Liquid Ring Vacuum Pumps: Types, Selection & Ordering a Nash Replacement
How it works
A liquid ring pump is a positive-displacement machine, not a dynamic one. An impeller sits eccentrically in the casing; sealing liquid thrown outward by rotation forms a ring against the casing wall. Because the impeller is off-centre, the space between two blades opens up over the first half-turn — pressure there falls and gas is drawn in — then closes over the second half-turn, compressing that gas until it is pushed out through the discharge port. Every revolution repeats the cycle in every blade cell, so the flow is continuous.
Two consequences follow from that geometry, and they explain most of what buyers ask us:
- The liquid does the sealing, so wet and dirty gas is not a problem. Condensable vapour entering the pump simply condenses into the ring; slugs of liquid that would wreck a dry pump pass through. This is why liquid ring machines dominate paper, sugar, mining filtration and condenser extraction.
- The liquid's vapour pressure sets the floor. With 15 °C water you can reach roughly 33 mbar abs; with hot water in a summer plant, far less. Nothing about the pump changes — the physics of the seal liquid does. That is why we always ask for the seal-liquid temperature before quoting, and why chilled liquid or a Roots booster is the route to deeper vacuum.
Compression also generates heat, and part of the ring leaves with the discharged gas, so the pump needs a continuous supply of cool make-up liquid — once-through, partially recovered, or a fully closed loop with separator and heat exchanger.
Single-stage vs two-stage
Single-stage pumps (our 2BE1, 2BV, NEV2, SX, XL and SK types) cover most duties down to about 33 mbar abs. Two-stage and two-stage cone types (TXV, DLV, AT/TC, CL, Premier 2620 class) reach lower — roughly 25 mbar — because the second stage starts from an already-reduced pressure.
Inside a single-stage machine the parts that matter are few: pump body, impeller, the port (distribution) plates at each end, shaft, and the covers. Clearance between impeller and port plate is set by a locating bearing, and that clearance is what decides internal leakage from discharge back to suction — which is to say, it decides the attainable ultimate pressure. A worn port plate shows up as vacuum that will no longer hold, long before anything sounds wrong.
Valve plates on the port plates let gas escape as soon as the pressure between the blades reaches discharge pressure, instead of over-compressing it to the geometric limit. That is an efficiency feature, not a sealing one: it cuts the power drawn at deep vacuum.
Shaft sealing is either gland packing or a mechanical seal. Packing is flushed and cooled by sealing water fed through a port in the cover, is cheap to re-pack in the field, and is expected to weep slightly. A mechanical seal runs leak-free and suits clean or regulated duties, but wants cleaner seal liquid and is a workshop job to change. Cone-type two-stage machines add the cone itself as a wear part.
What to specify to select a pump
- Required flow (m³/h) and vacuum (mbar)
- Gas composition and whether it is wet / corrosive
- Sealing liquid available (usually water) and its temperature
- Frequency and voltage (50/60 Hz, 220–660 V)
- Quantity and target budget
Send us the original model if you are replacing a Nash, SIHI or Siemens unit — we match it to an equivalent and confirm with a drawing.
Ordering a dimension-compatible replacement
1. Send the original brand + model (or a nameplate photo). 2. We check flange size, orientation, bolt pattern and shaft height against the original drawing and reply with the equivalent + outline drawing. 3. You confirm the drawing, we quote. If the drawing doesn't match, we don't quote.
Where liquid ring pumps are used
Paper machine vacuum, power-plant condenser evacuation, distillation and solvent recovery, sugar crystallization, chemical process, water and general industry, mining and coal-bed duties.
Browse the pump series by capacity, or send us your duty — we reply within one working day with the equivalent and a drawing for your confirmation.