Drum Level Control Essentials

What is the Application?

As boilers convert water into steam to supply the plant’s load demand, more water must be supplied to the boiler to maintain the drum level at setpoint. In a modulating PID system, Boiler feedwater flow is controlled by the drum level controller by modulating a feedwater control valve which can be opened to give the boiler more water. The drum level control must have a way to measure the actual drum level in the boiler in order to maintain setpoint. The controller then compares the measured drum level to setpoint and adjusts the feedwater control valve to give the boiler enough water to stay on setpoint.

Why is This Needed?

One of the most critical parts of a boiler system is the drum level monitoring and feedwater supply system. The system must be designed to keep the boiler drum level from getting dangerously low or dangerously high. The drum level must always be above the top of the highest tubes in order to protect the boiler from overheating, which can damage to the tubes and cause a potentially life-threatening boiler meltdown. Feedwater controls must also be designed so that the control valve is “fail-closed” so that in the event of a valve, controller, or transmitter failure the boiler is not flooded, but instead shuts down when the low water safety limit is reached. Flooding a boiler can cause water hammer in the steam header which can cause catastrophic damage to the plant, and can put plant personnel in danger.

How Does it Work?

Good: 1-Element Drum Level Control

There are three types of drum level PID controls: 1-element, 2-element, and 3-element. 1-element control is the simplest type of PID control. It only looks at the current drum level reading and adjusts the feedwater control valve to maintain setpoint. The problem with 1-element control is that when the boiler firing rate is increased, the drum level will actually rise or “swell” despite more of the water being turned to steam. This causes a 1-element control to start closing the feedwater control valve. If boiler steam production is increasing and the feedwater input is decreasing, the drum level will not stay on setpoint and the boiler may lockout on a low water condition.

The opposite happens when a boiler’s firing rate is decreased. Despite making less steam with the same amount of water input, the drum level will temporarily drop or “sink” causing the drum level controller to start opening the feedwater control valve. When this happens, the boiler will get too much water and overfill.

To better understand how “sink and swell” works, think about the fact that when water boils and turns to steam it is converted from a liquid state to a gas which is less dense. Even though the mass of water + steam is the same, there is a higher % of steam which makes the water level appear to rise. Because the drum level controller is only looking at the measured drum level, it has no way to know if the measured rise is because of “swell” or if the boiler really has too much water.

Better: 2-Element Drum Level Control

To account for “sink and swell” in the boiler drum, a more advanced control system can be used. 2-Element drum level control adds a steam flow input to let the PID controller know if the boiler is increasing or decreasing steam production. When the boiler firing rate is increased, a 2-element drum level control system will see the increased steam flow signal and start opening the feedwater control valve to account for boiling more water. Once the steam flow has leveled out, the controller will then adjust the control valve output to bring the drum level back to setpoint if it is not there already.

This control scheme works very well in boiler plants with a steady and consistent water supply pressure because the flow through a valve at each valve position is repeatable. For applications where the water supply pressure can vary because of plant load changes and quantity of pumps running, a more sophisticated control scheme is needed.

Best: 3-Element Drum Level Control

The 2-Element drum level control system can be further improved by adding a feedwater flow signal input. Steam flow meters and feedwater flow meters are often setup to measure lbs. of steam and water. The idea here is that 1 lb. of water into the boiler will convert into 1 lb. of steam output. In the 2-element system the controller knows the lbs. of steam flow out of the boiler. By adding an input for lbs. of feedwater flow into the boiler, the controller can modulate the feedwater control valve to make the feedwater flow coming in match the steam flow out. If the drum level is below or above setpoint, the controller will adjust to give the boiler more or less feedwater until the drum level is back on setpoint.

Preferred’s Drum Level Controls

Preferred has decades of experience with 3-element drum level control both in full-package boiler control systems and standalone drum level controllers. The BurnerMate Universal, BurnerMate TS, and BurnerMate PLC complete boiler control systems all include 3-Element drum level control. For standalone drum level controls, Preferred offers the PCC-IV programmable loop controller and the JC-43D2 3-Element drum level controller, each with a user friendly touchscreen for easy operation and setup.

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