Hydrotest pressure and/or MOP with elevation changes in pipeline

[engr2GW]

Hi,
I have a question about test pressure and/or MOP setting for pipeline when you have significant elevation changes
1. what is a rule of thumb of how much the elevation change should be for liquid line before you start putting it into consideration for adjusting either test pressure or MOP?

2. if there is significant change in elevation, is it test pressure or MOP you adjust?

3. what is the reference for the elevation change, highest vs. lowest poing or do you assume where your test equipment is going to be and use that as your datum/reference?

4. when you determine the lowest point (or the highest point) and the line is such that you don't want to devide it into segments, what do you do? add the psi value to the MOP for highest point and/or subtract from the MOP for the lowest point? and then determine test pressure to cover all?

Any help is really appreciated.
thanks a lot!





As much as possible, do it right the first time...

 

[LittleInch]

In answer to your questions my take on this is;

1), There is no rule of thumb, but I would not recommend you change design pressure by less than 15 to 20 bar, so for liquid line, you're looking at elevation change of 200 to 250m.

2) What you change is design pressure and everything runs from there

3) You can choose any point so long as you fix it. Start point of pipeline or lowest point is probably the best. Always fix the lowest point first and then work up. Remember that for liquid lines, you needs to think about hydro contrition and locked in sections and draw the pressure line as a flat head line when working out what the pressure is at the lowest point. Do not use the operating preassure fall over distance.

4) You follow the design code which will state where the pressure is to be measured art and then make sure the pipe can stand the pressure. This might mean you need thicker pipe at the lower sections.

My motto: Learn something new every day

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

 

[BigInch]

I don't think I really understand your questions. It looks like you are making a hydrotest plan, but you CANNOT "adjust MAOP" when making a hydrotest plan. Setting that is a design issue, not to be changed during testing. Are you designing the pipeline and determining hydrotest pressure now too?

1.) As I said, you must not "adjust test pressure or MAOP" Those are determined by the design pressure required for a segment, or in many cases the whole pipeline. Design pressure is that pressure at any given point required for flow, but it must not be less than the hydrostatic head (when the pipeline is not flowing) above any point when there is product in the pipeline. Note that products may be less dense than water and give a correspondingly lower static pressure head. MINIMUM test pressure is 1.25 (or occasionally some other factor) x Design Pressure. See your pipeline design code for the test pressure multiplication factor required in any given area class, or if it is a liquid line designed to B31.4, it will be 1.25. See your code.

2.) When designing a liquid pipeline, MAOP may be different for different segments of the pipeline, because static pressure may vary significantly with elevation. Thus the engineer responsible for design and pipe wall selection may use different wall thicknesses at different elevations. S/He must include pressures required for flow as well as static head when not flowing and will determine when it is economical to use different wall thicknesses. The BTC pipeline used many different wall thicknesses ranging from 7mm to 12mm, depending on elevation, as elevations on that pipeline ranged from 0 to 2800 meters. The elevation ranges over which each wall thickness was used were determined by the wall thicknesses available from the pipe mill and the total pressure that each could sustain and their position along the pipeline, as friction pressure drop varied from distance since the last pump station, as well as elevation.

3. When you have high and low elevations and you are determining required operating pressure, your operating pressure at any point along the pipeline is a combination of elevation and friction heads. Operating, pressures, especially at the end of a pipeline, or at the suction of a pump station can be lower than static head, but you must never design for a lesser pressure than when the pipeline is not flowing.

4. If you have a short pipeline with large elevation changes, it may not be worth the effort to design using multiple pipe walls with different design pressures, but maybe it is. It's simply a question of extra trouble in design, or spend a bit more money on pipe wall. You could use a constant wall for the whole length if you wanted to. Gas pipelines often do that, because there is not much pressure change with elevation due to differences in gas density, but do remember that gas pressure may remain very high along the whole pipeline, even at the top of a mountain, if compressors remain on while a pipeline discharge valve is closed.

5.) Once you know the design pressure, hydrotest pressures can be determined. As I mentioned, usually 1.25 x design pressure. Setting up a hydrotest plan with varying elevations must be done between two limits,
1.) the minimum required test pressure and
2.) the maximum possible test pressure.
The minimum is the design pressure x test pressure factor.
The maximum is not defined by a pressure (under B31.4), but is the pressure that can cause a pipe to pass it's yield limit of 2%.
Take a liquid pipeline with design factor of 0.72 x SMYS
minimum test pressure is 1.25 x 0.72 = 0.90 or 90% of Yield Pressure, pressure where the specified minimum yield stress (SMYS) is reached. The maximum test STRESS would then be approximately 1.02 x the SMYS. You can back-calculate what pressure that is, but remember that B31.4 does not actually define that as a pressure. It defines the max test pressure as that pressure which causes a measured deformation of 1.02 in/in.
Now that you have those two pressures, min test, and approximate pressure resulting in 1.02 SMYS, you can set your maximum an minimum elevations accordingly to each wall thickness and each pipe of perhaps even different yield stresses that make up your pipeline.

Independent events are seldomly independent.