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Boiler Drum Level Control

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.

Single-Element Drum Level Control

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

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

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

 

11 Responses to “Boiler Drum Level Control”

  • Brent:

    If you’re trying to design a feedforward for an integrating process, such as a boiler steam drum level, how do you set up the lead-lag? In your book the FFlead should be the time constant of the process response and the FFlag should be the time constant of the disturbance response. But there is no time constant in an integrating process. My guess would be to use the integration rates instead of the time constant. Is this correct? Thanks.

  • Brent,
    The integration rates you refer to are equivalent to the process gain of an integrating process. So if there were to be a difference between the rate at which the drum level changes after a change in steam flow versus feedwater flow, you would compensate for that with the feedforward’s gain. Generally, if your steam and feed flows are measured accurately, the integration rates will be the same, so the FF gain will be 1.0.

    You ask a good question about tuning the lead-lag. Generally, on an integrating process (excluding drum level), any lags in the process will show up as dead time because of the way we model the process. So you will set your lead equal to the “dead time” after a change in control action, and your lag equal to the “dead time” after a disturbance.

    However, for drum level it is not so straightforward because of the drum level’s inverse response. From my experience, people normally don’t bother with a lead-lag on drum level control. Sam Dukelow suggested using a lag on the steam flow signal to compensate for the inverse response. I have not tried it out, so I can’t speak to its effectiveness. If you tune boiler controls and don’t have Sam Dulelow’s book, I higly recommend getting it.

  • ajit laware:

    What are major limitations of PID controller for boiler-drum level ?
    Can we use any robust controller like H2, H infinitey or sliding mode controller ?
    Whether this research beneficial for idustry ? In what point of view ?

  • Ajit,
    1. The limitations of PID for drum-level control result from the drum’s inverse response. High controller gains (that can normally be used on level control of non-surge tanks) cannot be used on drum level because the loop goes unstable very easily. The same goes for using derivative control mode.
    2. You could probably get slightly better response with a properly-designed model-based controller, provided that the inverse response is modeled accurately. I have not seen this used in practice. The standard design is to use a feedforward from steam flow because it gives a response vastly superior to the capabilities of any feedback control.
    3. I don’t think improving feedback control for drum level will be widely adopted in industry because: a) a feedforward will still be the primary control action, b) industry is reluctant to use advanced control technologies where its benefits are marginal (especially the power industry).

  • Benny:

    I worked at a power plant, the drum level was the classical three element control system using circa 1950’s pneumatic controls. The controllers were completely worn out due to their age (40 years in service). I put in a proposal to have them upgraded to digital controls. Two units were retrofitted. Unit 2 went into service with very little fuss. Unit 1, however, made me pull my hair. Through luck I found out that the non-return valve (NRV) between the economizer inlet and the feed pumps was defective (not closing), After it was repaired the loop worked flawlessly, it even kept the drum level close to set point after 3 coal feeders out of 5 tripped during a test. In a nutshell don’t only look at your transmitters, controllers and final control elements keep an eye on anything in those pipes such as NRV’s.

  • DOST MUHAMMAD:

    Why at the start up of boiler the level of drum is control by single element control and on which stage or load it should be change over to three element control?

  • Muhammad, Flow measurements for feedwater and steam get less accurate as the flow rates decrease. Therefore, only single-element (drum level) control is used under low flow conditions. You can switch to three-element control when the flow measurements become more accurate, typically around 25% of maximum flow.

  • d lakshmanudu:

    i worked in powerplant , i would like to know some doubts in 3 element controller.
    when it is stedy with controlling boiler drum level . if suddendly steam flow become zero, what wil be action of three element controller . can you explain ?.
    wht is the mean of compensation of steam with pressure and temperature. if compensation element became vary too much , what will happen?.

  • Lakshmanudu:
    1) Under three-element control, the feedwater flow controller’s setpoint is set by the steam flow measurement plus the bias from the drum level controller. As a result, if the steam flow indication becomes zero, the setpoint to the feedwater flow controller will become equal to zero (plus the bias from the drum level controller, which is normally just a fraction of the actual steam flow rate). So, if you lose steam flow indication, your feedwater flow will go to virtually zero unless the control logic protects you from this situation. Normally, if the steam flow measurement goes bad, the feedwater flow controller will be forced to manual by the control logic to prevent an incorrect change in feedwater flow.
    2) Steam flow is most often indicated as a mass flow rate which is calculated from the measured volumetric flow rate multiplied by steam density. If the steam pressure or temperature changes, the steam density changes, and the calculated mass flow rate will change. If your temperature or pressure measurement is inaccurate, the mass flow calculation will consequently be inaccurate.

  • TendaiM:

    Good Day

    . The problem is I have a pressurized boiler and the feed water is pumped into the boiler using a pump which gets its input from a frequency inverter. The frequency inverter gets its input from a controller which monitors the water level in the boiler. The concern however is that the pump is only able to overcome the boiler pressure, hence pump water, into the boiler after a certain frequency, which thus is giving the problem because pump in excessive amounts than is required.

    The controller being used is a PI controller. We cannot replace the pump and we need to develop a revised water regulation algorithm. I am working on a model to calculate the expected amount of water to get into the boiler by using a look up table of speed at which pump can pump water versus the boiler pressure. Since I know pump capacity I can then calculate the amount of water which will be pumped in at a frequency at which the pump can overcome the boiler pressure.

    Also I can find from a level transmitter the amount of water which has to go into the boiler, hence when water is eventually pumped, I just return the error amount back to the reservoir.

    However I feel that they might be a better and cheaper way of control. Maybe introduce a derivative term.

    Can you please help me out on this one.

  • It seems to me you are over-complicating the problem. Why do the original PI controls not work?

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