Inlet and unloader valves for screw compressors

What does an inlet valve do on a screw compressor?

On an oil-injected rotary screw compressor the inlet valve sits between the intake filter and the airend suction port. It is the main capacity-control device on most fixed-speed machines. When it is open, the airend draws its full volume and the compressor delivers its rated free air delivery (FAD). When it closes, the airend is starved of air and delivery falls to near zero while the motor keeps running.

The term “unloader valve” is used loosely in the trade. On a screw package it usually means the inlet valve assembly together with the blow-down (vent) valve that releases pressure from the separator tank when the machine unloads. On a reciprocating compressor the unloader is a different device that holds the suction valve plates open. This page covers screw-compressor inlet and unloader arrangements; for piston machines see our page on reciprocating unloader systems.

Most inlet valves do three jobs:

• Capacity control: open for load, closed for unload, and on some machines part-open to modulate delivery.
• Start-up protection: staying closed so the motor can start against a light load, particularly in star-delta starting.
• Shutdown protection: closing quickly at stop so oil and air do not reverse out through the intake filter. Many designs include a non-return disc or plate for exactly this reason.

Common inlet valve designs and how they are actuated

Different manufacturers, and different generations of the same brand, use quite different valve mechanisms. Knowing which type you have tells you what can fail and what can be rebuilt.

Actuation is almost always pneumatic, taken from the separator tank or the discharge side, and switched by a solenoid valve under the controller. Older machines may use a mechanical pressure switch and a pneumatic regulator for modulation. Newer machines use a pressure transducer and an electronic controller that switches the solenoid and, on some designs, reads a position or limit signal back from the valve.

Load/unload versus modulation: why the valve matters for energy

How the inlet valve is controlled has a direct effect on specific power, the electrical input per unit of air delivered.

• Load/unload control: the valve is either fully open or fully closed. When unloaded, the blow-down valve vents the separator so the machine idles at low pressure. Unloaded power is typically in the range of a quarter to a third of full-load power, which is why long unload periods waste energy.
• Modulation: the valve throttles the intake to match demand. Delivery falls, but power does not fall in proportion, because the airend still works against a high pressure ratio. Modulation keeps pressure steady but is inefficient at low loads.
• Modulation with unload: the valve throttles down to a set point and then unloads fully. This is a common compromise on older fixed-speed machines.
• Variable speed drive: capacity is controlled by motor speed and the inlet valve mainly opens and closes at start, stop and minimum speed. The valve still needs to seal properly.

If the valve does not close fully on unload, the compressor keeps pumping and the blow-down valve dumps that air to atmosphere. The result is high unloaded power, oil carry-over through the vent silencer and a separator tank that never fully depressurises. A valve that opens slowly or only part-way robs the plant of FAD and makes pressure sag at peak demand.

Symptoms and their likely causes

Most inlet valve complaints fall into a handful of patterns. The table lists what we usually look at for each, in rough order of likelihood. The valve itself is often not the first cause.

How we diagnose an inlet or unloader problem before replacing parts

Swapping an inlet valve because the machine will not load is an expensive guess. The valve is one link in a control chain: controller, pressure signal, solenoid, control-air path, actuator, valve mechanism and blow-down. Our engineers work along that chain in order.

1. Record the machine identity: make, model, serial number, running hours, controller type and the fault history shown on the display.
2. Watch a full cycle: start, load, unload, idle and stop, noting pressures and timings on the gauge and controller.
3. Check the signal: compare the transducer or pressure-switch reading against a calibrated test gauge.
4. Check the solenoid: confirm the coil receives the correct voltage on command and that the valve actually shifts. A coil can energise while the plunger stays stuck.
5. Check the control-air path: look for cracked nylon tubes, leaking fittings, blocked orifices and choked control-air filters. Small leaks here commonly cause slow loading.
6. Check the actuator and mechanism: with the machine isolated and depressurised, inspect the piston or diaphragm, spring, seat, plate and linkage for wear, varnish and damage.
7. Check the blow-down valve: confirm it vents the tank on unload and at stop, and closes on load.
8. Decide: rebuild with a seal kit, replace individual parts, replace the assembly, or correct a control fault. We explain the reason for the decision.

All work inside the valve follows lock-out and full depressurisation of the separator tank. A screw package holds stored pressure and hot oil even after the motor stops.

Repair kit or complete valve: how to decide

Many inlet valves are designed to be rebuilt. A service kit typically contains the O-rings, piston seals, diaphragm, valve plate or seat and, sometimes, a spring. Whether a kit is the right answer depends on what the inspection finds.

Varnish inside the inlet valve is a warning sign rather than just a valve fault. It usually points to high oil temperature, an overdue oil change or a lubricant that is not suited to the duty. Fixing the valve without dealing with the cause brings the same fault back.

Identifying the right inlet valve or kit

Inlet valves that look alike can differ in port size, mounting pattern, spring rate, internal orifice, solenoid voltage and actuation logic (normally open or normally closed). Fitting a valve because it “looks the same” can leave a machine that loads at the wrong time or not at all. We identify parts from the machine, not from a photograph alone.

• Compressor make, model and serial number from the nameplate.
• Part number cast or stamped on the valve body, or on a label, including any revision letter.
• Solenoid coil voltage and type (AC or DC) printed on the coil.
• Controller type, since the same compressor model may have been built with different control systems.
• Whether the machine is load/unload, modulating or variable speed.
• The old part or kit packaging, where available.

Manufacturers frequently supersede part numbers. A superseded valve may need a different solenoid, adapter or control-line connection. We confirm the fit before we quote.

Genuine, OEM-equivalent and rebuilt options

As an independent multi-brand specialist we can usually offer more than one route. A genuine part from the compressor manufacturer is the reference for fit and function. Quality aftermarket kits and valves made to match the original specification are a practical choice for many machines, especially older ones. Rebuilding the existing valve with a good seal kit is often the most sensible answer when the body is sound. We tell you which option we are quoting and why, so you can weigh cost against the criticality of the machine.

Checks after fitting a new or rebuilt valve

A valve job is not finished when the bolts are tightened. Before handing the machine back we run it through its control sequence and check that it behaves correctly.

• The machine starts with the valve closed and loads only when commanded.
• Load and unload pressures match the set points, measured on a test gauge.
• The blow-down valve vents the tank on unload and at stop.
• No oil comes from the intake at stop or from the blow-down silencer.
• Full-load current and delivery pressure are consistent with normal operation for that machine.
• Control-air lines and fittings are leak-free.
• Modulating machines hold a steady pressure without hunting.

We record what was found, what was replaced and the readings taken, so the next service has a baseline.

Inlet valve types and how each behaves when unloaded

The design table above shows how each valve moves. What matters for energy, oil carry-over and diagnosis is what happens to the pressures inside the machine when the valve closes. The figures are typical for oil-injected screw compressors at about 7.5 bar(g); confirm against the manual for the model.

Two practical points follow. First, a machine that never blows down after unloading is still running against full sump pressure: unloaded power then stays near 40–60% instead of falling to a quarter or a third. Second, a valve that does not close fully on unload shows as oil from the blow-down silencer and higher unloaded current, not as a pressure problem.

Part-load power: a worked comparison of control modes

Modulation sounds efficient because delivery follows demand smoothly. The physics says otherwise. A screw airend has fixed displacement, so throttling the inlet reduces inlet pressure, and the airend still compresses to full discharge pressure.

Why modulation wastes power. At 7.5 bar(g) discharge (8.51 bar(a)) and full inlet (1.013 bar(a)), the pressure ratio is 8.51 ÷ 1.013 = 8.40. Throttling to 50% flow lowers inlet pressure to about 0.507 bar(a), so the ratio rises to 8.51 ÷ 0.507 = 16.8. Ideal adiabatic power is proportional to mass flow × (r^0.286 − 1):

• Full load: 1.0 × (8.40^0.286 − 1) = 0.838
• 50% flow: 0.5 × (16.8^0.286 − 1) = 0.620
• Ratio: 0.620 ÷ 0.838 = 0.74. Half the air still needs 74% of the ideal gas power.

That 74% is the ideal figure, and real machines are worse. Mechanical, fan and oil-churning losses do not fall with flow, so a real modulating screw compressor typically draws about 85–90% of full-load power at 50% flow. A common planning approximation is power fraction ≈ 0.70 + 0.30 × capacity fraction, which gives 85% at half flow and sits at the favourable end of that range.

Worked example (illustrative). A 55 kW fixed-speed screw compressor at 7.5 bar(g) runs 6,000 hours a year at an average 50% demand. Electricity is taken at ₹8.50/kWh (illustrative tariff).

If the modulating machine draws 90% at half flow, the gap grows to (0.90 − 0.625) × 55 = 15.13 kW, 90,750 kWh and ₹7.71 lakh a year. On the planning approximation, at 75% average demand the gap narrows to 0.925 − 0.8125 = 0.1125 of 55 kW, or 6.19 kW and about 37,100 kWh a year. Modulation is least wasteful near full load and most wasteful at the low demand typical of night shifts and weekends.

The load/unload figure depends on the inlet valve closing fully and the sump blowing down promptly. With a small receiver the machine cycles often, never completes blow-down, and its real unloaded power approaches the modulation curve. Where demand varies widely, compare both modes with a variable-speed machine using the specific power comparison calculator and the notes on unload and idle losses.

Inlet valve service kit: contents and interval

Kits differ by make and valve design, but a complete inlet valve service kit for an oil-injected screw compressor usually contains the parts below. Check the kit against an exploded drawing before the valve is opened, so the machine is not left stripped waiting for a missing O-ring.

Interval. Many makes schedule an inlet valve kit at between 4,000 and 8,000 running hours, often at the same visit as the minimum-pressure valve kit. Bring it forward on hot, dusty sites, after any oil-overheating event, or when the on-site test below shows slow response. Always use the correct lubricant on seals: the wrong grease can swell them.

How to test an inlet valve on site

These checks take under an hour with basic instruments: a clamp meter, a compound (vacuum and pressure) gauge on the airend inlet test port where one is provided, the machine’s own sump and line gauges, and a stopwatch. Follow the lock-out and depressurisation procedure before fitting any gauge.

1. Baseline at full load. Record line pressure, sump pressure and motor current at set pressure. Inlet vacuum at full load should be small, typically a few tens of millibar, rising with a dirty filter.
2. Load response. From unloaded, open a drain or valve to drop pressure to the load point and time from the load signal to full current. A slow ramp suggests low control pressure, hardened seals or varnish.
3. Unload behaviour. At the unload point, the inlet gauge should swing to deep vacuum, typically below −0.8 bar(g), at once. A shallow vacuum means the plate is not closing fully.
4. Blow-down. Time the sump pressure falling to its unloaded level. Compare with the manual or with the reading taken when the valve was new; a time that has doubled points to a blocked blow-down orifice or a sticking solenoid.
5. Unloaded current. Read it after blow-down is complete. Unloaded current is usually a higher percentage of full-load current than unloaded power is of full-load power, because power factor falls at light load; compare current with your own baseline, not with the power figure.
6. Stop test. Stop the machine and watch the air filter. Any oil mist or air reversal means the non-return function is failing.
7. Modulating valves only. Close off demand slowly and record sump pressure and current at several delivery points. Hunting of more than about ±0.2 bar points to a worn diaphragm, linkage play or a contaminated regulator.

Record every reading after a rebuild; it becomes the baseline for the next test and for the post-fitting checks described above.

How Reddy Compressors helps

Reddy Compressors has served industry from Secunderabad since 2002. We are an independent multi-brand specialist working on screw compressors from Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic, Gardner Denver, Sullair, Doosan, Kirloskar, Bobcat and many others. We are not an authorised distributor of these brands. We have tie-ups with all major and reputed brands.

For inlet and unloader problems we can diagnose the control chain on site, rebuild valves with suitable kits, supply replacement valves, solenoids and control-line parts, and correct controller settings. We identify the part from your machine details and confirm fit before quoting. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spare parts across India. If a machine is down for longer than you can afford, we can also discuss a temporary rental compressor.

Plants in and around Hyderabad run compressors hard through hot, dusty summers. High ambient temperature raises oil temperature, which speeds up varnish and seal ageing inside inlet valves, while dust loading on intake filters adds restriction. We take these local conditions into account when we advise on rebuild intervals.

What to send us

• Compressor make, model and serial number, ideally a clear photo of the nameplate.
• A photo of the inlet valve, showing any cast or label part number.
• Solenoid coil details (voltage and AC or DC).
• The symptom: will not load, will not unload, cycling, hunting or oil from the intake.
• Controller type and any fault messages, plus running hours.
• Your location and how critical the machine is to production.

Frequently asked questions

Why is my screw compressor not loading?

A screw compressor that runs but will not load usually has a control fault rather than a failed valve. Common causes are a solenoid that is not energising or is stuck, a cracked or leaking control-air tube, a blocked orifice, a pressure transducer reading wrongly, or a seized inlet valve piston. Check the controller command and solenoid first, then the control-air path, before opening the valve itself.

Why does my air compressor not unload when it reaches pressure?

If pressure keeps climbing past the unload set point, the controller may not be seeing the correct pressure, the solenoid may be stuck in the load position, or the inlet valve may be jammed open by a broken spring, varnish or a damaged plate. This is a safety concern because the safety valve becomes the only protection. Stop the machine and have the control chain checked.

What is the difference between an inlet valve and an unloader valve?

On a screw compressor the inlet valve controls how much air enters the airend, while the unloader or blow-down valve releases separator tank pressure when the machine unloads or stops. The two work together and are sometimes built into one block. On reciprocating compressors, an unloader is a separate device that holds the suction valves open to stop compression.

Can an inlet valve be repaired with a kit instead of replacing it?

Yes, in many cases. If the valve body, piston bore and seat are in good condition, a seal kit with new O-rings, piston seals, diaphragm and sometimes a plate or spring restores normal operation. If the bore is scored, the seat is damaged or the linkage is badly worn, a kit will not hold and the assembly should be replaced.

Why does oil come out of the compressor air filter when it stops?

Oil at the intake filter after shutdown usually means the inlet valve is not closing quickly or fully enough to stop the reverse flow of air and oil from the airend. The non-return plate or its seal may be worn, or the blow-down may be too slow. An over-filled oil sump can make it worse. Fix it promptly, because oil-soaked filters restrict airflow.

How often should a compressor inlet valve be serviced?

There is no single interval for every machine. Manufacturers usually specify an inlet valve kit at a fixed number of running hours, often alongside a major service. Hot ambient conditions, high oil temperature, frequent load cycling and dusty intake air shorten seal life. Follow the manufacturer schedule for your model and inspect sooner if loading becomes slow or erratic.

Can a distributor support me with an inlet valve for my compressor model?

Support from any supplier, whether a distributor or an independent multi-brand specialist, starts with correct identification: compressor make, model and serial number, the valve’s part number or casting marks, the solenoid coil voltage and the control mode. With those, a supplier can confirm whether the genuine part, a matched equivalent or a seal kit fits. Send clear photos of the valve fitted and removed, as described in the identification section above.

What pressure should the sump fall to when an inlet valve unloads?

On most oil-injected screw compressors the sump, or separator, pressure blows down to a low level, typically about 1.5–3 bar(g), within a short time of unloading. That residual pressure keeps oil circulating while the airend runs unloaded. If sump pressure stays near line pressure, unloaded power stays high and the cause is usually the blow-down solenoid, a blocked orifice or an inlet valve that is not closing fully.

Is modulation or load/unload control more efficient?

Load/unload is usually more efficient at part load, provided the receiver is large enough for complete blow-down. Modulation throttles the inlet, so the airend compresses less air over a higher pressure ratio. At 50% demand a real modulating machine typically draws about 85–90% of full-load power, while load/unload averages about 62.5% with prompt blow-down. For a 55 kW machine over 6,000 hours the difference is about 74,000–91,000 kWh a year.

Related pages

• Compressor service kit finder: which kit, what is in it, when it is due
• Sensors, transducers and solenoids
• Minimum-pressure and check valves
• Compressor service kits
• Reducing unload and idle losses
• Find my compressor part
• Reciprocating compressor unloader systems