Blowdown / vent valve

Dumps sump pressure at unload and shutdown so the airend can restart against no load.

Also called: vent valve · 3SV · depressurisation valve

Normally open to vent with no control air/power — fail-safe, so a dead machine cannot sit fully pressurised.

How it works

Either an integrated port in the load solenoid or a separate pilot-operated valve. On unload it opens the sump to atmosphere through a muffler/orifice, dropping sump pressure to roughly 15 – 30 PSIG (unloaded) or 0 (stopped). Some machines use a fixed orifice to control how fast it dumps.

Specifications

Vent time

Full sump to 0 PSIG in 30 – 120 s depending on volume

Orifice

1.5 – 4 mm, often behind a small screen

Silencer

Sintered muffler — clogs with varnish

Parts you can order

• ASCO (Emerson)

  Series 210 general service

  8210G094

  $130 – $280
• Generic / OEM

  Blowdown valve & vent muffler kit

  By port size (1/4 to 1/2 in.)

  $25 – $140

Field test procedure

1. Time the blowdown at stop. Much slower than the OEM figure = plugged orifice or muffler.
2. Hissing while loaded = the vent is leaking, wasting flow and running the airend hot.
3. Never attempt a restart until the sump gauge reads 0 — restarting pressurised destroys couplings and motors.

Symptoms & probable causes

• Motor trips on restart
  Sump not vented, airend starting against pressure.
• Continuous vent noise while running loaded
  Vent valve seat leaking or solenoid stuck.
• Oil misting from the vent
  Blowdown venting too fast, foamed oil, or overfilled sump.