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!

 

[1gibson]

What is the elevation in your area?

 

[SalvationTsai]

Dear good 1gibson, the elevation of my location is at the sea surface, thank you for your fast reply!

 

[TenPenny]

Do I get this correctly, you are calculating NPSHa, and then trying to compare with the reading on a pressure gauge at the suction of the pump?

You cannot read NPSHa on a gauge.

 

[SalvationTsai]

Dear good Tenpenny, I am sorry for using wrong way to calculate the pressure of the pump suction side, and could you give me some hint to calculate it correctly?
And I am not not understand why NPSHa is a wrong way to calculate it.
Thank you for your teaching!

 

[LittleInch]

Salvation. What you calcualted (NPSHA) is the amount of head in absolute terms that the fluid can supply at the pump suction flange. The pump needs to have a certain amount of head in absolute terms (even though it is negative in atmospheric terms) - NPSHR. NPSHR must be > NPSHA in order for the pump to be able to "suck" the liquid out of the pipe without it vapourising. If you're reading 0.2 barg, then it looks to me like either the static pressure flowing pressure reading or maybe your 2m of friction losses are exagerated as the guage will only measure pressure of the liquid and will not measure the vapour proessure as a negative quantity. The fact that it may be vapourising does not affect it's actual pressure.

If you think about a pan of water which is cold, then heated up until it boils. The pan of water once it boils doesn't weigh any less than the cold pan (within a few percent and before it all boils off of coourse). Therefore the pressure at the bottom of your pan of water stays the same. However if you tried to suck up the water through a straw (don't try this..) you would get mostly steam. Not sure if that helps, but in essence you're not measuring the same thing as noted above.

My motto: Learn something new every day

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

 

[Artisi]

SalvationTsai

Hydraulic Lesson 1. Pumps do not suck.

Lesson 2. A fully primed impeller is capable of lowering the pressure in the impeller eye.

Lesson 3. Flow will only happen if the pressure at the pump inlet (NPSHa)is higher than NPSHr.

In your calculations;
2 metres of pipe will not result in 2 metres of head loss - your flowrate has been given as 4.8kg/hr - at this flowrate the friction loss would be near enough to zero.
A rough estimate for your NPSHa is around 6 metres - what does the pump require at the flowrate.

The gauge reading you are seeing is the 1.5 metre static head less a few losses between the tank and your gauge.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[77JQX]

One difference between your measurement and NPHSa is the vapor pressure of water. The point of the NPSHa calculation is prevent going vapor in the eye of the impeller. After taking into account static head and friction, is the pressure at the impeller eye above vapor pressure? To keep it simple, the NPSHa calculation subtracts vapor pressure - to be sure the result is above zero (in theory). However, for real pumps the frictional losses near the impeller are difficult, if not impossible, to calculate. Therefore, manufacturers publish the NPSHr at the pump inlet, and the NPSHr value captures the friction in the pump inlet. So the net result is that when NPSHa is greater than NPSHr, the pressure at the impeller eye is greater than the vapor pressure of the fluid. One could just as easily not subtract vapor pressure in the NPSHa calculation, and say that NPHSa(without VP subtracted) > (NPSHr + vapor pressure). Or we could say NPSHa(without VP subtracted) - NPSHr > Vapor Pressure. All say the same thing. But subtracting the vapor pressure is the convention in the NPHSa calculation, and we all stick to it. The vapor pressure is just the minimum pressure below which we can't drop, and have the pump operate properly.

Which is just a long way to say the vapor pressure shouldn't be subtracted when calculating the actual inlet pressure.

And I suspect the assumption of 2m of friction losses in the 2m line are also too high.

 

[DubMac]

NPSHR values are NOT published for the pump inlet, they are "published" at the impeller centerline; if you are on the edge, you must take this elevation distance between the gauge and impeller centerline into account.

How accurate is your gauge anyway?

repeating Artisi....one of the cardinal rules, written in stone, in pump physice.....PUMPS DO NOT SUCK. If you start considering they do, you will surely walk down wrong paths.

I'm not sure where to even start with the "don't consider vapor pressure"....hopefully I just read that wrong.

 

[SalvationTsai]

Dear good LittleInch:
Your description on “NPSHR must be > NPSHA”, in my humble opinion, isn’t it should be NPSHA > NPSHR in order to avoid the cavitation?
And I really thank you for your vivid explanation (cold and hot water in pan) on why the vapor pressure should not be added in when calculating the gauge pressure reading, I do figure it out!

Dear good Artisi:
Thank you for your lesson 2 and 3, I will keep it in mind, but I don’t know why the lesson 1 said that the pump do not suck? In my humble opinion I thought a pump sucks up the water(theoretically 10m at best), and deliver the fluid to a higher level.
Could you please correct my concept?

Dear good 77JQX:
Thank you for your clear description on the relationship of whether the vapor pressure is within or without on the equation of NPSHa and NPSHr.
But it is indeed a clear explanation on why the vapor pressure shouldn’t be subtracted when calculate the actual inlet pressure.

I started to sum up the above three good seniors teaching. If my water source surface level is 2m, and H(suction pipe loss)=0m, and then my theoretical pressure 0.2kg/cm^2 will matches the practical gauge reading on the suction side!

But what will happen if my water source surface level is under the pump centerline about 3m?
In my hypothetical question, friction loss is zero, fluid type is a hot water 80 Celcius.
And then my NPSHa“1atm-H(static)-H(vapor)-H(suction pipe loss)” would be=
10m-3m-4.83m-0m=2.17m
And the practical gauge reading on the pump suction side should be -0.3kg/cm^2.
Thus if my pump’s NPSHr is 1m(less then the 2.17), then my pump could suck up the water and do the normal operating;
And if my pump’s NPSHr is 5m(higher then the 2.17), then my pump could not suck up the water and won’t be able to do the normal operating.
Is my above concept right?

Thank you for your patient for reading my question, thank you again!

 

[SalvationTsai]

Dear good DubMac:
I really don't know the concept on the "PUMPS DO NOT SUCK", could you please hint me on this?
Many of my book I found in the library do describe on the pump, said the pump have suction side and deliver side, please I really wish to figure it out, and son't want to stray on the pumping engineering.

 

[Artisi]

Pump suction is a WRONG term it has and does and will always cause a lot of confusion, the correct term is pump inlet. whenever coming across this term you should put your pen through the word and write pump inlet over it.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

You're quite right - I got that the wrong way round - sorry.

There are issues with having water below the suction inlet of the pump, as a centrifugal pump won't work at all unless you have flooded suction. You can get special centrifugal pumps that will self prime, but lets not go there at the moment.

A better example for the one you note above is that the pump is much further away and the losses in the pipework when you flow equal 3m. Then yes, you are essentially correct. With an NPSHR of 1m your pump should pump, but with 5m, will cavitate wildly and not work properly.

Beware that the NPSHR measurement is a function of performence and cavitation, which will eat your impellor in a matter of days, can occur at a few meters ABOVE NPSHR. Therefore you should always have a margin of 2-3 m min above NPSHR to avoid cavitation. Ask the pump supplier for his onset of cavitation curve if you are operating close the NPSHR margin.

Oh and a bit less of the "senior" bit...

My motto: Learn something new every day

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

 

[DubMac]

A centrifugal CAN WORK even if the "inlet" fluid level is below the pump itself; and it doesn't necessarily have to be a designed self-priming pump. This concept is somewhat central to our discussion here.

The fluid is not "sucked" into the inlet, it travels there because it is moving from from a higher pressure zone to a lower pressure zone. The "mystery force" allowing this to happen is atmospheric pressure.

The fluid is not "sucked" in, it is pushed in. Take that and chew on it for awhile.......

 

[Artisi]

or suck on it for awhile....

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[TenPenny]

"The fluid is not "sucked" into the inlet, it travels there because it is moving from a higher pressure zone to a lower pressure zone. The "mystery force" allowing this to happen is atmospheric pressure."

Out of curiosity, how does this differ from any other 'sucking' motion, whether a vacuum cleaner or a woman with a straw?

 

[Artisi]

It doesn't differ - same principal applies, pressure is lowered by the "sucking" and flow takes place because of the "mystery force" - atmospheric pressure.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[DubMac]

Yes, there are many times the words sucking and blowing are used rather interchangeably, but I guess what really matters is who or what is responsible for initiating the sucking or blowing action. Thats about as far as I want to go with this discussion.

 

[LittleInch]

I fully agree a centrifugal can work with liquid level below inlet level, just that unless you do it right, you can have problems starting the pump if the inlet line has drained down too far. Also it does tend to make vaporisation / cavitation issues come to the fore.

I did use the word suck in inverted commas simply to illustrate to the OP what I meant as this is the most easliy understood way of expressing this concept, even though not strictly correct. I will try the "inlet" instead of suction line/header/flange where I can.

My motto: Learn something new every day

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

 

[TenPenny]

So we have just determined that pumps do, indeed, 'suck', insofar as anything 'sucks', because they create a lower pressure which allows atmospheric pressure to push the liquid into the pump.