Steam-Water-Oxygen Cleaning and Passivation Print version

Technology of the method

Implementation of SWOCP technology, developed by SPE Energetika i Nauka, is a comprehensive and consistent process which is maintained in several stages, beginning from the boiler manufacturing, installation, or repair—before putting it into operation: Stage-0, Stage-1, Stage-2.

Stage-0

Fig. 5. Mechanical cleaning of the pipes by a metal brush

Organization of “clean” manufacturing, storage, installation and repair of the boiler and steam-water circuit of the power unit. The purpose of Stage-0 is a significant reduction of the initial contamination at the beginning of treatment which includes the following process: production → transportation → storage → assembly → repair and putting into operation.

This stage is implemented in co-operation with the manufacturers at the stage of designing and manufacturing of the boiler and also during its transportation, storage at the customer’s site and includes the following work:

development and inclusion of the necessary elements of the SWOCP scheme into the project;
control of manufacturing of the boiler heating surfaces and collectors using pipes made of pure metal without dross and parking corrosion—the initial state of the inner surfaces is controlled at the boiler factory;
control of an effective drying by blowing heated air through the blocks of boiler heating surfaces that went through hydraulic tests at the factory;
exclusion of moisture penetration of the inner surfaces during transportation and storage of blocks and tubes of the boiler at the customer’s site;
organization and performance of the installation works that ensure the prevention of getting foreign objects into the equipment, mechanical cleaning with pre-assembly of the boiler — “clean installation”;
cleaning by mechanical means with the obligatory blowing of individual blocks of the boiler tubes with compressed air using a special receiver with the volume of 6—8 m³;
obligatory testing for patency of tubes of all the heating surfaces by passing the metal ball through them at the factory as well as at pre-assembly of the boiler blocks;
implementation of multiple air purging of individual circuits in the final phase of Stage-0, using the disruptive diaphragms;
after the installation of the boiler filling it with water for the hydraulic tests with an obligatory preceding wet-oxidative treatment by low-potential side steam no earlier than 1—1,5 month before the first firing for SWOCP.

Stage-1

Includes a wet-steam cleaning by side steam (30—100 t/h, 1.0—1.3 MPa) from the launch boilers with subsequent high-speed water flushing of the feed circuit together with HPH, injection unit of the boiler, injection pipes in FPRDS and PRDS, water economizer and furnace waterwall screens. Washing is performed by CPW using standard condensate electric pumps and drain pumps of LPH.

Fig. 6. Cleaning of water economizer chambers

The purpose of Stage-1 is to remove large foreign particles (burr, foreign objects, scale, sand) and water-soluble deposits from all the above-mentioned circuits and to carry out degreasing. In addition to that, at least 20—30% of iron-oxide deposits are removed from the pipe inner surfaces and the remaining deposits are considerably softened.

The scheme of cleaning before the post-installation blowdown provides special measures of prevention of damage to the sharp edges of the orifice meter. The “filling” of regulators, back-flow valves, and throttling devices is removed. Special blowdown devices (filter-sieves) are installed as well as inserts into FPRDS, PRDS, steam valve, and other armature for the possibility of limited use during blowing. Bearing units of temporary pipelines connection, designed to supply and discharge steam and CPW and to avoid cutting and welding of the main pipelines, are installed.

Requirements to the equipment prior to the stage:

feed circuit with HPH, the steam-water circuit and the main steam pipelines with FPRDS and PRDS should go through a unit test and be in a working state with the possibility of thermal transfer upon heating; rough insulation and brickwork of the boiler and steam pipelines should be done;
a series of procedures reduce the cooling of the boiler through the draft should be performed;
condenser and condensate circuit should be accepted, cleaned and washed, condensate and drain pumps of LPH should be tested; the condenser should be filled by CPW using standard scheme.
activities aimed at prevention of getting steam and water into the turbine and clogging drains of scavenged steam pipelines should be performed;
special temperature inserts in the screen panels should be installed with readings displayed on the temperature recorder of metal tube walls;
standard instruments of temperature control of the drum metal, pressure and level in the drum should be used.

Fig. 7. Cleaning of dead-end sections of water economizer by pressured air

The efficiency of steam blowdown is provided by partitioning of the heating surfaces (water economizer and furnace waterwalls) into individual circuits and the transfer of side scavenging steam in the steam-water mixture with the moisture level of 30—50%. This leads to a 3—5 times increase of mass flow (ρV) for blowing the furnace waterwalls up to ρV ≈ 300—500 kg/(m² × s).

During the water flushing with condensate pumps trough a standard scheme a mass speeds reach the following levels:

for feed pipelines ρV ≈ 3000—5000 kg/(m² × s);
for water economizers ρV ≈ 1500—3000 kg/(m² × s);
for waterwalls ρV ≈ 1500—3000 kg/(m² × s).

At the initial stage of washing, as well in a situation of a strong increase of contamination, the scheme and the technology provide a discharge of polluted CPW from a washing circuit prior to the condenser. This is relevant for the washing of all washing circuits.

Implementation is carried out at the final stage of introducing the boiler, 1,5—2 months prior to the first launch.

The time of Stage-1 implementation is 40—50 hours after installation, and 8—12 hours after repairs. Expected efficiency of pipes cleaning is at least 25—30%.

Upon the completion of flushing and washing the temporary scheme is dismantled, efficiency of the treatment is evaluated, standard scheme is restored and a series of hydraulic tests are maintained. Then goes preparation for the first launch with implementation of SWOCP and steam testing of the boiler.

Stage-2

Conducting a Steam-Water-Oxygen Cleaning and Passivation of the boiler with an effective scavenging of primary and secondary boiler superheaters and main steam pipelines at the first start-up and operation of the boiler at 10—30% capacity.

The purpose of Stage-2 is to implement the final phase of the steam-water circuit cleaning obtaining the required heating surfaces contamination level, achieving the required quality of steam with the polished “mirror” control, and forming resistant protective magnetite film on the entire surface of the treated circuit.

The main elements of the blowing scheme are exhaust pipes. Removable polished plate-mirrors are installed in the temporary pipeliness after disconnecting valves. The purging is continued until the quality of mirrors reaches VGB-R 513 norms. The cleaning of the system is deemed accomplished if the area of 40 × 40 mm mirrors:

is free of dents with the diameter more than 1 mm;
doesn’t contain more than 4 dents with the diameter of 0.5 mm;
doesn’t contain more than 10 dents with the diameter of 0.2—0.5 mm;
has dents without significantly upraised edges.

Fig. 8. Steam exhaust during cleaning

If the reduction of noise in the exhaust pipes is necessary, special nozzles for cooling down (slowing down) the steam are installed.

Prior to firing during the filling of the boiler with water a hot washing of the condensate-feed circuit is maintained (according to a separate program). The washing is carried out through a standard scheme thread-by-thread with a flow rate of 150% (from the nominal) per thread with 120—150 ºC feed water and held until the cessation of blinding of fine filter screens at the entrance to feed electro pump, but not less than 2—3 times.

The start-up of the boiler is carried out with a gradual stepwise increase in fuel consumption (load). Oxygen is dozed into the feed water and furnace waterwalls.

On each level of thermal load (7%, 10%) impulse blowing of steam superheater and direct steam pipelines is carried out.

After 14—16 hours of oxidative blowing at a 15% thermal load with the concentration of iron oxides in steam of less than 300 mg/kg an oxidizing cleaning of furnace waterwalls starts.

During the boiler operation in the condition of implementing SWOCP the following parameters are maintained:

Fig. 9. Blowdown #1. The impact of contamination particles on a polished control plate

primary circuit pressure ≤ 60 kgf/cm²;
secondary circuit pressure ≤ 40 kgf/cm²;
primary and secondary steam temperature is 200—450 ºС;
cold reheat temperature is 150—300 ºС.

Furnace waterwalls cleaning is held during the next 16—18 hours, alternately dozing oxygen into drain risers, with the boiler working at 15% load (excluding time for 3 boiler stops for 10—14 hours to accumulate CPW in the condensate storage tank). Every 3—4 hours of dosing the cycle is opened and within about 2 hours the superheater and steam pipelines are blown at 20% and 25% load. At the same time polished control plates are installed into all drain steam pipes.

The main criteria for purging the primary superheater:

reliable cooling of the secondary superheater;
creation of blowdown regime with the maximum efficiency.

The final (control) blowdown is performed after accumulation of CPW supply in the condenser and the condensate storage tank at 30% load during 4 hours in two cycles. The energy of flow in all parts of superheater and steam pipelines should fit the formula:

Рис 10. Blowdown #7. Polished control plate. The quality of cleaning complies with VGB-R 513 norms

D_b² × V_b ≥ 1.2D_n² × V_n,

where:
D_b — steam consumption during blowdown;
V_b — marginal volume of steam during blowdown;
D_n — nominal steam consumption;
V_n — nominal marginal volume of steam during blowdown.

This condition for the primary superheater and direct steam pipelines can be fulfilled at a pressure in the drum of no more than 25 kgf/cm² at the blowdown regime.

These design parameters can be obtained by short-time opening of the safety valves of direct steam.

In the final part of SWOCP implementation the main steam stop valves are closed, the pressure in the boiler rises to the level of 120—130 kgf/cm², oxygen is dosed into the feed water pipeline prior to the boiler and into the drain risers of furnace waterwalls. A thorough passivation of all the treated steam-water circuits is conducted, reaching certain indicators of water regime. Then the pressure in the boiler increases up to the value of testing and adjustment of direct steam safety valves, valves are adjusted, and the boiler stops.

The time of SWOCP implementation is 3.5—4 hours after installation, and 1—1.5 hours after repairs. Expected efficiency of cleaning is 80—85%. Final pipes contamination is no more than 50 g/m². The quality of steam prior to the turbine complies with VGB–R 513 norms.

Recovery of the scheme and assessment of cleaning quality

After the steam-water-oxygen treatment is performed, its effectiveness is to be assessed. Tubes samples are cut in the same places where the original contamination was determined, and effectiveness of cleaning and corrosion resistance of obtained protective magnetite film is evaluated on each sample.

When restoring the scheme, a special attention should be paid to the cleaning of dead-end sections of the cleaning scheme — burr and mechanical inclusions are inacceptable. The drum of the boiler, deaerator, and all water strainers on filters are thoroughly cleaned.

According to the results of implementation a final conclusion is prepared. Then the boiler starts-up and reaches nominal parameters.

Start-up and subsequent operation of power unit

After SWOCP prior to the boiler start-up washings of feed-economizer circuit, HPH, and furnace waterwalls feed pumps are held (until figures are stabilized, but not less than one hour).

The first launch of the boiler is conducted by a special program. During turbine operation a special attention is paid to purity of steam, the work of the stop valves of high-pressure cylinder and medium-pressure cylinder, as well as pressure difference in the sieves.

During subsequent operation an implementation of SWOCP is recommended using standard thermal scheme without temporary elements.

The main provisions of such post-installation SWOCP technology:

it is implemented at start-up and operation of the boiler at 15—30% load with a pressure in the boiler of 5.0—10.0 MPa; time of the cleaning is 8—12 hours, the required amount of oxygen is 12—15 tons;
oxygen is dozed into the feed water and waterwalls for 6—8 hours with the cleaning and passivation; washing (cleaning) of the steam superheater is conducted together with discharge of steam through FPRDS by short-time opening of direct steam.

Such an approach of treatment of heating surfaces will ensurethe good condition of the protective oxide film on the pipes, and, consequently, will help maintain reliable operation of power equipment metal.

Energetika i Nauka

Scientific-production Enterprise