Steam Blowing of Boiler

1. BLOWING OUT STEAM LINES
INTRODUCTION
The purpose of blowing the steam lines prior to starting up a new unit is to remove any foreign
matter remaining in the equipment (i.e. super heater) and steam piping after erection is
completed. Considerable damage could result if such foreign matter was allowed to enter the
plant steam system during initial operation.
On older units the need for steam line blowing should be considered following major pressure
parts repairs, where the possibility of introduction of foreign material into the system exists.
Since prevention of damage to the plant steam system is the prime concern, the responsibility
for determining the effectiveness of the steam line blowing operation rests with the plant
operator. During the steam line blowing process the unit should be operated in accordance
with the vendor recommended procedures, with all control systems and protective interlocks
functioning.
The principle behind steam blowing lines clean is that the thermal cycling (heating/cooling)
and high velocity gas flowing through the line will "shock" the pipe and tend to break the mill
scale and weld slag away from the pipe wall. Ideally, steam blowing should be done prior to
the system being insulated to maximum the thermal cycling of the equipment. If the system is
insulated, a greater time between blows may be necessary to allow the system to cool.
Ideally, to obtain optimum cleaning, the flow conditions in the system during steam line
blowing should equal those during normal operation at maximum load. Since it is impossible
to exactly duplicate these conditions when blowing through the piping to atmosphere, it is
desirable to produce equivalent conditions by using lower pressure steam with a flow rate
such that the product of steam flow times velocity will equal that under normal full load
conditions. The determination of the total obtainable flow quantity must be based on flow
resistance in the entire system, including the temporary piping. This determination is normally
made by the designer of the blowing system.
The temporary piping is normally equipped with a shutoff valve that is used as a blowing
valve. With this arrangement, the superheater and the upstream portion of the temporary
piping are maintained at drum pressure at all times when not actually blowing.
When a boiler and its associated steam piping have been erected, there is a possibility that
debris, such as welding beads and dirt may have been left in the tubes and piping. In addition,
these parts may have an internal coating of oxide or mill scale, which will flake off during initial
temperature changes.

2. To avoid any hazard of such material causing obstruction or damage to machinery, it is
recommended that arrangements are made to blow the piping systems to atmosphere before
the final connections are made and steam is passed to the plant. This blowing through is
done with steam which is exhausted through adequate sized, open ended low pressure
temporary piping. The steam blowing operation cleans the system and provided the steam of
requisite purity for the plant.
This low pressure temporary piping must be connected as near as possible to the terminal
point of the section of the system being purged. In some cases an appropriate valve cover
may be removed and the temporary piping led off the valve chest. The run should be as short
as possible consistent with steam discharging freely to atmosphere with out causing danger
to personnel or damage to structure and equipment.
If a basket type strainer is installed in the steam piping it must be removed prior to steam
blowing and subsequently replaced.
The precautions listed below must be strictly observed.
REQUIREMENT OF TEMPORARY STEAM LINE FOR BLOWING
The diameter of temporary discharge piping shall not be less than the diameter of the
respective line to be steam blown. The spool pieces should have an opening equal to the
diameter of the respective temporary pipeline. The temporary discharge piping and supports
shall be capable to withstand steam blowing parameters.
PROCEDURE FOR BLOWING
There are two alternative procedures for blowing out the steam lines to the plant.
One is to raise pressure in the boiler to just below the permissible pressure in the temporary
atmospheric discharge piping, gradually open the boiler stop valve whilst increasing the firing
rate to maintain pressure during the blow, then gradually close the stop valve whilst reducing
the firing rate.
This procedure has the advantage that the blowing period may be as long as is necessary to
clean the steam piping thoroughly.
Usually the waste steam discharged to atmosphere will be dark in colour when blowing starts,
and will become clear as foreign matter is removed from the system. Several successive
bows should be made with an adequate cooling period between each, as the associated
temperature changes will assist in loosening mill scale.
The alternative procedure is the puffing method. The boiler is raised to around 40kg/cm2g
pressure or just below the permissible pressure in the temporary atmospheric discharge
piping. Shut off the firing.
Ensure that the drum water level is at the lowest level.

3. Open the boiler stop valve quickly; water level in the drum swings upward. The level may
even go beyond visible limit. Permit the boiler pressure to drop about 10.5 to 14kg/cm2g. The
change in steam saturation temperature is kept less than 42deg.C. Then quickly close the
stop valve.
This method has the advantage that the sudden change in steam pressure produces a
sudden change in saturated steam temperature causing a thermal shock which helps to
loosen the adhered scale from the inside of tubes and piping. The steam expands quickly
through the system as the pressure drops. The scales are than removed by the expanding
steam. How ever, all the pressure parts, including the drum, are subject to thermal shock, and
to prevent excessive stresses and differential expansions, the change in saturation
temperature should not exceed 42deg.C.
To ensure thorough cleaning, a number of successive blows must be made due to the limited
blowing period permitted by the allowable pressure reduction.
For both procedures, experienced observation can determine when sufficient blowing through
has been effected. Clean metal target plates are rigidly secured in the path of the discharging
steam at a suitable distance from the pipe end.
These are placed in position for evaluation of debris carry over along with the steam after the
initial few blows when the steam is visibly clean. After each blow the target plates are
inspected for any pitting by the debris.
Target plate assembly is located in the temporary exhaust pipe and placed nearest to the
permanent pipe work. Availability of sufficient number of target plates is to be ensured. Stain
less steel plates having mirror finish shall be used in final blows. In preceding blows, target
plates of aluminium shall be used. Further blows are given until they indicate that no debris
remains in the piping. It is a standard practise to limit the number of blows to 6 to 8 per day at
an interval of 2 hours between 2 consecutive blows and with an overnight cooling.
A complete record of the events and observations are to be maintained.
STEAM BLOWING COMPLETION CRITERIA
Steam blowing can be declared complete only after ensuring cleanliness of target plates
mounted in the temporary exhaust pipe. As already mentioned, for evaluating the cleanliness
and termination point of the steam blowing, aluminium target plates are to be used initially.
For the final stages, target plates of stain less steel having mirror finish are to be used.
The highest velocity of steam being at the centre of pipe, the effectiveness of steam blowing
is judged by the absolute pitting on the target plate in the central zone i.e the area covered by
three quarters of the pipe diameter.

4. The piping is considered clean if there are not more than five pitting on the target plate in the
central zone and the edges are not deformed. Besides, there should be no pitting in the rim
zone i.e the area other than the central zone. This should be achieved in three consecutive
plates.
PRECAUTIONS
To prevent excessive strain on pressure parts and danger to personnel, the following
precautions must be strictly observed.
- The securing arrangements of permanent piping must be complete, or adequate
temporary bracing installed. For thermal expansion of pipes, proper allowance must
be given. First test steam blow should be given at lower pressure and the temporary
piping should be surveyed for proper expansion and supports.
- The temporary low pressure piping exhausting to atmosphere will be subject to strong
reaction forces during the blow and must be adequately secured. The exhaust can be
diverted at an angle of 30deg. from horizontal to reduce reaction force on temporary
piping.
- During the blowing period steam and debris will be issuing from the discharge pipe at
high velocity and the location and direction of this must be carefully considered in
terms of safety. The steam must be exhausted to the atmosphere outside the boiler
house and so directed that personnel and equipment are not damaged or
endangered. The area should be cordoned off and warning/safety notices displayed
prominently.
- Since it is necessary to have a high steam velocity through the system being cleaned
during the blowing period, the temporary piping exhausting to atmosphere must be of
generous size so that it will not significantly restrict the flow.
- The boiler pressure used for blowing out the lines must not exceed the design
pressure of the low pressure temporary piping and fittings used during the blowing
period. The temporary piping should have minimum 1in 100 slopes towards exhaust
for removal of condensate and should be well supported. This should have proper
provision for thermal expansion.
- During pressure raising and blowing, all NRV internals, control valves and flow
nozzles, orifice plates if any in the blow circuits are to be removed and spool pieces
to be provided.
- An adequate reserve of feed /DM water must be kept readily available to replace the
blowing losses.
- Water level in the boiler must be carefully watched and kept under proper control at
all times, particularly as feed heaters may not be in commission. Special care must
be taken to prevent priming and carry over. If the water level drops below the bottom
of the gauge glass, there is a possibility of quenching the hot drum with cold feed
water when regaining level, and causing excessive thermal stresses. The
temperature of the water entering the steam drum should be monitored at all times. It
is recommended that the differential between water leaving economiser and drum
shell temperature shall not excede20deg.C.
- All the permanent pipelines and temporary pipelines passing through working area
should be insulated before the blowing.
- All permanent hangers and supports for various lines are to be erected and set for
proper values as per design figures. All locks and restraints are to be removed from
pipelines,
- Provision should be made for tightening of flanges of holding arrangements for target
plates. Spare gaskets for flanges are to be kept ready.

5. - In addition to usual light up instruments, provision is to be made for monitoring /
recording pressure and temperature at outlet of boiler and before temporary exhaust.
- The area around the exhaust point should be cordoned off. For warning sound, prior
to blow, siren or PA system can be utilised.
- Effective communication and lighting system should be ensured at the vulnerable
areas.
- Boiler should be ready in all respects including
a. Interlocks, protections, alarms, annunciation and instruments
b. Chemical dosing system
c. Insulation of furnace and flue gas duct
d. Fire fighting system as per designed scheme
e. Fuel oil firing equipment
- Chemical cleaning of the pressure parts internals should be complete prior to steam
blowing.
- All drain connecting piping are to be left open to atmosphere and are not to be
connected permanently to drain vessels to ensure proper and quick draining. After
checking drain pots for choking, the drain lines are to be connected permanently to
vessels after steam blowing.
General Pre-Cleaning Procedures
1. Check all pipe anchors and guides for adequate support of piping, including the
temporary vent pipe.
2. Insure that all lines have been inspected and hydrotested.
3. Remove all control valves, desuperheater nozzles, flow elements, thermo wells,
pressure gauges, orifice plates, safety valves and instruments that may be damaged
during the steam blow.
4. Insure all vent pipes have adequate drains and that these drains are routed to a safe
location.
5. Install temporary pressure gauges as required to monitor the progress of the steam
blow and make arrangements for to record pressure readings.
6. Establish adequate radio communication between the control room and the operator at
the shutoff valve. Back this up with a system of visual communication, such as lights or
flags, since audio communication may be difficult due to high noise levels at the steam
shutoff valve.
7. Insure personnel traffic is controlled in area during steam blow. Insure the area near the
vent pipe is clear of all personnel during the steam blow. Protect neighbouring
equipment from damage due to the steam vents.
8. Individuals operating the steam blow valves (especially near the vent pipe) should wear
double ear protection.

6. General Cleaning Procedure
1. Discontinue all firing and gradually open the temporary blowoff valve, to blow through
the steam system and out the temporary blowoff piping.
Caution: The first blow should always be done at reduced pressure, in order to check out the
temporary piping system, its supports and anchors.
2. When the drum pressure has dropped to the value corresponding with a 100°F (56°C)
saturated steam temperature decrease, close the temporary blowoff valve and refire the
unit to re-establish blowing conditions.
3. Repeat the above cycle until it is considered that cleaning is satisfactory as indicated by
inspection of impact specimens during the final blows.
Post Cleaning Procedures
1. Drain the temporary and permanent piping.
2. Remove temporary spools, blind flanges, temporary vent piping, etc., and reinstall all
components removed prior to steam blow.
Cleanliness Criteria
The turbo-generator vendor or the primary end user of the steam will generally specify the
criteria to which the system should be cleaned. If steam is used in a turbo-generator the
following criteria is generally used:
 Two successive targets must result in mirror finishes that meet the specified criteria.
 After a cooling down period of at least 12 hours, two more successive steam blows
must result in mirror surfaces that meet the specified criteria.
Criteria
 No indentation exceeding 0.8 mm diameter
 Indentation exceeding 0.4 mm diameter to number not more than 2 per 2500 mm² of
the target plate surface
 Indentations exceeding 0.2 mm to number not more than 10 per 2500 mm² of the
target plate surface
 Indentations less than 0.2 mm to be well dispersed and nowhere present in
concentration
S V NAGESH