Is suction casing velocity of 20 fps too high?
Is suction casing velocity of 20 fps too high?
(OP)
Typical suction piping design velocities are given as 4 - 7 fps (Crane TP410). What about the velocity in the plane of the face of the suction nozzle? Pump selections by pump application engineers are often much higher, 20 fps for example. Is there a guideline for a minimum pump suction size for an application? My application is 3000 to 6000 gpm at 110 feet, chilled water.





RE: Is suction casing velocity of 20 fps too high?
One problem I can think of is vortexing if you are drawing from a vessel. For example, at a suction velocity of 15 FPS, approx. 15 Ft. of suction submergence is required otherwise air can be entrained.
RE: Is suction casing velocity of 20 fps too high?
This is a closed system application and pulling from a 30 inch header 18 feet overhead.
RE: Is suction casing velocity of 20 fps too high?
I would be sure to check the available NPSH against that required by the pump, since as velocity increases, NPSH available decreases due to increased piping losses.
RE: Is suction casing velocity of 20 fps too high?
Since this is a closed chilled water system, the NPSHa is many times greater than the NPSHr. The velocity is very low, ~ 7 fps in the suction piping except at the reducer just prior to the suction flange. At the suction flange the velocity is very high, but only for a distance of a few inches, so that translates into an insignificant pressure drop.
- Ken
RE: Is suction casing velocity of 20 fps too high?
For your application, the suction head of 18 feet alone appears to be inadequate for a 4500 GPM average flowrate if your closed system is not pressurized and your pump running speed is more than 900 RPM synchronous. Calculations based on pump specific speed using Stepanoff's Sigma parameter equation 12.14 give, for 4500 GPM and 110 ft. of head, approximate (and not certainly conservative) NPSHR values of 20.5, 35.1 and 88.3 ft. for pump running speeds of 1164, 1746, 3492 RPM (all 3% motor slip). For suction piping velocities of 4, 7 and 20 fps, the required suction pipe diameters at 4500 GPM are 21.5, 16.2 and 9.5 inches, respectively. Where does your cited large NPSH margin come from?
RE: Is suction casing velocity of 20 fps too high?
The system is a pressurized closed loop. The NPSHa is 150 to 170 ft. The pumps are 3000 gpm and 6000 gpm. The NPSHr for the pumps are 21.3 and 10.2 respectively.
I have found out that some pumps are designed as high suction energy pumps (HSE) and have velocities in the 20 fps range. They apparently perform adequately as long as the NPSHa is 3-4 times the HPSHr.
- Ken
RE: Is suction casing velocity of 20 fps too high?