How to Choose a Liquid Ring Vacuum Pump in 2026? — How to Calculate Vacuum Level, Capacity, and Operating Requirements
1. Vacuum Level Reference: Absolute Pressure, Gauge Vacuum, and Vacuum Percentage
| Target Condition | Absolute Pressure | Gauge Vacuum at Sea Level | Approximate Vacuum Percentage | Typical Engineering Use |
| Atmospheric pressure | 101.3 kPa abs | 0 kPa(g) | 0% | Reference pressure before evacuation |
| Coarse vacuum | 40 kPa abs | -61.3 kPa(g) | 60.5% | Vacuum conveying, rough degassing, and priming |
| Medium vacuum | 10 kPa abs | -91.3 kPa(g) | 90.1% | Filtration, evaporation, and vessel evacuation |
| Deep vacuum with cool seal water | 5 kPa abs | -96.3 kPa(g) | 95.1% | Drying, deaeration, and process holding |
| Approximate water-ring limit at 20°C | 2.34 kPa abs | -99.0 kPa(g) | 97.7% | Approximate thermodynamic limit for water-sealed operation |
Conversion formulas:
Gauge vacuum = Absolute atmospheric pressure − Absolute suction pressure
Vacuum percentage = [1 − (Absolute suction pressure ÷ Atmospheric pressure)] × 100
At sea level, atmospheric pressure is approximately 101.3 kPa absolute. Actual atmospheric pressure changes with altitude and weather.
2. Water Temperature and Theoretical Vacuum Limit
| Seal-Water Temperature | Approximate Water Vapor Pressure | Practical Meaning for a Water-Sealed Pump | Recommended Engineering Action |
| 10°C | 1.23 kPa abs | Provides the lowest theoretical vapor-pressure limit in this range | Use when deep vacuum and stable cooling are required |
| 15°C | 1.71 kPa abs | Suitable for many deep-vacuum applications | Maintain a controlled supply and monitor temperature at the pump inlet |
| 20°C | 2.34 kPa abs | Common design reference for water-ring systems | Allow operating margin above the vapor-pressure limit |
| 30°C | 4.24 kPa abs | Deep-vacuum capability is significantly reduced | Improve cooling or select a higher-temperature sealing liquid |
| 40°C | 7.38 kPa abs | May prevent operation at low absolute pressures | Do not size the pump using cold-water performance data |
The vapor pressure of the sealing liquid establishes a practical lower limit. A liquid ring pump cannot normally maintain a stable suction pressure below or very close to the vapor pressure of its sealing liquid.
3. Capacity Calculation and Sizing Method
| Calculation Item | Formula or Method | Required Input | Example |
| Required pumping speed | Effective speed, S = Gas throughput, Q ÷ Suction pressure, P | Gas load in Pa·m³/s and target pressure in Pa abs | 0.12 Pa·m³/s ÷ 8,000 Pa = 0.000015 m³/s |
| Convert pumping speed to hourly capacity | Capacity in m³/h = Speed in m³/s × 3,600 | Calculated speed in m³/s | 0.000015 × 3,600 = 0.054 m³/h |
| Practical sizing margin | Select approximately 25% to 40% above the calculated duty point | Uncertainty in leakage, vapor release, temperature, and fouling | 54 m³/h calculated process load → approximately 68–76 m³/h minimum selected capacity |
| Gas-load conversion | 1 mbar·L/s = 0.1 Pa·m³/s | Gas load commonly stated in mbar·L/s | 1,200 mbar·L/s = 120 Pa·m³/s |
| Capacity conversion | 1 m³/h = 0.5886 ft³/min | Metric capacity requiring conversion to CFM | 100 m³/h ≈ 58.9 CFM |
Worked example:
If the process gas load is 1,200 mbar·L/s and the required suction pressure is 8 kPa absolute:
Gas load = 1,200 × 0.1 = 120 Pa·m³/s
Required speed = 120 ÷ 8,000 = 0.015 m³/s
Required capacity = 0.015 × 3,600 = 54 m³/h
With a 30% design margin: 54 × 1.30 = 70.2 m³/h
Select a pump whose certified performance at 8 kPa absolute is at least approximately 70 m³/h, rather than relying only on its maximum free-air capacity.
4. Typical Process Duty and Initial Capacity Range
| Application | Typical Target Pressure | Typical Capacity Range | Important Load to Include | Initial Selection Guidance |
| Small vessel evacuation | 20–60 kPa abs | 5–50 m³/h | Vessel volume, evacuation time, and leakage | Prioritize rapid evacuation and stable control |
| Vacuum filtration | 10–40 kPa abs | 20–250 m³/h | Filter cake resistance, filtrate vapor, and continuous air ingress | Check performance at the actual operating pressure |
| Deaeration and degassing | 2–20 kPa abs | 50–500 m³/h | Dissolved gas release, foaming, and liquid vapor | Use a separator or demister where carryover is possible |
| Evaporation | 5–40 kPa abs | 50–1,000 m³/h | High vapor load and condenser performance | Size from the vapor mass flow, not vessel volume alone |
| Vacuum drying | 2–20 kPa abs | 50–750 m³/h | Moisture evaporation rate and non-condensable leakage | Confirm that the condenser removes most condensable vapor |
| Priming and liquid transfer | 20–80 kPa abs | 10–150 m³/h | Pipe volume, liquid temperature, and entrained liquid | Provide liquid-separator protection at the suction inlet |
The capacity ranges above are preliminary engineering ranges, not guaranteed pump performance. Final selection must use the pump curve at the specified suction pressure, seal-liquid temperature, rotational speed, and gas composition.
5. Operating Requirements Checklist
| Requirement | Recommended Design Check | Why It Matters | Typical Acceptance Target |
| Seal-liquid temperature | Measure temperature at the pump inlet and outlet | Vapor pressure rises rapidly as temperature increases | Keep the temperature within the selected pump curve conditions |
| Seal-liquid flow | Verify minimum and maximum flow at operating pressure | Insufficient liquid reduces sealing; excessive flow increases power and separator load | Follow the certified operating range for the selected pump |
| Gas composition | Identify air, oxygen, corrosive gases, solvents, and condensable vapors | Materials, safety classification, and sealing liquid may need to change | Confirm compatibility with all process gases and liquids |
| Inlet liquid carryover | Install a suction separator, knockout pot, or demister when required | Liquid slugs can damage the pump and overload the motor | No uncontrolled liquid slugging at the pump inlet |
| Discharge backpressure | Calculate separator, condenser, silencer, and piping pressure losses | High discharge pressure increases power and may reduce capacity | Keep backpressure below the pump’s certified limit |
| Motor power | Check absorbed power at the complete operating envelope | Power demand can change with suction and discharge pressure | Motor service factor should cover the maximum expected duty |
| Control method | Consider a variable-frequency drive, throttling valve, or seal-liquid control | Process demand often changes during evacuation and holding | Maintain stable pressure without excessive energy consumption |
| Noise and vibration | Check foundation, alignment, piping support, and operating speed | Two-phase flow and cavitation-like operation can increase vibration | Meet the applicable site and machinery vibration limits |
6. Selection Decision Matrix
| Selection Question | If the Answer Is “Yes” | Recommended Direction |
| Is the target pressure close to the vapor pressure of the seal liquid? | Stable vacuum may be difficult to maintain | Lower the seal-liquid temperature, change the liquid, or select another vacuum technology |
| Does the process contain a large amount of condensable vapor? | The pump may experience a high thermal and volumetric load | Use a condenser or separator upstream and include remaining vapor in the gas-load calculation |
| Is the gas corrosive or chemically reactive? | Standard materials may not be suitable | Specify compatible metallurgy, elastomers, coatings, and seal liquid |
| Does the duty cycle change significantly? | Full-speed operation may waste energy during holding | Consider variable-speed control or an automatic vacuum regulation system |
| Is liquid carryover possible? | The pump may be exposed to liquid slugs or solids | Add inlet protection and confirm separator drainage arrangements |
| Is the required capacity stated only at free-air conditions? | The figure may not represent actual process performance | Use the certified capacity at the required suction pressure and seal-liquid temperature |
7. Final Data Required Before Ordering
| Data Category | Minimum Information to Provide | Preferred Unit |
| Vacuum duty | Minimum, normal, and maximum suction pressure | kPa abs or mbar abs |
| Capacity | Gas load at the normal and worst-case operating points | m³/h, Nm³/h, kg/h, or mbar·L/s |
| Evacuation requirement | Vessel volume and required time from atmospheric pressure to target pressure | m³ and minutes |
| Vapor load | Condensable vapor type, mass flow, temperature, and expected condensation rate | kg/h and °C |
| Seal liquid | Liquid type, supply temperature, flow, quality, and disposal or recirculation method | °C and m³/h |
| Discharge conditions | Discharge pressure, piping length, separator pressure loss, and vent arrangement | kPa(g) or kPa abs |
| Installation conditions | Altitude, ambient temperature, available voltage, frequency, and hazardous-area classification | m, °C, V, Hz |
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