Design pressure and pressure thrust

[curtis2004]

Hi Everyone,

I have question regarding pressure thrust calculations of an expansion joint. I have design pressure of my piping system stipulated 150 psig @ 86 F. It's steel piping installed in intake and discharge of a centrifugal pump. Intake pressure is 10' of water head max, and discharge pressure of a pump is 25 psig operating. Pump curve shows dead head of 33 psig. The pumps is isolated with molded rubber expansion joint both inlet and discharge.

What pressure would you use for pressure thrust calculations? Pdesign = 150 psig or Pdead = 33 psig? At first glance the answer is to use design pressure. However, what is the point to use design pressure if we know that pressure would never rise above pump dead head pressure. Is there any code requirements or guidelines which stipulate this?
For 12" size nozzle it will make big difference...

Thank you,
Curtis

 

[BigInch]

Coming back at you.
Well what use is design pressure, if it is so high it is impossible to reach. You can/should set design pressure no higher than pump dead head pressure. Code requirements do not set any requirement to use a design pressure above the highest possible pressure.

In any case "design pressure" means design pressure and you must use design pressure, whatever it is, as the worse case load.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[bimr]

You have answered your own question. Yes, use the pump dead head pressure as the design pressure.

 

[curtis2004]

Hi there,

Design pressure is set by piping specification. The contractor accepted bid specs and it is binding contract right now.

I think I can use pump's dead head for pressure thrust calculations, although it is imperative to use design pressure for pipe stress analysis.

In case of rotary pumps, it would have been a different story though...

Regards,
Curtis

 

[LittleInch]

It's also important to use the dead head pressure that uses the maximum valvue.

Maybe you've already considered this, but pump curves normally come in m head differential, not psi. hence you need to add the inlet head (pressure) which exists at no flow, not always the same thing as the "normal" inlet head as it has no friction losses. Hence your dead head pressure could be significantly above 33psig.

The key issue is whether the pump could see any higher pressure due to failure upstream or downstream. Only then can you decide to use what is the Maximum Operating Pressure as your figure instead of the design pressure. It is not normally considered good practice to have items of equipment below the design pressure within a piping system without changing the design pressure to the lowest strength item as these things could easily get forgotten and someone in the future puts in a new pump or system thinking the design pressure is 150psig and your expansion joint goes bang....

So I disagree with bimr and essentially agree with BI - change the design pressure to a more reasonable figure or use the design pressure - no other option is allowable, IMHO.

My motto: Learn something new every day

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

 

[DSB123]

Curtis2004,
You say the design pressure is set by the piping specification. Are you sure the pipe specification is not just giving Pressure/Temperature limits for the pipe spec use and not really specifying the Design Pressure of a system.