How to maintain flowrate and pressure head in a pump?

[erobz]

You need to give me the inlet conditions ( head), tank fluid, temperate, and a pump curve would be nice. There are too many paths to take with this little bit of info to troubleshoot.

On second thought, just tell me about the fluid properties, no need to tell me what’s it is exactly. Your pump performance curve is tested with water(likely) . If the fluid has a viscosity different from water the performance curve could need adjustment.

To get the inlet conditions without a guage, need to know the tank water height and where the pump is physically placed relative to the water level.

 

[Nitheeswar]

The relative height between pump and water level in tank is 1.6 m

 

[erobz]

The relative height between pump and water level in tank is 1.6 m

Ok, so subtract 1.6 m from 32m. Your differential is about 30.4 m. What flow rate does your pump curve suggest it is running at 30.4 m?

 

[Nitheeswar]

We are using old pump, there are no pump curve for that. It has only head range and flow rate at particular lpm.please find attached image

 

[erobz]

We are using old pump, there are no pump curve for that. It has only head range and flow rate at particular lpm.please find attached image

If your tank has a regular cross-sectional area its a simple matter to determine the average flow rate. You mark the top of the water, let it run for a minute (if tank capacity permits). And measure the height again.

$$ \bar Q = \frac{A_{tank} \Delta h }{ \Delta t } $$

Since you don't have a pump curve, or cant find one. Its old, perhaps modified from nameplate. It would be a challenge to verify its performance with so little information. As far as I can tell you are already operating outside of nameplate head range.

 

[Lnewqban]

Irrespective of pump output conditions, if we are maintaining pressure 30 m at nozzles, we are getting the 500 lpm, the flowrate only changes if pressure is below 30m

What is the problem with that?
Is the pump satisfying the nozzles or not?

The nameplate of your pump states that it can deliver 14 lps (840 lpm) at a head of 24 m.
If a gauge located at the discharge of the pump shows 32 m of water column (45.5 psi), then it can only deliver less than 840 lpm (let's assume 630 lpm).

In actuality, it will deliver only the amount of water that the nozzles let go through.
The motor will consume less electricity (Amps) and the water may get a little warm.

Please, see how similar pumps behave.
Note the ranges of delivery pressures and volumes.

Copied from:
https://www.crompton.co.in/product/...agricultural-pumps/centrifugal-monoset-pumps/

 

[Nitheeswar]

If we assume it is delivering 630 lpm ,the nozzle can only permit 500 lpm.
What is effect of that 130 lpm?

 

[erobz]

If we assume it is delivering 630 lpm ,the nozzle can only permit 500 lpm.
What is effect of that 130 lpm?

If we assume the pump is delivering 630 lpm, the nozzle are dischaging 630 lpm. You are really hung up on this idea that the nozzels are designed to only output 500 lpm. No one cares what you think the design specs are for your nozzles!

Nozzle: “Hey pump, destroy mass because some engineer miss-designed me!"

Pump: “…”

 

[Lnewqban]

If we assume it is delivering 630 lpm ,the nozzle can only permit 500 lpm.
What is effect of that 130 lpm?

There is no effect, because those 130 lpm are not happening.

Please, take a look at the red curve in the diagram shown in post #4.
Q is the volume of liquid that is delivered by the pump at different pressures.

Note how the pump produces a maximum discharge pressure when Q=0, and it remains close to that value for small amount of flow.
Our pump may be working in a similar flexible area of performance, in which the flow self-adjusts according to the restriction of the nozzles, keeping a maximum discharge pressure.

That is how centrifugal pumps work.
You can fully close the discharge using a gate valve (Q=0 condition), and your gauge will still show the maximum discharge pressure that the pump can produce.

Because the power taken from the motor is proportional to the product of pressure and flow (P x Q), our pump will consume less power in that zero-flow condition.

Your nozzles are acting as a partially open valve, limiting the flow.
Your pump will naturally adjust to that limited flow, increasing the pressure accordingly (which is good for the performance of your nozzles).

All you need to do is to watch the temperature of the water downstream the pump, which tends to increase in those conditions.
If it gets warm, there is no problem.
If it gets too hot, it could damage the seals and the bearings of the pump.