NPSHA 1

[TECHDOC]

Hi..
IF NPSHA IS ABOVE NPSHR BY MARGIN OF 3FT,WILL THIS BE OK FOR ALL OPERATING PUMPS AT THEIR RATED CONDITION????CONDITIONS MAY VARY viz.PUMP MAY BE HAVING NEGATIVE OR POSITIVE SUCTION LIFT.AND ALSO THE VISCOSITY MAY REDUCE.....

 

[BigInch]

No. First of all a percentage (of up to 25%) over the required head is better than just adding some absolute amount. There is also a relationship between your specific product and cold water. Not all products behave entirely the same. There can also be some hydraulic considerations from piping and equipment configurations that the usual calculations of pipe, fitting, valve head loss and vapor pressure to get NPSHA don't include. Type of pump also matters.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[TECHDOC]

tnx BigInch...
let me give you some more details.....
the flow is 225m3/hr....
the vapour pressure is 4 hPa.
specific gravity is 0.87...
the NPSHr is 4.4m........
can you explain me more?

 

[JJPellin]

We would need to know the basis of the NPSHa calculation. Is this determined at actual operating flow, temperature, pressure, suction vessel level, etc.? Or, is it calculated at limits such as minimum level, maximum temperature, etc.? As BigInch noted, you should normally require a minimum margin of X feet or YY percent, whichever is larger. For hydrocarbon, we require 3 feet or 15%. But, our margin is calculated with conservative assumptions about vapor pressure and suction vessel level. If you don't have conservative assumptions built in to th NPSHa calculation, then you should require a greater margin (feet and percent).

Johnny Pellin

 

[SNORGY]

TECHDOC,

There are two measures that you can use to evaluate your pump a bit more closely in this regard: suction energy and suction specific speed.

The attached link may be of benefit.