Expansion Tank Sizing

Procedure [2] – ASHRAE Diaphragm Vessel Specific Volume & Acceptance Factor Calculator

1. System Definition & Pipe Water Volume

Pipe MaterialNominal Dia [mm]Length [m]Area Cross [m²]Volume [m³]Volume [L]
Total Pipe Water Volume: 0.00 L

2. Equipment Water Volume

Equipment DescriptionQuantityVolume / Each [L]Total Volume [L]
Total Equipment Water Volume: 0.00 L

3. ASHRAE Diaphragm Tank Sizing

L
°C
°C
%
m
kPa
%
Pre-Charge P₁
(Gauge)0 kPa
Max Oper P₂
(Gauge)0 kPa
Acceptance
Factor (AF)0.00
Pipe Stretch
(-3αΔt)0.00%
Net Fluid Expansion Volume 0.00 L
Calculated Tank Capacity (No Margin) 0.00 L
Final Vessel Volume
(Includes Safety Margin) 0.00 L

Calculation Methodology – Procedure [2]

Unlike Procedure [1] which uses an empirical heuristic multiplier, Procedure [2] applies the strict fluid dynamics methodology outlined in the ASHRAE HVAC Systems and Equipment Handbook for Diaphragm Expansion Tanks. This determines the exact physical vessel size required to prevent system over-pressurization.

Vt = Vs × [ ( ν2 / ν1 ) - 1 ] - 3αΔt 1 - ( P1 / P2 )

1. The Acceptance Factor (Denominator)

A 500L diaphragm tank does not hold 500L of water; it is mostly filled with compressed air. The Acceptance Factor (AF) dictates the usable internal volume. It is calculated using Boyle's Law based on absolute pressures: 1 - (P₁ / P₂).

2. Intelligent Pipe Stretch (- 3αΔt)

As water heats up and expands, the metal pipes containing it also stretch. This slight increase in total pipe volume reduces the amount of fluid forced into the expansion tank. Because this calculator knows exactly how much Copper vs. Carbon Steel you selected in the Pipe Table, it calculates a Weighted Linear Coefficient of Thermal Expansion (α) specifically for your network, mathematically shaving off the exact expansion offset.

3. Glycol Expansion Penalty

Propylene Glycol significantly alters the density and specific heat properties of the fluid. A 30% glycol mix can expand up to 25% more than pure water over the same temperature delta. The calculator dynamically applies a volumetric multiplier based on your glycol percentage input to prevent undersizing freeze-protected systems.

Calculating Expansion Tank Pre-charge Pressure

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

Pi = [ Hm × 10 ] + 20 kPa
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 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 70°C, to prevent premature diaphragm failure. If water temperature is higher than 70°C, 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 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 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 pre-charge 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.