Pressure-drop audit: find where compressed air loses pressure

Why pressure drop costs money

When a machine at the end of the line complains of low pressure, the quickest response is to turn the compressor up. It works, but it is expensive. Every extra bar of generation pressure costs about 7% more compressor power, and it raises the pressure everywhere else in the plant, so every leak and every unregulated user consumes more air as well.

In most plants the compressor is producing enough pressure. The problem is that some of it is being thrown away on the way: a choked filter, a dryer running beyond its rated flow, an undersized header, a long narrow hose or a quick-release coupling that is far too small. A pressure-drop audit puts a number on each of these losses and ranks them.

A good rule of thumb used across the industry is that the total drop from compressor discharge to the furthest point of use should be a small fraction of operating pressure, typically under about 10%. Many plants we visit are well above that.

How a pressure-drop audit works

Pressure drop depends on flow. A filter that loses almost nothing at night may lose a great deal at full production. A gauge reading taken with nothing running proves nothing. The audit therefore measures under representative flow and in segments.

1. Map the system. Draw the path from each compressor through aftercooler, receiver, dryer, filters, main header, ring main, branches, drops, hoses and regulators to the critical users.
2. Choose the measurement points. Place tappings at the inlet and outlet of each major component and at the start and end of each piping segment.
3. Use matched instruments. Pressure transducers or gauges of known accuracy, preferably logged at the same time, so the difference between two readings is real rather than instrument error.
4. Log through a full production cycle. Include peak shifts, start-up and any batch processes that draw large amounts of air.
5. Record compressor state and flow. Loading, unloading, which machines are running, and flow where a meter is fitted.
6. Calculate the drop per segment at peak and average flow and compare it with what that component should lose.
7. Rank and fix the worst restrictions, then re-measure to confirm the improvement.

Where pressure typically disappears

In our experience the last few metres – the hose, coupling and regulator at the machine – are often responsible for more loss than everything upstream. They are also the cheapest to fix.

Common findings and fixes

• Choked filter elements. Replace on differential pressure, not only by calendar. Fit differential gauges where missing.
• Overloaded dryer. When plant demand has grown, the dryer may now run above its rated flow, losing pressure and dew point. A second dryer or a correctly sized replacement fixes both.
• Undersized header. Looping the main into a ring, adding a parallel line or upsizing the worst section often solves low-pressure complaints across a whole bay.
• Hose and coupling restrictions. Shorter, larger-bore hoses and full-flow couplings at high-demand tools.
• Old galvanised or black steel pipe. Internal corrosion reduces bore and adds roughness; aluminium or other modern systems can replace the worst sections.
• Partly closed isolation valves. Surprisingly common after maintenance work.
• Regulators set too low or too small. A regulator undersized for the flow drops pressure sharply when the tool runs.

Pressure drop vs pressure profile: what is the difference?

A pressure profile looks at how pressure varies over time at one point – for example, how deep the dips are when large users start. A pressure-drop audit looks at how pressure differs between points along the system at the same moment. Both matter. The profile tells you there is a problem; the segmented drop tells you where it is. We usually combine them with power and flow logging so that every finding can be linked to energy and to production.

What to do before raising the setpoint

1. Check which users actually need the highest pressure and how much air they use.
2. Measure the drop to those users under load.
3. Remove the avoidable restrictions on that path.
4. Consider a local booster or a dedicated small compressor if one user truly needs a much higher pressure than the rest.
5. Add storage close to large intermittent users, so short peaks do not pull down the whole system.
6. Only then reconsider the compressor setpoint – often it can come down, not up.

How results are verified

A pressure-drop finding is only useful if the fix can be shown to work. After corrective action we repeat the measurements at the same points, under similar production conditions, with the same instruments. The before-and-after drop for each segment, the new pressure at critical users and any reduction in the compressor setpoint are recorded together. If the setpoint is lowered, power logging shows the effect on energy, using the plant’s own data rather than an assumed saving.

Indian plant conditions that affect pressure drop

Several local factors push pressure losses up in Telangana, Andhra Pradesh and across India. High summer ambient temperatures raise dryer inlet temperature and cut dryer capacity. Dust loads filters faster. Plants that have expanded in stages often still run on the original header sized for a fraction of today’s load. Condensate left in poorly drained pipes reduces bore and corrodes steel. An audit takes all of this into account rather than relying on design drawings.

Instruments and good measurement practice

The difference between two pressure readings is often small, so instrument quality decides whether a finding is real. We use transducers or test gauges of known accuracy and range suited to the working pressure, check them against each other at the same point before installing them apart, and log them on synchronised timestamps. Tappings should be on straight runs away from bends and valves, clean and leak-free. Readings are recorded in bar gauge (with psig where useful) and every chart states the flow and compressor state at the time. Where flow meters are not fitted, compressor load data and power logging help estimate the flow through each segment.

How Reddy Compressors helps

Reddy Compressors, from Tarnaka in Secunderabad, carries out compressed-air audits including pressure-drop, pressure profiling, leak detection and power and flow logging for plants across Telangana and Andhra Pradesh, with project support elsewhere in India where feasible. We are an independent multi-brand specialist, so our recommendations are not tied to selling one brand of compressor, dryer or filter.

• We map your system, place measurement points and log under real production.
• We give a ranked list of restrictions with practical fixes.
• We supply and fit filter elements, drains, dryers, piping sections, couplings and regulators where needed.
• We service dryers and compressors of ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Kirloskar and other makes.
• We re-measure after the work so you can see what changed.

What to send us

• A simple sketch or layout of your compressor room and main piping.
• Compressor, dryer and filter makes, models and ratings.
• Current compressor setpoints and the pressure at the machines that complain.
• Which users are critical and when they run.
• Any recent changes: new machines, new lines, added shifts.

Frequently asked questions

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

A common target is that the total drop from compressor discharge to the furthest critical user stays within about 10% of operating pressure at peak flow, and many well-designed systems achieve less. Each component should be within its own rated drop at its actual flow. More important than a single figure is knowing where your losses are, because a few restrictions usually account for most of the gap.

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

Because pressure is being lost between them. Typical culprits are a choked filter, an overloaded dryer, an undersized or corroded header, a partly closed valve, or a small hose, coupling or regulator at the machine itself. Measuring pressure at several points while the machine is running will show which segment is responsible. Turning up the compressor hides the problem and raises energy use.

Does increasing compressor pressure cost more electricity?

Yes. As a widely used rule of thumb, each extra bar of discharge pressure needs about 7% more compressor power for the same flow. Higher system pressure also increases the air lost through leaks and unregulated users. That is why removing pressure losses and then lowering the setpoint is usually one of the most effective energy measures in a compressed-air system.

How is a pressure-drop audit carried out?

We map the system, choose measurement points at the inlet and outlet of each major component and along the piping, and log pressure with calibrated instruments through a representative production period. Compressor state and flow are recorded alongside. The drop for each segment is then calculated at peak and average demand, ranked, and checked again after corrective work to confirm the improvement.

How often should compressed-air filters be changed?

Change them on condition as well as time. A differential pressure gauge across each filter shows when the element is restricting flow; replace it when the drop reaches the manufacturer’s recommended limit, or at the stated interval, whichever comes first. A choked filter can waste more energy in a year than the element costs, and coalescing filters also lose oil-removal performance with age.

Can a bigger pipe really fix low pressure?

Often it can, when the pipe is the restriction. Pressure loss in pipe rises steeply with flow, so a header sized for the original plant may be badly overloaded after expansion. Converting a dead-end main into a ring, adding a parallel feed or upsizing the worst section can recover pressure across a whole area. An audit confirms whether piping is really the problem first.

Related pages

• Compressed air audit and measurement hub
• Pressure profiling
• Distribution losses
• Pressure optimisation
• Filter lifecycle and differential pressure
• Compressed-air pipe sizing calculator