Expansion Tank Sizing, Commissioning & Maintenance

Sizing an Expansion Tank

Careful calculation of the expansion tank size is critical to the correct functioning of the system.

Expansion Coefficient
Calculate the expansion coefficient for your system by calculating the difference between the cold system water temperature (heating off) and the max working temperature.

°CCoefficient
00.00013
100.00025
200.00174
300.00426
400.00782
500.01207
600.0145
650.01704
700.0198
750.02269
800.0258
850.02899
900.0324
950.0396
1000.04343
expansion graph
Heating System

The expansion tank sizing formula is as follows (based on Boyles Law):

Vt   =e x C
1 – (Pi/Pf)

where:

Vt=Expansion Tank size
e=Expansion coefficient
Pi=Initial charge pressure (absolute) of vessel.
This pressure must not be lower than the hydrostatic pressure at the point where the tank is connected to the system.
Pf=Maximum operating pressure (absolute) of the pressure relief valve
taking into account any differences in level between the vessel and the safety valve.
C=Total water capacity of the system in litres:boiler, pipework, radiators etc
(as a general approximation, C is between 4 and 8 litres for every kW of boiler output)
Note: Calculations must be done in Absolute Pressure
e.g. 100kPa = 200kPa absolute.

In standard heating systems:

e=0.04318 (Tmax = 99°C, Tmin = 10°C, Δt = 89ºC, C = 0.035)
Cooling System

The vessel sizing formula is as follows (based on Boyles Law):

Vt   =e x C
1 – (Pi/Pf)

In standard cooling systems:

e=0.011 (Tmax = 50ºC, T min =4ºC)
Pi=Maximum plant pressure, corresponding to the maximum
achievable temperature, equal to the ambient temperature,
which is recommended to be fixed at 50ºC
Pf=The final working pressure achieved at minimum
temperature, using 4ºC
Example
C=500 litres
Pi=150kPa (250kPa Abs)
Pf=400kPa (500kPa Abs)
Vt=0.04318 x 500 = 43.2 litres
1 – (250/500) = 0.5

43.2 divided by 0.5 = 86.4L

Select the next largest sized tank = 100 litres

Calculating Expansion Tank Pre-charge Pressure

Please use the below calculation to correctly determine the expansion tank pre-charge pressure:

Pi=[Hm x 10] + 20kPa
where:
Pi=Initial charge pressure (absolute) of vessel
Hm=System height (metres) above the location of the
expansion tank

Installation

  1. The expansion tank must be installed on the suction side of the system pump and preferably in the coolest part of the system e.g. on return to boiler.
  2. Ensure water entering the tank is less than 70ºC, to prevent premature diaphragm failure. If water temperature is higher than 70ºC, an intermediate tank must be installed between the expansion tank and the system.
  3. The expansion tank must be installed with a lockable service valve and drain point. This is to ensure the tank can be serviced properly in the future.
  4. The expansion tank must be installed with a pressure relief valve between the tank and the lockable service valve, to protect the tank from overpressure situations.
  5. The pressure relief valve rating must be no higher than the safe working pressure of the expansion tank.

Commissioning

Please follow the below 4 step process for commissioning an expansion tank:

  1. Disconnect
    1. Isolate the expansion tank from the system via the lockable service valve. This is crucial to ensure an accurate pressure reading.
    2. Disconnect from the system and drain the tank.

  2. Test
    1. Calculate the correct expansion tank pre-charge pressure.
    2. Test the pre-charge pressure in the expansion tank via the Schrader valve.

  3. Charge
    1. Charge the expansion tank to the correct pressure (see note on calculating pre-charge pressure) via the Schrader valve, using an air compressor or nitrogen canister.
    2. Recheck tank charge to ensure pre-charge pressure is holding. If a leak is found, the Schrader valve or the expansion tank will need replacing.

  4. Reconnect
    1. Reconnect the expansion tank to the system.
    2. Re-pressurise the system and check for leaks.

Maintenance

Please follow the below 5 step process for maintaining an expansion tank:

  1. Inspect
    1. Perform a visual check of expansion tank to ensure no obvious damage or corrosion is present.
    2. To check the integrity of the diaphragm, press
      down the Schrader valve. If water exits the valve, the diaphragm has ruptured and the expansion tank will need replacing.

  2. Disconnect
    1. Isolate the expansion tank from the system via the lockable service valve. This is crucial to ensure an accurate pressure reading.
    2. Disconnect from the system and drain the tank.

  3. Test
    1. Calculate the correct expansion tank
      (see note on calculating pre-charge pressure)
    2. Test the pre-charge pressure in the expansion tank via the Schrader valve.

  4. Charge
    1. Charge the expansion tank to the correct precharge pressure via the Schrader valve, using an air compressor or nitrogen canister.
    2. Recheck tank charge to ensure pressure is holding. If a leak is found, the Schrader valve or the expansion tank will need replacing.

  5. Reconnect
    1. Reconnect the expansion tank to the system.
    2. Re-pressurise the system and check for leaks.