Expansion Vessel Purging
Purpose
To drain water from the air side of expansion vessels and restore their ability to handle pressure changes.
Required Equipment
Nitrogen or air pump, pressure gauge, hose, drain container
Required PPE
Gloves, safety goggles, overalls
Required Tools
Spanners, pump with gauge, pressure gauge for Schrader valve, drain hose
Step-by-Step Method
- Turn off the vessel from the system using the isolation valve.
- Open the drain valve on the water side. Let all the water drain out.
- Connect a pressure gauge to the Schrader valve (like a tyre valve) on the air side.
- Check the air pressure.
- If WATER comes out of the Schrader valve - STOP. The diaphragm is broken. The vessel must be replaced. Do NOT pump it up.
- If pressure is low and no water - pump it up with nitrogen or air.
- Set the pressure to the system design pressure (usually 1.0-1.5 bar - check the paperwork).
- Close the drain valve.
- Slowly open the isolation valve to put it back in service.
- Watch the system pressure over the next heating cycle to make sure it works.
- Write down the old pressure, new pressure, and vessel condition.
Pass Criteria
Air pressure at the right setting. No water at Schrader valve. System pressure stays steady through the heating cycle. No pressure relief valve discharge.
Fail Criteria
Water at Schrader valve (diaphragm broken). Pressure will not hold (diaphragm leak or Schrader valve fault). System pressure still unstable. Pressure relief valve still discharging.
Common Problems & Corrections
Water at Schrader valve - Diaphragm broken. Replace vessel. Pressure will not hold - Check Schrader valve for leaks. Replace if bad. If valve is OK, diaphragm has a small leak - replace vessel. System pressure still unstable - Check for multiple waterlogged vessels. Check the charge pressure against the system design. Check for system leaks. Vessel too small - Check vessel size against system volume. Add more vessels if needed.
Warnings
If water comes out of the Schrader valve, the diaphragm has failed. Do NOT pump it up. Replace the vessel. Do not go above the maker's maximum pressure. Make sure the vessel is fully isolated before purging.
Relevant Legislation
BG50 (BSRIA), Pressure Equipment Regulations 2016