Would over-rated motor damage pump-screw?

Q – What role would moment of inertia of the motor play in damaging, rather, shearing the power screw of a triple screw pump? Salient details are –

1. Pumped liquid – lubricating oil
2. Capacity 160 lpm
3. Rated pressure – 180 bar
4. Drive – 60 kW, 2900 rpm
5. Mounting – Vertically into a 36 m³ oil tank. Minimum submergence recommended to avoid vortex-formation, air entrainment, dry running is 40 mm. The recommendation is not always followed.
6. Constructional features – There is no packed gland or mechanical seal. Pump shaft or the power screw has a balance piston on the driver side, with close clearance between balance piston and pump casing. Leakage if any, maximum 0.5 lpm, would flow over the pump and drip back into the oil tank.
7. Material strength and diameter of power screw – 24mm, 16CrMnS5, with minimum UTS of 570 MPa

by C. S. Cowlagi, anucool16@rediffmail.com

Answer – To get a clarity for myself, on the role of moment of inertia of the motor, I would paraphrase the question a little differently. “Though a pump requires only 60 kW motor, would a 100 kW motor, inadvertently or even wantonly connected, cause damage, rather shearing of the pump shaft?” My logical answer would be “No”.

A 100 kW motor would have much higher moment of inertia than a 60 kW motor. If the shaft has suffered seizure, a 100 kW motor has rather, a better chance of overcoming the seizure and make it run than a 60 kW motor.

The root cause of the failure seems to be the seizure of the shaft than excessive moment or torque imparted by the motor.

Standards for motors specify pull-up or starting or locked rotor or breakaway torque to be 150 percent of full-load torque. This testing is typically done by locking the rotor. This actually simulates a seized shaft and demonstrates capacity of the motor to overcome seizure.  A motor does try to overcome a seizure. Excessive torque may get imparted in this effort of the motor to overcome the seizure. But the demand for excessive torque comes from a seized shaft. If the shaft is not seized and does not demand any exccssive torque, the motor will not impart, by its own volition, any torque more than what the driven shaft demands. If the driven shaft demands only 10 kW load from a 60 kW motor, the motor would provide just as much. That is what is called as part-load running of the motor. Efficiency of the motor would of course be poor in such part-load running. Motors have power demand of their own, even when running on no load or zero load. So, motors can run all the way from zero load or no load to full load and somewhat beyond full load, which is overload. Basic fact is that motors respond to the demand. They do not impose load on to the driven equipment. They impose load on to the supply system, not on the driven system.

Root causes of the shear of a driven shaft would be misalignment, thermal load, seizure. Possible causes for a shaft of a screw pump to suffer seizure would be dry running.

 

Incidentally, if both the power screw and the idler screw have same metallurgy and are, of course, running in frictional contact, they are susceptible to suffer electrolytic galling and consequently a seizure, more so in dry running.

 

To prevent dry running in the given installation, it seems that a level controlled interface with the motor’s starter would be a good protection. The other check should be on using dissimilar metallurgies to avoid electrolytic galling at surfaces in frictional contact.