Minimum Require GPM for Pump / NPSHa

[Ultganon2437]

Hello there, I have moved into a new position that has to do with piping and pumps in a ship construction industry and part of my assignment is to verify a pump and make sure that everything will work fine. I am just learning about pumps as id previously worked in 3d modeling.

My question is that a client has spec'd out a certain pump that they want to use, whose minimum required GPM is based off the MVR standards for bilge pumps. I'm not sure how it applies but I have heard that a 10% safety margin is supposed to be used. Is this 10% supposed to be added to the minimum required GPM to find a correct pump or as long as the NPSHa is above NPSHr by a factor of 10% then all is good?

To go a little more in depth the minimum required discharge is 480 GPM and the client has chosen a pump with a max rating of 490 and shutoff head of 47 ft which is good, but if the 10% is added to the 480 GPM then that would make it 528 GPM and based on the pump curve then the shutoff head would need to be higher based on the impeller they have chosen.

So what is the correct approach to follow? Any help is appreciated, Thanks

 

[Artisi]

You need to dig a little deeper into what is what, is 480 the flow required to be a specific operating condition, or just some arbitory number, what is the head required from the pump at this flow?
You also need to understand the inlet condions regarding NPSHA/R, that's actual not assumed,
So, establish real numbers before adding margins, this way you get a properly selected pump for a specific duty, not a pump that will over or under perform in operation.

You have given a min flow of 480 and a max flow of 490 and a shutoff head of 47 ft (shut head to pump guys this means at zero flow) doesn't really tell us what the client wants, might be very clear and understandable if the thinking was further explained.


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.)

 

[1503-44]

You need BEP requirements to select the proper pump.

 

[LittleInch]

What's "MVR"? Don't assume we are familiar with all your abbreviations. "Minimum variable Rate"?? Marine something?

Don't add margins without knowing first the true requirement otherwise you find yourself adding margin on margin and ending up with a pump 30% bigger than you need that works poorly.

You need to see what the system curve looks like first.

With NPSH, a margin of 2-3m is recommended. Percent doesn't work. NPSH doesn't mean the cavitation limit so you could be NPSHA> NPSHR, but your pump is sitting there busily cavitating itself to death.

Operating pumps close to the manufacturers recommended minimum flow is bad practice and inefficient.

You need to understand the system and the operation so that you get the best pump you can for the duty.

We have about 10% of the required information to help you further.



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

 

[Artisi]

Think the OP has all that's necessary other than the important info like the flow, the discharge and inlet static conditions are and an idea of the pipe layout - or a full understanding of what is required - but never mind, we have solved tougher one's than this

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.)

 

[1503-44]

Yes. I'm moving on.

 

[SandCounter]

Normally margin is added to the pump power, not head or flow. You mention MVR, are you referring to ABS MVR? I do not see the 10% requirement in those rules for bilge pumps.

 

[1503-44]

We normally buy such that the impeller can be upsized by 10%. 10% additional power comes with.