What are the requirements for uniform flow into a pump suction?
In general, the flow of liquid into any pump should be uniform, steady and free from swirl and entrained air. Lack of uniformity can cause the pump to operate away from the optimum design condition/curve and at a lower hydraulic efficiency.
Unsteady flow causes the load on the impeller to fluctuate, which can lead to noise, vibration, bearing problems and fatigue failures of pump shafts.
The negative impact of these phenomena on pump performance depends on pump specific speed and size, as well as other design features of the pump that are specific to the given pump. A comprehensive assessment of which pump types may be expected to withstand a given level of adverse phenomena with no ill effects has not been performed. However, in general, larger pumps and axial flow pumps (high specific speed) are more sensitive toadverse flow phenomena than smaller pumps or radial flow pumps (low specific speed).
American National Standards Institute (ANSI) standards for the design of pump free surface intakes and suction cans (ANSI/HI 9.8 Rotodynamic Pumps for Pump Intake Design), and for pump piping (ANSI/HI 9.6.6 Rotodynamic Pumps for Pump Piping), provide standard practices for the design of pump intakes and piping that provide flow profiles into the pump that will not adversely affect the pump.
However, it is not always possible to design intakes or piping per the standard designs, and there are instances where pumping equipment is large and additional consideration is required.
In these cases, the standards mentioned allow for physical model studies to be conducted with stated acceptance criteria to qualify the design per the standards.
A physical hydraulic model is a reduced-scale replicate of the geometry that controls approach flow patterns, operated according to certain similitude laws for flow, velocity and time. The physical model testing qualifies the intake and pump piping design to the standard by ensuring that:
- the surface vortices and subsurface vortices are below a certain classification and frequency
- the swirl angles are consistent and below a certain degree
- the time-averaged velocities at a point in the throat of the inlet bell or at the pump suction are within a certain percentage of the cross-section velocity
- the flow for double suction pumps are equal (within a specified percentage) on each side of the pump
For more information on designing for uniform flow entering the pump, refer to ANSI/HI 9.6.6 and ANSI/HI 9.8 at www.pumps.org.