Unit Startup Procedure for Subcritical Corner Fired Boiler

Want to know about the various processes involved while starting a unit? Then you are at the right place because we have stuffed all the details involved in the start-up of a Sub-critical Tangential Firing unit. Below we have explained the start-up procedure types and various processes involved in case of tangential fired sub-critical boiler type plant.

The operation department in the plant is responsible for the start-up and shut-down activities. The start-up or light-up activity is a major activity because to start the unit you have to keep various things in mind as well as you have to remember the sequence also and there are a lot of parallel activities running at the time of light up.Before we start explaining you about the various processes in the Unit Start-up, Let us explain you about the various types of start-up procedures we follow depending on certain conditions. The light up/start-up activity is mainly categorized into 3 types;

Cold Startup

Warm Startup

Hot Startup

For the startup activity every plant is having their different theory and design. The start-up procedures are mainly categorized depending on the Turbine conditions. As Turbine is the most crucial equipment in the plant, more attention is given to the turbine. If precautions are not taken properly during start-up then it may lead to turbine damage. The nomenclatures for various types of start-up procedures are different depending on the designs. Like in case of Chinese design we see the terms like Extreme / Very Hot Startup and Very Cold Startup and The BHEL (Indian Design) categories for startup are;

Cold Startup: (If the HPT (high pressure turbine) casing metal temperature is less than 150 deg. C. then it is called the cold startup)

Warm Startup: (If the HPT casing metal temperature is between 150 – 350 deg. C. then it is called the warm startup)

Hot Startup: (If the HPT casing metal temperature is above 350 deg. C. then it is called the hot startup)

Very Hot Startup:  in some plant very hot startup is a new mode of light up. In which if HPT casing metal temperature is between 350 – 450 deg. C. then it is called the hot startup. And above 450 deg. C. of metal they called very hot startup. (**HPT – High Pressure Turbine)

In the same manner, in some plant shutdown timing of boiler is also important for light up activity. For example after 48 hour of shutdown the light-up is called the Cold Startup, but in the same time it also has to satisfy the turbine casing metal temperature characteristic/criteria. From 8 to 48 hours of shutdown is called Warm Startup and before 8 hours of shutdown it is called the Hot Startup. This light up and shutdown terms vary from plant to plant and their data may also be different for different startups.

The main difference between all the start-up types are that the steam temperature criteria. The Steam temperature should be 50°C more than the turbine metal temperature. As compared to Cold Startup, in Hot Startup, the soaking time (time required to maintain the differential expansion in the turbine within a permissible limit) of the turbine is less because in Hot Startup, already sufficient amount of metal temperature is there.

In Cold Startup, the load increase rate is also less as compared to hot startup. Because in cold startup there is a chance of thermal deformation of the turbine (Thermal Stress) so we require gradual heating of the material. The time for the Cold Startup is approximately 6 to 8 hours, for Warm Startup this time is 3 to 6 hours and for Hot Startup , this time is 1-3 hours .Apart from that we also check various parameters like;

Vibration of Turbine

Bearing Temperature of Turbine

Lube Oil Temperature

Axial Shift of the Rotor

Steam Temperature and Pressure

The procedures for all startup are same. Suppose if any emergency comes in which we have to trip generator and turbine, but if HP-LP bypass is capable to take that much amount of flow then we may not trip the boiler. We mainly see the turbine metal temperature and according to that we light up the unit. If HP-LP bypass is not there, then we have to trip the boiler. If within 8 hour we have to again start the unit and our metal temperature is above 350 °C then we go for Hot Startup. If the boiler temperature is sufficient to coal ignition temperature then we start feeding coal slowly otherwise we go for oil ignition first. After that we increase speed and load according to the curve given by manufacture.

So the main concept is that irrespective of the time and duration, first you have to see the Turbine Metal Temperature and according to that we light up the unit.

Below mentioned are the start-up processes in sequence for the Cold Start-up of a Tangential Fired Sub-critical Boiler Unit

1. Charging of Auxiliary Equipments from the Station Transformer
2. Charging of Service Water from the Storage Tank
3. Starting of Service Air and Instrumentation Air Compressors
4. Charging of DM Make-up System
5. Opening Boiler Side Vents & Drains
6. Filling of Boiler Drum with Boiler Fill Pump or any other arrangement
7. Starting of AC Scanner Air Fan
8. Starting of APH(Air Pre Heaters A & B)
9. Starting of one ID (Induced Draught) Fan followed by the respective FD (Forced Draught) fan or in some case both the FD and ID are started.
10. Establish 30% to 40% of Air Flow by controlling the FD Fan
11. Put the Furnace Pressure Control on Auto after adjusting the furnace pressure to -10 mmwc to -30 mmwc by controlling the ID (Induced Draught) Fan.

(** -10 mmwc is for 210 MW capacity Unit)

12. Start the LDO system and charge the lines

(** Charging means to make the system or equipment ready for operation)

13. Initiate Purging of Boiler
14. After controlling the Furnace DP, go for Light-up with LDO(Light Diesel Oil) with a AB Elevation Pair 1-3 and 2-4
15. Next step is to Fill up the Feed Storage Tank(FST)
16. Then go for Deaerator & GSC (Gland Steam Cooler) Heating and Pegging by Auxiliary Steam.
17. Close all the Drum Vents,superheater Vents and Drains
18. After FST charging, go for charging the Cooling Water System and Cooling Water Pump.
19. Then go for Filling of Hotwell.
20. Now, charge the Condensate Extraction Pump and charge the Condensate Cycle.
21. Charge the LP Heaters from the Water Side
22. Put the Deaerator level & Hotwell level control on Auto
23. Before proceeding to the next step, it is now time to make the generator system ready

(** If hydrogen cooling generator then Seal oil system and hydrogen cooling system has to be started)

24. Now, take the 30%/50% FRS line (**Lower Capacity Line) in service.
25. Take the Turbine Lube oil in Service for the Turbine and after starting JOP (Jacking Oil Pump) put the Turbine on Barring Gear.
26. Start one BFP (Boiler Feed Pump) for Low load operation.

Now as the Condensate & the Feed Water lines are in service, you can go for Turbine Rolling Process now. We also go for interlayer heating of turbine casing for minimizing the differential expansion. The Turbine Rolling processes are different and temperature dependent, depending on the design of the turbine. The turbine rolling criteria in Chinese design are different from Indian designs, but the objective behind the Rolling Process is same in all the designs.

Significance of Rolling Process:

In the Rolling Process, Sufficient time is given to the turbine to increase its parameters like; Temperature of the Casing, Blades & Flow in order to limit the Differential Expansion within a permissible limit.

Differential Expansion:

It is defined as the relative difference in expansion of the Casing and Rotor Unit.It can be both Positive and Negative.

Initially, for Turbine Rolling as we supply steam to increase the parameters like temperature, pressure & flow, the expansion of both rotor and casing units occur. As the rotor is of lighter mass than the casing unit so initially the expansion of the rotor is more than the casing unit. This is called Positive Expansion.

After a certain amount of time, the casing expansion becomes more than the rotor expansion and this is called as Negative Expansion.

In the Rolling Process, the objective is to maintain the Positive & Negative Differential expansions in a permissible region to avoid any contact between the rotor and the casing, which may lead to erosion or blade damage.