Determining Pressure Rating (PN) of Valves and Pipes

[Paul_LebKnight]

Hello guys,

A new comer to this forum. Hope you're all doing well.

For as long as I remember, the rule of thumb for determining the PN of pipes and valves is 1.5 times the working pressure in the network. But lately I've been digging deeper. So here's my question:
How do you guys determine the pressure rating of the pipes/valves (discharge side of the pump):
A - 1.5 x pump head
or (another factor) x pump head
B- 1.5 x pump shut-off head
or (another factor) x pump shut-off head
C- 1.5 x (maximum surge pressure with surge protection)
or (another factor) x (maximum surge pressure with surge protection)

Any specific code of standard that mandates this?
Thanks!
Paul

 

[LittleInch]

Yes. The design code of the piping system will help you.

In general you aim to set the design pressure higher than the higgest pressure the system can see, which for a pumped liquid system is usually pump shut off with the highest pump inlet pressure. How much higher is you to you but 10% is fairly common.

Surge is often seen as an occasional event for which you can accept a certain overpressure for short periods or you protect by relief valves.

It's all part of process design. Abd lots of other things can impact the design pressure.

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

 

[Paul_LebKnight]

Hey. Many thanks for your swift reply.
So basically the best choice for PN would be the (shut-off head) x (a certain safety factor).
And surge is not necessarily accounted for since it's a temporary and instantaneous occurrence.
(though I make sure that the PN is greater than the max surge, even without a safety factor, to be on the safe side)

But I got the sense you were saying: [(shut-off head) + (max suction pressure)] x safety factor ??

I've wondered about that. When I normally selected my pump, it's at the intersection of the system curve with the pump curve. And, as we know, the system curve already reflects the suction pressure from its starting point at the Y-axis. So, to get the pressure at the pump discharge, do we really need to ADD the suction pressure to the pump produced head? (the intersection point already reflects the suction pressure, is what I am saying)

Hope you're having a great pre-new year weekend !

Paul

 

[mk3223]

IMO, the line PN of the pump discharge is adequate once it's higher than the pump shut-off head, since too many safety margins added typically in the pump hydraulic calculation. For example, the safety margins in the calculation includes the pressure drop of equipment, pipe, or control valve, and static head, etc. And, the another added safety factor on top of the pump shut-off head seems an unnecessary over design.
Unless, the Owner may require to have an additional pressure head into the current condition for the future system upgrade.

 

[Artisi]

Unless the pump inlet is under high pressure you could ignore it in this case.

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]

" as we know, the system curve already reflects the suction pressure from its starting point at the Y-axis. "

Errr, no. That may be why you are confused.

The system curve is the curve of head vs flow for the flow along a particular pipe. If there is a static element to the flow, e.g. pumping to tank say 20m higher than your pump, then your system curve would start at the 20m point.

This has nothing to do with inlet or suction head /pressure coming into the pump.

It's all about specifics, if the pump has a high differential head ( say 300m) then if the difference between min inlet head and max static head is say 10m then it's basically negligible.

But each system is different.

"But I got the sense you were saying: [(shut-off head) + (max suction pressure)] x safety factor ??"
Correct.
Your system design pressure needs to allow for the maximum pressure your system can see, even in shut off condition.

Your shut off head from your pump curve is the DIFFERENTIAL head to which you need to add the inlet head in order to get your discharge head. You may already have added this - I don't know there are no details or examples in the thread - but it's usually best to separate out inlet head + pump differential head and then total it up.

If this is a fairly simple system, say an atmospheric tank, a pump, a pipeline, another tank, then your original A would be a good start point for most operations, but the devil is always in the detail.

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