🔍 Key Concepts

• Pneumatic dual-element control systems and how they combine two different measured variables • Difference between proportional control, reset (integral) action, and simple on/off control • Why boiler feedwater regulation needs smooth correction of both level and steam flow, not just a binary response

💭 Think About

• Think about what happens in a main propulsion boiler if the water level drifts slowly off setpoint over time. Which control action is designed specifically to eliminate a steady, long-term offset? • Which type of controller would give smoother, more continuous valve movement: a simple on/off (two-position) controller, or one that adjusts the valve in proportion to the error and can also "wind up" to remove residual error? • In a dual-element system (typically steam flow and drum level), would a purely proportional system be enough to hold level tightly during long, steady steaming, or would something else be added to correct small persistent deviations?

✅ Before You Answer

• Identify which options involve continuous control (not simple on/off) suitable for main propulsion boiler feedwater valves. • Recall which control action is responsible for eliminating steady-state error in industrial control systems. • Eliminate any options that describe binary or two-position operation, which would tend to cause excessive cycling in boiler drum level.