Pressure increase as diameter increases

[CPorion]

I'm having a heck of a time wrapping my mind around a pump replacement I've been tasked with. Currently we have a pump which operates at 200GPM at 220#. It immediately goes into a pressure reducing valve to take it down to 110# because that is the working pressure we need. The system piping is 3" and the relief setpoint is 150#. I need to replace this pump and remove the reducing valve to save energy. So I spec a 200GPM 110# pump. The discharge flange however is 1.5". I start thinking about bernoulli's equation and now I'm concerned the change in pipe diameter will cause the pressure downstream to increase based on the reduction in fluid velocity and lift the relief valves. Is this something I need to account for in my pump spec? Based on my calculations the change in diameter would take the fluid pressure from 110# to 330#. I'm just a simple HVAC engineer please help.

 

[LittleInch]

Velocity head is usually ignored as its so small so suspect something wrong in your calculations.

So , no, there is no need to account for this small change in diameter.

 

[katmar]

I agree with LittleInch that you have an error in your Bernoulli calculation. Assuming that you are pumping something similar to water, the pressure recovery due to Bernoulli in going from 1.5" to 3" will be about 6 psi. There would also be a friction loss of about 3.5 psi through the 1.5" to 3" reducer so pressure gauges mounted before and after the diameter change would show a pressure increase of only 2.5 psi.

Pump curves usually use Total Dynamic Head (TDH) in metres or feet rather than psi for the vertical axis - unless the pump will only ever be used for one type of liquid. If you want 110 psi of static pressure at the start of the 3" section then you need this 110 plus the 0.5 psi of velocity head in the 3" pipe and the 3.5 psi friction loss in the reducer making a pump TDH of 114 psi or 263 feet if you are pumping water.

The velocity in the 3" pipe will be around 8.7 ft/s giving the velocity head of 0.5 psi, and as LittleInch pointed out this is negligible and usually within the safety margin included by the pump manufacturer. It would only be in a critical situation where you would perform the detailed calculation I have shown above to get an accurate TDH specification. In my experience, off-the-shelf pumps always out-perform the published curves and it is far more common to have to install restriction orifices during commissioning than it is to find that the pump is giving too little pressure.