Most new power plants nowadays are producing steam to run a steam turbine. A steam turbine is a sensitive system containing turbine blades. The steam quality is very important as impurities in the steam can deposit on the turbine blades and imbalance the turbine. To ensure good steam quality, the cleanliness of the system is of great importance, from the start of the erection of a steam generator of a new power plant. The cleanliness of the steam and water parts of a steam generator is achieved by acid cleaning or boil-out combined with steam blow, the so called pre-commission cleaning. The reason a typical pre-commission cleaning of a steam generator consists of a chemical cleaning and steam blow, is the fact that mill scale/silicates are easier removed with chemical cleaning, which ensures less steam blow cycles are required in a later stage. This technical bulletin goes deeper into the methods of steam blow
Steam blowing for a steam generating plant requires careful planning and the establishment of a steam blow program, a so called pre-engineering. One of the important parts of the engineering is the determination of the velocity of the steam inside the system. This is called K-factor. Only from a certain velocity or K-factor the steam has sufficient energy to remove the particles from the system. The K-factor is dependent on the dimensions of the boiler like production of steam, pressure, temperature, diameter of the steam lines and length of the steam lines. When the K-factor is known, the dimensions of the temporary piping and silencers can be determined. The turbine itself will of course not be a part of the steam blow and will be bypassed with the temporary steam blow lines. Steam blow operations have to be monitored. As the size and number of particles are important, these are measured using “target plates”. These plates are mirror like steel (or other prescribed materials) plates, which are placed in the temporary steam lines. The target plates are inspected regularly during operation. The size and number of particles on the mirror plate are a part of the cleanliness criteria.
With shock blow, the pressure inside the steam generator is raised to a certain maximum and reaching the required pressure, a temporary sacrificial valve is opened quickly. During this pressure release the K-factor or steam velocity is reached, but for a short period of time. Due to release of the pressure and temperature, the steam generator has to start again. The steam blowing operation described should last approximately four to six hours, including start-up and shutdown of the steam generating plant. Normally not more than one steam blowing operation should be effected daily in order to ensure adequate cooling of the system.
With continuous blow there is no pressure build up, but after calculations, the K-factor is reached continuously. During the continuous steam blow the target plates are checked on impacts of particles.
|Continuous Blow||Exhaustive (Puff) blow||Remarks|
|Steam Velocity||The system is designed and blown at maximum velocity in the entire system.||Full system not subjected to Max velocity. Max velocity is achieved close to outlet of temporary pipe only.||Higher velocities achieved in the entire system in CB. This facilitates better cleaning and removal of contaminants from the whole system.|
|Effective CFR||Assured >1.1 calculated based on Max operating conditions throughout the blow duration of minimum 12 Hours.||May be reached at selected points only for few seconds.||The duration of Max CFR in Puff blow is inadequate for cleaning deeper inside the line and hence cleaning cannot be guaranteed and evaluated by Puff blow. A clean target does not mean a clean line unless the CFR is achieved everywhere at least for the time needed for a debris at the beginning of the pipe to reach the exit.|
|Noise||Less than 90 db||Exceed 130 db||Work may continue undisrupted in nearby areas when CB is in progress.|
|Duration||Duration is more closely defined due to engineered process, typically 12 to 48 hours max.||Unknown duration typically requires 40+ blows which take over 10+ days. Operation in the nearby areas also interrupted.||Significantly reduces commissioning time.|
|Thermal Cycling||On line without stopping the blow.||Blow interrupted||Thermal cycling is achieved in 1-1.5 Hrs reducing cleaning time.|
|Target Insertion||Safe on line using pneumatic inserters without stopping blow.||Saves much time in shooting multiple targets for better QA.|
|System Loading & Stress||LP high velocity blow. Hence minimum stress & load on boiler and supports. No anchors required.||Boiler subjected to high stresses and reaction forces. Piping requires strong expensive anchoring.||Minimum effect on Boiler life on using CB.|
|Cost Savings||Excellent saving in overall cost.|