Pump sump

Many pump installations possess an open vessel, a pump sump, from which the pump receives the liquid to be transported via a suction pipe. The pump performance stated by the manufacturer is based on an uninterrupted flow to the pump inlet. Defective design of pump sumps and suction pipe will result in inferior pump performance and increased running costs. The commonest faults affecting the function of a pump due to defective pump sump design are:

• admixture of air
• the formation of foam
• the uneven distribution of velocities in the pipe inlet
• the formation of undesirable turbulence and too low pressure at the pump inlet

All these faults are dependent on the properties of the pumped liquid and lead to different degrees of reduced pump performance for various types of pumps. Viscous liquids and suspensions demand as a rule the utmost care with regard to the design of the suction side of the pump installation.

In order to prevent the admixture of air as far as possible, the mouth of the pump inlet should be located below the surface of the liquid. In certain cases, however, the intake must be placed above this level. Figure 1.4a illustrates
different arrangements of pump intakes.

Low pressure at the pump intake with consequent risk of cavitation can be avoided by means of small losses in the suction pipe and restricted suction lift. Whenever possible, the pump is placed to advantage below the level of the liquid in the sump, thereby permitting the liquid to flow downwards into the pump (supply head).

Figure 1.4a Pump sump intakes

The volume of the pump sump represents a compromise between, on the one hand, economic factors associated with low cost for the plant and, on the other hand, some of the criteria: maximum permissible number of starts per hour in the case of on-off regulation or adequate stopping time for venting.

By way of example of the effect of liquid properties in this respect, mention can be made of the different rates of ascent of air-bubbles in oil and water. The design of the pump sump should be such as to ensure even flow velocity of the liquid. Local high velocities in crease the risk of the formation of turbulence and ad mixture of air. Areas of very low velocity have a tendency to collect particles which may be present in the liquid. A sensitive place is the connection of the suction pipe to the pump. There the risk of unfavourable flow is considerable.

Figure 1.4b illustrate examples of unsuitable designs. Figure 1.4c illustrate examples of suitable designs.

Figure 1.4b Less successful connections of suction pipes (pumpportal.com)

Although the numerical values stated in Figure 1.6 (below) refer to water, the design of the connections are also applicable to other liquids.

Recommended designs for the connections of suction pipes. The numerical values refer to water and a single pump.