Pipeline Hydrotest Pressure Calculation 1

[jonjose123]

We have a 52" Gas Pipeline API 5L X65 With wall thickness of 20.5 mm.
Our question is how do we calculate the Hydrotest pressure at the highest point and at the lowest point along the route. The length of pipeline is 8 km.

We have calculated teh max Hydrotest pressure as P = 2*WT*SMYS/OD.

Need help in calculating the pressure at the highest and lowest point

 

[zdas04]

That is actually the simplest part of the arithmetic. The hydrostatic pressure is P=ρgh+P_applied.
Density of pure water is 62.4 lbm/ft³ or 1000 kg/m³, if you don't know "g" you have no business doing pipeline calcs, and "h" is the change in elevation from the highest point on the system to the lowest point on the system regardless of intervening ups and downs.

The design standard that you are using has equations for calculating minimum wall thickness. The MAWP is an input to the calculation, not an output. You have to SPECIFY an MAWP to determine a wall thickness, not the other way around. The equation in ASME B31.8 looks like your equation except it includes a design factor (which increases the required wall thickness based on population density), a longitudinal joint factor, and a temperature deration factor. Using the whole equation you calculate a minimum wall thickness for your chosen MAWP and then find a pipe that is thick enough to include that wall thickness after you deduct your corrosion allowance.

The minimum test pressure is a factor (greater than 1.0) of MAWP as specified either by the code you are using or by company policy. Often the code will specify a number like 1.1 times MAWP, but company policy will require 1.5 times MAWP, you have to use the code or a higher value.

If your elevation change is significant, then doing hydrotests at all can be a problem. It would likely be worth your time to read Pneumatic Testing. I don't mean to start the whole hydrostatic vs. pneumatic testing discussion again, but the linked paper goes into the issues of hydrostatic testing when elevation change is not trivial.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

You calculate the test pressie in accordance with the relevant section of whatever design code your pipeline is beyond designed to. This tells you what walk thickness you can use, design factors etc.

Gas Pipelines are notorious for having limited ability to handle diffwrenches in elevation. Often you are limited to around 100m difference from high point to low point.

So what is your design code and design pressure?



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[zdas04]

LittleInch,
Where did you get that? There are hundreds of gas pipelines crossing the Rocky Mountains with WAY more than 100 m of elevation change. I've personally build gas gathering pipes with more than 400 m elevation change.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Sorry,a bit more of an explanation. What I meant was the difference in elevation before you need to section the pipeline into discrete parts to be able to hydrotest. I know you advent pneumatic testing where this means a lot of golden welds and I tend to agree. In 8 km as per the OP this isn't likely but you never know. ....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jonjose123]

Hi All,

What I understood is :

1. The maximum pressure Pmax will be at the lowest point
2. Pmax is determined from the Hoop Stress eqn.
3. (Usually 95% SMYS BUT may vary)
4. The Hyrotest Pressure at the Highest Point is Pmax - Pelev.
5. Pelev is the static pressure due to the difference in elevation between the low point and the high point

If this is the case, and we have a test section of 8 km and the elevation difference between highest and lowest point is 22 m, I understand that as per above the lowest point will have the highest test pressure.

 

[LittleInch]

Points below:

1. The maximum pressure Pmax will be at the lowest point - Correct
2. Pmax is determined from the Hoop Stress eqn. - Correct
3. (Usually 95% SMYS BUT may vary) - 95% is quite low, 100% is common and sometimes 105% for the highest stress at the lowest point
4. The Hyrotest Pressure at the Highest Point is Pmax - Pelev. Correct, but the correct way of doing this is to fix the test pressure according to the design code you're using and set this at the highest point, which is what the code normally requires. Therefore PMax = P test + P elev. If P max exceeds 100% SMYS then you may need to section the pipeline into different test sections.
5. Pelev is the static pressure due to the difference in elevation between the low point and the high point. Correct

If this is the case, and we have a test section of 8 km and the elevation difference between highest and lowest point is 22 m, I understand that as per above the lowest point will have the highest test pressure. Correct, but you need to determine and fix the test pressure at the HIGHEST point. You also need to calculate what the pressure needs to be at your test point (start or finish) to achieve the test pressure at the highest point.

With only 22m difference in elevation you shouldn't have a big issue, but if it was 220m then you probably would.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[jonjose123]

Hi Little Inch

Thanks for the clarification;

The Hyrotest Pressure at the Highest Point is Pmax - Pelev. Correct, but the correct way of doing this is to fix the test pressure according to the design code you're using and set this at the highest point, which is what the code normally requires. Therefore PMax = P test + P elev. If P max exceeds 100% SMYS then you may need to section the pipeline into different test sections.-- As per code the test pressure shall be 1.25 times the design pressure. based on this PMax = P test + P elev is calculated which is at the highest point . accordingly the max hoop stress is calculate and compared against 100% SMYS. IS the approach correct?

 

[LittleInch]

Yes.

You should find that with a Design Factor of 0.72, test to 1.25 x 0.72 = 0.9. Therefore your hoop stress at the high point is 90% of SMYS, but may be a bit less if your pipe is a bit thicker than you need.

Thus there is >10% of SMYS to allow for changes in elevation before you get past 100%. This normally equates to between 80 to 120m of elevation difference, but each system needs its own calculation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.