Pressure Relief Valve

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  • Technical Course

    Pressure Safety Valve

    Prepared by

    Bassem BALGHOUTHI

    Date January 2012

    Rev 00

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    Scope This technical course covers the sizing and selection methods of pressure relief valves

    used in the typical process industries. It helps engineers and designers understand the

    basic design of different types of pressure relief valves and rupture disks, and increase

    their knowledge in selection and sizing.

    The selection section contains the explanation for the suitability of types of pressure relief

    valve used in various applications.

    All the important parameters used in this guideline are explained in the definition section

    which helps the reader understand the meaning of the parameters and the terms.

    The theory section includes the sizing theory for the pressure relief valves for gas, 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 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. Selection of Pressure Relief Valves ................................................................................ 7

    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

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

    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 worth to be asked when you are going to specify details of PSV.

    What type of PSV we will have for our process requirements?

    Is there any easier way for PSV sizing (PSV calculation) rather than calculate it

    manually?

    What kind of material shall be chosen for our process 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;

    Sizing and Selection) about Safety Valve definition: A safety valve is a spring loaded pressure

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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 A1 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 keep 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 (Fs), process fluid from the

    pressurized system (PVAN), and backpressure (PBAN). The PV is the pressure due to the changes

    over the pressurized system, and the PB 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 Fs is

    having main contribution to the force balance, and have a positive direction along the PB. The

    overpressure in the pressurized system will increase the magnitude of the PV, and eventually it

    will affect the balance of the pressure force, and hence the sum of the PBAN and the Fs will be

    less than the PVAN. 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

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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 inlet temperature of 450 F

    and the newer designs are available for a maximum inlet temperature of about 1000F 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.

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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.

    M

    TZ

    PKKCK

    WA

    bdc 1

    eq 1

    Where,

    A : Effective discharge area relief valve, in2

    W : Flow through the device, Ib/hr

    C1 : Coefficient determined from an expression

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    )1(

    )1(

    )1

    2(5201

    k

    k

    kkC 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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    )1

    (

    1 1

    2

    k

    k

    cf

    kP

    P

    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 (167F) [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, P1, 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 (167F) [348 K (75C)].

    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 case,

    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).

    P1 = 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

    manufacturers 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 P1 1500 psia (10,339 kPaa) and P1

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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, P1, 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 = Reynolds 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.

    = 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: 60F 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:

    GC

    BHQ

    500

    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 60F.

    H = total heat transfer rate, in British thermal units per hour. For heat exchangers, this can

    be taken as the maximum exchanger duty during operation.

    G = specific gravity referred to water = 1 .O0 at 60F. 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 15C 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:

    0.82

    wAF00432Q

    Q/Hl3.6W

    Where

    Aw Total wetted surface area of vessel (m)

    Q Heat input (W)

    Hl Latent heat of the liquid exposed to fire (kJ/kg)

    F Factor due to insulation

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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=650R

    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

    400C (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 100F

    (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.

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    References

    1. API RP 521 Guide for Pressure-Relieving and Depressuring Systems, American

    Petroleum Institute, 1220 L Street, NW, Washington, DC 20005.

    2.API Standard 527 Commercial Seat Tightness of Safety Relief Valves with Metal-to-Metal

    Seats, American Petroleum Institute, 1220 L Street, NW, Washington, DC 20005.

    3. API RP 520 Recommended Practice for the Design (Part I) and Installation (Part II) of

    Pressure Relieving Systems in Refineries, American Petroleum Institute, 1220 L Street, NW,

    Washington, DC 20005.

    4. ASME Boiler and Pressure Vessel Code, Section I and Section VIII, American Society of

    Mechanical Engineers (ASME), New York, NY.

    5. ANSI B31.3 Chemical Plant and Petroleum Refinery Piping, The American Society of

    Mechanical Engineers (ASME), 345 East 47th Street, New York, NY 10017.

    6. GPSA code, Section 5, Relief System