WikiSIS

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**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>


====== Gases ======
{{ :wiki:microbellen.png?200| }}

Gases can be dissolved in water. The gas molecules are trapped in between the water molecules. The amount dissolved depends on the temperature and the pressure. The behaviour of dissolved gasses in liquids was first discovered by William Henry and the law is named after him. [[https://en.wikipedia.org/wiki/Henry%27s_law|Henry´s Law]] says that the higher the pressure and the lower the temperature the more gas can be dissolved in the water.
It, therefore, follows that when the water is heated or the pressure drops gases come out of solution. They come out as tiny bubbles often referred to as Micro-Bubbles which give the water a milky appearance.
In a heating system, water is heated in the boiler. Therefore this is the hottest place in the installation where the microbubbles are released. However, Henry´s Law also says that gases come out of solution when the pressure drops. In a system, the lowest pressure is at the top of the installation where the static pressure is zero. Therefore more gases can be released at the top of an installation whether it is a heating or cooling system.
Dissolved gases in the form of micro-bubbles lead to inefficient systems as they reduce heat transfer and pump efficiency. When large amounts of gas come out of solution the micro-bubbles collect and become air pockets for example at the top of radiators further reducing efficiency.

The dissolved gases in fresh water is air which also contains oxygen. Normal tap water contains approximately 10mg/l but that can vary from area to area. Therefore a lot of dissolved air can lead to additional corrosion. The dissolved oxygen reacts with the steel in the system and is quickly consumed by the chemical process called corrosion. Air separators, deaerators or even vacuum degassers cannot prevent corrosion as the oxygen reacts quicker than these products can remove it. What is expelled is mainly inert nitrogen. However, left in the system this \\ 
can still cause nuisance problems such as cold radiators, poor circulation gurgling noises and general inefficiencies.

===== Micro-Bubbles =====
Micro-Bubbles
Micro-bubbles are formed when dissolved gases come out of solution due to an increase in temperature or a drop in pressure in accordance with Henry´s Law.
As the name already suggests they are tiny bubbles that give the water a milky appearance. It is sometimes referred to the champagne effect.
Removing such small bubbles, which have very little buoyancy in flowing water is difficult. The best way to remove them is to let the water rest and the bubbles will slowly rise to the top. The water will clear from the bottom upwards.
Bubbles also tend to adhere to surfaces where they collect and combine (coalesce) into larger bubbles. These larger bubbles gain in buoyancy and rise.
These two effects of adhesion and bubbles rising in still water are utilised in micro-bubble separators to trap them and expel them from the system.
If micro-bubbles remain in the system they will collect at places of low pressure and low flow. For example in radiators at the top of the heating installation. Here they can collect and grow into larger bubbles and form air pockets.


===== Filling and Venting =====

=== Enclosed Air Pockets ===

Air pockets may form in systems where there is low or no flow or the system hasn´t been adequately vented during filling. They can however also accumulate when there is a negative pressure and air is sucked in.
While filling the installation with water an equal amount of air which is occupying the internal volume of the system needs to be expelled. Commonly automatic air vents are being used at the various high points of an installation to automatically expel air while the system is being filled. The filling process should be done slowly to ensure that as much air as possible is being expelled. If the filling happens too fast it is likely that air pockets will be trapped and that air is mixed into the water in the form of bubbles. Fast filling also means higher pressure which in turn means that more gas can be absorbed by the water (Henry´s Law)
When the system is heated for the first time large amounts of air will come out of solution due to the increase in temperature (Henry´s Law).
The air that comes out of solution is in the form of microscopic bubbles. (Micro Bubbles) Due to their low buoyancy they will be carried by the flowing water and not rise to a high point unless the circulation is stopped. In older systems where the water velocity was much lower than in modern systems micro-bubbles could rise against the flow and collect at high points where thy could be vented manually or automatically.However in modern system with much smaller pipe diameters the flow is much faster and bubbles get flushed around the system. When the flow stops the bubbles will rise and create air pockets for example at the top of radiators.

=== Automatic Air Vents ===

Automatic air vents can be very useful to automatically expel air from a system when it is being filled.However once a system is running they cannot get rid of micro-bubbles and can even let air in if the pressure in the system becomes negative! All AAVs use a float operated valve to expell air. When air collects in the air vent the water level drop and the float drops with it. Connected directly or indirectly to the valve mechanism, the dropping float opens the valve and air is expelled. Now the water level rises again and pushes the float upwards in turn closing the valve.
The air however can only be expelled if the system pressure is higher than the atmospheric pressure. If this is not the case air will be sucked in through the open valve. **This will greatly accelerate corrosion in the system!**



Some low cost air vents where the float is rigidly coupled to the valve are prone to leaking. A lot of water can be lost through leaking air vents. This in turn leads to the pressure in the system dropping. If it drops to a level that there is no more pressure at the top of the system air will be sucked in through all the air vents leading to accelerated corrosion.

{{:aav.jpg?200|  }} {{:leaking_aav.jpg?200|}}

=== Micro-bubble Separators ===
{{ :microbubble_separator.jpg?150|}}
Unlike air vents micro-bubble separators are designed to catch the smallest bubbles that circulate through the system. They have a larger diameter than the pipework which slows the flow. The internal mesh further reduces flow and bubbles become attached. More and more bubbles accumulate to form larger bubbles. Once they are large enough the buoyancy can now overcome the flow of the water and the bubbles rise to the to of the separator where they are expelled through an automatic air vent. Micro-bubble separators should be placed at a location where most of the air comes out of solution. The ideal point according to Henry´s Law is the one with the highest temperature and lowest pressure.This is normally in the flow close to the boiler. That is why sometimes these type of separators are called thermal separators.



=== Pressure Step Degasser ===
When the static height above the boiler exceeds 15m the pressure will keep air in solution despite the fact that the water is at its hottest.That means even if a micro-bubble separator is installed air will still come out of solution at the top of the building where the pressure is at its lowest.
For tall systems so called pressure step degasser are used. They are not installed in line but in parallel where they ´sample´  the water into a vessel.This sample is then isolated from the system and depressurised allowing all the dissolved gases to come out of solution.Once the water is degassed it is flushed back into the system. This cycle repeats every minute or so until the system is completely degassed.

{{:drukstapontgasser-01.jpg?400 |}}

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Devices such as micro-bubble separators, pressure step degassers, automatic air vents etc. **cannot prevent corrosion**. The oxygen in the air bubbles reacts very quickly with the steel it comes into contact with.This happens faster than the separators can expell the air. If the expelled ´air´is analysed it is found to be mainly consisting of nitrogen.
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