What does a temporary air changeover actually have to achieve?
Start by naming the arrangement, because each one has a different flow path and a different failure consequence. Temporary air may replace the whole compressor house during an overhaul, feed one isolated section while a header is modified, top up a healthy station that cannot meet peak demand, or keep only essential users alive during a shutdown.
Then define success at the point where the plant receives the air, not at the rental compressor outlet. A machine showing 7 bar (101.5 psig) on its own gauge tells you nothing about the pressure at the far end of a 60 m temporary hose, after a dryer, filters and a reducing tee. Write down:
- the minimum and maximum acceptable pressure at the receiving header;
- the net flow the users need after dryer purge, drains and hose losses;
- the air quality required, stated as an ISO 8573-1 class where it matters (instrument air, food contact, pharma);
- how long the plant can tolerate a pressure dip during transfer, if at all;
- how long stable operation must be observed before the permanent station is released for work.
Separate “connected” from “ready for service”. A package can be on site and piped up while the dryer is still reaching dew point or the electrical supply is still being checked. The schedule should show when acceptable air is actually available.
Survey the installed system and check the drawing against reality
Piping drawings in Indian plants are often years out of date. Before choosing a connection point, walk the line with the drawing in hand and confirm:
- where each compressor, dryer, filter and receiver actually discharges, and which valves really isolate what;
- pipe material and size at the candidate tie-in point, and the flange or thread standard;
- non-return valves that may hold pressure on one side of an isolation;
- drain points, relief valves and gauges that will be needed during the transfer;
- any users that cannot tolerate an interruption, such as instrument air to control valves, breathing air or process purges.
Mark up the drawing with what you find. The marked-up copy, not the original, becomes the basis for the changeover plan.
Where should the temporary compressor connect?
The right connection point depends on what the temporary air must replace. The table summarises the usual options.
| Connection point | Typical use | Advantages | Watch-outs |
|---|---|---|---|
| Spare flange or tee on the main receiver | Full station replacement | Uses existing storage to buffer transients; downstream treatment can stay in service | Receiver must be isolable from the machines being worked on; check relief valve setting |
| Main header downstream of the dryers | Dryer or compressor house shutdown | Bypasses equipment under maintenance | Temporary package must bring its own dryer and filters to the required class |
| Branch header to one section | Supporting one building or line | Smaller package, shorter hose run | Back-feeding into the rest of the network if isolation is incomplete |
| Dedicated hot-tap or new tie-in valve | Repeated or planned future rentals | Clean, repeatable interface for future shutdowns | Needs a planned outage and qualified work to install the first time |
If the plant expects regular shutdowns, a permanent rental tie-in valve with a blind flange, gauge point and drain, sized for the full station flow, often pays for itself in saved preparation time on every later job.
Isolation, trapped pressure and interface ratings
Isolation must not depend on a compressor’s control system. A stopped machine with its controller in standby can restart on a pressure signal. The equipment being worked on needs positive isolation: a closed and locked valve, a blind or a spool removed, under the plant’s permit-to-work and lock-out procedure.
Identify every place where pressure can be trapped: between two closed valves, behind a non-return valve, inside a receiver or a dryer vessel, and in the temporary hose itself. Each one needs a known vent route and a gauge to prove it is at zero before anyone breaks a joint. Compressed air stores a great deal of energy; a hose whipping loose or a flange releasing under pressure causes serious injury.
Check that everything in the temporary path is rated for the highest pressure it can see. That includes hoses, couplings, manifolds, adaptors and any reducing fittings, and it means the rental compressor’s maximum unload or relief pressure, not its working pressure. Whip checks (safety restraints) are needed on every hose coupling. Thread and flange standards must match; improvised adaptors are a common weak point.
Confirm air quality before the process sees it
A diesel portable straight off a trailer delivers hot, saturated air with some oil carry-over. That may be acceptable for a construction tool but not for instrument air or a pharma process. Before the temporary source is opened to the plant:
- run the package against a blow-off or a test load until the aftercooler, separator and dryer have stabilised;
- check pressure dew point at the outlet; a refrigerated dryer typically gives about +3 °C, a desiccant dryer typically −20 °C or −40 °C;
- confirm filter elements are new and drains are working;
- purge the temporary hoses and manifold so that rust, dust and water from storage do not go into the plant header.
Where the plant requires oil-free air, specify an oil-free rental package rather than relying on filtration alone. Our temporary air treatment page covers dryer and filter selection in more detail.
How to choose the transfer strategy
There are three common ways to move users from the permanent station to temporary air.
- Parallel then isolate. Pressure settings are arranged so the temporary machine takes load, then the permanent compressors are stopped and isolated. This gives the smoothest transfer but needs a careful look at control interaction.
- Planned short interruption. The plant agrees a short window with no air. The permanent station is stopped, the connection is made, and the temporary source is started. This is simpler but only works where the users can stop safely.
- Section by section. Users are moved over one branch at a time using branch valves. This suits large sites with a ring main and several buildings.
The strategy is set by the plant design and what the process can tolerate, not by what is convenient for the rental crew.
Capacity, storage and pressure through every stage
Check that net capacity is adequate in every transfer state, not only at steady running. Typical traps:
- Charging the temporary volume. Empty hoses, a new receiver and a dryer vessel must be pressurised before they deliver anything. If they are opened to a live header, they pull the header down.
- Dryer purge and drains. A heatless desiccant dryer uses a significant share of the dried air for regeneration. Size the compressor for the users plus that purge.
- Restart events. When a production line restarts after a break, demand can jump well above the average. Storage and spare capacity carry the plant through that period.
- Pressure drop in the temporary path. Long, small-bore hoses lose pressure quickly at high flow. Keep runs short and size the hose for the full flow.
Plan pressure settings for both sources together. If the temporary compressor is set lower than the permanent station, it will sit unloaded and contribute nothing. If it is set much higher, it may push through equipment not rated for that pressure. Remember that every extra bar of generation pressure costs about 7% more power, so do not set the temporary source higher than needed just to be safe.
Roles, readiness checks and a realistic fallback
Before transfer day, agree who operates which valve, who watches which gauge, who decides to stop, and how they talk to each other (radio channel, phone numbers). One person should control the sequence.
Turn the plan into observable readiness checks that can be ticked off on the day:
- temporary package running, loaded against a test outlet, with stable pressure and dew point;
- hoses purged, restrained and leak-checked;
- isolation valves identified, tagged and proven operable;
- gauges in place at the temporary outlet, the tie-in point and a remote user;
- fuel or electrical supply confirmed for the full planned duration;
- fallback ready: permanent station still able to restart, or a second temporary machine on standby.
Define stop conditions in advance, for example pressure at a critical user falling below an agreed limit, an unexpected alarm on the temporary package, or a leak at the interface. When a stop condition is reached, the team reverts to the fallback without debate. Our page on redundancy and failure management covers standby arrangements.
Running on temporary air and switching back
While the plant runs on temporary air, log pressure at the receiving header and a remote user, the package’s discharge temperature (for an oil-injected screw typically about 75–95 °C), fuel level and dryer dew point. Summer ambient of 40–45 °C, and above 45 °C on open sites, pushes portable packages hard, so watch for high-temperature trips, keep cooling air paths clear and shade the package where the layout allows.
Switching back to permanent supply is a changeover in its own right. Before removing the temporary source:
- confirm the repaired or overhauled permanent equipment has been commissioned and runs stably;
- restore the permanent pressure settings and bring the station on line in parallel where the design permits;
- unload and stop the temporary package, isolate it, vent the hose and prove zero pressure;
- disconnect, fit blinds or close and cap the tie-in valve, and record the final state;
- remove temporary tags and locks only once each step is complete.
On our own hire jobs we service the package every 300 running hours, including an oil change, so longer shutdowns need that interval built into the plan.
How Reddy Compressors helps
Reddy Compressors has supplied, rented and serviced compressors since 2002 from our office at Tarnaka, Secunderabad. We work across makes, including ELGi, Atlas Copco, Ingersoll Rand, KAESER and Chicago Pneumatic, as an independent multi-brand specialist rather than an authorised dealer. For a tie-in and changeover we can:
- review your drawing and connection options with your engineers, and visit site where needed;
- supply rental compressors from our own fleet, 50–2,000 cfm and 2–30 bar (29–435.1 psig), diesel and electric, with hoses, manifolds, aftercoolers and dryers as required;
- provide operators and service engineers during the transfer and the hire period;
- help plan the switch back to your permanent station, and service or overhaul that station through our team of over 20 service engineers and technicians.
Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply rental equipment across India. Your plant’s own permit, lock-out and valve-operating procedures always govern the work on site.
What to send us
- A short description of the work: what is being shut down, and for how long.
- Required pressure, flow and air quality at the users, and any users that cannot be interrupted.
- The piping drawing or a sketch, with photos of the proposed connection point and its size and flange or thread type.
- Details of the existing compressors, dryers and receivers (make, model, nameplate photos).
- Site location, access for the package, and the available power or fuel arrangements.
- Your planned dates and the window allowed for transfer.
Frequently asked questions
Can a rental compressor be connected without shutting down the plant?
Often yes, if there is a spare valved connection on the receiver or header that can be opened while the plant runs. The temporary package is started on a test load, the hose is purged and pressurised, then the valve is opened and pressure settings are arranged so it takes load. If no such connection exists, a short planned interruption is usually needed to fit one, and that is the best moment to install a permanent rental tie-in point.
What size hose do I need for a temporary air connection?
It depends on the flow, the length of the run and how much pressure drop you can accept. As a rule, keep runs short and avoid small-bore hose at high flow, because losses rise steeply with velocity. A large station usually needs several hoses into a manifold or a single large-bore line. Every hose and fitting must be rated above the compressor’s maximum pressure and restrained with whip checks. Send us the flow, pressure and distance and we will size it.
Why did plant pressure drop when we opened the temporary air valve?
The most common reason is that the temporary hoses, receiver or dryer were not fully pressurised before the valve was opened, so the plant header emptied into them. Other causes are a temporary compressor set at a lower pressure than the permanent machines, a dryer still regenerating, or too much pressure drop in an undersized hose. Charge the temporary system first, match the settings, and open the valve slowly while watching a remote gauge.
Do I need a dryer with a rental compressor?
For most factory networks, yes. A portable compressor delivers saturated air, and free water will reach users and the existing pipework. If your plant already has dryers downstream of the tie-in point that remain in service, those may be enough. If the temporary air enters after the plant dryers, or the users need instrument or process quality air, the rental package needs its own aftercooler, separator, dryer and filters.
How do we isolate our compressor safely during an overhaul?
Do not rely on the compressor controller or a stop button. Close and lock the discharge isolation valve, isolate and lock off the electrical supply under your permit system, vent the trapped air between the machine and the closed valve, and prove zero pressure on a gauge before any joint is opened. Check for non-return valves that may hold pressure in sections you expect to be empty.
How long can a plant run on rental compressed air?
As long as the package is properly serviced and supplied with fuel or power. Rental compressors run for weeks or months on shutdown and long-term hire jobs. On our hire jobs we carry out an oil change and servicing every 300 running hours and keep operators on shift where the job needs it, so the planned duration simply sets the service schedule and fuel logistics.
Related pages
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.