Piping Design Section 02

There are several important considerations in the design of a suction supply tank or sump. It is imperative that the amount of turbulence and entrained air be kept to a minimum. Entrained air will cause reduced capacity and efficiency as well as vibration, noise, shaft breakage, loss of prime, and/or accelerated corrosion. The free discharge of liquid above the surface of the supply tank at or near the pump suction can cause entrained air to enter the pump. All lines should be submerged in the tank, and baffles should be used in extreme cases as shown in Fig. 4.

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Fig. 4 Keeping Air Out of Pump
Improper submergence of the pump suction line can cause a vortex, which is a swirling funnel of air from the surface directly Into the pump suction pipe. In addition to submergence, the location of the pipe in the sump and the actual dimensions of the sump are also important in preventing vortexing and/or excess turbulence.

For horizontal pumps, Fig. 5 can be used as a guide for minimum submergence and sump dimensions for flows up to approximately 5000 gpm.

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Fig. 5 Minimum Suction Pipe Submergence and Sump Dimensions

Baffles can be used to help prevent vortexing In cases where it is impractical or impossible to maintain the required submergence. Fig. 6 below shows three such baffling arrangements.

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Fig. 6 Baffle Arrangements for Vortex Prevention
On horizontal pumps, a bell should be used on the end of the suction pipe to limit the entrance velocity to 3 - 8 feet per second. Also, a reducer at the pump suction flange to smoothly accelerate and stabilize the flow into the pump is desirable.

The submergence of the suction pipe must also be carefully considered. The amount submergence required depends upon the size and capacity of the individual pumps as well as on the sump design. Past experience is the best guide for determining the submergence. The pump manufacturer should be consulted for recommendations in the absence of other reliable data.

For larger units (over 5000 GPM) taking their suction supply for an intake sump (especially vertically submerged pumps), requires special attention. The following section (intake System Design) addresses these larger pumps.