Power station instrument, service and ash conveying air

Where is compressed air used in a power station?

Thermal (coal and lignite), gas-based, combined-cycle, biomass and waste-to-energy plants all depend on compressed air, although the users vary with technology and owner design. Hydro stations and solar parks need much less, mainly for governors, braking, cleaning and maintenance. A typical coal-fired unit has the widest mix:

Instrument air: protect the controls first

Loss of instrument air on a running unit can move control valves and dampers to their fail-safe positions, trip feed-water or draught systems, and in the worst case trip the unit. Instrument air is therefore treated as a critical utility with its own generation, treatment and priority, not as one branch of plant air.

• Dry it properly. Most stations specify heatless or heated desiccant dryers to about −40 °C PDP for instrument air, because lines run outdoors, across the boiler house and through areas that cool at night. Refrigerated dryers at a typical +3 °C PDP are generally reserved for service air.
• Keep it oil-free. Oil-free screw or centrifugal compressors are common for instrument air. Where oil-injected screw machines are used, they need coalescing and activated-carbon filtration that is monitored and changed on time.
• Give it priority. Service air should be taken through a priority (pressure-sustaining) valve, which closes as header pressure falls so instruments keep their air.
• Store enough air. Receivers sized to carry the critical load for the time it takes a standby compressor to start and load, or for operators to act, are a basic safeguard. See air receiver sizing.
• Monitor it. Header pressure, dew point and dryer status should alarm in the control room, not just on the local panel.

Service air and how it differs

Service air supports maintenance, cleaning, workshops and general utility duties. It matters, especially during overhauls, but it can be interrupted for short periods. Problems arise when service air shares a header with instrument air without protection. A burst hose, someone using an open blowgun for cleaning, or a large tool can drag down instrument pressure at the worst moment. Good designs either use separate compressors and headers, or share generation with a priority valve and a clear rule that no cross-connection can be opened without permission.

Ash handling conveying air

Fly ash collected under ESP or bag-filter hoppers is commonly moved to silos by dense-phase pneumatic conveying. Vessels below the hoppers fill with ash, are pressurised with compressed air, then push the ash through a pipeline to the silo. This is one of the largest air users in a coal-fired station.

1. Size from the conveying system design. Air flow and pressure depend on ash rate, pipeline length and bends, and the vessel cycle. The conveying system supplier sets these numbers, and the compressor station must meet them at the vessels, not just at the compressor outlet.
2. Dry the air. Fly ash is hygroscopic and fine. Wet conveying air causes build-up and choking in pipelines and fluidising pads. Refrigerated drying is a minimum, and many plants use desiccant drying.
3. Keep conveying separate from instrument air. Conveying vessels draw very large, fast batches of air that cause pressure swings. Dedicated conveying compressors and receivers stop these swings from reaching the controls.
4. Allow for abrasion and dust. Compressor intakes near ash areas need good filtration and more frequent filter changes.
5. Plan standby. If ash cannot be evacuated, hoppers fill and the unit may need to reduce load. Duty-standby conveying compressors, or a rental tie-in, protect generation.

Bottom ash systems are often hydraulic, but some plants use dry bottom ash or pneumatic duties. Silo fluidising and unloading usually rely on low-pressure blowers. More detail is on dense-phase conveying air supply.

Typical compressed-air architecture

Cross-connections between these systems are useful during emergencies but dangerous if left open. Keep each one valved, labelled, normally closed, and recorded in the operating procedures.

Construction-phase and shutdown temporary air

Before a station’s own compressors are commissioned, and whenever units are overhauled, temporary air carries much of the work. Typical uses include:

• Boiler erection and structural work: grinding, impact wrenches, air hoists.
• Surface preparation and painting of structures, ducts and tanks, which need aftercooled, moisture-separated air.
• Pipeline cleaning, drying and pneumatic testing, which may need high-pressure or high-volume packages.
• Temporary instrument air for early commissioning, before the permanent instrument air plant is handed over, with a dryer and filters on the rental package.
• Annual overhauls and capital maintenance, when service-air demand jumps far above normal and permanent compressors may themselves be under overhaul.

Our team has worked on this kind of duty. On a thermal power project in Karnataka, we hired out a 400 cfm electric compressor and a 160 kVA DG set to a mechanical construction contractor. We delivered and commissioned them, trained the operators, and handled preventive and breakdown maintenance. We have also worked on the power project at Muthukur in Nellore district, Andhra Pradesh, for the plant developer under its EPC contractor. These projects were delivered by our team across Reddy Generators and Reddy Compressors.

For a shutdown, brief the hire early. Agree the air demand per work front, the pressure needed, any air-quality need (for example, painting or temporary instrument air), the duration, and who operates and refuels the machines. On hire jobs we service every 300 running hours. See temporary air for planned shutdowns.

Energy and efficiency

The compressed-air plant is a small share of a station’s auxiliary power, but it runs all year, so savings add up. The usual opportunities are:

• Reducing header pressure to what the devices really need. Each extra bar costs about 7% more compressor power.
• Repairing leaks at actuator tubing, filter-regulators and hose connections, which are common in hot, vibrating areas of the boiler and turbine buildings.
• Using dew-point-dependent switching on desiccant dryers, so purge air is not wasted at low load.
• Stopping open blowgun cleaning on instrument headers.
• Sequencing compressors so one machine trims while others run fully loaded or stop.
• Considering heat recovery where a regular hot-water use exists.

Pressure, power and flow logging during a normal week and during a start-up gives the facts needed for these decisions. See compressed air audits.

Indian context

India’s thermal fleet is spread across coal-bearing and coastal regions, including Telangana, Andhra Pradesh, Karnataka, Maharashtra, Chhattisgarh and Odisha. Gas, solar and hydro add more variety. Conditions are harsh for compressors. Summer ambient temperatures of 40–45 °C, and above 45 °C on open sites, high humidity on the coast, and ash and coal dust everywhere all reduce compressor and dryer capacity and block filters and coolers. Screw compressors normally discharge at about 75–95 °C. In a hot, dusty compressor house, fouled coolers soon push them to high-temperature trips. Ventilating the compressor house, ducting cool intake air, and cleaning coolers on a schedule are basic needs, not extras.

Common problems and fixes

Sizing instrument and service air for a thermal unit: A worked example

Instrument air is sized from an inventory of the devices that use it, not from a rule of thumb per megawatt. The example below is for the boiler and turbine island of a single 250 MW coal-fired unit (illustrative). The device counts and bleed rates are typical inputs; replace them with the figures from your own instrument schedule and valve data sheets.

The dryer’s purge air must be added on top of the net demand. A heatless desiccant dryer that purges about 15% of its inlet flow needs 6.26 ÷ 0.85 = 7.37 m³/min (260 cfm) from the compressors. A blower-purge heated dryer at about 3% purge needs 6.26 ÷ 0.97 = 6.46 m³/min (228 cfm). The 0.9 m³/min difference runs every hour of the year. To compare dryer types on running cost, see the purge consumption worksheet.

Service air for the same unit is set by the largest users that run at the same time:

• Maintenance tools such as impact wrenches, grinders and chipping hammers: 8 tools × 0.8 m³/min × 50% running at once = 3.2 m³/min.
• Blow-off and cleaning points: 10 × 0.3 m³/min × 30% running at once = 0.9 m³/min.
• Air-heater soot blowing during start-up, where compressed air replaces auxiliary steam: take the figure from the soot-blower data sheet. 6 m³/min is used here (illustrative).

That gives a total of 10.1 m³/min (357 cfm). The soot-blowing demand is short and occurs only during start-up, so it is usually met by bringing a standby compressor online, not by installing extra permanent capacity.

Ash conveying, in brief. Dense-phase fly ash conveying air dwarfs both of these figures. As a cross-check (illustrative), take a heat rate of 2,400 kcal/kWh and coal of 3,500 kcal/kg with 40% ash. The unit burns 250,000 × 2,400 ÷ 3,500 = about 171 t/h of coal, which gives about 69 t/h of ash. Taking 80% of it as fly ash gives about 55 t/h. At a solids loading of 20–30 kg of ash per kg of air, conveying needs 1,830–2,740 kg/h of air. At 1.204 kg/m³ that is about 25–38 m³/min of free air, four to six times the instrument air demand. The conveying contractor’s cycle design fixes the actual figure. For the conveying side, see dense-phase conveying air supply.

Redundancy and hold-up for instrument air

The receiver hold-up method is set out on the refinery utility air page. What differs in a power station is the event the hold-up has to cover. After a unit trip with loss of the station supply, the instrument air compressors restart only when the emergency diesel-backed board is live and its load sequence reaches them. That can take several minutes, while dampers and valves still need air to reach their safe positions. Set the hold-up time from that recovery path, not from the start time of a standby compressor on a healthy board.

Worked example (illustrative): with the 6.3 m³/min net demand above, receivers charged to 8.0 bar(g) and a 5.5 bar(g) minimum header pressure, each minute of hold-up needs 6.3 × 1.013 ÷ (8.0 − 5.5) = 2.55 m³ of dry receiver volume. A 10-minute requirement therefore needs about 25.5 m³.

Hold-up only works if the stored air is kept for instrument use. Place the dry receivers downstream of the dryers and the priority valve, with a check valve upstream of them. This stops service air users and a depressurising dryer tower from drawing the stored air back.

For temporary cover arrangements, see instrument air outages. For vessel sizing, see receiver sizing.

Air quality classes for power-station duties

ISO 8573-1 classes are written as particles:water:oil. Specify the class at the point of use, then build the treatment train to meet it. Fly ash and coal dust make the particle class harder to achieve than in most plants. The dust is hard and abrasive and it loads intake filters quickly, so filter maintenance has to be scheduled around it.

For the instrument air intake, use a heavy-duty or two-stage intake filter. Draw air from the side of the building away from the coal yard and the ash silos. The prevailing wind direction should be checked for both the pre-monsoon and the monsoon season. For the full filter and dryer arrangement, see air quality treatment architecture.

Water-cooled or air-cooled compressor house?

A power station has cooling water available, so water cooling is a real option. It is not always the better one. About 90% of a compressor’s shaft power leaves through its oil and air coolers, and that heat has to go somewhere.

Worked example (illustrative): one 132 kW (177 hp) compressor rejects about 0.9 × 132 = 119 kW.

• Air-cooled: to limit the room temperature rise to 10 °C, with air at 40 °C (1.127 kg/m³), the house needs 119 ÷ (1.127 × 1.005 × 10) = 10.5 m³/s of ventilation. That is about 37,800 m³/h for each machine, ducted so that hot air is not drawn back in during a 45 °C summer.
• Water-cooled: for a 10 °C water temperature rise, the machine needs 119 ÷ (4.18 × 10) = 2.84 kg/s, about 10.2 m³/h. Three machines need about 31 m³/h. The room still needs ventilation for motor and radiated heat.

Where water cooling is chosen, feed the compressor coolers from the closed DM cooling water loop, which the station’s plate heat exchangers already separate from cooling tower water. If only tower water is available, add a dedicated plate heat exchanger. Do not use open cooling tower water directly. A scaled cooler loses capacity long before it trips the machine.

How Reddy Compressors helps

We are an independent multi-brand compressed-air specialist based at Tarnaka, Secunderabad, serving industry since 2002. We sell, rent and service machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and others. We are not an authorised dealer for any brand. We have tie-ups with all major and reputed brands. For power stations and power projects, we:

• Supply new and used or refurbished compressors, dryers, filters and receivers, and brand-new compressors to site for long-term projects.
• Hire out diesel and electric compressors from our own fleet, 50 to 2,000 cfm and 2 to 30 bar, for construction, commissioning, overhauls and emergency instrument-air cover, with operators where needed.
• Service and overhaul compressors and dryers, and provide AMC cover, with more than 20 service engineers and technicians covering all of Telangana and Andhra Pradesh.
• Supply spare parts identified from the machine’s make, model, serial number and nameplate, with fit confirmed before we quote.
• Carry out air audits covering pressure, leakage, dew point and specific power.
• Supply equipment, rentals and spares across India, with project-based site support where feasible. Generators for temporary power can come from Reddy Generators, which is under the same management.

Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

Our track record. We have supplied compressors to leading companies in power generation across Telangana, Andhra Pradesh and beyond – new and used machines, long-term rental, and breakdown cover when a plant’s own compressor fails – backed by our own service team.

What to send us

• Station or project type and location, and whether it is in operation, under construction or in shutdown.
• The duty: instrument air, service air, ash conveying, or temporary air for specific work.
• Required flow (cfm or m³/min), pressure (bar or psig) and air quality, including dew point.
• Nameplate photos of existing compressors and dryers if you need service or spares.
• Hire duration, running hours per day, and whether operators are needed.
• Power available on site (electric supply or diesel required) and access for delivery.

Frequently asked questions

What is the difference between instrument air and service air in a power plant?

Instrument air feeds control valves, dampers, positioners and actuators, so it must be dry, clean and oil-free, typically dried to about −40 °C PDP, and it must never lose pressure. Service air serves tools, cleaning and workshops, so it can be less dry and can be interrupted briefly. Plants protect instrument air with separate generation or a priority valve that cuts service air first.

What dew point is required for power plant instrument air?

Many stations specify a pressure dew point of about −40 °C, which corresponds to ISO 8573-1 water Class 2, using desiccant dryers. This keeps water from forming in lines that run outdoors, through cooler areas or past air-conditioned control rooms. The exact value comes from the station’s design basis and instrument makers’ requirements, so check the owner’s specification before changing dryers.

Why does my fly ash conveying line keep choking?

Common causes are wet conveying air, too little air reaching the vessels, and a conveying cycle that no longer matches the ash rate. In hot weather, compressor output falls and dryers struggle, so check the pressure at the vessel, dryer performance and compressor capacity. Also check that conveying air is not being drawn down by other users through an open cross-connection.

Can a rental compressor supply temporary instrument air?

Yes, if it is set up correctly. Oil-free or oil-injected rental compressors can feed a temporary desiccant dryer and coalescing filters to meet the instrument air specification, connected at a tie-in point near the receiver. Test the dew point and oil content before switching over, and keep the permanent system isolated and tagged while the rental runs.

What size compressor is needed for a power plant shutdown?

It depends on the work fronts, not the size of the unit. Add up the tools, blasting nozzles and other users that will run at the same time, at their working pressures, then allow for hose losses and some margin. A single overhaul often uses several 400–600 cfm portable compressors spread around the boiler, turbine and ash areas rather than one large machine.

Should ash conveying air share compressors with instrument air?

It is better kept separate. Conveying vessels take large, fast batches of air that make header pressure swing, and instrument air should not see those swings. Where generation is shared for cost reasons, use separate receivers and dryers, a priority valve on the conveying branch, and enough standby capacity to carry instrument air if a compressor trips.

What is a service air compressor in a power plant?

A service air compressor supplies general plant air for maintenance tools, cleaning, blow-off points, workshops and, on some units, air-heater soot blowing during start-up. It runs at about 7 bar(g), with a refrigerated dryer at about +3 °C pressure dew point. Because a short interruption is acceptable, it is often separate from instrument air. Where the two share compressors, a priority valve shuts off service air first when header pressure falls. Where service air compressors back up instrument air, their air must pass through the instrument air dryers and filters. Size it from the largest users that run at the same time, not from the total connected load.

What does an instrument air system consist of?

A power-station instrument air system has duty and standby compressors, an aftercooler and separator, and desiccant dryers to about −40 °C pressure dew point. It also has coalescing and particulate filters, and dry receivers sized for hold-up behind a check valve. A priority valve sheds service air first, and a ring main or dual header distributes air with local filter regulators at each instrument. Dew point and header pressure alarms report to the control room. The standby compressor should start on falling header pressure, and duty and standby should be fed from different electrical sections.

How much compressed air does a thermal power plant use?

It has to be built up from the station’s own device lists, but the proportions are consistent. In the illustrative 250 MW unit on this page, instrument air is about 6.3 m³/min (221 cfm) net, or about 7.4 m³/min (260 cfm) at the compressors with a heatless dryer. Service air is about 10 m³/min when start-up soot blowing runs. Dense-phase fly ash conveying needs about 25–38 m³/min, four to six times the instrument air. Fluidising air normally comes from blowers, so it is sized separately.

Should power-station compressors be water-cooled or air-cooled?

Water cooling suits large compressor houses where the station has a reliable closed-cycle cooling water system. A 132 kW machine needs only about 10 m³/h of water, compared with about 37,800 m³/h of ventilation air if it is air-cooled. Never feed the coolers directly with raw cooling tower water, because it scales them. Use a plate heat exchanger to separate them. Air cooling suits smaller machines and remote ash-handling houses. It needs ducted exhaust, filtered louvres and regular fin cleaning, because fly ash packs the coolers.

Related pages

• Industrial applications
• Temporary air for instrument air outages
• Temporary air for planned shutdowns
• Desiccant air dryers
• Dense-phase conveying air supply
• Verified temporary air projects