Expansion Vessels

• 21 Jan, 2024

Testing and Maintaining the Assembled Expansion Vessel
An expansion vessel is a device that helps to regulate the pressure and volume of water in a closed heating or cooling system. It consists of a steel shell that contains a flexible membrane or diaphragm that separates the water from a gas, usually air or nitrogen. The gas acts as a cushion that absorbs the changes in the water volume due to temperature variations.
After assembling an expansion vessel, it is important to test and maintain it regularly to ensure its proper functioning and safety. Testing and maintaining an expansion vessel involves checking and adjusting the pre-charge pressure, inspecting and replacing the membrane or diaphragm, and checking and replacing the gas.
Checking and Adjusting the Pre-Charge Pressure
The pre-charge pressure is the pressure of the gas in the expansion vessel when the system is cold and the water is not expanding. The pre-charge pressure should match the system pressure at the coldest temperature. If the pre-charge pressure is too high or too low, it can affect the performance and safety of the system and the expansion vessel.
To check and adjust the pre-charge pressure, the following steps should be followed:

• Attach a pressure gauge to the Schrader valve on the top cover of the expansion vessel.
• Record the pre-charge pressure reading and compare it with the manufacturer’s recommended pre-charge pressure.
• If the pre-charge pressure is lower than the recommended value, use a tire inflator with a pressure gauge to inflate the expansion vessel to the desired pressure.
• If the pre-charge pressure is higher than the recommended value, use a Schrader valve core tool to release some gas from the expansion vessel until the desired pressure is reached.
• Detach the pressure gauge from the Schrader valve and replace the dust cap.
• Reconnect and refill the expansion vessel to the system, using the isolation valve and the filling loop.

The pre-charge pressure should be checked and adjusted at least once a year or whenever the system pressure is changed.
Inspecting and Replacing the Membrane or Diaphragm
The membrane or diaphragm is the flexible rubber component that separates the water from the gas in the expansion vessel. The membrane or diaphragm can be damaged by wear and tear, corrosion, or puncture, causing the water and the gas to mix and the expansion vessel to lose pressure.
To inspect and replace the membrane or diaphragm, the following steps should be followed:

• Isolate and drain the expansion vessel from the system, using the isolation valve and the drain valve.
• Unscrew and remove the expansion vessel from the system or its bracket.
• Open the expansion vessel and remove the damaged membrane or diaphragm.
• Install the new membrane or diaphragm, making sure it is properly aligned and secured.
• Reassemble the expansion vessel, ensuring all connections are tightened and secure.
• Reconnect and refill the expansion vessel to the system, using the isolation valve and the filling loop.

The membrane or diaphragm should be inspected and replaced at least every five years or whenever the expansion vessel loses pressure.
Checking and Replacing the Gas
The gas in the expansion vessel is usually air or nitrogen, which acts as a cushion for the water expansion and contraction. The gas can be contaminated or humid, causing corrosion, leakage, or pressure loss in the expansion vessel.
To check and replace the gas, the following steps should be followed:

• Isolate and drain the expansion vessel from the system, using the isolation valve and the drain valve.
• Depressurise the expansion vessel by pressing the Schrader valve core tool on the valve.
• Disconnect the expansion vessel from the system or its bracket.
• Open the gas valve on the top cover of the expansion vessel and release the gas.
• Close the gas valve and detach the gas source or the purging device.
• Reconnect and refill the expansion vessel to the system, using the isolation valve and the filling loop.

The gas should be checked and replaced at least every two years or whenever the expansion vessel shows signs of corrosion, leakage, or pressure loss.