Who this page is for: plant engineers, maintenance managers and safety officers who already look after their air receiver and want the same discipline applied to the hidden pressure vessel inside every oil-injected screw compressor: the air-oil separator vessel.
Key facts
- An oil-injected screw compressor has at least two vessels at full system pressure: the separator vessel inside the canopy and the receiver outside it. Usually only the receiver is on the plant’s inspection list.
- The separator vessel typically holds an air-oil mixture at about 70–100 °C. The receiver typically holds air at about ambient plus 10–15 °C after the aftercooler, which is still above 50 °C in a Telangana summer.
- At 10 bar(g) (145.0 psig), a 500-litre receiver stores about 680 kJ of expansion energy and a 40-litre separator gas space about 54 kJ, a ratio of 12.5 to 1 (illustrative; see the worked example).
- The chemical energy in 20 litres of compressor oil is more than ten thousand times the separator vessel’s stored pneumatic energy.
- Separator elements are typically replaced when their differential pressure reaches about 0.7–1.0 bar (10.2–14.5 psi differential), or at the rated hours, whichever comes first.
- The minimum-pressure valve typically holds the separator vessel at about 4–5 bar(g) (58.0–72.5 psig) to drive oil circulation before any air reaches the plant.
- A relief valve protects a vessel only if its set pressure is at or below the design pressure and its capacity covers the full free air delivery of every compressor that can feed it.
Two vessels, two different jobs
Oil injected into a screw airend seals, cools and lubricates, so what leaves the airend is a hot air-oil mixture, typically of the order of 5–10 litres of oil per minute for every m³/min of air. The separator vessel strips the oil back out, stores it and returns it to the airend. It is part of the lubrication circuit that happens to be at full discharge pressure.
The receiver, downstream of the aftercooler, meets short peaks from storage, steadies control and drops out condensate. Sizing is covered on receivers and storage, and daily checks and generic inspection on air receiver inspection and safety. This page does not repeat that material; it contrasts the two vessels and focuses on the forgotten one.
It is forgotten because, behind panels and pipework, it does not look like a vessel, and it gets opened as routine work. That routine is where the injuries happen.
Inside the separator vessel: operating conditions
Separation happens in three stages:
- Primary separation. The mixture enters tangentially or strikes a baffle, and the sudden drop in velocity typically drops well over 95 % of the oil into the sump.
- Coalescing. A glass-fibre element gathers the remaining aerosol into droplets.
- Return. A scavenge line takes the collected oil from inside the element back to the airend.
Foaming undermines all three. Air leaves solution as pressure falls at each unload, and the foam is carried through the element as liquid. Wrong or mixed oils, overfilling and rapid cycling make it worse.
| Component | Function | What goes wrong if neglected |
|---|---|---|
| Coalescing element | Brings oil carry-over down to a few mg/m³ | Blocks (rising airend discharge pressure), ruptures (oil floods downstream) or loses its earthing path (static build-up in oil mist) |
| Scavenge line and orifice | Returns oil collected inside the element | A blocked orifice floods the element. The element then gets wrongly blamed for the carry-over |
| Minimum-pressure valve | Typically holds about 4–5 bar(g) (58.0–72.5 psig) for oil flow and acts as a non-return valve | If it sticks open, oil pressure is low at start and element velocity is excessive. If it leaks back, plant air refills the stopped vessel |
| Blowdown valve | Vents the vessel at unload, and to near zero after stop | If it fails closed, the vessel stays pressurised after shutdown. If it fails open, the machine cannot build pressure |
| Package safety valve | Overpressure protection for the vessel and airend | Seized, painted over, re-set by guesswork or replaced with a lower-capacity valve |
| Oil level sight glass | Shows the level at the condition the maker specifies | Misread while the oil is foaming or hot, which leads to overfilling and carry-over |
| Thermostatic valve | Bypasses the oil cooler until the oil reaches its opening temperature (typically about 70–85 °C, set by the maker), keeping oil-out temperature well clear of both condensation and the high-temperature trip (typically about 110–120 °C) | If it sticks open, water condenses in the oil. If it sticks closed, the oil overheats and oxidises |
Indian sites push this vessel harder than the temperate conditions most packages are designed for:
- Ambient temperatures of 42–46 °C in May eat into the cooling margin.
- Cement, road and crusher dust can plug oil coolers within weeks, and vessel temperature creeps up as they block.
- Monsoon intake air is so humid that a lightly loaded machine running cool condenses water into its own sump.
- Grid trips and generator changeovers mean frequent hot shutdowns, each relying on the blowdown valve.
How the separator vessel fails, and how it hurts people
| Hazard | Mechanism | Early sign | Prevention |
|---|---|---|---|
| Internal corrosion | Oil-out temperature runs too close to, or below, the pressure dew point of the air in the vessel, so water condenses, the oil emulsifies and the steel rusts above the oil line | Milky oil, rust in the oil filter, short oil life in the monsoon | Hold oil-out temperature above the maker’s minimum, test the thermostatic valve, avoid long light-load running |
| Fire or explosion | Overheated oil oxidises and cracks into a flammable mist. A hot spot, glowing carbon or static discharge then ignites it | Rising discharge temperature trend, dark oil, burnt smell, varnish on the element | Correct oil, clean coolers, working high-temperature trip that is never bypassed, earthed elements |
| Hot-oil spray | Filler cap or element cover opened while pressure remains, because blowdown failed, the minimum-pressure valve leaked back or the machine was only just stopped | Hiss at the cap, gauge above zero, oil at the cap thread | The depressurisation sequence below, every time |
| Carbon and varnish | Long oil intervals, high temperature and mixed oils lay lacquer on walls, element and scavenge orifice | Rising element differential, blocked scavenge sight glass | Change oil on condition and hours, and flush when changing oil type |
| Overpressure from a blocked element | The controller often senses pressure downstream of the element, so vessel and airend pressure rise above what it “sees” | Element differential above the maker’s limit, typically 0.7–1.0 bar (10.2–14.5 psi differential), higher motor current at the same outlet pressure | Log the differential at every visit, keep its alarm live, keep the safety valve correctly sized and free |
Oil at 85 °C escaping under even 1 bar(g) (14.5 psig) causes deep burns. Typically it happens to someone who “only needed to top up” a minute after stopping.
How the air receiver fails
Receiver hazards develop slowly, over decades. Condensate behind a failed or neglected drain pits the bottom of the shell and the drain nozzle. Overpressure typically follows a change: a new, higher-pressure or rented compressor is tied in, and nobody checks the receiver’s design pressure or whether the relief valve can pass the combined flow. The worst finds are receivers with no safety valve, or with an isolating valve between the vessel and its safety valve.
A second-hand receiver with no nameplate, test certificate or fabrication record has unknown design pressure, material and thickness. Treat it as unknown until a competent person has examined and tested it. Long-term integrity is covered on air receiver lifecycle integrity.
Separator vessel and receiver side by side
| Item | Air-oil separator vessel | Air receiver |
|---|---|---|
| Design pressure | Set by the package maker, typically 10–16 bar(g) (145.0–232.1 psig) on industrial screws | Chosen by the purchaser, commonly 10–12 bar(g) (145.0–174.0 psig) for plant air |
| Contents | Air, oil mist, liquid oil charge, dissolved air, sometimes water | Air, water vapour, condensate, traces of oil |
| Temperature | Typically about 70–100 °C oil-out | Typically about ambient plus 10–15 °C |
| Typical volume | Tens of litres on small packages, a few hundred litres on large ones | Hundreds of litres to several m³ |
| Relief device | Package safety valve supplied with the compressor | Receiver safety valve, sized for every compressor that can feed it |
| After shutdown | Blown down to near zero, typically within about one to two minutes (check the maker’s figure) | Stays at system pressure until deliberately isolated and vented |
| Who inspects routinely | Compressor service engineer, at every service | Plant staff daily, and the service engineer at visits |
| Statutory examination | Confirm with your competent person whether, and how, it falls within the examination scope | Competent person, at the intervals set by the applicable rules, the design code and the insurer |
| Documents | Package manual, vessel nameplate, maker’s test certificate, element and oil specifications | Nameplate, fabrication and hydraulic test certificate, safety valve record, examination reports |
A compressor package is two pressure vessels and a fire load sharing one canopy. Each vessel needs its own design limit, its own relief device, its own records and its own safe opening procedure.
What to check at service and at statutory examination
Service keeps the machine inside its design envelope. Statutory examination is a competent person’s independent judgement of fitness for working pressure. A good service record helps the examiner, but it does not replace the examination.
| Check | Separator vessel at service | Receiver at service | At statutory examination |
|---|---|---|---|
| Pressure and temperature | Element differential, oil-out temperature, loaded and unloaded vessel pressure | Working pressure compared with design pressure | Safe working pressure confirmed against design data and condition |
| Relief device | Set pressure on the tag, lever free, not blanked | As for the separator vessel, plus no isolating valve upstream | Set pressure and capacity verified; valve tested or replaced |
| Blowdown and minimum-pressure valve | Time from stop to near-zero pressure; opening pressure and back-leak | Not applicable | Reviewed as part of the protective system |
| Internal condition | Look inside at every element change for rust, varnish and sludge | Condition of the drained water: rust colour, sludge, oil | Internal inspection where accessible, thickness survey, hydraulic test if required |
| Nameplate and fittings | Nameplate legible, gauge working, element earthing intact | Nameplate legible, no new nozzles or welds | Nameplate matched to records; repairs and modifications reviewed |
A suitable record format is on our compressor log book format page.
Safe depressurisation before opening either vessel
Work within your permit system (see safety isolation and permit to work); these vessel-specific steps are the ones skipped.
- Stop with the normal stop button, not the emergency stop, so the designed unload and blowdown sequence runs.
- Lock off the electrical isolator with your own padlock and tag. On sites with generator back-up, make sure no automatic changeover can re-energise the machine.
- Close and lock the compressor’s discharge valve to the plant. Do not rely on the minimum-pressure valve to stop plant air flowing back.
- Watch the separator gauge fall to zero. If it stalls above zero, the blowdown valve or its silencer is faulty. Open nothing, and vent through the maker’s designated point.
- Let the oil cool to below about 50 °C before opening any oil-side part, or longer if the maker specifies.
- Prove zero a second way, because gauges stick. Crack the filler cap a quarter turn and listen. If it hisses, close it and investigate.
- For the receiver, lock closed every inlet and the outlet, open the drain and a top vent, and confirm zero both on the gauge and by the vent going silent.
- Look for hidden feeds: a second compressor, a ring main looping back, or a rental unit on a temporary tie-in.
Modifications, replacement elements and pressure settings
No welding or new nozzles on either vessel, even for a sensor, without design-authority approval and competent-person acceptance.
Replacement elements. A replacement must match the original’s pressure, flow and collapse ratings and its dimensions. It must also have an intact earthing feature, usually a conductive gasket or a staple through it. A cheaper element without an earthing path lets static build up inside a vessel full of oil mist. Reject any element that has been dropped or crushed. See separator parts and oil separator performance.
Pressure settings. The maximum safe pressure of a package is set by its weakest component, not by the controller: separator vessel design pressure, safety valve setting, airend rating, motor full-load current, and the minimum-pressure valve and element ratings. Going from 8 to 10 bar(g) (116.0 to 145.0 psig) costs about 12–14 % more energy (6–7 % per bar), cuts free air delivery and can bring the separator vessel close to its safety valve setting, which then weeps and gets screwed down. Review any pressure change for both vessels and both relief devices, and record it. Our safety valve identification page sets out what may be adjusted on site.
Blowdown valves. A replacement blowdown valve must vent at the rate the maker intended. If it is undersized, the vessel stays pressurised for longer. If it is oversized or fitted without its silencer, it causes foaming and oil discharge at every unload. See blowdown valve compatibility.
Documents and records to keep
Keep one file per vessel that an examiner or new maintenance manager can follow unaided:
- Nameplate data, photographed and transcribed: maker, serial number, year, design pressure and temperature, test pressure, capacity and the design code referred to.
- Test certificate from manufacture, plus any later hydraulic test records.
- Safety valve data: set pressure, rated capacity in m³/min (and cfm), size, and each test or replacement. Write the total FAD of the feeding compressors alongside, so the comparison is visible.
- Examination reports, with any conditions or reduced working pressure.
- Repair and modification history, with approvals, and thickness readings taken at fixed, marked points.
- For separator vessels: element part number, fitting date and hours, differential pressure at removal, and oil grade and change dates.
When buying used equipment, ask for this file first (see used compressors).
The Indian legal position, kept general
Pressure plant in Indian factories has long been regulated under the Factories Act, 1948 and the factory rules made by each state, which cover safe working pressure, protective fittings, and periodic examination and testing by a competent person. The Occupational Safety, Health and Working Conditions Code, 2020 consolidates the Factories Act and other workplace-safety laws. The central government notified 21 November 2025 as the date on which the Code’s provisions came into force, but the detailed rules under it, particularly each state’s rules, are being notified in stages, so the framework is in transition. Until your state’s rules under the Code are in place, confirm with your factory inspectorate or competent person which requirements are being applied to pressure plant at your site. Whatever the framework, the occupier remains responsible for keeping pressure plant safe and examined.
We deliberately give no intervals, test pressures or section numbers, because they depend on the state, the vessel’s classification, its design code and the insurer. Ask your competent person or factory inspectorate whether separator vessels fall within the examination scope. Rules from other countries that appear in imported marketing material do not apply to installations in India. A design code on a nameplate shows how a vessel was built, not that it complies with Indian law.
Worked example (illustrative)
Question: how much energy is stored in a 500-litre receiver and in a separator vessel with a 40-litre gas space, both at 10 bar(g) (145.0 psig)? What does the hot oil add?
Method: isentropic expansion to atmosphere, with absolute pressures and γ = 1.4:
E ≈ p1 × V / (γ − 1) × [1 − (pa / p1)^((γ − 1) / γ)]
- Absolute pressures: p1 = 10 + 1.01325 = 11.01325 bar abs = 1,101,325 Pa; pa = 101,325 Pa.
- Ratio: pa / p1 = 0.092003. Exponent: 0.4 / 1.4 = 0.285714. Then 0.092003^0.285714 = 0.505758, so the bracket is 0.494242.
- Receiver, V = 0.5 m³: 1,101,325 × 0.5 / 0.4 = 1,376,656 J. Multiplied by 0.494242, that gives 680,401 J, or about 680.4 kJ.
- Separator gas space, V = 0.04 m³: 1,101,325 × 0.04 / 0.4 = 110,133 J. Multiplied by 0.494242, that gives 54,432 J, or about 54.4 kJ.
- Comparison: 680.4 / 54.4 = 12.5, which equals the volume ratio 500 / 40, because the pressure is the same.
In plain terms: 680 kJ would lift about 69 tonnes through one metre (680,401 / (1,000 × 9.81) = 69.4). The separator’s 54 kJ would lift about 5.5 tonnes, which is still enough to throw a filler cap or element cover with lethal force.
What the oil adds: say the vessel holds 20 litres of mineral compressor oil at about 0.86 kg/litre, which is 17.2 kg. At about 42 MJ/kg heat of combustion, that oil carries 17.2 × 42,000 = 722,400 kJ. This is about 13,300 times the separator vessel’s pneumatic energy (722,400 / 54.4) and about 1,060 times the receiver’s (722,400 / 680.4). Pressure energy is released in a fraction of a second. Oil energy is released only if the oil ignites, and in this vessel the oil is hot, atomised, mixed with compressed air and touching an element that can hold static charge. That is why the vessel needs temperature protection, earthed elements and the correct oil, as well as a safety valve.
Illustrative figures only. The real gas space depends on oil level and geometry, and the calculation ignores heat transfer and oil flashing.
Pressure vessel register for a compressor station
One column per vessel: every separator vessel and every receiver, including standby and rental machines.
| Field | Vessel 1 | Vessel 2 | Vessel 3 | Vessel 4 |
|---|---|---|---|---|
| Site tag and location | ||||
| Type (separator vessel or receiver) | ||||
| Compressor it belongs to or is fed by (make, model, serial) | ||||
| Vessel maker, serial number, year | ||||
| Volume (litres) | ||||
| Design pressure (bar g / psig) and temperature (°C) | ||||
| Test pressure and date on certificate | ||||
| Maximum working pressure in use (bar g / psig) | ||||
| Safety valve set pressure (bar g / psig) | ||||
| Safety valve capacity (m³/min / cfm) | ||||
| Total FAD of compressors feeding it (m³/min / cfm) | ||||
| Isolating valve between vessel and safety valve? (must be no) | ||||
| Nameplate photographed / certificate on file | ||||
| Last statutory examination: date and examiner | ||||
| Next examination due (per competent person) | ||||
| Last element change and differential at removal (bar) | ||||
| Blowdown time to near zero after stop (s) | ||||
| Drain type and condition (receiver) | ||||
| Thickness readings at marked points (mm) | ||||
| Repairs or modifications, approval reference | ||||
| Recorded by and date |
Exceptions and cautions
- Tank-mounted and vane machines may use different separator layouts. Follow the manual.
- Oil-free compressors have no separator vessel, but still feed a receiver.
- On portable diesel units, check vessel mounts and nozzles more often, and never open the vessel where oil can reach a hot exhaust.
- Synthetic oils differ in foaming and flash point. Never mix oil types; flush when changing.
- The values on this page are typical. The maker’s manual and your competent person take precedence.
Common mistakes
- Registering every receiver and leaving out every separator vessel.
- Opening the filler cap straight after a stop without proving zero.
- Screwing down a weeping safety valve instead of finding out why pressure is reaching it, often because of a blocked element.
- Fitting an element without an earthing path, or re-using a crushed gasket.
- Raising the pressure setting without checking both vessels and both relief devices.
- Tying a rental or second compressor into a receiver without checking that the relief capacity covers the combined FAD.
- Putting an undocumented second-hand receiver straight into service.
- Treating a service report as an examination report.
How Reddy Compressors helps
Reddy Compressors has been an independent multi-brand compressed-air specialist in Secunderabad since 2002. We sell new, used and refurbished compressors, and provide rental, service, AMC, overhaul and spare parts for all major makes, including Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic and others. We are suppliers, not an authorised dealer of any make. We have tie-ups with all major and reputed brands.
Our team of more than 20 service engineers and technicians changes elements and oil, services minimum-pressure and blowdown valves, and fits safety valves with matched set pressure and capacity. We also supply receivers, dryers, filters and piping. Our own rental fleet of 50–2,000 cfm (1.4–56.6 m³/min) at 2–30 bar can keep a plant running while a vessel is out for examination. We serve Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad, and supply across India. The statutory examination itself stays with your competent person.
What to send us
- Make, model, serial number and running hours of each compressor.
- Photographs of every receiver nameplate and, where visible, each separator vessel nameplate.
- Photographs of each safety valve tag.
- Present unload pressure setting and the pressure your plant actually needs.
- Last element change date and differential reading, if logged.
- Symptoms: oil consumption, oil in lines, high temperature, weeping safety valve, slow blowdown.
Use our service and maintenance enquiry. Call/WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
Is the separator vessel inside my compressor a pressure vessel?
Yes. It holds air and oil at full discharge pressure, often 7 to 13 bar(g) (101.5 to 188.5 psig), at 70 to 100 degrees Celsius. Confirm with your competent person how it is treated in your factory’s pressure-plant examination.
Watch
Short explainers on this topic
Slide-by-slide engineering explainers from Reddy Compressors. More in the video library.
Why is the separator vessel more of a fire risk than the receiver?
It contains hot, partly atomised oil mixed with compressed air. Overheated or carbonised oil, or a static discharge from an element without an earthing path, can ignite the mist. A receiver normally holds only air and condensate near ambient temperature.
How long should I wait before opening the oil filler cap?
Follow the maker’s manual. As a working rule, stop normally, isolate, confirm the separator gauge reads zero, let the oil cool below about 50 degrees Celsius, then crack the cap a quarter turn and listen for a hiss before removing it.
When should the separator element be changed?
At the maker’s rated hours or when differential pressure reaches the maker’s limit, typically about 0.7 to 1.0 bar (10.2 to 14.5 psi differential), whichever comes first. Change it sooner if carry-over rises or the oil has been contaminated.
Can I raise my compressor’s pressure setting?
Only after checking the separator vessel design pressure, package safety valve, airend and motor ratings, and the receiver and its relief valve. Each extra bar also costs about 6 to 7 percent more energy and reduces delivered air.
Does a service visit count as the statutory examination?
No. Service keeps the machine within its limits. A statutory examination is an independent assessment by a competent person of whether the vessel remains fit for its working pressure. Keep separate records.
Is it safe to buy a second-hand receiver without papers?
Not until it has been examined. Without a nameplate or certificate, nobody can state its design pressure or remaining thickness. Have a competent person examine and test it before use, and keep the report.
Do foreign pressure-equipment rules mentioned in brochures apply in India?
No. Installations in India are governed by Indian law and the applicable state rules. A design code on a nameplate describes how the vessel was built, not whether it meets Indian requirements.
Related pages
- The complete compressed-air system
- Air receiver inspection and safety
- Air receiver lifecycle integrity
- Air-oil separation performance
- Compressor safety valve identification
- Blowdown valve function and compatibility
- Minimum-pressure valves
- Safety isolation and permit to work
- Oil-injected rotary screw compressors
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.