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How to fit a water separator on an air compressor

Updated: October 2026

The new trap is on the bench, the compressor has already cut out, and you are about to screw it in. How to fit a water separator on an air compressor, once the thread and the pressure limit already match, is a short sequence: bowl down, arrow toward the tool, seal only that matching thread, drain before the bowl fills, and depressurise before you open anything. The [water separator comparison](/hub/compressors/water-separator) is where the six listings are lined up. This page assumes you already chose one. A qualifying purchase from our links can earn us a commission at no extra cost to you. Guidance is as of 15 October 2026. We have not fitted these traps in a lab.

The new trap is on the bench, the compressor has already cut out, and you are about to screw it in.

Our top picks

CYTGOUPER CYTGOUPER AF2000-02

Best overall

CYTGOUPER CYTGOUPER AF2000-02
9.4/10about £10.69
View offer
Tiardey Tiardey TD-AF002

Alternative

Tiardey Tiardey TD-AF002
8.8/10about £8.99
View offer
nutmanel nutmanel 003

Rank 3

nutmanel nutmanel 003
8.6/10about £12.34
View offer
On this page
  1. 1.Point the bowl down and follow the arrow
  2. 2.Seal a thread you have already matched
  3. 3.Drain the bowl before it fills
  4. 4.Depressurise, then stop if you cannot isolate the joint
  5. 5.The sequence once the unit matches

Point the bowl down and follow the arrow

Atlas Copco UK’s moisture-trap note is the orientation that matters: fit the separator vertically, and follow the flow. TOPRING says the same thing. The bowl points down, with room for a container, and the arrow runs from the compressor toward the tool. If you reverse it, liquid is asked to fall out of air moving the wrong way. Support the pipe so the bowl is not hanging on two threads. Leave clearance under the drain. TOPRING states that a separator does not lower the dew point, so a long hose can condense again after the bowl. Placement for paint sits on the water trap for spray painting page.

Seal a thread you have already matched

PTFE tape, or a face seal on a parallel thread, is for a thread you have already matched. It does not convert NPT into BSP. If the listing still disagrees with itself, go back to the water separator buying guide rather than taping over the doubt. Tiardey’s title says 1/4 inch NPT and a bullet says G1/4. On a matching taper, a few turns of tape in the direction of the thread are the usual hobby seal. Stop when the joint is firm. Tape will not hold Tiardey above 90 psi, and it will not turn a German Type coupling into PCL Standard.
CYTGOUPER CYTGOUPER AF2000-02

Testsieger

CYTGOUPER CYTGOUPER AF2000-02
9.4/10about £10.69
View offer

Drain the bowl before it fills

A full bowl stops separating. Drain into a container while you can still see air space in the bowl. Tiardey has a release valve, PRITOK’s title says hand drain, and a UK review of CYTGOUPER describes a sprung valve. Use the control that is actually there. nutmanel and Hakeeta do not describe a drain. Do not invent a knob, or unscrew the bowl while it is still up to pressure. Draining the separator does not empty the receiver. Do not pour the milky waste to a storm drain.

Depressurise, then stop if you cannot isolate the joint

HSE leaflet INDG261 says systems should be depressurised before maintenance. Switch the compressor off, open a blow gun or the tank drain until the gauge reads zero, and only then unscrew a bowl. The HSE blow-down FAQ gives 30 psi, about 2.1 bar, as a cleaning guide and warns of embolism. That figure is not the install pressure. Keep compressed air off your skin. A separator does not cancel a workplace written scheme. If the bowl is cracked, if you cannot isolate the joint, or if the air is for breathing, stop and call a competent person.

The sequence once the unit matches

If the thread and the stated pressure already match, fit the bowl downhill with the arrow toward the tool, seal only that thread, and drain before the bowl fills. If the gauge is not at zero, do not unscrew anything. If the bowl is cracked or you cannot isolate the joint, stop and call a competent person. A dryer is not the next fitting step.

Common questions

Which way round does the water separator go?
The flow arrow points from the compressor toward the tool. Atlas Copco and TOPRING both treat direction as part of the fit. If you reverse it, the bowl is trying to catch liquid from air moving the wrong way. Look for the arrow before you tape the threads. A trap with no arrow should be checked against its own sheet, not guessed from the ports.
Does the bowl point up or down?
Down. Fit the separator upright so liquid can fall into the bowl, with clearance under it for a container. A bowl aimed uphill cannot store what the body knocks out. If the only way to seal the thread leaves the bowl high, the pipe run is wrong. Mr R. Luton’s note on a Noosverl is that practical version of the mistake.
Should I use PTFE tape on the threads?
Only on a thread you have already matched. Tape helps a taper seal. It does not turn NPT into BSP, and it does not fix Tiardey’s title, which says NPT while a bullet says G1/4. On a parallel BSP thread the seal is usually a face or a washer. Stop if the joint only seals when you force a plastic body.
How do I drain the bowl?
Put a container under the drain, then open the control the listing describes. Tiardey has a release valve, PRITOK’s title says a hand drain, and a UK review of CYTGOUPER describes a sprung valve. nutmanel and Hakeeta do not describe a drain, so do not invent one. The liquid is often oily. Do not pour it to a storm drain.
Will draining the separator empty the compressor tank?
No. The bowl only holds liquid that has fallen out at that fitting. The receiver is a separate vessel and still needs its own drain. You can empty the trap and leave the tank wet, or drain the tank and leave a full bowl, which then stops separating. Treat them as two jobs.
What do I do before I unscrew the bowl?
Switch the compressor off and depressurise until the gauge reads zero. That is the order in HSE leaflet INDG261. Do not unscrew a bowl at cut-out to check it. If you cannot isolate the joint, or the bowl is cracked, stop and get a competent person. The 30 psi figure in the HSE blow-down FAQ is a cleaning guide, not the pressure to work at.

Related pages

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Water trap for spray painting: liquid in the airline

Sources

Public UK sources used for this comparison. Listing prices, stars, and review counts are from amazon.co.uk on 15 October 2026.

  1. Atlas Copco UK — moisture trap (retrieved 15 October 2026)
  2. BCAS BPG 104 — filtration and drying of compressed air (retrieved 15 October 2026)
  3. Tanair — water traps (retrieved 15 October 2026)
  4. HSE INDG178 — written schemes of examination (retrieved 15 October 2026)
  5. HSE INDG261 — compressed air safety (retrieved 15 October 2026)
  6. GOV.UK — Pressure Equipment (Safety) Regulations 2016 (retrieved 15 October 2026)
  7. PCL — Standard and XF-Euro coupling profiles (retrieved 15 October 2026)
  8. Spray Guns Direct — filtering compressed air for spray painting (retrieved 15 October 2026)
  9. BCAS — condensate fact sheet (retrieved 15 October 2026)