NPSHR Test

[ojsturbo]

In NPSHR tests, when a suction valve is piched down to reduce NPSH, the pump flow is of course reduced accordingly. How can they keep the flow unchanged ? The 3% drop-off curves are for constant flow and speed.

 

[Artisi]

The NPSH test is undertaken by balancing the discharge pressure and the inlet pressure to maintain the same flow rate.

 

[ojsturbo]

Thanks, Artisi.
If they have to open the discharge valve in order to return to original flow, I do not think they can get many data points of NPSH for a constant flow rate. Do they experience some limitations on the number of points available? How about changing the static lift instead of modulating suction valve?

 

[BigInch]

He didn't say that. Balance discharge and inlet pressure means both control of suction head and control of discharge head. You only have one flowrate for any one differential head value anyway, or v/v, so there is only one head data point for each flow, or only one flow data point for each head. If you want another data point, at a different flow, you change the flow, if you want to change the differential head, control the suction head and/or the discharge head to do so.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ojsturbo]

BigInch, please allow my ignorance. What I want to know is how to keep a constant flow in the NPSHR test to get the 3% drop-off curves while piching down suction valve. For example, at an initial test point, there is one set of flow-head-NPSH at constant speed. To get a second point of reduced NPSH at the same speed, they close the suction valve a little bit to decrease NPSH available. But they will also get a reduced flow. From Artisi's reply, I understood that they have to open a discharge valve to return to the original flow. Am I still misunderstood?

 

[BigInch]

You have to reduce the suction head into the pump while maintaining the same flow. Maintain the flow from the source as you adjust the valve in the suction line, by setting the source's flow control valve. You do have either an automatic, or manual FCV, that you can adjust to maintain a given source flowrate, right?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ojsturbo]

Thanks again, BigInch. You mean they need two suction valves at least for the test; one is near the pump impeller suction for reducing NPSH, and the other is to control the source flow rate far upstream. They do not modulate the discharge valve. I still wonder if this means the same as Artisi's reply.

 

[BigInch]

Well, this is one way to do it.

You must have some method of flow contol whatever it may be, such being able to hold flow while you reduce the suction head at the pump.

The suction piping for example can be designed such that, as you lower the inlet pressure into the suction piping from the source, and as you lower the suction piping outlet pressure (the inlet pressure into the pump), flow in the suction piping remains constant. To do that you must hold the suction piping's differential pressure constant (suction piping inlet pressure - suction piping outlet pressure). That will hold flowrate in the suction piping constant and send that into the pump, even as the suction head into the pump is also lowered. You want to be able to do that for all the flowrates you want to check.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ojsturbo]

Thank a lot, BigInch. Please allow my ignorance again. I wanna add once again what I think about the test procedure to make it sure.
1) A pump is running at flow(A) and speed(B) with TDH(C).
2) When the suction valve is piched down a little bit to reduce NPSH, the pump TDH will be rising a little bit because of reduced flow.
3) By opening the second valve (either another suction valve or a discharge valve), the flow will be recovered to flow(A) with TDH(C).
4) Repeat 2) and 3) until 97% of TDH(C) is obtained.
5) Calculate NPSHA at 4), and it becomes NPSHR of the pump for flow(A), speed(B).
6) Repeat 1) to 5) for each flow and speed in the operating range.
Am I thinking correctly?

 

[BigInch]

I think that'll do it. With two guys maybe you can adjust both valves 2 & 3 at the same time.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[racookpe1978]

What if ... (For an ideal situation = no budget limits or time limits. Har, har.) ... Try putting the suction (supply) tank as a vented (open top even) tank that could be raised or lowered, connected to the test pump suction by a flex hose arrangement.

So, to change the NPSHa to simulate different conditions (without changing any suction or discharge valve positions), you raise the tank up or down from its baseline point. If the flex hose is the same length, and the resulting bends as the tank moves are not too extreme, then the flow resistance remains about the same -> so the flow "should" remain about the same.

Or: use a pressurized tank, and change the air pressure above the tank to simulate different NPSHa values. Then a flowmeter in the suction can tell you the impact on higher or lower pressures on flow as well. After all, I understand that you want to find out the pump flow changes as suction pressure changes, not on how water inlet flow changes based on tank pressure.

 

[BigInch]

Or simply adjusting the fluid level in the tank might work with only 1 valve in the suction line, but then maybe you need to add some piping & valves to adjust the tank level. Or you could raise or lower the pump.... or .. change the diameter of the suction pipe.... or have two or three parallel suction lines, open more of them to increase suction head....

There are a number of ways to do this, just pick the easiest for you.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[dabluffrat]

NPSHr testing is a two man operation using vacuum tank, suction pressure gauge (inHg), discharge pressure gauge (psi), and a flowmeter on the pump discharge. Start the pump with the tank open to atmosphere, take initial readings at the test flow, then begin drawing a vacuum on the tank. As the vacuum pump pulls suction, one operator reads and records suction and discharge pressure readings at constant intervals (every 2 inHg record corresponding disch psi). The other operator is watching the flowmeter and adjusting the valve accordingly to maintain a relative constant flow.

Did you know that 76.4% of all statistics are made up...