Determination of Pump PSHL Set Point

[123engr]

I am designing a pumping system that involves me providing the set point for the high and low pressure switch at the pump discharge? The pump operating discharge pressure for the pump is 110 psig.

Is there a rule of thumb that can be used pending detail pump characteristic from the vendor to determine the pump shut-off head?

Thanks for your anticipated cooperation.

I am what I am by His grace

 

[Artisi]

assuming the pump will operate near to its BEP I would add 20% and call this its shut-off head until further detail comes available.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Assuming it's a centrifugal pump, I agree with the above. However normally the PSH is set by the piping system such that only if the inlet pressure rises and hence outlet rises a lot more would the pressure switch operate. It's not common to see the PSH set below the shut off head with "normal" inlet head or is this what you're trying to find??

For PSL, you're probably looking about the same, i.e. 20% below the BEP is the point you wouldn't want to go below in normal operation at this stage until you get a proper pump curve.

My motto: Learn something new every day

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

 

[123engr]

Thanks littleInch and Artisi. am grateful.
LittleInch, the answer to your comment is Yes.

I am what I am by His grace

 

[BigInch]

IT IS NOT A MATTER OF A RULE OF THUMB. THERE ARE SPECIFIC REQUIREMENTS.
IF it is a pipeline design project (ASME B31.4 OR B31.8), you can set the maximum discharge pressure for NO HIGHER than 10% above the allowed operating pressure. OTHERWISE SEE THE DESIGN CODE BEING USED FOR THE PIPING SYSTEM.

Independent events are seldomly independent.

 

[LittleInch]

With all respect BI, the actual wording in 31.4 section 403.3.4 is the any surge or variation to normal poressure " shall not exceed the internal design pressure at any point in the piping system and equipment by more than 10%." The (maximum) allowed operating pressure may be set at the design presusre or may be set well below it - we don't know.

B31.8 has no such wording, but as it's for gas and the OP is talking about liquid so we'll pass on that for the time being....

This is not the same as saying that the shut off head for the pump has to be < 10% of normal operating pressure - it can be anything it likes so long as it is less than the pipe / pipeline design pressure IMO, which was my point. I think the OP has acknowledged that he just wants to set his PSH level somewhere above pump shut off pressure, but below pipe / pipeline design pressure.




My motto: Learn something new every day

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

 

[SNORGY]

I think you really need the head-capacity curve before you make this determination. Plus, you need to know the variations in suction pressure for this to work well, if at all. I wouldn't try to make this work with a PSHL in the pump discharge unless you have consistent suction pressure. I would either use a differential pressure switch across the pump or separate switches (one on either side of the pump) that are tied into a PLC such that the pump trips on either a wide gap in the sensed pressures or a narrow gap in the sensed pressures. With respect to pump shut off versus MOP, again you need to see how high your suction pressure can get, but I would suspect that the MOP is higher than worst case shutoff pressure.

 

[BigInch]

There's obviously a lot of confusion about design pressure, MAOP, operating pressure etc. in the international world.

Test Pressure / Test factor = MAOP, which in turn should be higher than design pressure. Design pressure is something engineers use to design the pipeline ONLY. It is not used for anything after the pipeline is placed into service. After hydrotest, the MAOP = test pressure/test factor is the only number of significance and is not to be exceeded by any steady state pressure. Any pressure encountered above that MAOP must be only in the context of the system returning back to or below MAOP, thus a transient pressure. A shutdown switch to a pump HI, or HI HI, can be set as high as 10% over MAOP as that would be the maximum pressure allowed under a transient situation.


Independent events are seldomly independent.

 

[BigInch]

If pump shutoff head is less than MAOP, the addition of pressure relief systems should be considered. If pump shutoff head is less than MAOP, no pressure relief system is required. That's the best "Rule of Thumb"; keep it less than MAOP, OR make your pipe design pressure, test pressure and resulting MAOP > pump PSHi

Independent events are seldomly independent.

 

[LittleInch]

I agree there's a lot of confusion about some of these terms which is why you need to go back to the written word in the design codes.

I disagree with the statement made above "MAOP, which in turn should be higher than design pressure" This is counter to what is written in e.g. ANSI B 31.4 which states "The pipe and components at any point in the pipeline shall be designed for an internal design pressure that shall not be less than the maximum steady state operating pressure at that point, or less than the static head pressure at that point with the pipeline in a static condition." [my italics]. Whilst this is what is in 31.4, most other codes have something similar.

MAOP can equal Design pressure, but should NEVER be higher than it. There seems to be a belief in many quarters that there is some sort of requirment to set MAOP 5% or 10% BELOW design pressure, but this is in my experience simply custom and practice by some operators and not manadated in any code. Providing that the test pressure was determined using the design pressure, the MAOP might initially be set lower, but can then be raised at any time up to the design pressure, but no further.

The design pressure rmeains the design pressure before, during and after installation and in nearly all circumstances remains the highest pressure that the pipeline should be able to see in normal operation (barring hydrotest).

In your second post above , first sentence did you mean to say "..head is more than MAOP..." ?? If so then I fully agree with that post. I'm sure we've all seen systems where someone has designed the piping / pipeline based on the duty point of the pump without realising that everything will trip if the flow reduces even by a few percent...

SNORGY - I've seen pump shutoff heads much higher than MOP, which is why you set the PSH and PSHH at the piping limit or below, not anything really to do with the pump due to much higher inlet pressures than "normal". This happens sometimes on start-up, especially start-up of a booster station when steady state flow is happening. Yes you need the curve, but the OP hasn't got it yet and hence just wants to see what sort of range he is looking at.

My motto: Learn something new every day

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

 

[BigInch]

LittleInch, Design pressure has to be at least MAOP, but making it higher than MAOP is a waste of steel. Hence design pressure should EQUAL MAOP for a cost effective design. Make it higher if you want, but I don't. Sometimes I might have to, but only if I am forced to buy a commercially produced pipe wall that is greater than the "design pressure" I require for process. A maximum setting for any transient condition must be less than or equal to 110% MAOP. If the PSH shut down being set is for a transient condition, then 110% MAOP is the highest possible setting. If PSH is being set for a steady state case, then it cannot be set higher than 100% MAOP.

Independent events are seldomly independent.

 

[LittleInch]

BI,

I actually think we're saying the same thing and I agree - not setting MAOP at the design presusre is a waste and I don't either. Re-read what you posted earlier and then tell me if you meant to say "MAOP, which in turn should be higher than design pressure"??.. Did you mean to put a "not" in between "should ... be higher" ??

Your post immeadiately above says "Design pressure has to be at least MAOP", so therefore DP canot be less than MAOP - I AGREE.

I also AGREE with the words on the PSH setting.

However the wording in many codes uses design pressure not MAOP (though some do use MOP just to confuse everyone), as does 31.4 which says the 10% above rule applies to the Design Pressure, not MAOP. May be splitting hairs, but they are important hairs.

LI

My motto: Learn something new every day

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

 

[SNORGY]

LittleInch,

You are correct. I suppose I have seen it too, but I forgot about what I did about it. In that instance, we tied the VFD control logic to the discharge pressure to slow the RPMs down so that the reduced speed shutoff head was below MOP.
I think I got confused by the whole PSHL thing...

 

[BigInch]

I know we have to be saying the same thing, because I know that you know how to do this and ... there is no other way that it can be done.
We have to break it down to the simplest form. PSHi can be set anywhere below the pipe's "maximum pressure" for operating purposes, inclusive of 0, but nothing less than maximum "pipe pressure" makes any economic sense, ie. as in why design for 1000, if you set the pumps to stop at 999.

Independent events are seldomly independent.

 

[Artisi]

Just want to hop in here and say, the original question was "Is there a rule of thumb that can be used pending detail pump characteristic from the vendor to determine the pump shut-off head?" nothing mentioned about piping systems etc - all very academic but maybe wasted on this question. So I'll stick with my original answer - add 20% in the interim of selecting or specifying a pump.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Accepted, but it is kind of fun to go a little bit off topic ever now and then....

My motto: Learn something new every day

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

 

[BigInch]

Wee were ignoring that question and concentrating on the first. Actually I have a general rule of thumb for shutoff head as being around 1.275 x Head at BEP. That usually suits my purposes for initial pipe system design.

Independent events are seldomly independent.

 

[LittleInch]

For relatively small pumps like the OP has I would agree, but for big main line units I thought API 610 had a max 10% rise from rated duty point??

My motto: Learn something new every day

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

 

[BigInch]

That would apply to the highest point of a curve that actually rises above shutoff head. Most curves begin at a shutoff head, stay relatively flat, or have a low rate of descent for awhile, then begin a continuous descent. A "rising curve" starts at shutoff head and increases to a max, before beginning its descent.

Independent events are seldomly independent.