GB/T 24921.1-2010 PDF English
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GB/T 24921.1-2010: Sizing, ion and installation of pressure relieving valves for petrochemical industries -- Part 1: Sizing and se-lection
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GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 23.060.99
J 16
Sizing, selection and installation of pressure relieving valves
for petrochemical industries - Part 1.Sizing and selection
Issued on: AUGUST 09, 2010
Implemented on: DECEMBER 31, 2010
Issued by. General Administration of Quality Supervision, Inspection and
Quarantine of PRC;
National Standardization Administration.
Table of Contents
Foreword... 3
1 Scope... 4
2 Normative references... 4
3 Terms and definitions... 4
4 Type characteristics... 6
4.1 Structural form... 6
4.2 Characteristics... 9
4.3 Type selection... 11
5 Size determination... 12
5.1 General requirements... 12
5.2 Effective area and effective displacement coefficient... 12
5.3 Back pressure... 12
5.4 Cold state test differential pressure... 14
5.5 Discharge pressure... 14
5.6 Determination of the size of the pressure relief valve for gas... 18
5.7 Determination of the size of pressure relief valves for steam... 24
5.8 Determination of the size of pressure relief valves for liquids... 25
5.9 Determination of the size of pressure relief valves for liquid/steam two-phase media
... 28
Appendix A (Informative) Example of size determination of gas media during critical
flow... 29
Appendix B (Informative) Example of determining the size of gas medium during
subcritical flow... 31
Appendix C (Informative) Example of determining the size of gas medium by
alternative method... 33
Appendix D (Informative) Example of determining the size of steam medium during
critical flow... 35
Appendix E (Informative) Example of determining the size of liquid media that require
displacement verification... 36
Appendix F (Informative) Sizing of pressure relief valves for two-phase media... 38
Sizing, selection and installation of pressure relieving valves
for petrochemical industries - Part 1.Sizing and selection
1 Scope
This Part of GB/T 24921 specifies the terms and definitions, type characteristics and
size determination of pressure relief valves for gas, steam, incompressible fluids and
two-phase flow media for petrochemical industry.
This Part applies to pressure relief valves, which have a set pressure of not less than 0.1
MPa for petrochemical industry.
2 Normative references
The provisions in following documents become the provisions of this Part through
reference in this Part of GB/T 24921.For the dated references, the subsequent
amendments (excluding corrections) or revisions do not apply to this Part; however,
parties who reach an agreement based on this Part are encouraged to study if the latest
versions of these documents are applicable. For undated references, the latest edition of
the referenced document applies.
GB/T 12241 Safety valves - General requirements
GB/T 12242 Pressure relief devices - Performance test code
GB/T 24920 Steel pressure relief valves for petrochemical industries
3 Terms and definitions
The terms and definitions established in GB/T 12241 and GB/T 12242, as well as the
following terms and definitions, shall apply to this Part.
3.1 Pressure relief valve
3.1.1
Pressure relief valve
It is a pressure relief device designed to reclose after returning to normal working
conditions to prevent the medium from continuing to flow out.
5 Size determination
5.1 General requirements
5.1.1 Reasonable consideration shall be given to various unexpected events that may
cause overpressure, so as to determine the conditions required for overpressure
protection and the size and type of pressure relief valve to be used.
5.1.2 Estimate the pressure generated by various unexpected events that lead to
overpressure and calculate the medium displacement required to be released. When
calculating the displacement, the process flow chart, materials, pipelines, containers
and equipment design specifications are required.
5.1.3 The release requirements under a series of operating conditions (including fire
conditions and non-fire conditions) that require overpressure protection shall be
analyzed and confirmed in detail.
5.2 Effective area and effective displacement coefficient
5.2.1 In the corresponding calculation formulas of 5.6, 5.7, 5.8, the effective area and
effective displacement coefficient are used to preliminarily determine the size of the
pressure relief valve.
5.2.2 The effective area and effective displacement coefficient are only used for
preliminary selection calculations and have no direct relationship with the design of
specific valves. The actual flow area of the valve that finally meets the use requirements
is usually larger than the standard effective area; the rated displacement coefficient is
smaller than the effective displacement coefficient.
5.2.3 The rated displacement coefficient is determined by multiplying the average
coefficient obtained by test verification by 0.9; its value is usually smaller than the
effective displacement coefficient (especially for valves for steam media, wherein the
effective displacement coefficient is 0.975).
5.2.4 When the pressure relief valve is selected, the actual flow area and rated
displacement coefficient of the valve are used to verify the rated displacement of the
valve. The verified displacement shall reach or exceed the displacement calculated by
the corresponding formulas of 5.6, 5.7, 5.8, so as to verify whether the valve has
sufficient displacement to meet the application requirements.
5.3 Back pressure
5.3.1 Back pressure can cause changes in opening pressure, reduction in flow rate,
5.4 Cold state test differential pressure
5.4.1 The cold state test differential pressure includes corrections for operating
conditions such as temperature and back pressure.
5.4.2 When a conventional pressure relief valve is tested at a set pressure on a test bench
at room temperature and used under high temperature working conditions or constant
back pressure, the set pressure needs to be corrected.
5.4.3 For the adjustment of the cold state test differential pressure, for conventional
pressure relief valves under constant back pressure, the required set pressure shall be
subtracted from the additional back pressure; for balanced pressure relief valves, the
additional back pressure has no effect on the set pressure; for pressure relief valves with
a discharge temperature exceeding 120 °C or below -59 °C, a temperature correction
factor for the set pressure is required for correction, the manufacturer shall be consulted.
5.5 Discharge pressure
5.5.1 Determination of discharge pressure
5.5.1.1 The allowable excess pressure shall be determined according to the
accumulation pressure allowed by the relevant specifications; the allowable excess
pressure shall be determined according to the different relationships between the set
pressure and the maximum allowable working pressure of the system to be protected.
When the set pressure is equal to the maximum allowable working pressure, the
allowable excess pressure is equal to the allowable accumulation pressure (see Figure
5).
5.5.1.2 When designing, the atmospheric pressure corresponding to the ground altitude
shall be taken into account.
5.5.1.3 The method for determining the discharge pressure of the pressure relief valve
for liquids is similar to that for the pressure relief valve for steam, or according to the
requirements of the order contract.
5.5.1.4 According to the relationship between the pressure relief valve and the pressures
of the system to be protected, the restrictions on the maximum accumulation pressure
and set pressure of the pressure relief valve are shown in Table 1.
Appendix A
(Informative)
Example of size determination of gas media during critical flow
A.1 Working conditions
A.1.1 Due to operating errors, the required flow rate W of hydrocarbon mixture is 24260
kg/h.
A.1.2 The main components of hydrocarbon mixture are butane (C4) and pentane (C5);
the molecular weight M of hydrocarbon mixture is 65.
A.1.3 The discharge temperature is 75 °C, that is, (T = 273 + 75 = 348 K).
A.1.4 The set pressure of the pressure relief valve is 517 kPa (gauge pressure), which
is the design pressure of the overpressure protection device.
A.1.5 The back pressure is 101.3 kPa (absolute pressure).
A.2 Parameters
A.2.1 The allowable accumulation pressure is 10%.
A.2.2 Discharge pressure, pd = 517 × 1.1 + 101.3 = 670 kPa (absolute pressure).
A.2.3 The calculated compression coefficient Z is 0.84.(When the compression
coefficient cannot be determined, Z = 1.0 can be taken).
A.2.4 The critical flow pressure (according to Table 7) is 670 × 0.59 = 395.3 kPa
(absolute pressure).
A.2.5 Cp/Cv = k (according to Table 7) is 1.09, according to Table 8, C = 326.
A.2.6 The back pressure correction factor of the displacement, Kb is 1.0.
A.2.7 Comprehensive correction factor, Kc is 1.0.
A.2.8 Effective displacement coefficient, when used for preliminary calculation, Kd is
0.975.
A.2.9 Since the back pressure of 101.3 kPa (absolute pressure) is less than the critical
flow pressure of 395.3 kPa (absolute pressure), the size of the pressure relief valve is
determined according to the critical flow formula [see formula (2) and 5.6.3].
A.3 Calculation
A.3.1 The effective discharge area of a pressure relief valve is calculated according to
Appendix B
(Informative)
Example of determining the size of gas medium during subcritical flow
B.1 Operating conditions
B.1.1 Due to operating errors, the required flow rate W of the hydrocarbon mixture is
24260 kg/h.
B.1.2 The main components of the hydrocarbon mixture are butane (C4) and pentane
(C5); the molecular weight M of the hydrocarbon mixture is 65.
B.1.3 The discharge temperature is 75 °C, that is (T = 273 + 75 = 348 K).
B.1.4 The set pressure of the pressure relief valve is 517 kPa (gauge pressure), which
is the design pressure of the overpressure protection equipment.
B.1.5 The constant back pressure is 379 kPa (gauge pressure). For conventional
pressure relief valves, the set pressure shall be corrected by adjusting the spring load
according to the value of the constant back pressure. In this example, the cold test
differential pressure is 138 kPa (gauge pressure).
B.2 Parameters
B.2.1 The allowable accumulation pressure is 10%.
B.2.2 Discharge pressure, pd = 517 × 1.1 + 101.3 = 670 kPa (absolute pressure).
B.2.3 The calculated compression coefficient Z is 0.84.(When the compression
coefficient cannot be determined, Z = 1.0 can be taken).
B.2.4 The critical flow pressure (according to Table 7) is 670 × 0.59 = 395.3 kPa
(absolute pressure).
B.2.5 The accumulation pressure is 517 × 0.1 = 51.7 kPa (gauge pressure).
B.2.6 The total back pressure is 379 + 51.7 = 431 kPa (gauge pressure). (Absolute
pressure is 532.3 kPa).
B.2.7 Cp/Cv = k (according to Table 7) is 1.09.
B.2.8 Comprehensive correction factor, Kc is 1.0.
B.2.9 Effective displacement coefficient, when used for preliminary calculation, Kd is
0.975.
Appendix C
(Informative)
Example of determining the size of gas medium by alternative method
C.1 Working conditions
C.1.1 Due to operating errors, the required flow rate W of hydrocarbon mixture is 24260
kg/h.
C.1.2 The main components of hydrocarbon mixture are butane (C4) and pentane (C5);
the molecular weight M of hydrocarbon mixture is 65.
C.1.3 The discharge temperature is 75 °C, that is, (273 + 75 = 348 K).
C.1.4 The design pressure of the overpressure protection device is 517 kPa (gauge
pressure), which is the set pressure of the pressure relief valve.
C.1.5 The constant back pressure is 379 kPa (gauge pressure). For conventional valves,
the spring assembly is adjusted according to the value of the constant back pressure
obtained. In this example, the cold test differential pressure is 138 kPa (gauge pressure).
C.2 Parameters
C.2.1 Allow 10% accumulation pressure.
C.2.2 Discharge pressure, pd = 517 × 1.1 + 101.3 = 670 kPa (absolute pressure).
C.2.3 The calculated compression coefficient Z is 0.84.(If the compression coefficient
cannot be calculated, Z = 1.0 can be taken).
C.2.4 The critical flow pressure (according to Table 7) is 670 × 0.59 = 395 kPa (absolute
pressure).
C.2.5 The system accumulation pressure is 517 × 0.1 = 51.7 kPa (gauge pressure).
C.2.6 The total back pressure is 379 + 51.7 = 431 kPa (gauge pressure). (Absolute
pressure is 532.3 kPa).
C.2.7 Cp/Cv = k (according to Table 7) is 1.09, according to Table 8, C = 326.
C.2.8 The comprehensive correction factor, Kc is 1.0.
C.2.9 The effective displacement coefficient, when used for preliminary calculations,
Kd is 0.975.
C.2.10 The ratio of back pressure to discharge pressure pb/pd (critical pressure ratio) is
Appendix D
(Informative)
Example of determining the size of steam medium during critical flow
D.1 Operating conditions
D.1.1 Due to operating errors, the required flow rate W of saturated steam to the
atmosphere is 69615 kg/h.
D.1.2 The design pressure of the overpressure protection device is 11032 kPa (gauge
pressure), which is the set pressure of the pressure relief valve.
D.1.3 Accumulation pressure is 10%.
D.2 Parameters
D.2.1 Discharge pressure, pd = 11032 × 1.1 + 101.3 = 12236 kPa (absolute pressure).
D.2.2 Effective displacement coefficient Kd is 0.975.
D.2.3 Back pressure correction factor of flow, Kb is 1.0.
D.2.4 Comprehensive correction factor, Kc is 1.0.
D.2.5 Pressure correction factor Kn = [0.02764 (12236) - 1000]/[0.03324 (12236) -1061]
= 1.01.
D.2.6 Overheat correction factor Ksh is 1.0.
D.2.7 Effective displacement coefficient, for preliminary calculation, Kd is 0.975.
D.3 Calculation
D.3.1 Substituting the above conditions and data into formula (9), the effective
discharge area of a single pressure relief valve is obtained as follows.
D.3.2 Using the flow channel codes "D" to "T" in GB/T 24920, and comparing with the
calculated A, the standard flow channel effective area is selected as 1186 mm2,
meanwhile the corresponding flow channel code is "K".
correction factor is only applicable to balanced pressure relief valves;
Kc - Comprehensive correction factor when a bursting disc is installed at the inlet of
the pressure relief valve; Kc = 1.0 when no bursting disc is installed. When the
bursting disc and the pressure relief valve are installed in combination and the
combined comprehensive correction factor is not given, Kc = 0.9.
F.2.1.2 Example
In this example, the following discharge requirements are known.
a) The required flow rate of the crude oil two-phase medium flow caused by the
operating error is 477430 pounds per hour. This flow occurs on the outlet side of
the condenser;
b) The temperature at the inlet of the pressure relief valve is 200 °F (659.7R);
c) The pressure relief valve is set at 60 psig, which is the design pressure of the
equipment;
d) The outlet back pressure is 15 psig (29.7 psi) (absolute pressure) [additional back
pressure is 0 psi (gauge pressure); the discharge back pressure is 15 psi (gauge
pressure)];
e) The specific volume of the two-phase medium at the inlet of the pressure relief
valve is 0.3116 cubic feet/pound.
In this example, the following data can be obtained.
a) The allowable accumulation pressure is 10%;
b) The discharge pressure is 1.10 × 60 = 66 psig (80.7 psi) (absolute pressure);
c) The percentage of back pressure (gauge pressure) to set pressure (gauge pressure)
is (15/60) × 100 = 25%.
The back pressure correction factor Kb = 1.0 (derived from Figure 10). Since the outlet
discharge back pressure is greater than 10% of the set pressure, a balanced pressure
relief valve shall be used.
Step 1.Calculate the Omega ω parameter.
Because the crude oil two-phase medium flow medium is a multi-component flash
medium with a nominal boiling point range greater than 150 °F, formula (F.3) is selected
to calculate the Omega parameter ω. Using a process simulation computer to perform
isenthalpic (adiabatic) flash calculations, the estimated specific volume value at 0.9 ×
80.7 = 72.63 psi (absolute pressure) is 0.3629 cubic feet/pound. The parameter ω
obtained by formula (F.3) is.
A - Effective discharge area required for the pressure relief valve, in square inches;
Q - Volume flow rate, in gallons per minute;
Kd - Effective displacement coefficient to be obtained from the manufacturer. When
initially determining the size, Kd = 0.65 can be used for subcooled liquids and Kd =
0.85 can be used for saturated liquids;
Kb - Liquid back pressure correction factor to be obtained from the manufacturer.
When initially determining the size, Figure 10 can be used. The back pressure
correction factor is only applicable to balanced bellows pressure relief valves;
Kc - Comprehensive correction factor for equipment with explosion-proof
membranes installed upstream of the pressure relief valve. When no explosion-proof
membrane is installed, Kc = 1.0.When the explosion-proof membrane and the
pressure relief valve are installed together, but no recognized value is given, Kc =
0.9.
F.2.2.2 Example
In this example, the following discharge requirements are known.
a) The required propane volume flow rate due to pump line blockage is 100 gpm;
b) The pressure relief valve is set at 260 psig, that is, the design pressure of the
equipment;
c) The total outlet back pressure is 10 psig (24.7 psi) (absolute pressure) [additional
back pressure = 0 psi (gauge pressure), discharge back pressure = 10 psi (gauge
pressure)];
d) The temperature at the pressure relief valve inlet is 60 °F (519.67R);
e) The density of liquid propane at the pressure relief valve inlet is 31.92 pounds per
cubic foot;
f) The specific heat of liquid propane at constant pressure at the pressure relief valve
inlet is 0.6365 Btu/lb·R;
g) The saturation pressure of propane at 60 °F is 107.6 psi (absolute pressure);
h) The specific volume of liquid propane at saturation pressure is 0.03160 cubic feet
per pound;
i) At saturated pressure, the specific volume of propane vapor is 1.001 cubic feet per
pound;
j) At saturated pressure, the latent heat of vaporization of propane is 152.3 Btu per
Kc - Comprehensive correction factor for equipment with explosion-proof disks
upstream of the pressure relief valve. When no rupture diaphragm is installed, Kc =
1.0.When the rupture diaphragm and the pressure relief valve are installed together,
but no recognized value is given, Kc = 0.9.
F.2.3.2 Example
In this example, the following discharge requirements are known.
a) The operating pressurization requires a flow rate of 153830 lb/hr to the diesel
hydrotreater (GOHT);
b) The temperature at the pressure relief valve inlet is 450 °F (909.67R);
c) The pressure relief valve is set at 600 psig, which is the design pressure of the
equipment;
d) The total downstream back pressure is 55 psig (69.7 psi) (absolute pressure)
[additional back pressure = 0 psi (gauge pressure), discharge back pressure = 55
psi (gauge pressure)];
e) The two-phase specific volume at the pressure relief valve inlet is 0.1549 cubic
feet per pound;
f) The mass fraction of steam and gas at the pressure relief valve inlet is 0.5596;
g) The mixed specific volume of steam and gas at the pressure relief valve inlet is
0.2462 cubic feet per pound;
h) The mole fraction of gas in the vapor phase is 0.4696.The non-condensable gases
in the diesel hydrotreater system include hydrogen, nitrogen, hydrogen sulfide;
i) Since the specific heat ratio k is unknown, k = 1.0 can be taken.
In this example, the following data can be obtained.
a) Excess pressure 10%;
b) The discharge pressure is 1.10 × 600 = 660 psi (gauge pressure) (674.7 psi)
(absolute pressure);
c) The back pressure (gauge pressure) percentage = (55/600) × 100 = 9.2%.
Since the downstream back pressure is less than 10% of the set pressure, a conventional
pressure relief valve shall be used. Therefore, the back pressure correction factor Kb =
1.0.
Step 1.Calculate the inlet void ratio αo
......
Source: Above contents are excerpted from the full-copy PDF -- translated/reviewed by: www.ChineseStandard.net / Wayne Zheng et al.
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