re: System Design Pressure Upstream and Downstream of a Pump

[TurbineBlade]

HI,

I came across with a pump + Heat exhanger closed cooling system having the following service conditions in the flowsheet:

At the Pump Suction:
Normal: T= 140 F P = 10 psig
Design T= 200F P = 100 psig

At the Pump Discharge:
Normal: T=140F P = 90 psig
Design T=200F P = 145 psig

The max. pump shut-off head is 180 ft. Over-pressure protection is done by interlocking the main isolation valve with a pump drain valve.

What determines the design pressure? Is it provided by the pump manufactures? Shouldn't one use the design pressure at the pump discharge as a bounding value for the suction side?

Thanks

 

[BigInch]

By "Design Pressure" I think you mean maximum allowable operating pressure, which is determined by the mfgr.

The maximum allowable suction pressure DOES NOT have to equal to the pump discharge pressure. You should find the pump's specification sheet or check with the mfgr (or better, both). Suction and discharge pressures may be different.

You should have a Pressure Relief Valve WITHOUT any interlock for hi pressure protection.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[quark]

There seems to be some problem with the data.

Your normal conditions indicate the differential pressure across the pump as 80 psig which is equal to 184 ft. This is beyond the shut off head.

 

[BigInch]

Could be, but you really don't know that. You assume he is pumping water with DEN = = 62.4 pcf.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[TurbineBlade]

hI,

The system is recirculating heavy water, desnity is approx. 10% higher than water. To give a background, this is an exisitng system (25 yrs old) that need gate valves replacement. This system has a huge tank with heating elements, a pump, a HX, and the assoicated isolation gate valves. The function of this system is mainly for cooling of the heating elements. There are rupture discs (brust at 20 psig) mounted on the big tank for pressure relief. The heat tank is located upstream of the pump. When I was reviewing the process conditions for this system, it confused me seeing the design conditions for the pump.

I checked to pump curve, it shows a 250 ft total dynamic at zero flow. At normal operating conditon, it provides 180 ft and 940 L/s.

What factors determine the design pressure and temperature upstream of the pump?

Thanks again for your help

 

[TurbineBlade]

Hi,

Of course, 240 ft = approx. 100 psig for heavy water at the normal temperature. Is that how the design pressure is set upstream of pump?

 

[BigInch]

If you need to install a pump it is because you want to raise the pressure of your fluid. Generally you're going to know what pressure and temperature are available and you will also know to what pressure you will need to raise the fluid to get it to do whatever you can't do with it at its existing pressure. That determines the design condition of the pump, for both suction and discharge pressure. How much fluid you will need, determines the design flowrate. The highest of the inlet temperature or discharge temperature will determine the design temperature. Then you take that to the pump manufacturers and you see what they are willing to build for you.

Did that answer the question?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[stanier]

Of course when the pump trips and you get a high pressure transient you will have another number to give to the pump manufacturer.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[BigInch]

Excellent point.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[TurbineBlade]

Hi,

Thanks BigInch and others for the insights.