What is an Expansion Vessel?

An expansion vessel—sometimes called a hydraulic accumulator—is a critical component in water systems that helps manage pressure fluctuations. These vessels contain a gas cushion that absorbs sudden changes in pressure, thereby protecting the system. While “pressure vessel” is a commonly used term, it’s worth noting that not all pressure vessels function in the same way.

Depending on the system design, expansion vessels can serve two distinct purposes:

1. As hydraulic accumulators, they act as energy storage units, delivering supplementary flow to reduce pump cycling and extend pump lifespan.
2. As thermal expansion compensators, they accommodate the increased volume of water as it heats—typically expanding by less than 4%—to prevent overpressure and system damage.

Types of Expansion Vessels

The most widely used types are bladder-type accumulators. These include a flexible bladder that separates the water and gas sections inside the vessel. When water enters, it compresses the gas on the other side, creating a cushioning effect.

Manufacturers may offer different internal designs, such as:

• Bladder accumulators – Prone to stretching, which can cause creases or partial collapse if not properly configured.
• Diaphragm accumulators – Typically more stable in form, with reduced wear and tear on the membrane.

⚠️ Note: Valves claiming to induce internal water flow should not be mistaken for true expansion vessels with dedicated supply and discharge points.

Pre-Charging and Maintenance

Most vessels are pre-charged by the manufacturer using nitrogen, which helps preserve the membrane’s longevity.

Maintenance Tips:

• Typical pre-charge pressure is around 3 bar, but actual values should be set during system commissioning.
• Annually, isolate and depressurize the vessel before checking its internal pressure via the test valve.
• Ensure pressure remains stable for 15 minutes before recharging with water.

A recent study showed that around 10% of expansion vessels had failed membranes, highlighting the importance of regular inspection. Leaking membranes may allow water into the gas chamber or result in gas pressure loss.

Flushing and Isolation

To reduce stagnation—especially in large or rarely used vessels—install isolation and drain valves. Periodically flushing the vessel helps prevent microbial growth and ensures the unit remains functional.

A good flushing cycle involves:

1. Isolating the vessel.
2. Draining water.
3. Allowing full gas recharge.
4. Repeating the water charging process.

Final Thoughts

Expansion vessels are essential for system stability and safety, especially in hot water circuits. Their design, maintenance, and inspection directly affect their lifespan and system performance.

By following these best practices, you ensure compliance, efficiency, and safety in your water management systems.