Steam drum level is one of the major contributors of boiler trips and downtime. Effected with a specific phenomenon (swell and shrink) it is difficult to optimally control the drum level.

Babcock conducted a study to determine the best control configuration to control steam drum level and how to optimise it. The basic process and analysing method will also be described.

In natural circulation boilers the steam drum is used to separate steam from water. Scrubbers and mechanical equipment are used to perform this function. Once the steam is separated it will flow to the superheated and main steam equipment. The outflow of steam is replenished by adding feed water to the steam drum.

The natural circulation within the boiler will cause the added feed water to flow from the steam drum through the downcomers to the boiler furnace. When the water reaches saturation temperature it will flow back to the steam drum due to a change in density. The mixed steam and water will be separated in the steam drum which will restart the cycle.

Steam drum demand is controlled by the downstream process e.g. turbine. The downstream equipment regulates the steam flow based on the required load. When the downstream equipment reduces/increases load the boiler steam drum pressure is affected.

The change in pressure will change both the density and boiling point of the water in the boiler circuit. The combined reactions of the density and boiling point change will cause the level in the steam drum to rapidly increase/decrease. The increase and decrease in the water level caused by pressure change is commonly known as swell and shrink reaction. These reactions will be in the opposite direction as caused by the imbalance of in and out flow of the steam drum.

If the steam output from the boiler is suddenly reduced the pressure will increase in the boiler circuit. This increase of pressure will cause the drum level to initially shrink. After the shrink function is complete the drum level will increase due to the higher inflow than outflow.

The inverse is true when the steam output is increased suddenly from the boiler. Unstable energy input to the boiler can also causes changes in pressure which will result in the same swell and shrink reaction.

The swell and shrink reactions immensely affects the controllability of the steam drum level.

The objective of the control loop is to maintain the water level at close proximity to 50% of the steam drum capacity. Water in the steam drum must not be allowed to decrease/increase below specified limits.

When water is increased above the high trip limit, water will carry over into the steam line and superheater systems. If water decreases below the low trip limit, water tubes can run dry and could cause severe damage to boiler pressure parts.

Drum level, feed water flow and steam flow are the main elements used in steam drum level control. Combinations of the elements are used in conjunction with cascade and feed-forward methods to control the steam drum level. Single, two and three element control are the commonly known configurations.

Babcock conducted a study to determine the advantage of using three-element and two-element control configurations on a steam drum level application. The testing compared the two configurations to single-element control. Two Identical high pressure industrial Babcock boilers were used during the testing.

Both boilers are connected to a common header which supplies a single turbine unit. The superheated steam was supplied to the turbine at 66 Barg 490°C with a combined capacity of 110 t/h.

Boiler 1 was used as the baseline for the testing. The steam drum level configuration on boiler 1 was left on single-element control during the test period. The configuration on boiler 2 was adjusted to two and three-element control configurations and compared to the boiler 1.

Control loop tuning software Protuner was used to obtain tuning values for all the controllers. The following section will explain control loop configuration and performance testing of the different control methods.

The single-element control loop is indicated in Figure 1. The steam drum level signal is used as the process variable and the output of the controller is used to control the feed water flow to the steam drum. The set-point of the controller is configured to 50% of the drum level capacity. The steam drum pressure is used for density correction for the drum level measurement.

A standard open loop step change was used to analyse the process response. The control loop was placed in manual and the feed water valve was opened with a step change. The test data was analysed and the fast proportional and integral settings were selected from the program for the control loop.

To test the tuning values the controller was placed in automatic and a step change was induced on the set-point signal. The test result is indicated in Figure 2. The results indicated that the fast tuning settings was the optimal selection for the controller. The single-element was installed on boiler 1 and will be used as the base line for the testing.

The standard two-element controller is indicated in Figure 3. This unit includes a feed water controller in cascade to the drum level controller. The drum level controller output is the feed water controller set-point. The feed water flow into the steam drum is the process variable for the secondary control loop.

Tuning optimization started with the secondary loop (feedwater control loop). A step change was induced on the feed water valve and the feed water flow was recorded. From the results, the data was analysed and the fast proportional and integral setting was selected for the flow loop.

After the tuning parameters were obtained the flow controller was updated with the new values and the tuning process was done on the primary loop (level control loop).The primary loop had to be retuned due to the addition of the cascade flow control loop. The flow loop was set to automatic and the drum level loop was set to manual.

A step change was introduced to the output of the drum level controller and the drum level was recorded. The data was re-analysed and the tuning values were updated with the fast proportional and integral settings.

The two-element control was placed in operation within boiler 2 after the tuning values were updated. Exceptional process control was achieved with the flow control loop as can be seen from in Figure 4.

After the two-element control was placed into operation on boiler 2, a load change was recorded on the boiler. The common turbine, supplied only by the two boilers, tripped. The trip was caused by a turbine fault, thus the boilers didn’t trip and remained in service.

During a turbine trip, the steam supply valve is closed and the turbine by-pass line is opened. The turbine supply valve closes quicker than the by-pass valve can open. Thus during the change from turbine to by-pass the steam flow is momentarily stopped.

The turbine trip causes an upset in the boiler steam drum levels as indicated in Figure 5.

The reaction of the boilers proved that the two-element configuration is faster than single-element configuration. Table 1 indicates the summary of the reaction for both boilers.

By comparing the two reactions the low points are identical to one another. The only notable difference is the maximum magnitude and settling time.

The maximum reaction magnitude on boiler 1 was 4% higher than on boiler 2. This difference caused boiler 1 to activate the high drum level alarm. The two-element control on boiler 2 retained the process below the alarm limit.

The two-element control on boiler 2 also stabilized 8 min before the single-element control on boiler 1. The load change confirmed that the two-element control configuration reduces the settling time and reaction magnitude compared to the single-element configuration.

The two-element control configuration is a simple and effective method to control the steam drum level. With the correct configuration and tuning values, the drum level can be controlled within operating boundaries. The two-element control retained and stabilized the process more efficiently than the single-element control.

The single-element control functioned relatively well with reference to its simple construction. Correct tuning was however essential for obtaining the response.

Normally the feed water flow is in any case measured in the boiler process. Thus without major physical changes or financial expense to the plant, single-element control can be upgraded to two-element control. The only changes will be within the boiler control system.

In the next edition of SA Instrumentation & Control the three-element control configuration will be explained and tested.