Suction recirculation can undermine operation of a centrifugal pump at low flows. The pump’s suction specific speed and suction energy can provide insights about potential difficulties (see: “Cut Pump Speed to Cut Problems"). Variable frequency drives (VFDs) frequently can provide a solution (“Consider VFDs for Centrifugal Pumps").
Fixing suction recirculation problems often costs money. Typically, getting the funding requires convincing the holder of the purse strings of what’s the real cause of a problem.
Many people have trouble grasping the idea that flow can go backward within a centrifugal pump. Over the years, I’ve often struggled to find a fundamentally correct but simple explanation.
First, we must realize that what’s simple to a mechanical engineer may appear complicated to a chemical engineer. So, let’s explain the phenomenon in ways both groups can understand.
For mechanical engineers, a centrifugal pump increases the pressure of a liquid stream. The natural direction of fluid flow is from high pressure to low pressure. Fluid flow through the pump occurs because the rotating impeller provides velocity to generate pressure. The velocity gradient creates the discharge pressure.
In a centrifugal pump, liquid enters the suction eye at the center of the impeller disc. The liquid changes velocity before exiting the pump at the impeller periphery. To deal with the geometry of flow from the center to the edge of the impeller and changes in velocity, the flow passage shape changes. Figure 1 shows an end-on view of an impeller that’s rotating counter-clockwise and Figure 2 shows a side view.
Inlet flow contacts the impeller’s leading edge at a specific incidence angle (Figure 1). Vector analysis of liquid flow directions shows the incidence angle affects eddy formation. As flow rates drop, the eddies formed become larger. Eventually, eddies can create partial flow from the pump discharge to the pump suction. The local flow pattern follows the outline shown in Figure 1.
For chemical engineers, it’s better to consider how pump flow patterns interact with material balance boundaries. A series of material balance boundaries through the pump always will have the same net flow as the pump suction flow. If pump suction flow drops, the net flow through any cross section of the pump drops. Because typical fluids are incompressible at most conditions, this creates an absolute requirement that average velocity in any cross section varies linearly with flow rate.
However, close to the pressure edge (the leading surface) of the impeller, impeller speed sets liquid velocity. If average liquid velocity must drop but velocity in that area of the flow passage is close to constant, velocity in other areas must fall even more than the average. At some point, when average velocity decreases enough, the flow direction in areas far from the impeller’s leading edge must reverse to meet the average velocity requirement. Figure 2 shows a schematic of net flow in a pump at its best efficiency point (BEP) and at a low flow condition (0.25 of the BEP). At low rates, the flow passages in the pump are simply too big. Nevertheless, they must be filled with liquid. Flow recirculation results.
Flow recirculation can damage the impeller due to cavitation caused by vaporization in the low-pressure regions that recirculation creates. Flow recirculation also stresses pump components with unbalanced forces and vibration.
Both mechanical and process changes can reduce the consequences of flow recirculation.
Mechanical solutions focus on the pump. For instance, impeller-volute-geometry matching and vane-angle, leading-edge and inlet-eye modifications, as well as pump speed changes all can improve pump flexibility. However, each affects efficiency, discharge head and capacity differently.
One process modification adds a recirculation loop to keep the pump out of the low flow region. Recirculation systems require extra equipment (piping, restriction orifices, control valves, etc.). In my experience, many flow control loops are abandoned due to maintenance costs or ignorance of their importance.
Another process modification provides excess suction head to the pump. This helps prevent cavitation-like damage. Even the low-pressure regions in the pump have sufficient head to keep the fluid above its bubble point. However, extra suction head doesn’t solve stress and vibration problems. At some point, pump vibration may exceed good practice values. Extra stress and vibration decrease mean time between repair and mean time between failure. Maintenance costs rise with high vibration and stress. Reduced operating speed lowers vibration and pump loads.
While it may not be a perfect solution, switching to a VFD can benefit nearly every centrifugal pump service suffering from inlet recirculation.
Picking the right option requires a thorough analysis of both mechanical and process constraints and costs.
ANDREW SLOLEY is a Chemical Processing Contributing Editor. You can email him at ASloley@putman.net