In the original plant configuration, the auxiliary steam that fed the gland sealing system during start-ups was supplied by the Heat Recovery Steam Generator (HRSG) High Pressure (HP) steam line (derived from main HP admission steam line). However, this system was giving several problems during operation, especially not availability of hot-start function as well as unstable plant performance during warm and cold starts causing frequent plant trips.
With the originally delivered solution, the auxiliary steam was produced in delay with respect to the plant needs, and more important with insufficient parameters for the hot start-up of the power plant which means hot start-up was not possible. This design was also limiting the operational flexibility of the Power plant and was causing swelling issues in the LP drum. In order to enable the hot-start-up function, to increase the plant stability during startups and to fully minimize the swelling issues with the LP drum, TE-TO carried out a project for installation of an electrical steam generator and all related connections to the existing plant, that feeds steam to the Gland steam sealing system, as an alternative to the original one.
Also there is the benefit of quick restart of the plant in case of trip which decreases the possible high penalties for the grid system operator. The choice of auxiliary electric steam boiler (AESB) instead of natural gas auxiliary boiler was driven by the following reasons:
- AESB does not have emission problems. So it does not need any emissions authorization
- AESB is fed by power connection, meanwhile a gas fired boiler installation implies modifications in natural gas distribution system, including hazardous area study
- AESB can be installed in the ST building which is currently unclassified area
- AESB is supplied already preassembled on skid, which implied a short installation time once all the connections at skid battery limits are ready
- AESB is a simpler installation and has faster delivery time than a gas fired boiler.
The project for basic design of the complete system was given to Tractabel Engineering – Italy, while the project for design and construction of the boiler was carried out by ATTSU Group – Spain. For the on-site installation and procurement of equipment (pipes, valves, etc…) needed for Balance of Plant, a local Macedonian company was engaged – DMB Construction. Electrical connection of the AESB, along with new dry transformer, UPS and 110 VDC power supply were performed by Macedonian company Spektartermo. During all phases of the Project, engineers from TE-TO AD were including, making sure that the design, the performed works and the system functionality will match the Power Plant needs as best as possible.
The preassembled boiler skid supplied by ATTSU included the following equipment: an electrical saturated steam boiler model GE?2400/17, two electrical super heaters model RVE?100/17, a deareator tank, model DT?1, to prepare the feeding demi?water to boiler, an additives pump for PH correction and O2 scavenger, a set of pumps to feeding the boiler with water, an automatic valve to vent and condition the steam, an automatic purge system for pipelines, a blowdown tank, model DF?5, an electrical cabinet with a PLC control and every accessories needed for the safety and correct operation of the boiler installation.
The main maximal outlet parameters of the AESB to the Power Plant tie-in point are:
| Pressure | 15 barg |
| Temperature | 295 oC |
| Flow | 2.4 t/h |
The first start-up of the system and complete system logic and commissioning was carried out by TE-TO AD engineers with support from ATTSU Group.