SiteIt's Monday, 6am. The first shift fitter walks the compressor house. Both compressors are loading. The dryer's alarm light is on again. Nobody looks at the receiver, because nobody ever does.
Someone cracks the manual drain on the bottom. Brown water runs out for a full minute. The auto drain has been stuck shut for weeks. The water's been going down the air main into every valve and cylinder on site.
That receiver is a big steel tank full of stored energy, and the law treats it that way. Most sites get the statutory exam booked. Where they slip is the daily care, and the repairs done in a hurry when the air goes down.
Is your receiver in the regulations?
The Pressure Systems Safety Regulations cover compressed air above 0.5 bar* (HSE, PSSR). Size then decides how much applies.
Multiply the pressure in bar by the internal volume in litres. If every vessel in the system comes in under 250 bar litres*, the rules on written schemes, exams and records don't apply (Schedule 1, Part II). A 500 litre receiver at 10 bar is 5,000 bar litres. That's well over, and it needs a written scheme. HSE lists a compressed air receiver at or over 250 bar litres* as likely to need one (INDG178).
Under 250* isn't a free pass. The duties to know the safe operating limits, give operating instructions and keep it properly maintained still apply. Steam gets no size exception at all.
The compressor itself usually stays out of the scheme. HSE says "you don't normally need to include the compressor associated with an air receiver" (INDG178). The receiver, its safety valve and the pipework where a failure could hurt someone go in. Our post on written schemes of examination covers how the scheme, the exam and the report fit together.
Know the numbers on the plate
You can't run the system until you've set its safe operating limits (regulation 7). For a receiver that mostly means its maximum pressure. The code of practice says basic information "should be permanently marked on the vessel" (L122, paragraph 70).
If the plate's painted over or missing, find out. HSE says that if the limits weren't supplied, a competent person can advise, "for example your employers' liability insurer" (HSE, pressure systems).
Then check the compressor's cut-out pressure sits below it, with the safety valve set to protect it. If someone has wound the pressure switch up to cure low pressure at the far end of the factory, you've found a bigger problem.
Leave the safety valve alone
HSE's leaflet is short on safety valves. Keep them in good working order. Make sure they discharge to a safe place. Once set, they should not be altered "except by an authorised person" (INDG261).
On most sites the valve gets a pull of the ring now and then. Nobody looks at it again until it lifts. If yours lifts often, HSE says the system may be over-pressurising or the valve may not be working (INDG261). Our guide to compressor faults covers the usual causes. Don't plug it, gag it or swap it for whatever's in the stores.
Drain it, every day
Regulation 11 says you must give operators "adequate and suitable instructions" for running the system, and for emergencies (regulation 11). For compressed air, the code of practice lists what those should cover. That includes "draining of receivers, intercoolers, aftercoolers, pipework". It also covers lubricants, housekeeping round the protective devices, and the danger of removing covers before residual pressure has been vented (L122, paragraph 154).
A stuck auto drain is a common fault in a compressor house, and one of the cheapest to catch. Put a daily drain check on the shift sheet. Water left in the receiver rusts it from the inside, and what gets past ends up in your valves and cylinders. If your pneumatics are sticking, start with filters, regulators and lubricators.
Working on it
Depressurise first. HSE says systems "should be depressurised before maintenance work is carried out" (INDG261). Isolate the compressors, lock off the isolating valves, and vent the receiver to zero. Then prove it with the gauge before anyone cracks a fitting. A closed valve on the outlet doesn't empty the tank.
Repairs carry their own duty. The employer of whoever does the repair must make sure it doesn't create danger or affect any protective device (regulation 13). The code of practice says repairs should take into account "the original design specification" and whether the protective devices are still adequate (L122, paragraph 174).
Our view: nobody welds anything to a receiver shell, and nobody fits a bigger outlet or a different safety valve, without the competent person agreeing first. A bracket for a dryer isn't worth a vessel you can't trust.
When the compressor dies
It's 2am and the lead compressor has gone. The standby can't hold the site. Production is losing pressure across every line at once. Our post on compressed air going down site-wide covers the first hour.
You'll probably hire one in. A towed compressor set with its own receiver counts as a mobile system (L122, paragraph 40). Its owner must mark the next exam date on it (regulation 9). HSE says the user of hired kit "should make sure that the WSE is in place" and that the certificate is current (HSE, PSSR). Check it on the low loader, before it's coupled to your main.
Then think about who's fixing your own machine. A good shift fitter can change a drain or a filter. A failed airend, a controller fault or a VSD trip is often a job for someone who works on compressors every week. Know who that is before 2am, because every hour the air's down, the whole site is stood.
What to do on Monday
- Work out the bar litres for every receiver on site. Check the ones over 250* are on a written scheme.
- Read the plate on each receiver and check the compressor cut-out and safety valve settings against it.
- Add a daily drain check to the shift sheet, and test every auto drain this week.
- Write down who you'll call for a compressor breakdown and who you'll hire from, with numbers.
* The 0.5 bar relevant fluid threshold and the 250 bar litre exception correct as of 1 October 2026. Check the Pressure Systems Safety Regulations on legislation.gov.uk and HSE's PSSR page for the latest. Paragraph numbers refer to L122, second edition (2014).