19 Sep, 2018

Mechanical Engineering Terms

Collection of mechanical engineering Terms of the Week including examples and expanded definitions.

Best Efficiency Point (BEP)

Definition:

The flow on a centrifugal pump at which the efficiency is highest. Centrifugal pump vibration varies with flow, usually being a minimum near BEP and increasing as flow is increased or decreased.  

A centrifugal pump's operating flow range can be divided into two regions, one termed the best efficiency or preferred operating region, over which the pump exhibits low vibration, the other termed the allowable operating region, with the limits, both high and low, defined as those flowrates at which the pump's vibration reaches a higher (but still acceptable‖ level). Operating outside of the allowable or preferred operating regions will result in increased seal failures and shorter bearing life. 

 

Balance Piston 

Definition:

A device that is used to reduce the axial thrust of a multi-stage pump or compressor.  The most common method of balancing opposing forces is with a balancing drum or piston.  A solid metal cylinder is fitted to, and made a part of, the shaft just behind the last impeller, and is fitted with a labyrinth seal.  The labyrinth seal allows a small amount of the full discharge pressure gas to leak across the piston and returned to the compressor or pump suction through a balance line.

 

Labyrinth Seal

Definition:

A mechanical seal that restricts leakage by forcing the gas or liquid to flow across a tortuous path.  Some example applications are interstage seals in multi-stage pumps and compressors to minimize internal leakage, and a component of balance pistons that are used minimize the axial thrust across the pump or compressor.

 

Volute   (single stage pump)

Definition:

The volute of a centrifugal pump is the casing that receives the fluid being pumped by the impeller, maintaining the velocity of the fluid through to the diffuser. As liquid exits the impeller it has high kinetic energy and the volute directs this flow through to the discharge. As the fluid travels along the volute it is joined by more and more fluid exiting the impeller but, as the cross-sectional area of the volute increases, the velocity is maintained if the pump is running close to the design point. If the pump has a low flow rate then the velocity will decrease across the volute leading to a pressure rise causing a cross thrust across the impeller that we see as vibration. If the pump flow is higher than design the velocity will INCREASE across the volute and the pressure will decrease according to the first law of thermodynamics. This will cause a side thrust in the opposite direction to that caused by low flow but the result is the same - vibration with resultant short bearing and seal life.  

 

Fatigue

Definition:

F. Braithwaite defined fatigue as “the cracking of metals under repeated loadings”.  Cracking occurs when the stress loadings are higher than the endurance limit.  To avoid fatigue failures make sure the cyclic stresses are below the endurance limit.