How does a rotary piston vacuum pump work?
Inside the pump, an eccentric drives a piston that rolls around the inside of a cylinder. A hollow slide valve attached to the piston passes through a pivoting guide and acts as the inlet port. As the piston turns, the space on the inlet side grows and draws gas in; the space on the other side shrinks and pushes gas out through a discharge valve, which is covered by oil. Oil fills the small clearances between piston and cylinder, so the pump can reach a low pressure in a single stage.
Compared with a rotary vane pump, there are no thin vanes sliding in slots. The piston and cylinder are heavy, with generous clearances held by the eccentric. That is why rotary piston pumps have a long reputation for tolerating process dirt, occasional liquid slugs and continuous heavy duty in industrial plants.
Oil has four jobs in this pump:
- Sealing the clearances so that gas cannot leak back from discharge to inlet.
- Lubricating the piston, bearings and slide.
- Cooling, by carrying the heat of compression to the casing or an oil cooler.
- Carrying contamination, which is useful until the oil is so contaminated that it stops sealing.
Where are rotary piston pumps used?
Typical industrial duties in India include:
- Vacuum drying and impregnation of transformers, motors and cables.
- Vacuum degassing of oils, resins and metals.
- Vacuum distillation and solvent recovery in chemical and pharmaceutical plants, often with condensers and traps upstream.
- Freeze-drying and vacuum ovens.
- Vacuum furnaces and heat treatment, often with a Roots booster ahead of the pump.
- Large-chamber evacuation for coating and testing.
In many of these duties, a vacuum Roots booster is mounted ahead of the pump to reach lower pressures or higher speed. The rotary piston pump then acts as the backing (fore) pump.
How to choose: work from the pump curve, not a single number
A pump’s nominal displacement (for example, in m³/h) is its swept volume at atmospheric inlet. Real pumping speed falls as inlet pressure drops, and every model has its own curve. Selection follows a clear method:
- Define the working pressure at the process, in mbar absolute, for each phase (pump-down, steady process, final dry-out).
- Define the gas load: air leaks, process gases and vapours released, in mbar·l/s or kg/h, for each phase.
- Allow for the line: pipe diameter and length reduce effective speed at the process; at low pressure, conductance can dominate.
- Read the required speed at the pump inlet from the gas load divided by the inlet pressure, then find a model whose curve gives that speed at that pressure with margin.
- Check pump-down time for the chamber volume from the curve, not from nominal displacement.
- Qualify the medium: vapours, corrosives, solvents, particles and liquids decide oil, materials, gas ballast and traps.
A simple screening formula for pump-down from atmosphere with no gas load is t ≈ (V / S) × ln(p1/p2), with V the chamber volume, S the effective speed in the same volume units per unit time and p the start and end pressures. It underestimates time at low pressures, where outgassing and leaks take over; use the maker’s curve and add margin.
Rotary piston, rotary vane, dry and liquid ring compared
| Pump type | Typical strengths | Typical limits |
|---|---|---|
| Oil-sealed rotary piston | Rugged, tolerant of dirty duties, large displacements in one stage, long service life with good oil care | Heavy, some vibration, oil contamination and oil mist must be managed |
| Oil-sealed rotary vane | Compact, quiet, low ultimate pressure, widely available | Vanes and slots less tolerant of particles and liquids; oil care critical |
| Dry screw or claw | No oil in the pumping chamber, no oil contamination of process, low waste oil | Higher purchase cost, sensitive to some deposits, temperature management needed |
| Liquid ring | Handles wet, saturated and condensable loads well; nearly isothermal | Ultimate pressure limited by seal-liquid vapour pressure; seal-liquid treatment and disposal |
The right choice depends on the process. Where oil contact with process vapour is unacceptable, or waste-oil disposal is costly, a dry pump may justify its price. Where the duty is dirty, wet or heavy, a rotary piston pump with good traps and oil care can be the most dependable option.
Gas ballast and condensable vapours
When a pump compresses gas containing water or solvent vapour, the vapour can condense inside the pump before the discharge valve opens. The liquid then mixes with the oil, emulsifies it and ruins its sealing. A gas ballast valve admits a controlled amount of air into the compression space, so the discharge valve opens before the vapour reaches its condensation pressure.
Points to understand about gas ballast:
- It raises the ultimate pressure of the pump, so it is used during the vapour-heavy part of the process and closed when the deepest vacuum is needed.
- Each model has its own water-vapour tolerance with ballast open; read it from the maker’s data, not from a similar pump.
- Running the pump warm helps, because hot oil holds less condensate.
- For heavy vapour loads, a condenser or cold trap ahead of the pump is better than relying on ballast alone.
Contamination, solvents and materials
Solvent compatibility cannot be generalised across rotary piston designs. Every pump has its own oil, seals, gaskets, metals and exhaust separator, and each can be attacked differently. Before committing a pump to a solvent or corrosive duty:
- List every chemical that can reach the pump, including cleaning agents and upset cases.
- Ask the maker to confirm compatibility of oil, elastomers, metals and separator media in writing.
- Choose the oil type the maker approves for that duty (mineral, synthetic or special inert oils).
- Fit inlet filters for particles, and traps or condensers for vapours and liquids.
- Plan oil changes by condition: colour, viscosity, water content and acid content tell you when to change.
Exhaust, oil mist and installation
Oil-sealed pumps release an oil mist with the exhaust. An exhaust oil-mist separator captures most of it and returns oil to the pump, but the exhaust should still be piped to a safe outdoor point, especially where process vapours are toxic or flammable. Where solvent vapour is pumped, the exhaust treatment, hazardous-area classification and emissions rules are decided by the plant’s competent engineers.
Installation points that matter in Indian plants:
- A firm base and flexible connections, because rotary piston pumps transmit vibration.
- Good ventilation; hot rooms in summer raise oil temperature and shorten oil life.
- Inlet isolation and vent valves so the pump can be warmed up and run on ballast before the process.
- Exhaust piping that falls away from the pump, with a drain, so condensate does not run back.
- Space to change oil and service the discharge valves and separator.
Maintenance and common faults
| Symptom | Likely causes | First checks |
|---|---|---|
| Cannot reach usual vacuum | Contaminated or emulsified oil, leaks in the system, worn discharge valves, ballast left open | Oil condition, blank-off test at the pump inlet, ballast position |
| Milky or emulsified oil | Water vapour condensing in the pump | Run warm with ballast, add condenser or trap, change oil |
| Oil mist from exhaust | Saturated or damaged separator element, high inlet pressure for long periods | Separator element, operating pressure profile |
| High oil temperature | Hot room, blocked cooling fins or cooler, high continuous gas load | Ventilation, cooler, gas load |
| Noise or knocking | Bearing wear, slide guide wear, liquid slug | Vibration, bearing condition, inlet trap |
A blank-off test is the simplest diagnostic: close the inlet at the pump and read the pressure it reaches. If the pump alone is good, the problem is in the system (leaks, outgassing or vapour load).
How Reddy Compressors helps
We are an independent multi-brand compressed-air, blower and gas-compressor specialist at Tarnaka, Secunderabad, serving industry since 2002, and not an authorised dealer of any brand. We have tie-ups with all major and reputed brands. We sell new and used/refurbished rotary piston and other vacuum pumps, rent them, service them and supply their spares, working alongside the plant’s process team.
- We help you put the duty into a clear enquiry: working pressures, gas and vapour loads, chamber volume, line sizes and the chemicals involved.
- We identify existing pumps from the nameplate, model and serial number and supply spares, separator elements and approved oils, confirming fit before quoting.
- We supply and service the Roots and other blowers that often form part of vacuum systems, and the compressed-air utilities that support the process.
- We can review a vacuum system that has lost performance, starting with a blank-off test and oil condition, and show whether the problem lies in the pump or the system.
Our service teams cover Telangana and Andhra Pradesh, and we supply spares across India. Solvent compatibility is confirmed by the pump maker in writing, and hazardous-area decisions stay with the plant.
What to send us
- Process type and the working pressure needed in mbar absolute, for each phase.
- Chamber or vessel volume and the pump-down time you need.
- Gases, vapours, solvents and any particles or liquids that can reach the pump.
- Pipe size and length from the process to the pump.
- For an existing pump: nameplate photo, oil type, the last oil change and the problem you are seeing.
Frequently asked questions
What is the difference between a rotary piston and a rotary vane vacuum pump?
Both are oil-sealed and positive-displacement. A rotary vane pump uses thin vanes sliding in rotor slots; a rotary piston pump uses a heavy eccentric piston rolling inside a cylinder with a slide valve. The piston design is generally more tolerant of dirt, liquid slugs and heavy continuous duty, while vane pumps are smaller, quieter and common for lighter and cleaner duties.
Why is my rotary piston vacuum pump oil turning milky?
Milky oil means water has condensed inside the pump and emulsified with the oil. It happens when the process releases water vapour and the pump runs cold or without gas ballast. Run the pump to full temperature with the ballast open during wet phases, fit a condenser or trap upstream for heavy vapour loads, and change the oil, because emulsified oil no longer seals properly.
How often should I change the oil in a rotary piston vacuum pump?
It depends on the duty. A clean, dry duty may run a long time between changes, while solvent or wet duties may need frequent changes. Follow the maker’s guidance as a starting point and then change by condition: darkening, a change in viscosity, water content, acid build-up or loss of ultimate vacuum all show the oil has stopped doing its job.
Can a rotary piston pump handle solvent vapours?
Often yes, with the right oil, materials, gas ballast and traps, but compatibility cannot be assumed from one design to another. Some solvents attack oils, seals or separator media. List every chemical that can reach the pump and ask the maker to confirm the oil, elastomers, metals and exhaust arrangement in writing. Exhaust treatment and hazardous-area decisions stay with the plant’s engineers.
Why does my vacuum pump not reach its rated vacuum?
Start with a blank-off test: isolate the pump inlet and see what pressure the pump reaches alone. If it is poor, look at oil condition, gas ballast position, discharge valves and wear. If the pump alone is good, the problem is in the system: leaks, outgassing, vapour load or a pipe that is too small or too long for the pressure you need.
Do I need a Roots booster with a rotary piston pump?
A Roots booster on top of a rotary piston backing pump increases pumping speed at lower pressures and can lower the achievable working pressure. It is common on vacuum furnaces, large drying chambers and degassing plants where pump-down time matters. Size the combination from both makers’ curves, and include bypass or pressure-switch control so the booster starts only when the pressure is low enough.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Vacuum pump-down time and required pumping speed
First check of pump size for a vessel: pump-down time at a given effective speed, or the speed needed for a required time, and the speed needed to hold a pressure against a gas load. Default: 2 m³ from 1,013 to 50 mbar(a) at 100 m³/h, with the speed needed for 3.6 minutes.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Evacuated volume incl. piping and connected equipment (unit below): 2
- Volume unit: m³
- Start pressure (mbar a): 1013
- Target pressure (mbar a): 50
- Pump speed at its inlet, average over the range from the curve (unit below): 100
- Line conductance, same unit as the speed (0 = no line loss): 0
- Speed and conductance unit: m³/h
- Continuing gas load: leaks + outgassing + vapour (0 = none): 0
- Gas load unit: mbar·m³/h
- Required pump-down time, for the speed check (min; 0 = skip): 3.6
- Liquid water in the vessel or product?: No (dry)
- Temperature of that water or wet product (°C): 20
Result
- Evacuated volume: 2 m³ = 2,000 litres
- Effective speed at the vessel: 100 m³/h = 27.78 L/s (no line loss entered)
- Pressure ratio p₀ ÷ p₁: 20.26 ln = 3.009
- Ideal pump-down time (no gas load): 3.61 min (217 s, 0.0602 h)
- Effective speed needed for 3.6 min: 100.3 m³/h = 27.86 L/s
- Pump speed entered against the need: about equal (within 2%, the rounding of a first estimate) 100 m³/h entered, 100.3 m³/h needed
Method. Ideal pump-down: t = (V ÷ S_eff) × ln(p₀ ÷ p₁), with V the whole evacuated volume (vessel, piping, filters, condensers), absolute pressures and S_eff the average effective speed over the range. Effective speed through a line: 1/S_eff = 1/S_pump + 1/C. A continuing gas load Q (leaks, outgassing, vapour, as pressure × volume flow) sets a floor p_floor = Q ÷ S_eff; with that load the time becomes t = (V ÷ S_eff) × ln((p₀ − p_floor) ÷ (p₁ − p_floor)), and a target at or below the floor is never reached. Holding duty: the effective speed needed to hold a pressure p against Q is Q ÷ p. Required speed for a target time: S_eff = V × ln(p₀ ÷ p₁) ÷ t (solved numerically when there is a gas load), then the pump speed that gives it through the line. Units: 1 L/s = 3.6 m³/h; 1 mbar·L/s = 3.6 mbar·m³/h. Water boils when the pressure falls below its vapour pressure (Magnus formula: about 23 mbar at 20 °C, 42 mbar at 30 °C). Constant speed is assumed: real pumps lose speed towards their ultimate pressure, so treat the result as a lower bound and check the maker’s curve, or use the banded tool.
What to take from it. If the target is near the floor Q ÷ S_eff, find and cut the gas load (leaks, wet product, a condenser) before buying a bigger pump; if the line conductance is close to the pump speed, fix the line first.
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.
Vacuum line conductance and effective speed
Work out the conductance of a round vacuum line in viscous or molecular flow, add valves and traps in series, and see how much of the pump speed actually reaches the chamber.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pump speed at its inlet (L/s): 100
- Line inside diameter (mm): 40
- Line length (m): 3
- Flow regime: Decide from pressure and diameter
- Mean pressure in the line (mbar a): 0.001
- Other series elements: valves, traps, filters (L/s; 0 = none): 0
- Gas load for the required-speed check (mbar·L/s; 0 = skip): 0
Result
- Mean free path of air at this pressure: 66 mm
- Knudsen number λ ÷ d: 1.65 regime from this: molecular
- Line conductance (molecular): 2.58 L/s = 9.29 m³/h
- Total series conductance: 2.58 L/s
- Effective speed at the chamber: 2.52 L/s = 9.06 m³/h, 2.5% of the pump speed
- Conductance against pump speed: 0.03 times: the line, not the pump, is the limit
Method. Long round pipe, air at about 20 °C, d and L in cm: molecular flow C ≈ 12.1 × d³ ÷ L L/s; viscous (laminar) flow C ≈ 135 × d⁴ × p̄ ÷ L L/s with p̄ the mean pressure in mbar. Regime from the Knudsen number Kn = λ ÷ d, mean free path λ ≈ 0.066 mm at 1 mbar, inversely proportional to pressure: viscous below Kn 0.01, molecular above 0.5, transitional between (this tool then adds the two, an indicative figure; use maker’s data). Series elements: 1/C_total = Σ 1/Cᵢ. Effective speed: 1/S_eff = 1/S + 1/C_total. Required effective speed for a gas load: S_eff = Q ÷ p (the mean line pressure is taken as the chamber pressure for this check). 1 L/s = 3.6 m³/h. Short pipes (L less than about 20 d) need an entrance correction; near atmospheric pressure flow may be turbulent or choked and the laminar formula is optimistic.
What to take from it. Size the line so its total conductance is several times the effective speed you need, at the lowest pressure you run: a pump twice as large on a poor line changes almost nothing.