What the blowdown valve does
On an oil-injected screw compressor, the separator vessel holds compressed air and oil. When the machine unloads or stops, the inlet valve closes and the blowdown valve opens to release separator pressure, usually through a silencer and sometimes back to the inlet. This reduces the power drawn while unloaded, protects the motor from starting under load and prevents oil being forced out through the inlet. The minimum-pressure/check valve keeps the system pressure from flowing back while the separator vents.
Reciprocating and oil-free machines use related arrangements, such as unloaders and vent valves, but the principle is similar: release trapped pressure so the machine runs unloaded and restarts easily.
What volume is being vented?
The valve normally vents only the separator, oil circuit and pipe up to the minimum-pressure valve, not the plant receivers. If system pressure falls when the machine unloads, the check or minimum-pressure valve is leaking back. The time to vent depends on that trapped volume, the valve’s flow capacity, any orifice or silencer restriction and the pressure at the moment of unloading.
Common valve arrangements
| Arrangement | How it works | Points to check |
|---|---|---|
| Electrically actuated solenoid | Controller energises or de-energises a coil to open or close the vent | Normally open or normally closed type, coil voltage AC or DC, coil condition, controller output |
| Pneumatically piloted | A pilot signal from a small solenoid or the inlet valve moves a piston or diaphragm | Pilot line condition, small orifices, diaphragm and seal wear |
| Integrated with the inlet valve | Vent opens mechanically or pneumatically as the inlet closes | Inlet valve kit condition, linkage, internal passages |
Many machines are designed so that loss of power opens the vent, making the safe state an unloaded, depressurised machine. A substitute valve that closes on loss of power reverses this and can create a hazard.
Normal sequence: start, load, unload and stop
- Start: inlet valve closed, blowdown open, motor accelerates without load.
- Load: blowdown closes, inlet valve opens, separator pressure builds and the minimum-pressure valve opens to the system.
- Unload: inlet valve closes, blowdown opens, separator pressure falls to the unloaded level set by the design.
- Stop: after an unloaded run-on, the motor stops and the separator is fully vented.
Comparing this expected sequence with what the pressure gauge and controller show is the quickest way to locate a fault.
Symptoms and likely causes
| Symptom | Possible causes |
|---|---|
| Air blowing from the silencer while loaded | Valve seat or seal worn, debris on the seat, coil or pilot signal wrong, wrong valve type fitted |
| Machine will not load or builds pressure slowly | Blowdown stuck open, inlet valve fault, control signal missing |
| Restart trips or heavy motor start | Blowdown not opening, blocked silencer or orifice, pressure not falling before restart |
| Oil mist from the silencer | Venting too fast, high oil level, separator issue, saturated silencer |
| System pressure falls when the machine unloads | Check or minimum-pressure valve leaking back, not the blowdown valve itself |
| Long unloaded running with high pressure in separator | Partially blocked vent path or slow-acting pilot |
Sustained leakage costs money
A blowdown valve that leaks while loaded wastes compressed air continuously and may carry oil to the silencer. The compressor runs longer to make up the loss. Listen at the silencer during loaded running; any steady flow is a fault. A worn seal kit is often the cause, but debris from a deteriorating separator or a damaged seat can also prevent closing.
Electrical checks
- Confirm the coil voltage and type (AC or DC) match the controller output.
- Measure coil resistance and check for overheating or burnt insulation.
- Check connectors and cable glands for moisture and corrosion, especially in humid or dusty compressor rooms.
- Check the controller output and any relay between controller and coil.
Pneumatic pilot lines
Small pilot lines and orifices block with oil varnish, rust and dirt. A blocked pilot may stop the valve opening or slow it enough to cause a restart trip. Inspect and clean pilot lines and filters at service, and replace hardened tubing.
Seals and materials
The valve sees hot oil-laden air at around 75–95 °C during normal running, plus repeated pressure cycling. Seal materials must suit both the temperature and the lubricant. If the oil type has been changed, check that valve seals are compatible.
Replacing a blowdown valve
Before ordering, confirm:
- Make, model and serial number of the compressor, and the original valve part number.
- Normal state: normally open or normally closed.
- Coil voltage and connector, or pilot arrangement.
- Pressure and temperature rating at or above the machine’s maximum.
- Port sizes, thread standard and flow direction.
- Flow capacity and any orifice or silencer in the vent line.
After fitting, run the machine through start, load, unload and stop, check vent time and separator pressure, and listen for leakage while loaded. Isolate, lock off and depressurise the machine before any work on the valve; never rely on the blowdown valve itself to make the machine safe.
Silencers and vent routes
The vent silencer reduces noise and catches oil mist. It can block over time, slowing venting and causing restart problems, or become saturated and discharge oil. Replace silencers at service intervals or when oily, and make sure the vent route is not restricted by modifications.
Interactions with other valves
A fault that looks like a blowdown problem often starts in another valve. The minimum-pressure/check valve must hold system pressure while the separator vents; if it leaks back, the blowdown valve vents system air too, and the fault looks like a blowdown problem. The inlet valve must close as the vent opens; if it sticks partly open, the machine keeps drawing air while unloaded and the separator pressure does not fall as expected. The pressure transducer that tells the controller when to load and unload must be accurate and located where the design expects, or the load and unload points drift. Check these together when diagnosing.
Preventive checks at service
- Listen at the silencer while loaded for any steady flow.
- Time the vent after unload and compare with earlier records.
- Inspect and replace the seal kit or valve at the interval suited to duty and condition.
- Clean pilot lines, orifices and strainers.
- Check the coil, connector and cable for heat damage or moisture.
- Replace the silencer if it is oily or restricted.
On our portable hire compressors, which we service every 300 running hours, these checks form part of the routine visit.
A simple diagnostic sequence
- Note the controller messages and the separator and system pressure readings at the time of the fault.
- Observe one complete start, load, unload and stop cycle, listening at the silencer.
- Check whether the control signal to the coil or pilot changes when the machine loads and unloads.
- With the machine isolated and depressurised, inspect the silencer, pilot lines and valve seat.
- Check the minimum-pressure valve and inlet valve if the blowdown valve itself is in good order.
- After repair, repeat the full cycle and record vent time for future comparison.
This sequence separates electrical, pneumatic and mechanical causes quickly and avoids replacing a good valve.
Valve states and separator pressure through one cycle
The blowdown valve never works alone. In each phase it works with the inlet valve and the minimum pressure valve (MPV). Reading the separator gauge and the line pressure together, phase by phase, shows which of the three valves is at fault. The pressures below are typical for a 7–8 bar(g) oil-injected screw compressor. The design of your machine sets the exact values.
| Phase | Inlet valve | Blowdown valve | Minimum pressure valve | Separator pressure | If it is wrong |
|---|---|---|---|---|---|
| Start (star-delta or soft start) | Closed | Open | Closed | Near 0 bar(g) | Pressure above about 0.5–1 bar(g) at start means the separator was not vented. The result is heavy starting and overload trips |
| Loading | Opening | Closed | Closed until about 4–4.5 bar(g), then open | Rises quickly to the MPV setting | Pressure stalls below the MPV setting: the blowdown valve is stuck open or the inlet valve is not opening |
| Loaded | Open | Closed | Open | Line pressure plus the separator and MPV loss, usually about 0.3–1.0 bar above line pressure | Hiss from the silencer: the blowdown valve is passing |
| Unloaded | Closed, with only a small bleed | Open, often through an orifice | Closed, acting as a check valve | Falls to the unloaded level, typically about 1.5–3 bar(g) | If line pressure also falls, the MPV is leaking back. If separator pressure stays high, the vent path is restricted |
| Stop | Closed | Open | Closed | Falls to 0 bar(g) | Pressure remains after the run-on: the valve is not opening, or the silencer is blocked |
Unloaded power is lowest when the separator vents fully to its unloaded level. If the vent path is partly blocked, the separator stays at a higher pressure, and the machine draws more power every unloaded minute without any alarm.
How long blowdown should take: A worked calculation
At separator pressure, the vent orifice runs choked: air passes at sonic speed. While the flow is choked, pressure decays exponentially, p = p0 × e−t/τ. For air at about 85 °C, the time constant is τ ≈ V ÷ (Cd × A × 220). Here V is the gas volume in the separator and piping in m³, A is the orifice area in m², Cd is the discharge coefficient (about 0.65 for a sharp orifice) and τ is in seconds.
The 220 m/s comes from the choked mass flow, which is Cd × A × ρ × 0.685 × √(RT), where ρ is the gas density in the separator, R is 287 J/(kg·K) for air and T is in kelvin. At 85 °C, √(287 × 358) = 321 m/s, and 0.685 × 321 = 220 m/s. Because the mass in the separator is ρ × V, the density, and with it the pressure, falls at a rate proportional to itself. The gas is taken to stay near oil temperature (isothermal), which the hot oil makes a fair assumption. The flow stays choked until the separator falls to about 1.92 bar(a), about 0.9 bar(g). Below that it slows. The figures below were found by stepping the full calculation through to the end.
Worked example (illustrative): a 37 kW compressor with 50 L of gas space above the oil, a 4 mm vent orifice, and a start at 8.0 bar(g), which is 9.013 bar(a).
- A = π ÷ 4 × 0.004² = 1.257 × 10⁻⁵ m².
- τ = 0.050 ÷ (0.65 × 1.257 × 10⁻⁵ × 220) = 27.8 s.
- The choked phase to 1.92 bar(a) lasts 27.8 × ln(9.013 ÷ 1.918) = 43 s.
- The total time to fall to 0.2 bar(g) after a stop is about 57 s. Unloading from 8.0 to 2.0 bar(g) takes about 31 s.
The time scales with V ÷ d². The same machine behaves like this if the effective vent bore shrinks:
| Effective vent bore | Unload 8.0 → 2.0 bar(g) | Stop 8.0 → 0.2 bar(g) | Likely reason |
|---|---|---|---|
| 4 mm | about 31 s | about 57 s | Healthy baseline in this example |
| 3 mm | about 54 s | about 101 s | Silencer partly clogged, or the wrong orifice fitted |
| 2 mm | about 122 s | about 228 s | Silencer heavily oil-loaded, or the pilot or orifice varnished |
Time the blowdown when the machine is known to be healthy, and keep that figure as the baseline. Going from a 4 mm to a 3 mm bore makes the blowdown (4 ÷ 3)² = 1.78 times as long. A rise of more than about half over the baseline justifies inspecting the silencer, orifice and pilot line. If the restart delay is shorter than the stop time, and the controller does not also check separator pressure before a restart, the machine will try to restart against pressure.
Fault modes: stays open, leaks, will not open
The symptom table above starts from what the operator sees. This table starts from the failure itself and gives the measurement that confirms it.
| Fault mode | What you see and hear | Evidence on gauges | Check | Usual fix |
|---|---|---|---|---|
| Stays open | Loud, continuous blowing from the silencer on a load command; slow pressure build-up, or none where the vent has no restricting orifice | Separator pressure rises slowly or stalls below the MPV setting | Measure the coil voltage at the plug on a load command. Voltage present: a mechanical fault, such as a torn diaphragm or debris on the seat. No voltage: a controller or wiring fault | Seal kit or valve, after removing the debris source |
| Leaks while loaded | Steady hiss from the silencer, and an oily silencer face | Loaded pressure normal, but the machine runs loaded longer | Listen or use an ultrasonic detector at the silencer while loaded. Compare loaded hours with the previous log | Seat and seal kit |
| Will not open | No vent sound on unload or stop; heavy start or overload trip on restart | Separator stays at the loaded or unloaded pressure after stopping | Confirm the coil drops out. Check the pilot line and orifice for varnish, and look for a blocked silencer | Clean the pilot line, replace the silencer, or replace the valve |
| Opens slowly | Long vent after a stop; occasional restart trips in hot weather | Blowdown time well above the baseline | Time the vent and compare it with the table above | Replace the silencer, clean the orifice |
| Vents too fast | Oil spray at the silencer after every stop | Separator reaches 0 bar(g) far faster than the baseline | Check whether the restricting orifice is missing or has been enlarged | Refit the correct orifice |
What a small leak costs. A worn seat that leaks like a 1.5 mm hole at 8 bar(g) and 85 °C passes about 2.2 g/s of air. That is about 0.11 m³/min (3.9 cfm) of free air. At a typical 6 kW per m³/min, it is 0.66 kW of compressor power. Over 4,000 loaded hours a year that is 2,640 kWh. At an illustrative tariff of ₹8.50/kWh it costs about ₹22,440 a year, from one valve.
Oil mist at the silencer: Why fast venting foams the oil
Hot compressor oil under pressure holds dissolved air. The amount is roughly proportional to the absolute pressure, about 8–10% of the oil’s volume (as free air) for each bar. When the separator vents suddenly, that air comes out of solution as foam. The foam rises towards the vent connection, and the outgoing air carries oil droplets into the silencer. For this reason, many designs deliberately restrict the vent with a small orifice or a two-stage arrangement.
- A high oil level reduces the space above the oil, so the foam reaches the vent. Check the level only after the machine has stopped and fully vented.
- Wrong or degraded oil with poor anti-foam properties produces more foam. Mixing oil types makes this worse.
- A saturated silencer releases the oil it has trapped as drips or spray. A sintered or absorbent silencer that is oil-wet should be replaced, not washed and refitted.
- The vent route must not discharge inside the enclosure or near the cooling-air intake. Oil mist drawn into the cooler fins combines with Indian summer dust to form a hard crust.
Verifying a replacement valve before fitting
| Item | How to verify on the bench | Common mistake |
|---|---|---|
| Normal state (NO or NC) | With the coil unpowered, apply low-pressure air of 1–2 bar(g) to the inlet port. Repeat with the coil powered at its rated voltage. An NO valve passes air unpowered and closes powered | Fitting an NC valve where an NO valve was used, so the machine cannot vent on power failure |
| Coil voltage and type | Read the coil marking, then measure at the plug on the machine | Fitting a 24 V DC coil on a 24 V AC circuit, or the reverse |
| Orifice and flow | Gauge the orifice bore with a drill shank and compare it with the original | A 3 mm orifice in place of a 4 mm one gives 1.78 times the blowdown time |
| Pressure rating | Check that the marking is at least the safety valve setting | Fitting a valve rated for general utility pressure on a high-pressure package |
| Seal material and temperature | Confirm the seals are rated for the discharge temperature and for the oil used. FKM fluoroelastomer seals are common in hot, oil-laden service, and nitrile seals are limited to about 100 °C | Fitting nitrile seals on a machine that runs hot or uses an ester or PAG synthetic oil |
| Thread form | Count the threads per inch. ¼” and ⅜” BSP have 19 threads per inch and NPT has 18. At ½” and ¾”, both have 14, but the thread angles differ (55° for BSP, 60° for NPT) | Forcing an NPT fitting into a BSP port, which seals poorly and cracks bodies |
| Flow direction and pilot | Check the arrow on the body and the minimum pilot pressure | Fitting the valve reversed, so it passes air when closed |
For related parts, see minimum pressure valves, inlet valves and sensors and solenoids.
How Reddy Compressors helps
Our service engineers and technicians work on screw compressors of all major makes across Telangana and Andhra Pradesh from our base at Tarnaka, Secunderabad. As an independent multi-brand specialist, not an authorised dealer but with tie-ups with all major and reputed brands, we:
- Diagnose loading, unloading and restart faults on site, including blowdown, inlet and minimum-pressure valves.
- Supply blowdown valves, seal kits, coils, silencers and inlet valve kits, identified from your machine details and confirmed for fit before quoting.
- Include valve checks in preventive maintenance and AMC visits.
- Provide rental compressors while repairs are completed.
What to send us
- Nameplate photograph with serial number.
- Photographs of the blowdown valve, its label and coil.
- Description of the symptom and when it occurs.
- Controller alarm or trip messages.
- Working pressure and running hours.
Frequently asked questions
Why is air continuously blowing out of my compressor silencer?
If air flows from the silencer while the compressor is loaded, the blowdown valve is not closing fully. Common causes are a worn seal or seat, debris holding it open, a faulty coil or pilot signal, or a wrong replacement valve. Check that the control signal changes when the machine loads, then inspect the valve seat and seals. Constant venting wastes air and power.
Why does my screw compressor trip when restarting?
A common cause is that separator pressure has not vented before restart, so the motor starts against load. The blowdown valve may not be opening, or a blocked silencer or pilot line may slow venting. Other causes include a faulty inlet valve, low supply voltage or a motor fault. Check separator pressure before the restart and the blowdown operation first.
Is a blowdown valve normally open or normally closed?
It depends on the machine design. Many screw compressors use a valve that opens when power is removed, so a power failure leaves the machine vented and unloaded. Others use a different logic within the control circuit. Always replace the valve with one of the same normal state, coil voltage and rating, because reversing it can prevent safe venting or loading.
How long should a compressor take to blow down?
There is no single figure; it depends on the separator volume, unloading pressure, valve capacity and silencer restriction. The maker’s design sets the expected time, and the controller often has a restart delay matched to it. A noticeable increase in venting time compared with when the machine was healthy usually points to a partly blocked silencer, orifice or pilot line.
Why does oil come out of the blowdown silencer?
Oil mist from the silencer can result from a high oil level, venting too rapidly, a separator problem or a silencer that has become saturated over time. Check the oil level with the machine stopped and vented, inspect the silencer, and look for separator or scavenge line faults. Persistent oil discharge should be investigated, not just cleaned up.
Can I fit a blowdown valve from a different brand?
Possibly, if it matches the original’s normal state, coil voltage and type, pressure and temperature rating, port size, thread standard, flow direction and flow capacity, and its seals suit your lubricant. Fit alone is not enough. Send us the nameplate and valve photographs and we will identify a suitable genuine or aftermarket part and confirm fit before quoting.
What is compressor blowdown?
Compressor blowdown is the controlled venting of the pressurised air trapped in an oil-injected screw compressor’s separator vessel, oil circuit and internal piping. It happens every time the machine unloads, when it falls to a low unloaded pressure, and every time it stops, when it falls to near zero. Blowdown lets the motor restart without load, cuts unloaded power and protects the inlet valve from oil being forced out through it. It does not empty the plant’s receivers or pipework, which the minimum pressure valve isolates. A healthy blowdown takes the same time on every stop.
What is the blow down valve in a compressor and where is it fitted?
The blowdown valve is a small solenoid-operated or air-piloted valve connected to the separator vessel, upstream of the minimum pressure valve. It often sits on or next to the inlet valve block, and it vents through a silencer, sometimes through an orifice that sets the venting rate. Many screw compressors use a normally open valve, which closes when the machine loads and opens on unload, on stop or on loss of power. On piston compressors, a related unloader valve vents the delivery pipe so the motor restarts unloaded.
Why does my air compressor blow down every time it unloads?
This is normal. When pressure reaches the unload setting, the inlet valve closes and the blowdown valve opens. Separator pressure then falls to the unloaded level, typically about 1.5–3 bar(g), and the machine idles at low power. A hiss for a few tens of seconds at each unload is expected. It is a fault if the air keeps blowing while the machine is loaded, if line pressure also falls during unload (a leaking minimum pressure valve), or if the blowdown takes much longer than its healthy baseline.
Can I block or remove the blowdown valve?
No. Without a working blowdown, the separator stays pressurised after a stop. The motor then restarts against pressure, causing overload trips and heavy starting current. Oil can be forced back through the inlet valve into the air filter. Unloaded power also rises, because the machine idles against a higher separator pressure. If the valve leaks or sprays oil, repair the seat, replace the seal kit, renew the silencer or correct the oil level. Never plug the valve or its vent port.
Related pages
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.