NPSHa using pressure gauge

[Marbig]

Hi everyone,

This might be embarrassing, but it has me stumped. I am looking at a pump that is possibly cavitating. What I want to see is if the NPSH available is meeting what the pump was selected for.

So the pump is designed for a NPSH of 1.5m of sea water (this is the duty point on the pump curve). Now what I want to see is what NPSH is the pump actually working with.

With the pump operating the gauge on the suction side of the pump reads 0.145 bar. What I was doing was converting this to meters of sea water using P = ρ.g.h but I dont think this necessarily equates to the NPSH.

Any help or guidance would be appreciated.

 

[Artisi]

How about posting the pump performance curve along with a schematic of the installation.

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.)

 

[georgeverghese]

In this case, assuming you've got this PG sitting right next to the pump suction nozzle, the NPSH available would be equal to the absolute pressure head less the saturation vapor pressure head for seawater at this operating temp:

ie NPSH =(101.3e3 + P - Pv)/(rho.g), where P is the reading you've got off this static pressure guage ( which is you say 0.145barG?), while Pv is the vapor pressure of water at this operating temp, atmospheric pressure= 101.3e3Pascals.

Units in the SI case would be Pascals for pressure and vapor pressure, kg/m3 for water density, g = 9.81m/sec2. We typically ignore the kinetic energy head contribution of the flowing stream to the NPSH in this approximation, assuming it is insignificant compared to the other contributors.

Must say this NPSHr of 1.5m sounds lowish for most centrifugal pumps, typically 2-3metres for low diff head pumps, unless you've got a special low NPSH first stage impeller.

 

[bimr]

Here is a method to calculate the NPSH of your pumping system.

Link to Mc Nally

 

[Marbig]

Thanks everyone! Really appreciate the replies

NPSH = (101.3e3 + P - Pv)/(rho.g)
=(101.3e3 + 14.5e3 - 2.4e3)/(1030x9.8)
= (113.4e3)/(1030x9.8)
=11.23m

NPSHa = 11.23m
NPSHr = 1.5m

I think I am missing something here or doing something wrong.

Description of the installation:
The pump sits below the water line (sea) and it also discharges below the waterline. The suction point of the pump sits approx 2.0m below the waterline.

Unfortunately I cannot access the pump curve at the moment. But there is a curve that shows head (m) vs flow and the head used for the design point is 1.5m on that curve.

Could they possibly be using a different 'head' or suction lift? What am I missing?

 

[LittleInch]

Can you sketch out as a section view the system and where your guage is?

From your description the pump inlet is below sea level?

Then if the inlet line is short, an npsha of 11.5m is perfectly feasible as described.

From the data we have at the moment I don't think your pump is cavitating but it's difficult to say. Is the pump operating on the far right of the curve? Cavitation and NPSHR are not the same thing. The cavitation curve is above npsh and sometimes a lot higher.

Is there a possibility of a restriction or blockage between the pressure guage location and the pump inlet flange?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

Your pump could also be cavitating because it is operating at a very low flow rate (below minimum continuous stable flow). In my experience suction recirculation cavitation cannot be suppressed by operating with a large NPSH margin.

Your calculation looks right based on your description. I would consider this an adequate NPSH margin if you are operating above MCSF and below about 120 percent of BEP.

Is there any chance you are getting a vortex and drawing in air?

Johnny Pellin

 

[georgeverghese]

2metres submergence seems plenty, but check if the flow is vortexing into the pump suction, drawing in air along with the seawater. What were the pump vendor's installation recommendations re minimum required submergence / vortex breaker provisions? Looks like we need to see a sketch of the inlet sump / feed arrangement. What type of centrifugal pump is this? Sounds like a submersible and not a vertical turbine from your description. NPSHa calcs look okay, so you may be pulling in air for some reason.

 

[Artisi]

As usual, a complete lack of useful information to even start any meaningful comments.

Could be a very simple solution if given the basics:

Pump type
Performance curve
Current flow and head
Sketch of installation showing relative levels etc.
Onset of the problem, old installation previously operating ok, new installation, pump has been overhauled and reinstalled.





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.)

 

[jrrocks]

Marbig, what fluid temperature you have considered? Vapour pressure 2.4kPa indicates fluid temperature below 1°C. Note that higher fluid temperature will result in reduced NPSHa.
Considering 30°C water temperature, it would result in 4.2m NPSHa.

I would like to know further. Please keep posted.