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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”.
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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. |
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The design pre-charge pressure is chosen in such a way that evaporation, pump cavitation and underpressure are avoided at all times at any location in the system. Hysopt calculates the pre-charge pressure based on the maximal static height above vessel, plus an additional safety margin.
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The calculation of the pre-charge pressure assumes that the expansion vessel is located at the suction side of the circulation pump. If this is not the case, a manual correction by adding an additional pressure differential is necessary. The additional pressure differential equals the pump’s design differential pressure for the worst-case scenario. If well-understood, the additional pressure can also be limited to the pressure that ensures no evaporation nor cavitation anywhere in the system. It is advised to always place the expansion vessel at the suction side of the circulation pump whenever possible. |
Final pressure
The final pressure represents the pressure (gauge pressruepressure) at the expansion vessel when the complete system volume is heated to the maximal temperature.
During design, we choose the final pressure to be equal to the maximal allowable pressure at the expansion vessel location to minimise the required expansion vessel volume. However, the final pressure can also be chosen lower than the maximal allowable pressure, which will result in an oversized expansion vessel.
If the user overwrites the design volume, we will recalculate the final pressure following its definition, namely the pressure when the complete system is at the maximal temperature. This value will deviate from the maximal allowable pressure at the vessel location, which arises from the safety valve properties.
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The final pressure calculation assumes that the pump head, if a pump is located between the vessel and the safety relief valve, is negligible compared to the discharge pressure of the safety valve. If this is not the case, a manual correction of the final pressure is necessary by subtracting the maximal pump pressure from the final pressure. |
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The acceptance factor represents the percentage of the expansion vessel that can effectively be used to take in any fluid. The acceptance factor is not 100% due to the minimal and maximal pressure limitations of the system, and is thus derived from the pre-charge and final pressure.
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Expansion automats | Constant pressure expansion vessels Expansion automats are devices that keep the pressure in the expansion vessel constant by taking away gas or water volume in a controlled manner in as a response to thermal expansion. As a result, expansion automats allow to use a greater part of the gross volume of an expansion vessel in a useful way. The acceptance factor can be set equal to the maximal acceptance factor that the membrane can withstand. If you want to calculate the required vessel volume using an expansion automat, manually enter & lock the maximal acceptance factor of your expansion vessel in the parameter “acceptance factor”. |
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If the pre-charge pressure from the manufacturer is different than the calculated design pre-charge pressure, re-enter the pre-charge pressure of the manufacturer as “design pre-charge pressure” and recalculate the design volume. The design volume might increase as a result of the less optimal pre-charge pressure from the manufacturer. |
5. Warnings & Errors
Warnings
The final pressure and the acceptance factor are recalculated due to part selection or a locked design volume. The recalculated final pressure is x bar. The safety valve setting would allow a maximal pressure at the vessel location of x bar.
The design volume of the expansion vessel is overwritten, therefore the calculations of the final pressure and maximal pressure at the vessel are recalculated based on the locked volume of the expansion vessel.
Acceptance factor and design pre-charge OR final pressure OR design volume are locked simultaneously, causing inconsistent results. Please unlock one the parameters.
The acceptance factor and pre-charge OR final pressure OR design volume are locked simultaneously, this will cause inconsistent calculations. Therefore it is advised to unlock one of the parameters.
Pre-charge pressure (x kPa) opens safety valve (x kPa)
The “Pre-charge pressure” is higher than the “Design discharge pressure”, therefore the safety valve will open at the “Pre-charge pressure”. This is not a good design, because the safety valve will always be open. Check the input parameters and make sure that the “Design discharge pressure” is higher than the “Design pre-charge pressure”.
Errors
Expansion vessel too small (expected x l but installed x l): The recalculated acceptance factor (100%) and final pressure (infinity) are invalid due to insufficient vessel size
The design volume of the expansion vessel is overwritten, therefore the calculations of the final pressure and maximal pressure at the vessel are recalculated based on the locked volume of the expansion vessel. In this case the locked design volume of the expansion vessel is to low, therefore resulting in invalid calculations.
To solve this error: Unlock the design volume of the expansion vessel or fill in a volume that is higher than the expected value.
The final pressure (x kPa) cannot be lower than the pre-charge pressure (x kPa) in practical situations. Please verify the inputs, increase the maximal allowable pressure if possible or consider other design configurations (see wiki).
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This error can occur in the following situations:
The “Design discharge pressure” parameter being too low, therefore the calculation of the “Final pressure” is not sufficient.
The “Final pressure” is locked with a value that is lower than the “Pre-charge pressure”.
The “Design pre-charge pressure” is locked with a value that is higher than the “Final pressure”.
To solve this error (based on the situations above):
Increase the “Design discharge pressure” so that the final pressure is sufficient.
Unlock the “Final pressure”
Unlock the “Design pre-charge pressure”
6. Example Sizing Scenarios
Standard Sizing (No Special Conditions)
On default the Hysopt software will calculate the design volume and pressures of the expansion vessel based on ideal design conditions. These are:
The expansion vessel is on the pump(s) suction side.
The pump differential pressure is negligible compared to the safety relief valve setting.
Pump is not located between the expansion vessel and the safety relief valve.
Additional pressure from the pump is considered in the “Pre-charge” OR “Final pressure” calculation depending on the location of the pump(s) and/or the expansion vessel.
Example model
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Calculations
Determination of the system volume
Currently, the user has to determine the system volume himself. The Hysopt software can determine the total water volume of the pipes and thermal storage. These values can be found on the information label when hovering over the expansion vessel. These calculated values must be manually added to the total water volume of the other components (e.g. Radiators, Boilers, etc.). The other parameters of the expansion vessel are calculated by the Hysopt software.
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Total water volume of the pipes = 0.25 m³
Total water volume of thermal storage = 1 m³
Total water volume of radiators = 1 m³
Total water volume of boiler = 0.2 m³
Determination of the max/min system temperatures for expansion coefficient
On default the minimal system temperature is 4°C, if the system has a lower minimal temperature it is advised to use a glycol mixture. On default the maximal system temperature is based on the supply temperature in the model + 5°C. In this example the supply temperature of the system is 70°C, therefore the software will use 75 °C as the maximum system temperature.
Minimal system temperature = 4°C
Maximal system temperature = 75°C
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Example Sizing Scenarios
Example 1: Standard Sizing (No Special Conditions)
On default the Hysopt software will calculate the design volume and pressures of the expansion vessel based on ideal design conditions. These are:
The expansion vessel is on the pump(s) suction side.
The pump differential pressure is negligible compared to the safety relief valve setting.
Pump is not located between the expansion vessel and the safety relief valve.
Additional pressure from the pump is considered in the “Pre-charge” OR “Final pressure” calculation depending on the location of the pump(s) and/or the expansion vessel.
Example model
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Calculations
Determination of the system volume
Currently, the user has to determine the system volume himself. The Hysopt software can determine the total water volume of the pipes and thermal storage. These values can be found on the information label when hovering over the expansion vessel. These calculated values must be manually added to the total water volume of the other components (e.g. Radiators, Boilers, etc.). The other parameters of the expansion vessel are calculated by the Hysopt software.
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Total water volume of the pipes = 0.25 m³
Total water volume of thermal storage = 1 m³
Total water volume of radiators = 1 m³
Total water volume of boiler = 0.2 m³
Determination of the max/min system temperatures for expansion coefficient
By default, the minimal system temperature is 4°C. If the system has a lower minimal temperature, it is advised to switch to a glycol mixture.
By default, the maximal system temperature is based on the supply temperature in the model + 5°C. In this example the supply temperature of the system is 70°C, therefore the software will use 75 °C as the maximum system temperature.
Minimal system temperature = 4°C
Maximal system temperature = 75°C
The expansion coefficient e is subsequently calculated with the formula below:
The density of water at the minimal and maximal system temperature can be determined by using density tables. A density table is shown as example below:
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Density at minimal system temperature = 999.97 kg/m³
Density at maximal system temperature = 974.84 kg/m³
Expansion coefficient e = 2.51%.
Determination of the maximal static height and safety valve height above the expansion vessel
The user needs to fill in these value depending on the configuration of the system. In this example, the values of these parameters are:
Maximal static height above expansion vessel = 15 meter
Safety valve height above expansion vessel = 3 meter
Determination of Design discharge pressure, Design pre-charge pressure and Final pressure
The pre-charge pressure is calculated with the formula below:
Where:
p0,gauge = Pre-charge pressure
pst = Static height pressure
pd = Vapour pressure, this factor is negligible at full liquid phase state.
Mpre = 30 kPa = Pre-charge pressure margin.
ρmin_temp = density of the water at the lowest system temperature
g = gravitational constant
Hmax,static = Maximal static height of the system above the expansion vessel
By default, the software uses a pre-charge pressure margin of 30 kPa. If you want to apply another pre-charge pressure margin, you can manually calculate the pre-charge pressure and lock the value.
Pre-charge pressure = 177.15 kPa
The final pressure in the design situation is calculated with the formula below:
where:
pfinal = Final pressure
pmax,sv = Design discharge temperature
Msv = Safety valve setting margin
ρmax = Density of the water at the highest system temperature
Hsv,vessel = Static height of the safety valve above the expansion vessel
By default, the Design discharge pressure of the safety valve is set to 350 kPa (=3.5 bar). If this valus is different for your situation, you can adjust the value of the “Design discharge pressure” at the safety valve component. The default safety valve setting margin is 0.9. If you want to change this pressure margin, you need to manually calculate your desired final pressure, fill in that value and lock the final pressure.
The density of water at the minimal and maximal system temperature can be determined by using the table below:
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Density at minimal system temperature = 999.97 kg/m³
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Determination of the Design volume of the expansion vessel
At last, the design volume of the expansion vessel is calculated with the formula below:
Expansion coefficient (e) = 2.51%.
Determination of the maximal static height and safety valve height above the expansion vessel
The user needs to fill in these value depending on the configuration of the system. In this example the values of these parameters are:
Maximal static height above expansion vessel = 15 meter
Safety valve height above expansion vessel = 3 meter
Determination of Design discharge pressure, Design pre-charge pressure and Final pressure
The pre-charge pressure is calculated with the formula below:
where:
Vsystem = Total water volume of the system
e = Expansion coefficient
wr% = water reserve factor
ηg = Acceptance factor
The default water reserve factor is 0.5%. If the user want to change this parameter, Design volume of the expansion vessel needs to be manually calculated and filled in.
Where:
p0,gauge = Pre-charge pressure
pst = Static height pressure
pd = Vapour pressure, this factor is negligible at full liquid phase state.
Mpre = 30 kPa = Pre-charge pressure margin.
ρmin_temp = density of the water at the lowest system temperature
g = gravitational constant
Hmax,static = Maximal static height of the system above the expansion vessel
On default the software uses a pre-charge pressure margin of 30 kPa, if a different margin is desired, the user must manually calculate the pre-charge pressure and lock it.
Pre-charge pressure = 177.15 kPa
The final pressure is calculated with the formula below:
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Example 2: Expansion vessel at discharge side of pump
If the expansion vessel is at the discharge side of the pump, additional pressure from the pump must be considered in the “Pre-charge pressure” calculation. The additional pump pressure will have an impact on the calculations of the “Design volume” of the expansion vessel.
Example model
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In this example, the same system configuration is used except for the location of the expansion vessel. This means that most of calculated parameters will not be affected. Only the pre-charge pressure, acceptance factor and design volume of the expansion vessel will be affected by having the vessel at a different location, more specifically at the discharge side of the pump. To take into account the pump influence correctly, you need to manually add the pressure from the pump(s) to the calculated “Pre-charge pressure” in order to calculate the correct design volume of the expansion vessel.
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Calculations
Pre-charge pressure
The pre-charge pressure can be calculated with the formula below, if the vessel stands at the discharge side oof the pump:
whereWhere:
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ΔPpump=
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pmax,sv = Design discharge temperature
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Msv = Safety valve setting margin
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ρmax = Density of the water at the highest system temperature
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Hsv,vessel = Static height of the safety valve above the expansion vessel
On default the Design discharge pressure is set on 350 kPa, this value differs based on the installed safety valve, the user must overwrite this value. The default safety valve setting margin is 0.9. If the user want to change this parameter, the final pressure needs to be manually calculated and filled in.Pump pressure in kPa, in this example the pump pressure is 14.41 kPa
It is possible that the “Design pre-charge pressure” exceeds the “Final pressure” and/or the “Design discharge pressure”. In this cases you must manually fill in a higher value for the “Design discharge pressure”.
Design volume of the expansion vessel
The design volume of the expansion vessel is calculated with the formula below:
Determination of the Design volume of the expansion vessel
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In this situation the acceptance factor ηg differs than with the standard sizing due to the change in Pre-charge pressure, therefore increasing the required volume of the expansion vessel.
where:
Vsystem = Total water volume of the system
e = Expansion coefficient
wr% = water reserve factor
ηg = Acceptance factor
The default water reserve factor is 0.5%. If the user want to change this parameter, Design volume of the expansion vessel needs to be manually calculated and filled in.
Expansion vessel at discharge side of pump
If the expansion vessel is at the discharge side of the pump additional pressure from the pump is considered in the “Pre-charge pressure” calculation. The pump pressure will certainly have an impact on the calculations of the “Design volume” of the expansion vessel.
Example model
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Example 3: Pump Between Vessel & Safety Valve
If the pump is located in between the expansion vessel and the safety relief valve, the final pressure is limited by the pump pressure. The pump pressure will certainly have an impact on the calculations of the “Design volume” of the expansion vessel.
Example model
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In this example the same system configuration is used, only the location of the pump is changed. This means that most of calculated parameters will not be affected. Only the final pressure, acceptance factor and design volume of the expansion vessel will be affected. Currently, the user have has to manually add subtract the pressure from the pump(s) with the calculated “Pre-charge “Final pressure”, to calculate the correct design volume of the expansion vessel.
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Calculations
Pre-charge Final pressure
The pre-charge pressure can be final pressure is calculated with the formula below:
Wherewhere:
ΔPpump= Pump pressure in kPa, in this example the pump pressure is 14.41 kPa
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In this situation the acceptance factor ηg differs than with the standard sizing due to the change of Pre-charge Final pressure, therefore the volume of the expansion vessel differs.
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Pump Between Vessel & Safety Valve
If the pump is located in between the expansion vessel and the safety relief valve, the final pressure is limited by the pump pressure. The pump pressure will certainly have an impact on the calculations of the “Design volume” of the expansion vessel.
Example model
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In this example the same system configuration is used, only the location of the pump is changed. This means that most of calculated parameters will not be affected. Only the final pressure, acceptance factor and design volume of the expansion vessel will be affected. Currently, the user have to manually subtract the pressure from the pump(s) with the calculated “Final pressure”, to calculate the correct design volume of the expansion vessel.
Calculations
Final pressure
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Example 4: Expansion automat (Constant pressure vessels)
When using an expansion automat, the required design volume of the expansion vessel can be lowered due to the higher efficiency of the expansion vessel. In this case the acceptance factor of the expansion vessel is significantly higher, depending on the maximal efficiency of the expansion automat. Currently, the user needs to manually fill in the acceptance factor in the Hysopt software.
Example model
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Calculations
Design volume of the expansion vessel
The design volume of the expansion vessel is calculated with the formula below:
where:
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In this situation the acceptance factor ηg depends on the maximal efficiency of the expansion automat. In this example an acceptance factor of 95% is used.
It is possible that the “Design pre-charge pressure” exceeds the “Final pressure” and/or the “Design discharge pressure”. In this cases you must manually fill in a higher value for the “Design discharge pressure”.
Design volume of the expansion vessel
The design volume of the expansion vessel is calculated with the formula below:
In this situation the acceptance factor ηg differs than with the standard sizing due to the change of Final pressure, therefore the volume of the expansion vessel differs.
Expansion automat
When using an expansion automat, the volume of the expansion vessel can be lowered due to the higher efficiency of the expansion vessel. In this case the acceptance factor of the expansion vessel is significant higher, depending on the maximal efficiency of the expansion automat. Currently, the user needs to manually fill in the acceptance factor in the Hysopt software.
Example model
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Calculations
Design volume of the expansion vessel
The design volume of the expansion vessel is calculated with the formula below:
In this situation the acceptance factor ηg depends on the maximal efficiency of the expansion automat. In this example an acceptance factor of 95% is taken into account.
Overview design volume expansion vessel
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Standard sizing
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expansion vessel at discharge side of pump
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pump in between expansion vessel and safety valve
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Expansion automat
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Design volume of expansion vessel
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0.197 m³
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0.216 m³
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0.208 m³
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0.078 m³
Overview design volume expansion vessel
Standard sizing | expansion vessel at discharge side of pump | pump in between expansion vessel and safety valve | Expansion automat | |
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Design volume of expansion vessel | 0.197 m³ | 0.216 m³ | 0.208 m³ | 0.078 m³ |
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Warnings and Errors
Warnings
The final pressure and the acceptance factor are recalculated due to part selection or a locked design volume. The recalculated final pressure is x bar. The safety valve setting would allow a maximal pressure at the vessel location of x bar.
The design volume of the expansion vessel is overwritten, therefore the calculations of the final pressure and maximal pressure at the vessel are recalculated based on the locked volume of the expansion vessel.
Acceptance factor and design pre-charge OR final pressure OR design volume are locked simultaneously, causing inconsistent results. Please unlock one the parameters.
The acceptance factor and pre-charge OR final pressure OR design volume are locked simultaneously, this will cause inconsistent calculations. Therefore it is advised to unlock one of the parameters.
Pre-charge pressure (x kPa) opens safety valve (x kPa)
The “Pre-charge pressure” is higher than the “Design discharge pressure”, therefore the safety valve will open at the “Pre-charge pressure”. This is not a good design, because the safety valve will always be open. Check the input parameters and make sure that the “Design discharge pressure” is higher than the “Design pre-charge pressure”.
Errors
Expansion vessel too small (expected x l but installed x l): The recalculated acceptance factor (100%) and final pressure (infinity) are invalid due to insufficient vessel size
The design volume of the expansion vessel is overwritten, therefore the calculations of the final pressure and maximal pressure at the vessel are recalculated based on the locked volume of the expansion vessel. In this case the locked design volume of the expansion vessel is to low, therefore resulting in invalid calculations.
To solve this error: Unlock the design volume of the expansion vessel or fill in a volume that is higher than the expected value.
The final pressure (x kPa) cannot be lower than the pre-charge pressure (x kPa) in practical situations. Please verify the inputs, increase the maximal allowable pressure if possible or consider other design configurations (see wiki).
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This error can occur in the following situations:
The “Design discharge pressure” parameter being too low, therefore the calculation of the “Final pressure” is not sufficient.
The “Final pressure” is locked with a value that is lower than the “Pre-charge pressure”.
The “Design pre-charge pressure” is locked with a value that is higher than the “Final pressure”.
To solve this error (based on the situations above):
Increase the “Design discharge pressure” so that the final pressure is sufficient.
Unlock the “Final pressure”
Unlock the “Design pre-charge pressure”