A decade ago, there were only a few boiler manufacturers in India. However, numerous manufacturers are now available to supply boilers and their auxiliaries. In our experience with troubleshooting boiler issues, we often encounter abnormalities related to engineering concepts that have been proven through practice.
In this paper, we will discuss a case study on the commissioning challenges faced with a 20 TPH AFBC boiler and the steps taken to successfully bring the boiler online. The history of the boiler is as follows:
- Initially, the bed was not fluidized even at the fan’s full RPM. After that, the customer decided to increase the air nozzle hole size.
- Following the revision of the air nozzles, the bed was successfully fluidized at 85% of the rated RPM.
- Once the boiler was started, the user faced difficulties in maintaining operation for even 8 hours. In a short period, bed clinker formation occurred.
- The user was unable to reach the rated steam outlet temperature.
The following points were observed during our visit.
- Abnormal APH outlet ducting
- Oversized fluidized bed
- No doors are provided for fluidized bed inspection and operation
- Looping of water circuit drain line with superheater headers
Abnormal Ducting
Ducting designs were undersized, which led to pressure drop and its design having sharp bends that would lead to additional pressure loss. The provided fan is sufficient for the desired steam flow, but due to additional pressure losses, the bed was not able to be fluidized.
The APH outlet duct was undersized, and its outlet duct did not cover the complete APH outlet area, which caused a high-pressure drop.
Oversized Fluidized Bed
The fluidized bed area was calculated for designed fuel and found to be oversized so, two lines of air nozzles were plugged and a refractory wall was constructed to reduce bed size.
For smooth fluidization, velocity should be 2.3-2.5 m/sec, if the designed fluidization velocity is below the limit, then excess air is required to match the fluidization limits.
As designed, fluidization velocity was less, and fan ducting systems were undersized with a high-pressure drop, which caused light-up failures.
No Door Provided For Furnace Inspection And Operation
Inspection doors with clear space are mandatory for the operation and inspection of the furnace. In this boiler, no inspection doors were provided for bed charging in the front & rear sides. Due to this fluidized bed is not visible to operators.
We had designed & developed openings without affecting boiler circulations.
Drain Line Arrangement
Boiler design parameters were checked and superheater area requirement was also checked for rated steam temperature output. Found provided area is sufficient for the rated steam temperature output, but in actual steam, the temperature did not reach the rated temperature.
During troubleshooting, the boiler drum drain was directly connected with the superheater header drain, during light-up time drum drain was kept open by boiler operators, which caused direct water circulation to the superheater, here superheater acted as an evaporator.
Valve-01 outlet was connected before Valve-02, this caused direct water injection to superheater headers whenever Valve-01 was been operated. So, we disconnected these lines and separately terminated them with a blowdown tank.
Conclusion,
Involving consultants with expertise in design, engineering, and process during the design and project stages can help eliminate commissioning problems.
Author:
Mr R.Nagarajaprasath – Director
Mr B.Pravin Raj & V.Nitesh Kumar – Sr.Engineer
Boilertech Energy Services
