hydrogen leakage and purity gek 45941

GEK 45941cRevised 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 forevery possible contingency to be met in connection with installation, operation or maintenance. Shouldfurther information be desired or should particular problems arise which are not covered sufficiently forthe purchaser’s purposes the matter should be referred to the GE Company.

© 2002 GENERAL ELECTRIC COMPANY

GEK 45941c Hydrogen Leakage and Purity Formulas

I. GENERAL

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

II. GAS CONSUMPTION

Gas is depleted from the generator by both leakage from pressure-containing parts, i.e., frame and endshield joints, weld joints and piping, and by entrainment into the seal oil.

Total gas consumption (Lt) when the unit is in operation or on turning gear is

Lt = Le + Lo (1)

where:

Le = gas leakage from generator frameLo = 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 gaspressure (M) and temperature (T), in units of ft3/day (m3/day), @ STP, i.e. expressed as if at65�F (18.3�C), and at atmospheric pressure, (29.92 in. Hg), (101 kPa), (1.03 kg/cm2)

H = duration of test, (hr)

V = gas volume of the generator system, (ft3) or (m3)

B1 and B2 = initial and final barometric pressures for the test, (in. Hg), (kPa), (kg/cm2)

M1 and M2 = initial and final generator test pressures, measured with respect to atmosphericpressure, (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

2

Hydrogen Leakage and Purity Formulas GEK 45941c

NOTE

The following notes will aid when using the different systems of units mentionedabove for equation (2). Funny symbols show up instead of quotes below.

Volume - The “volume units” used for the generator volume (V) set the “volumeunits” per day that the generator leakage (Lt) will come out in. For example, Vexpressed in (ft3), (m3) respectively, results in an Lt in (ft3/day), (m3/day), etc.

Pressure - The constant of 234 in equation (2) applies only when pressures areexpressed in (in. Hg). This constant becomes 69.0 when pressures are in (kPa),and 6766 for pressures in (kg/cm2).

The pressure decay test can be done in air or hydrogen, while the unit is operating on turning gear or atstandstill. It is important that the measurements of pressure and temperature be made as accurately as pos-sible. 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-hourtest. This will generate several consumption rates which can be averaged. This procedure will help elim-inate errors due to measurement tolerances.

Equivalent hydrogen leakage can be determined from the air pressure decay results. Hydrogen will leakthrough joints or holes at a much higher rate than air but will be lost through entrainment in the seal oilonly 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 lessthan 5 atmospheres. Thus, the loss of air from the generator casing (Loa) by entrainment into the seal oilfor equilibrium conditions, is determined from the following equation:

��

(3)

where:

Loa = air entrained in seal oil, (ft3/day)

P = Absolute gas pressure in generator, expressed in atmospheres, where 1 atmosphere = 14.7psia (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 accountthe 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)

3


P = generator gas pressure, expressed in atmospheres, where 1 atmosphere = (101 kPa), (1.03 kg/cm2)

Q = air flow, (litres/s)

The amount of hydrogen that would be entrained if the test were run in hydrogen, considering that thevolume solubility is 7%, is given by

��

(4)

and

Loh = 1.4 Loa (5)

where:

Loh = hydrogen entrained in seal oil, (ft3/day)

Sf = 1

which, in metric units, becomes:

Loh = 6.04 PQ

Loh = hydrogen entrained, (m3/day)

P = generator gas pressure, expressed in atmospheres, where 1 atmosphere = (101 kPa), (1.03 kg/cm2)

Q = oil flow, (litres/s)

Subtracting the entrained air from the total air consumption leaves that portion which has leaked throughthe 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 molecularmake-up.

therefore:

Leh = 3.38 Lea (7)

where:

4


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 consumptionless than can be expected during normal operation. This is because the seal oil flows are greater when theunit 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 otherthan the test pressure the following correction can be applied to the leakage portion:

� �

�

�

�

�

(9)

where:

Pd = desired pressure, (psig), [kPa (gauge)], [kg/cm2 (gauge)]

Pt = average test pressure, (psig), [kPa (gauge)], [kg/cm2 (gauge)]

p = atmospheric pressure, (14.7 psi), (101 kPa), (1.03 kg/cm2)

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 pressurein 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 thehydrogen in the generator. This is done by admitting fresh hydrogen and discharging an equal amount ofgas to atmosphere. Assuming perfect diffusion of the entering hydrogen with the gas in the generator, theamount of fresh hydrogen of purity S0% required to increase the hydrogen purity from S1% to S2% is

� ��

� �

� �

(10)

where:

Qh = amount of fresh hydrogen of purity S0% added at atmospheric pressure, (ft3), (m3)

P = absolute pressure in generator (atmospheres) i.e., 14.7 psia = 1 atmosphere, 29.4 psia = 2 atmospheres,etc.

S1 = initial generator purity, (%)

5


S2 = final casing purity, (%)

V = gas volume of the generator, (ft3), (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 finalpurity with the machine in operation and assuming that S0 = 99.6%. From these curves it is possible to de-termine the amount of fresh hydrogen required to produce a desired increase in generator hydrogen purity.

6


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 givenincrease in hydrogen purity would be less, by a factor of K, than the amount given by equation (10). Seeequation (11) in Figure 1. Values of K are found in Figure 1.

V. HYDROGEN PURITY WHEN EMERGENCY SEAL OIL SYSTEM IS IN OPERATION (FORVACUUM — TREATED SEAL OIL SYSTEMS ONLY)

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

7


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 admittingfresh 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 ofair entering the generator with the sealing oil, assuming 10% adsorption:

�

(12)

where:

Qa = rate of air entering generator @ STP (ft3/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:

8


�

� �

� �

where:

qa = rate of air entering generator @ STP, (ft3/h), or (litres/s)

qh = rate of hydrogen required to maintain purity, (ft3/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 (ft3) or(m3).

9


GE Power Systems

General Electric CompanyOne River Road, Schenectady, NY 12345518 • 385 • 2211 TX: 145354

10

hydrogen leakage and purity gek 45941

Documents