Pre-charged expansion tanks
Pre-charged expansion tanks are available in a range of sizes to accommodate various system volumes and are supplied to order pre-charged with nitrogen to the pressure required for the intended installation. The pre-charged expansion tanks come in two types.
The basic type features a non-replaceable diaphragm which spans the middle of the tank. The water in the tank is in contact with the tank’s metal inner surface which makes it prone to corrosion and therefore unsuitable for potable water.
The other type of pre-charged expansion tank contains a replaceable butyl (bag type) diaphragm. Because the water in the tank is completely contained inside the membrane, the risk of corrosion is avoided and the tank is suitable for systems containing untreated water.
Examples of this type of expansion tank include Duraflex Cobalt.
Regular maintenance required for pre-charged expansion tanks includes complete draining and pressure testing at least annually.
Pump-controlled expansion tanks
Pump-controlled expansion tanks operate as pressure-maintaining stations with automatic water make-up and deaeration. Typically, a pump-controlled expansion tank comprises a tank fitted with a replaceable butyl (bag type) diaphragm, a pump and a control panel. This type of system represents a superior method of system pressure management and takes care of pressure control, degassing, and water-makeup with BMS interface capabilities.
Pressure control with this type of system is much more fine-tuned and up to 90% of the tank’s capacity can be utilized, compared to less than half that volume with a pre-charged tank (depending on system pressure). This saves a significant amount of plant room space and piping. One pump-controlled expansion tank can be substituted for four pre charge vessels.