation valves shall be provided to ensure efficient servicing and limit exposure to potential harmful substances. For requirements regarding isolation, reference is made to TR1951. For working requirements/procedures regarding testing reference is made to OM02.04, “Certify safety valves”. The maintenance testing interval shall be defined in accordance with the premises described below. The initial test interval for pressure relief valves shall be maximum 12 months. The first increase in test interval shall not be considered before the valve has a minimum of three approved certification tests according to OM02.04. Further increase in test interval requires another three approved certification tests. When considering adjustments to test interval (up or down), all pressure safety valves within the same group shall be included, and the groups shall be established and assessed in the following order of precedence: - Equal valves in parallel applications (e.g., parallel trains or similar process facilities). - All valves in a multiple valve arrangement - Single valve if no comparable units are available within the plant/installation. All valves in the same group shall have equal test intervals. Project development (PD), Technical and professional requirement, TR3004, Final Ver. 2.01, valid from 2012-08-06 Page 9 of 13 Validity area:




The test interval for the valve and its group shall be reduced to the previous interval (e.g. from 36 to 24 months) if a test fails. The test interval shall not be increased by more than 12 months each time, and the interval shall not exceed 48 months. Failure of a valve with 12 months interval will require an analysis that may impose the need for design changes, addressing failure modes and root cause, valve type, manufacturer and comparable units. Evaluations shall be performed by skilled personnel. (e.g. technical discipline responsible). The safety valve shall be overhauled at level L2 if two previous inspections have been carried out without an overhaul, or if the test interval is 36 months or more. If pilot-operated valves are used, the pilot valve shall always be recertified according to level L2 requirements in OM02.04 to ensure that dismantling for a thorough shop inspection, cleaning and repair is performed. If a pilot-operated pressure relief valve has been in use (confirmed lift) the pilot shall be recertified (level L2). If pilot-operated valves are equipped with a pilot supply filter (PSF) this shall be changed every time the valve is recertified. PSFs shall also be changed after every time the valve has been used. Relief devices shall be in addition to any equipment internal relief device (e.g. internal relief device on positive displacement pump) to assure that they are included in proper maintenance and recertification programmes. Relief devices mounted on equipment for equipment protection (i.e. not for overpressure protection) shall have the same test interval as the equipment. These devices do not have the same test documentation requirements as for other pressure relief devices (e.g. M4 certificate). Pressure relief valves are defined as Safety Critical Elements and their performance shall be followed up in accordance with the requirements given in TR1055/TR2237 and reference to GL0114 for monitoring of safety critical failures. 2.9

Documentation

The datasheets for pressure relief devices shall contain all relevant information specified in API RP 520 Part 1 (Annex A for rupture disks and Annex D for pressure relief valves in 8th edition)




3

Additional information

3.1

Definitions and abbreviations

3.1.1

Definitions


The definitions are based on those given in API RP 520, which should be consulted for further guidance. Accumulation

The pressure increase over the maximum allowable working pressure of the equipment, expressed in pressure units or as a percentage of maximum allowable working pressure (MAWP) or design pressure. Maximum allowable accumulations are established by applicable codes for emergency operating and fire contingencies.

Backpressure

The pressure that exists at the outlet of a pressure relief device as a result of the pressure in the discharge system. Backpressure is the sum of the superimposed and built-up backpressures.

Blowdown

The difference between the set pressure and the closing pressure of a pressure relief valve, expressed as a percentage of the set pressure or in pressure units.

Overpressure

The pressure increase over the set pressure of the relieving device. Overpressure is expressed in pressure units or as a percentage of set pressure. Overpressure is the same as accumulation only when the relieving device is set to open at the maximum allowable working pressure of the vessel.

Pressure Relief Device

A device actuated by inlet static pressure and designed to open during emergency or abnormal conditions to prevent a rise of internal fluid pressure in excess of a specified design value. The device also may be designed to prevent excessive internal vacuum. The device may be a pressure relief valve, a non-reclosing pressure relief device, or a vacuum relief valve.

Required overpressure

The pressure increase over the set pressure, at which the safety valve attains the lift specified by the manufacturer, usually expressed as a percentage of the set pressure. This is the overpressure used to certify the safety valve. A non-reclosing pressure relief device actuated by static differential pressure between the inlet and outlet of the device and designed to function by the bursting of a rupture disk. A rupture disk device includes a rupture disk and a rupture disk holder.

Rupture disk device

3.1.2

Abbreviations

API

American Petroleum Institute





ASME

American Society of Mechanical Engineers

BPVC

Boiler and Pressure Vessel Code

GL

Statoil Guideline

Ft

Feet

ISO

International Organization for Standardization

Lb

Pounds mass

Lb/MMSCV

Pounds mass per million standard cubic feet

PRD

Pressure Relief Device

PSV

Pressure Safety Valve (PRV, Pressure Relief Valve, used in API RP 520, some installations use PZV)

PSF

Pilot Supply Filter

RP

Recommended Practice

TR

Statoil Technical Requirement

WR

Statoil Work Process Requirements

3.2

Changes from previous version

Document updated to include US units. Section 2.8 updated to include requirements/guidance in how to adjust test intervals based on test results. Reference for requirements regarding securing of valves in correct position is changed from TR1951 to TR2315 “Valve Locking, Interlocking and other Postion Securing Systems”




3.3


References 

ASME, ”Boiler and Pressure Vessel Code, Section VIII, Rules for Construction of Pressure Vessels”



API RP 520, “Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries” Part 1-Sizing and Selection, 8th edition



API RP 520, “Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries” Part 2-Installation, 5th edition



API Std 526, “Flanged Steel Pressure Relief Valves”



API RP 576, “Inspection of Pressure-Relieving Devices”



API 2000 (ISO 28300, identical), “Venting Atmospheric and Low-pressure Storage Tanks”, 6th edition



Gas Processors Suppliers Association, “Engineering Data Book”, 11th Edition



ISO 4126, “Safety devices for protection against excessive pressure”



“Performance Standards for Safety Systems and Barriers - Offshore”, (TR1055)



“Piping Engineering”, (TR1951)



“Safety design for Onshore Plants” (TR2237)



“Process Technology, Technical requirements and standards” (TR3000)



“Process Safety” (TR3001)



“Flare, Vent and Drain” (TR3002)



“Safety Relief Valves & Rupture Discs” (TR3014)



“Valve Locking, Interlocking and other Position Securing Systems” (TR2315)



“Technology Qualification” (WR1622)



“Safety Critical Failures“ (GL0114)



TR3004 Pressure Relief Devices

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