API 14c - other pumps - high pressure trip on discharge

[Justice100]

Hi all,

API 14c states that a high pressure trip on the pump (others) discharge is not required if the “Maximum discharge pressure does not exceed 70pc of the maximum allowable working pressure of the discharge piping”.

It also states a psv on the discharge is not required if “Maximum discharge pressure does not exceed the maximum allowable working pressure of the discharge piping”.

I am used to designing pump systems to the latter psv statement but I can’t understand the 70pc part regards the high pressure trip? Why is a larger margin required?

 

[1503-44]

One is a trip, the other is pressure relief.
Pressure relief is not required unless you exceed pipe allowable pressure.
RP 14C does not say that the trip shall be set at 70% of pipe allowable pressure, just that the pump must have a trip IF you operate at >= 70% of pipe allowable. You can set the trip to equal 100% pipeline allowable pressure.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

That section - A7.2.2.1 ? - is a bit convoluted.

It appears to em to say if you have other pumps connected into a manifold there should be a PSH on that manifold where if your particular pump is <70% of the MAWP then it isn't needed.

This is based on some twisted logic IMHO and assumes that your inlet pressure doesn't increase or if it does you take account of that as well.

But if you pump is <70% then its either too small or the pressure is too high.

I've never liked 14C - think its well out of date and has some odd ball things which are not explained or notes added to let you see what it's trying to do.

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

 

[Justice100]

Thanks both. For clarity I am taking max pump pressure as suction vessel psv set point plus pump dead head.

1504 - I was really concerned with the implication that to avoid a high pressure trip you need to set the design pressure of the system at max presume/0.7.

Little inch - 7.2.2.2.
I’ve read that section 10 times now and I still don’t understand it. Agree 14c is very short on explanation.

 

[1503-44]

Do what it says. Exactly what it says. Don't read things into it that are not there.
And as with all codes or design guides, they are minimum requirements. If you need a more robust design, do not hesitate to provide it.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[georgeverghese]

"For clarity I am taking max pump pressure as suction vessel psv set point plus pump dead head."
I would use this as the value of max discharge pressure relevant to PSV requirement criterion for this downstream pump
For the PSH requirement, I would use the PSH setting at source + max static head + shutoff head at downstream pump to be the max normal discharge pressure.

In both case, use the max possible density of the pumping fluid, which can also be what you would get if there was loss of some interface level control at the source vessel (which in most cases occurs often)

 

[hassann]

Why is a larger margin required?

good question

in my opinion the concept of 70 percent is acceptable. because the pump shut off pressure is normaly 25-40 percent greater than normal operating pressure. but for pressure vesseles (you are right) and this margin is almost 15-20 percent.

 

[1503-44]

There is no prescribed margin stated.

RP 14C does not say that the trip shall be set at 70% of pipe allowable pressure, just that the pump must have a trip IF you operate at >= 70% of pipe allowable. You can set the trip to equal 100% pipeline allowable pressure.

At a 100% setting (of pipeline or vessel), there is no margin.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[hassann]

There is no prescribed margin stated.

API 14c says if maximum down stream pumping is below 70 pc MAWP, there is no need to trip the pump.
The pressure set point for trip the pump depend on some parameters such as ESDV closure time, liquid or gas media, distant of ESDV to equipment that lead to Significant margin below 100 percent. (for example 80 percent not 100 percent ).

 

[1503-44]

You do not have to trip the pump at less than downstream max rated pressure. Only if you go over.
In fact tripping the pump at any pressure might in itself cause overpressures. Perhaps you want to run at reduced pressure, so that tripping the pump will not cause overpressure. That is one way to handle the overpressure problem, but there is no 14c requirement to run at reduced pressure, as there may also be other ways to keep surge pressure from going too high, even if operating at maximum design pressure. 14c does not require running at reduced pressure. It only requires that pressure be maintained <= max pressure.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[hassann]

You do not have to trip the pump at less than downstream max rated pressure.

the pump shoud be tipped at less than downstream piping max rated pressure. (mawp)
No reseon is acceptable to have a pressure equal or above mawp. Always there is a safety margin below mawp to preventing over pressure.

In fact tripping the pump at any pressure might in itself cause overpressures.

i dont understand why trippping the pump may cause overpressure? please say an example.

14c does not require running at reduced pressure. It only requires that pressure be maintained <= max pressure.

You're telling me. to prevent piping damage/failure the maximum operating pressure (MOP) should be less than MAWP.

finally one question:
there is a PD pump with down stream piping mawp of 100 bar what is the psv set poing and pump trip set point?
answer : psv set point is 100 bar and pump trip set point is smaller than 100 bar that should be calculated based on many parameters that i mentioned in previous.

 

[1503-44]

Sure. No problem.

1) The pump can be tripped at any pressure, as long as all piping remains <= max allowed operating pressure. There can be a safety margin, if you want (or need) to have one, but there is NO requirement to have one. A Relief valve alone may be sufficient to prevent overpressure, even if the pump remain running. The safety margin is already there from the pipe yield pressure being much higher than the maximum allowed pressure.

2) Tripping a pump causes rapid stop of velocity in the pipeline. That can initiate strong waterhammer pressure waves. Very high pressures can occur, up to about 150% of normal pressure. It can also cause vacuum pressures to develop, potentially vaporizing the pumped liquid and filling the pipe with liquid vapors.

3) I am telling you that pipe can operate at its maximum design pressure. It is only necessary to keep pressures at or less than the maximum allowed operating pressure. You can do that by stopping the pump, assuming waterhammer pressures are minor, or by relief valve set = max allowed operating pressure, or by other means, such as a surge control valve diverting flow to a relief tank, or by a slowly operated shutoff valve.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[1503-44]

4) You can find many cases where pump discharge pressure at low flow exceeds pipe maximum operating pressure. In fact we'll above pipe max pressure. In that case, you would not be able to operate your pump with a safety margin as you suggest. In those cases we would probably install a discharge pressure control valve, or maybe a VFD controller to keep overpressure from reaching the pipe.
You may choose to operate a pump at lower pressure than maximum pipe pressure, if you want to, but it is not an efficient use of pipe capacity, so it is usually not the best option, especially for long pipelines. That pipe is best used to its maximum capability and operating at reduced pressure does not do that at all. It is not a good use of pipe money.

 

[hassann]

dear 1503-44 (Petroleum). may I ask a question. How familiar are you with api rp 14c?

A Relief valve alone may be sufficient to prevent overpressure,...

The safety margin is already there from the pipe yield pressure being much higher than the maximum allowed pressure.

with respect to you, above mentioned are out of api rp 14c and are not comply with api rp 14c.

 

[1503-44]

I admit I could be relying on older information, however it cannot contradict Federal Regulations,and it would be secondary to ASME B31 series codes that specifically address pipe design. Even if it does contradict those, RP means recommended practice, so nothing in it is mandatory.

14c, as available on the Internet.

https://archive.org/details/gov.law.api.14c.2001/page/n47/mode/1up

Please see A.7.2.2.1
It references maximum allowable working pressure (MAWP here, or MAOP in B31.1,3,4 and 8) which means that there is no other "margin" required.

A PRV will control overpressure, however it is recommended to have at least 2 means of doing so. A pump trip could be one, a high pressure shut off valve could be another. A flow surge valve could be another. Not installing any pump that could reach the overpressure limit is still another.

If you could please post the paragraphs you say are disagreeing with my comments, I would be happy to re-evaluate my opinion.



--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."