Pressure Relief Valve

Pressure Safety Valve

Technical Course

Prepared by

Bassem BALGHOUTHI Date Rev

January 2012

00

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Scope This technical course course covers the sizing and selection selection methods of pressure relief valves used in the typical process industries. industries. It helps engineers and designers understand understand the basic design of different different types of pressure relief valves and rupture disks, disks, and increase their knowledge in selection and sizing. The selection section contains the explanation for the the suitability of types types of pressure pressure relief valve used in various various applications. All the important important parameters parameters used in this guideline guideline are explained in the the definition definition section which helps the reader reader understand understand the meaning of of the parameters and the terms. The theory section section includes the sizing sizing theory for the pressure relief relief valves for gas, steam, steam, and liquid services and several methods of installation for pressure relieving devices. In the application section, four cases examples are included by guiding the reader step by step in pressure relief valve sizing s izing for difference applications.


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Table of Contents

Scope ........................................... ................................................................. ............................................ ............................................ ............................................. ........................... .... 2 1.

Introduction ........................................... .................................................................. ............................................. ............................................. .............................. ....... 4

2.

Definition ............................................ .................................................................. ............................................ ............................................ .................................. ............ 4

3.

How does it work? ........................................... .................................................................. ............................................. ......................................... ................... 5

4.

................................................................. ...................................... ............... 7 Selection of Pressure Relief Valves ..........................................

5.

4.1.

Back Pressure Consideration ............................................. .................................................................... ......................................... .................. 7

4.2.

Conventional Pressure Relief Valve .......................................... ................................................................. .................................. ........... 8

4.3.

Balanced Pressure Relief Valve .......................................... ................................................................. ......................................... .................. 9

4.4.

Pilot Operated Relief Valves .......................................... ................................................................. ........................................... .................... 11

4.5.

Rupture Disc ........................................... ................................................................. ............................................ ........................................... ..................... 13

............................................................... ............................................ .................................... .............. 14 PSV Sizing Procedure .........................................

5.1.

Sizing for Gas or Vapor .......................................... ................................................................ ............................................ ............................ ...... 14

a) for Critical Flow ............................................. ................................................................... ............................................ ........................................... ..................... 14 b) for Subcritical Flow ............................................ .................................................................. ............................................ .................................... .............. 21 5.2

Applications ............................................ .................................................................. ............................................ ........................................... ..................... 24

5.3

Sizing for Steam Relief ........................................... ................................................................. ............................................ ............................ ...... 25

5.4

Sizing for liquid .......................................... ................................................................. ............................................. ....................................... ................. 28

5.5. Sizing for Thermal Relief Valve ............................................. .................................................................... ....................................... ................ 34 5.6

Sizing for fire ........................................... ................................................................. ............................................ ........................................... ..................... 35


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1. Introduction In the daily operation of petroleum processing plant, overpressure can happen due to incidents like a blocked discharge, fire exposure, tube rupture, check valve failure, thermal expansion that can happen at process heat exchanger, and the failures can occur. This can lead to a major incident in plant if the pressure relief system is not in place or not functional. Is very important to properly select, size, locate and maintain the pressure relief systems to prevent or minimize the losses from major incident like fire or other issues. Pressure relief system is used to protect piping and equipment against excessive overpressure for equipment and personnel safety. Pressure relief systems consist of a pressure relief device, flare piping system, flare separation drum and flare system. A pressure relief device is designed to open and relieve excess pressure; it is re-closed after normal conditions have been restored to prevent the further flow of fluid. Pressure Safety Valve (PSV) is one of safety devices in oil and gas production facility, which ensure that pipes, valves, fittings, and pressure vessels can never be subjected to pressure higher than their design pressure. Therefore, the selection of PSV to be installed must be conducted in a careful and proper manner. These are the questions questions worth to to be asked when you are going to specify specify details of PSV. PSV. 



What type of PSV PSV we will have for for our process requirements? Is there any easier way for PSV sizing (PSV calculation) rather than calculate it manually?



What kind of material material shall be chosen chosen for our process process requirements? requirements?

Prior to the PSV selection, it would be better if we know how the PSV works which will lead us in understanding of critical parts of PSV. Then, the PSV selection process can be done with awareness of some strong points.

2. Definition Cited from API 520 part 1 ( Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries; ) about Sa fety Valve definition: “ A safety valve is a spring loaded pressure pressure Sizing and Selection Pressure Safety Valve

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relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. A safety valve is normally used with compressible fluids. ” Figure 1 shows Conventional PSV, which is purposed for description only.

Figure 1: Conventional Pressure Safety Valve ( Taken from API 520 part 1 )

3. How does it work?


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Figure 2: Sketch of Pressure Relief Valve

How does the PSV work? Figure 2 is a simple sketch of pressure relief valve which shows the disc held in the closed position by the spring. When system pressure reaches the desired opening pressure, the pressure force of the process fluid pass through the inlet and then it is acting over Area A 1 equals the force of the spring, and the disc will lift and allow fluid to flow out through the outlet. When pressure in the system returns to a safe level, the valve will return to the closed position. Certain area of the disc and nozzle will allow certain amount of the gas/liquid volume. The area of the nozzle (so called as “Orifice”) needs to be calculated in order to have proper amount

flow of the process fluid. This certain area has been standardized in API 526 (Flange Steel Pressure Relief Valves) and designated into certain alphabetic as shown on Table 1.


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Table 1 Orifice Area

Since PSV will most likely to be in closed position, it is a good idea to choose some kind of “seal” between disc and the nozzle to ke ep the process fluid from leaking to the outlet of the

PSV.

4. Selection of Pressure Relief Valves 4.1. Back Pressure Consideration Types of PSV are created due to existence of backpressure. The effect of backpressure can be depicted by Figure 3 which incorporate forces from spring (F ), process fluid from the s pressurized system (P V AN ), and backpressure (P B AN ). The P V is the pressure due to the changes over the pressurized system, and the P B is the pressure which exist in the outlet of the PSV, we recognize this as a back pressure. As you may see, that the spring – denotes with the F s – is having main contribution to the force balance, and have a positive direction along the P B. The overpressure in the pressurized system will increase the magnitude of the P V , and eventually it will affect the balance of the pressure force, and hence the sum of the P B AN and the Fs will be less than the P V AN. The spring, which holds the disk and isolates the pressurized system into


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the outlet of the PSV, is moving upward and the disk will not contain the pressurized system anymore.

Figure 3: Effect of Backpressure to the set pressure (Taken from API 520 part 1)

An extreme example, in the closed position, if backpressure is high enough to compensate the force pressure of process fluid, the force resultant will be zero, in other words the PSV will remain close. In this condition, the PSV is not successfully to full fill its function. We will examine types of PSV. 4.2.

Conventional Pressure Relief Valve

This type of PSV is the simplest one as you may see on Figure 4. Usually, this type of PSV is used whenever the existence of back pressure is relatively small (less than 10% of set pressure), or nearly zero. Due to its low immunity to back pressure, the conventional type outlet is vented into atmospheric, and mostly, the fluid to be vented is non-hazardous fluid i.e.: water steam.


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Figure 4: Conventional Pressure Safety Valve ( Taken from API 520 part 1 )

4.3.

Balanced Pressure Relief Valve

Balanced pressure relief valve is a spring-loaded pressure relief valve which is consisted of bellows or piston to balance the valve disc to minimize the back pressure effect on the performance of relief valve. Balanced pressure relief valve is used when the built-up pressure (back pressure caused by flow through the downstream piping after the relief valve lifts) is too high for conventional pressure relief or when the back pressure varies from time to time. It can typically be applied when the total back pressure (superimposed + build-up) does not exceed <50% of the set pressure. Typical balanced pressure relief valve is showed in Figure 5. Based on API RP 520(2000) the unit of the balanced pressure relief valve to overcome the back pressure effect is explained as when a superimposed back pressure is applied to the outlet of valve, a pressure force is applied to the valve disc which is additive to the spring force. This added


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force increases the pressure at which an unbalanced pressure relief valve will open. If the superimposed back pressure is variable then the pressure at which the valve will open will vary (Figure 1). In a balanced-bellows pressure relief valve, a bellows is attached to the disc holder with a pressure area, AB, approximately equal to the seating area of the disc, AN. This isolates an area on the disc, approximately equal to the disc seat area, from the back pressure. With the addition of a bellows, therefore, the set pressure of the pressure relief valve will remain constant in spite of variations in back pressure. Note that the internal area of the bellows in a balanced-bellows spring loaded pressure relief valve is referenced to atmospheric pressure in the valve bonnet. The interior of the bellows must be vented through the bonnet chamber to the atmosphere. A 3/8 to 3/4 in. diameter vent hole is provided in the bonnet for this purpose. Thus, any bellows failure or leakage will permit process fluid from the discharge side of the valve to be released through the vent.


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Figure 5: Bellows Pressure Relief Valve ( Taken from API 520 part 1 )

4.4. Pilot Operated Relief Valves A pilot-operated pressure relief valve consists of the main valve, which normally encloses a floating unbalanced piston assembly, and an external pilot as shown on Fig.6. The piston is designed to have a larger area on the top than on the bottom. Up to the set pressure, the top and bottom areas are exposed to the same inlet operating pressure. Because of the larger area on the top of the piston, the net force holds the piston tightly against the main valve nozzle. As the operating pressure increases, the net seating force increases and tends to make the valve tighter. This feature allows most pilot-operated valves to be used where the maximum expected operating pressure is higher than 90% of MAWP The pilot type has a sensing line and its function is transmitting the built-up pressure that may exist in the pressurized system to the pilot valve. As the pressure in the pressurized system is increasing and reaching the set pressure, the pilot valve will actuate the PSV spring inside the main valve to pop up the PSV. Due to the actuator has no direct contact with the venting system the valve will not relatively be affected by backpressure. Moreover, this type of PSV has a wide range of spring setting, it will be an advantage if we want to change the set pressure on a wide range alternatives.


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Figure 6. Typical pilot-operated valve

The advantages of pilot-operated pressure relief valves are (a) capable of operation at close to the set point and remains closed without simmer until the inlet pressure reaches above 98% of the set pressure; (b) once the set pressure is reached, the valve opens fully if a pop action pilot is used; (c) a pilot-operated pressure relief valve is fully balanced, when it exhausts to the atmosphere; (d) pilot-operated pressure relief valves may be satisfactorily used in vapor or liquid services up to a maximum back pressure (superimposed plus built-up) of 90% of set pressure, provided that the back pressure is incorporated into the sizing calculation; (e) A pilot operated valve is sufficiently positive in action to be used as a depressuring device. By using a hand valve, a control valve or a solenoid valve to exhaust the piston chamber, the pilot-operated PR valve can be made to open and close at pressures below its set point from any remote location, without affecting its operation as a pressure relief valve. (f) Pilot-operated pressure relief valves can be specified for blow down as low as 2%. The disadvantages of pilot-operated pressure relief valves are (a) Not recommended for dirty or fouling services, because of plugging of the pilot valve and small-bore pressure-sensing lines. If the pilot valve or pilot connections become fouled, the valve will not open. (b) A piston seal with the “O” ring type is limited to a maximum i nlet temperature of 450 °F

and the newer designs are available for a maximum inlet temperature of about 1000°F in a limited number of valve sizes and for a limited range of set pressures. (c) Vapor condensation and liquid accumulation above the piston may cause the valve to malfunction. Pressure Safety Valve

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(d) Back pressure, if it exceeds the process pressure under any circumstance (such as during start-up or shutdown), would result in the main valve opening (due to exerting pressure on the underside of the piston that protrudes beyond the seat) and flow of material from the discharge backwards through the valve and into the process vessel. To prevent this backflow preventer must be installed in the pilot operated pressure relief valve. (e) For smaller sizes pilot operated pressure relief valve, it is more costly than spring loaded pressure relief valve. Pilot-operated relief valves are commonly used in clean, low-pressure services and in services where a large relieving area at high set pressures is required. The set pressure of this type of valve can be close to the operating pressure. Pilot operated valves are frequently chosen when operating pressures are within 5 percent of set pressures and a close tolerance valve is required. 4.5. Rupture Disc A rupture disk consists of a thin diaphragm held between flanges. The disk is designed to rupture and relieve pressure within tolerances established by ASME Code. Rupture disks can be used in gas processing plants, upstream of relief valves, to reduce minor leakage and valve deterioration. In these installations, the pressure in the cavity between the rupture disk and the relief valve should be monitored to detect a ruptured disk. In some applications a rupture disk with a higher pressure rating is installed parallel to a relief valve. A rupture disk is subject to fatigue failure due to operating pressure cycles. To establish recommended replacement intervals, consult rupture disk supplier. Rupture disks should be used as the primary relieving device only if using a pressure relief valve is not practical. Some examples of such situations are: (a) Rapid rates of pressure rise. A pressure relief valve system does not react fast enough or cannot be made large enough to prevent overpressure, e.g., an exchanger ruptured tube case or a runaway reaction in a vessel.


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(b) Large relieving area required. Because of extremely high flow rates and/or low relieving pressure, providing the required relieving area with a pressure relief valve system is not practical. (c) A pressure relief valve system is susceptible to being plugged, and thus inoperable, during service.

5. PSV Sizing Procedure 5.1. Sizing for Gas or Vapor Critical and Subcritical Gas Flow:

The distinction between critical and subcritical gas flow is very important in order to distinguished the sizing formula. If the back pressure is bellow the critical pressure Pc the flow is called Critical flow otherwise is called Subcritical flow . PbPc Subcritical flow

a) for Critical Flow Formula below is used to estimate the required effective discharge area for relief valve when the flow into the relief valve is critical flow.

A 

W

TZ

CK c K d K b P 1

M

Where, A : Effective discharge area relief valve, in2 W : Flow through the device, Ib/hr C1 : Coefficient determined from an expression


eq 1

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C 1  520 k (

2

) k  1

( k 1) ( k 1)

eq 2

K=Cp/Cv Kd :Effective coefficient of discharge. For preliminary sizing, the following values are used: :0.975 when a pressure relief valve is installed without a rupture disk in combination, :0.62 when a rupture disk is installed in accordance with pressure relief valve. P1 :Upstream relieving pressure, psia, is the set pressure plus the allowable overpressure plus atmospheric pressure. Kb : Capacity correction factor due to back pressure. It applies for balanced bellows valves only, for the conventional and pilot operated valves, use a value for Kb equal to 1.0. Kc : Combination correction factor for installations with a rupture disk upstream of the pressure relief valve. Value is 1.0 when a rupture disk is not installed and is 0.9 when a rupture disk is installed in combination which does not have a published value. T1 : Relieving temperature of the inlet gas or vapor, R (°F+460) Z : Compressibility factor for gas. MW : Molecular weight for gas or vapor at inlet relieving conditions. The compressibility factor of Gas Z can be determined using the following figure after determining the reduced temperature and the reduced Pressure which are: The reduced temperature Tr= T/Tc Reduced Pressure Pr=P/Pc Where: Tc and Pc are respectively the critical temperature and the critical pressure


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Table 2: Values of Coefficient C1


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Table 3: Values of K for Gases


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Figure 7: Constant Back Pressure sizing Factor, Kb, for conventional PSV (vapor and Gases only)

b) for Subcritical Flow Subcritical flow is occurred when the ratio of back pressure to inlet pressure exceeded the critical pressure ratio Pcf/P1.


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 2    P  k  1 1

P cf

(

k k 1

)

Where

Pcf : Critical flow Pressure, psia Under this condition the formula used for calculation the required effective discharge area of device is:

Using S.I unit

where

Where: F2 : Coefficient of subcritical flow k : Ratio of the specific heats r : Ratio of back pressure to upstream relieving pressure, P2/P1 P2 : Total back pressure, psia F2 can be taken also by the bellow figure:


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Figure 8: Values of F2 for Gases


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5.2

Applications Exercice 1: In this example, the following relief requirements are given:

a) Required hydrocarbon vapor flow, W , caused by an operational upset, of 53,500 lb/hr [24,260 kg/hr]. b.) The hydrocarbon vapor is a mixture of butane (C4) and pentane (C5). The molecular weight of vapor, M , is 65. c) Relieving temperature, T , of 627 R (167°F) [348 K]. d) Relief valve set at 75 psig [517 kPa], which is the design pressure of the equipment. e)Back pressure of 14.7 psig [101.3 kPag] Exercice 2:

In this example, the following data are derived: a)Permitted accumulation of 10%. b)Relieving pressure, P 1, of 75 x 1.1 + 14.7 = 97.2 psia [670 kPa]. c) calculated compressibility, Z , of 0.84. (If a calculated compressibility is not available, a Z value of 1.0 should be used.) d) Critical flow pressure (from Table 7) of 97.2 x 0.59 = 57.3 psia (42.6 psig) [395 kPaa]. Note: Since the back pressure (0 psig [0 kPag]) is less than the critical flow pressure (42.6 psig [294 kPag]), the relief valve sizing is based on the critical flow equation (see Equation 3.2 and paragraphs e) Cp/Cv = k (from Table 7) of 1.09. From Table 8, C = 326. f)Capacity correction due to back pressure, Kb , of 1.0. g) Capacity Correction for rupture disk, Kc = 1.0 Exercice 3:

In this example, the following relief requirements are given: a) Required hydrocarbon vapor flow, W , caused by an operational upset, of 53,500 lb/hr (24,260 kgs/hr).


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b)The hydrocarbon vapor is a mixture of butane (C4) and pentane (C5). The molecular weight of the mixture, M , is 65. c) Relieving temperature, T , of 627 R (167°F) [348 K (75°C)]. d) Relief valve set at 75 psig [517 kPag], which is the design pressure of the equipment. e) Constant back pressure of 55 psig [379 kPa]. The spring setting of the valve should be adjusted according to the amount of constant back pressure obtained. In this ca se, the valve spring should be adjusted to open in the shop at a CDTP of 20 psig [138 kPag]. 5.3

Sizing for Steam Relief Pressure relief devices in steam service that operate at critical flow conditions may be sized using the following equations: Using U.S units

using the S.I unit

Where: A = required effective discharge area, in.2 [mm2] W = required flow rate, lb/hr (kg/hr). P 1 = upstream relieving pressure, psia (kPaa). This is the set pressure plus the allowable

overpressure plus the atmospheric pressure. Kd = effective coefficient of discharge. For preliminary sizing, use the following values:

= 0.975 when a pressure relief valve is installed with or without a rupture disk in combination,


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= 0.62 when a pressure relief valve is not installed Kb = capacity correction factor due to back pressure. This can be obtained from the manufacturer’s literature or estimated from Figure 30. The back pressure correction factor

applies to balanced bellows valves only. For conventional valves, use a value for Kb equal to 1.0 back pressure of a magnitude that will cause subcritical flow. Kc = combination correction factor for installations with a rupture disk upstream of the

pressure relief valve = 1.0 when a rupture disk is not installed, = 0.9 when a rupture disk is installed in combination with a pressure relief valve and the combination does not have a published value. KN = correction factor for Napier equation.

=1 where P 1 <1500 psia (10,339 kPaa), =

0.1906 P 1  1000 0.2292 P 1  1061

Where where P 1 >1500 psia (10,339 kPaa) and P1<3200

psia KSH = superheat steam correction factor. This can be obtained from Table 9. For saturated steam at

any pressure, KSH = 1.0.


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Table 3: Superheat Correction Factors, KSH


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Exercice 4: In this example, the following relief requirement is given:

W = saturated steam at 153,500 lb/hr (69,615 kgs/hr) at 1600 psig (11,032 kPag) set pressure with 10% accumulation. Note: that the set pressure is equal to the design pressure in this example a. Relieving pressure, P 1, of 1600 x 1.1 + 14.7 = 1774.7 psia (12,236 kPaa). b. Effective coefficient of discharge, Kd , of 0.975. c. Back pressure correction factor, Kb , of 1.0 for conventional valve discharging to atmosphere. d. Capacity Correction for rupture disk, Kc = 1.0, since there is no rupture disk. e. Correction factor for the Napier equation, KN , of [0.1906(1774.7) – 1000]/[0.2293(1774.7) – 1061] = 1.01. f. Superheat steam correction factor, KSH , of 1.0. 5.4

Sizing for liquid

The procedure for obtaining capacity certification includes testing to determine the rated coefficient of discharge for the liquid relief valves at 10% over pressure. Valves in liquid service that are designed in accordance with the ASME Code which require a capacity certification may be initially sized using the following equations: Using the US unit

Using the S.I unit


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A = required effective discharge area, in.2 (mm2). Q = flow rate, U.S. gpm (liters/min). Kd = rated coefficient of discharge that should be obtained from the valve manufacturer. For

a preliminary sizing, an effective discharge coefficient can be used as follows: = 0.65 when a pressure relief valve is installed without a rupture disk in combination, = 0.62 when a repture disc is installed. Kw = correction factor due to back pressure. If the back pressure is atmospheric, use a value

for Kw of 1.0. Balanced bellows valves in back pressure service will require the correction factor determined from Figure 9. Conventional and pilot operated valves require no special correction. Kc = combination correction factor for installations with a rupture disk upstream of the

pressure relief valve = 1.0 when a rupture disk is not installed, = 0.9 when a rupture disk is installed in combination with a pressure relief valve and the combination does not have a published value. Kv = correction factor due to viscosity as determined from Figure 10 or from the following

equation:

G = specific gravity of the liquid at the flowing temperature referred to water at standard

conditions. p1 = upstream relieving pressure, psig (kPag). This is the set pressure plus allowable

overpressure. p2 = back pressure, psig (kPag).


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where R = Reynold’s Number. Q = flow rate at the flowing temperature, U.S. gpm (liters/ min). G = specific gravity of the liquid at the flowing temperature

referred to water at standard conditions. μ = absolute viscosity at the flowing temperature, centipoise. A = effective discharge area, in.2 (mm2) (from API Std 526 standard orifice areas). U = viscosity at the flowing in Saybolt Universal seconds, SSU


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Figure 9: Capacity Correction Factor, Kv, Due to Viscosity


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Figure 10: Percent of Gauge Backpressure


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Figure 11: Relief Valve designation

Exercice 5

Guiven Fluid: Fuel oil Required capacity: 1200 gpm Set pressure: 150 psig back pressure: Atmospheric Overpressure: 10c% Inlet Tempreture: 60°F Viscosity: 850cP Specific gravity 0.993 Find the correct size standard orifice to meet the given requirement?


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5.5. Sizing for Thermal Relief Valve Hydraulic expansion is the increase in liquid volume caused by an increase in temperature. It can result from several causes, the most common of which are the following: a. Piping or vessels are blocked-in while they are filled with cold liquid and are subsequently heated by heat tracing, coils, ambient heat gain, or fire. b. An exchanger is blocked-in on the cold side with flow in the hot side. c. Piping or vessels are blocked in while they are filled with liquid at near-ambient temperatures and are heated by direct solar radiation. The above equation detailed in the section 5.4 can be used in the sizing for the thermal expansion:

in this case the liquid flow rate is calculated by the following equation:

Q

BH 500GC

Where: Q = flow rate at the flowing temperature, in US. galllon per minute. B = cubical expansion coefficient per degree Fahrenheit for the liquid at the expected

temperature. This information is best obtained from the process design data; however, Table 4 shows typical values for hydrocarbon liquids and water at 60°F. H = total heat transfer rate, in British thermal units per hour. For heat exchangers, this ca n

be taken as the maximum exchanger duty during operation. G = specific gravity referred to water = 1 .O0 at 60°F. Compressibility of the liquid is usually ignored.


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C = specific heat of the trapped fluid, in British thermal units per pound per degree Fahrenheit. Typical values of the liquid expansion coefficient, B, at 15°C are: Table 4: Liquid expansion coefficients

5.6 Sizing for fire Safety relief valve sizing is based on external fire criteria. The selected PSV for the application in gas service shall have an effective discharge area equal to or greater than the calculated value by the same formula used in paragraph 5.1. For the external fire case, relieved flow rate through the SRV is calculated using the following formulas:

Q



W

0.82

43200 F A w 

3.6 Q/Hl

Where Aw

Total wetted surface area of vessel (m²)

Q

Heat input (W)

Hl

Latent heat of the liquid exposed to fire (kJ/kg) Factor due to insulation

F


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6. Application 1 Exercice 1 A safety valve must be sized for a flow rate of 5 l/s (79.25 gpm) of Glycerin (G=1.26, viscosity= 1410 Cp). the set pressure is 10 barg (147 psig) with 10% accumulation and Atmospheric back pressure Find the correct size of the psv? using the software Solution:

The preliminary size of the PSV is 0.27 in2 (using Kv=1)

The effective API area is 0.307 in2

Re= 357.9 Kv= 0.8359 A=0.323 in2 The effective API area is 0.503 in2

Selected orifice G

Exercice 2 A safety valve is required for a vessel containing natural gas (methane M=16.04 lb/lbmol, Tc= -116 F, Pc= 673 psia) Flow= 22600lb/hr T=650°R Set pressure=80 psig over pressure 10%

Find the size of the PSV?


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Solution Tr=1.895 Pr=0.152 Z=0.98 Pb/Ps= 0.1837 Kb=1 P/Pc= 0.548 C=345.65 A=4.14 in2 Orifice N Exercice 3 We consider the same data as Exercice 2 with Set Pressure 20 psig and back pressure 24.7 psig and Z=1 Size?

Solution r=Pb/P1= 0.655 Pcf/P1= 0.548 Subcritical flow F2= 0.779 A=11.73 in2 orifice R

Exercice 4


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it is required to install a PSV for a heating hot oil at a set pressure 12 barg (174 psi) and 400°C (1211.7 R) with a flow rate 12.5m3/h (55.03 gpm). The back pressure is 2 barg. this vessel is exposed to the sun , we consider that the solar radiation is about 1040 w/m2 The vessel is vertical with a diameter 2.7 m and the height is 4m

Find the size?

Solution

The required orifice is D

Exercice 5

A vertical vessel with spherical ends at a pressure of 200 psig contain benzene at 100°F (559.7R). The vessel has a diameter of 15ft a length of 40 ft and elevation of 15 ft. The maximum fluid level is 12 ft F eff= 10

solution


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Glossary This section contains common and standard definitions related to pressure relief valves. It is in

accordance with generally accepted terminology. accumulation – a pressure increase over the maximum allowable working pressure (MAWP)

of the equipment being protected, during discharge through the pressure relief valve, usually expressed as a percentage of MAWP. Compare with overpressure . actual discharge area – the net area of a selected orifice which dictates the pressure relief

valve relieving capacity. back pressure – the static pressure existing at the outlet of a pressure relief valve due to

pressure in the discharge system. balanced safety relief valve – a pressure relief valve which incorporates means of minimizing

the effect of back pressure on the operational characteristics (opening pressure, closing pressure, and relieving capacity). blowdown – the difference between actual lifting pressure of a pressure relief valve and actual

reseating pressure expressed as a percentage of set pressure. blowdown pressure – the value of decreasing inlet static pressure at which no further

discharge is detected at the outlet of a pressure relief valve after the valve has been subjected to a pressure equal to or above the lifting pressure. built-up back pressure – pressure existing at the outlet of a pressure relief valve caused by the

flow through that particular valve into a discharge system. chatter – abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in

which the disc contacts the seat. closing pressure – the value of decreasing inlet static pressure at which the valve disc

reestablishes contact with the seat or at which lift become zero.


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coefficient of discharge – the ratio of the measured relieving capacity to the theoretical

relieving capacity. constant back pressure – a superimposed back pressure which is constant with time. conventional safety relief valve – a pressure relief valve which has its spring housing vented

to the discharge side of the valve. The operational characteristics (opening pressure, closing pressure, and relieving capacity) are directly affected by changes in the back pressure on the valve. design pressure – the value selected for the design of equipment for the most severe condition

of coincident pressure and temperature expected in normal operation, with provision for a suitable margin above these operating conditions to allow for operation of the pressure relief valve. The design pressure usually becomes the maximum allowable working pressure. discharge area – see actual discharge area. effective discharge area – a nominal or computed area of flow through a pressure relief valve,

contrasted to actual discharge area. For use in recognized flow formulas to determine the required capacity of a pressure relief valve. flow capacity – see rated relieving capacity. flow-rating pressure – the inlet static pressure at which the relieving capacity of a pressure

relief valve is measured. inlet size – the nominal pipe size of the inlet of a pressure relief valve, unless otherwise

designated. lift – the actual travel of the disc away from the closed position when a valve is relieving. maximum allowable working pressure – (1) the pressure determined by employing the

allowable stress values of the materials used in the construction of the equipment. It is the least value of allowable pressure value found for any component part of a piece of equipment for a given temperature. The equipment may not be operated above this pressure and consequently, it is the highest pressure at which the primary pressure relief valve is set to open. (2) the maximum gage pressure permissible at the top of a pressure vessel in its normal operating position at the designated coincident temperature specified for that pressure. nozzle constant, nozzle coefficient - a variable in the standard gas and vapor sizing formula

which is dependent on the specific heat ratio of the fluid. See equation 6, Figure 2, or Table 8.


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operating pressure – the service pressure to which a piece of equipment is usually subjected. orifice area – see actual discharge area. outlet size – the nominal pipe size of the outlet of a pressure relief valve, unless otherwise

designated. overpressure – a pressure increase over the set pressure of a pressure relief valve, usually

expressed a percentage of set pressure. Compare with accumulation . pilot-operated pressure relief valve – a pressure relief valve in which the major relieving

device is combined with and is controlled by a self-actuated pressure relief valve. pressure relief valve – a generic term for a re-closing spring loaded pressure relief device

which is designed to open to relieve excess pressure until normal conditions have been restored. rated relieving capacity – that portion of the measured relieving capacity permitted by the

applicable code of regulation to be used as a basis for the application of a pressure relief valve. relief valve – a pressure relief valve actuated by inlet static pressure and having a gradual lift

generally proportional to the increase in pressure over opening pressure. It is primarily used for liquid service. relieving pressure – set pressure plus overpressure. safety valve – a pressure relief valve actuated by inlet static pressure and characterized by rapid

opening or pop action. It is normally used for steam and air service. safety relief valve – a pressure relief valve characterized by rapid opening or pop action, or by

opening in proportion to the increase in pressure over the opening pressure, depending on the application. It may be used in either liquid or compressible fluid applications based on configuration. set pressure – the value of increasing inlet static pressure at which a pressure relief valve begins

to open. superimposed back pressure – the static pressure existing at the outlet of a pressure relief

valve at the time the valve is required to operate. It is the result of pressure in the discharge system from other sources.


Pressure Relief Valve

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