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Three Types of Expansion Tanks | MEP Engineering

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Eric Baluch
ebaluch@tesengineering.com

Expansion tanks are used in closed loop hydronic systems, such as in boiler systems or chilled water systems.  In closed loop piping systems, when the fluid temperature rises, expansion of the fluid occurs.  In boiler systems, the water is brought in cold and is heated by the boiler causing expansion.  If a chiller is turned off within a chilled water system, the temperature of the water will rise.  Expansion tanks allow expansion and contraction of the fluid within these systems without damaging components of the systems by regulating the pressure of the fluid in the system.  Without an expansion tank the pressure would rise and blow pressure relief valves.

There are three main types of expansion tanks used:

The first is a plain steel tank that is open to that atmosphere.  In atmospheric expansion tanks, the water and air in the tank are in direct contact.  Smaller models of atmospheric tanks can cost less than other styles of expansion tanks due to their simplicity.  Atmospheric expansion tanks can also withstand higher temperatures than other expansion tank types because there is no rubber barrier in the tank.  The plain steel expansion tank can be purchased with the ASME option, ensuring higher pressure ratings and quality.  However, plain steel tanks also have a higher installed cost because they are much larger than other tank types, often requiring them to be mounted from the ceiling of a mechanical room.  Plain steel tanks have higher maintenance costs.  The level indicators that are required for the atmospheric tank often develop leaks that force air out of the tank and the tank eventually is completely filled with water, leaving no room for expansion.  Corrosion within the system can occur because air on the atmospheric side of this type of expansion tank is dissolved in the water, causing rust in the pipes.

The second type of tank is a bladder style tank.  In a bladder tank, there is a rubber bladder that expands and contracts as fluid expands and contracts within the system.  In bladder tanks, there is no direct contact between the air in the tank and the fluid in the bladder which will reduce corrosion compared to a atmospheric tank.  Bladder tanks can typically fill 100% with fluid, reducing the size of tank needed.  The bladder is typically removable and/or replaceable allowing for simpler and less expensive repairs or inspections.  Bladder tanks also have an ASME option available.  They can also be floor mounted in the vertical position which can save valuable space.  Bladder tanks can also be purchased with FDA approved Butyl bladders allowing for use in potable water systems.  However, bladder tank usually have a higher initial cost and have a lower maximum allowable temperature than plain steel tanks because of the material of the bladder.

The third type of tank is a diaphragm tank.  In a diaphragm tank, there is a diaphragm that separates the fluid in the top of the tank and the air in the bottom of the tank.  Diaphragm tanks are similar to bladder tanks in that there is no direct contact between the fluid and air in the system, which reduces corrosion.  Diaphragm tanks require less maintenance and have a lower installation cost.  They can be floor mounted in the vertical position to save space and also come with an ASME option.  Unlike bladder tanks, diaphragm tanks have a limited acceptance volume, requiring a larger tank.  The diaphragm is not replaceable and the diaphragm tank has a lower maximum temperature than the plain steel tank because of the material of the diaphragm.