Why Does Pump Head Decrease with Constant Velocity?

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Discussion Overview

The discussion revolves around an experimental issue related to a pumping system where the head has decreased despite constant velocity. Participants explore potential causes for this phenomenon, including equipment malfunctions, system configuration, and measurement accuracy.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes a drop in head of 0.3 Kpa/m in a pumping system while maintaining the same velocity, questioning the correlation between head and velocity.
  • Another participant suggests that a partially clogged gauge could be responsible for the observed pressure drop.
  • A participant refutes the clogging theory, stating that the line and transducers are clean and that the pressure dropped suddenly, which they believe contradicts Bernoulli's law.
  • Concerns are raised about potential leaks on the suction side, which could lead to a decrease in discharge pressure.
  • A participant expresses frustration after spending a significant amount on pump components without resolving the issue, emphasizing the inability to achieve the same pressure despite consistent velocity.
  • Questions are posed regarding the location of the discharge pressure measurement and the configuration of the discharge line.
  • Another participant emphasizes the need for more detailed information about the system to provide effective assistance, including specifics about loads, piping setup, and pump data.
  • Discussion includes the idea that changes in line restrictions, such as valve adjustments, could affect pressure measurements.
  • Participants inquire about the calibration status of the pressure and velocity measurement devices.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the cause of the pressure drop, with multiple competing views and suggestions remaining unresolved.

Contextual Notes

Participants highlight the need for additional information regarding the system setup, measurement accuracy, and potential changes in line restrictions, indicating that assumptions about the system's behavior may not be fully addressed.

Who May Find This Useful

Individuals involved in pump system design, maintenance, or troubleshooting, particularly those dealing with pressure measurement and fluid dynamics in engineering contexts.

discreet544
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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;
 
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That's just under 2 psi over the entire length. I suspect something's partially clogging your gauge.
 
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 !
 
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.
 
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 ?
 
Also I changed the suction hose !
 
Where are you measuring the discharge pressure? Are you measuring it near the pump, or nearer the end of the discharge line?
 
The discharge pressure is a few cm ahead of discharge valve ! The loop is all horizontal !
 
discreet544 said:
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.
 
  • #10
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.
 
  • #11
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...
 
  • #12
What are you using to measure the pressure, when was it last calibrated, ditto velocity?
 

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