Liquid Ring Vacuum & Compressors

Have you ever wondered how vacuum pumps work? It’s all part of a process. Here’s what it looks like:

LRVPs are rotary displacement pumps which operate to an essentially isothermal process. An impeller is mounted eccentrically to the axis of a casing, and the liquid ring, which consists of service liquid, circulates concentrically inside the axis of the casing. The seal liquid leaves and re-enters the impeller buckets in a piston-like motion. Gas is then aspirated into the liquid ring via the suction port. The void created in the centre holds gas drawn in through the inlet port of the pump. As the gas travels around the pocket of air in the centre, it is compressed. This compressed air is then fed back out through the discharge port.

LRVPs can achieve a maximum vacuum of 95% barometric pressure, dependent on the type of sealant liquid used.

When operating at altitudes higher than sea level, atmospheric pressure decreases, which means that the higher you are, the less vacuum you can achieve. It’s important to take note of this as it affects vacuum pump performance. While flow is not affected, operation at higher altitudes reduces the capacity and maximum vacuum levels achievable.

Air and gas temperature also affect the performance of a vacuum pump. Dry, hot air causes water to evaporate, requiring a capacity increase. Saturated air, on the other hand, creates condensation, reducing the required capacity.

Hot or cold seal water affects vacuum pump performance in different ways. Dry air and hot water results in reduced capacity as there is vapour pressure in the impeller buckets. Dry air and cold water, however, sees an improvement in capacity. Saturated air and hot water has no effect on capacity if water temperature is equal to process temperature, but it can cavitate.
Saturated air and cold water has a condensation effect, which shows a reduction in capacity.

Problems typically seem with this system are:
– Carry over of high amps and vibration
– Pressure drops – drop in vacuum and capacity
– Flooding the pump
– Water or product from the pump exhaust

Troubleshooting tips:
1. Vacuum: If there is no vacuum, it could be due to a missing liquid ring, or leaks. If you do not have enough vacuum, more airflow is required, and you will need to increase pump speed. If you’re experiencing too much vacuum, reduce the airflow and bleed air into the system.
2. High Amps – kW: The seal water flow rate is too high. The pump could be flooded, or there might be restricted or blocked discharge, there could be a carry over from process, the bearings could be faulty, or the gland packing is too tight.
3. Pump Temperature – delta T: The ideal temperature is between 5 – 7C. If it is higher, increase the seal water flow rate. If it’s lower, reduce seal water flow rate.
4. Noisy Pump: It could be cavitation, meaning the seal water temperature is too high, or the vacuum level is too high, or the pump is too big. If it is the bearings, there could be no lubrication, the bearing could be damaged, or there could be high vibration due to misalignment.
5. Leakage affecting pump performance: There could be leaks in the pipes or the gland packing could be hard or dry which allow air leaks.

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