11 replies to this topic

[farid.k]

Hye there. Again just a simple question regarding pump. I just go through a pump datasheet that will be submitted to vendor for purchase. I found that the figure of total suction pressure is smaller than NPSHa. Is it possible? From my understanding, NPSHa is the minimum pressure at pump nozzle minus the liquid vapour pressure. Thus, total suction pressure should be always greater than NPSHa of pump. Any comment?

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[Art Montemayor]

 

The Net Suction Head Available (NPSHa) should ALWAYS be larger than the Net Suction Head Required (NPSHr).  The NPSHa is a characteristic of the system and the NPSHr is a characteristic of the specific pump.  The pump manufacturer controls the NPSHr and you, as the system designer, control the NPSHa.  It is your duty to make sure the NPSHa is greater than the NPSHr.

 

Contrary to what you state, the net total suction pressure available to the pump should be always greater than the pump’s NPSHr (NOT the NPSHa).  I think you have your NPSH’s reversed.

   Understanding Net Positive Ssuction Head.pdf   90.94KB   141 downloads

 

[xavio]

farid k,

 

The hydraulic calculation is incomplete, it seems that someone is too lazy to finish the calculation.

Total minimum suction head = H suction vessel + H liquid (min.) - H friction

H friction is missing!

 

Minimum NPSHA = H suction (min) - H vapor pressure.

H vapor pressure is missing too!

 

I believe the NPSHA written there is just a random value, probably from another calculation.

Nothing's wrong with your concept, something's wrong with the guy preparing the sheet.

 

Good luck.

 

xavio

[breizh]

Farid,

 

Probably good to read , on top of literature provided.

 

Breizh

Edited by breizh, 20 June 2014 - 01:29 AM.

[farid.k]

Dear Art Montemayor

 

None of my words mention NPSHr. i don’t even bother that value since that value is from vendor. My duty is only to state the value of NPSHa to the vendor so that the vendor will come out the pump with lower NPSHr value for at least 1 meter. I just confuse the term “total minimum m suction head” and “net positive suction head available” (NPSHa) (in the previous attached datasheet). Both terms should be in the datasheet that come from hydraulic calculation and for me total minimum suction head should be slightly higher than NPSHa (since to get the NPSHa must minus the vapor pressure). For me, total minimum suction head is the pressure at pump nozzle at the rated flow rate and the lowest tank level. Can refer typical process pump datasheet that mention suction pressure/ head and NPSHa. Appreciate to advice. Thanks

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•  00405-02-PR-DAS-1002.pdf   333.59KB   78 downloads

[xavio]

farid.k,

 

Our esteemed admin Mr. Montemayor reminded you about the importance of having sufficient NPSH margin, which you seem to understand already. That's good.

 

With such crystal clear understanding you should be able to solve the hydraulic calculation problem you first raised.

 

Good luck.

 

xavio

[Padmakar Katre]

Dear Art Montemayor

 

None of my words mention NPSHr. i don’t even bother that value since that value is from vendor. My duty is only to state the value of NPSHa to the vendor so that the vendor will come out the pump with lower NPSHr value for at least 1 meter. I just confuse the term “total minimum m suction head” and “net positive suction head available” (NPSHa) (in the previous attached datasheet). Both terms should be in the datasheet that come from hydraulic calculation and for me total minimum suction head should be slightly higher than NPSHa (since to get the NPSHa must minus the vapor pressure). For me, total minimum suction head is the pressure at pump nozzle at the rated flow rate and the lowest tank level. Can refer typical process pump datasheet that mention suction pressure/ head and NPSHa. Appreciate to advice. Thanks

Hi,

You should always bother the value of NPSHr too although it is in vendor domain. Your hydraulic estimates should be reasonably good to find multiple vendors and cheaper pump models which can cater to NPSHa you have specified in your design sheet.

 

If it is a saturated liquid, source system pressure has no meaning and it does not go into any calculations but the source design pressure does. Suction pressure is estimated based on Normal Liquid Level minus the friction, suction strainer losses and any other losses if any. NPSHa is estimated based on either Low liquid level or sometimes on conservative approach based on vessel BTL minus all suction losses and further subtracting the pump center line elevation from grade.

 

Yes you are right that NPSHa is lower than suction pressure on account of NLL & LLL.

Max suction pressure is estimated based on source design pressure, HLL and no credit for suction losses (these are really minor).

 

One piece of advice, before releasing the datasheet into market with vendors, check their catalogs for possible models availability rather waiting for vendor feedback which may call for increase in source vessel elevations.

[farid.k]

Padmakar S Katre:

 

how to bother the value of NPSHr?  we can estimate?

 

 

thanks

[fallah]

farid.k,

 

You may bother the value of NPSHR before finding and selection the proper pump, because it should be lower enough than NPSHA....Anyway there are the methods such as using Stepanoff's sigma equation to estimate the expected NPSHR at design flow...

[breizh]

http://www.cheresour...ed-calculation/

 

Farid ,

Consider this link

 

Breizh .

[Padmakar Katre]

Hi,

Whenever you are involved in equipment hydraulics, a strong advice is get in touch with suppliers or vendors. Yes you can get NPSHr from pump vendor catalogs which I feel you should have with you if you work on pump hydraulics. It is so simple and vendors will share these catalogs which will contain the relevant data. 

[MJ94]

I think that minimum suction head is =  abs pressure in vessel + height in pump centre - Pressure drop 

 

so in your case it should be = 2.438 + .85 -  total head loss (which is not given )

 

and NPSHA = 3.288 + vel head (can be neglected)- vapour pressure of liquid at that temperature