Q. What does the expression discharge recirculation signify in rotodynamic (centrifugal) pumps? When does it occur, and is it harmful to the pump?
A. Discharge recirculation is circulatory flow in the discharge area of impellers that can impose large forces on the impeller shrouds, resulting in random unbalanced axial forces and high thrust. Mechanical vibration and bearing failures can also occur.
Discharge recirculation occurs when rotodynamic pumps are operated at low flow rates, typically less than about 50 percent of the best efficiency flow rate. Under these conditions, the entire liquid volume passing through the impeller is unable to exit the pump casing and is recirculated in a random fashion on both sides of the impeller shrouds. This random action on the large surface area of the impeller shrouds creates pulsating unbalanced forces on the impeller that result in equal, unbalanced loading on the thrust bearings.
The consequences can be premature failure of thrust bearings. In addition, the pulsation can cause unstable movement of shaft seals that can lead to premature seal failures.
When operation at low flow rates is an infrequent operating condition, a correction for this problem is to add a recirculation line from the pump discharge to the suction so the pump can operate at the best efficiency flow rate. Some users also mitigate the situation by installing close clearance rings in the pump casing near the impeller's discharge diameter.
If the pump frequently operates at low flow rates, the system should be reviewed to determine whether an impeller diameter reduction is appropriate. Variable speed drives may also be a consideration.
Q. When starting a rotodynamic (centrifugal) pump, should the pump discharge valve be open or closed?
A. The answer depends on the design specific speed of the pump.
When primed and operated at full speed with the discharge shutoff valve closed, a low or medium specific speed centrifugal pump (below values of 120 [6,000]) requires much less power input than when it is operated at its rated flow rate and head with the valve open. For this reason, it is advantageous to close (or nearly close) the discharge valve when the pump is started.
The input power required at shutoff on higher specific speed pumps (values above 120 [6,000]) may equal or exceed the power required with the discharge valve open. Starting with the discharge valve closed is, therefore ,not recommended.
System start-up or shutdown requirements often necessitate brief shut-off operation of most centrifugal pumps. Prolonged operation at shut-off is harmful because of:
a. Increased vibration level affecting the bearings
b. Increased radial thrust and resultant stresses in the shafts and bearings of centrifugal volute type pumps
c. Heat buildup resulting in a dangerous temperature rise of the liquid handled and of pump elements in contact with it
d. Excessive cavitation and accompanying damage resulting from internal recirculation
CAUTION: Operation of a rotodynamic (centrifugal) pump with the suction valve closed (discharge valve open) may cause serious damage and should not be attempted.
WARNING: Operation of a rotodynamic (centrifugal) pump with both valves closed for even brief periods of time is an unacceptable and dangerous practice. It can rapidly lead to a violent pump failure.
Q. We recently received an order for baseplate mounted end suction pumps with motors included. The motors were mounted on several thin shims that appeared flimsy. Should the motors be mounted directly on the baseplate?
A. No. It is common practice to mount motors on shims to provide vertical alignment of the shaft coupling prior to start-up. Alignment adjustment is usually made on the motor to avoid repositioning the pump piping.
The following description is from the HI Standard ANSI/HI 1.3, Rotodynamic (Centrifugal) Pumps for Design and Application:
A minimum of 3 mm (0.125 in) of shim pack thickness shall be provided under all drivers. Because of the pad height and parallelism tolerances, it is often necessary to design for a shim pack thickness of 9 mm (0.375 in) under the driver to ensure that the minimum of 3 mm (0.125 in) will be achieved.
Baseplates are not normally designed with shims under the pumps, although drive trains involving three pieces of equipment may require shims under the pump for alignment. Good engineering practice limits the number of shims in a pack to five or less. Installation instructions shall emphasize the importance of minimizing shims.
Fasteners used in driver mounting holes should be 3 mm (0.125 in) in diameter smaller than the nominal mounting hole diameter where possible. The minimum fastener size for mounting equipment is 9 mm (0.375 in), except on 215 frame and smaller motors. When washers are used under fasteners, they shall be either hardened or extra thick.