NPSHA calculation

[behbood]

I am checking the NPSHA relation with the data, included in the Mechanical datasheet of pump, which is used in a petrochemical site.
the NPSHA=8.7 m is insisted in the datasheet, but the Psuc is
Psuc,norm=2.16 bara,
Psuc,max=2.18 bara
Psuc,min=1.06 bara
Psuc,des=2 barg
Pump design temperature=85 C
Pumping Temperature=40~50 C
Vap. Pressure=0.12 bara (at pumping T)
Relative Density=0.988

when calculating the NPSHA using the above quantities, the resulted NPSHA differs from the one, included in the datasheet.
would you please inform me which one is true?

NPSHA=(Psuc-Pvap)/(1000*S.G.*g)

 

[BigInch]

You should use the lowest expected suction pressure to be safe when calculating NPSHA.
You have a minimum of (1.06 -0.12) barg + 1 bara of = 1.94 BarA of NPSHA. That's equivalent to some 20 meters of NPSHA, so the 8.7 m would seem to be wrong.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[JJPellin]

Our standard method for calculating NPSH(available) is to perform the calculation at normal suction drum pressure. The difference would be driven by suction level. Our specifications require the use of bottom nozzle or bottom tangent for level. This may be well below low level or even low-low level. Depending on the distance from low level to bottom tangent, the NPSHa may be much lower than you would calculate using minimum suction pressure. If they are using the combination of the highest temperature (highest vapor pressure), lowest drum pressure and lowest level, then this may be excessively conservative. Excessive conservatism in NPSH may drive you to purchase a pump with a very high suction energy and a very high suction specific speed, or a very large, very low rpm pump which may have very low efficiency. I would discourage the use of multiple parameters at the worst possible condition. If the combination of conditions specified for the calculation will never really occur all at the same time, it would not be a good idea to design for that combination.

Johnny Pellin

 

[BigInch]

Even if engineers recognize the interdependence of design parameters, for example that flow, temperature and pressure are all required to specify only one fluid operating point, typical data sheets that ask for more than one single value for each parameter to be given, such as minimums and maximums for P, Q & T, actually make it near impossible to select proper equipment. Instead of only one design point you would specify with one value for each parameter, with mins and maxs you see that there are no longer just two points, but that you can actually draw up to 8 possible combinations of data points, each describing a possible fluid operating point, now making it necessary to design for the full envelop those 8 points describe, because the data sheet does not tell you which of those combinations are valid design points.

Data sheets didn't use to be that way, having only one box for each value and that required that the specifying engineer know what he was doing and preselect the worst case, or most conservative operating point and write down those corresponding values. Today it seems that they just want to copy all the data from the computer run and paste it into the boxes on the data sheet, no matter how disassociated those data values might be from any given actual operating point.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[casflo]

Which is the NPSHR (required NPSH) given in the nechanical data sheet of the pump?
The NPSHR must be provided by the pump manufacturer and the NPSHA is provided by the pump purchaser.
May be that the 8.7 m be the NPSHR instead the NPSHA.

casflo

 

[Pumpsonly]

You only provided the pressure detail. NPSHA calculation need to take elevation diff.between liquid level and pump impeller center line and also suction piping and fitting losses into the consideration.

Your Psuc, des=2.0barg as relate to Psuc,max=2.18 bara certainly is a mistake.

 

[DubMac]

Pump manufacturers (reputable ones) only gaurantee ONE operating point; you will find yourself in trouble more often than not if you don't let user know that up front.

NPSH = S - Hfs - Hi Ps - Pvp

where:
S = static head between suction liquid level and pump centerline
Hfs = friction loss in suction line
Hi = entrance loss in suction line
Ps = pressure on suction liquid surface
Pvp = vapor pressure of liquid

all units in feet of liquid

 

[DubMac]

Sorry,

Left symbol out of equation, should be:

NPSH = S - Hfs - Hi + Ps - Pvp

 

[BigInch]

Pumpsonly, since it was the pressures at suction, not head, they are direct measurements with no additional elevation differences to account for (unless there was an additional elev difference between gauge point and CL) he didn't mention.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Pumpsonly]

BigInch
As my experience ,the suction pressure in pump data sheet is normally the pressure on the surface of the liquid or suction vessel pressure. The OP may have misread the data as indicated in the 2 values. It would be help a lot if the OP can post the data sheet.

Psuc,des=2 barg = 3.03barA is > Psuc,max=2.18 bara

DubMac, you are mixing head and pressure in your formula.

 

[DubMac]

Pumpsonly,

I should have been clearer that the same units should be used throughout the equation; either head in units of feet, or pressure in units of PSI. Convention in the industry uses "feet of head" for NPSH calculations.

There is no difference between head and pressure; the quantity is just expressed in different units; pressure in PSI, head in feet of liquid.

And as we all know from our Pumps 101, to convert from one to the other:

Head, ft. = (2.31 X PSI) / S.G. of fluid

 

[BigInch]

No way.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso