Boiler Drum Level Control July 3, 2010
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
Posted in 7. Control Strategies
9 Responses to “Boiler Drum Level Control”
Brent: July 1, 2012 at 11:10 pm 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.
Jacques: July 10, 2012 at 4:51 pm 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: August 11, 2012 at 11:50 pm 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 ?
Jacques: August 12, 2012 at 8:15 am 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: September 14, 2012 at 1:17 pm 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: September 22, 2012 at 7:31 am 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?
Jacques: September 22, 2012 at 6:50 pm 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: December 8, 2014 at 5:30 pm 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?.
Jacques: December 10, 2014 at 9:08 pm 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.
Search
PID for Dummies "I personally have a few hundred dollars worth of books on controllers, PID algorithms, and PID tuning. Since I am an engineer, I stand a chance of understanding some of it. But where do you go if you want to understand PID without a PhD? very good material about PID which simplifies understanding. What follows here is an excerpt from his reprinted with his permission. I don't make a dime off this endorsement. I simply provide this introduction because I material to be an excellent simplified discussion of PID." -- Jim Hogenson
Of A Feedback Control System
lassic block diagram of a process under PID Control.
Control Solutions' VP4-2310 prov mix of I/O points and a soft PLC graphical programming that inclu PID blocks. Check it out here!
Web enabled, programming in B
Check it out here
Basics of PID Control (Proportional+Integral+Derivative) The PID features found in the control loops of today’s controllers have enabled us to achieve much greater accuracy in our commercial control systems at an attractive price compared to that available only a few years ago. When setting up PID loop control, achieving proper operation can be difficult because of the complex setup parameters and the need to understand the sequence of implementing them. Proper operating control may be defined as “the ability to control a variable at a given setpoint within an acceptable degree of accuracy.” This is not an easy feat, because of the dynamics of a control system. If not properly set up, abrupt changes in setpoint or system loading can cause system controls to oscillate or control with excessive error between setpoint and actual control point. The period of the loop (oscillation) is the time from peak to peak. All control loops have a tendency to oscillate because of the built-in timing constants of the control system components and the dynamically changing variables such as setpoint shifts or load changes. Typical period values encountered in control system loops would be in the range of 30 seconds to twenty minutes. All loops can be made to oscillate by setting the throttling range too low (loop gain too high). Loop oscillation is undesirable in control systems and is easily eliminated by increasing the proportional band of the loop. Commonly referred to as the throttling range (TR), proportional band is defined as the amount of change in the controlled variable required to drive the loop output from 0 to 100%. Systems subjected to abrupt changes in load or setpoint will typically require a wider proportional band to achieve stability in control during these system upsets. Very quick system response times, such as those found in static pressure control, will require much wider proportional bands to prevent “overshoot,” the most common cause of oscillation. The gain of the loop is inversely proportional to the throttling range or proportional band. In general, decreasing the throttling range will increase the amount of over shoot. Conversely, the larger the throttling range, the slower the loop will respond. Gain is the ratio of output change (%) over the measured variable change (%) that caused it.
Where PB is the proportional band. Example: If the PB is 20%, then the gain is 5. A 3% change in the error signal (setpoint- process variable) will result in a 15% change in a controller’s output, due to the proportional action. If gain is 2, then the PB is 50%. A common characteristic of proportional control is an error between the setpoint and control point, which is referred to as offset or droop. As the system load and/or proportional band increases, so does throttling range. For instance, with 10 degree throttling range and 100% loop output, the actual control point will be offset 5 degrees from setpoint. Offset is an undesirable characteristic of proportional only control loops and is easily eliminated by adding Integral Action. The integral component of a control loop has the effect of continuing to increase or decrease the output as long as any offset or droop continues to exist. This action drives the controller in the direction necessary to eliminate the error caused by the offset.
Integral, or reset, adjusts a controller’s output in accordance with both the size of the deviation from setpoint and the time it lasts. A controller accomplishes this correction by determining the amount of error that exists between the actual value of the controlled variable and the value of the loop setpoint, then acting as though it were automatically resetting the setpoint by this error amount over a specified time interval. Integral action is sometimes defined as “repeats per minute of rest toward setpoint.” The range or adjustment is typically in the area of .01 to 2.0 repeats per minute. Integral works by causing the controller output to move in the direction of setpoint by an amount equal to the difference between the loop output when setpoint is equal to control point (assume 50%) and the actual loop output caused by offset. Consider a loop that is at setpoint when its output is at 50%. If offset or error causes the loop output to be at 20% (a proportional term of 30%), an integral value of one repeat per minute will change the loop output 30% per minute in a direction to bring the control point back to setpoint. Note the loop output change occurs in increments throughout the minute. (The size of these increments depends on the controller.) If a loop block has an update time of five seconds, the 30% change resulting from integral action of one repeat per minute will occur in 12 steps of approximately 2.5% each. Remember that the system’s dynamics will change with each increment of integral action. The loop may reach setpoint well before the full 30% integral change is achieved. This is what the term “repeats per minute” relates to when used in the context of integral action. Derivative action adds the effect of the error’s differential or rate of change. This means when an error changes by more than a given percentage during a specified time period, a portion of the error is added to the calculated output to boost the output response. Typically, using derivative action is effective only if the loop can respond to a “surge” in the output very quickly. Derivative action observes how fast the actual condition approaches the desired condition and produces a control action based on this rate. This additional action anticipates the convergence of actual and desired conditions. In effect, it counteracts the control signal produced by the proportional and integral terms. The intended result is a reduction in overshoot. As a general practice, the loop control we encounter in HVAC control does not require the use of derivative control. It is difficult to determine exactly how to set it up; an improper setup can cause more harm than good. Derivative action is more commonly used in the process control industry, which typically involves equipment with extremely rapid response times and large overshoots. To review, proportional only controls cannot hold a process at the exact setpoint. A proportional offset is always present because the control output is 0% at setpoint. Any load on the system will cause the control point to be offset from the setpoint. The greater the load on the system, the further the control point will be offset from the setpoint and under maximum load, this error will approach the throttling range. SeeFigure 1.
Figure 1. Adding integration results in much less error than proportional only. See Figure 2.
Figure 2. On proportional plus integral controls, the amount of correction may become too large if the system load exceeds the capacity of the equipment. When the actuated device (valve or damper) is fully open or closed, and the setpoint still cannot be reached, the integration error continues to grow. The result is called “integral windup.” Properly sized equipment is extremely important to avoid integral windup. When properly set up, PID features in the control loops of today’s controllers enable you to achieve high accuracy at a very affordable price. The following guidelines will help you overcome the complexity of PID setup parameters to achieve proper operation.
Step 1.
Before stabilizing the loop by increasing the throttling range (TR), measure the period of oscillation-the time (in minutes) from one peak to the next (one complete cycle).
Step 2.
Next, achieve loop stability using proportional control only. Do this by increasing the TR attribute value until the loop control is stable with no oscillation, and then add an additional 10% to avoid future oscillation. Do not hesitate to increase TR if necessary, because some loops, such as mixed air, may require a TR of 25 degrees or more to achieve stability. If stability cannot be achieved by increasing the TR, the mechanical system installation and design should be reviewed because the addition of integral and/or derivative action to an unstable control loop can only cause further instability.
Step 3.
Once stable loop operation is achieved using proportional only control, widen the TR attribute value by 20 to 30% in preparation for adding integral.
Step 4.
Use the following formula to calculate the integral value to be used. It will provide a good starting point for integral action:
Step 5.
Monitor loop control to evaluate response. If response is slow with integral action, increase the “I” value slightly. It may be necessary to upset the loop to get a good test of loop response. Changing the setpoint to simulate a sudden change in load, then observing the time required to reach the new setpoint can do this. It is generally not recommended to exceed 1.0 for integral, so it is better to start too small than too large. Experience shows that numbers between 0.1 and 0.5 are usually effective in providing “close” control.
Step 6.
Typically, the control loops used in the HVAC industry do not require derivative action. Derivative action is generally not recommended because an improper derivative value will produce worse control than none at all. Experience proves that proportional and integral control can achieve precision. If derivative is required, use the following formula to determine the derivative value:
Many DDC systems offer an automatic self-tuning loop feature that eliminates the need to time the loop period, calculate the proper integral value, and select the correct proportional band. While self-tuning loops appear to offer the ideal solution to achieving good control and saving time, caution must be exercised when using the self-tuning loop feature, especially in more complex control strategies. For instance, the use of self-tuning loops in systems requiring the sequencing of two or more control valves and/or dampers can inadvertently cause an overlap that turns on the heating and cooling at the same time. This can happen when the self-tuning loop results in an excessively wide throttling range, effectively reducing or eliminating the amount of intended separation between the heating and cooling devices. Another word of caution is never to use the self-tuning loop feature to tune a loop used to control two, two-position heating or cooling devices in sequence. Such a control strategy is usually dependent on a specific throttling range value necessary to obtain the desired sequencing results. Self-tuning loops tend to ratchet PID parameters (throttling range, integral, and derivative) upward at a relatively quick rate in an attempt to achieve stable operation. If any of these values overshoot for any reason, it will generally take much longer for the algorithm to bring them back to more realistic values. When using the self-tuning loop feature, be sure to monitor system performance long enough to be certain the entire control system operates properly and functions as a system. Once the self-tuning PID values have stabilized and system operation has been monitored for proper and stable control, the self-tuning feature may be turned off to prevent unexpected system disturbances from changing the PID parameters in the future.
The basic principles of PID control and self-tuning PID loops are the same among all DDC control systems. However, specific details and algorithm design may vary from one manufacturer to the next. PID control represents a significant advancement in the controls industry. It is a very effective technique for providing precise control. Although PID control is a relatively complex feature, control engineers and technicians will find that well-designed products also make it user friendly. It is important to understand what PID control can do for your operation and to learn how to set up an effective PID control loop. While an improper setup is likely to result in unnecessary callbacks, a properly tuned PID control loop will deliver satisfaction.
