{{:wiki:logo_wikisis-testsite_achtergrond-grijs.gif?nolink&100|}} WikiSIS is the place where you can find all the answers to preventing the Sick Installation Syndrome. {{ :leaking_aav.jpg?200|}} ====== Introduction to WikiSIS ====== Many heating and cooling systems suffer from chronic problems.No matter whether they are small domestic systems or large commercial systems in hospitals or airports. Many of the symptoms are known to all installers and maintenance companies, like sticking thermostatic radiator valves, frequent pump failures, blocked valves, noisy boilers, leaks, increasing energy consumption, dropping system pressure and requiring frequent top-ups, black sludge and reddish-brown water. Often these problems are seen as normal and unavoidable. Left untreated they will inevitably lead to component and in the end even complete system failure. Power flushing and chemical treatment often fail to cure the problem and frequently make the problem worse. **Don't just accept it!** It **is** possible to have systems that don`t suffer frequent breakdowns, that work efficiently and reliable over its entire life., This Wiki is dedicated to eradicating sick systems. The result will be a permanently healthy and efficient installation. ====== The Sick Installation Syndrome ====== Sick Installation Syndrome is a collective term given to water-related problems in heating and cooling systems. The common symptoms of SIS are well known such as reddish, brown or black water, seized pumps, noisy or failed boilers, leaks, stuck valves, cold radiators, gurgling noises and clogged pipes. Systems showing these symptoms suffer premature component failures, higher maintenance costs and greatly reduced efficiency. The aim of this WikiSIS is to explain why systems suffer from SIS and how the root causes can be identified and rectified. The good news is that SIS can easily be prevented from attacking your heating or cooling system in the first place without any additional equipment or chemicals and at no or little extra cost. {{:wiki:container4.jpg?100|}} ===== What Causes The Sick Installation Syndrome? ===== All the symptoms described above can be linked to one thing: They are caused by corrosion and its associated effects. Corrosion comes in many forms and can be a very complex subject. In heating and cooling systems, the most common form by far is [[wiki:dokuwiki|Uniform Corrosion]] {{:wiki:filmvorming_door_roest_en_inhibitor_in_radiatorkraan.jpg?100|}} ===== Why do Systems Corrode? ===== All systems containing metals such as steel, brass, copper etc. corrode. Low levels of corrosion in itself are not a problem. It only becomes a problem if there is continued or accelerated corrosion. There are a number of possible causes for continued or accelerated corrosion rates. Uniform corrosion, which is by far the most common form in heating systems, can only occur in the presence of oxygen. Simplified it can be said that the more oxygen is present in the system the higher the rate of corrosion. Without oxygen, corrosion will stop. Therefore it is vital to prevent oxygen from entering the system. There are a number of ways oxygen can enter a system. **Dissolved in the initial fill or topping up water** **Trapped as air in the system after filling** **Diffusion through non-diffusion tight materials such as plastics and rubbers** **Low system pressure allowing air to be sucked in at high level** The amount of oxygen that enters the system and the corrosion that is caused by these mechanisms varies greatly. [[Research]] has shown that incorrect pressure control leading to air being sucked into the system at high level leads by far to the most corrosion. How this happens is not immediately obvious and therefore often not identified as a cause of the corrosion. In the majority of closed heating and cooling installations, an expansion vessel controls the pressure. It therefore follows that the expansion vessel plays a vital role in the fight against corrosion and the Sick Installation Syndrome. ====== 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: **Ve=e.Vs** Ve=Expansion Volume Vs= System Volume e=Coefficient of Expansion ==== The Expansion Vessel (Fixed Charge)==== 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 Tutorial 2 Fixed charge vessels are sometimes also referred to as variable pressure expansion vessels. ==== Membrane Expansion Vessel ==== 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. {{:expansion_tank.jpg?100|}}{{:cutaway_membrane_vessel.jpg?100|}} ==== 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 discuss 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_sd.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. {{:wiki:statico_sg.jpg?100|}} ==== Expansion Vessels (Constant Pressure)==== 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. {{:compresso_c10_f_connect.jpg?100|}} 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. {{:transfero_tv_connect_front.jpg?100|}} ==== Correctly Sizeing 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. **P1.V1=P2.V2** ==== System Volume ==== 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 ==== Once the system volume Vs is known the expansion volume Vn 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 tables. It is important to take into account if glycol or other additives have been added to the water which may change the coefficient. Example: Vs=1000l tmin=0C tmax=70C e=0.0226 Ve=1000.0.0226 Ve=22.6l ==== Water Reserve ==== 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. The reserve Vwr=0.005.Vs (min. 3l or more) In accordance with EN12828 Example: Vwr=1000.0.005 Vwr=5 l ==== Vessel Acceptance or Pressure Factor ==== 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. **Pf=(pe+1)/(pe-p0)** ==== Static Height ==== 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 barg 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?200|}} ==== Gas Fill Pressure ==== 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. **P0=Hst+0.3 barg** 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: 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 ==== Vessel Size Calculation ==== 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. **Vn>(Ve+Vwr)Pf** Pf=Pressure Factor Pe=End (hot) Pressure P0=Min or gas fill pressure Hst=Static hight above vessel Pa= Cold fill pressure P0=Hst/10+0,3 bar Psv= Safety valve lift pressure **Example:** Hst=10m P0=10/10+0.3 P0=1.3 bar Psv=3.0 bar Pe=2.5 bar Pf=(2.5+1)/(2.5-1.3) Pf=2,92 Now the vessel size can be calculated Vn>(Ve+Vwr).Pf Vn>(22.61+5).2.92 Vn>80.62l In this example the next biggest vessel of 100l should be selected ==== 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 **Vwr=Vn/Pf-Ve** Example: Vwr=100/2.92-22.61 Vwr=11.6l ==== Cold Fill Pressure ==== 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: **Pa=P0+0.3 barg** In the example, a bigger vessel was chosen to accommodate an increased reserve of 11.6l To ensure that this reserve is pushed into the vessel the revised cold fill pressure needs to be calculated. **Pa=(Vn(P0+1)/(Vn-Vwr))-1** Pa=(100(1.3+1)/(100-11.6))-1 Pa=1.6 barg ====== Corrosion ====== In general, corrosion of a material can be considered to be an interaction of the material (e.g. steel, brass, aluminium) with its environment (e.g. water). This causes damage to the material, resulting, among other things, in a deterioration of the mechanical strength of the material. The most common types of corrosion are the impairment of metal surfaces by oxygen and water in the air (dry corrosion), such as the rusting of iron or the formation of a green layer of copper carbonate on copper. Corrosion can, however, also occur in an aqueous environment and at high temperatures, whereby ceramic materials and even polymers can be affected. In an aqueous environment, the interaction between the material and water is of an electrochemical nature, which means that the phenomena that occur are governed by both chemical and electrical laws. The most common corrosion phenomenon in heating engineering is still the deterioration of steel through oxygen. In systems where there is little or no steel, the oxygen will react with the next least noble metal. The main component of steel is iron (Fe), which, when brought into contact with pure water, tends to allow the iron particles on its surface dissolve in the water as divalent, positively charged particles (iron ions Fe2+). This can happen because their energy level in water is lower than in the parent material. The steel is then left with the remaining negative particles (electrons e), and, as a result, is now negatively charged. The overall system, with the parent material and iron ions dissolved in water, thereby remains electrically neutral. As more and more iron ions become dissolved in the water, and as the parent material, therefore, becomes more and more negatively charged, an increasing potential difference will be created between the steel and the positive iron particles that have been transferred to the solution. Over time, this difference becomes so significant that it prevents any further iron particles dissolving in the water: the chemical energy that is released through the dissolution is insufficient to overcome the increasing difference in electrical potential. At first sight, an electrochemical equilibrium between the negatively charged steel and the positive dissolved particles is thereby created. In this situation, the impairment of the steel element would be limited to a small loss of material. In practice, however, we find that the water used in central heating installations is very different from the ideal, pure water described above. It contains many dissolved substances, including oxygen. The latter will cause several chemical reactions in the vicinity of the negative parent material, resulting in the consumption of electrons on the one hand, and the oxidisation of the iron ions dissolved in the water on the other, causing them to disappear from the aqueous solution as insoluble substances. Due to this combination of reactions, the original electrical equilibrium is upset, with the result that iron particles will again leave the parent material. In this way, the deterioration of the steel element will continue as long as oxygen remains present in the system. Corrosion problems with steel in heating installations can therefore almost always be traced back to the influx of oxygen into the installation. By applying the mass balance to the reaction of iron with oxygen, it is possible to determine the quantity of iron that will be affected by a given quantity of oxygen: each gram of oxygen that is present in the water reacts with 2.6 grams of iron, and forms 3.6 grams of fine, granular black iron oxide, which is called magnetite because it can be magnetised. ===== Controlling Corrosion ===== {{ :ph.jpg?400|}} As already mentioned corrosion is a complex process. The chemistry of the water also plays a part in the corrosion potential of the installation. Factors such as pH, conductivity, dissolved minerals and salts all have an influence. Analysing the water and 'optimising' its values can be beneficial but can differ a lot depending on the materials of the components such as brass, copper and Aluminium. As a rule, the likelihood of corrosion decreases greatly with a fall in conductivity of the water. PH values in the alkaline range are advantageous with regards to the durability of copper and steel but not for aluminium. There is a common misconception that by replacing steel in systems corrosion problems will not occur. However, this is not correct as the oxygen will also react with the other metals in the system. Often it is not possible to avoid steel altogether and these few parts will be under severe attack. It, therefore, makes sense to have as much steel in an installation as possible which will consume the dissolved oxygen quickly and without damaging components. Buffer tanks which are usually made from steel and carbon steel pipework should be incorporated as much as possible. Note that the amount of magnetite formed is always the same as it is directly related to the amount of oxygen that reacts. More detailed Information and recommendations can be found in the VDI 2035[[http://www.vdi.eu/engineering/vdi-standards/vdi-standards-details/?tx_wmdbvdirilisearch_pi1%5BsearchKey%5D=2035&tx_wmdbvdirilisearch_pi1%5Bmode%5D=1&tx_wmdbvdirilisearch_pi1%5BsingleSearch%5D=1]]. ===== Corrosion Caused by Incorrect Pressure Control ===== Correct pressure control is vital in closed heating and cooling systems. If the pressure drops it is possible that a negative pressure or vacuum will exist at the very top of the system. If the pressure in the system at any point is less than the surrounding atmosphere air may be drawn into the installation. The amount of air drawn in can be very substantial if automatic air vents have been fitted at the top of the system. But also smaller amounts of air can be sucked in through screwed joints or valve seal glands etc. The oxygen contained in the air will greatly accelerate the corrosion process. It is therefore paramount that a positive pressure must be maintained everywhere in the system at all times by the expansion vessel. All expansion vessels will lose gas charge over time. The time it takes depends on the type of vessel and it´s quality. Gas charge is lost by diffusion of gas through the membrane or through the crimped seals and air valves. ==== Charging and Re-Charging the Vessel With Gas ==== When the vessel is fitted for the first time it is vital that the pressure is checked and set to P0 for the specific installation. This is best done before the system is filled with water. If the pre-charge pressure is set to high or too low, it cannot maintain reliable system pressure. === Pre-Charge Pressure Too Low === When the pre-charge pressure is too low too much water will enter the vessel while the system is still cold. When the water heats up and expands, the expanded volume will be pushed into the vessel. However, as there is now insufficient volume left for the expanded water (the acceptance of the vessel is now much too high) the pressure increases beyond the calculated end pressure Pe. (Pressure Factor Pf too high). In the worst case, the pressure will be high enough that the safety valves open and spill water to limit the pressure. At this stage, the heating system will function normally and a casual inspection of the pressure gauges will show that everything is normal. However when the system now cools there is insufficient water volume and pressure in the expansion vessel to maintain a positive pressure at the top of the installation. Air may now be sucked in leading to accelerated corrosion. At this stage, systems are often topped up with water to re-pressurise them. However, this will start the cycle from new with the additional corrosion potential due to the added fresh water. === Pre-Charge Pressure Too High === This is the opposite scenario but with the same effect. If the gas pressure in the vessel is too high not sufficient or even no water enters the vessel during the cold fill. When the water heats up the pressure rises too high as it needs to overcome the excessive gas pressure in the vessel. Once again the pressure may exceed the safety valve pressure Psv. When the system cools again there is now no or insufficient water in the vessel to compensate for the reduced volume and therefore pressure cannot be maintained. It is worth remembering that a vessel that has no water in it cannot pass its pressure onto the system. As soon as the vessel is empty the system pressure at the top of the installation will be zero and can quickly become negative resulting in air being sucked in. === Checking and Re-Charging Expansion Vessels with Gas === All vessels will lose pre-charge pressure over time. Therefore the pre-charge pressure needs to be checked at regular intervals at least once per year. It is not possible to determine the state of the gas pressure by looking at a system pressure gauge. These read system pressure, not gas charge pressure. Therefore the vessel needs to be isolated from the system by closing the lockshield valves which are commonly fitted. Then the vessel needs to be drained. Once empty the gas pressure can be measured with a tyre gauge. {{:bv000003.jpg?100|}} === Vessel is Sized Incorrectly === If the vessel is sized too small it cannot accommodate the expanded water volume. When the system heats up and water is forced into the vessel the pressure will increase rapidly as the pressure factor will quickly be exceeded. This will result in safety valves lifting and spilling water to control the maximum pressure. Now when the system cools again there is not enough water in the vessel to maintain the pressure at the top of the installation and air may be drawn in leading to accelerated corrosion rates. On the other hand, if the vessel is sized too large full pressure control is still guaranteed. It, therefore, follows that **a vessel cannot be too large**. ===== Corrosion Caused by Topping up ===== Drinking water contains approx 10mg/l of dissolved oxygen which starts to react with the steel in an installation as soon as it comes into contact with it. When the installation is being filled for the first time the corrosion process starts immediately. Provided the system is properly vented after filling and positive pressure is maintained the oxygen dissolved in the fill water will soon be used up by the corrosion process without causing any harm to the system. (Note: in a system which contains only plastic or non-ferrous materials and very little steel, all the corrosion will be concentrated on this part.) Oxygen dissolved in the fill water will only become a problem if the system is continuously topped up to compensate for water losses. Water may be lost in heating systems for a number of reasons. The obvious ones are leaks or frequent maintenance where the system is being drained and re-filled. The designer of the installation should ensure that components such as pumps can be replaced without having to drain the complete system. Automatic water make-up can make leaks less obvious unless water consumption is closely monitored. The less obvious cause of losing water is down to poor pressure control where water is lost through safety. The German VDI2035 allows a maximum of 2x the system volume during the lifetime of an installation. If water loss exceeds 10% per annum this should be urgently investigated. ===== Corrosion Caused by Diffusion ===== Oxygen [[https://en.wikipedia.org/wiki/Diffusion|diffusion]] is a well know problem with some plastics and rubbers. Many modern plastic pipes are therefore now coated with oxygen diffusion [[https://en.wikipedia.org/wiki/Diffusion_barrier|barrier]] to reduce oxygen diffusion. Multi-layer pipes using an aluminium foil virtually eliminate this phenomenon altogether. However other materials such as rubbers also allow oxygen to diffuse from the atmosphere into the system water. Components made of rubber such as hoses and bellows are often used as flexible connections but constitute a relatively small overall surface. Another rubber component found in virtually every system is the membrane in the expansion vessel. The membranes need to be elastic so that they can stretch when the vessel takes in water. The ´stretchy´ rubbers such as EPDM are much more open to diffusion than non-elastic rubbers such as butyl. That is why some vessels are charged with nitrogen instead of air. Others use bags and not membranes which do not need to stretch and can, therefore, use more diffusion tight rubbers e.g. Butyl. It is often not understood that gases can diffuse through materials even if the pressure is higher on the water side than it is on the atmospheric side. The fact that some materials are water-tight but not gas-tight can be seen with high tech fabrics such as Goretex but usually, with fabric, the pressure is equal on both sides. However, as the dissolved oxygen in water is consumed by corrosion the partial pressure of oxygen in the water is lower than the partial pressure of oxygen in the atmosphere. Now if the molecular structure allows oxygen molecules to penetrate, oxygen will diffuse from the outside into the water even if this is at higher pressure. If not prevented, this is a continuous cycle as the diffused oxygen is consumed by the corrosion process and replenished from the atmosphere. ===== Corrosion Caused by Incorrect Pump Position ===== The job of a pump or circulator is to move the water around the system. In doing so the pump has to overcome only the friction losses and not the static hight pressure of the system. Therefore there is a higher pressure at the discharge side of the pump compared to the suction or return side. The difference in pressure is referred to as the pump head. Due to the friction losses in the system, the pressure reduces around the system. It follows that there is a point in the system where the pressure is the same no matter if the pump runs or not, this is called the neutral point. This point is always the connection point of the expansion vessel. (provided there is a single expansion vessel connection) It is therefore important that the expansion vessel is connected on the suction side of the pump. That way the pump has to push the water around the system and generates a positive pump head. If the vessel is on the discharge side, the pump it has to ´suck´ the water around the system. This can cause negative pressure at high points and air entering the system as a consequence. When this happens the remedy is frequently to install a bigger pump which only aggravates the problem. ===== Corrosion Monitoring ===== All system corrode to some extent. If the corrosion rate is low this is not detrimental to the installation. However, the corrosion levels can rise quickly when there are problems such as a defective expansion vessels or water is lost due to a leak. It is often not recognised that oxygen reacts very quickly when it comes into contact with steel. A good example where fast corrosion is visible are the brake discs on a car which go rusty shortly after a rain shower. It, therefore, makes sense to monitor the corrosion rate of a heating system contineously and during the entire lifetime of the system. Nowadays there are low cost highly accurate sensors on the market which record the corrosion rate and sound an alarm if a set threshold is exceeded. Corrosion sensors are an early warning system for problems such as: * a defective membrane in the expansion vessel * loss of gas pressure in the expansion vessel * loss of pressure due to leaks * excessive topping up due to leaks or maintenance * excessive ingress of oxygen due to permeable materials such as plastics and rubbers. * high oxygen levels * Incorrect pump position ===== Types of corrosion Monitors ===== There are a number of ways that corrosion can be monitored either directly or indirectly. Corrosion coupons is a direct method whereas water sampling and analysis is an indirect method. Both methods share the same problem as they are only snapshots of the condition or corrosion activity when the water sample is taken or when the coupons are measured. Modern sensors have made it possible to monitor the system and water condition continuously and record this data to give a complete picture over the lifetime of an installation. They can pick up small changes in corrosion activity or changes in water condition which allows operators to investigate the causes and act on them much more quickly thus preventing costly corrosion damages. ==== Direct Corrosion Monitors ==== Risycor is a patented direct corrosion sensor that is based on a corrosion coupon but can measure the mass of the coupon online and continuously. It is a low-cost sensor that reacts sensitively to increasing oxygen levels and therefore increased corrosion in the system water. Corrosion speed is logged every 7 hours and stored in the memory. The data can be downloaded locally or remotely via a cloud portal. The generated graphics can easily be analysed and show if corrosion levels are safe and if not help with diagnosing the problems. {{ ::x257_20190206.png?400 |}} === Indirect Corrosion Sensors === There are a number of systems on the market that use sensors to measure water quality such as pH, conductivity, dissolved oxygen, redox potential etc. Data from these sensors is the recorded and stored and made available locally or via the cloud. The disadvantage of these systems is that they are costly and therefore more suited to large installations and they require a high level of expertise and experience to analyse the large amounts of data they generate. Examples of these systems are: Hevasure Fe-Quan {{ ::fequan.jpg?400 |}} ====== Gases ====== 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. Henry's Law.[[https://en.wikipedia.org/wiki/Henry%27s_law|Henry´s 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. {{:wiki:microbellen.png?100|}} 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 quicken 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. {{:wiki:gases_in_radiators.jpg?300|}} ===== 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 {{:aav.jpg?100|}} 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. {{:leaking_aav.jpg?200|}} === Micro-bubble Separators === 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. {{:microbubble_separator.jpg?100|}} === 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. {{:servitech.jpg?100|}} 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. ==== Chemical Water Treatment and Water Conditioning==== The condition of the fill and system water can have a marked effect on the corrosion potential. Water is a cheap and plentiful medium and is, therefore, the preferred liquid. Water isn't just water and water chemistry is very complex. It can be highly corrosive to steel and other metals if certain values such as pH and conductivity are unfavourable. There are different methods of achieving the right water chemistry. The water can be either conditioned or chemically treated or both. There are many water treatment chemicals on the market. Chemical dosing can be beneficial under certain circumstances. However, it should be viewed like medication and only used to fight very specific problems. A healthy system does not need any chemical additives. If problems occur it is always better to find out the causes and rectify them before resourcing to chemical dosing.