corrosion
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corrosion [2019/06/27 13:58] superman |
corrosion [2019/07/01 14:06] (current) superman [Direct Corrosion Monitors] |
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| - | **WikiSIS** is the place where you can find all the answers to preventing the [[wikisis|Sick Installation Syndrome]]. | + | **WikiSIS** is the place where you can find all the answers to preventing the [[wikisis|Sick Installation Syndrome (SIS)]]. |
| **Topics that affect SIS:** | **Topics that affect SIS:** | ||
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| When the pre-charge pressure is too low too much water will enter the vessel while the system is still cold **[STEP 1]**. 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. ([[pressurisation#Vessel Acceptance or Pressure Factor (Pf)|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 **[STEP 2]**. 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 **[STEP 3]**. 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.\\ | When the pre-charge pressure is too low too much water will enter the vessel while the system is still cold **[STEP 1]**. 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. ([[pressurisation#Vessel Acceptance or Pressure Factor (Pf)|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 **[STEP 2]**. 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 **[STEP 3]**. 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.\\ | ||
| {{:wiki:inflation-pressure-too-low.gif|}} | {{:wiki:inflation-pressure-too-low.gif|}} | ||
| - | === 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. | ||
| + | === 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 **[STEP 1]**. 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 **[STEP 2]**. 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 **[STEP 3]**. | ||
| + | {{:inflation-pressure-too-high.gif|}} | ||
| === Checking and Re-Charging Expansion Vessels with Gas === | === Checking and Re-Charging Expansion Vessels with Gas === | ||
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| - | {{:bv000003.jpg?100|}} | + | {{:bv000003.jpg?300|}} |
| === Vessel is Sized Incorrectly === | === Vessel is Sized Incorrectly === | ||
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| + | {{:bijvulset-01.png?250|}} | ||
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| ===== Corrosion Caused by Diffusion ===== | ===== 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. | + | 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 [[https://en.wikipedia.org/wiki/Diffusion_barrier|diffusion 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. | 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. | 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. | ||
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| 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. | 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. | ||
| + | {{:diffusie_evenwicht-01.png?400|}} | ||
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| - | 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: | + | 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 continuously 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 | * a defective membrane in the expansion vessel | ||
| * loss of gas pressure in the expansion vessel | * loss of gas pressure in the expansion vessel | ||
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| ==== Direct Corrosion Monitors ==== | ==== 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. | + | [[https://www.resus.eu/en/operation|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. | 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. | Corrosion speed is logged every 7 hours and stored in the memory. The data can be downloaded locally or remotely via a cloud portal. | ||
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| Examples of these systems are: | Examples of these systems are: | ||
| - | Hevasure | + | * Hevasure |
| - | + | * Fe-Quan | |
| - | Fe-Quan | + | |
| {{ ::fequan.jpg?400 |}} | {{ ::fequan.jpg?400 |}} | ||
corrosion.1561636716.txt.gz · Last modified: 2019/06/27 13:58 by superman
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