Why steaming blowing is done
The steam blowing operation cleans all the debris in the superheater, reheater and the steam pipe line connecting the turbine. During the maximum continuous rating load of the turbine the drag force in the pipe line can dislodge particles loosely adhering to the tubes internal surface, causing great damage to the turbine blades. Guidelines given by various boiler designers and agencies like CEGB, etc., to effectively carry out this operation focus on creating a drag force created on the inside surface of tubes or pipes which is much higher than that occurring during the maximum continuous rating load of the turbine. Ratio of the drag created during steam blowing to the drag occurring during the maximum continuous rating load is called the disturbance factor.
How steam blowing is done
Steam blowing guidelines focus on creating a drag force on the surface of tubes or pipes much higher than that which occurs during the maximum continuous rating load of the turbine. The ratio of the drag created during steam blowing to the drag occurring during the maximum continuous rating load is called the disturbance factor. Boiler designers and commissioning groups prefer to keep a disturbance factor of about 1.6.
The puffing method
The puffing method
As steam blowing is one of the initial stages of start-up operation, extra care must be taken to maintain the boiler operating parameters as per the commissioning procedure.
- The boiler is slowly brought up to the required pressure level
- Do not exceed the furnace outlet temperature as the reheater is dry and has to be protected
- Boiler downward expansion should be noted as the pressure is raised to 40 kg/cm2
- The firing rate must be slowly increased, keeping in mind the drum temperature differential
- Keep the drum level slightly above the normal level as when the blowing is started the water level will raise and go beyond sight then as the blow proceeds it will drop below lowest sight level if water level maintained is at or below normal.
- In the first stage of blowing the first blow is done at much lower pressure to check the temporary pipe supports and anchors.
- The temporary valve provided is opened to blow through the superheater, the main steam pipe and the temporary pipe.
- The first few blows, generally three blows, are done without any target plate till the color of steam is clear. Then the preliminary target or impact plates made of stainless steel material 40 to 50 mm wide, 25 mm thick and of length to suit the pipe diameter is placed to capture the indentation of the particles going along with the steam.
- The target plate must be placed just after the mainstream line in the temporary pipe but not too far and the velocity at this place must be around 245 m/s.
- While blowing the fire is killed and the pressure is dropped to around 15 kg/cm2; this ensures a good temperature differential for any adhering material to dislodge.
- The boiler is started after closing the steam blowing valve and the process is continued.
- The final target plate of turbine blade is placed once the preliminary target plate is in acceptable form, and the blowing is continued till the indentations are acceptable to the turbine manufacturer.
- After the mainstream line is completed the other stages are done in the similar way to achieve a clear target plate of acceptable quality.
Continuous Blowing Method
The procedure adopted in the continuous method of steam blowing is the same as the puffing method but for the fact that the pressure is maintained constantly during the blow. This reduces the reactionary forces on the temporary pipe and also the stresses on the boiler system are lower.
However, many boiler designers and commissioning groups adopt the puffing method.
During steam blowing
- The thick wall components like the drum, headers and piping are subjected to much higher thermal stresses than the normal operation of the boiler, so it is prudent to keep the number of blows to the minimum required for effective cleaning.
- As a rule of thumb it is good to keep the number of blows less than 50 a day.
- The temporary pipe is designed for a much lower pressure than the boiler system and hence the operators should take extra care not to exceed the pressure beyond what is required for blowing.
- Care must be taken in the forced circulation boiler. If the drum level is very low and the pump losses its suction then the boiler must be tripped and the blow must be terminated
- It is not possible to avoid carryover of solids to the superheater during steam blowing, hence boiler water should not be treated with non- volatile chemicals
- The temporary supports provided should be watched and every day before starting the boiler they must be inspected fully to avoid any catastrophe.