Degasser PRE-V (page 1 of 3)

Oxygen solubility in water Air can be dissolved in water without any visible airbubbles. Occurrence of visible bubbles may indicate that the temperature and the pressure are such that the saturation point bas been reached. lf not, the air-bubbles will disappear. However, air-bubbles may appear anywhere in the plant. In e.g. local vacuum zones, restrictions, pumps etc, and aggregate in parts where it is calm, in e.g. batteries and heat exchangers. A degassing issue will appear. The solubility of oxygen in water is defined by the air-pressure and by the water temperature. The phenomenon is described by Henry's Law. Air is mainly composed of oxygen and nitrogen in fixed proportions. Oxidation processes in the plant will in the long run cause damage to the installed equipment. In order to prevent this, the presence of oxygen must be eliminated to the extent possible. One possible solution is to degas the water in the system and significantly reduce the quantity of air/oxygen.	3-step degasser
In PREMA's degasser this is accomplished in three steps. A continunus flow of water is led through the PRE-V equipment. In which the water is degassed in a 3-step process of successive pressure reductions. This process affects neither the pressure nor the flow in the main system. Drainage and emptying can be accomplished without interruption of the main system operation. The pump and the automatic valves in the equipment cooperate for an optimal degassing effect irrespective of the main system pressure and flow variations. STEP 1 The targeted pressure level is dependent on the current system pressure. The pressure reduction is made in a way that keeps the flow constant if the system pressure is lower than 1.0 bar the pressure reduction is insufficient and the degassing effect wili fail to appear. For successful degassing in step 1, the system pressure must exceed 1.0 bar. Another important function of step 1 is to protect the pump from accumulations of air and thus prevent a possibie breakdown. STEP 2 The purpose is to estabiish a negative pressure by further reducing the pressure from the level established in step 1. The dissolved gas will not be released in this step due to the negative pressure. The air-bubbles will consequently follow the water flow into the pump. STEP 3 In step 3 the pressure is reduced even further from the level established in step 2. The degassing effect is strong since a big partial negative pressure is obtained in the pump blading. The air-bubbles are not dissolved into the water immediateiy because of the hysteresis effect. Consequently there will be enough time to separate and blow off the gas that is set free. The degassing process During the degassing process, small bubbles of air are set free. These bubbles consist, among other things, of oxygen. The bubbles are caught in a net in the collecting vessel, in which it is calm. The accumulated air is blown off by means of a conventional degasser (equipped with non-return valves). This process reduces the saturation ratio for the water and thus ellminates accumulation of air in heat exchangers, batteries, radiators, baffles etc. Magnetite Removal of oxygen from the system water also prevents the formation of magnetite. Magnetite is a black oxide of iron, which is formed when the material in pipes and fittings corrode. The magnetite colors the water almost completely black. Pump wheels, garniture of fittings and parts of iron erode when magnetite precipitates in the system water. The result is requirements for major repairs and thus high costs. Plants in which black system water bas been observed is at risk. A continuous process of formation of magnetite is active and batteries and heat exchangers have already started to deteriorate. It is important to immediately call for professional cleaning of the system in order to eliminate oxygen and to slow down the oxidation process. It is advsable to equip also new installations with a degasser in order to prevent corrosion, break down and expensive replacement of equipment. Thls ensures big cost savings and prevents interruption of operation due to formation of magnetite. Magnetite might have a flaky structure, especialiy in the presence of calcareous water. The flakes might get stuck in heat exchangers, batteries etc and can only be removed by using strong solvents. If the result from cleaning is not successful, the apparatus must be replaced. It is much easier to handle free magnetite, since it can be removed by using magnetic devices. Strong acidification of the system water takes place during the process of formation of magnetite. The acidification accelerates the oxidation process. Therefore it is generally advisable to renew the system water when a degasser is installed. In "cold" systems the freeze protection deteriorates as glycol is consumed. During this process there is a risk of growth of aerobe bacteria.
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