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| Sulphuric Acid on the WebTM | Technology Manual | DKL Engineering, Inc. |
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Knowledge for
the Sulphuric Acid Industry Introduction
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Introduction
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.
An indication of the cleanliness is obtained by impact specimens,
installed in the blowoff piping during the final blows. Suitably mounted and
supported, polished square bar stock has been effectively used for this purpose. The
degree of pitting of the specimen surface following a blow is used to determine the end
point of the blowing cycle. This procedure is general in nature and as such should be adapted to the specific conditions and procedures existing at the site at the time the operation is performed. Safety The process or steam blowing imposes abnormal and severe conditions
upon the boiler, steam equipment and steam piping. Large, rapid temperature changes
occur during each blowing cycle. This cycling of temperature is far more severe than
is incurred in normal operation. Thermal stresses may be excessive in the heavy wall
portions of the system such as steam drums, headers and piping. It is prudent to
consider this fact when performing the blowing procedure and to limit the number of blows
to the minimum consistent with cleaning the system. Since the temporary steam blowing piping may be designed for lower
pressure than the boiler, care must be used to prevent over pressuring this piping during
the entire steam line blowing operation. The pressure in the temporary piping should
be monitored continuously and operators be alert to prevent over pressure. It is recommended that a means of over pressure protection be provided
in the temporary piping, such as, safety valves set at the design pressure of the
temporary piping. Noise
levels will be very high during the steam blow. Hearing protection is mandatory for
those in the immediate vicinity of the steam outlet. An area at least 100 m from the
blow point must be roped off to prevent personnel from approaching too close. Equipment and Supplies The following equipment will be required:
Procedure The unit is started in the normal manner. All normal
recommendations and limitations with respect to fuel firing equipment, air handling
equipment, drains and vents, etc., should be followed, as if the unit were being started
for normal operation. As for any new unit, the steam blowing operation is the first
occasion that the unit is fired at any significant rate. Consequently, the startup
as well as the steam line blowing must be conducted with great care. The normal
startup rate does not apply; the unit must be brought up much slower, while all equipment
is checked and expansion movements monitored closely. If the unit is equipped with a
fired superheater, the 1000°F (538°C) furnace exit gas temperature limitation must not
be exceeded when the unit is fired. The same general precautions taken on any new
unit for this period of operation apply equally here. The drum level will be subject to extreme fluctuations during the
blows. As the temporary blowoff valve is opened, the drum level will rise rapidly
and may go out of sight in the gauge glass. As the blow progresses the drum water
level will re-appear and may drop out of sight. Therefore, it is important that the
drum level is established at or slightly above normal operating level before the start of
each blow. A small amount of feedwater flow should be established before the start
of each blow and the feedwater flow should be increased as soon as the water level drops
back in sight, in order to prevent excessive low water level. When the drum pressure reaches the value calculated to produce the
desired blowing flow quantity, the blowing procedure can be started. It is difficult to avoid carryover from the steam drum to the
superheater during the steam line blowing operation. Therefore, boiler water should
not be treated with non-volatile chemicals during this process, to avoid deposits of solid
materials into the superheater. Firing
of all fuels must be discontinued during all blows. 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, thermowells, 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.
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.
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.
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.
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:
Criteria
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Copyright© 2005,
2006, 2007, 2008 DKL
Engineering, Inc., All Rights Reserved |
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. superheater) 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.