How does a liquid ring vacuum pump work?
The impeller sits off-centre in a cylindrical casing that is partly filled with seal liquid, usually water. As the impeller turns, the liquid is flung outwards and forms a ring that follows the casing wall. Because the impeller is eccentric, the space between the hub and the liquid ring grows on one side of each revolution and shrinks on the other.
Where the cells grow, gas is drawn in through the suction port in the port plate or cone. Where the cells shrink, the liquid ring acts like a piston, compresses the gas and pushes it out through the discharge port together with some of the seal liquid. The liquid then has to be separated from the gas, cooled and returned.
Three consequences follow from this principle, and they shape every design decision:
- Near-isothermal compression. The gas is in intimate contact with a large mass of liquid, so the heat of compression and any condensation heat go straight into the seal liquid. Discharge temperatures stay close to the liquid temperature, which is why liquid ring pumps are favoured for flammable and heat-sensitive vapours.
- Tolerance of liquid and dirt. Slugs of condensate, carried-over process liquid and light particles pass through the pump with the seal liquid instead of scoring close clearances.
- Vapour-pressure limit. The ring liquid evaporates into the suction cells. Once suction pressure approaches the liquid’s vapour pressure, the pump pumps its own seal liquid vapour, capacity collapses and cavitation begins.
Why seal-liquid temperature sets the vacuum you can reach
This is the single most misunderstood point on liquid ring duty. Water has a vapour pressure of about 17 mbar at 15 °C, about 42 mbar at 30 °C and about 74 mbar at 40 °C. A pump that reaches a comfortable 50 mbar(a) with cool water in winter may struggle and cavitate in a Hyderabad summer when the cooling tower or once-through supply arrives at 32–35 °C and the ring warms further from the heat of compression.
In practice, designers keep a working margin above the vapour pressure of the ring liquid at its actual operating temperature, not at the temperature printed on a datasheet. Manufacturers publish capacity curves at a reference seal-water temperature, commonly 15 °C, together with correction factors for warmer water. Those corrections are large, so they must be applied before a model is accepted.
| Seal water temperature | Approximate vapour pressure of water | What it means in service |
|---|---|---|
| 15 °C | about 17 mbar | Typical reference for published curves |
| 25 °C | about 32 mbar | Noticeable loss of capacity at deeper vacuum |
| 30 °C | about 42 mbar | Common Indian cooling-water temperature; check correction factors carefully |
| 40 °C | about 74 mbar | Deep vacuum duty becomes impractical without chilling or a different liquid |
Options when the vacuum is too shallow include chilling the seal liquid, using a lower-vapour-pressure liquid compatible with the process, adding a gas ejector or a Roots booster ahead of the pump, or reviewing whether a dry technology suits the duty better.
Single-stage or two-stage?
Single-stage liquid ring pumps are the workhorses for rough vacuum, typically from atmospheric down to roughly 130–150 mbar(a) with good efficiency. Two-stage pumps pass gas through a second impeller and are used where the duty sits nearer 30–100 mbar(a), and where the seal liquid conditions allow it. Below the practical liquid ring range, the usual arrangement is a booster or an air ejector in front of the liquid ring pump rather than a bigger liquid ring pump.
The choice should come from the operating pressure across the real cycle. A pump that spends most of its life at 400 mbar(a) on a filter or deaeration duty is a different selection from one that must hold 60 mbar(a) on a dryer.
Seal-liquid circuit: once-through, partial recovery or total recovery
The liquid circuit is as important as the pump. The three common arrangements are compared below.
| Circuit | How it works | Where it suits | Watch-outs |
|---|---|---|---|
| Once-through | Fresh water enters, leaves with the discharge and goes to drain | Abundant clean water, harmless gas, small pumps | High water use, effluent load, water temperature varies with season |
| Partial recovery | Discharge separator returns part of the liquid; some fresh make-up cools the ring | Moderate water availability, tolerant process | Make-up rate and blowdown must be controlled to hold temperature and quality |
| Total recovery (closed loop) | Separator, pump and heat exchanger return all liquid; heat is rejected to cooling water | Solvent duty, scarce water, contaminated gas, non-water seal liquids | Heat exchanger sizing, fouling, level control, accumulation of condensed process liquids |
For closed loops, the heat exchanger must remove the motor shaft power that ends up in the liquid plus the heat released by condensing vapours. Undersized coolers are a frequent cause of vacuum drifting off in the afternoon.
Process gas, condensables and materials
Begin with what is being removed, not with pump size. Write down the dry gas load (air leakage and non-condensables), the condensable load (water or solvent vapour), the operating pressure and temperature at the vessel, and the composition during start-up and upset conditions.
- Condensables help and hinder. Vapour that condenses into a cooler ring increases effective pumping capacity, but it also heats the ring and, in a closed loop, accumulates as liquid that must be drained.
- Pre-condensers. A condenser ahead of the pump often reduces the load sharply and protects the ring temperature.
- Materials. Cast iron with standard seals suits clean air and water vapour. Acidic, chloride-bearing or solvent duties may need stainless steel or other alloys and compatible mechanical seals; this is a maker’s materials review, not an assumption.
- Solids and scaling. Hard water deposits scale on the casing and port plate, reducing clearances and capacity. Treat the make-up water or blow down the loop.
- Hazardous media. Flammable vapours need the area classification, motor and instrument certification and exhaust routing set by the plant’s process safety team and the manufacturer.
Where liquid ring pumps are used in India
Typical duties include vacuum filtration and drying in pharmaceutical and bulk-drug plants in Hyderabad, Genome Valley and the Visakhapatnam pharma zone; solvent recovery and distillation in chemical plants; paper machine felt and suction box duty; sugar and distillery vacuum pans; condenser air extraction and priming on power and process plants; deaeration; and plastics extrusion venting. In all of these, wet gas and carryover are normal, which is exactly where the liquid ring principle earns its place.
Where the process demands a dry exhaust, very low operating pressure or no contact between gas and liquid, a dry screw or claw pump may be the better choice. See our vacuum selection guide for the comparison.
Common problems and what we check
| Symptom | Likely causes | What to check |
|---|---|---|
| Vacuum shallower than expected | Warm seal liquid, low seal flow, air leakage into the system, worn clearances | Seal liquid inlet temperature and flow, system leak test, pump curve at actual temperature |
| Rumbling or rattling noise | Cavitation near vapour pressure | Operating pressure versus vapour pressure; fit an anti-cavitation bleed if the maker recommends one |
| High motor current | Too much seal liquid, flooded casing at start, scale build-up | Seal flow setting, drain before start, internal inspection |
| Capacity falls over months | Scale, corrosion, erosion of port plate | Water quality, materials, inspection of impeller and port plate |
| Exhaust carrying liquid | Separator undersized or level control fault | Separator level, drain valve, overflow routing |
How Reddy Compressors helps
We are an independent compressed-air and utilities engineering business based at Tarnaka, Secunderabad, working since 2002. On vacuum duty we help you define the operating pressure, pumping speed and gas load, review the seal-liquid circuit and cooling, and compare technology and supplier offers on the same basis. We sell new and used/refurbished liquid ring vacuum pumps, rent them, service them and supply their spares, as an independent multi-brand specialist.
Our service engineers work across Telangana and Andhra Pradesh. For an existing liquid ring pump, send the make, model and nameplate and we will plan its inspection, service and spares. We also sell, rent and service Roots blowers, which are commonly used as boosters ahead of liquid ring pumps, and we look after the compressed-air system on the same site.
What to send us
- Process and what the vacuum does (filtration, drying, distillation, deaeration, priming)
- Required operating pressure in mbar(a), and start-up or upset pressures
- Dry gas load and condensable vapour load, with composition
- Seal liquid, its supply temperature in summer, and current circuit type
- Existing pump make, model, nameplate photo and running hours
- Area classification and any material or effluent restrictions
Frequently asked questions
Why is my liquid ring vacuum pump not reaching the required vacuum?
The most common cause is warm seal liquid. As water warms, its vapour pressure rises and the pump starts pumping its own vapour, so capacity at deep vacuum falls sharply. Check the seal water inlet temperature and flow first, then look for air leaks into the vessel and pipework, scale inside the casing and worn clearances. Compare the reading with the maker’s curve corrected to your actual water temperature, not the 15 °C reference.
What is the lowest pressure a liquid ring vacuum pump can reach?
With water at around 15 °C, two-stage liquid ring pumps typically work down to roughly 30–35 mbar(a), and single-stage pumps are most efficient above about 130–150 mbar(a). With warmer water the practical limit rises. For deeper vacuum, a gas ejector or Roots booster is normally added in front of the liquid ring pump, or a dry technology is considered for the duty.
Can I use something other than water as the seal liquid?
Yes, provided it is compatible with the process gas, the pump materials and the seals. Solvent-recovery plants sometimes use the process solvent itself or a low-vapour-pressure liquid so that condensed vapour does not contaminate the loop. This needs a closed-loop circuit, a correctly sized heat exchanger, the manufacturer’s agreement and a review by your process safety team.
Why does my liquid ring pump make a loud rattling noise?
A gravel-like rattle usually indicates cavitation: the pump is operating too close to the vapour pressure of the seal liquid, and vapour bubbles collapse violently against the impeller and port plate. Raising suction pressure slightly, cooling the seal liquid or fitting the maker’s anti-cavitation connection normally cures it. Left alone, cavitation erodes the impeller and port plate.
Is a liquid ring pump better than a dry screw vacuum pump?
Neither is better in general. Liquid ring pumps are robust, simple and very tolerant of wet, dirty and condensing gas, and they keep discharge temperatures low. Dry screw pumps reach deeper vacuum, use no seal liquid and produce no contaminated effluent, but need more care with condensation, deposits and internal temperature. The process gas and the effluent rules usually decide.
How much seal water does a liquid ring vacuum pump need?
It depends on pump size, operating pressure, gas load and how much heat must be removed, so the figure must come from the manufacturer’s data for your duty. Once-through circuits use the most water. Partial and total recovery circuits cut consumption sharply but need a separator, level control and, for total recovery, a heat exchanger sized for the full heat load.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.