Why is instrument air different from plant air?
Plant air drives tools, cylinders and blowing. Instrument air drives the final control elements of a process: control valves, on-off valves, dampers, positioners, I/P converters, pneumatic controllers and some analysers. These devices have small ports, fine nozzles and close-fitting spools. Water, oil and particles that a breaker would never notice can make a positioner hunt, a solenoid valve stick or a valve fail to move when the process needs it.
That is why instrument air is normally dried to a low pressure dew point and filtered to a tight class, typically specified under ISO 8573-1. A common practice is to keep the pressure dew point well below the lowest temperature any instrument line will see, so water never condenses in the tubing. Many plants use desiccant dryers, typically giving −20 °C or −40 °C pressure dew point, rather than refrigerated dryers at about +3 °C, for this reason. Check your plant’s own specification and use it as the target for the temporary supply.
Restoring header pressure is therefore not proof that instrument air has been restored. The temporary supply has to deliver the right quality and stable pressure where the instruments are.
Define the outage and the process state
Start with why instrument air is being lost, because each cause draws a different boundary for the temporary supply.
| Cause of outage | What is still usable | Temporary supply approach |
|---|---|---|
| Instrument-air compressor failure | Receiver, dryer, filters (if healthy) | Rental compressor feeding upstream of the existing dryer, within its ratings |
| Dryer failure | Compressors, receiver | Rental dryer and filters, or a packaged compressor with its own drying, downstream of the failed dryer |
| Contaminated header (water or oil) | Possibly only sections of the network | Independent clean supply to critical sections while the header is drained and cleaned |
| Planned receiver or dryer inspection | Everything else, by plan | Rental tied in and proven before the isolation |
| Electrical outage at the air station | Air network and treatment | Diesel rental, or electric rental on another supply or DG set |
Next, operations decides the process state the air must support: continued production, reduced rate, controlled shutdown, maintenance hold or restart. The air demand is very different in each. During a controlled shutdown, several actuators may need to stroke together, which can draw more air for a short time than the normal running average.
Do not assume that keeping every valve energised is the safest goal. Some valves are designed to move to a safe position on loss of air. The process owner and instrumentation team must decide the correct response. The temporary supply must never defeat an intended protective action or cause an uncontrolled restart.
Map the users and their air dependencies
Before sizing anything, list the instruments and actuators that depend on the affected air supply. A useful register records:
- Tag and service of each control valve, on-off valve, damper, positioner and analyser.
- Required supply pressure range at the device.
- Normal steady consumption (bleed from positioners and I/P converters) and stroke demand.
- Failure response on loss of air, loss of signal and loss of power. These are separate events, and “fail closed” should be checked on the installed assembly, not assumed from the service name.
- Local regulators, accumulators, volume tanks, non-return valves and lock-up devices between header and device.
- Which devices are safety-related and who must approve any change to their supply.
This register tells you which users must be on the temporary supply, which can be valved off, and how much air the temporary system really has to deliver.
How much temporary instrument air is needed?
Instrument-air demand has two parts. Steady demand is the continuous bleed from positioners, I/P converters, pneumatic controllers and purge users, plus leakage. Transient demand is the air used when valves stroke, especially during start-up, shutdown or a trip, when many may move at once.
- Take steady demand from the existing compressor’s loading, a flow meter, or a sum of device bleed figures from datasheets.
- Estimate the largest credible simultaneous stroke demand from the actuator volumes and the sequence operations will follow.
- Decide how much of that transient demand will come from a receiver rather than the compressor. A receiver on the temporary supply smooths these peaks and gives time if the rental trips.
- Set the delivery pressure from the minimum needed at the most demanding device, plus losses through the dryer, filters, hoses and regulators.
- Add a margin for leaks, and remember that the air used to purge and regenerate a heatless desiccant dryer is taken from the compressor’s output.
The last point is often missed. A heatless desiccant dryer uses a significant share of the dried air as purge, so the compressor must be sized for the instrument demand plus the dryer’s purge. Our duty specification guide covers the general method.
The temporary treatment train
For instrument air, the treatment train matters more than the compressor. A typical temporary package for an outage includes:
| Stage | Purpose | Notes |
|---|---|---|
| Compressor (oil-free or oil-injected) | Supplies the air | Oil-free removes the oil source; oil-injected needs coalescing and carbon filtration |
| Aftercooler and moisture separator | Remove bulk water and lower inlet temperature to the dryer | Essential in summer ambients of 40–45 °C, and above 45 °C on open sites |
| Pre-filter | Protects the dryer from liquid and particles | Automatic drain checked each shift |
| Desiccant dryer (or refrigerated where the specification allows) | Reaches the required pressure dew point | Sized for actual inlet temperature and pressure, not catalogue conditions |
| After-filter and, where needed, activated carbon | Remove desiccant dust and oil vapour | Differential pressure watched across the hire |
| Receiver and regulator | Stores air for transients, steadies delivery pressure | Rated for the pressure, with a relief valve |
If the plant’s own dryer and filters are healthy, the rental can feed them instead. Check their rated flow, maximum inlet temperature and pressure first. See temporary air treatment trains and, where oil must be excluded, temporary oil-free air supply.
Tie-in, changeover and verification
For a planned outage, the temporary supply should be connected, run and proven before the permanent station is isolated. For an unplanned outage the same steps apply, only faster.
- Agree the tie-in point, isolation valves and non-return valve with the plant’s instrumentation and operations teams.
- Run the temporary package on a vent or blow-down until dew point, filter condition and pressure are steady.
- Purge the hose and connection so desiccant dust, water or debris do not enter the instrument header.
- Open the temporary supply into the header with the permanent supply still available, where possible, so both sources are live for a short overlap.
- Isolate the permanent source and confirm pressure and dew point at representative critical devices, not just at the tie-in.
- Log the changeover time, readings and who approved it.
Watch for back-feeding. Without a non-return valve, the temporary supply can pressurise a section that maintenance technicians believe is isolated. Our tie-in and changeover guide covers this in more detail.
Reliability during the outage
A temporary instrument-air supply often protects the whole process, so plan for it to fail. Consider:
- Standby compressor. Two rental machines, one running and one ready, or an N+1 arrangement for larger demand. See redundancy and failure management.
- Receiver reserve. Enough stored air to allow an orderly response if the running unit trips.
- Power. Diesel rentals avoid dependence on the plant feeder; electric rentals may need a DG set or an alternative supply.
- Alarms. Low-pressure and high-dew-point alarms at the temporary package, visible to the control room.
- People. A named person or our operator on each shift checking drains, dew point, filter differential and fuel.
Our hire machines are serviced every 300 running hours, and our service engineers attend faults on them. For longer outages we plan servicing with you so it does not coincide with a critical process step.
Restoring permanent instrument air
When the permanent compressor or dryer returns, check it before releasing the rental. Confirm normal operation, stable discharge temperature, normal dryer cycling and dew point, and clean filters. Then reverse the changeover sequence with both sources briefly live, isolate the temporary supply, and depressurise before disconnecting. If the outage was caused by contamination, inspect and drain the header before relying on the permanent source. See demobilisation and return.
How Reddy Compressors helps
We hire oil-free and oil-injected compressors from our own fleet, with aftercoolers, dryers, filters, receivers and hoses arranged as a complete instrument-air package. We size the package from your instrument demand and the dryer’s purge, and prepare it in our workshop at Tarnaka, Secunderabad.
Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply rental compressors across India. We have supplied compressors on breakdown to leading pharmaceutical manufacturers in Hyderabad and Telangana, where clean, dry air is a production need. We also service and repair instrument-air compressors and dryers of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands as an independent multi-brand business, not an authorised dealer. We have tie-ups with all major and reputed brands. See our dryer service work.
Call +91 99590 60718 or +91 92468 88812, WhatsApp +91 99590 60718, or email reddycompressors@gmail.com. Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017.
What to send us
- Cause of the outage (compressor, dryer, contamination, power, planned inspection) and expected duration.
- Instrument-air demand in cfm or m³/min, and normal header pressure in bar or psig.
- Required pressure dew point and air-quality class, or your plant specification.
- Whether the existing dryer, filters and receiver can stay in service.
- Make, model and nameplate photo of the failed compressor or dryer.
- Power available at the tie-in position, or whether you need diesel.
- Site location, access and distance to the tie-in point.
Frequently asked questions
Can I rent a compressor for instrument air?
Yes. We hire oil-free and oil-injected compressors with aftercoolers, dryers, filters and receivers as a complete temporary instrument-air package. The package is sized for your instrument demand plus the dryer’s own purge air, and connected so that the air reaching your valves and positioners is dry, clean and at stable pressure. Send your demand, pressure and dew point specification.
What dew point do I need for temporary instrument air?
Use your plant’s instrument-air specification. As a general practice, the pressure dew point should be well below the lowest temperature any instrument line will reach, so water never condenses in tubing. Many plants use desiccant drying, typically −20 °C or −40 °C pressure dew point. A refrigerated dryer typically gives about +3 °C and may be acceptable only where the specification allows.
Does instrument air have to be oil-free?
Instrument air must be free of oil to the class your specification sets, because oil fouls positioners, I/P converters and solenoid valves. An oil-free compressor removes the source. An oil-injected rental can be used with coalescing filters and activated carbon to reach the required class, but the filters must be sized correctly and watched through the hire. We can supply either.
Why are my control valves misbehaving after a dryer failure?
When a dryer fails, water and sometimes oil reach the instrument header. Moisture in positioners, I/P converters and small solenoid valves causes sticking, hunting and slow response. Drain the header low points, check filters, and put a temporary dryer or a dried rental package in service. Once dry air is restored, devices that were flooded may still need cleaning.
How do I change over to a temporary instrument-air supply safely?
Run and prove the temporary package on a vent first, purge the hose, then open it into the header while the permanent supply is still live where possible. Isolate the permanent source and check pressure and dew point at critical devices. Fit a non-return valve to prevent back-feeding. The plant’s instrumentation and operations teams must approve each step.
How large a receiver should a temporary instrument-air system have?
Size it to cover the largest credible simultaneous valve stroke demand and to give time for an orderly response if the running compressor trips. That depends on actuator volumes, the pressure band you can accept and how quickly a standby can take over. Your instrumentation engineer should confirm the hold-up time needed; we size the receiver and compressor to suit.
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.
Receiver storage sizing
Estimate the receiver volume needed to ride through a short burst of demand without the pressure falling below the limit.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Burst demand above compressor output (m³/min free air): 5
- Burst duration (s): 20
- Allowed pressure fall (bar): 0.7
- Receiver volume you already have (litres; 0 to skip the hold-up check): 1520
Result
- Receiver volume needed: 2,412 litres
- 2.41 m³
- Practical size (with 25% margin): 3,016 litres
- Hold-up time of the receiver you have: 12.6 s before the pressure falls by 0.7 bar
Method. V = Q × t × P_atm / ΔP, where Q is the shortfall in free air (m³/min), t the time (min), ΔP the allowed pressure fall (bar) and P_atm = 1.013 bar. The same relation gives the hold-up time of an existing receiver: t = V × ΔP / (Q × P_atm). Isothermal; for a real system add a margin.
What to take from it. A receiver buys time. It does not create capacity: once it is spent the pressure keeps falling until the compressors catch up.
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.
Adsorption dryer purge air
A desiccant dryer uses some of the dried air to regenerate. Put a cost on that purge.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Dryer flow (m³/min): 12
- Purge fraction (%): 15
- Specific power (kW per m³/min): 7
- Running hours per year: 6000
Result
- Purge air: 1.8 m³/min
- Compressor power for purge: 12.6 kW
- Purge energy per year: 75,600 kWh
- Purge cost per year: Enter your electricity tariff (cost per kWh) above
Method. Purge flow = dryer flow × purge fraction. Energy = purge flow × specific power × hours × tariff. Heatless dryers are commonly in the range of 15–20% purge; heated and blower-purge types use less. Use the datasheet value for the dryer in question.
What to take from it. Purge air is generated by the compressor and paid for like any other air. Use the dryer’s measured purge rather than the nameplate where you can.