Why Does Pump Head Decrease with Constant Velocity?

[discreet544]

Hi Folks;

I am facing a problem with a real experimental pumping case. I am running a schedule 40, commercial steel, 45m length, 2" diameter pipeline for water. Since past month, suddenly, the head of the pumped dropped by 0.3 Kpa/m, however, the velocities are the same. I wonder is it possible to get less head, but the same velocity. Shouldn't these two be correlated ?

Thanks;

 

[DoggerDan]

That's just under 2 psi over the entire length. I suspect something's partially clogging your gauge.

 

[discreet544]

Sure not ! I've check step by step all the line. it is completely clean and transducer's are also fine; no clogging ! Within 1 week, pressures suddenly dropped ! While the velocities are still the same. It is obviously violating Bernuli's law !

 

[meplumber]

Have you checked for a leak on the suction side? I have had that happen a time or two.

It usually shows up as a slight decrease in discharge pressure.

 

[discreet544]

Today I spent $4000 to change all the pump impeller and plates and sealings and orings and ... But the problem is still there ! OMG ! I am getting crazy ! I can make the same velocity, but not the same pressure ! Even for pure water ! I am now in a real trouble ! I can't make any more guess ! Is there any help, please ?

 

[discreet544]

Also I changed the suction hose !

 

[turbo]

Where are you measuring the discharge pressure? Are you measuring it near the pump, or nearer the end of the discharge line?

 

[discreet544]

The discharge pressure is a few cm ahead of discharge valve ! The loop is all horizontal !

 

[turbo]

The discharge pressure is a few cm ahead of discharge valve ! The loop is all horizontal !

Where is the discharge valve? Is it very near the pump or nearer to the end of that long line? 45m is quite a long line and a lot of stuff can happen along that path. You are making it very hard to help you, due to the lack of information on the system. Please provide more information and fewer exclamation marks.

 

[Travis_King]

You spent $4000 to jump 2psi? How precise is this system? I'm sure you know that pumps ride on a curve, losing 2psi only amounts to a few GPM.

I agree with turbo. We need more information if we are to help you.
--What are the loads on the pump system (i.e. heat exchangers, suction side valves/tees/wyes, etc)
--What are you pumping into?
--What is the pump discharge head?
--Suction head?
--How is the piping at the suction side set up?
--How is the piping at the discharge side set up?
--Any reducers? (Concentric or eccentric)
--Water temp?
--What data can you take from the pump? Amps? Power draw? Hz?

Help us help you. And don't spend money on an impeller unless you absolutely have to. Same goes for packing and orings.

 

[Q_Goest]

If the restriction in your line decreases, pressure drop will also decrease, so any change in line restriction including valve adjustments will change the pressure drop you measure. Can the pressure drop be changed by changing the adjustment on a hand valve? You mention a discharge valve...

 

[Jobrag]

What are you using to measure the pressure, when was it last calibrated, ditto velocity?