What does the seal liquid actually do?
Inside a liquid-ring pump, an eccentrically mounted impeller throws the operating liquid outwards into a ring. Between the impeller blades and that ring, gas pockets grow on the suction side and shrink on the discharge side, drawing gas in and pushing it out. The liquid therefore does four jobs at once:
- It forms the moving seal that creates the compression spaces.
- It seals the clearances between impeller and casing.
- It absorbs the heat of compression and any heat carried in by the process gas.
- It condenses part of any incoming vapour and collects contamination.
The liquid leaves the pump with the discharged gas and must be separated, cooled, conditioned or replaced. Its density, viscosity, vapour pressure, solids content, dissolved gases and corrosivity all affect performance. Water is common, but it is not suitable for every process or every pump material.
Why seal-liquid temperature sets the vacuum
Every liquid has a vapour pressure that rises with temperature. The pump cannot hold a suction pressure close to the seal liquid’s vapour pressure; as the two approach, the liquid starts to flash, capacity falls and cavitation can damage the impeller and casing.
| Water temperature | Approximate vapour pressure of water | What it means in practice |
|---|---|---|
| 15 °C | about 17 mbar(a) | Good margin for deep rough vacuum |
| 25 °C | about 32 mbar(a) | Typical design point in many plants |
| 35 °C | about 56 mbar(a) | Noticeably reduced capacity at low suction pressure |
| 45 °C | about 96 mbar(a) | Common in Indian summers without adequate cooling; deep vacuum becomes difficult |
These are standard properties of pure water. Dissolved solvent, salt, glycol or product changes the vapour pressure, so use the actual liquid composition and the pump maker’s performance and cavitation curves, not a generic margin.
Colder is not always better. Lower liquid temperature can increase cooler duty and utility use, raise viscosity, and cause freezing or fouling of some products. Choose the temperature from the full process and equipment picture.
Once-through, partial or total recirculation?
| Arrangement | How it works | Advantages | Drawbacks |
|---|---|---|---|
| Once-through | Fresh liquid enters, used liquid leaves after separation | Simple, coolest possible liquid, contamination does not build up | High water use and a continuous effluent stream |
| Partial recirculation | Part of the separated liquid is reused, topped up with fresh make-up | Lower water use, moderate complexity | Temperature and contamination rise; needs balancing |
| Total recirculation | Liquid is cooled in a heat exchanger and reused, with small make-up | Minimum water use and effluent, suits solvents and special liquids | Needs a cooler, pump, controls and contamination management |
In water-scarce parts of Telangana and Andhra Pradesh, and wherever effluent consent is tight, total recirculation is often preferred. A once-through system still needs a separator, level control, vent and an authorised discharge route; a drain connection is not proof that discharge is permitted.
Drawing the system boundary
Draw the loop from the pump suction flange to every final gas and liquid destination. Include the casing, seal-liquid inlet, any separate shaft-seal flush, discharge separator, reservoir, cooler, circulation pump, strainers, control valves, make-up, overflow, purge, drains, vents, gas outlet, sampling points and any condensate entering from the process.
Mark which streams cross the package boundary. A skid may contain the pump and separator but rely on site cooling water, make-up water, power, exhaust treatment and effluent handling. Record who is responsible at each connection.
Define inventories: the pump’s working volume, the separator’s operating level band, cooler and pipe volume, and any trapped sections. A separator’s nameplate volume is not the same as its usable inventory.
Balancing heat and liquid flow
The cooler must reject the heat of compression, heat from the incoming gas and the latent heat of any vapour that condenses in the ring. The basic relationship is Q = m × cp × ΔT, where Q is heat in kW, m is liquid mass flow in kg/s, cp is specific heat and ΔT is the temperature rise.
Worked example
Assume a hypothetical total heat load of 50 kW to be removed with water (cp about 4.18 kJ/kg·K), allowing a 10 K rise across the pump.
- Required liquid flow = 50 ÷ (4.18 × 10), about 1.2 kg/s.
- That is roughly 4.3 m³/h of seal water through the pump.
- In total recirculation, the cooler must remove the same 50 kW, so its cooling-water side and approach temperature must be checked against summer conditions.
- Condensed process vapour adds to the reservoir, so overflow or purge must remove that volume, plus any contamination.
The figures illustrate the method only. The pump maker’s data sets the required seal-liquid flow and temperature for the selected model.
Contamination, chemistry and disposal
Whatever the process sends into the pump ends up in the seal liquid: condensed solvents, acids, product dust, dissolved gases. Over time this changes vapour pressure, causes corrosion or scaling, and makes the liquid harder to dispose of.
- Sample the liquid at defined points and intervals, and trend the results.
- Set a purge or replacement rate that keeps contamination within the limits agreed for the pump and the effluent route.
- Check water hardness; scale in the casing and cooler reduces performance.
- Where solvent is recovered, separate it before the liquid returns to the pump.
- Confirm the disposal route and consent for every liquid stream leaving the system.
Where process vapour condenses in the ring, count it once in the liquid balance. Do not treat it as a separate source as well, or the balance will not close.
Operating states to design for
A steady data sheet cannot show how the system behaves at start-up or during an upset. List the states and the response for each: cold start, warm-up, minimum and peak load, high condensable load, cleaning, product change, blocked cooler, loss of cooling water, high separator level, low make-up, high contamination, shutdown, drain-down and restart.
For each state, note suction pressure, gas and vapour load, seal-liquid temperature and flow, separator level, cooler duty and where the gas and liquid go. Alarms and trips come from the pump maker’s instructions, the risk assessment and the site’s control philosophy.
Symptoms and what to check
| Symptom | Likely causes |
|---|---|
| Vacuum worse than expected | Seal liquid too warm, low seal flow, contaminated liquid with higher vapour pressure, air leaks |
| Noise and vibration like gravel in the pump | Cavitation from operating too close to the liquid’s vapour pressure |
| Rising seal-liquid temperature | Fouled cooler, reduced cooling water, extra condensing load, low circulation |
| Liquid in the gas outlet | Separator level too high, undersized separator, foaming |
| Scale or corrosion in the casing | Hard water, acidic contamination, insufficient purge |
Temperature rise on its own does not identify the cause. Check flow, pressure, level and cooler data before increasing make-up or changing a set point.
Keep a simple log that pairs suction pressure, seal-liquid supply and return temperature, separator level, cooling-water temperature and purge rate with the date and production state. Over a few weeks, that record shows whether falling vacuum follows the season, a product change, cooler fouling or a gradual build-up of contamination. It also gives the pump maker and service engineer real data to work from, which shortens fault-finding and avoids unnecessary part changes.
How Reddy Compressors helps
Liquid-ring pumps often sit alongside compressed-air systems, blowers and cooling equipment in pharmaceutical, chemical, paper, food and power plants across Telangana and Andhra Pradesh. As an independent multi-brand specialist based at Tarnaka, Secunderabad, we help plants look at the whole utility picture.
- We review seal-liquid loops for temperature, flow, cooling and contamination problems and give a clear written finding.
- We help balance make-up, recirculation and purge against the maker’s data.
- We supply, rent and service the compressors and blowers that serve the same process areas, and supply their spares.
- Our team of more than 20 service engineers and technicians supports sites across both states, and we supply equipment and spares across India.
We also sell new and used/refurbished liquid ring and other vacuum pumps, rent them, service them and supply their spares. Settings follow the vacuum-pump maker’s instructions, and effluent and disposal approvals stay with the site.
What to send us
- Pump nameplate photos and the maker’s data sheet if available.
- Required suction pressure and gas and vapour load.
- Seal-liquid type, supply temperature and flow.
- Arrangement: once-through, partial or total recirculation.
- Cooling-water supply temperature in summer.
- Symptoms and any recent liquid analyses.
Frequently asked questions
Why is my liquid ring vacuum pump not reaching full vacuum?
The most common cause is seal liquid that is too warm, because the pump cannot pull below the liquid’s vapour pressure. Low seal flow, contaminated liquid, air leaks in the suction line and worn clearances are other causes. Measure seal-liquid temperature at the pump inlet, check flow and cooler performance, and compare suction pressure with the maker’s curve.
What temperature should liquid ring seal water be?
The pump maker’s data for your model sets the design temperature, often around 15–25 °C for water. Higher temperatures raise vapour pressure and reduce capacity at low suction pressure. In Indian summers, make sure the cooler or water supply can still hold the design temperature when ambient and cooling-water temperatures are at their highest.
What causes cavitation in a liquid ring pump?
Cavitation occurs when suction pressure approaches the seal liquid’s vapour pressure, so the liquid flashes into bubbles that collapse violently. Warm seal liquid, contamination that raises vapour pressure and operating below the pump’s minimum suction pressure are the usual causes. It sounds like gravel in the pump and erodes the impeller and casing. Many pumps use an anti-cavitation connection.
Is total recirculation better than once-through?
It uses far less water and produces less effluent, which suits water-scarce sites and solvent duties. However, it needs a cooler, circulation pump, controls and a purge to keep contamination under control, and the liquid runs warmer than fresh supply. Once-through is simpler but consumes more water. The best choice depends on water cost, effluent limits and process vapours.
How much make-up water does a liquid ring pump need?
In once-through operation, the full seal flow from the maker’s data. In recirculation, make-up replaces liquid lost in the gas outlet, purge and overflow, and helps control temperature and contamination. Condensed process vapour adds liquid rather than removing it. Balance the loop using the maker’s flow figure, the purge rate and measured levels.
Can Reddy Compressors look at our vacuum and compressed-air utilities together?
Yes. We regularly review compressed-air systems, blowers and the cooling and utility interfaces around them, and we can include liquid-ring seal-liquid loops in a site visit. We report on temperature, flow, cooling and contamination issues, while pump settings remain as specified by the pump maker.
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.
Liquid-ring seal-liquid flow and vapour-pressure margin
Seal-liquid flow needed to carry the heat out of a liquid-ring pump for an allowed temperature rise, the cooler duty, and how close the required suction pressure is to the vapour pressure of the liquid.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Total heat to remove: compression + gas + condensing vapour (kW): 50
- Seal-liquid specific heat (kJ/kg·K): 4.18
- Allowed temperature rise across the pump (K): 10
- Seal-liquid density (kg/m³): 1000
- Seal-liquid supply temperature (°C): 25
- Required suction pressure (mbar a): 60
- Process vapour condensing in the ring (kg/h; 0 = none): 0
Result
- Seal-liquid mass flow: 1.196 kg/s = 4,306 kg/h
- Seal-liquid volume flow: 4.31 m³/h = 71.8 l/min
- Cooler duty in total recirculation: 50 kW
- Seal-liquid outlet temperature: 35 °C
- Water vapour pressure at supply temperature: 31.6 mbar a at 25 °C
- Water vapour pressure at outlet temperature: 56.1 mbar a at 35 °C
- Margin: suction pressure − vapour pressure at outlet: 3.9 mbar (suction is 1.07 × the vapour pressure)
- Result: Too close within the 1.5 × screening margin
Method. Mass flow m = Q ÷ (c_p × ΔT) (kW, kJ/kg·K, K → kg/s); volume flow = m ÷ ρ × 3,600 m³/h. In total recirculation the cooler rejects the same heat Q. The ring liquid is hottest at the outlet (supply + ΔT); the pump cannot hold a suction pressure close to the liquid’s vapour pressure there. Water vapour pressure (Magnus formula): about 17 mbar at 15 °C, 32 at 25 °C, 56 at 35 °C, 96 at 45 °C. Screening value used by this tool: suction pressure at least 1.5 × the vapour pressure at the outlet temperature; closer than that, capacity falls and cavitation is likely. Pure water only: dissolved solvent, salt or glycol changes the vapour pressure, and the maker’s performance and cavitation curves govern. Condensing process vapour adds to the liquid inventory and must leave by overflow or purge.
What to take from it. Check the summer case: warmer supply water raises the vapour pressure and can take a working pump into cavitation without any fault in the pump.
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.