The unsung heroes of boiler control

When it comes to boiler control, selecting the right types of valve actuators and positioners is key to ensuring safe, efficient and reliable performance whilst also minimizing environmental impact through reduced emissions. Naas deJager, Global Product Manager for ABB’s actuator and positioner products, looks at some of the key challenges that can arise in boiler control applications and how they can be solved by choosing the right actuator for the job.

The growing awareness of the impact of human activities on the environment, coupled with initiatives such as the Paris Agreement and tightening legislation, is putting industrial companies under growing pressure to find ways to minimize their emissions.
For companies that rely on combustion-based processes, there is a need to ensure that the right conditions are achieved for optimum efficiency. Combustion optimization has become an absolute necessity for maximizing performance and efficiency as well as reducing environmental impact and increasing time between maintenance shutdowns.

However, maintaining optimum conditions can be difficult, with challenges including rapid changes in process conditions, variations in fuel quality and the impact of plant shutdowns and start-ups all needing to be handled by the final elements used to control combustion plants

Widely used throughout many power plants to provide precise and continuous positioning of the final control elements used to control flows of air and other variables, actuators and positioners can make a big difference to combustion performance.

The following is a look at some of the key applications in power plants where using high performance actuators and positioners can help ensure the highest levels of boiler control.

As complex pressurised systems, boilers require careful handling during start-up and shutdown. Minimizing thermal stresses caused by expansion and contraction and reducing fuel costs are just some of the factors that demand careful and accurate control of steam and water flows to achieve the required operating conditions.

Best practice for boiler start-up requires very small quantities of steam to be introduced over a prolonged period. To achieve this requires highly accurate control of the start-up valve.

Used only for start-up or shutdown, start-up valves will often remain in the same position – either at zero or 100 percent – for months on end. Over time, this can result in corrosion of the valve seats and other components exposed to the process, which in turn can increase the amount of break-away force needed to actuate the valve when it needs to open or close. In most cases, any problems in achieving this force will only become apparent when the valve needs to move, potentially increasing the risk of a safety issue if too little or too much steam is applied to the system. Introducing excessive steam into a system during start-up, for example, can increase the risk of expansion, where pipework can be damaged by the sudden increase in pressure as it hits the weakest point in the line.  Ensuring precise control of the start-up valve to avoid this and other problems is therefore of paramount importance.

Eliminating the problems posed by valve seat corrosion requires the use of actuators that can withstand the impact of breakaway force. One solution is to use a device with double-rated force/torque. The variable force/torque design of ABB’s Contrac actuator ensures that only the required energy is delivered, minimizing the strain placed upon it when varying the valve position. The inclusion of integrated diagnostics also helps to immediately detect any potential faults, allowing operators to take the necessary remedial action.

With a dead zone of only ±0.05 %, these types of actuators can also provide the high accuracy needed to ensure optimum control of steam and water flows during boiler start-up and shutdown.