SiteIt's Monday morning after a cold weekend. Half the cylinders on the end-of-line packer are sluggish. Two valves won't shift without a tap. Someone cracks a fitting and water spits out of it.
Nobody's looked at the filter-regulator on the side of the machine for months. The bowl is full to the top with brown water. The auto drain has been choked with sediment since the summer.
Most pneumatic faults on a machine start upstream of the machine. The air prep unit is supposed to catch them. When it's ignored, every cylinder and valve after it pays the price.
Why it matters more than it looks
HSE's compressed air guidance is blunt about it. Dirty and wet air and worn components are among the most common causes of hazards with air powered equipment (HSG39, paragraph 159)*. The same guidance says each machine should have its own filter, regulator, gauge and lubricator set, so the machine can be set up without affecting anything else on the supply. Kept in good order, that set limits wear on seals in actuators and valves (HSG39, paragraph 109)*.
So a slow cylinder or a sticky valve is often an air prep problem in disguise. If you're chasing one, our guides to sticking or slow cylinders and solenoid valve faults both start here.
The filter: drain it, then look at the element
Start with the bowl. If there's water in it, drain it. Then ask why the auto drain didn't. HSG39 suggests draining water from the drain points at the end of each day, and checking the drain valves aren't choked with sediment (HSG39, paragraph 163)*. Auto drains clog with the same dirt they're meant to get rid of. Strip and clean them, or replace them.
If there's a lot of water at every machine, the problem is further back. Look at the compressor's dryer, the receiver drains and the drain legs on the main. Wet air from the compressor house will beat any filter at the machine. Our guide to air receivers covers the legal side of the receiver itself.
Then the element. A clogged element starves the machine. You'll see the pressure on the gauge dip hard every time the machine cycles. The guidance says filters should be checked weekly, and any that are clogged or restricting flow too much should be replaced (HSG39, paragraph 164)*. Some units have a pressure drop indicator on top. If yours do, use them.
The bowl that can burst
This is the bit most people don't know.
Most filter and lubricator bowls are clear plastic, usually polycarbonate. HSG39 says the properties of the plastic can be affected by solvents, alcohols, cleaning solutions or synthetic lubricants. It says to clean polycarbonate bowls with clean rags. It also warns that even minor knocks can lead to damaged bowls exploding, and calls it good practice to shield them (HSG39, paragraph 89)*.
On a shop floor that means a few things. Don't spray degreaser or brake cleaner on an air prep unit. Don't wipe it down with a rag that's been in the solvent tray. Make sure lubricator oil is compatible with the bowl as well as the kit it feeds, which HSG39 also flags (HSG39, paragraph 71)*. Look at every bowl for crazing, cracks and cloudiness. If it's crazed, replace it. Keep the metal guard on, or fit one.
And before you take a bowl off, isolate and vent the unit. Every machine's air prep should sit after a stop valve so it can be worked on safely (HSG39, paragraph 88)*.
The regulator: set it, mark it, lock it
The regulator is where fault-finding goes wrong. A cylinder runs slow, someone winds the regulator up, and it runs again for a while. The real fault is still there, and now the machine is running at a pressure nobody chose.
HSG39 says that on machines needing pressure below mains, the regulator should be checked weekly to make sure it hasn't been tampered with and that the right pressure is held (HSG39, paragraph 165)*. It also says adjustable controls should have some tamper-resistant protection, matched to the risk. That runs from a simple snap lock up to a lockable enclosure (HSG39, paragraph 90)*.
So write the set pressure next to the gauge. Use the knob lock. If the pressure keeps getting turned up on one machine, treat that as a fault report and find out why.
The lubricator: more isn't better
Lubricators cause more arguments than any other part of the set.
The guidance says lubricators should be topped up weekly with the correct grade of oil, and checked to make sure they're giving adequate but not excessive lubrication (HSG39, paragraph 164)*. Too much oil ends up in silencers, on the product and on the floor. Too little and seals wear.
Here's our view. Check the valve and cylinder makers' data before you fit or remove a lubricator. Many modern components are greased at the factory and are meant to run dry. In our experience, once oil has washed that grease out, the component needs oil for the rest of its life. Decide per machine, write it down, and stick to it. A lubricator that's empty on a machine that needs one is worse than no lubricator at all.
Food and pharma lines are a special case. The guidance expects very high filtration there, and specialist advice for very clean or sterile air (HSG39, paragraph 88)*. Check what your customers' standards say before anyone tops up a lubricator with whatever's in the stores.
Put it on the PPM
HSG39 sets out daily, weekly and monthly checks, but it also says its frequencies are a guide only and should be adjusted in the light of experience (HSG39, paragraph 145)*. So put the air prep on your PPM schedule, then adjust it by what you find. A unit that's full of water every Monday needs looking at more often, and so does the air feeding it.
It's cheap work. A walk round with a rag and a spanner takes minutes. Missing it costs you sluggish machines all week, valves that need a tap and a breakdown at 3am that nobody can find. And while you're walking round, listen. The same walk will find the leaks you can hear.
What to do on Monday
- Walk every air prep unit on site. Drain any water, and note which auto drains have failed.
- Check every bowl for crazing or cracks, and replace any that are damaged.
- Write the set pressure next to each regulator gauge, and lock the knobs.
- Add a weekly air prep check to the PPM for your busiest machines.
* Paragraph numbers and check intervals correct as of 3 November 2026 and refer to HSG39, second edition (1998). Check HSE's HSG39 page for the latest.