Smaller DN on pump than on pipe

[TSN79]

When I select a water pump for a pipe, I select the pump that matches the pipes nominell diameter. A DN100 pump for a DN100 pipe for instance. It only seems natural to me. But I see that others usually go down on the pump size, a DN65 or DN80 pump for a DN100 pipe for instance. Why is this acceptable?

 

[FredGarvin]

Honestly, sizing a pump based on the size of its ports is not a good practice. Size the pump for the system it is going in.

 

[Q_Goest]

Hi TSN,
Are you familiar with NPSH? Take a look at these references if not:
http://www.mcnallyinstitute.com/11-html/11-12.html
http://en.wikipedia.org/wiki/NPSH

From the first reference:

The definition of NPSHA is simple: Static head + surface pressure head - the vapor pressure of your product - the friction losses in the piping, valves and fittings.

Note this is NPSHA which is simply the “actual” pressure above saturation pressure. This can be compared to your pump’s NPSH.

For a pump to operate without cavitating, the pressure at the suction must be above the fluid’s vapor pressure by the amount of NPSH.

For example: Your fluid is water at 212 F, so this will boil at 14.7 psia. Let’s say you have 10 feet of head (about 5 psi) and the water is inside a tank at 20 psig. If the system were static, the pressure at the pump would be the 20 psig plus 5 psi static head, total 25 psi. This is well above the 14.7 psi that it boils at, call it 10.3 psi. But now let’s say that under flowing conditions we calculate there is an 8 psi pressure loss to the pump suction. We now only have 10.3 – 8 = 2.3 psi NPSHA. If your pump’s NPSH rating is more than 2.3 psi, it will cavitate. If the pump’s NPSH was 5 psi for example, then one thing you could do is simply increase the suction pipe so that the pressure loss wasn’t as great.

There’s another reason for calculating pressure drop going into a pump suction and that’s because centrifugal pumps create a dP as given by the charts/graphs for the specific pump. Finding the operating point of the pump requires you do an analysis on the system both upstream and downstream of the pump. By changing the pipe diameter upstream or downstream, you can change the operating point at which the pump will function.

Hope that helps.