relationship of velocity to power

[pumposervicio]

Hey guys,
One consultant asked me for 1200mm dia pump. Did my work and came out with only 900mm dia. So I tried to talked him out to use the 900 instead of the 1200. Less cost initially and lower operating cost in terms of power. He didn't agree. He said he took into considerations the velocity of water. The larger the diameter the lesser the the velocity and also the power.Is it true?
Isn't it that P=(0.163*Q*H*sG)/Eff and V=Q/A but the V is only used in the System Loss and nothing to do with the power of the pump?
Please help? And thanks a lot.
Best regards,

 

[BigInch]

Well, supposedly you are trying to provide power to move fluid through the system, right? Your equation is power required to lift fluid to a given energy level or head, the total of which must include any system losses due to fluid friction, if the fluid is supposed to move from one place to another. You can't optimize a pump design or make a proper pump selection without knowing the system curve.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Is the consultant talking about inlet diameter, impeller diameter or some other dimension - are you designing a pump for a specific duty or selecting from a set of data from a family of pumps or a particular manufacturer?

Pump power is flow X head X SG / pump efficiency as you have stated - all things being equal, pump size has little to do with efficiency, although a bigger diameter impeller may run at a slower speed and "might" put you into a more efficient area on the pump curve within a family of pumps or manufacturer.

A full hydraulic study with 1 or 2 recommendations backed by a report should be acceptable to any qualified consultant who knows what he / she is doing.

I once had the mis-fortune to come across a "consultant" who rejected a pump on the certified hydrailic test because the pump tested out at 2% higher on efficiency - his justification was - "it wasn't as specified".

We were talking about a unit that was 250Kw drive - and he wasn't interested in the power save over a years operation.

A nicely worded but very pointed fax to his company soon sorted that little problem.