Replaceable-Bladder Expansion Tanks: When to Replace the Bladder Instead of the Tank

A failed expansion tank doesn't always mean a failed vessel. On a tank built with a replaceable bladder, the steel shell is the long-lived part and the bladder is the wear item. When the bladder gives out, you may be able to replace just that, instead of paying for a whole new ASME tank and the labor to swap it.

How the Tank Works

The shell holds an air cushion, pre-charged to a set pressure. A flexible bladder separates that air from the system water. As the water heats and expands, it pushes against the bladder and compresses the air. When it cools, the air pushes the water back out. Because the bladder keeps the water and the air apart, the steel never touches oxygen-rich air and water together, and the air can't be absorbed into the system.

The Wessels Lines in Our Catalog

• FXA series. ASME-certified replaceable-bladder tanks, such as the FXA-500 and the FXA-2000, plus the FXA-HP 800L. The listings describe these for commercial and industrial water service, including booster systems and well supplies.
• NA series. The 12NA33 and 14NA63 are classic compression tanks. They have no bladder at all. The air cushion sits directly on the water surface, so the design relies on keeping the right air-to-water level in the tank.
• Replacement bladders. We carry bladders for FXA tanks from the FXA-130 up through the FXA-2500, plus the NLA-1000.

How to Tell the Bladder Has Failed

• Water comes out of the tank's air valve when you press it. That's the clearest sign the bladder has ruptured.
• The tank feels heavy and full right to the top, with no air cushion left, which is what a waterlogged tank looks like.
• The pump short-cycles, or pressure swings sharply every time the system heats and cools.
• The relief valve discharges, because the expansion has nowhere to go.

Before you condemn the bladder, check the pre-charge. Pressure on the air side has to be tested with the tank isolated from system pressure. A low pre-charge produces some of the same symptoms and costs nothing to fix. If you want the full picture of how tanks are sized and charged, our expansion tank guide covers it.

Replace the Bladder or the Tank

Replacing the bladder makes sense when the shell is sound: no corrosion, no leaks, no damage. You keep the vessel, the piping connections, and the original installation. Replacing the whole tank makes sense when the shell itself is corroded or damaged, or when the tank was undersized to begin with, since a new bladder won't fix a sizing problem. The bladders in our catalog run from under $1,000 on the smaller FXA models to over $7,000 on the largest, so the savings depend on how big the tank is. Follow the manufacturer's procedure for the actual swap, which involves isolating, draining, and depressurizing the tank first.

Matching the Bladder

A bladder is specific to its tank model. The FXA-600 bladder, for example, is made for the FXA-600 tank and not for the one next to it in size. One thing to confirm: the listings in the FXA family describe the tanks for potable and booster service, while the replacement bladder listings describe closed-loop heating and chilled water use. Check your tank's nameplate and confirm that the bladder you order is rated for your fluid before buying, since potable and hydronic applications don't always call for the same material.

National Boiler Supply carries Wessels replaceable-bladder tanks and replacement bladders. Send us the model number off the tank's nameplate and we'll match the bladder.