Natural Gas Pipeline Commisioning Calculations Problem

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dingelde
Posts: 2
Joined: Fri Apr 05, 2013 9:24 pm

Natural Gas Pipeline Commisioning Calculations Problem

Post by dingelde »

Hello,

I am a Canadian Engineer who works with a local Gas Company. The problem at hand is our company has an old spreadsheet that we have been using for many years. Unfortunately, no one can seem to follow the calculations of it. I have been going through the calcualtions, and I have hit a snag. Maybe someone here can help me out?

First, some background. For those who are not in the industry, when we install new pipe, we have to commision the line, which means pushing all the air out and replacing that air with natural gas. At the end of the new pipeline is a flare stack, where we ignite the gas and let if flare for awhile to ensure that the gas line is at 100% gas. The procedure is such that we 1. Install and air test the new gas line 2. Connect the flare assembly 3. Squeeze the existing main 4. Cut and fuse the existing main to the new main 5. Partially release squeezer and wait for flare.

Step 5 is the calcualtion part. Ultimately what we want to know, is 1. How long will it take to commsion the line to 100% gas? 2. What is the velocity of the gas in the pipeline? The spreadsheet that I have attached includes the velocitys through the flare stack, hose, and stub/valve as well.I apoligize for the length of this, but I felt it was necessary back ground info.

As for the specifics of my problem:

In the attached spreadsheet, you can ignore the info on stub/valve, hose, and flare stack. Just look at the pipeline section. This spreadsheet has the user manually input a purge rate (line 18). Then it takes the purge rate and gets the "purge time" (simple velocity = distance/time) (line 56). Now here is the issue. To get the Purge Volume to Achieve Average Flow, it takes the volume of the pipeline (volume=areaXlength) (line 91) and multiplies it by 2. All subsequent calculations are used using this doubled volume. Which means the velocity in the pipe (line 92) is double that of the purge rate, which makes no sense.

Any chance any one here might have some insight into this? I really appreciate it, thanks!
dingelde
Posts: 2
Joined: Fri Apr 05, 2013 9:24 pm

Re: Natural Gas Pipeline Commisioning Calculations Problem

Post by dingelde »

Apparently I can not attach the spread sheet. So here is the bare bones of it:
GIVEN
Pipeline length: 827 m
outside diameter: 88.000 mm
wall thickness: 7.630 mm
Purge rate: 0.00311 (min/meter)

Calculate

Pipeline a. Inside Diamter (od-2wt) = (88-2*7.63) = 72.74 mm
b. AREA = pi*r^2 = pi* (72.74/2)^2= 4155.626 mm^2 or 0.004156 m^2
c. Volume (area*length) = 0.004156* 827 m = 3.437 m^3

Purge Time = 0.00311 * 827 m = 2.57 minutes


Ok, this is where it goes south for me:

Purge volume to achieve average flow: Pipeline volume*2 = 3.437 m^3 * 2 = 6.874 m^3
Average Flow: Purge volume/purge time = 6.874/2.57 = 2.675 m^3/min or 160.51 m^3/hr

Does that make sense to anyone? They use this average flow to calculate everything else, including pipeline velocity and inlet pressure.
admin
Site Admin
Posts: 375
Joined: Mon Feb 08, 2010 7:47 pm

Re: Natural Gas Pipeline Commisioning Calculations Problem

Post by admin »

For me it makes no sense also.
Where did you get purge rate of 0.00311 min/meter?
Pipe flow calculations - since 2000
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