Expansion vessel
Introduction
An expansion vessel (also called an expansion tank) is a key component in any closed-loop hydronic system. It accommodates changes in the fluid volume resulting from thermal expansion of the system’s volume during operation, thereby preventing excessive pressure in the system. Without an expansion vessel, the heated fluid would have nowhere to expand to leading to dangerously high pressures and system damage.
Hysopt focuses on a diaphragm expansion vessels of which the internally topology is depicted below for a system using water as heat transfer medium:
It is important to adequately design the expansion vessels in order to avoid air infiltration, evaporation of the liquid, pump cavitation and excessive pressures anywhere in the system under all, valid operating conditions. Practically, in the Hysopt software, the expansion vessel base circuit is also used to adapt the type of fluid used in the system. (e.g. glycol or ethanol mixtures)
Expansion vessel Base Circuit
Below is a screenshot of Hysopt’s Expansion Vessel with parameter list. The expansion vessel can be found in the heating library and cooling library under “System peripherals”.
Hysopt only provides an expansion vessel in the return pipe for reasons of good practice. Expansion tank membranes are sensitive to continuously high temperatures above 70°C (158°C), so it is recommended to place the component in the return pipe to avoid excessive temperatures.
Parameter list
General
Mixture and Brine
Brine: Define the fluid mixture (e.g., water or water+glycol). If none, water is used as fluid type.
Mixture: define the concentration of mixture as part of the water+glycol mixture.
Specifying the heat transfer medium
It is important to specify the correct heat transfer medium in your system as brine mixtures have other heat transfer properties than water. Hysopt will automatically propagate the specified medium throughout the hydronic loop.
System Volume
The total volume of fluid in the system (pipes, equipment, etc.).
Minimal System Temperature / Maximal System Temperature
Defaulted to 4 °C and 90 °C, respectively. Used to determine the fluid’s density extremes.
Expansion Coefficient
Calculated from the fluid densities at the minimal and maximal temperatures. Shown as
0 %
by default and can be overridden by the user if needed.
Maximal Static Height above Vessel
The static head of water/mixture above the expansion vessel. Used to determine the minimum required pre-charge pressure.
Safety Valve Height above Vessel
The relative height of the safety valve compared to the vessel. Affects the maximal allowable pressure at the vessel’s location, which influences the final pressure during design.
Safety Valve Parameters
Design Discharge Pressure
The design discharge pressure at which the safety valve is expected to open.Effective Discharge Pressure
Only relevant if using the part catalogue: The effective discharge pressure of the part-selected safety relief valve.
Expansion Vessel Parameters
Design Pre-Charge Pressure
The theoretically calculated pre-charge pressure that is necessary to ensure the minimum pressure in the system to avoid issues like evaporation and cavitation.Final Pressure
The maximum allowed gauge pressure at the vessel location (often derived from the safety valve setting). If the vessel is oversized, the final pressure is recalculated and will be lower than the maximal allowable pressure as derived from the safety valve.Acceptance Factor
The fraction of the gross vessel volume actually usable to accommodate expansion (calculated from pre-charge and final pressures).Design Volume
The theoretically calculated volume needed (gross size) for the vessel to handle the net expansion volume.Installed Volume
If a manufacturer’s part or catalog selection is made, the actual installed tank volume is displayed here.Installed Pre-Charge Pressure
If a manufacturer’s part or catalog selection is made, the actual installed pre-charge pressure of the selected tank is displayed here.
Expansion vessel sizing
The expansion vessel needs to be adequately sized to avoid harmful situations at any location in the hydronic loop. Properly sized and properly pre-charged expansion vessels will:
Avoid air infiltration at any point in the system
Avoid evaporation of the fluid at any point in the system
Avoid pump cavitation
Keep the pressure below the maximal allowed pressure at all locations in the loop (at components, at piping, …)
Below is a step-by-step procedure to size the expansion vessel for your situation
Step 1: Determine the inputs
Brine and Mixture
As the thermal expansion properties depends on the type of medium used, enter the properties of the heat transfer medium used in the system.
If the heat transfer medium is water, you can ignore this step.
System Volume
The system volume is the sum of all internal volumes of production units, end units, low loss headers, piping, storage tanks and all other system components that are part of the corresponding hydronic loop.
The total system volume must be determined upfront. To assist in the determination of the pipework volume and the storage tank volume, you can consult the “info message” after running optimise components , as shown below.
The sum of the pipework volume and thermal storage volume is insufficient to consider as total system volume. The system volume encapsulates the internal volume of all system components that are part of the corresponding hydronic loop.
Design Discharge Pressure
The design discharge pressure represents the pressure setting at which the system’s safety valve would open. This pressure is seen as the maximal allowable pressure in the system at the location of the safety valve.
The safety valve’s setting itself can be determined by investigating the maximal operating pressure of all system components (pipework PN, production unit maximal pressure, …). Hysopt does not support (yet) this determination so it is advised to consult a professional guideline or standard to determine the safety relief valve sizing and setting.
Maximal static height above the vessel
The maximal static height above the vessel represents the height difference between the expansion vessel (= reference height) and the highest point of the particular hydronic loop. The used convention is shown in the picture:
The maximal static height matters because the fluid’s pressure declines the higher the fluid climbs. Therefore, the highest system location will be the critical point when the system is at rest at which evaporation and underpressure must be avoided.
Safety Valve Height above vessel
The safety valve height above vessel is the height difference between the expansion vessel (=reference height) and the system’s safety valve. If the safety relief valve is installed at a different height than the expansion vessel, its setting must be recalculated to reflect the pressure at the expansion vessel’s location.
The applied conven