Head rise to Shut off 5

[rotaryguy]

Hello guys,

It came to know that head rise to shut off ratio should be between 110% to 120%. I think upper limit of 120% stands fore pump downstream sizing. but dont understand why ratio should be not be less than 110%?

Regards
Rotaryguy

 

[chemjock]

Do you want a really flat pump curve?

 

[rotaryguy]

may be silly question what happens if i have flat curve!

 

[cvg]

flat curve (horizontal) will give approximately the same head output at various flow rates. Since the total dynamic head of your pump and piping system increases with increasing flow, the pump could move off the end of it's pump curve easily and will not operate at the BEP. Vertical curve will pump nearly the same output at various different heads. This may be useful if you want a fixed output. Most pumps are neither flat or vertical, but something in between - depending on the type of pump.

http://www.mcnallyinstitute.com/02-html/2-03.html

 

[Artisi]

The main problem I see with a "flat" curve is that a very small increase or decrease in head can result in a very large shift in flowrate, this can give a very unstable operation especially if the head is variable ie, fluctuating inlet condition.

 

[BigInch]

If your system curve isn't flat, its no problem. If your system curve is also nearly flat, you might have trouble reaching a steady state flow without a given control setting it to a perticular flowrate.

A flat curve would only work well with a high resistance system curve that had no flat areas in its curve. You'd have to stay well away from low end flows where curves have flat regions.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[JJPellin]

I work in an oil refinery. Most of our centrifugal pumps are flow controlled with a control valve pinching on the discharge. We have a requirement for head rise to shut-off for two reasons. First, a flat curve is much harder to control using our preferred method. A small move of the control valve would produce a large change in flow. This can adversely affect control stability. Our second reason is based on the fact that almost all of our pumps are installed in pairs piped in parallel. We prefer not to run the pumps in parallel, but must do so when starting/stopping pump and switching from one to the other. And, when process conditions require more flow that one pump can produce, we will run the pumps in parallel on a longer term basis. Pumps with very flat curves are more likely to have problems running in parallel. A relatively small change in pump condition could shift the curve up or down. Then, when the pumps are run in parallel at the same head, one could be running at excessively high flow and the other could be running below minimum flow.

Johnny Pellin

 

[rotaryguy]

Thank you JJ ,Big, ARtisi and CVG