Pipeline Dewatering Calculations 1

[Shaneholl]

Hello All,

I am working on calculating the size of an air compressor that needs to be purchased to push a pig through a pipeline.

I am new to pigging and not familiar with the calculations. I have found some online and have been able to get that I should need compressor that is able to push 345 CFM. However, I would like verification.

Provided data:

Dewater by running the fill pigs with compressed air backwards to the initial fill header. Following dewatering remove the test headers and run foam drying pigs or, in the case of short length or small diameter lines, blow with compressed air until accepted by the Inspector. When running foam pigs the line will be considered dry when a pig is received dry.

Following drying, operate all inline valves to 1/4 open position and open all drain valves while maintaining at least 10 psi air pressure on the pipeline.


Variables Provided:
Pipeline Variations: 15-22" Diameter
Pipeline Length: 1-18 Miles
Be able to open all drain valves and inline valves(1/4 open) while being able to maintain at least 10 psi air pressure on the pipeline.

Excel Information that I have come up with (Values are not identical to what needs to be done, this was just a trial):

Some issues:
Not sure the flow rate out with drain valves/inline valves open
Not sure what the required pressure is - I assume 10 psi?
Are there any variables that I am missing?
Should I do a force balance and counter the pig friction force as well as force to push water through?


Free Air Volume of the Pipe
Diameter (Inches): 22
Length (Miles): 18
Radius (Inches): 11
Cubic Inches in one cubic foot: 1728
Length (Inches): 1140480
Free Air Pipe Volume (Cubic Feet): 250887.5893

Air Compression Ratio
Required Pressure (PSI): 10
Atmospheric Pressure (PSI): 14.7
Sum of Pressure (PSI): 24.7
Pipe Site Elevation (Feet): 4500
Pipe Site Atmospheric Pressure (PSI): 12.45827875
Compression Ratio: 1.982617382
Total Compressed Air required to increase pressure in this or any other pipe is: 497414.0955

Time As Factor (CFM): 345.4264552

Height (Meters): 1371.6
Height (feet): 4500
Pipe Site Atmospheric Pressure (Pa): 85896.80811
Pipe Site Atmospheric Pressure (PSI): 12.45827875


Any help would be greatly appreciated!

 

[zdas04]

The big issue in moving drying pigs with an air compressor is not so much flow rate as pressure. For drying, people often assume that the only hydrostatic number they have to consider is the relative elevation between the sites (i.e., the intervening hills are irrelevant). This is true when you first open the dewatering valve, but not after that. The first thing that happens is water from the last high point to the valve starts to drain, but it is exceedingly unusual for it to create a siphon that will pull the uphill water along with it. What I do in picking a compressor for dewatering is add up the height of every uphill (from the start of that uphill to the top), multiply the total height by 0.433 psi/ft (9.81 kPa/m). Add to that 3 times the weight of the pig (to account for friction, in 20-inch pigs this can be significant). The result in any line that is very long with an average elevation of 4500 ft is going to be much more than 10 psig. I take the result of the resistance calcs and round them up to the next 50 psig (i.e., if it works out to 105 or 145 I call it 150 psig). Now you have a realistic pressure capability.

How fast do you want the pig to run? Numbers around 5 ft/s are pretty common. What is the dV of the pipeline space in one minute if the pig is running at 5 ft/s? It will move 300 ft/min. Multiply that times the cross-sectional area of the pipe and you get the CFM you need at atmospheric pressure. Multiply that times the pressure you calculated above divided by local atmospheric pressure and you have the CFM you need.

So for a 20-inch line that needs 200 psig to ensure that it will dewater you need:

Capacity = (π/4)*(20/12)^2*(5 ft/s * 60 s)(200+12)/12=11,500 CFM at suction conditions

Change it to 1 ft/s and the number goes down to 2300 CFM. Using compressors to dewater big lines is a common activity and I regularly see machines installed that cannot achieve the required discharge pressure and they stall out in an uphill. This allows significant water to leak through the pig, and the second run requires the same pressure as the first run.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[LittleInch]

I agree with David and he has provided a very good explanation. The biggest thing missing from your data is a profile. What you need to consider when de-watering or clearing a liquid line is that your pig is essentially a moving pump station. Hence when it is at the bottom of a hill you need to push that water all the way to the top of the highest hill in front of the pig otherwise nothing will happen. On the converse, when it is at the top of the highest hill all the water provides is frictional back pressure and if you have the same pressure at the bottom of the hill your velocity will increase hugely.

This is not a simple task and requires experienced operators and an air compressor man enough for the worst case, but control over outlet pressure is required to even out these differences. Basically you need to have flow control / online measurement to maintain a more or less fixed liquid outflow which will mean the pressure at your receiving point varies as the pig goes up and down hills.

Keeping the pig going at 1-2 m/sec is key to good we-watering - too fast and it leaves water behind and all sorts of bad things can happen, too slow and the pig starts to jump and water leaks past as zdas 04 says.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Shaneholl]

I appreciate the replies. I think they were helpful but I would like to verify that I created the excel sheet correctly. One of my issues is that I am getting a different number depending on which way I calculate the total CFM. I used 2 methods and while the numbers are within a couple hundred CFM, I would like verification that there is not an error.

Could you review the excel sheet that I created, I attempted 2 methods, the 1st is from another online source.

I appreciate all of the feedback so far, it has been very helpful.

 

[LittleInch]

It look fairly basic as it ignores the z factor and is based on ideal gas law.

Also if you're getting pedantic, the Internal radius of a 24" pipe isn't 12", more like 11.5 or 12" minus your wall thickness.

It would also help if you used terms such as psig and psia and scfm to make it clear what units all your numbers are in.

your spread sheet also has no allowance for elevation (el=0), whereas your initial post says 4,500ft

I don't follow method 2 very well, but in line 86 you seem the be dividing the diameter (B34) divided by the atmospheric pressure ??. Also you have mixed units. if you turn the diameter into feet (B34/12) and square that I get an almost identical answer.

However that is only on a calculation basis, but your assumptions will bear no resemblance to reality, i.e. the pig speed is very low (1.1 ft/sec), and the pressure will vary quite wildly unless you have a very good back pressure controller on the far end. you need a lot more pressure when you have 18 miles of pipeline full of water than you do at 1 mile of pipeline, even if it is as flat as pancake, which I doubt.

you can always throttle a compressor back, but difficult to make it go faster....

I think you need to first do one or two of these with someone who has done lots of them otherwise you are in great danger of ignoring the advise we're giving you and ending up doing it wrong and either not clearing the line or clearing it far too fast.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Shaneholl]

Thank you for the input.
I may adjust the sheet for the actual Internal Radius once I get the basics.

The excel sheet is for a general reference, but the job is for a 4500 ft elevation.

Method 2 was the method that zdas04 had specified above. Yes that was an error, I should have put the (24[in]/12[in/ft]).
Using the equation above that zdas04 provided, I still had a couple questions. I was concerned about the (200+12)/12. I understand the 200 is PSIG however I am not sure of the 12’s. Doing dimensional analysis does not seem to help as I end up with [ft^3]*[psig+12]/12 which doesn’t equal CFM.

I am trying to find somebody locally that has done this before, but most of the guys I know are completions, reservoir, or production. They don’t get into the midstream aspect of the industry. Our manager keeps insisting that if I can get him the compressor CFM requirements, he will be able to handle the rest.