High Radial Thrust
High radial thrust is often the common root cause of a single-stage pump failure. The pressure distribution around the pump casing is rarely uniform and leads to a radial force that deflects the shaft. The radial force is lowest at the BEP and increases at capacities away from BEP to a maximum at shutoff.
High radial load causes vibration, which can shorten seal and bearing life and, in extreme cases, can fatigue shafts. Industry standards limit radial deflection for single-stage pumps to 0.002 inches. This level of deflection is low enough to prevent most shaft failures, but seal life can be shortened if operated at low flow for extended periods of time.
If a pump does not have adequate flow, it can build up discharge pressure. When this pressure reaches a certain level, the pump cannot overcome it and the fluid starts to reverse flow. The reverse momentum causes the pump speed to slow down and the impeller slips backward, building pressure again. The fluid shifts back and forth. This cycle will repeat and could cause premature wear of thrust bearings.
Low-flow operation results in a mismatch of flow incidence angles in the impeller and diffuser vanes. This can lead to the formation of vortices that shake the rotor assembly at sub-synchronous frequencies. Long-term vibration can result in fatigue of impeller shrouds or diffuser plates.
At reduced flow, centrifugal pumps can experience a flow reversal where the fluid turns and flows back upstream. This results in internal recirculation, which is often referred to as suction recirculation. Internal recirculation is often a difficult problem to understand. It occurs at reduced flow rates when more liquid approaches the eye of the impeller than can pass through the pump.
Every pump has a point where recirculation begins—a point inherent to the impeller’s design. Internal recirculation causes the formation of vortices with high velocities at their core and a lowering of the static pressure at that location. This leads to cavitation, pressure pulsations and noise that can interfere with the pump’s operation and damage the impeller.
The location of the cavitation damage indicates that the problem is either internal recirculation or classic cavitation resulting from low NPSH. If the damage is on the inlet side of the impeller vanes, the cause is classic cavitation. If the damage is in the hidden pressure side of the vanes, the cause is internal suction recirculation.
Minimum Flow Protection Systems
End users can employ three main methods of minimum flow protection: continuous bypass, automated flow-controlled recirculation and self-contained automatic recirculation valves (ARVs). Each has its advantages and disadvantages, which are outlined in Table 1.
A continuous bypass system circulates liquid continuously regardless of the system demands for fluid. A fixed orifice in the bypass piping reduces the pressure and is sized to bypass adequate liquid to protect the pump.
The required NPSH increases as the pump operates further out on its performance curve, so the addition of a continuous flow volume will often require a larger pump/driver.
Because of the economic disadvantage during operation, continuous recirculation should be confined to small-volume, low-head pump applications. When the energy to bypass minimum flow exceeds 10 brake horsepower, an alternate method can often be justified.
Automated Flow-Controlled Recirculation
Another approach is to install an instrumented flow-control loop, which opens (bypasses) liquid at low flows and closes when process demand exceeds pump minimum flow. A typical system includes a flow meter, bypass control valve with its related automation, and mainline check valve. An orifice or other backpressure-creating device may be required to prevent flashing in the bypass valve and return pipe.
These valves have multi-purpose built-in functions, including mainline check valve, flow-sensing elements, bypass flow-control valve and bypass pressure-reducing valve.