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In researching failure of liquid ring vacuum pumps, operating on Vacuum Pan & Evaporator duties, Kirton Mechanical Equipment have established that at certain times the pumps operate at a very unfavourable duty point, often well within the cavitation zone. 

This is partly due to saturated vapour, at approx 55°C being carried over from the condenser & condensing out within the liquid ring, which is anywhere between 25°C (Winter) & 34°C (Summer) .

Snifter ValveRefer Daltons Law of partial pressures which applies. 

The resultant increased vacuum within the pump creates a situation where the pump is starved of inert air, which it requires to complete each compression cycle (liquid ring pumps compress air in order to create vacuum). The pump, when starved of inert air, will draw in service liquid to fill the void. This overfilling creates pressure peaks as & when the pump tries to discharge the excess water through discharge ports which are designed for an air water mix. 

This is the major cause of second stage impeller failure, in two stage vacuum pumps, in the sugar Industry. 

By fitting an adjustable vacuum breaking snifter valve to the suction casing on each pump one can remedy this situation. This is done by setting the snifter valve to open at between 15% & 10% absolute (-85Kpa & -92Kpa) & bleeding atmospheric air into the pump. The bleeding of this air into the pump will satisfy the pumps demand for inert air for compression & hence stabilise the duty point at between 15% abs & 10% abs. The duty point whilst hunting between these two points will, subject to service liquid not being too hot, remain outside the cavitation zone. 

We have now manufactured snifter valves for all the two stage pumps & recommend that all due consideration be given to fitment of this safety device. We are of the opinion that this is the first & most important step that one should take insofar as protecting liquid ring vacuum pumps against system variables.


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