G = Specific gravity of the gas or vapor.
Kb = Capacity correction factor due to back pres- sure. Standard valves with superimposed (constant) back pressure exceeding critical see Table T7-1 on page 7-3. For bellows or Series BP valves with superimposed or variable back pressure, see Figure F7-2 on page 7-5. For pilot valves see discussion on page 7-4.
P1 = Relieving pressure, kiloPascals absolute. This is the set pressure (kPa) + overpressure (kPa) + atmospheric pressure (kPaa).
T = Absolute temperature of the fluid at the inlet, degrees Kelvin (°C + 273).
Q = Required relieving capacity, standard cubic meters per minute (sm3/min).
Z = Compressibility factor (see Figure F7-1 on page 7-2). Use Z = 1.0 if value is unknown.
6 - 6
An "N" orifice valve with an effective area of 2800 square millimeters is the smallest standard size valve that will flow the required relieving capacity. From Crosby Cata- log No. 310, select a 4N6 Style JOS-46 valve with a Type C lifting lever and alloy steel spring. Standard materials of construction are satisfactory for this superheated steam application.
EXAMPLE #3
Saturated Steam at a Relieving Pressure Greater than 1500 psig(103 Barg)
Required Capacity: 39,900 kg/hr saturated steam Set Pressure: 18,960 kPag
Overpressure: 10%
Back Pressure: Atmospheric Special Requirement: Open Bonnet
Relieving Pressure: P1 = 18,960 kPag + 1896 kPa + 101 kPa = 20957 kPaa
From Figure F7-4
on page 7-6: Capacity Correction Factor, Kn=1.154
A = 190.4 W K P1 Ksh Kn Kb A = 190.4 (39,900)
0.975 (20957) (1) (1.154) (1) A = 322 sq.mm
A "G" orifice valve with an effective area of 325 square millimeters is the smallest standard size valve that will flow the required relieving capacity. From Crosby Catalog No. 310, select a 2G3 Style JOS-H-76 valve with a Type C lifting lever and alloy steel spring. Standard materials of construction are satisfactory for this saturated steam application.
EXAMPLE #1
Saturated Steam (kg/hr)
Required Capacity: 9750 kg/hr saturated steam Set Pressure: 1550 kPag
Overpressure: 10%
Relieving Pressure:P1= 1550 kPag + 155 kPa + 101 kPa = 1806 kPaa
Back Pressure: Atmospheric A = 190.4W
K P1 Ksh Kn Kb A = 190.4 (9750)
= 1054 sq.mm 0.975 (1806) (1) (1) (1)
A "K" orifice valve with an effective area of 1186 square millimeters is the smallest standard size valve that will flow the required capacity. From Crosby Catalog No.310, select a 3K4 Style JOS-15 valve with a Type C lifting lever. Standard materials of construction are satisfactory for this saturated steam application.
EXAMPLE #2
Superheated Steam (kg/hr)
Required Capacity: 49,200 kg/hr superheated steam Relieving Temperature: 400C
Set Pressure: 3670 kPag
Relieving Pressure: P1=3670 kPag + 367 kPa + 101 kPa = 4138 kPaa Back Pressure: Atmospheric
From page 7-8: Capacity Correction Factor, Ksh = 0.844
A = (190.4) (49,200)
= 2751 sq.mm 0.975 (4138) (0.844) (1) (1)
The following formula is used for sizing valves for steam service at 10% overpressure. This formula is based on the empirical Napier formula for steam flow. Correction factors are included to account for the effects of super- heat, back pressure and subcritical flow. An additional correction factor Kn is required by ASME when relieving pressure (P1) is above 10,340 kPaa.
A = 190.4 W K P1 Ksh Kn Kb Where:
A = Minimum required effective discharge area, square millimeters.
W = Required relieving capacity, kilograms per hour.
K = Effective coefficient of discharge. K = 0.975 P1 = Relieving pressure, kiloPascals absolute. This
is the set pressure (kPaa) + overpressure (kPa) + atmospheric pressure (kPaa).
Ksh = Capacity correction factor due to the degree of superheat in the steam. For saturated steam use Ksh = 1.00. See Table T7-2 on page 7-8 for other values.
Kn = Capacity correction factor for dry saturated steam at set pressures above 10346 kPaa and up to 22,060 kPaa. See Figure F7-4 on page 7-6.
Kb = Capacity correction factor due to back pres- sure. For conventional valves with superim- posed (constant) back pressure exceeding critical see Table T7-1 on page 7-3. For bellows valves with superimposed or variable back pressure see Figure F7-2 on page 7-5.
For pilot valves, see discussion on page 7-4.
Steam Sizing
10% Overpressure (kg/hr)
EXAMPLE #2
Liquid, liters/minute
Fluid: Castor Oil
Relieving Capacity: 380 liters/minute
Set Pressure: 1450 kPag
Overpressure: 10%
Back Pressure: 240 kPag (constant) Relieving Temperature: 15C
Specific Gravity: 0.96 A = 15.9 Q √ G
Kw Kv √∆P Where:
A = Minimum required effective discharge area, square millimeters
Q = 380 liters/minute
G = 0.96
Kw = 1.0 from page 7-5 Kv = 1.0 for non-viscous fluid
∆P = 1450 kPag + 145 kPa - 240 kPag = 1355 kPa A = 15.9 (380) √ 0.96
= 160.8 sq.mm (1) (1) √1355
A number 8 orifice with an effective area of 198 sq.mm is the smallest Series 900 OMNI-TRIM valve that will flow the required relieving capacity. Since the back pressure is constant a conventional Style JOS or Series 900 valve can be used. Therefore, from Crosby Catalog No. 902 select a Series 900 OMNI-TRIM 981105M-A.
EXAMPLE #1
Liquid, liters/minute
Fluid: Sodium Trisulfate
Relieving Capacity: 475 liters/minute
Set Pressure: 690 kPag
Overpressure: 10%
Back Pressure: 0 - 207 kPag (built-up) Relieving Temperature: 16C
Specific Gravity: 1.23 A = 15.9 Q √ G
Kw Kv √∆P Where:
A = Minimum required effective discharge area, square millimeters
Q = 475 liters/minute
G = 1.23
Kw = .866 (Figure F7-3 on page 7-5) Kv = 1.0 for non-viscous fluid
∆P = 690 kPag + 69 kPa - 207 kPag = 552 kPa A = 15.9 (475) √1.23
= 412 sq.mm (0.866) (1) √ 552
An "H" orifice valve with an effective area of 506 square millimeters is the smallest standard size valve that will flow the required relieving capacity. Since the built-up back pressure exceeds 10% a bellows style valve, Style JBS, is required. From Crosby Catalog No. 310, stan- dard materials were selected. Therefore, Model Number is 1-1/2H3 Style JLT-JBS-15 valve with a Type J Cap.
Liquid Sizing
Spring Loaded Valves
Styles JLT-JOS, JLT-JBS, Series 900 and Series BP
Where:
A = Minimum required effective discharge area, square millimeters.
G = Specific gravity of the liquid at flowing conditions.
Q = Required relieving capacity, liters per minute at flowing temperature.
∆P = Differential pressure (kPa). This is set pressure (kPag) + overpressure (kPa) - back pressure (kPag).
Kv = Flow correction factor due to viscosity of the fluid at flowing conditions (see page 7-7).
Kw = Capacity correction factor due to back pressure on bellows or Series BP valves on liquid service.
Refer to Figure F7-3 on page 7-5.
Note: See page 7-25 for information on two phase flow.
The following formula has been developed for valve Styles JLT-JOS, JLT-JBS, Series 900 and Series BP pressure relief valves using valve capacities certified by the Na- tional Board of Boiler and Pressure Vessel Inspectors in accordance with the rules of ASME Boiler and Pressure Vessel Code Section VIII. This formula applies to, and is to be used exclusively for sizing Styles JLT, Series 900 and Series BP pressure relief valves for liquid service applications.
Valve sizing using this formulation is not permitted for overpressures less than 10%.
A = 15.9Q √ G Kw Kv √∆P
6 - 8
A = 12.16Q √ G Kv √ ∆P Where:
A = Minimum required effective discharge area, square millimeters.
G = Specific gravity of the liquid at flowing condi- tions.
Q = Required relieving capacity, liters per minute at flowing temperature.
∆P = Differential pressure (kPa). This is the set pressure (kPag) + overpressure (kPa) - back pressure (kPag).
Kv= Flow correction factor due to viscosity of the fluid flowing conditions (see page 7-7).
Note: For optimum operation, fluid viscosity should be no greater than 300 SSU, and in this case Kv = 1.0 may be used.
Note: See page 7-25 for information on two phase flow.