Pressure drop across pump discharge valve.

[Abhi1712]

Hello everybody,


Centrifugal Pump performance test (ANSI / HI 1.6 standards)
Gate valve: full open; Water Flowrate: 260 GPM(+/-1); Discharge pressure before gate valve 60 PSI; Discharge pressure after gate valve 0 PSI. Why there is 60 psi pressure drop across the valve?

I have tried following experiment to solve problem… but all fail.
1. I have isolated discharge line with blind at end point. Pressure drop is 0. Both pressure gauge showing same 60 psi reading. I have observed back pressure and installed Check / one way / non returing valve before the gate valve & after gate valve(one at a time). But was showing pressure drop 60 psi.
2. Submerged tank inlet line isolated above water surface level by draining some water. But was showing pressure drop 60 psi.
3. Checked operatibility of pressure gauges. Those are good. But was showing pressure drop 60 psi.
4. Dismentaled all lines and pipe below gauges. All are clean & clear. But was showing pressure drop 60 psi.
5. Checked gate valve functionality it was Ok. Also replace gate valve with ball valve. But was showing pressure drop 60 psi.
6. Swaped position of flowmeter & gate valve.
Gate valve full open: pressure drop across valve 0 PSI
gate valve 50% open: pressure drop 100 PSI
gate valve 75% open: pressure drop 80 PSI
Please find attached document.

 

[ione]

Do you discharge to an atmospheric tank?
If so what do you expect if not atmospheric pressure (0 psig) plus static head due to water column inside the tank upon condition discharge pipe is submerged.

 

[Pumpsonly]

1) I am sure your 1st PG location is not in accordance with HI 1.6.
or any other standard. The tapping point can not be immediately after an elbow.

2) When your flow meter is before the gate valve and the 2nd PG.
the pressure drop is including the flow meter as well. In other words, you flow meter has very high pressure drop. Is it turbine meter?

 

[Abhi1712]

Thank you for ur reply.
To IONE: yes discharge goes to atmospheric tank. You are right I did not get anything in that condition. I was varing conditions.

To PUMPSONLY: They have not indicated Pressure gauge location in HI 1.6 standard. Can you please suggest how much distance should be between taping pipe and elbow. My discharge pipe nominal size is 2 inch?
My flowmeter is turbine meter. I found therotically for 260 GPM flow pressure drop is around 7-8 PSI. Is turbine meter give large pressure drop?

I would like to mention about pipe size at discharge side:
1)My discharge line upto 2nd pressure gauge is 2 inch nominal size. (as shown in figure 1 of provided document) all of my PG1, PG2, Flowmeter, valve on that line.
2) Then I connected hosepipe of 3 inch nominal size upto tank (as shown in figure 2)
3) My 3rd pipe going toward tank top is of 4 inch nominal dia. (as shown in figure 3)

Is this condition makeing large pressure drop?

 

[Pumpsonly]

Abhi1712,

HI 1.6 will tell you the tapping point should be on a straight pipe
min. 2 pipe diameter from the pump suction / discharge flange.
Please get a copy if you do not have one. Understand that you are a pump dealer or packager, your set up is not to perform a proper performance test of the pump.

Turbine meter is high dp type.

As ione pointed out, the friction losses and the static head of the discharge piping after the gate valve ( back pressure)in your initial set up is probably too low to be measurable with the pressure gauge you have.

 

[Abhi1712]

Dear PUMPONLY,

I really appreciate your guidance. I have changed taping point for pressure gauge. As you stated, I require digital pressure gauge to measure minor losses. Am I right?

But 60 PSI pressure drop is too much. This might be majorly because of Turbine meter. Right?

Any more suggestions are welcome.

 

[quark]

Second gauge showing 0 psi is little doubtful to me. Your tank seems to be around 3 meters high, so that should read 4 to 5 psi. Nevertheless, that doesn't explain 60psi at pump discharge.

What is that black hose before the meter doing? Is there any flow through it?

 

[Pumpsonly]

The flow meter manual should include a chart / curve showing the dp verses flow.


You mentioned:
6. Swaped position of flowmeter & gate valve.
Gate valve full open: pressure drop across valve 0 PSI
gate valve 50% open: pressure drop 100 PSI
gate valve 75% open: pressure drop 80 PSI

Did see a change in the flow meter reading?

Was the PG #2 before all after the flow meter?

 

[Abhi1712]

To QUARK:
Black hose before meter is a priming point. I prime pump by closing discharge valve and move air from that hose.

To PUMPSONLY:
PG#2 was before flow meter
PG1 & PG2 are in PSI. by varying discharge gate valve from fully open to fully closed I got following readings. The flow meter dP in manual is showing 8 PSI.

Flow(GPM) : 254 220 200 180 160 140 100 60 20 0
PG1(before valve): 70 84 93 100 108 115 128 138 148 151
PG2(after valve) : 66 45 34 21 19 18 10 6 5 4

 

[cvg]

something doesn't jive. you said you had 60 psi pressure drop across the valve at full flow, 260 gpm with downstream pressure of zero. however in your table above you show just 4 psi pressure drop across the valve at full flow with approximately 254 gpm and downstream pressure of 66. I don't see a problem with this last table.

 

[Pumpsonly]

Ok let us analyse your table.

1) At 0 flow, valve closed. PG1 is the closed valve head of the pump.PG2=4 PSI reading is the static pressure of the water column in the discharge piping between PG2 and the entry point at the tank.

2) At 254 GPM, pressure before flow meter= PG2 =66 PSI.
Therefore pressure loss across flow meter = 66-4 =62 PSI.

3) In your 1st post, you mentioned there was a 60 psi pressure difference between PG1 and PG2 with the valve and the flow meter in between.

Now do you see the closeness of the 2 figures and what they represent? --->>> the pressure loss across the flow meter.

There could be some pressure in PG2 (4 PSI ?) in your first test run but you did not notice it.

In your first post you mentioned:

"I have isolated discharge line with blind at end point. Pressure drop is 0. Both pressure gauge showing same 60 psi reading."

This must be a mistake also as it should be about 151 PSI as shown in your tabulation at 0 flow.

The 8 PSI dp mentioned in the flow manual is at what flow rate?

 

[Abhi1712]

To PUMPyour help.SONLY & CVG:

I have solved the problem. It was because of increaseing pipe diameter in discharge side. Initially there is 2" pipe after second PG there is 3" pipe dia. This is root cause of problem. Thank you for help.

 

[Pumpsonly]

Abhi,

I don't get you.

 

[Abhi1712]

I would like to mention about pipe size at discharge side:
1)My discharge line upto 2nd pressure gauge is 2 inch nominal size. (as shown in figure 1 of provided document) all of my PG1, PG2, Flowmeter, valve on that line.
2) Then I connected hosepipe of 3 inch nominal size upto tank (as shown in figure 2)
3) My 3rd pipe going toward tank top is of 4 inch nominal dia. (as shown in figure 3)

for experimental basis I have disconnected 2" pipe from PG1 and installed gate valve after PG1 and immediately connected to 3" discharge hose pipe and followed by 3rd pipe going toward tank top is of 4 inch nominal dia. Pump have started with fully open valve. PG1 was showing 0 PSI reading. As per thumb rule pump discharge line has to be always small pipe size compare to suction side. My Suction side pipe dia is 4".

This was root cause of problem.

 

[Pumpsonly]

Abhi,

Your rule of thump pump discharge line must be always smaller than suction pipe is incorrect.