HYDROGEN LEAKAGE AND PURITY GEK 45941

GEK 45941c Revised August 2002

GE Power Systems

Hydrogen Leakage and Purity Formulas

These instructions do not purport to cover all details or variations in equipment nor to provide for every possible contingency to be met in connection with installation, operation or maintenance. Should further information be desired or should particular problems arise which are not covered sufficiently for the purchaser’s purposes the matter should be referred to the GE Company. © 2002 GENERAL ELECTRIC COMPANY COMPANY

GEK 45941c


I. GENERAL

These instructions contain formulas for calculating the amount of gas leakage from the generator and for determining how much hydrogen must be added to the generator to obtain a desired increase in hydrogen purity. The formulas for gas leakage are used when the generator is being tested for leaks before being placed in operation. II. GAS CONSUMPTION

Gas is depleted from the generator by both leakage from pressure-containing parts, i.e., frame and end shield joints, weld joints and piping, and by entrainment into the seal oil. Total gas consumption (L t) when the unit is in operation or on turning gear is Lt = Le + Lo

(1)

where: Le = gas leakage from generator frame Lo = gas entrainment in seal oil III. PRESSURE DECAY TEST

Total gas consumption of the generator can be determined by means of a pressure decay test. (2)

where: Lt = total gas consumption from the generator that occurs with the generator at an average gas pressure (M) and temperature (T), in units of ft 3 /day (m 3 /day), @ STP, i.e. expressed as if at 65 F (18.3 C), and at atmospheric pressure, (29.92 in. Hg), (101 kPa), (1.03 kg/cm 2) H = duration of test, (hr) V = gas volume of the generator system, (ft 3) or (m3) B1 and B2 = initial and final barometric pressures for the test, (in. Hg), (kPa), (kg/cm 2) M1 and M2 = initial and final generator test pressures, measured with respect to atmospheric pressure, (in Hg),(kPa), (kg/cm2) M = 0.5 (M1 + M2) = average gas pressure during test T1 and T2 = initial and final gas temperatures for the test, (C ) T = 0.5 (T1 + T2) = average gas temperature during test

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GEK 45941c NOTE

The following notes will aid when using the different systems of units mentioned above for equation (2). Funny symbols show up instead of quotes below. Volume - The “volume units” used for the generator volume (V) set the “volume units” per day that the generator leakage (L t) will come out in. For example, V expressed in (ft3), (m3) respectively, results in an L t in (ft3 /day), (m 3 /day), etc. Pressure - The constant of 234 in equation (2) applies only when pressures are expressed in (in. Hg). This constant becomes 69.0 when pressures are in (kPa), and 6766 for pressures in (kg/cm 2). The pressure decay test can be done in air or hydrogen, while the unit is operating on turning gear or at standstill. It is important that the measurements of pressure and temperature be made as accurately as possible. Relatively small errors in measurements have a very significant effect on the calculated consumption. Readings should be taken periodically during the test period; for example, every 4 hours during a 24-hour test. This will generate several consumption rates which can be averaged. This procedure will help eliminate errors due to measurement tolerances. Equivalent hydrogen leakage can be determined from the air pressure decay results. Hydrogen will leak through joints or holes at a much higher rate than air but will be lost through entrainment in the seal oil only slightly faster. It is, therefore, necessary to separate the two components of total air consumption, i.e., air leaked and air entrained. The volume solubility of air in oil is 10%. This value is relatively independent of pressure at pressures less than 5 atmospheres. Thus, the loss of air from the generator casing (L oa) by entrainment into the seal oil for equilibrium conditions, is determined from the following equation: (3)

where: Loa = air entrained in seal oil, (ft 3 /day) P = Absolute gas pressure in generator, expressed in atmospheres, where 1 atmosphere = 14.7 psia (29.92 in. Hg), 2 atmospheres = 29.4 psia, etc. Q = seal oil flow to gas side seals, (gpm) Sf = saturation factor = 0.5 (this factor has been determined empirically; it takes into account the amount of air actually adsorbed in the seal oil) In metric units, equation (3) becomes: Loa = 4.32 PQ where: Loa = air entrained, (m3 /day)

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GEK 45941c


P = generator gas pressure, expressed in atmospheres, where 1 atmosphere = (101 kPa), (1.03 kg/cm 2) Q = air flow, (litres/s) The amount of hydrogen that would be entrained if the test were run in hydrogen, considering that the volume solubility is 7%, is given by (4)

and Loh = 1.4 Loa

(5)

where: Loh = hydrogen entrained in seal oil, (ft 3 /day) Sf = 1 which, in metric units, becomes: Loh = 6.04 PQ Loh = hydrogen entrained, (m 3 /day) P = generator gas pressure, expressed in atmospheres, where 1 atmosphere = (101 kPa), (1.03 kg/cm 2) Q = oil flow, (litres/s) Subtracting the entrained air from the total air consumption leaves that portion which has leaked through the frame and associated piping. Lea = Lta - Loa

(6)

where: Lta = total air consumption Lea = air which has leaked through the frame and piping A 98% mixture of hydrogen and air will leak 3.38 times faster than air because of the difference in molecular make-up. therefore: Leh = 3.38 Lea

where:

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(7)


GEK 45941c

Leh = hydrogen which has leaked through the frame and piping Total equivalent hydrogen consumption is the sum of entrained and leaked gas from equations (5) and (7): Lth = Loh + Leh

(8)

Pressure decay tests performed with the unit on turning gear or at standstill will yield values of consumption less than can be expected during normal operation. This is because the seal oil flows are greater when the unit is at speed. Gas lost through entrainment will increase in proportion to gas side seal oil flow. Equations (2) or (8) give total gas consumption at the test pressure. To determine gas consumption at other than the test pressure the following correction can be applied to the leakage portion: (9)

where: Pd = desired pressure, (psig), [kPa (gauge)], [kg/cm 2 (gauge)] Pt = average test pressure, (psig), [kPa (gauge)], [kg/cm 2 (gauge)] p = atmospheric pressure, (14.7 psi), (101 kPa), (1.03 kg/cm 2) Le = leakage at desired pressure L’e = leakage at test pressure Pressure correction for the entrained portion of consumption is made by substituting the desired pressure in equation (3) or (4). IV. INCREASING THE HYDROGEN PURITY

During operation of a hydrogen-cooled generator it is sometimes necessary to increase the purity of the hydrogen in the generator. This is done by admitting fresh hydrogen and discharging an equal amount of gas to atmosphere. Assuming perfect diffusion of the entering hydrogen with the gas in the generator, the amount of fresh hydrogen of purity S 0% required to increase the hydrogen purity from S 1% to S2% is (10)

where: Qh = amount of fresh hydrogen of purity S 0% added at atmospheric pressure, (ft 3), (m3) P = absolute pressure in generator (atmospheres) i.e., 14.7 psia = 1 atmosphere, 29.4 psia = 2 atmospheres, etc. S1 = initial generator purity, (%)

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GEK 45941c


S2 = final casing purity, (%) V = gas volume of the generator, (ft 3), (m3) S0 = purity of supply gas, (%) Qh /PV= machine volumes of gas, gas at same pressure as in generator

NOTE Qh and V must be chosen so that they are in the same consistent set of units, i.e., either (ft3) or (m3). In Figure 1, values of Qh /PV have been calculated from equation (10) for different values of desired final purity with the machine in operation and assuming that S 0 = 99.6%. From these curves it is possible to determine the amount of fresh hydrogen required to produce a desired increase in generator hydrogen purity.

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GEK 45941c

Figure 1. Quantity of Hydrogen Required to Increase Gas Purity from Equation (10).

With the generator at standstill or on turning gear, the amount of fresh hydrogen required to produce a given increase in hydrogen purity would be less, by a factor of K, than the amount given by equation (10). See equation (11) in Figure 1. Values of K are found in Figure 1. V. HYDROGEN PURITY WHEN EMERGENCY SEAL OIL SYSTEM IS IN OPERATION (FOR VACUUM — TREATED SEAL OIL SYSTEMS ONLY)

During operation of the emergency seal oil system, the shaft seals are supplied with oil that has not been vacuum-treated. Air from the oil will be released into the generator casing. This will gradually reduce the hydrogen purity. The generator must be scavenged periodically with fresh hydrogen in order to keep the hydrogen purity at a satisfactory value. Scavenging will be necessary any time the vacuum pump is not operating properly. Figure 2 shows the estimated drop in hydrogen purity for a typical generator when the seals are supplied with untreated oil. This graph shows that the hydrogen purity will decrease at the rate of about 2% in 8 hours. 7

GEK 45941c


Figure 2. Decrease in Machine Purity with Time; Seals Supplied with Untreated Oil

To overcome this drop in purity, the casing must be scavenged at regular intervals. This is done by admitting fresh hydrogen to the generator and discharging gas from the generator to the atmosphere. The rate of which hydrogen must be added to the generator may be determined from the estimated rate of air entering the generator with the sealing oil, assuming 10% adsorption: (12)

where: Qa = rate of air entering generator @ STP (ft 3 /h) Q = oil flow to the gas side of seals, (gpm) In metric units, equation (12) becomes: qa = 0.36 Q where: qa = rate of air entering generator at STP, (litres/s) Q = oil flow to seals, (litres/s) The amount of hydrogen needed during the emergency period is:

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GEK 45941c

where: qa = rate of air entering generator @ STP, (ft 3 /h), or (litres/s) qh = rate of hydrogen required to maintain purity, (ft 3 /h), (litres/s) S0 = purity of fresh hydrogen supplied, (%) S2 = desired final purity in the casing, (%)

NOTE qh and qa must be chosen in the same consistent set of units, i.e., either in (ft 3) or (m3).

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GEK 45941c


GE Power Systems General Electric Company One River Road, Schenectady, NY 12345 518  385 • 2211 TX: 145354

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HYDROGEN LEAKAGE AND PURITY GEK 45941

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