Theoretical calculation does not match the practical gauge reading on pump suction side 1

[SalvationTsai]

Greetings every senior engineers!
I have a basic question on how to exactly calculate the pressure on the pump suction side, and then matches the practical pressure gauge reading on pump suction side?

First I am using the NPSHa equation.
1atm+H(static)-H(vapor)-H(suction pipe loss)=NPSHa <absolute value>

1atm: I convert it to approximately 10m
H(static): the water source surface level is above the pump centerline for 1.5m
H(vapor): the water is 80 Celsius, so the vapor pressure is 4.83m
H(suction pipe loss): the length is about 2m, carbon steel pipe, and the inner diameter is 36mm, in this part I would like to assume the pipe loss is 2m.

So sum up the above values, 10m+1.5m-4.83m-2m=4.67m < absolute value>
Then 4.67m-10m=-5.33m=-0.533kg/cm^2 <gauge value>
But in the practical gauge reading on the pump suction side is +0.2kg/cm^2.
Thus I don't know why my theoretical value does not matches the practical gauge reading? and what makes the difference between it?

Later I found another question, I found that there is no any contribution by the pump itself, in my assumption there ought to be a force created by the pump which helps the pump to suck in the water.
Thus I would like to ask is it the factor I missed which cause my theoretical calculation can't matches the practical gauge reading on the pump suction side?

And if it does, how could I calculate the sucking in pressure generated by pump?
My centrifugal pump's rated Q is 4.8kg/hr, rated H is 120, 3600 rpm.

Thank you for reading my lengthy description of my question, truly thank you!

 

[LittleInch]

Salvation,

You're getting very close. Pump systems usually incorporate an nrv in the system to prevent back flow through the pump, which is a bad thing.

First diagram yes,.

Second one, also correct, but if the nrv doesn't leak (some seal very well), then the pump would sit at high pressure with possible damage to and leakage from the seals.

If the nrv was on the discharge side and no inlet nrv, then the water might drain out as air leaks into the pump via seals which are designed to keep liquid from getting out as opposed to air getting in. This might be very slow or not happen, but you always need to think about "what if" situations including drain down or loss of liquid level.

Having reread what I wrote I can see why you were a bit confused, hope the above helps.

LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SalvationTsai]

Dear good LittleInch:

Thank you for verifying my 2 diagrams, and I had re-drawn the second diagram(with the NRV at discharge side), please kindly see it once more, thank you very much!
But I don't quite know where & how the air comes from? I thought the pipe is full of water, or is it leaked (from the outside atmosphere) into the seal(NRV)?

I do sometimes confused, but it is due to my poor English and novice at pumping engineering, but I really feel fortunate that a lot of people are willing to teach me, especially dear good you.
Best and warmest regards.

 

[LittleInch]

Salvation,

You're getting very very close now.

It's also me assuming you know things. Pumps normally have seals around the shaft to allow it to rotate while full of fluid. The seals are various types - mechnical seals, chevron seals, stuffing boxes etc - which are designed to stop liquid getting out, not air getting in.

IF you have a situation like your last drawing with an inlet pipe liquid level lower than the pump and no NRV on the inlet line, then the pump at rest will be at a lower pressure than atmospheric. Normally then someone isolates the pump on the discharge side. In that instance, a small amount of air can leak into the pump through the pump shaft seal and slowly the pump and inlet line will empty. I told you this just so that if it happened to you you could see why the pump might not re-start after some time because all the fluid had leaked out due to a small air leak in the pump, despite being isolated from the discharge line and no leaks in normal operation.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SalvationTsai]

Dear good LittleInch:

Thank you for your clear explanation on the seals of the shaft, I used to assume the seal is on the NRV!
I am really happy and glad I could now finally clear on this "idea of a possibility of air leaks into the pump shaft's seal", then in the very end, water in the inlet pipe line will be fully emptied, and the pressure in the pump should be equal to the atmosphere!

Really thank you, and because of your kind teaching, I do learn something new today and the last few days, sincerely thank you!

Best and Warmth regards.

 

[LittleInch]

A pleasure. It's sometimes not easy to explain things by posts and many times people don't respond, so your feedback and questions have been very good.

LI

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way