My liquid ring pump will not pull the vacuum it used to — what do I check first?

Measure the seal-liquid temperature before you touch the pump. On seven or eight cases out of ten that is the whole answer, and it costs nothing to check.

The reason is physical, not mechanical. A liquid ring pump cannot pull below the vapour pressure of its own seal liquid — the liquid simply boils in the suction zone and fills the cells with its own vapour. With water at 15 °C the floor is about 33 mbar; at 30 °C it is roughly 42 mbar; at 40 °C you are near 74 mbar. A plant that changed nothing but let the cooling water creep up in summer has a pump that looks broken and is behaving exactly as designed.

Check the seal-liquid flow next, for the same reason. Too little flow and the ring runs hot and thin; too much and it fills the cells and throttles capacity. The rated flow is in the manual and is usually a surprisingly specific number — it is not a range to be eyeballed.

Only when temperature and flow are right should you open the machine, and then the first thing to measure is the porting face. On a flat-port machine (2BE, 2BV, NEV2, SK, SX) that is the port plate; on a cone machine (AT, ATTC, TC, CL, 2SK) it is the cone. The face wears, the axial clearance to the impeller opens, and gas leaks internally from discharge back to suction. Noise and motor current do not change, which is why this is the part most often missed.

The cone machines have a repair route the flat-port machines do not: the cone position is adjustable, so taking up wear can restore performance without replacing the porting part. Check how much axial adjustment is left before you quote a new cone.

Send the pump model, the serial number, the seal liquid and its temperature, the current ultimate pressure and whether the shaft is packed or has a mechanical seal. We will tell you whether this is a cooling problem or a clearance problem before you spend anything.