Vertical Can Pumps NSPHr

[mechengr77]

New to the pumping field and trying to find an answer to what I thought was a simple problem. The pump curve that I was given has a NSPHr at the centerline of the impeller first stage of about 18.5 feet. Our datasheet that we submitted stated that we have 8 feet of NPSHa (taken at the centerline of the nozzle). Do I need to get the distance from the centerline of the nozzle to the centerline of the first stage and subtract that from the NPSHr to get an NPSHr that works under the idea that NPSHa needs to be greater than NPSHr?
Thanks

 

[pmover]

from api610, 10th edition:

3.28
net positive suction head NPSH
total absolute suction pressure determined at the suction nozzle and referred to the datum elevation, minus the vapour pressure of the liquid
NOTE It is expressed in metres (feet) of head of the pumped liquid.
3.29
net positive suction head available NPSHA
NPSH determined by the purchaser for the pumping system with the liquid at the rated flow and normal pumping temperature
3.30
net positive suction head required NPSHR
NPSH that results in a 3 % loss of head (first stage head in a multistage pump) determined by the vendor by
testing with water

5.1.10 The vendor shall specify on the data sheets the NPSHR based on water [at a temperature of less than 65 °C (150 °F)] at the rated flow and rated speed. A reduction or correction factor for liquids other than water (such as hydrocarbons) shall be applied. The purchaser should consider an appropriate NPSH margin in addition to the NPSHR specified. An NPSH margin is the NPSH that exists in excess of the pump’s NPSHR. It is usually desirable to have an operating NPSH margin that is sufficient at all flows (from minimum continuous stable flow to maximum expected
operating flow) to protect the pump from damage caused by flow recirculation, separation and cavitation. The vendor should be consulted about recommended NPSH margins for the specific pump type and intended service.

In establishing NPSHA, the purchaser and the vendor should recognize the relationship between minimum continuous stable flow and the pump’s suction-specific speed. In general, minimum continuous stable flow, expressed as a percentage of flow at the pump’s best efficiency point, increases as suction-specific speed increases. However, other factors, such as the pump’s energy level and hydraulic design, the pumped liquid and the NPSH margin, also affect the pump’s ability to operate satisfactorily over a wide flow range. Pump design that addresses low-flow operation is an evolving technology, and selection of suction-specific speed levels and NPSH margins should take into account current industry and vendor experience.
Unless otherwise specified, the datum elevation shall be the shaft centreline for horizontal pumps, the suction nozzle centreline for vertical in-line pumps, and the top of the foundation for vertically suspended pumps.

so, yes the distance is required in order to obtain npshr/a or you need to coordinate the npsh datum level with suppliers to establish the npsha/r.

good luck!
-pmover

 

[mechengr77]

pmover,
Thanks for the feed back. Another question has come up that has me thinking. Below are 3 different vertical can pumps. From what you mentioned, pumps 2 and 3 make since in the spec sheet NSPHr is less than the graph because of the centerline being referenced. But how can pump 1 increase (more than double) the NSPHr from the graph?

Pump Graph NSPHr Spec Sheet NSPHr Spec Sheet NSPHa
1 21 44 Ample
2 37 28 34
3 19 5 8

 

[dang3114]

who could help me to explain more about NPSH, I do not know why the NPSHr must be less than NPSHa atleast 1m. I know it comes from avoiding cavitation but can not know how to buid that NPSHa formular. For example, we often use the Bernulli equation to buid almost formular in hydraulic calculation.
If I have the function of NPSH, I will have its graph and understanding NPSH deeply.
Thank you very much.
Dang.

 

[TD2K]

The 1 m difference between NPHSR and NPSHA is a recommendation to provide a safety factor.

Calculations aren't exact, fluid properties are not exact and the 1 m is a common value I've seen. Some people use more, some people use a % with a minimum 'do not exceed' value.