h2 Dew Point

H2 drier dew point A seal oil system for the generator was developed to keep the hydrogen from leaking along the rotor shaft (Figure 1). The seal ring floats on the shaft and does not rotate. It receives makeup oil from the bearing lubricating oil system. Seal oil enters the seal housing at about 5 psig above the machine hydrogen gas pressure. The seal oil flows in both directions along the shaft. Most oil (air-oil) flows toward the bearing (zero psig) and some oil (hydrogen-oil) flows toward the hydrogen (90 psig) thus blocking the flow of hydrogen along the shaft. When the air-oil mixes with the bearing lubricating oil, some hydrogen gas becomes entrained in the hydrogen-oil. In most generator manufacturer’s systems, lubricating oil and hydrogen seal oil are combined; in others the oils are separated with minor interaction between the two. Water in lubrication oil causes a plethora of problems. The amount of moisture allowed in the lubricating oil of a typical turbine/generator is specified by ASTM D95 as 2000 ppm (0.2%). Most users would like to see moisture concentration below 1000 ppm. Lubricating oil moisture affects hydrogen dew point. This 1000 ppm lubricating oil is at risk of mixing with the seal oil where it can come in contact with the hydrogen gas. Hydrogen gas at -60°F dew point is hygroscopic and looks like a dry sponge in the presence of oil with 1000 ppm water. In service, the hydrogen gas dew point usually is maintained between -15°F and +32°F depending on the following: •The generator manufacturer, •The condition of the seal oil system clearances and operation, •The size of the generator, •The hydrogen gas pressure, and •A few other operating conditions. Experience has proven that water in the hydrogen seal oil is the most common source of moisture for hydrogen dew point deterioration. Seal oil moisture of less than 50 ppm is necessary to maintain hydrogen dew points below +15°F, as required. Experience further suggests that seal oil moisture above 50 ppm will elevate the hydrogen dew point, and just 250 ppm of water will increase the dew point above acceptable levels. Retaining Rings. Around 1960, generator manufacturers began using a non-magnetic, stainless steel material, ASTM A289, Class B, commonly known as 18Mn-5Cr, for the generator rotor retaining rings. The rings retain or hold the rotor copper winding in place on the rotor. If the retaining ring were not in place, the winding would be flung from the rotor due to the centrifugal force caused by the rotor speed (typically 3600 rpm). In the body of the rotor, the windings are held in place by wedges in the winding slots. In Europe during the 1980s, there were a number of in-service failures by retaining rings. The reason for the failures was identified as “stress corrosion cracking” caused by the presence of condensed water on the highly stressed retaining rings. Manufacturers urged users to replace the rings with a different ring material, ASTM A289, Class C, known as 18Mn-18Cr. A unit outage for replacing retaining rings can last three to four weeks naturally, and a retaining ring failure could destroy the generator. Utilities became very sensitive to moisture in the hydrogen gas. Most utilities established a dew point upper limit of +32°F. Dew points higher than this might allow moisture in the hydrogen to condensate on the retaining rings when unit is out of service. A Solution The major source of water in the turbine/generator lubricating oil is water from poor steam seals on the turbine. Turbine/generator lubricating oil and the hydrogen seal oil should be routinely analyzed Page 1

H2 drier dew point for moisture to maintain it at levels in the range of 10 ppm (0.001%). If the seal oil moisture content rises above 250 ppm, the dew point of the hydrogen may exceed acceptable limits. It is necessary to find the source of the seal oil moisture and eliminate it in order to avoid potential problems. The dew point of hydrogen is the temperature to which a given volume of hydrogen must be cooled, at constant barometric pressure in order for any water vapor present in the hydrogen to condense into liquid water. The condensed water is called dew, and hence the term. The dew point, therefore, is a saturation point – i.e., it is the temperature at tmospheric pressure at which the hydrogen gas can hold no more water vapor. Therefore, the dewpoint is a measure of the water content of any gas and it is read in degrees of temperature (but always at a base pressure – atmospheric in this case). The dewpoint measurement in temperature can be converted to other units of measure, such as parts per million (mass), parts per million (volume), or lbs/cubic feet. Within the power industry it is common practice to use hydrogen as a direct coolant for the generator stator windings. Hydrogen is used because it has a high heat transfer capacity and is more efficient at transferring heat than other mediums. The re-circulation of hydrogen within an electrical enclosure removes heat from the generator, transferring it via a heat exchanger into a secondary cooling circuit which usually uses demineralized water. Often this de-mineralized water is then cooled either by sea water or river water. As it is not possible to hermetically seal the generator set castings, there is a potential for moisture to ingress from the surrounding atmospheric air through packings and gaskets. Similarly, as the heat exchanger becomes more porous with age, it will allow moisture to get into the hydrogen. It is, therefore, essential that the dew point of the hydrogen be monitored and maintained at a safe margin, as a build-up of moisture in hydrogen can lead to a risk of what is termed “flashover”. An increase in the dewpoint simply means that additional water vapor has been picked up by the hydrogen – usually through atmospheric leaks or exposure to water. Therefore, simply increasing the pressure of the system does not alleviate or reduce the quantity of water vapor in the hydrogen. You simply must take steps to remove the excess water vapor in the closed circuit. Yes, it is possible to lower the dewpoint at the same relative pressure. There are manufacturers of equipment that does just that. Usually, for low dewpoints (as in your case) one usually employs an adsorption process unit to “dry” the hydrogen. There are charts and calculators for determining the mass quantity of water vapor contained in the circulating hydrogen. You can find these through the Internet. Go to: http://www.phymetrix.com/Software.htm and you can download a dewpoint calculator. 1.seal oil flow, 2.Condition of the seal oil rings and springs, 3.accumulated impure gas in the generator, 4.Scavenging flow, 5.purity of supply hydrogen cylinders- but it is rare, 6.Hydrogen analyzers condition, 7.High flow solenoids operation after enable the scavenging, 8.Hydrogen gas flow measuring units(rota meters are responding according to change in gas flow), 9.Any CO2 valves line up to the generator or any passing(Normally CO2 system is in lined up condition to purge the system up to the CO2 system solenoid valve and ensure this solenoid bypass having no passing). Page 2

H2 drier dew point Generators which do not have vacuum treated seal oil should be provided with a means of continuously bleeding out a small flow of generator gas from each of the two seal oil drain enlargements. The seal oil drain enlargements are where air contamination will be introduced into the generator because air comes out of the solution from the seal oil. The gas control valves will automatically introduce clean hydrogen into the generator casing when gas is bled out, with the result of maintaining generator gas purity at an acceptable level. Sources of Contamination * Possible sources of contamination of the end cavity gas are excessive seal oil flow- need to measure the actual oil flow and it should be within 3 ltrs to 20 ltrs/min * poor seal oil draining, * insufficient scavenging- We are maintaining maximum scavenging to improve the purity but no significant change observed * excessive air in the seal oil supply. * The low purity in the casing may be due to a leaky CO2 valve. CO2 valves often corrode due to an, interaction of CO2 with humidity causing an acid to form, and so are susceptible to internal leaks. * Moisture in the gas analyzer probe may cause erroneous readings. An aluminum moisture indicator should be upstream of the gas analyzer probe, and will warn of moisture contamination. Moisture is often removed from the gas sample by a molecular sieve filter. This special type of filter traps carbon dioxide and bleeds it out over a day or so. Therefore, if the carbon dioxide from the purge operation was inadvertently routed to the filter, then the reading will erroneously show low purity for about a day. An increase in the dewpoint simply means that additional water vapor has been picked up by the hydrogen – usually through atmospheric leaks or exposure to water. Therefore, simply increasing the pressure of the system does not alleviate or reduce the quantity of water vapor in the hydrogen. You simply must take steps to remove the excess water vapor in the closed circuit.Yes, it is possible to lower the dewpoint at the same relative pressure. There are manufacturers of equipment that does just that. Usually, for low dewpoints (as in your case) one usually employs an adsorption process unit to “dry” the hydrogen.There are charts and calculators for determining the mass quantity of water vapor contained in the circulating hydrogen. You can find these through the Internet. Go to: http://www.phymetrix.com/Software.htm and you can download a dewpoint calculator. 1.seal oil flow, 2.Condition of the seal oil rings and springs, 3.accumulated impure gas in the generator, 4.Scavenging flow, 5.purity of supply hydrogen cylinders- but it is rare, 6.Hydrogen analyzers condition, 7.High flow solenoids operation after enable the scavenging, 8.Hydrogen gas flow measuring units(rota meters are responding according to change in gas flow), 9.Any CO2 valves line up to the generator or any passing(Normally CO2 system is in lined up condition to purge the system up to the CO2 system solenoid valve and ensure this solenoid bypass having no passing). Page 3

H2 drier dew point Generators which do not have vacuum treated seal oil should be provided with a means of continuously bleeding out a small flow of generator gas from each of the two seal oil drain enlargements. The seal oil drain enlargements are where air contamination will be introduced into the generator because air comes out of the solution from the seal oil. The gas control valves will automatically introduce clean hydrogen into the generator casing when gas is bled out, with the result of maintaining generator gas purity at an acceptable level. Sources of Contamination * Possible sources of contamination of the end cavity gas are excessive seal oil flow- need to measure the actual oil flow and it should be within 3 ltrs to 20 ltrs/min * poor seal oil draining, * insufficient scavenging- We are maintaining maximum scavenging to improve the purity but no significant change observed * excessive air in the seal oil supply. * The low purity in the casing may be due to a leaky CO2 valve. CO2 valves often corrode due to an, interaction of CO2 with humidity causing an acid to form, and so are susceptible to internal leaks. * Moisture in the gas analyzer probe may cause erroneous readings. An aluminum moisture indicator should be upstream of the gas analyzer probe, and will warn of moisture contamination. Moisture is often removed from the gas sample by a molecular sieve filter. This special type of filter traps carbon dioxide and bleeds it out over a day or so. Therefore, if the carbon dioxide from the purge operation was inadvertently routed to the filter, then the reading will erroneously show low purity for about a day.

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