Compressed air line filter troubleshooting: is it the element?

Most problems blamed on a compressed air line filter come from somewhere else. The element is at fault in three cases: its pressure drop has reached the change limit, its pressure drop reads lower than a new element should, or liquid is getting past it because the bowl is not draining. Water after the filter row, low pressure at the tools with a normal filter gauge, and an element that fills with oil within weeks all point upstream or downstream of the filter. Read the differential pressure gauge first and check the drain second. Those two checks settle most cases before you open the housing.

This guide works through the faults you are most likely to see on a row of line filters, what each one means, and what to check before you order a new element. If you have just fitted an element such as SPM-INF-1061, our guides to fitting a line filter element and how long a line filter element lasts cover the installation and the service plan. This one is for when something looks wrong.

Start with two readings

Before you change anything, take two readings with the compressor loaded and air flowing at a normal rate.

• The pressure drop across each housing. Read the differential gauge or indicator. Compare the figure with what a new element of that grade reads, and with the change limit. The MTA manual for its HEF filter range gives the limit as 0.6 bar for particulate and coalescing grades.
• Whether each drain discharges. Open the manual drain for a second, or watch an automatic drain through a cycle. The MTA manual calls for both checks at least once a week.

Our catalogue data gives the new element pressure drop by grade. Across the in-line filter elements in our database that carry the figure, cellulose 3 µm elements read 10 mbar new, glass fibre 1 µm elements read 20 mbar, most 0.1 µm elements read 50 mbar, 0.01 µm elements read 80 mbar and activated carbon elements read 60 mbar. A reading far from those figures in either direction tells you something.

Pressure drop climbs fast after a change

A new coalescing element that reaches its limit in weeks instead of months has met more dirt or liquid than it was built for. The element is doing its job. The question is where the load comes from.

• The drain is not discharging. A coalescing element drains the oil and water it collects off its outer face into the bowl. If the drain is blocked or the float is stuck, the bowl fills and the lower part of the element sits in liquid. Clear or replace the drain before you fit another element.
• A fine element has no stage ahead of it. A 0.01 µm element such as the Atlas Copco PD60 starts at 80 mbar, so it has the least room to climb of any grade in our range. MTA describes its 3 µm grade as a pre-filter for line filters of a finer grade, and its 0.1 µm grade as a pre-filter in front of a refrigeration dryer. On a dirty line, a 3 µm particulate element such as SPM-INF-1131 or a 0.1 µm element such as the Atlas Copco DD60 in front of the fine stage takes the bulk of the load.
• The element is soaked in oil. If you pull the old element and it is heavy and dripping, the oil came from the compressor. A coalescing filter removes oil aerosol that has got past the compressor's separator. It is not a substitute for one. Check the compressor's oil level, separator pressure drop and scavenge line. Our guide to diagnosing oil in the air line walks through those checks in order.

Pressure drop reads lower than a new element

A low reading looks like good news. It is not. An element that reads below its new figure, or a gauge that reads zero at full flow, means air is going round the media instead of through it.

• The element is not seated. Push-fit elements seal on an O-ring in the filter head. If the O-ring is missing, rolled or dry, or the element was not pushed fully home, air passes between the element and the head. Shut the housing off, depressurise it slowly, and check the seat. MTA calls for a light coat of silicone free grease on the O-ring and sealing faces.
• The media has split or collapsed. MTA warns that pressure must be raised and lowered gradually to avoid damage to the element. A housing opened to line pressure in one go, or vented fast through the bowl, can tear or crush the media. Inspect the old element if you still have it.
• The gauge has stuck. With the line isolated and vented, a differential gauge should read zero. If it reads the same with and without flow, the gauge is the fault.

On a carbon housing, a low or steady gauge proves nothing about the carbon. Activated carbon removes oil vapour by adsorption and stays at the same pressure drop when it is full. Atlas Copco gives its QD+ carbon element a change interval of 2,000 operating hours or one year.

Oil past the filter row

Find out what kind of oil it is before you change anything.

• Liquid oil after a coalescing filter means the element is bypassed, overloaded or sitting in liquid. Check the bowl level, the drain and the seat as above. Then check the grade. MTA rates its 3 µm grade for emulsion and particles and gives no residual oil figure for it. It rates its 0.1 µm grade to a maximum residual oil content of 0.1 mg/m³ and its 0.01 µm grade to 0.01 mg/m³.
• Oil smell or taste after a carbon stage means the carbon is full, too hot, or meeting liquid. The carbon elements in our range are rated to 45 °C, against 65 °C for the glass fibre and cellulose elements. MTA states that oil and water aerosols reaching an activated carbon filter make its adsorption void, which is why Atlas Copco puts a DD+ and PD+ pair or a UD+ filter before the QD+ carbon stage.

If the bowl is dry, the element is sound and seated, and oil still gets through, the compressor is putting out more oil than the row was built to handle. Fix the source. A bigger filter only moves the problem.

Water after the filter row

A line filter removes liquid. Atlas Copco describes its DD+ and PD+ elements as removing oil aerosol, wet dust and water drops. Water that is still vapour when it reaches the filter passes through, then condenses further down the pipe as the air cools. No filter element stops that.

So check the drain first. A stuck drain lets the bowl fill until water is carried over the top of the element. If the drain works and water still appears downstream, the fault is moisture removal, not filtration. That is the job of a dryer. MTA places its 0.1 µm grade in front of a refrigeration dryer and its 0.01 µm grade after one.

Drains that blow, and heads that whistle

Two noises worry people after a filter change, and both have a plain cause.

• The drain blows air at start up. This is normal on a float drain. The MTA internal float drain opens when the pressure in the bowl is below 0.6 bar and stays open until the filter is pressurised again. Once the line comes up to pressure it should close. A drain that blows air continuously at full line pressure is faulty and wastes compressed air.
• The head whistles when you start to unscrew the bowl. Stop. MTA filter heads have a small calibrated hole that whistles to warn you the housing is still under pressure. Close the inlet valve and vent the housing fully before you go on.

Low pressure at the tools

If tools at the end of the line are short of pressure but every filter gauge reads within its range, the filters are not the cause. Measure pressure at the compressor outlet, after the filter row and at the point of use. The drop is wherever the readings change most: leaks, a pipe or hose too small for the flow, a regulator set low, or a compressor set point that has been changed.

Before you order a new element

• Note the reading on each gauge, and the date of the last change from the label on the bowl.
• Check that each drain discharges and that each bowl is empty.
• Look at the old element: dry and grey, soaked in oil, split, or crushed. Each one points at a different cause.
• Read the maker and code from the element end cap or the housing. Every element we list is in the in-line filter collection, and most cross-reference to Atlas Copco and other makers' codes. The Atlas Copco QD60 type carbon element SPM-INF-1061 is see current price today.

If the code is worn off or you cannot match the housing, send us the dimensions of the old element and the housing model through our quote request page.

Frequently asked questions

Why does my new line filter element read lower than the old one ever did?
A new element should read close to its grade's new figure: 10 mbar for 3 µm cellulose, up to 80 mbar for 0.01 µm glass fibre. A reading below that means air is bypassing the media. Check the O-ring and that the element is pushed fully home.

Can a line filter remove water from compressed air?
It removes liquid water drops. It cannot remove water vapour, which condenses downstream as the air cools. Water after a filter row with a working drain is a dryer problem.

Why does the filter drain blow air when the compressor starts?
A float drain stays open while the bowl is below about 0.6 bar and closes as pressure builds. Air at start up is normal. Air blowing at full pressure means the drain needs attention.

How do I know if the carbon element is finished?
Not from the gauge. Activated carbon holds the same pressure drop when it is full. Change it on hours or date: Atlas Copco gives 2,000 operating hours or one year for its QD+ element. An oil smell downstream means it is overdue or the stages before it are failing.

My coalescing element fills with oil every few weeks. Should I fit a finer grade?
No. A finer element loads up even faster. The oil is coming from the compressor, so check its separator, oil level and scavenge line first.