**WikiSIS** is the place where you can find all the answers to preventing the [[wikisis|Sick Installation Syndrome (SIS)]]. 

**Topics that affect SIS:**
  * <fs 90%>[[Pressurisation|Pressurisation]]</fs>
  * <fs 90%>[[Corrosion|Corrosion]]</fs>
  * <fs 90%>[[Gases|Gases]]</fs>
  * <fs 90%>[[chemical_water_treatment_and_water_conditioning|Chemical Water Treatment and Water Conditioning]]</fs>

====== Pressurisation ======

The function of pressurisation is to maintain the pressure in the system between pre-determined limits.Traditionally an open header tank above the system was used to take up expanded water volume and maintain pressure.In modern systems this task is performed by a sealed expansion vessel using a bladder or membrane. A gas cushion in the vessel maintains the pressure. In larger commercial systems sometimes a compressor or pump are used to regulate the system pressure.

==== Expansion ====
When water changes its temperature its volume will change. Water is not compressible and therefore in a closed heating system, this expanded volume needs to be accommodated to ensure the system does not burst. In most modern heating installations this function is performed by the expansion vessel.

The volume of expansion in a system can be calculated:

<WRAP box 250px left>

**Ve = e.Vs** 

  * <fs 80%> Ve = Expansion Volume </fs>\\
  * <fs 80%> Vs = System Volume </fs>\\
  * <fs 80%> e  = Coefficient of Expansion </fs>\\
</WRAP>

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===== Fixed Charged Expansion Vessels (variable pressure) =====


There are many different types of expansion vessels. In smaller systems the membrane type with a fixed gas charge are the most commonly used.The advantage of an expansion vessel over an open header tank is that it is sealed from the atmosphere preventing oxygen entering the system. A rubber membrane separates the gas charge from the water side. The working principle of a fixed gas charge vessel can be seen in this [[https://youtu.be/NO7r2aiNlRU|clip]]. Fixed charge vessels are sometimes also referred to as variable pressure expansion vessels.

==== Membrane Expansion Vessel ====

{{ :cutaway_membrane_vessel.jpg?120|}}
Membrane expansion vessels are the most commonly used especially in domestic heating installations. Typically a vessel consists of two dished ends which are connected by a rolled circumferential seam. The membrane is clamped by this rolled connection which acts as a seal. The membranes are usually made from an elastic rubber such as EPDM as it needs to stretch to take up expanded water from the system. The membrane divides the vessel into two chambers. One side is filled or pressurised with a gas (pre-charge pressure) while the other side contains the system water. As the membrane is flexible the pressure from the gas side is transferred to the waterside as soon as the water enters the vessel. EPDM membranes are not gas tight and over time gas will diffuse through the membrane into the water. To ensure that this effect does not contribute directly to corrosion vessels are sometimes charged with nitrogen instead of air. However, pressure loss will still occur and \\ 
regular re-charging is required.



{{ :wiki:statico_sd.jpg?110| }}

==== Bladder Expansion Vessel ====

Some higher quality vessels make use of a bladder instead of a membrane. The design of the bladder means that rubber does not have to stretch and therefore a more diffusion tight butyl rubber can be used. 
In this case, the gas pocket is between the vessel and the bladder whereas the water is inside the bladder. Due to the fact that butyl is more diffusion tight, they can be charged with normal air. The construction of these vessels is often discus shaped and the two halves are welded not crimped. The combined effect of a welded construction and the Butyl bladder is that they maintain their gas charge much longer than other vessels.


{{ :wiki:statico_sg.jpg?100|}}
==== Bag Expansion Vessel ====
These are similar in construction to the bladder but for larger vessels from 140l to 5000l.
Larger vessels are usually cylindrical and stand upright. The rubber bag is suspended from the top of the vessel. The gas charge is on the outside of the bag while the water is on the inside.\\ 
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===== Constant Pressure Expansion Vessels =====

There are some systems which are able to regulate the gas charge in order to keep the pressure constant no matter how much water they have taken up. In construction, they are similar to a bag vessel but when the system water expands and water is forced into the vessel a relief valve will spill air out of the vessel to maintain a constant pressure. When the system cools and spills back into the system a compressor maintains the gas pressure.
Mainly used for large systems it is important that they have a diffusion tight membrane as the compressor works with air and not nitrogen.

{{:schema_s_-en.jpg?600 |}}{{:compresso_c10_f_connect.jpg?120|}}

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Another type is the spill and fill system.
These systems do not use a gas charge at all but maintain a constant system pressure by spilling the water through a solenoid actuated valve and filling it back into the system with a pump. The whole process is regulated by a pressure sensor which activates either the spill valve or the fill pump.
The big advantage of spill and fill systems is the nearly 100% vessel acceptance which makes them best suited to compensating for very large expansion volumes. Diffusion of oxygen into the system water is still a problem with these systems as the air side of the vessel is open to atmosphere. This problem is aggravated when the vessel is also used as a degassing system. Water is constantly flushed through the vessel increasing the chance that oxygen will diffuse and enter the system.

{{:pompdrukbehoud.jpg?600 |}}{{:transfero_tv_connect_front.jpg?140|}}

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===== Correctly Sizing the Expansion Vessel to EN12828 =====

The expansion vessel must be large enough to contain the expanded water volume at maximum temperature and still have a sufficiently large gas pocket to control the pressure within set limits. [[https://en.wikipedia.org/wiki/Boyle%27s_law|Boyle's law]] says:

<WRAP box 150px left>

**P<sub>1</sub>V<sub>1</sub> = P<sub>2</sub>V<sub>2</sub>**
  * <fs 80%> P = Pressure </fs>\\
  * <fs 80%> V = Volume </fs>\\
</WRAP>




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==== System Volume (Vs) ====

To get an accurate value all individual components of the system should be added up. Water contents of the various pieces of equipment are usually available from the manufacturers.
There are also tables available that help to estimate the system volume based on the duty of the boiler or chiller. But care needs to be taken if systems have unusually long pipe runs or have additional buffer tanks. These need to be added separately.

If the volume is estimated it is recommended to err on the high side. This may mean that the expansion vessel is larger than required but this doesn’t matter. In fact, the additional capacity can be utilised to increase the water reserve (Vwr).

==== Expansion Volume (Ve)====

Once the system volume (Vs) is known the expansion volume (Ve) can be calculated. It depends on the coefficient of expansion of the liquid at the maximum temperature difference.
The expansion coefficient for water for various temperature differences can be determined from [[http://www.wikisis.org/wiki/lib/exe/fetch.php?media=thermal-expansion-eng.jpg|tables]] (in %). It is important to take into account if glycol or other additives have been added to the water which may change the coefficient.

++++Example |
<WRAP box 330px left>

**<fs 80%> Vs = 1000l\ \ \ \ \ t<sub>min</sub> = 4°C\ \ \ \ \ t<sub>max</sub> = 70°C\ \ \ \ \ e = 2.22% </fs>**
  *  Ve = 1000l * 2.22% \\
  *  Ve = 22,2l\\
</WRAP>
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==== Water Reserve (Vwr)====

In addition to the expanded water, the vessel also has to accommodate a reserve. The reserve ensures that when small amounts of water are lost from the system then Expansion vessel can still transfer its pressure from the gas cushion to the water side. An empty vessel cannot maintain system pressure.

<WRAP box 250px left>
**Vwr = Vs * 0.005 **(=min. 3l or more)
  * <fs 80%> Vwr = Water Reserve </fs> \\
  * <fs 80%> Vs = System Volume</fs> \\
</WRAP>

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++++Example|
<WRAP box 330px left>
**<fs 80%> Vwr = 1000l </fs>**
  *  Vwr = 1000l * 0.005 \\
  *  Vwr = 5l\\
</WRAP>
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==== Vessel Acceptance or Pressure Factor (Pf) ====

Every fixed gas charge vessel has an acceptance. The acceptance is the amount of water relative to the total vessel size it can absorb without exceeding a maximum pressure. In other words, the vessel must leave room for the gas side. The more the fixed gas filling is compressed the higher the pressure in the vessel and thus the system.

The acceptance of a vessel is dependant on the working pressure range of the system. That is why the acceptance is determined by the pressure factor Pf. To calculate the pressure factor it is first necessary to determine the gas fill pressure P0 and the maximum or end pressure Pe.

<WRAP box 250ox left>
**Pf = (Pe+1) / (Pe-P0)**
  * <fs 80%> Pf = Pressure factor </fs>\\ 
  * <fs 80%> Pe = End Pressure </fs>\\ 
  * <fs 80%> P0 = gass fill pressure </fs>\\ 
</WRAP>\\ 
==== Static Height (Hst)====


The static pressure Hst of an installation is determined by its height from the lowest to the highest point. This pressure is due to the weight of the column of water. E.g. a 10m high column of water will generate a pressure of 1 bar g at the bottom of the column. At the top of the column the pressure is 0 bar g.
In order to calculate the correct gas fill pressure of an expansion vessel the static height above the point of installation of the expansion vessel Hst needs to be determined as accurately as possible.

{{:column_of_water_pressure.jpg?300|}}

==== Gas Fill Pressure (P0)====
The gas fill pressure is the pressure of the gas pocket inside the expansion vessel. It is very important that this is set correctly. Most expansion vessels are supplied pre-charged to a `standard´pressure usually 1 or 1.5barg. However, this pressure must always be checked before installation and adjusted to suit the specific system it is installed in. The position of the expansion vessel in the system also needs to be taken into account. 

<WRAP box 250px left>
**P0 = Hst/10 + 0.3 barg**
  * <fs 80%> P0 = Gass Fill Pressure </fs>\\ 
  * <fs 80%> Hst = Static Height </fs>\\ 
</WRAP>\\ 
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The 0.3 barg additional pressure will ensure that the system maintains a positive pressure at all times at the highest point of the installation.


++++Example| 
<WRAP box 600px left>
  * If the vessel is installed at the bottom of a 10m high system: P0 = 1.3 barg\\ 
  * If the vessel is installed at the top of the same system: P0 = 0.3 barg\\ 
</WRAP>
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==== Vessel Size Calculation (Vn)====
With the system volume Vs, the expansion volume Ve, the water reserve Vwr and the gas fill pressure P0 determined the nominal size of the expansion vessel Vn can be calculated.

<WRAP box 250px left>
**Vn > (Ve + Vwr) * Pf **
  * <fs 80%> Vn = nominal size expansion vessel </fs> \\
  * <fs 80%> Ve = Expansion Volume </fs> \\
  * <fs 80%> Vwr = Water Reserve </fs> \\
  * <fs 80%> Pf = Pressure Factor </fs> \\
</WRAP>

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++++Example|
<WRAP box 500px left>
**<fs 80%> Hst = 10m \ \ \ \ \ Vs = 1000l\ \ \ \ \  e = 2.22% \ \ \ \ \ Pe = 2,5bar </fs>**
  *  Ve = 1000l * 2,22% = 22,2l\\
  *  Vwr = 1000l * 0,005 = 5l\\
  *  P0 = 10m/10 + 0,3bar = 1,3bar\\
  *  Pf = (2,5bar+1) / (2,bar - 1,3bar) = 2,92 \\
Now the vessel can be calculated:
  *  Vn > (22,2+5l)*2,92 = 80,3l \\
In this example the next biggest vessel of 100l should be selected. \\ 
</WRAP> \\ 
++++




==== Maximising the Reserve ====



If the next biggest vessel size is selected it is possible to use the extra capacity as an additional water reserve.

The new water reserve can be calculated

<WRAP box 250px left>
**Vwr = Vn / Pf - Ve **
  * <fs 80%> Vwr = Water Reserve </fs> \\
  * <fs 80%> Vn = nominal size expansion vessel </fs> \\
  * <fs 80%> Pf = Pressure Factor </fs> \\
  * <fs 80%> Ve = Expansion Volume </fs> \\ 
</WRAP>

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++++Example|
<WRAP box 500px left>
**<fs 80%> Vn = 100l \ \ \ \ \ Pf = 2.92 \ \ \ \ \  Ve = 22.2l  </fs>**
  *  Vwr = 100l / 2.92 - 22,2l = 12l
</WRAP> \\ 
++++


==== Cold Fill Pressure (Pa) ====


The cold fill pressure Pa is the system pressure at the expansion vessel when the installation is filled but cold. The cold fill pressure must be sufficiently higher than the gas fill pressure of the vessel to ensure that the water reserve Vwr is pushed into the vessel.
The cold fill pressure is at least:

<WRAP box 250px left>
**Pa = P0 + 0,3barg **
  * <fs 80%> Pa = Cold Fill Pressure </fs> \\
  * <fs 80%> P0 = Gass Fill Pressure </fs> \\
</WRAP>

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In the above example, a bigger vessel was chosen to accommodate an increased reserve of 12l.
To ensure that this reserve is pushed into the vessel **the revised cold fill pressure** needs to be calculated.

<WRAP box 300px left>
**Pa = (Vn (P0+1) / (Vn-Vwr)) - 1 **
  * <fs 80%> Pa = Cold Fill Pressure </fs> \\
  * <fs 80%> Vn = Nominal size Expansion Vessel </fs> \\
  * <fs 80%> P0 = Gass Fill Pressure </fs> \\
  * <fs 80%> Vwr = Water Reserve </fs> \\
</WRAP>

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++++Example|
<WRAP box 500px left>
**<fs 80%> Vn = 100l \ \ \ \ \ P0 = 1.3 \ \ \ \ \  Vwr = 12l  </fs>**
  *  Pa = (100 (1.3+1) / (100-12l)) - 1 = 1.6 barg
</WRAP> \\ 
++++