Air Teaser: How Fast Can a Pig Travel?
By Ernie Parker, CFPAI, CFPSD, CFPS, CFPMM, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPE.
A gas pipeline was updated in my home area. The work included installing a line under the Minnesota River that runs to the residental area. In the final stages of the work, after checking for leaks or other problems, they ran a “pig” through the entire line to clean it before running natural gas through it. A pig is a plug that rubs the edges for cleaning. Normally a pig goes through at 0.2 MPa (30 psi) but may take as high as 0.55 MPa (80 psi).
The pressure in the pipeline will be 1.724 MPa (250 psi) but proof tested to 2.75 MPa (400 psi).
Using the followings specifications, how long would it take for the pig to travel the total distance at 0.2 MPa (30 psi ) and at 0.55 MPa (80 psi)?
Given:
Diameter: 607 mm (24 in)
Length: 2,134 m (7,000 ft)
Pressure required to move the pig: 0.2 MPa (30 psi) and at 0.55 MPa (80 psi)
Air compressor: 45,312 Nlm (1,600 scfm) screw compressor driven by a large diesel engine
psi = MPa x 145
See the Solution
Metric
Find area of pipe:
609.6²x 0.7854 = 291,864.19 mm² = 0.29 m²
Find volume of pipe:
0.29 m² x 2,134 m = 618.86 m3 x 1,000 = 622,840 liters
Calculate lpm: Find compression ratio:
(0.2 MPa + 0.101) / 0.1 = 3:1 C.R.
45,312 Nlpm / 3 C.R. = 15,104 lpm
Volume of pipe/lpm = 622,840/15,104 = 41.24 minutes
Calculate for 0.55 MPa:
C.R. = (0.55 + 0.101) /0 .1 = 6.51:1 C.R.
45,312 Nlpm / 6.51 = 6,960.37 lpm.
622,840 / 6,960.37 minutes = 89.5 minutes
Imperial
Find area of pipe:
24² x .7854 = 452 square inches.
Divide by 144 = 3.14 square feet.
Find the volume of air required to move the pig 7,000 feet: 3.14 x 7,000 = 21,991.2 cubic feet.
Calculate cfm: Find compression ratio (C.R.):
(30 psi + 14.7) / 14.7 = 3:1 C.R. 1,600 scfm / 3 = 533.33 cfm
Volume of pipe/cfm = time in minutes 21,991.2 / 533.33 = 41.23 minutes
Calculate for 80 psi:
C.R. = (80 + 14.7)/14.7 = 6.44 C.R.
1,600/6.44 = 248.36 cfm
21,991.2 / 248.36 = 88.54 minutes
Final answers = 41.23 minutes at 0.2 MPa (30 psi) and 89 minutes at 0.55 MPa (80 psi).
For previous Air Teaser problems, visit www.fluidpowerjournal.com/air-teaser.
