Despite air purification by Molecular Sieve, some amount
of water vapor and Carbon Dioxide will get past the Molecular
Sieve Driers and enter the Cold Box. In due course, any
Carbon Dioxide or water vapor that gets past through Molecular
Sieve Driers will be deposited as solid Carbon Dioxide (Dry
Ice - Sublimation temperature of - SO Deg. C.) or Ice (water
ice - freezing temperature a Deg.C.) within the tubes of
Heat Exchangers, inside the Valves, Expansion Valve, LO.
Pump Filter and inside the holes in sieve trays. These solid
deposits will restrict the flow of air and will be evidenced
by gradual.
increasing difference between Air Compressor discharge pressure
PC-4 and Air Pressure before V3, P-1. In case of excess
Carbon Dioxide the LO. Pump Suction Filter will get chocked.
Other symptoms of frosting are fluctuations of Pressure
and an increasing difficulty in maintaining the required
purity and rate of production. Ultimately the pipes become
so restricted that even when the compressor is working at
its rated pressure and flow, the amount of air that can
enter the plant is not sufficient to maintain production
and purity. When the above occurs, the plant must be defrosted
which is the process of melting out all of these accumulated
deposits.
Complete Defrost of Nitrogen gas
plants oxygen gas plants acetylene gas plants:
1: Initially drain all the liquid from
the Plant through R21, R22, R26, R27, R2S, R29 and R31.
2: Check that ,one of the molecular sieves
dryer is completely reactivated. This reactivated dryer,
'A' or 'B' should be kept as standby for final cooling and
production of Plant after defrost. The other spent drier
is to be used for complete defrosting.
3: Air circulation for defrosting should
be started at least 3 - 4 hours after liquid drain in order
to avoid sudden temperature change causing thermal shock.
4: Valves stem of expansion valves (V3,
V4, and V5) are taken out.
5: By pass valve (V1 )is dosed.
6: In Expansion Engine, lift the inlet
and outlet valve push rods by a lever. Place a metallic
piece of about 4mm. thickness in between Pistol plunger
and push rod. AT X see drawing No. UB/EXP/101
7: Close air inlet valve to the column
(V1/1).
8: Open all drain and analysis cocks (V6,
V7 and R21, R22, R26, R27, R 28, R29, R31)
9: Open valves (VII/1) of air supply to
defrosting heater (now off) and open valves 01 and 02.
10: Start the Air Compressor after ensuring
cooling water circulation as per Air Compressor manual and
adjust air pressure to about 20 to 30 Kgs. Icm2.
11: Air is made to circulate for about
for about one hour or as long as frosting on the expansion
valves (V3, V4, V5) disappears.
12: The water level in defrosting heater
is checked and if necessary, water is added up to the required
level and the defrosting heater is put on. Temperature should
never be allowed to reach more than 700C.
13: At this point, the warm air should
come out smoothly through all the drain and sampling ports.
The process continues till the outlet air from all the points
are hand warm, at no point of time the pressure at the (HP),
(MP) and (LP) gauge should cross 15 kg/cm2, 3 kg/cm2, 0.3
kg/cm2 respectively.
14: Bypass valves (VI) is also now opened
and cleaned.
15: When the air coming out the entire
opening is clean and dry, the heater is put off and the
valve (VII/1) is closed and (VI/1) is opened and flanges
(04) and (03) are loosened for cleaning the HP circuit.
The process is continued till the air coming
out of (V6, V7, 04, and 03) is clean and dry.
16: At this point it is advisable to defrost
the HP circuit by opening valves (VII/1) AND (03) and loosen
connection at (04) and open valves (V6 AND V7) and operate
for the half and hour in this mode.
17: At this point the defrosting is considered
over. Pressure lock test of the column is cond4cted to check
that there is no leakage and the distance piece between
the pastel plunger and the push rod of the expansion engine
is removed. The column is now ready to start.
