Karol Ostrowski | GRUPA WOLFF
In 2014, one of the companies of the EDF Polska Group launched an investment project consisting in equipping coal pulverisers with the so called steam extinguishing systems as designed for increasing explosive and fire safety in this part of the plant. Implementation of the project in the turnkey system was the responsibility of the Consortium of GRUPA WOLFF and BCE Systems.
In coal pulverisers, due to the specificity of the equipment operation (high operating temperature, the possibility of sparks of mechanical origin and the presence of oxygen and fuel in the form of coal dust and biomass), an ignition of air/dust mixture and, in consequence, explosion and/or fire may occur.
In the case of the described installation the investor has decided to use a two-stage protection mechanism based on the HRD type explosion suppression and decoupling systems as well as the above mentioned steam extinguishing systems. Both systems complement each other, thus creating a synergy effect.
The first phase of the work consisted in the design and delivery of the explosion suppression systems for EDF, Polska. It was completed by GRUPA WOLFF over the years 2012-2014. At the time, it was the largest investment project of the type in the energy sector in Europe.
The task of the implemented systems is to identify any source of explosion, then to suppress it immediately through injecting extinguishing powder into the pulveriser. In this case, pressure sensors with two measuring cells were used for explosion detection. This solution allows to avoid an incorrect operation of the system, for example, as a result of the impact of the product in one of the membranes (both membranes must indicate the same change in pressure over time at the same moment; the change is additionally characteristic for the explosion of a particular product – in this case, coal dust and biomass).
Steam fire extinguishing
One of the primary tasks of the steam fire extinguishing system is protection against the risks described above. To this end, prior to the pulveriser startup or shutdown processes, water steam with the appropriate parameters is fed to the pulveriser through specially designed nozzles. Thereby, two essential phenomena occur inside the pulveriser:
- steam acts as an inertising factor that reduces the oxygen concentration in the pulveriser,
- expanding steam reduces the temperature inside the pulveriser.
During the shutdown, nozzles of the pulveriser, as properly directed, clean the machine from the product, so that practically the whole product is removed from the inside. This eliminates the risk of ignition of product remnants during a restart of the plant.
Steam extinguishing also plays an important role during the normal operation of pulveriser plants. Due to application of temperature sensors and CO concentration, steam is also fed when temperature in the pulveriser rises excessively or product incandescence occurs.
Scope of the work
The work includes execution of necessary mechanical, electric and I&C designs as well as the subsequent erection and commissioning of the system. Interestingly, the combination of both systems, which further enhances the effectiveness of their operation, provides a worldwide unique solution – for instance, the steam introduced to the system puts out potential inflammatory focuses a moment after the HRD system suppressed an explosion and the temperature inside the pulveriser is high.
In view of steam parameters – 320°C and 1.8 MPa – the quality of the welds made was a very important part of the work. For that reason, the welds were subject to surveys of the VT, PT and RT types. Each weld had to be described and verified by a team of qualified engineers.
The RT test proved to be particularly difficult, chiefly due to a difficult access to the welds and time constraints; the work could only be carried out at night shifts, between 2 and 6 o’clock in the morning. Any delay could result in a postponement of the task for another 24 hours. Ultimately, all the work was completed on schedule.
The mechanical work also included:
- installation of special steam nozzles on pulveriser units,
- distribution of steam pipelines,
- insulation of pipelines.
In March 2015, GRUPA WOLFF completed the investment project on 12 pulveriser systems. Currently, the work at 6 subsequent installations is coming to an end.