We are facing many challenges today regarding estimation of the flow rate of any pipeline or hose installed in our plant. This paper just gives a brief of some equations used in the gas plants to calculate the leakage rate, minimum time required to get a shut-in test, and the maximum pressure drop for new lines. There are many equations to calculate the flow rate of gas through pipe line like: 1. Darcy formula using gas expansion factor which can be used in sonic and subsonic flow. 2. Crane isothermal flow equation 3. Weymouth isothermal flow equation 4. Panhandle isothermal flow equation This paper gives a a quick way to: 1. Calculate the leakage rate from gas line using pressure drop method. 2. Estimate the minimum time required to get a shut-in test on gas transmission line. 3. Estimation of the maximum pressure drop for new lines.

1. Everyday pipeline engineering calculations
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The transition from construction to
operation is the commissioning and startup.
Processing plant commissioning embraces
activities such as cleaning, flushing,
verifications, leak tests, performance
evaluation and functional tests essential for
bringing a newly installed plant or facility
into routine operation.
Correct commissioning is vital to the
satisfactory operation of any plant or facility
and it is essential that sufficient time and
resources be allocated to different stages of
the commissioning and startup.
Some equations, which can be used in
commissioning phase of chemical
projects, are listed below:
1. Estimation of time required to get a
shut-in test on gas lines:
Hm =
3D3
L
P1
Hm: minimum time necessary to
achieve an accurate test, hr
L: Length of pipe section under test,
mile
P1: initial test pressure, Psig
D: Internal pipe diameter, in
2. Calculation of maximum pressure
drop for new lines:
Pad =
HtP1
949D
Pad: acceptable pressure drop, Psig
Ht: Shut-in test time, hr
3. Determination of leakage from gas
line using pressure drop method for
pipe section of 1 mile under 1 hr test.
Ly = 9D2
[
P1
460 + t1
−
P2
460 + t2
]
Ly: Leakage rate, Mmcf per year
P1: pressure at the beginning of the
test, Psia
P2: pressure at the end of the test,
Psia
t1: Temperature at the beginning of
the test, F
t2: Temperature at the end of the test,
F.
4. Quick way for determination of the
gas flow rate:
• Calculation of the choke
pressure.
Pchoked
Po
= (
2
𝛾 + 1
)
𝛾
𝛾+1⁄
γ: Specific heat ratio
• If the downstream pressure is
less than choked pressure,
Sonic flow.
Qm = Co 𝐴Po
√ 𝛾gcM
RgTo
(
2
𝛾 + 1
)
(𝛾+1)
(𝛾-1)⁄
• If the downstream pressure is
higher than choked pressure,
sub-sonic flow.
Q =
500D3
√P1
2
− P2
2
√L
(
ft3
24 hr
)
References:
1. Chemical process safety
fundamentals with applications, 3rd
edition.
2. Pipeline rules of thumb handbook –
A manual of quick, accurate
solutions of everyday pipeline
engineering problems, 8th
edition.