Refinery compressed air: instrument, plant and turnaround air

Why refinery compressed air is a fourth utility

Alongside power, steam and cooling water, compressed air is one of the utilities a refinery cannot run without. Most process units fail safe on loss of instrument air: control valves move to their fail position, shutdown valves close or open, and the unit trips. That makes the instrument-air system part of the plant’s control and safety envelope, even though it is built from ordinary compressors, dryers and pipe.

The same compressor house usually also feeds plant (service) air for tools and hose stations, feed air for nitrogen generators, and sometimes air for packaged equipment such as analyser houses or burner management. Each of these users has a different consequence when pressure or quality is lost. The engineering task is to keep those consequences separate, so that a heavy demand from a hose station or a turnaround contractor never takes pressure away from control valves.

A refinery also stretches over long distances: unit pipe racks, tank farms, jetties or loading gantries, workshops and offsites. Pressure at the compressor discharge tells you little about what a control valve at the far end of a rack actually receives during peak simultaneous use. Good design starts from the user and works back to the source.

Where compressed air is used in a refinery

The table shows typical users, pressures and quality expectations. Values are typical of common practice; the site’s own specifications and package vendors’ data always govern.

How should instrument air and plant air be separated?

The core principle is functional segregation with a deliberate pressure hierarchy. Instrument air should be protected so that plant air can be shed first when supply is short. Common arrangements include:

• Separate compressors and headers for instrument and plant air, with a normally closed or controlled crossover.
• A shared compressor station with priority logic, where a pressure-sustaining or priority valve closes the plant-air branch when instrument-air header pressure falls below a set point.
• Dedicated instrument-air receivers downstream of the dryers, sized to give the control system time to reach a safe state if all compressors stop. Hold-up time is calculated from instrument-air demand, receiver volume and the pressure band between normal and minimum acceptable pressure.
• Non-return valves so a failure or depressurisation in one header cannot drain the other.
• No hose connections on instrument-air headers. Temporary tools connected to instrument air are one of the commonest ways an instrument-air system gets contaminated or starved.

The receiver hold-up question is worth working through. Usable air in a receiver is roughly its volume multiplied by the pressure drop allowed, divided by atmospheric pressure. A 10 m³ receiver allowed to fall from 7 to 4.5 bar(g) gives about 25 m³ of free air; at an instrument-air demand of 5 m³/min, that is about five minutes. The actual figure must come from the site’s measured demand and its safe-shutdown requirements.

Typical system architecture

1. Compression: oil-injected or oil-free rotary screw compressors in an N+1 arrangement for base load; large refineries may also use centrifugal machines. At least one unit on a separate electrical supply or with diesel drive improves resilience.
2. Aftercooling and condensate removal: refineries in coastal or humid locations generate a lot of condensate, which must be drained and treated for oil content before discharge.
3. Wet receiver to settle pressure and drop out water before the dryers.
4. Drying: twin-tower desiccant (heatless, heated-purge or blower-purge) for instrument air; refrigerated drying for plant air where a dew point around +3 °C is acceptable.
5. Filtration: pre-filters to protect desiccant, after-filters to catch desiccant dust, and coalescing filters for oil aerosols.
6. Dry receiver and distribution: instrument-air ring mains or radial headers to units, with local filter-regulators at each control device.
7. Monitoring: header pressure, dew point after the dryers, compressor status and alarms brought back to the control room.

For more on distribution down to control devices, see our page on instrument air distribution to control devices, and for dryer choice our guide to desiccant adsorption dryers.

Nitrogen generation feed air

Many refineries and terminals make nitrogen on site for purging, blanketing tanks and inerting equipment before maintenance. Membrane and PSA generators both depend on the quality and pressure of their feed air. Oil carry-over damages membranes and fouls carbon molecular sieve; liquid water does the same. Feed air is normally taken from a dedicated, well-treated branch with coalescing filtration and, often, an activated-carbon stage before the generator.

Flow matters too. Nitrogen output and purity depend on feed-air flow and pressure, so the compressor must deliver the generator’s design flow at its design pressure, with margin for the pressure drop through filters. During a turnaround, nitrogen demand for purging can rise well above normal, and temporary nitrogen generation or extra feed air is sometimes needed. Our page on nitrogen generator feed air covers sizing in more detail.

Turnarounds and shutdowns: temporary air

A planned turnaround changes the air demand completely. Process demand for instrument air may fall as units are isolated, but maintenance demand rises sharply: impact wrenches, grinders, needle scalers, vessel ventilation, line drying after hydrotesting, abrasive blasting and coating of vessels, columns, tanks and pipework. Much of this work runs round the clock and sits on the critical path.

Taking this demand from the permanent plant-air system is risky: it can starve whatever instrument-air users remain live and overload the station. The safer approach is a temporary air plan:

1. List the contractors, tools and blasting nozzles by area, with hours of use per shift.
2. Add up simultaneous demand at the pressure each user needs, then add hose losses.
3. Decide on diesel or electric temporary compressors for each area. Diesel units suit remote areas without temporary power; electric units suit fixed tie-ins.
4. Plan aftercoolers, moisture separators and, where the coating specification needs it, dryers for the blasting and coating spreads.
5. Agree hazardous-area zoning with the site so that portable engines are placed in safe areas, with spark arrestors and shutdown valves where the site requires them.
6. Set up refuelling, operators, and a service plan for round-the-clock running.
7. Arrange standby units so a single compressor failure does not stop a blasting spread on the critical path.

Our hub on temporary air for planned shutdowns explains changeover and tie-in in more detail.

Blasting and coating during turnarounds

Surface preparation is one of the largest compressed-air users at a refinery turnaround. A blast nozzle’s air consumption rises steeply with nozzle size and pressure, and productivity falls quickly if nozzle pressure drops. Practical points:

• Size compressors on nozzle consumption at the working pressure, plus hose and fitting losses, plus wear allowance: nozzles enlarge as they wear and consume more air.
• Use aftercooled air with moisture separation. Moisture causes abrasive clumping in the blast pot and flash rust on the prepared steel, and oil contamination ruins coating adhesion.
• Keep hose runs short and diameters generous; long small-bore hoses waste pressure at the nozzle.
• Plan for continuous duty in high ambient temperatures. Indian summers at coastal and inland sites push compressors close to their high-temperature shutdown limits, so placement, shade and cooling-air clearances matter.

Our team has supplied diesel portable compressors for a corrosion-protection coating contractor working at a public-sector oil refinery and at a petroleum installation in Kurnool, Andhra Pradesh. On round-the-clock coating hire work, our practice is to service hired machines every 300 running hours with an oil change and checks. See sandblasting compressor hire and our abrasive blasting air demand calculator.

Reliability, standby and rental cover

Refinery instrument air is normally designed with at least one standby compressor and one standby dryer train, so maintenance or a trip does not reduce supply below demand. Common weak points that defeat this redundancy include:

• A single dryer or filter train feeding all instrument air, with no bypass that keeps quality.
• All compressors on one electrical bus or one cooling-water supply.
• Standby compressors that have not been started or loaded for months.
• Drain valves that fail closed, sending water slugs into the dryers.
• Undocumented crossovers that let plant air back-feed instrument air.

When a permanent compressor fails or must be overhauled, temporary air can bridge the gap, but it must be treated to instrument quality before connection. A rental package for instrument-air duty normally includes the compressor, aftercooler, filtration, desiccant dryer and a tie-in point agreed with the site. See temporary air for instrument air outages.

Energy in refinery air systems

Compressed air is an expensive utility, and refineries often run more compressors than needed because nobody wants to risk instrument air. Practical gains usually come from:

• Fixing leaks on plant-air headers and hose stations, which are often the largest single loss.
• Avoiding excess header pressure: as a rule of thumb, each extra bar of discharge pressure costs about 7% more power.
• Coordinating multiple compressors with sequencing so one machine trims while others run at full load.
• Choosing purge-saving dryer types (heated or blower purge) for large instrument-air flows, because heatless dryers use a significant part of the air they dry.
• Measuring demand before adding capacity. A short demand and pressure logging exercise often shows the real problem is distribution or leaks, not compressor size.

Indian context

Indian refineries sit on the west and east coasts and inland, and refinery terminals and petroleum installations are spread across the country, including Andhra Pradesh and Telangana. Coastal sites face high humidity, salt-laden air and very high condensate loads; inland sites face summer ambient temperatures of 40–45 °C, and above 45 °C on open sites, dust, and occasional power disturbances. All of these push compressors and dryers towards their design limits. Intake filtration, ventilation of the compressor house, dryer sizing corrected for inlet temperature, and reliable condensate drains need extra attention in Indian conditions.

Common problems and fixes

Worked example: sizing instrument air from the device count

When a unit is new or being expanded, there is no flow record to measure, so instrument air is built up from the instrument index. The allowances below are illustrative planning figures. Replace them with vendor data for the positioners, purge systems and analysers actually installed, and check the steady bleed of modern digital positioners, which is often much lower.

1. Leakage allowance of 10%: 656 × 1.10 = 721.6 Nm³/h.
2. Future allowance of 20%: 721.6 × 1.20 = 865.9 Nm³/h at the dryer outlet.
3. Heatless desiccant dryer purge, assumed 15% of inlet flow: 865.9 ÷ (1 − 0.15) = 1,018.7 Nm³/h at the dryer inlet. A heated or blower-purge dryer would need far less purge air.
4. Convert to the ISO 1217 reference of 20 °C and 1 bar(a): 1,018.7 ÷ 60 × (293.15 ÷ 273.15) × (1.01325 ÷ 1.0) = 18.5 m³/min, about 652 cfm.
5. Correct for a hot inland site at 45 °C and 0.95 bar(a): 1,018.7 ÷ 60 × (318.15 ÷ 273.15) × (1.01325 ÷ 0.95) = 21.1 m³/min, about 745 cfm.

A displacement compressor delivers roughly the same volume at its inlet whatever the inlet conditions, so thin hot air means less mass. The site figure is about 14% above the reference-condition figure, so selecting on the reference figure would leave the plant about 12% short on a summer afternoon. The specification here is two compressors of at least about 21 m³/min each, one duty and one standby, at the pressure needed to hold the header after dryer and filter losses.

Instrument air quality: ISO 8573-1 classes and the reasoning

A typical refinery specification is ISO 8573-1 class [2:2:1] or [1:2:1], which gives particles, then water, then oil.

Why −40 °C pressure dew point. ISA-7.0.01 asks for a dew point at line pressure at least 10 °C below the minimum ambient temperature recorded at the site, and never above 4 °C. A refrigerated dryer at +3 °C pressure dew point would therefore need a site that never falls below 13 °C, and even then it leaves no margin for cold points: winter dawns at inland sites, air-conditioned analyser houses and vortex coolers in sample systems all run colder than the general ambient. Margin is the other reason. At 7 bar(g), −40 °C pressure dew point is about 16 ppm by volume of water, equal to about −57 °C at atmospheric pressure. Air at +3 °C carries about 950 ppm, roughly 60 times as much. A desiccant dryer that is drifting still delivers air far drier than any ambient condition, so an upset gives warning before water reaches a positioner. Fit a dew point transmitter at the dryer outlet, with an alarm set well inside the design value (for example −30 °C) and wired to the control room.

Receiver hold-up for a compressor trip: sizing to a required time

The worked example above checks how long an existing receiver lasts. The design question is the reverse: how much storage gives the hold-up time the safe-shutdown study requires?

Inputs (illustrative): present consumption including leakage is 721.6 Nm³/h, from the sizing example without the future allowance. The header normally runs at 7.0 bar(g). The minimum is 4.5 bar(g), which is the lowest pressure at which the worst-case actuator still strokes, plus its regulator loss, taken from the instrument datasheets. The required hold-up is 10 minutes, and the receiver is at 35 °C.

1. Consumption as free air at receiver temperature: 721.6 ÷ 60 = 12.03 Nm³/min × (308.15 ÷ 273.15) = 13.57 m³/min at 1.01325 bar(a).
2. Storage volume = flow × time × atmospheric pressure ÷ pressure fall = 13.57 × 10 × 1.01325 ÷ (7.0 − 4.5) = 55.0 m³.
3. Credit the header: 1,500 m of 4 in schedule 40 pipe (102.3 mm bore) holds π ÷ 4 × 0.1023² × 1,500 = 12.3 m³. The vessels must provide 55.0 − 12.3 = 42.7 m³.
4. Store high, regulate down: a dedicated receiver charged to 10.0 bar(g) and feeding the header through a pressure-reducing valve needs 13.57 × 10 × 1.01325 ÷ (10.0 − 4.5) = 25.0 m³. The higher storage pressure costs compression power, roughly 7% per extra bar. Charge only the reserve vessel to the higher pressure, not the whole air supply.

For comparison, a 10 m³ receiver plus this header gives (10 + 12.3) × 2.5 ÷ 1.01325 ÷ 13.57 = about 4.1 minutes. Two details decide whether the calculation holds in practice. First, the plant-air priority valve must close on low pressure so that plant air does not share the reserve. Second, a non-return valve downstream of the dryer, together with dryer logic that stops cycling on a compressor trip, prevents purge flow from draining the dry receiver.

Specifying an air package for refinery duty

A package bought to a general industrial specification usually fails at site inspection on the details below. Write each item into the enquiry, and let the site’s own engineering standards govern.

Packages for classified locations are discussed further on the hazardous-area package enquiries page.

Worked example: sizing temporary air for a turnaround

An illustrative user list for one unit’s turnaround is shown below. Blasting is planned at the worn-nozzle rate, because a No. 6 nozzle that wears to No. 7 bore draws about 30% more air.

Adding 10% for hose leaks and couplings gives 2,754 × 1.10 = 3,029 cfm, or 85.8 m³/min. Blasting needs about 7 bar(g) at the nozzle, and hose, separator and coupling losses can take 1 to 1.5 bar. That points to compressors of the 10 bar(g) class. Three duty units of about 1,200 cfm give 3,600 cfm, which is 19% above demand. Add a fourth as standby so that blasting does not stop for a single breakdown. Feeding blasting and tools from separate headers stops nozzle surges from starving the wrenches.

Plan fuel as well. A 1,200 cfm, 10 bar(g) diesel unit typically burns roughly 55 to 65 litres an hour at full load. Three units at 75% average load for 10 hours burn about 3 × 10 × 0.75 × 60 = 1,350 litres a day, and refuelling must be planned in the designated safe area. Breathing air for confined-space entry is a separate, specified system and must never be taken from this network. To check nozzle figures for your own nozzles, use the abrasive blasting air demand calculator.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressed-air business based at Tarnaka, Secunderabad, serving industry since 2002. We sell, rent and service compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes. We are not an authorised dealer for any brand. We have tie-ups with all major and reputed brands.

• Turnaround and shutdown air: diesel and electric compressors on hire from 50 to 2,000 cfm and 2 to 30 bar, from our own rental fleet, with operators where needed and service every 300 running hours on long hire.
• Blasting and coating spreads: compressors sized to nozzle demand, with guidance on aftercooling, moisture separation and hoses.
• Temporary instrument air: compressor and treatment packages to bridge outages or overhauls.
• Sales: new and used/refurbished compressors and dryers for plant, instrument and feed air, including brand-new machines for long-term project sites.
• Service, AMC and overhaul: over 20 service engineers and technicians covering Telangana and Andhra Pradesh, with project-based site support elsewhere in India where feasible.
• Spare parts: identified from make, model, serial number and nameplate, with fit confirmed before we quote; supplied across India.
• Air audits: demand, pressure, dew-point and leak measurement to find the real constraint before capacity is added.

Our track record. We have supplied compressors to leading companies in refining 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

• The duty: instrument air, plant air, nitrogen feed, or turnaround/blasting air.
• Flow (cfm or m³/min) and pressure (bar or psig) at the point of use.
• Air-quality requirement or coating specification, if any.
• Duration of hire, shift pattern and site location.
• Hazardous-area requirements and the safe area available for portable units.
• For existing machines: make, model, serial number and a nameplate photo.

Frequently asked questions

What pressure dew point does refinery instrument air need?

Many refineries specify instrument air dried by desiccant dryers to a pressure dew point around −40 °C, which corresponds to ISO 8573-1 humidity class 2. The aim is that water can never condense in instrument lines, even on cold nights or where lines run outdoors. The exact figure is set by the site’s own specification and control-valve vendors, so always confirm against the plant standard.

Can a refinery use plant air as backup for instrument air?

Only through an engineered crossover that keeps instrument-air quality. Plant air is usually dried less thoroughly and may carry oil and particles, so it should pass through instrument-grade drying and filtration before it reaches control devices. The crossover should be controlled, non-return protected and documented, so it cannot back-feed or contaminate instrument air in normal operation.

How much compressed air does a refinery turnaround need?

There is no standard figure; it depends on how many blasting nozzles, tools and drying jobs run at once. Build the estimate from contractors’ lists of users per area and shift, add hose losses and a margin for nozzle wear, and include standby units for critical-path work. A demand table agreed with contractors before mobilisation is the best way to avoid shortfalls.

Can diesel compressors be used inside a refinery?

Diesel portable compressors are widely used at refinery turnarounds, but only where the site’s hazardous-area rules allow. They are normally placed in designated safe areas, and the site may require spark arrestors, air-intake shutdown valves and gas detection. Always agree the location and any protection requirements with the site’s safety team before mobilisation.

Why does blasting need dry, oil-free compressed air?

Water in blasting air makes abrasive clump in the pot and causes flash rusting on freshly prepared steel, while oil contaminates the surface and weakens coating adhesion. Using aftercooled air with moisture separation, and drying where the coating specification demands it, protects surface preparation quality and reduces rework during tight turnaround schedules.

How is feed air for a nitrogen generator different from plant air?

Nitrogen generators need feed air at a steady design pressure and flow, with very low oil and water content, because membranes and carbon molecular sieve are damaged by oil and liquid water. Feed air normally comes from a dedicated, well-treated branch with coalescing and often activated-carbon filtration, rather than from a general plant-air header.

How do you size an instrument air system for a refinery?

Start from the instrument index: count control valves, on/off valves, purged enclosures and analysers, and apply vendor consumption figures to each. Add leakage, typically about 10%, a future allowance and dryer purge air. Then convert the total to free air at the hottest site conditions, not the reference conditions. Select duty and standby compressors that each meet the full figure. Size receivers separately, from the hold-up time the safe-shutdown study needs.

What ISO 8573-1 class is used for instrument air?

Refineries commonly specify class [2:2:1] or [1:2:1]. That means fine particle filtration, a pressure dew point of −40 °C or lower, and oil at or below 0.01 mg/m³. Class 2 oil, at or below 0.1 mg/m³, is sometimes accepted. The dew point follows the rule that it must stay at least 10 °C below the minimum ambient temperature at the site, with margin for cold points and dryer upsets. The oil and particle limits protect the small orifices in positioners and analysers.

What should industrial air packages for refineries include?

Expect compressors rated at site conditions with N+1 redundancy, aftercoolers, separators and duplicate dryers with dew point monitoring. Filters should be duplex. Controls should be hardwired to the plant control system with automatic standby start. Equipment must suit the area classification, and materials must suit the cooling water and the atmosphere. Pressure vessels need code certificates. A witnessed factory test and a complete document set should come with the package, and the site’s own engineering standards override any general list.

How long should instrument air last after a compressor trip?

The time comes from the site’s safe-shutdown study: long enough to start the standby and diagnose the trip, or to bring units down in a controlled way. Many sites ask for several minutes. Storage is then sized from consumption, the allowable pressure fall to the lowest workable actuator pressure, and the required time. Header volume counts. Charging a dedicated reserve vessel to a higher pressure cuts its size considerably.

Related pages

• Industrial applications engineering centre
• Oil and gas support: compressed air and nitrogen
• Petrochemical plant compressed air continuity
• Temporary air engineering centre
• Hazardous area compressor package enquiries
• Compressor AMC: annual maintenance contracts