Gas Pipeline Blowdown Time

Gas Pipeline Blowdown time Reference: "Simple Method Predicts Gas Line Blowdown Times", by M.H. Weiss, K.K. Botros, W.M. Jungowski, Oil & Gas Journal, Dec. 12, 1988

P t c  ln  i  Pa

  k   k  1      ln   k  1   2    k 1

 k  1  2( k 1) V    2  v  c  Ath  C d

Z k  R T MW

c

Z

1   Pi  5270  101.785SG   1   T 3.825    2

   f L  f  L   f  L  C f  a1  a 2  log 10    a3  log 10    a 4  log 10    D   D   D    2

 1   1   1    25.680     38.409    a1  1.0319  5.2735    Ar   Ar   Ar 

3

2

3

2

3

1 1 1 a2  0.2699417.304    86.415   144.77    Ar   Ar   Ar 

1 1 1 a3  0.2417512.637    56.772    88.351    Ar   Ar   Ar  2

1 1 1 a4  0.054856 2.6258   8.9593  12.139   Ar   Ar   Ar  Ar 

3

3

Ap Ath  C d

Note: The polynomials for the coefficients (a1, a2, a3, a4) are valid only for 3 < Ar < 30 and should not be used outside this range. Pc

k 1.20 1.25 1.30 1.35 1.40 1.45 1.50 1.55 1.60

ts 0.5645 0.5549 0.5457 0.5368 0.5283 0.52 0.512 0.5042 0.4968

t tot  t c  t s    V  C f

0.7371 0.7605 0.7833 0.8058 0.8278 0.8495 0.8707 0.8916 0.9122

where: tc =

Dimensionless Sonic blowdown time

Pi =

Initial Pressure of the gas pipeline, kPa (abs)

Pa =

Ap =

Atmospheric Pressure, kPa (abs) Ratio of Specific heats (Cp/Cv) Time constant, seconds Volume of the pipe being blown down, m3 (Pipe C/S area X Pipe length) sonic velocity, m/s compressibility factor Specific Gravity of Gas (Air =1) Absolute temperature of the gas in pipeline, K Universal Gas Constant (8314 J/kmol-K) Molecular Weight of the Gas = SG*28.96 (if only specific gravity of gas is known) Blowdown Correction Factor, dimensionless Darcy friction factor, dimensionless Length of pipeline, m Inside Diameter of the Pipeline, m Ratio of Pipe-to-effective-valve-discharge-area, dimensionless C/S area of pipe, m2

Ath =

Blowdown Valve discharge Area, m2

k= τV = V= c= Z= SG = T= R= MW = Cf = f= L= D= Ar =

Ath*Cd =

Notes: 1. For full-bore ball valves the "discharge area" will be considered the same as the blowdown line cross-sectional area 2. For reduced bore ball valves the "discharge area" should be considered based on the cross-sectional area of one size lower than the blowdown line size Blowdown Valve discharge coefficient, dimensionless (Use a value of 0.85 to 1 in absence of manufacturer's data) Effective valve discharge area, m2

Pc =

Critical Pressure Ratio

Cd =

k

 2  k 1 Pc     k  1 ts =

Dimensionless subsonic blowdown time

ttot =

total blowdown time, seconds

Darcy Friction Factor (Turbulent Flow) Pipe ID

f

mm 26.64 (1" STD)

0.023

40.9 (1.5" STD)

0.021

52.5 (2" STD)

0.019

77.92 (3" STD)

0.018

102.26 (4" STD)

0.017

154.08 (6" STD)

0.015

202.74 (8" STD)

0.014

254.46 (10" STD)

0.014

304.81 (12" STD)

0.013

336.56 (14" STD)

0.013

387.36 (16" STD)

0.013

438.16 (18" STD)

0.012

488.96 (20" STD)

0.012

590.56 (24" STD)

0.012

742.96 (30" STD)

0.011

895.36 (36" STD)

0.011

Exam ple Calculation: Estimate the total blowdown time for a 25 km, 36" STD pipeline which contains natural

gas at 6000 kPa(abs) and 10°C. The gas has a specific gravity of 0.6. The blowdown valve is a 8" full bore valve on a 8" blowdown line The atmospheric pressure is 100 kPa(abs). Inputs Pi = Pa = k= Cd = D= T= SG = f= BDV size = L= ts = Calculations Z= MW = V= c= Ath = Ap = Ar = tc =

6000 kPa(abs) 100 1.3 0.85 0.895 10 0.6 0.011 8 25000 0.7833

kPa(abs) input from external source m °C

inch m

23.6 3.49

a1 =

0.8514

a2 =

0.3229

a3 =

-0.1991

a4 =

0.0413846

Losses ttot =

Prepared by: Email:

Refer table

3 15728.1 m 390.4 m/s 2 0.031 m 2 0.629 m

2578.0 seconds

ttot =

specific gravity of gas (Air =1) manual input based on table for Darcy friction factor (if RB use one size lower than BD line size and if FB use line size)

0.87 17.4

τV =

Cf =

Input based on NPS & Pipe Schedule

1.06 11669.7 seconds 1.05 12253.2 seconds 204.2 minutes

(Time delay is based on losses in vent line and entrance) (including losses)

Ankur Srivastava Chemical Engineer [email protected]