Generator Air tightness test

R

Thread Starter

rtr

What is the pressure drop calculation for the loss in air pressure during Generator air tightness test and what is the allowable drop?
 
D
Refer to the OEM manual for your specific generator.

> What is the pressure drop calculation for the loss in air pressure during
> Generator air tightness test and what is the allowable drop?

The basic formula is:

L=238(V/H){[(P1+B1)/(273+T1)]-(P2+B2)/(273+T2)]}

Lh=3.38La

Where:

L = gas leakage
Lh = equivalent hydrogen leakage
La = gas leakage using air in the generator
H = duration of test in hours
V = volume of gas system in cubic feet
B1 & B2 = initial and final barametric pressure (inches of mercury)
P1 & P2 = initial and final generator gas pressure (inches of mercury)
T1 & T2 = initial and final generator gas temperature (degrees C)

The generator OEM manuals normally have the equations and further explanations.

What is allowable varies a bit. A reasonable value for a tight system with a system volume of 2500 cu ft is 350 cfd. However, higher leakages are acceptable under certain conditions. For example, a test at the end of an outage conducted as a very rough check with the unit not yet on turning gear will be substantially higher than the same unit once it is on gear with the seal oil system warm. But it can give a quick indication of gross problems when compared to similar data for similar units or past data for the same unit. This is one reason that a quick air test is often done on removing a unit from service. Obviously it is desirable to return a unit from an outage in better condition than when it was taken off for maintenance.
 
I

I Nyoman Bagus Yudha Dharma

Dear sir,

your posting here is very help me so much, but i want to ask you for another question related with your formula above. I still confuse until now.

1) what is the different between "Barametric Pressure (B)" and "Gas Pressure (M)", How we can collect the data for B value ?

2) does this formula only can be used on GE generator ?

I'm very appreciate if you reply my question here..

thanks for advance
 
S

soumitraban30

Hi,

Thanks, I followed the calculation and got a resultant as La=1154 Cu ft, is this per hour or per day? as we are dividing by 15, it may be per hour but the resultant looks too big.

Why to multiply by 3.38 for Lh from La?

Please guide.

Regards,
S Banerjee
 
Refer to the OEM manual for your specific generator.

> What is the pressure drop calculation for the loss in air pressure during
> Generator air tightness test and what is the allowable drop?

The basic formula is:

L=238(V/H){[(P1+B1)/(273+T1)]-(P2+B2)/(273+T2)]}

Lh=3.38La

Where:

L = gas leakage
Lh = equivalent hydrogen leakage
La = gas leakage using air in the generator
H = duration of test in hours
V = volume of gas system in cubic feet
B1 & B2 = initial and final barametric pressure (inches of mercury)
P1 & P2 = initial and final generator gas pressure (inches of mercury)
T1 & T2 = initial and final generator gas temperature (degrees C)

The generator OEM manuals normally have the equations and further explanations.

What is allowable varies a bit. A reasonable value for a tight system with a system volume of 2500 cu ft is 350 cfd. However, higher leakages are acceptable under certain conditions. For example, a test at the end of an outage conducted as a very rough check with the unit not yet on turning gear will be substantially higher than the same unit once it is on gear with the seal oil system warm. But it can give a quick indication of gross problems when compared to similar data for similar units or past data for the same unit. This is one reason that a quick air test is often done on removing a unit from service. Obviously it is desirable to return a unit from an outage in better condition than when it was taken off for maintenance.
 
Hello sir,

at my plant we used to run the test for half day, usually during night shift, so that in the morning we have the result and can either work on finding the leakage or start filling the gen with hydrogen.

don't forget to isolate scavenging and h2 analysers prior to the test. (Those are continuous flow to the vent!)

Also we find to have better result without cooling water running (if possible) because cooling water temp will cause the air in the gen to expand or contract.
Even if the formula is made to compensate this effect, it's better if you don't experience it.

i'll check the omm to find the official instruction
 
Hello sir,

at my plant we used to run the test for half day, usually during night shift, so that in the morning we have the result and can either work on finding the leakage or start filling the gen with hydrogen.

don't forget to isolate scavenging and h2 analysers prior to the test. (Those are continuous flow to the vent!)

Also we find to have better result without cooling water running (if possible) because cooling water temp will cause the air in the gen to expand or contract.
Even if the formula is made to compensate this effect, it's better if you don't experience it.

i'll check the omm to find the official instruction
thank you for your reply! We are currently struggling to hold pressure on CO2, leaking about 1 psi per hour. The formula above is meant to relate air to hydrogen. Do you know if there is a relationship out there for co2 and hydrogen? I want to swap to hydrogen but I expect the rate to be way too high since the leak rate on co2 is so drastic. I have an older westinghouse 110MW generator
 
Hi,

I don't know the formula for CO2,

official duration is 24 hour

for safety reason, the gen frame must not leak at all:
Create explosive atmosphere
And hydrogen fire is barely not visible by human eyes

you can check the frame with gas detector, since it's fill with CO2 and also with soapy water to detect leakage

Faulty manhole seal, or misplaced door can be the cause

Also check if gas is passing through stator cooling water, or cooling water.
Usualy stator cooling water pressure is above gas pressure, to ensure no leakage into the cooling circuit

but cooling water is under h2 pressure, that can be harder to detect

otherwise if the leakage is due to passing valve to vent, it's up to you to determine if the h2 consumption cost worth the reparation cost or not
 
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