Control Notes

A very common control problem, and one used in many examples elsewhere, is that of controlling the level in a boiler drum. Many industrial plants have boilers for generating process steam, and of course boilers are central to thermal power generation.

The boiler drum is where water and steam are separated. Controlling its level is critical – if the level becomes too low, the boiler can run dry resulting in mechanical damage of the drum and boiler piping. If the level becomes too high, water can be carried over into the steam pipework, possibly damaging downstream equipment.

The design of the boiler drum level control strategy is normally described as single-element, two-element, or three-element control. This article explains the three designs.

Single-element Control (Feedback Control)

One or more boiler feedwater pumps push water through one or more feedwater control valves into the boiler drum. The water level in the drum is measured with a pressure and temperature-compensated level transmitter. The drum level controller compares the drum level measurement to the set point and modulates the feedwater control valves to keep the water level in the drum as close to set point as possible. Variable-speed boiler feed pumps are sometimes used to control the level instead of valves.

The simple feedback control design described above is called single-element control, because it uses only a single feedback element for control – the drum level measurement.

Drum Level Controller Tuning

1. Integrating Process

From a controls point-of-view, the boiler drum is an integrating process. This means that any mismatch between inflow (water) and outflow (steam) will cause a continuous change in the drum level.

Integrating loops are difficult to tune, and can easily become unstable if the controller’s integral time is set too short (i.e. high integral gain). The process-imposed requirement for a long integral time makes the loop slow to recover from disturbances to the drum level.

2. Inverse Response

To further complicate matters, the boiler drum level is notorious for its inverse response. If the drum level is low, and more feedwater is added to increase it, the drum level tends to decrease first before increasing. This is because the cooler feedwater causes some of the steam in the evaporator to condense, causing the volume of water/steam to decrease, and hence the drop in drum level.

Conventional feedback control has difficulty in coping with this inverse response. A control loop using high controller gain and derivative action may work well in other level applications, but it will quickly go unstable on a boiler drum level. Stability is best achieved by using a low controller gain, long integral time, and no derivative. However, these settings make the controller’s response very sluggish and not suitable for controlling a process as critical as boiler drum level.

Major Disturbances

Drum level is affected by changes in feedwater and steam flow rate. But because of the very slow response of the feedback control loop, changes in feed flow or steam flow can cause very large deviations in boiler drum level. Single-element drum level control can work well only if the residence time of the drum is very large to accommodate the large deviations, but this is seldom the case – especially in the power industry. For this reason, the control strategy is normally expanded to also include feedwater and steam flow.

Two-element Control (Cascade Control)

Many boilers have two or three feed pumps that will be switched on or off depending on boiler load. If a feed pump is started up or shut down, the total feedwater flow rate changes. This causes a deviation in drum level, upon which the drum level controller will act and change the feedwater control valve position to compensate. As explained above, the level controller’s response is likely very slow, so switching feed pumps on and off can result in large deviations in drum level.

A faster control action is needed for dealing with changes in feedwater flow rate. This faster action is obtained by controlling the feedwater flow rate itself, in addition to the drum level.

To control both drum level and feedwater flow rate, cascade control is used. The drum level controller becomes the primary controller and its output drives the set point of the feedwater flow controller, the secondary control loop. This arrangement is also called two-element control, because both drum level and feedwater flow rate are measured and used for control.

Two-Element Drum Level Control

Three-element Control (Cascade + Feedforward Control)

Similar to feed flow, changes in steam flow can also cause large deviations in drum level, and could possibly trip the boiler. Changes in steam flow rate are measurable and this measurement can be used to improve level control very successfully by using a feedforward control strategy.

For the feedforward control strategy, steam flow rate is measured and used as the set point of the feedwater flow controller. In this way the feedwater flow rate is adjusted to match the steam flow. Changes in steam flow rate will almost immediately be counteracted by similar changes in feedwater flow rate. To ensure that deviations in drum level are also used for control, the output of the drum level controller is added to the feedforward from steam flow.

The combination of drum level measurement, steam flow measurement, and feed flow measurement to control boiler drum level is called three-element control.

Three-Element Drum Level Control

Low-load Conditions

Although three-element drum level control is superior to single- or two-element control, it is normally not used at low boiler loads. The reason is that steam flow measurement can be very inaccurate at low rates of steam flow. Once the boiler load is high enough for steam flow to be measured accurately, the feedforward must be activated bumplessly.

Stay tuned!

Jacques Smuts – Author of the book Process Control for Practitioners