Pump Curve with different values for one Pump

[nabeel3]

Dear pump experts

In my project, I have 3 vertical turbine pumps, controlled by PLC. Normal operation is if washing is required for 7 seawater screening Drums 2 pump will start. if only 3 Drum screens are in operation one pump only will start. 3 bar pressure and 52 m.cube /hr of water supply is required for each Drum for backwash operation. Our supplier has given me the following Pump curve. It shows If one pump is working the head is 52 m and Q= 210 m.cube/hr. But if two pumps are working Q= 420 m3/hr and H= 52m. Also shown are other values when one pump is in operation 288 cu.m /hr @29 m . That confused me . . I raised the following question to the Vendor to understand the curve

Qusetion) Dear Vendor, My understanding is that there is only one system curve whether one pump is working or two pumps are working. Only the pump curve adds up as you have shown in the pump curve. Since the System curve has a slope the flow couldn’t add up to 420 m3/hr. from 210 and 210 m3/hr. Otherwise, in parallel flow 210, m3/hr. of each 2 pumps will not give 420m3/hr. in combination.

Reply) In our Pump Curve, we represented N.2 system curves considering that you could increment the system curve slope using regulating valve on the discharge. If you haven’t regulating valve, when from two pumps running you have one pump running and the same system curve, the pump will work at 288m3/h @29m.
Reply Ends.......

There is no modulating Valve to control the flow to each Drum. We use a Motorised gate valve with just ON and OFF positions only.

I couldn't understand his reply. I could be wrong in my question. I am confused when he showed one pump with 210 m/hr and another curve showing one pump with different values,i.e 288m3/hr and 29 m head. How is this possible,one pump working with two performance?

 

[TugboatEng]

The vendor is correct. You gave two cases, one with 3 valves open and one with 7 valves open. There will be less resistance to flow with 7 valves open which is why there are two system curves.

 

[Rputvin]

Pump vendors make assumptions about system curves because they have no capability to analyze your piping - assuming, in part, that the pump will actually hit the design point is one of the bigger concepts they make assumptions about. If you need to hit the precise performance values from the design point, you need to create a pipe network that will allow for it to happen, the pump doesn't magically make those numbers in any piping system.

Turning on 1 pump in a system designed for 3 is a good way to get that 1 pump to run off its curve, as you're seeing with the red system curve in your picture.

You need to restrict the piping, through a control valve or some other method, to run 1 pump and force the blue system curve to exist temporarily, at least at the correct intercept - or deal with the higher flow, lower pressure you'll experience running 1 pump.

 

[nabeel3]

Dear Rputvin

When Only 3 or fewer drums are in operation and the other 4 are out of operation only 3 valves will open. But on normal condition, all 7 valves will open and 2 pumps will start even if all drum doesn't require washing.

Dear TugBoat

Thanks for your reply. But my question is why he is showing one pump with 29 m head and Q=288 in the red curve. When 7 valves are open both pumps will be working at 52 meter head and combined Q is 420. That is what i understand.

 

[EdStainless]

The issue that I see is that 2x3=7 is not how my math works.
If we assume that the piping is fairly symmetrical to each screen, and that the major pressure drop is across the screen, then the red (7 screen) flow curve has to be flatter than it is shown.
I am not exactly sure where it will fall but I would guess 450-460 at 47-48m is more likely.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[nabeel3]

Dear Edstainless,

The end of piping is open to atmosphere spray nozzles spraying water into the Drum Screen Mesh.Pump Head calculation attached

 

[LittleInch]

But my question is why he is showing one pump with 29 m head and Q=288 in the red curve.

Because the pump supplier doesn't know your system the way you do and is working on a worst case basis of only ONE pump operating when 7 valves are open.
He just showed you what would happen in only ONE pump was available.

The pump is only just able to cope and delivers the flow and pressure as stated.

7Pumps and suppliers don't know what is connected to them. They look at a range of pressures that the downstream system generates base don flow, hence the system curve. Interestingly the pump supplier has assumed a 9m static head lift or a 9m static inlet head into the pumps. Is this correct?

If you gave the vendor that document then he has used the data in it.



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

 

[nabeel3]

Little inch

yes , we have a requirement of static lift.

 

[LittleInch]

Be very careful here.

Your document states that there is a 9m elevation difference between pump discharge flange and the wash water spray line.
All pump data sheets use the pump discharge line as their reference point.

The pump data sheet will assume what ever it is told, so my guess is that the differential head is what is being presented here.

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

 

[nabeel3]

Little inch, Our Ground level is +6.5 m. As shown above and the previous pump elevation snap the spray pipe is about 1.2m (600+650-180 ) from Ground level. So It is the differential head. Otherwise, can we say the discharge head was calculated from pump centre line level, in this case, the impeller level.

 

[LittleInch]

Like I said, be careful because most pumps like this quote the differential head from the CL of the discharge flange to avoid this issue. Now how they account for the inlet water level I'm not certain, but check the definition on the pump data sheet.

The pump head calculation document says ""pump centre line elevation", without defining it and for a vertical turbine pump doesn't seem to be the correct way of thinking about it. This sort of description would normally be applied to a horizontal pump.

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

 

[nabeel3]

Reference level is Given as Impeller centre line . Please see attached

 

[LittleInch]

That's very odd and a poor way to describe it. Which impellor?

This also doesn't say anything about min submergence?

It seems to be a data sheet designed for a basic centrifugal pump being used for a vertical turbine pump to me, but if you're happy that the zero mark is that then so be it.

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