Boiler Systems

Supplying steam to individual units is a common element in almost all process industries. A boiler is very unique in that while most process units have individual target rates in production, a boiler must follow other process units and produce steam as it is consumed -- due to the difficulties in storing steam itself. This condition affects control strategies in their entirety, thus the loop tuning strategy of the boiler as well.

A Story from the Field

A customer was running several boilers with an integrated control schemes to maintain a common steam header pressure. In addition, a construction plan was prepared cogenerating units to offset high electricity costs. The cogeneration plan significantly affected the overall steam that would be demanded -- the operation style of the existing boilers needed to be changed, completely.

Most of the existing boilers were no longer in operation, but some would be running at minimum load and on constant standby. When the cogeneration tripped, the boilers were required to recover lost steam immediately to ensure continuous production. Electricity can be bought from grids, unfortunately steam cannot. Overall, this was an extraordinary challenge.

The existing control scheme and other information such as boiler specification, P&ID and tuning parameters were studied thoroughly. The existing control scheme was simple, but modern enough. The tuning parameters looked conservative, but unfortunately the trend charts were not available. I drafted the functional specifications of a new control scheme and made a proposal; there were two key points to this scheme:

1. Ramp up the boiler load when the cogeneration trips (fortunately existing load demand distribution circuit could be used with minor modifications)

2. Protect each control point during a rapid change of fuel feed. This would affect several control points, with the flue gas oxygen and steam temperature being the most serious - static and dynamic models were best to control the scheme

The draft was polished up by the customer and Yokogawa staff, and was implemented soon thereafter.  Using steam balance simulation, a target was set by the customer to reach a 47% load up within a three minute time frame, from its current load. 

A trial was conducted before implementation of the new control scheme with a 7% load up. With the original tuning parameters the control response was conservative and required manual interventions to pull the boiler back to normal.

The control loop tuning was started with the new control scheme. On the fourteenth trial we successfully met the target. The customer has confirmed that the boiler shows smooth and stable control with the 47% load up. Day-to-day operations were running smooth as well.

Summary

In most cases, control loop tuning is performed only when a process unit is being constructed from scratch. The tuning strategy is meant to fit the purpose of the process unit and its operation style. As operations and performance requirements change; a review of the control strategy is required, too. In the rapid pace of the business world today, this situation will most likely become more common.

In summary, this was a complicated situation, however, with specific tuning and minor adjustments to the application software, significant improvements in the overall operations process were achieved, the customer was happy and the overall project was a success.

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