Generator windings are cooled with hydrogen gas (H2). Hydrogen is used due to the large heat absorption capacity, which is approximately 2.7 times that of ambient air.
This gas must be kept moderately dry for several reasons. First, generators with a steel retaining ring having a composition of 18% Chromium (Cr) and 5% Manganese (Mn), are susceptible to stress failure in a high moisture environment. In this case, the problem can be solved by an expensive ring replacement* or by monitoring and drying the gas. Second, high moisture levels are suspected in the formation of lead carbonate. The lead is present in solder joints in the generator. Lead carbonate is hygroscopic and may provide a potential for arcing.
*New rings are 18% Cr and 18% Mn, and are claimed to be resistant to high moisture levels.
First, H2 gas is potentially explosive when mixed with O2 (air is 21% O2). Second, due to the presence of lubricating oil, the H2 gas may be oily. This is dependent on the system.
All in-line instruments can be made intrinsically safe for Class 1, Div 2 or Class 1, Div 1 areas. Some plants consider only the inside of H2 lines to be potentially explosive, and areas exposed to outside air is considered safe. However, many plants deem the areas around the H2 driers and lines as explosion proof and mark them as such with yellow lines painted on the floor. Check with the customer.
The XPDM portable is intrinsically safe and factory mutual (FM) approved for potentially explosive applications (Class 1, Div 1, Groups A, B, C, D).
Second, due to the potential for oil carryover, all in-line, continuous systems should be filtered with a good coalescing filter. If there is significant oil vapor, an activated carbon bed should be installed after the filter and before the sensor. In the case of the XPDM portable, a filter mounted on the unit is recommended. Oil carryover can be minimized by selecting a sample point high in the system.
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