Compressed-air piping service: leaks, pressure loss and corrosion

Why piping needs a service discipline, not just repairs

Most plants treat the air network as fixed infrastructure. It is a working asset that degrades. Threaded joints loosen with vibration and thermal cycling, galvanised iron (GI) pipe corrodes from the inside where wet air sits, supports sag, valves seize or become hard to reach, and “temporary” hose connections made during a breakdown are still there years later. Every new machine teed into an existing branch changes how the whole network behaves.

The usual reaction to low pressure at a machine is to raise the compressor setpoint. That hides the problem and pays for it every hour: as a rule of thumb, each extra bar of generation pressure costs about 7% more compressor power, and higher pressure also pushes more air out through every leak. A proper piping service separates three questions that are often mixed together:

• Performance – is pressure being lost between the compressor room and the point of use, and where exactly?
• Integrity – are the pipe, its joints and its supports still fit for the pressure, temperature and site conditions?
• Maintainability – can sections be isolated, drained, inspected and modified safely without shutting down the whole plant?

How do you find where pressure is being lost?

A single gauge reading in the compressor room tells you almost nothing about distribution. Static pressure with no flow will look perfect even in an undersized, leaking network, because pressure drop only appears when air is moving. The method we follow is a pressure profile under representative demand.

1. Choose the demand state. Measure during normal production, and ideally again at peak (shift start, simultaneous machine cycles, blow-down events).
2. Set measurement points in series. Compressor discharge, after the dryer and filters, the main header, the start and end of each suspect branch, and the inlet of the critical user’s regulator.
3. Measure at the same time. Readings taken one after another at different moments mislead because demand changes. Use logging gauges or transducers of similar accuracy, checked against each other at zero flow.
4. Split the loss. Attribute the total drop to the treatment train (dryer, filters), valves and fittings, straight pipe, drops and hoses, and the point-of-use regulator and quick couplings.
5. Compare with a budget. In a well-designed system, the drop from the compressor outlet to the farthest user is typically held to around 0.1 bar in the distribution itself, with treatment and point-of-use losses accounted for separately. Where your loss is several times that, the profile shows which segment is responsible.

The answer is frequently not the main pipe. A clogged filter element, a half-closed or failed ball valve, a long small-bore hose, or a quick coupling with a restricted bore can each take more pressure than a hundred metres of correctly sized header.

Typical sources of pressure loss and what they look like

Leaks: where they hide and how we repair them

Leaks are the most common reason a network “needs” more compressor capacity. In piping they concentrate at predictable points: threaded joints and unions, flange gaskets, valve stem packing, drain valves left cracked open, instrument tappings, hose clamps and quick couplings, and FRL units at machines. Many are inaudible over production noise, so we locate them with an ultrasonic detector, tag each one and record location, mechanism and size estimate.

Repair is more than tightening. A leaking threaded joint on old GI usually means corroded or worn threads, and over-tightening can crack the fitting. The correct sequence is:

1. Isolate the smallest practical segment and vent stored pressure to zero before work.
2. Identify why it leaked (thread damage, vibration, wrong sealant, gasket ageing, pipe strain).
3. Repair with materials and a joining method compatible with the rest of the line.
4. Repressurise in stages and check the repair with leak spray or ultrasound under pressure.
5. Close the tag in the leak register so the repair can be verified later.

Leak repair without a register tends to be undone within months as new leaks appear. A tagged, verified register turns it into a measurable programme.

Corrosion in GI pipe: repair or replace?

Galvanised iron is still the most common air pipe in Indian plants. It works, but the zinc layer is consumed where wet air sits, particularly in low points, dead legs and lines downstream of a dryer that is not delivering its dew point. Once internal rust starts, flakes travel downstream, block drains, load filters, score cylinders and damage valves. Externally, corrosion is worst where pipes run near wash areas, cooling towers or chemical handling, or pass through walls.

Corrosion is a symptom as well as a defect. Before replacing any section, check the dryer. A refrigerated dryer working properly typically gives a pressure dew point of about +3 °C; if it is bypassed, overloaded in summer ambient heat, or its drain has failed, liquid water reaches the network and new pipe will corrode as well.

When replacing, material choice (GI, aluminium modular pipe, stainless steel, or other rated systems) should follow the pressure, temperature, air quality required and site rules, and the pressure rating of pipe, fittings and seals must be confirmed from the manufacturer’s data. Mixing materials without considering joints and galvanic effects creates new leak points.

Drops, take-offs and condensate: the details that decide air quality

Point-of-use problems often come from how branches leave the header. Good practice is well established:

• Take branches off the top of the header (a “goose-neck”) so condensate in the main does not run into the drop.
• Slope mains gently in the direction of flow, typically around 1 in 100, towards drain points.
• Fit drain legs with automatic drains at low points, at the end of runs and before critical users; check that manual drains are not left cracked open, which is a leak.
• Extend drops below the user connection to form a drip leg, with the machine supply taken from the side.
• Give each user its own isolation valve, filter and regulator where needed, accessible without ladders.
• Avoid dead legs from removed machines; cap them close to the header or remove them.

Condensate collected from drains contains compressor oil and must not be poured into storm drains; it needs an oil-water separator or approved disposal route.

Valves, ring mains and maintainability

A network that cannot be isolated in sections forces a full plant shutdown for every repair, which is why leaks and corroded sections get left alone. During a piping service we map isolation points and check each valve for operation, passing (internal leakage), stem leakage, accessibility and identification.

• Main and branch isolation so a section can be worked on while the rest of the plant runs.
• Ring mains feed users from two directions, which reduces pressure loss and lets you isolate a segment of the ring with valves on both sides while supply continues.
• Bypass valves around dryers and filters allow servicing, but a bypass left open sends wet, dirty air into the network. Label and control them.
• Check valves between compressors and a common header must be correctly placed and working so a stopped machine is not back-pressurised.
• An isolation map at the compressor room showing valve numbers and what each one feeds saves time during breakdowns and makes safe lockout possible.

Should we upsize the pipe or add a ring main?

Upsizing is sometimes right, but only after the profile proves that straight pipe is the dominant loss at representative flow. Velocity matters: in main headers, designers typically aim for low velocities (often quoted around 6 m/s or less) to keep friction loss small and allow condensate separation. Before choosing a size, consider the current peak demand, realistic future demand, route length, number of fittings, and whether a cross-tie or ring would solve the problem with less new pipe. Our pipe sizing and pressure-drop calculators give a first estimate; the final size still needs engineering checks for the actual route and materials.

Upsizing a pipe will not fix a clogged filter, a restricting valve or a small hose at the user. That is why measurement comes first.

Supports, vibration and safe modification

Integrity problems are often mechanical. Pipes hung from cable trays, unsupported spans that sag and hold water, lines rigidly bolted to a vibrating compressor with no flexible connector, and pipe strain on equipment nozzles all lead to cracked fittings and joint leaks. Check supports, clamps and anchors, make sure flexible connections are rated for the pressure and temperature, and keep pipes clear of forklift routes or protect them.

Every modification should be recorded: route, size, material, joining method, pressure rating, new isolation points and the date of change. Without this record, the next engineer starts from guesswork. Joining methods such as welding, threading, press-fit or grooved couplings each have their own procedures and need competent installers; site permits for hot work and pressure testing apply.

How Reddy Compressors helps

Our team in Tarnaka, Secunderabad has worked with industrial compressed-air systems since 2002. As an independent multi-brand specialist, we look at the compressor, the treatment train and the piping as one system, whichever brand of compressor you run. For a piping service we:

• Survey the network and log pressure at the compressor, header and critical users under real demand.
• Carry out ultrasonic leak detection, tag leaks and prepare a repair register.
• Inspect GI and other pipe for corrosion, supports, valves, drops and drains, and check the dryer and filters that feed the network.
• Recommend repair, replacement, re-routing or ring-main options, with the reasoning shown.
• Arrange rental air from our fleet (50–2,000 cfm, 2–30 bar) where a section or compressor must be taken out of service for longer work.

Service teams cover the whole of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India. Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

What to send us

• A simple sketch or drawing of the network: compressor room, headers, main branches and critical users.
• Compressor make, model and set pressure, and the pressure each critical machine needs.
• Pipe material, approximate sizes and age, and any known leak or corrosion areas.
• Dryer and filter details, and photos of drains, drops and the compressor room.
• When the problem appears (all the time, peak shifts, one area) and any recent additions to the plant.

Frequently asked questions

Why is pressure low at my machine when the compressor shows full pressure?

Pressure is being lost between the compressor and the machine, usually in filters, valves, small drops, hoses or couplings, and sometimes in an undersized header. The loss only shows when air is flowing, so a reading at rest looks normal. Measure at several points at the same time during production to find the segment responsible, then fix that segment rather than raising the compressor setpoint.

How much pressure drop is acceptable in a compressed-air pipe network?

A common design target is to keep the distribution loss from the compressor room to the farthest user to around 0.1 bar, with dryer, filter and point-of-use losses counted separately. Real plants often lose far more. The exact limit depends on your users and pressure budget, but every extra bar of compressor pressure typically costs about 7% more power, so reducing losses pays back continuously.

Should I replace old GI pipe for compressed air?

Replace GI sections when you see repeated leaks along a run, thin or pitted walls, heavy internal rust at drains and filters, or when the line is also undersized. A single leaking joint can usually be re-made. Before replacing, fix the moisture source: check that the dryer is working and its drains are clear, otherwise new pipe will corrode in the same way.

Why is there water in my compressed-air lines?

Water in the lines usually means the dryer is bypassed, overloaded, or not reaching its dew point, or that drains have failed. A refrigerated dryer typically delivers a pressure dew point of about +3 °C. Low points without drains, branches taken from the bottom of the header, and dead legs collect condensate. Check the dryer, drains and take-off arrangement together.

Is a ring main better than a straight compressed-air header?

A ring main feeds each user from two directions, so pressure loss is generally lower and more even, and a segment can be isolated for repair while the rest of the plant keeps running. It suits plants with users spread around a building. A straight header can work well for short, simple layouts. The choice should follow a measured pressure profile and the plant layout.

How do I find compressed-air leaks in a noisy factory?

Use an ultrasonic leak detector, which picks up the high-frequency sound of escaping air even over production noise. Survey joints, valves, drains, hoses, couplings and FRL units, tag each leak with location and size, and record it in a register. Repair in priority order, then recheck under pressure. Surveys done during a non-production period make small leaks easier to hear.

Related pages

• Compressed-air distribution and piping design
• Pressure-drop audit
• Leak detection and air-loss management
• Air dryer service and dew-point performance
• Pipe network pressure drop calculator
• Working pressure and pressure budget