PART -IV
COMPLETION
CRITERIA
Stages 1a,
2a, 1c, 3a, 3b, 4a,4b
For the
first few blows Target plates need not be kept. Afterwards Target plates made
of Stainless steel of mirror finish
are to be used.
Target
plates are mounted in the temporary exhaust pipes nearer to the permanent line.
Steam
blowing can be declared complete only after ensuring cleanliness of target
plates. Following criteria is followed for declaring completion of steam
blowing:
Stage 1c & 2b
Minimum 8
(eight) numbers of blows are to be given.
Stages 4c to
4i
Steam
blowing is declared complete based on visual observation. Minimum three nos. of
blows shall be given for each sub stage.
No of impact >= 1 mm
Less than 4 impacts >= 0.5mm
less than 10 impacts >= 0.2mm
Total impact above 2mm size not exceeding 10 nos at the end of steam blow.
Impacts below 0.2mm size at the end of steam blow can be accepted provided they are well dispersed and uniform in distribution without creating a rough surface.
Minimum number of consequtive target plate is two.
The above criteria has been fixed by BHEL
for 660 MW Boiler. However there are
difference of opinion regarding completion
criteria. The First difference of opinion arises on metal of target
plate. Earlier days we have seen use of bronze target plate and it is malleable
material therefore slightest impacts are visible. Therefore steam blowing
operation gets continued longer time and disturbance factor calculation was not available with us. For any 200 to
250MW plant steam blowing operation took around 30 to 40 days. After that
target plate changed to Mild steel and now stainless steel. As per CEA
guidelines it is written as steel. Therefore all personnel carrying out steam
blowing should agree with the material.
A Few points
to be noted during steam blowing :
1. Four number
Electrical operated trip valves with quick opening and closing are needed to
conduct steam blowing. To standardize
the requirements 12”/1500 class for 250 MW and below and 14”/1500 class for
500MW and above are selected. Material of valve body selected is WC9.
2. Special precaution to be taken in cleaning of CRH lines .
As it is seen that Reheater tube failures happened due to debris deposited in
reheater tubes. Lines are to be checked thoroughly and some time it may happen
a small portion of pipe not cleaned during steam blowing operation. Therefore
pre cleaning and to maintain a log on that is necessary to avoid complication
at later dates.
3. The steam
blowing operation produces tremendous noise and certainly does noise pollution
to the maximum effect. In earlier days people are not bothered about such
pollution but now all are aware therefore steps to be taken to reuce this noise
pollution . Earlier days only ear caps were provided but now a days silencer
has been designed to reduce the noise.
The Steam Blowing Silencers are used
to reduce the noise produced by the valve (Structure born noise and Air born
noise) , when venting of pressurized and high velocity gas to the atmosphere,
especially , steam, air and nitrogen.
4. The low
point drain will be getting chocked at the time of initial operation. Therefore
the same to be cut and debris to be cleared immediately and again re –weld the
same after ensuring that steam flows through the drain.
5. Tools and other materials should be kept
ready for cleaning of BFP and CEP strainers. As initially there will be
frequent choking of strainer one after another
with minimum
time gap. If you are not prepared you can not keep the boiler running.
6. As we
will running the boiler through oil firing then Air heater soot blower is must.
7. Drum
level should be on manual to take care of swelling during blow off.
Disturbance Factor =
Qb2 x Vb
or
clearing force ratio Q2
MCR x VMCR
Qb - Steam flow during blowing
QMCR
- Steam flow at MCR
Vb - Sp. Volume of steam during blowing
VMCR - Sp. Volume of steam at MCR
Qb = 1.13 x A x sq.rt
of P/V
A = Area of Blow out
pipe at the end ( cm2)
P= Pressure at blow at
blow out piping end.
V= steam specific
volume at blow out piping end.
