sizing pressure relieving

8/7/2019 Sizing Pressure Relieving

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Sizing for Gas or Vapor Relief

Upstream Downstream

US Customary Units

A=Require eff

w= Required fl

C= Coefficient

Kd= Effective c

P1= Upstream r

SI Units

Kb= Capacity c

Kc= combinatio

T= Relieving te

Z= Compressib

M= Molecular

V= Required fl

G= Specific gr

Sizing for Critical Flow:The pressuredownstream of the nozzle is less than, or


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Example

d. Relieving valve set at 75 psig [517 kPa], which is the design pressure of the eq

Solution

a. Permitted accumulation of 10%, so

b. Relieving pressure, P1 = 75 + (75*0.1) + 14.7 = 97.2 psia [670 kPa]

c. Calculated compressibility, Z, of 0.84 (If a calculated compressibility is not avail

d. Critical flow pressure(from Table 7 ) of 97.2*0.59 = 57.3 psia (42.6 psig) [395 k

Note: Since the back pressure(0 psig[0 kPag])is less than the critical flo

the relief valve sizing is based on critical flow equation (see Equation 3.

e. Cp/Cv = k = 1.09(Table 7). From Table 8,,,at k=1.09, C=326or C=520[k(k/k+f. Capacity correction due to back pressure, Kb of 1.0

g. Capacity Correction for rupture disk, Kc = 1.0 (not installed the rupture disk)

The size of a single pressure relief valve is derive from Equation 3.2 as follows:

A = [53,500/(326*0.975*97.2*1.0*1.0)]*[(627*0.84)/65]^1/2

A=

Further calculated V, required flow through the device

w=

V= 5201.85 SCFM at 14.7 psia and 60 F C=Kd=

P1=

Kb=

Kc=

T=

Z=

M=

for Eq.3.3 V=

for Eq.3.4 G=

a. Required hydrocarbon vapor flow, W, caused by an operational upse, of 53,500

b. The hydrocarbon vapor is a mixture of butane (C4) and (C5). The molecular wei

c. Relieving temperature,T, of 627 R(167 F)[348 K]

e. Back pressure of 14.7 psia (0 psig) [101.3 kPaa(0 kPag)].Kb


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ctive discharge area of the device, in^2 or [mm^2]

ow through the device, lb/hr [kg/hr]

determined from an expression of the specific heat(k=Cp/Cv)

efficient of discharge. For preliminary sizing, use the following values:

Kd = 0.975 when a pressure relief valve is installed w/ or w/o a rupture disk in coKd = 0.62 when a pressure relief valve is not installed and sizing is for a rupture

elieving pressure, psia[kPaa]

,= Set pressure + allowable overpressure(see3.5) + atmosphere pressure

rrection factor due to back pressure, be obtained from te manufacturer's literature or

Kb= 1.0 for conventional and pilot operated valves(see 3.3 Back pressure)

* The back pressure correction factor applies to balanced bellow valve only

* See 3.6.3 for conventional valve applications with back pressure of a magnitude tha

n correction factor for installations with a rupture disk upstream of the pressurerelief

Kc= 1.0 when a rupture disk is not installed

Kc= 0.9 when a rupture disk is installed in combination with a pressure relief valve an

mperature of the inlet gas or vapor, R(F+460)[K(C+273)]

ility factor for the deviation of the actual gas from a perfect gas, a ratio evaluated at i

eight of the gas or vapor at inlet relieving conditions. Various handbooks carry tables

ow through the device, scfm at 14.7 psia and 60 F [Nm3/min at 0 C and 101.325 kPaa

vity of gas at standard conditions refered to air at standard conditions[normal conditi

qual to, the critical flow pressure, Pcf


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ipment

able, a Z value of 1.0 should be used.)

Paa]

pressure (42.6 psig[294 kPag]),

and paragraphs 3.6.1 and 3.6.2)

1)^(k+1)/(k-1)]^1/2

US unit/ [SI unit]

4.93in2/ [mm2]

53500lb/hr /[kg/hr]

326psia/ [kPaa]0.98

97.2

1

1

627 R(F+460)/ [K(C+273)]

0.84

65

scfm at 14.7 psia and 60 F/ [Nm3/min at 0 C and 101.325 kPaa]

Gfluid/(G=1.0 for air at 14.7 psia and 60 F/ [101.325 kPaa and 0 C])

0 lb/hr [24,260 kg/hr]

ght of vapor, M, is 65


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binationdisk in accordance with 3.11.1.2

preliminary sizing from Fig.30.

t will cause subcritical flow

alve(see 3.11.2)

d the combination does not have a published valve.

nlet relieving condition.

of molecular weights of materials, but the composition of the flowing gas or vapor is seld

]

ons]. In other words, G=1.0 for air at 14.7 psia and 60 F [101.325 kPaa and 0 C]


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m the same as that listed in tables. This value should be recieve from the process data. T


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ble 7 lists values for some common fluids, lbm/lbmole [kg/kmole]


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sizing pressure relieving

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