Is suction casing velocity of 20 fps too high?

[KANN]

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.

 

[fredb]

Hello,

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.

 

[KANN]

Fredb -

This is a closed system application and pulling from a 30 inch header 18 feet overhead.

 

[fredb]

Hello again,

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.

 

[KANN]

Fredb -

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

 

[vanstoja]

Guidelines for suction piping design for centrifugal pumps are provided by the Hydraulics Institute Standard and the Pump Handbook (Sect. 11 on Intakes and Suction Piping in the 1976 edition) among other sources in terms of inlet velocities rather than pipe sizes. Generally, recommended velocities are 2-7 fps. I've not seen any guidelines for transitional suction area velocity changes between piping, suction flanges and impeller eye diameters. Practically, complex high flowrate plants with multiple pumps and suction plenums such as electric powerplants cannot live with such low suction pipe velocities due to space, weight and cost considerations and must somehow accomodate pump suction and discharge velocities in the 20-30 fps range. Pressurized, closed systems with ample NPSH margins can easily accomodate such velocities. Low NPSH margin applications near or at two-phase flow conditions cannot do so as readily and need the lower suction velocities or else compensating measures like 1st stage impeller inducers.
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?

 

[KANN]

vanstoja -

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

 

[BigRed2]

Be careful here. Are we talking about an industrial or a commercial appication? Chilled water pump sounds like its part of a piping system connected for building space conditioning (offfices, classrooms, etc). High pump velocities can sometimes cause objectionable piping noise. Although you can always find a differing view, in the HVAC (pump) industry, 15fps SUCTION -or 25 fps DISCHARGE are generally considered the maximum acceptable velocities in order to limit noise. Although higher efficiences can sometimes be achieved by the pump designer by pushing the velocities higher, the noise issue can easily outweigh any theoretical energy savings that may be shown on paper.