Pressure drop in elevated pipeline during Hydrotest 4

[Sherin Achenkunju]

Can someone help me to calculate the pressure drop at the highest point in a pipeline during Hydrotest pressure holding?
We are constructing a portion of a new potable water pipeline that is 48" in diameter and the schedule is STD. we have an elevation difference of 9 meters between the lowest and highest point in the pipeline. The test pressure is 25 bars and the length of the portion of the pipeline is 480 meters. The test medium we use will be potable water. The line is in an inclined position as it is installed on sleeves under the bridge piers.

 

[LittleInch]

0.9 barg difference.

Usually you want the test pressure to be fixed at the highest point so the pressure at your lowest point will be 25.9 barg.

This should be well within your margins to not exceed yield.



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[1503-44]

You need to hold your test pressure of 25 bars at the highest point. Its 9 meters above the lowest point so the lowest points pressure must include the weight of water above it. 9m x 1000kg/m3 will add 0.9 bars to the low point's pressure. It must be held at a pressure of at least 25.9 barg there.

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[1503-44]

Good thing we got the same number.
Took me too long to type that on my tablet.
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[LittleInch]

We're a double team....

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[1503-44]

I want to turn off the auto correct feature. I have to edit everything at least once.

I 'll vote for that.

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[Sherin Achenkunju]

@LittleInch @1503-44... Thank you so much for your reply.
This is how the pipeline is positioned. The inclination that you could see on the stretch of 130 meters on the right side is just 1.5 meters.

So does that mean I have to add the weight of the water-filled in through the stretch of 350 meters with the test pressure to get the pressure at the lowest point ??.

 

[LittleInch]

Yes.

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[Sherin Achenkunju]

The pressure difference shall be calculated by Bernoulli's equation right ??
Ie, Pressure energy + potential energy + kinetic energy = constant. As the line remains static during the hydro test, kinetic energy shall be eliminated. Hence Pressure at highest point is equal to Pressure energy (25 bar) + Potential energy(1000 kg/m3 X 9.81 M/s2 X -9 meters)= 24.1 bar. So the pressure drop is 0.9 bar. I am guessing it right. Thanks, Both of you for your extended support. Kindly correct me if there is any correction in the calculation.

 

[LittleInch]

There's nothing wrong in your calculation but normally a test pressure applies at the highest point, not the lowest point.

Check your design code or specification.

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Also: If you get a response it's polite to respond to it.

 

[Sherin Achenkunju]

Okay, Noted. That is, 25.9 bar shall be given at the lowest to attain 25 bar at the highest point. Thank you so much

 

[1503-44]

Noticing that your pipe has a 48" D = 1.2m, which could raise the pressure difference. The 1.2 m should be included within the 9m height difference for the pressure calculation.

Is the height from bottom of pipe at low point to top of pipe at high point = 9m, or is it 9 + 1.2 = 10.2m?

If 10.2, then Pressure at the bottom of the lowest point on the pipe wall will be 26.2 barg when the pressure at top of top is 25.







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[Sherin Achenkunju]

I was considering the centerline of the pipe. It does mean that I should add up the diameter of the pipe to the elevation difference.

 

[1503-44]

I thought you might have been working with centerlines. Usually it does not matter, but in this case your diameter is large in relation to the height difference, so

Yes, technically, you should use high top of pipe to lowest bottom of pipe elevation. 10.2m So that would make your maximum pressure 25 + 1.2 = 26.2 bars.



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[Sherin Achenkunju]

Thank you so much for helping me to figure out this.

 

[1503-44]

Blessings

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[GD2]

Sherin,

Looks like you are new to pipeline hydro test. Don’t forget to calculate all aspects of a pipeline hydro test. What’s your pipeline code? Usually it prescribes strength and leak test by a pipeline code. Check also if the owner have their own test procedure. It’s a potable water pipeline as apposed to the industry construction code for hazardous fluid.

The code/ specification will also give you minimum test duration times in hours.

You must do all calculation of test pressure in terms of percentage of MOP and SMYS all the way to 150%. You much also assume/calculate a cushioning pressure.

You have already learned to calculate test pressures at high and low points.

By the way, who is writing the hydro test procedure- you or the Inspection? If you, don’t forget to get it reviewed and approved by your Supervisor.

GDD
Canada