Continue reading »]]> A panel on Future Perspectives of PID Control of was the highlight of the second day of the IFAC PID’12 conference. The panel chairman was Professor Karl Astrom who is widely known throughout the control community for his many contributions in process control. Willy Wojsznis was the speaker on the panel from Emerson.

The short presentation Willy gave on the panel was well received and sparked many questions from the audience. I recorded Willy’s presentation on this panel. The presentation is a little over 15 minutes in length and can be accessed using the viewer shown below.

Also, the viewer controls shown above may be used to download the presentation file.

All the panel presentations may be viewed and downloaded at the IFAC PID’12 web site.

Continue reading »]]> On the second day of the IFAC PID’12 conference, the paper that Dan Coyne, BP, and I wrote Improving PID Recovery from Limit Conditions was presented in the poster session. The paper addresses the performance of the PID under startup or during normal operation when the PID output becomes limited. Common techniques that have been utilized to reduce the time required to get to setpoint during process startup are reviewed in the paper. The response of the PID to conditions that limit PID operation during normal operating conditions is discussed for different implementation approaches. The recovery from a process saturation condition may be improved by modifying the PID operation to use a variable pre-load under limit conditions. The improvement that may be achieved is illustrated using a compressor anti-surge control application example. Variable pre-load is a feature of the PID block in DeltaV v11 and can be enabled on applications that commonly encounter limit conditions.

In the poster session on Thursday, March 29th, 2012 we presented key ideas from the paper. The reset component is realized in many commercial products using a positive feedback network as illustrated below.

When control operates at a limit condition for an extended period of time, the control parameter can overshoot the setpoint. The traditional approach to improve recovery is to provide a specified “Preload value” that is applied under a limit condition.

As addressed in the paper, rather than use a fixed pre-load value, the point at which control action should be taken to avoid overshoot can be calculated as illustrated for below for PI control.

The effectiveness of providing a variable preload to determine when control action is taken is illustrated in the paper using a simulated compressor surge control application.

Control Response for 60% reduction in Load with no Pre-load Applied in the PI Control is shown below.

Control response for 60% reduction in load with variable pre-load automatically applied in the PI Control

If you would like to learn more about this new PID functionality then the paper Improving PID Recovery from Limit Conditions may be freely downloaded from a web site created by IFAC.

Continue reading »]]> Willy Wojsznis is the primary author of a paper on Intelligent PID Product Design that was written for the IFAC PID’12 conference. The paper was scheduled to be presented in the first session of the conference on Wednesday, March 28, 2012. Unfortunately, Willy was delayed in getting to the conference. Thus, since I was a co-author of the paper, I presented this paper at the conference to a nearly packed room.

In the paper we define Intelligent PID design to include a PID algorithm with diverse standard options and algorithm extensions for wireless/event-driven control. The core of the PID intelligence is adaptive process modeling based on model switching and parameter interpolation. The developed process model is applied to loop tuning, adaptive control, loop performance evaluation and valve diagnostics.

The paper Intelligent PID Product Design may be freely downloaded from the web site set up by IFAC. Also, I recorded the paper presentation. The presentation is a little over 15 minutes in length and can be accessed using the viewer shown below.

Also, the viewer controls shown above may be used to download the presentation file.

Continue reading »]]> Field experience using WirelessHART in control applications may be summarized in the following manner:

• WirelessHART measurements may be used in closed loop control applications.  Window communications mode minimizes power consumption
• PIDPlus is a standard feature of DeltaV and works with standard WirelessHART devices.  Standard DeltaV Tools support Simulation and Tuning.
• The performance of PIDPlus in a wireless control network is comparable to PID with wired inputs.  PIDPlus handles lost communications better than conventional PID.
• PIDPlus tuning depends only upon process dynamics, not on wireless update rate

For best control performance process disturbances, the guideline provided for update period should be observed. If disturbance response is not critical then a slower update period may be used.

If you are interested in learning more about control using wireless devices, then a number of references are listed in the blog that I posted on February 20th, 2012.  Hope you found this series on control using wireless devices to be of help in evaluating and applyng this technology.

Continue reading »]]> The Separations Research Program was established at the J.J. Pickle Research Campus in 1984. This cooperative industry/university program performs fundamental research of interest to chemical, biotechnological, petroleum refining, gas processing, pharmaceutical, and food companies. CO2 removal from stack gas is a focus project for which WirelessHART transmitters were installed for pressure and steam flow control on a stripper column. A picture of the process is shown below.

Standard WirelessHART pressure and flow transmitters were installed to demonstrate and test control using WirelessHART and the PIDPlus. The control was modified to allow the operator to switch between control using WirelessHART and PIDPlus and the wired transmitters and PID. The stripper column pressure control is shown below for each manner of control.

The same dynamic control response was observed for SP changes as illustrated above. Original plant PID tuning was used for both wired and wireless control. The control performance for wireless vs. wired control is summarized in the following table.

Comparable control as measured by IAE was achieved using WirelessHART Measurements and PIDPlus vs. control with wired measurements and PID. However, the number of measurement samples with WirelessHART vs. Wired transmitter was reduced by a factor of 10X for flow control and 6X for pressure control – accounting for differences in test duration.

Continue reading »]]> Broadley James manufactures bioreactors and control systems (based on DeltaV) that are used in the life science industry. To explore the use of WirelessHART transmitters in bioreactor control, a portable Hyclone 100 liter disposable bioreactor was instrument with a Rosemount WirelessHART gateway and transmitters for measurement and control of pH and temperature. A WirelessHART transmitter was also installed to monitor pressure. The installation is shown below.

At this site the WirelessHART pH transmitter was configured for Window communication and the temperature transmitter was configured for Continuous communications. The temperature control provided by PIDPlus and WirelessHART transmitter is shown below of a batch run using mammalian cell culture.

Very precise pH control was achieved using PIDPlus and Window communications as illustrated below.

Continue reading »]]> The reliability of WirelessHART device communication has been well established. Even so, in the event of loss communication, the expected control behavior is of interest. The simulation environment was used to compare the behavior of the PIDPlus for loss of communications to a PID with a wired transmitter where the measurement value is frozen for a period of time. The response observed when the measurement was lost during a setpoint change is shown below.

The response observed when the measurement is lost after a process disturbance is shown below.

The Conventional PID provides poor dynamic response when the measurement is lost. As illustrated by these tests, the PIDPlus provides improved dynamic response under these conditions

Continue reading »]]> When the PIDPlus is used with a wireless transmitter in a control application the performance will be comparable to that achieved using a wired transmitter with a wired transmitter. To demonstrate this, the module shown below was created in DeltaV that allows the control using PIDPlus using s wireless communication to be compared to the control achieved using PID and a wired transmitter.

The simulated processes controlled by the PIDPlus and PID were identical and designed to utilize the same unmeasured disturbances. Also, using a module parameter, the setpoint value to the PID and PIDPlus can be changed at the same time. Using this capability, tests were conducted in which changes were made in the setpoint and unmeasured disturbance. The PID and PIDPlus control response is shown in the following trend.

For this test shown above, the communications Window trigger mode was utilized with a maximum update period of 10 seconds and a trigger value of 1%. As is illustrated, the control performance using wireless was comparable to that achieved using a PID and wired transmitter executing once per second. However, by using Window trigger mode, the wireless communications used by the PIDPlus were 96% less than the number of new measurement values provided by the wired transmitter to the PID. The impact of non-periodic measurement updates on control performance as measured by Integral of Absolute Error (IAE) is minimized through the use of PIDPlus for wireless communication as summarized below.

Continue reading »]]> In Solution Part 1 we examined that can be made in the PID reset to accommodate slow and/or non-periodic measurement update. To further enhance the response for continuous changes in setpoint the implementation of the PIDPlus may be modified as shown below.

This PIDPlus implementation is standard in DeltaV v12 and allows the reset calculation to automatically compensate for setpoint change and measurement update rate. When either PIDPlus implantation is used there is no need to modify tuning as sample rate changes i.e. reset is based strictly on the process response dynamics.
The derivative component of the PIDPlus is also modified to account for the fact that a new measurement value is not available each execution of the PID. The changes in the derivative calculation are shown below.

The derivative contribution is only updated when a new measurement become available. Also, the calculation is based on the elapse time since a new value was communicated.

Continue reading »]]> As mentioned my previous block, the communication capability of WirelessHART devices and a modification in the way the PID enable control using wireless devices. The standard communication techniques that are supported by a WirelessHART device are defined in HART 7 specification that has been adopted as an international standard, IEC 62591Ed. 1.0. The device may be configured to communication new measurement values using one of five defined burst message triggered modes. For control applications, the two communication techniques that best fit control applicatons are:

• Continuous – The device wakes up at a configured update period, senses the measurement and then communicates the value
• Window – The device wakes up at a configured update period, senses the measurement and then communicates the measurement if the specified trigger value is exceeded.

Window communication is the preferred method of communications for control applications since for the same update period Window communications will always require less power that Continuous communications. When window communication is selected, and then a new value will be communicated only if:

• the magnitude of the difference between the new measurement value and the last communicated measurement value is greater that a specified trigger value
• or if the time since the last communication exceeds a maximum update period.

Thus, the measurement is communicated only as often as required to allow control action to correct for unmeasured disturbances or response to setpoint changes. For Windowed mode you must specify an update period, a maximum update period, and a trigger value. The update period and the maximum update period are separate configured values. For best control performance process disturbances, the update period configured for Window communications should be ¼ the process response. If disturbance response is not critical then a slower update period may be used.
To provide best control, the PID must be restructured to correctly handle continuous measurement updates that are communicated much slower than 4X the process response time. Also, the PID must be able to work with the non-periodic measurement updates that are provided when Window trigger mode is configured. The PIDPlus capability included in DeltaV v11 is specifically designed to address control using wireless measurements. The implementation of the PIDPlus in DeltaV v11 is shown below.

The filter output used in the positive feedback network is calculated in the following manner when a new measurement is received.

Control execution is set much faster than measurement update. This permits immediate action on setpoint change and update in faceplate. PIDPlus tuning is based on the process dynamic (e.g. RESET= process time constant plus deadtime); PIDPlus reset automatically compensates for variations in the measurement update rate. No change in PID tuning is required for varying update rate. In Solutions Part 2 we will examine other PID enhancement for control using wireless devices.

Continue reading »]]> Emerson is one of the industrial sponsors of the Texas -Wisconsin – California Control Consortium (TWCCC). As explained on the TWCCC web site, this consortium carries out joint industrial-academic research in the areas of chemical process modeling, monitoring, control and optimization. The consortium meetings are scheduled periodically and rotated between the Texas, Wisconsin and California. The University of Texas sponsored the consortium meeting on March 5-6.

The sponsoring companies are invited to attend the consortium meetings. At this meeting, two papers were presented on research sponsored by Emerson in the area of continuous data analytics.

• Continuous Process Analytics – Professor Ricardo Dunia, graduate student Vinay Kumar

In the conference the results of a variety of other research projects were also presented. A list of the presentations given at this meeting is contained on the TWCCC conference web site.   Many of the leading companies in the process industry are members of the TWCCC and sponsor research projects. If your company is interest in joining the consortium then information on the how to join is contained on the consortium web site.

Continue reading »]]> There are significant difference in the frequency and manner in which anew measurement value is updated by a wired transmitter vs. a wireless transmitter. Thus, it is natural to question what impact this has whena wireless transmitter is used in closed loop control. Since most wireless transmitters are battery powered, it is desirable to minimize how often a measurement value is sensed and communicated to reduce transmitter power consumption. However, most multi-loop controller used in DCS systems today are designed to over-sample the measurement by a factor of 2-10X to avoid the restrictions of synchronizing the measurement value with the control. Also, to minimize control variation, the typical rule of thumb is that feedback control should be executed 4X to 10X times faster than the process response time which we will define as the process time constant plus process delay. Also, the conventional PID design utilized in DSC controllers assumes that a new measurement value is available each execution and that control is executed on a periodic basis. The measurement update and control execution that are typically assumed in a traditional control application using wired transmitters is illustrated below.

The conventional PID design (based on difference equation, z-transform) utilized in DSC controllers assumes that a new measurement value is available each execution and that control is executed on a periodic basis. When the measurement is not updated on a periodic basis, the calculated reset action may not be appropriate. If control is only executed when a new measurement is communicated, this could result in a delayed control response to setpoint changes and feedforward action on measured disturbances that occur between measurement updates. Also, as the PID execution period is increased the basic assumptions made in the PID design of the reset and derivative calculation may no longer be valid. For example, two common ways of implementing PI control are shown below.

In many cases the reset contribution of the PID is realized using a positive feedback network (top implementation) in which the time constant of the filter in this network defines the reset time in seconds per repeat. This approach is often taken since it supports the implementation of external reset for use in cascade and override application. When the reset is implemented as an integrator (bottom approach), then logic is used to avoid reset windup. However, using either approach, as the period of execution becomes significant compared to the process response time then the approximations in the reset implementation often breaks down and negatively impact control performance.
It may at first appear that there is no technical solution that minimizes how often a measurement is communicated without compromising control performance. In fact both requirements can be met using a combination of the communication capability of WirelessHART devices and a modification in the way the PID is implemented. The key to understanding how the PID must be modified is to realize that in the when the PID reset is implemented using a positive feedback network (top implementation above) that the filter time constant is a direct reflection of the process dynamic response. For example, when the Lambda PID tuning rules are used, then the reset (filter time constant) is set equal to the process time constant plus the process deadtime.

Continue reading »]]> The IFAC Conference on Advances in PID Control, PID’12, will be held in Brescia, Italy on March 28-30, 2012. As noted on the conference web site, the aim of this conference is to gather academic and industrial experts in the field in order to present the recent research developments in the design of PID controllers. In addition, the meeting aims at providing a perspective of the future requirements for PID controllers in industry. Emerson is one of the industrial sponsors of this conference.

Some of the top leaders in the process control industry will attend and speak at this conference. For example, Karl Johan Astrom, Senior Professor, Lund University, Sweden, will lead a panel on Future Perspectives of PID Controllers. Thus, many of the papers and panel discussions will be of interest to anyone actively involved in process control development and research.   At the PID’12 conference, Dr. Willy Wojsznis and I will give the following presentation on process control research and development by the DeltaV Future Architecture Team.

PID Advances in Industrial Control – Terry Blevins (plenary presentation)

Abstract: Major advances that improve control in the process industry have been made over the last ten years in the basic PID technology of modern distributed control systems. This paper addresses the impact that international standards have on control implementation and the tools utilized in industry for monitoring and commissioning PID control. Examples are used to illustrate how new technologies, such as model switching for process identification, have allowed manufacturers to introduce a new level of ease-of-use in tools developed for on-demand and adaptive tuning. This paper discusses PID modifications that improve the speed of recovery from process saturation conditions that are common in industrial applications. Also, details are provided on PID modifications that enable effective control with non-periodic measurement updates by wireless transmitters. Finally, prospective future directions for industrial PID controllers are sketched.

Intelligent PID Product Design – Willy Wojsznis, Terry Blevins, John Caldwell, Peter Wojsznis, Mark Nixon

Abstract: This paper outlines intelligent PID design for DCS. The design includes a PID algorithm with diverse standard options and algorithm extensions for wireless/event-driven control and for surge control. The core of the PID intelligence is adaptive process modeling based on model switching and parameter interpolation. The developed process model is applied to loop tuning, adaptive control, loop performance evaluation and valve diagnostics. A user-friendly interface provides insight into a loop’s current state and history events. The interface also provides advice about how to improve loop performance.

Improving PID Recovery from Limit Conditions – Terry Blevins, Dan Coyne(BP), Willy Wojsznis, Mark Nixon

Abstract: This paper addresses the performance of the PID under startup or during normal operation when the PID output becomes limited. Common techniques that have been utilized to reduce the time required to get to setpoint during process startup are reviewed. The response of the PID to conditions that limit PID operation during normal operating conditions is discussed for different implementation approaches. In particular under limiting conditions, anti-reset windup is automatically activated when a positive feedback network is used to create the reset contribution. For such implementations, the recovery from a process saturation condition may be improved by modifying the PID operation. An application example is used to show the impact of this modification on response speed and overshoot.

Continue reading »]]> Earlier this year, the Namur Working Group 4.15 invited Emerson and four other major manufactures of distributed control systems and wireless transmitters to present their activities and R&D work concerning closed loop real time control applications based on wireless technology. In this meeting with Working Group 4.15, each vendor was given one hour for their presentation and questions and answer. During each vendor’s presentation, only members of Working Group 4.15 were present. Namur requested that the presentations avoid general information and to concentrate on wireless control R+D activities and examples.

As detailed on the NAMUR web site, Namur is an international user association of automation technology in process industries. Member companies pool their experience to set user requirements on new devices, systems and technologies, and participate in national and international standardization bodies. NAMUR represents approx. 15,000 PCS experts, of whom approx. 300 are active in the 33 working groups that cover the fields of measurement & control, automation, communication, process control and electrical engineering over the entire life-cycle of systems, including planning, procurement, installation, operation as well as maintenance. Namur’s recommendations have significant influence within the process industry. Within Namur, the Working Group 4.15 has been established to address Wireless Automation. Members of this group are some of the top international experts on process control. Thus, it is a real honor to be given an opportunity address this group.

Over the last two years, Namur has sponsored field trails to evaluate the reliability of WirelessHART devices. The feedback I have gotten indicates the installations have shown the technology to be highly reliable. Many Namur member companies I have recently talked with have significant installations of WirelessHART devices in monitoring applications. However, often times where there are economic benefits for installing WirelessHART vs a wired transmitter there is also a requirement that the measurement be used in closed loop control. Thus, many Namur member companies are interested in knowing whether WirelessHART transmitters can be successfully used in closed loop control applications without compromising control performance.

To address questions that Namur had on doing closed loop control using wireless devices, I worked with Mark Nixon and Marty Zielinski to put together a presentation on Emerson’s research and development of PID modifications that enable control applications to be successfully addressed using WirelessHART transmitters. In particular this was an opportunity to discuss the PIDPlus capability in DeltaV v11.3 that is specifically designed to address control using wireless devices. Also, in the presentation we provided information on example installations where PIDPlus has been using with WirelessHART transmitters in closed loop control. This blog series on Control with Wireless devices will be based on the key points contained in our Namur presentation.

The meeting with Namur occurred earlier this month, February 16th , and was held at the BP Gelsenkirchen refinery in Germany. The outline of our presentation was:

• Wireless Impact on Control
• Modified PID for Wireless Measurements
• Performance Comparison to Wired Transmitter
• Addressing Lost Communications
• Test results – Field Installations
• Conclusion

The information presented in this blog series will follow the order shown above.  If you are considering using WirelessHART transmitters in closed loop control then I hope you find this blog series helps you successfully address your control applications.

Continue reading »]]> My blogs have a new home on the Control magazine website www.controlglobal.com. I have had a wonderful run here for the last 5 years with 230 posts. I came up with the name of this website to show the importance and interrelationship of modeling and control. Jim Cahill and Deb Franke designed the website and got me started in sharing my knowledge in the dynamic world of process control through social media. I had a great time and I thank Emerson for the support in this endeavor and my exploration via virtual plants of the many opportunities for process control improvement. In particular, I appreaciate the enthusiastic encouragement of Mark Nixon and Grant Wilson to push process control and social media constraints.

I chose the Control magazine for my new home to join Bela Liptak and Greg Shinskey who are frequent contributors. I had the honor of being inducted into the Control magazine Process Automation Hall of Fame the inaugural year with them.  I consider Liptak and Shinskey to be my best source of practical information on process control.

To see my blogs, go to the Control Global website and in the middle of the page from the “select a blog” pull down menu choose “Control Talk Blog” or go to “Community” tab on top and in the right side under blogs select “Control Talk Blog.” The content of the blogs will be highly technical and detailed almost like a mini white paper as seen here on this site. There will be links to my Control Talk column, Chemical Processing articles, Control articles, InTech articles, blogs on this website, and excerpts from existing or in-progress books. I have a new book on reactor control in-progress and I am doing studies via a virtual plant for the overdue fifth edition of Tuning and Control Loop Performance.

My first blog is “What is the Best PID Execution Time?“

Continue reading »]]> Work on the WirelessHART specification began over seven years ago. Since that time the WirelessHART specification has been published by the HART Communications Foundation. Also, in 2010 WirelessHART was approved by IEC as an international standard,  IEC 62591Ed. 1.0. From the very beginning, the WirelessHART design included the features required for both monitoring and control applications. When WirelessHART field devices are used in closed loop control, the PID must be modified to utilize the slow periodic or non-periodic exception reporting of measurement values. The PID in DeltaV v11 (released last year), includes the PIDPlus option that may be selected when using a WirelessHART transmitter in closed loop control.

Information on the PIDPlus feature and the performance that it enables when using wireless transmitters for control are addressed in the following:

• PID Advances in Industrial Control, IFAC Conference on Advances in PID Control PID’12, Brescia, Italy, 28-30 March 2012
• DeltaV v11 PID Enhancements for Wireless, DeltaV Whitepaper, August, 2010
• WirelessHART Successfully Handles Control, Chemical Processing, January, 2011
• Wireless – Overcoming Challenges of PID Control& Analyzer Applications, InTech, July/August, 2010
• PIDPlus An Enhanced PID Control Algorithm for Wireless Automation, AS-74.3199 Wireless Automation, Aalto University, Finland
• Incorporating Wireless Devices into Single-Use Disposable Bioreactor Design, 2009 Dhirubhai Ambani Life Sciences Symposium
• Improving PID Control with Unreliable Communications, ISA EXPO Technical Conference, 2006.
• Similarity-based Traffic Reduction to Increase Battery Life in a Wireless Process Control Network, ISA EXPO2005, Houston, TX

Ove the next month, this blog series on Control Using Wireless Devices will explore the PIDPlus technology and the control performance that it enables using WirelessHART devices.

Continue reading »]]> In January I posted a series of four (4) blogs that addressed the implement of an on-line compressor efficiency calculation. The example for this series was a compressor for a refrigeration system that used R134a refrigerant. If you have a similar compressor application in your plant then the information provided in these blogs may act as a guide in implementing this on-line calculation. In most cases, the tools commonly available in a distributed control system are sufficient to implement this on-line calculation. However, if you do not have time to implement this calculation for the variety of compressors and/or turbines in your plant then there are other ways to evaluate turbine or compressor operation efficiency that may prove to be easier to implement and support. One novel approach is based on cloud computing. MDC Technology (acquired by Emerson in Dec 2000) pioneered this approach long before Microsoft and other software companies discovered and started to promote the value of cloud computing.

This cloud based performance monitoring capability is marketed today (by Emerson) as AMS Performance Monitor. Web technology is used to collect, model, process and present performance information about critical plant equipment as illustrated below:

This capability may be used to monitor compressors (centrifugal and reciprocating) and calculate compressor efficiency (polytropic and adiabatic) as well as other compressor KPIs. These calculations are based on the American Society of Mechanical Engineers (AMSE) Performance Test Codes (PTCs) – i.e. PTC 10 for compressors. An example of the web interface to the compressor efficiency calculation is shown below.

Such a capability can be used to guide and improve plant operations.

Continue reading »]]> The week of January 8th, 2012 I traveled to our office in Costa Rica to teach the 9025 class. This class was created last year by Emerson’s education department to provide training on the basics of process control and instrumentation. The lecture material and student workshops included in the class are based on the book Control Loop Foundation – Batch and Continuous Processes. This class is normally scheduled for 4 ½ days. However, for the two classes I taught in Costa Rica, each class was condensed into 2 days. In two days it is possible to cover all the terms, concepts and techniques that a control engineer normally uses in the process industry. The students seemed to get a lot out of doing the workshops that are included in the book and on the book web site. Also, in this class I spent some time covering the chapter in the book on the development of dynamic simulations to support control system checkout and operator training. Many of the students in the class were especially interested in this area since they work on development of process simulations within the operator training group.

There were 20 students in each class. The training class room in Costa Rica and students in the second 9025 class are shown below.

It was a real pleasure working with the students in Costa Rica.  The city of San Jose is quite large. However, in most areas of the city some of the rich vegetation native to this climate has been preserved. Below is a picture showing a view of the city from my hotel window.

Continue reading »]]> Part 1 of the centrifugal compressor efficiency series addressed the measurements and parameters that must be calculated to determine compressor efficiency. In Part 2-3 of this series we examined one way of calculating the enthalpy and entropy of the gas stream at the compressor suction and discharge and isentropic enthalpy for the refrigerant R134a. Once these parameters are known, then the calculation of the compressor efficiency is very straight forward as shown in Part 1 of this series. Below is an on-line view of the completed compressor efficiency calculation for the refrigerant R134a.

When utilizing this module for on-line calculation of compressor efficiency in a DeltaV control system, the parameters shown for suction and discharge temperature and pressure would be changed to external references to the measurement values for suction and discharge pressure and temperature.

Later this year I will contribute the module shown above and associated documentation to the Emerson Application Exchange web site. Thus, through this web site the module will be freely available for download. If you have a DeltaV system and are responsible for a refrigeration compressor in your plant then I hope you find this module of value in improving your plant operation.

Continue reading »]]> A compressor going into stall is like jumping off a cliff with a bungee cord. If the bungee cord has no losses to dampen the oscillation, we have something akin to surge. A 0.5% drop in efficiency can occur for each surge cycle. Several surge cycles can occur due to delays and lags in high temperature, thrust, and vibration shutdown systems. In some compressors the damage is so severe after multiple surge cycles that rotors and seals need to be replaced. The cost of process downtime can be significant particularly when a compressor feeds parallel trains of equipment. The restart of exothermic fluidized bed reactors in the petrochemical industry may be the most hazardous mode of operation.

A precipitous drop in flow occurs in less than 0.05 seconds at the start of full surge. The oscillation period of 1 – 2 seconds is too fast for recovery by closed loop control. An open loop backup is needed to prevent a compressor trip. The culprit is what you don’t see on a compressor map.

Compressor maps typically show only the positive flow  negative slope portion of the characteristic curve possibly including the zero slope point. To the left of the zero slope is a positive slope portion preceded by a negative flow negative slope as depicted in Figure 3-1 in the excerpt Description-of-Surge from the Momentum Press book Axial and Centrifugal Compressor Control  

The negative flow negative slope can be measured by compressor manufacturers by feeding gas backwards from the discharge of the compressor. The positive slope section may be simply a 3^rd order polynomial fit between the negative flow negative slope and positive flow zero slope point of the characteristic curves. Users typically don’t get to see the curve to the left of the zero slope point that is the source of dynamic instability.

The precipitous drop in flow occurs when a discharge valve is closing and the operating point reaches the zero slope point. The positive slope provides positive feedback so fast the flow jumps horizontally on the compressor map to the negative slope in the negative flow region. The operating point then walks down the negative flow negative slope and at the start of the positive slope jumps horizontally to a point of positive flow negative slope. The result is a surge cycle shown in Figures 3-5a and 3-5b.

How do we deal with these exciting dynamics? Besides the vibration and temperature shutdown systems we need a surge controller and an open loop backup. The open loop back up has historically been triggered by a precipitous drop in flow to step open a fail open surge valve (e.g. vent or recycle valve). The open loop back holds its output or decays its output to give time for the feedback surge controller to take over smoothly. A frequent question is how fast does the feedback control loop need to be? Companies have built a business on saying the control system must have an analog controller or a high speed microprocessor with an execution time of 0.05 seconds or faster.

If the surge control system has an air actuated surge valve, even with volume boosters the pre-stroke deadtime is 0.1 sec with 2^nd order rate limited exponential lags of 0.2 sec for large compressors. The transmitter has a minimum lag of 0.1 sec. The minimum control loop reset time is about 1 sec. A 0.1 sec PID execution time adds 10% to the integrated error. The ultimate period is about 1 sec as well. A surge oscillation of 1-2 seconds is too fast to be attenuated so the job of the surge controller is to keep the compressor from getting close to the zero slope point. Feedforward from the downstream user feed flows dropping provides a helpful preemptive action for shutdowns in a parallel train. The surge controller should have minimal overshoot but not prematurely open the surge valve. The surge valve should be fast opening and slow closing. Dynamic reset limit, high speed readback of actual position, and an analog output (AO) closing setpoint velocity limit can be helpful if properly setup and tested. An open loop back up must kick in if the surge control can’t stop the operating point from reaching the zero slope point and be fast enough to prevent a shutdown on high thrust or vibration. If configured properely the open loop backup can be done in a module with a 0.1 sec execution time. Thus today’s DCS can be used for surge detection and prevention eliminating the need for special systems.

The key to an open loop back up system preventing even the start of surge is the use of a deadtime block to create a fast  train of rates of change of flow and pressure as noted in the March 4 post “A Calculation so Simple yet so Powerful”

If the rate of change of flow is negative, the open back up takes preemptive action when the rate of change of pressure approaches zero indicating the zero slope point is imminent. In this case an incremental opening of the surge valve can be used causing less disruption to downstream users.  

As I am listening to U2′s “One” I thought I would give you a heads up that I will be moving my blog to the Control magazine website to be “One” with my Control Talk column.  

“So Long, and Thanks for All the Fish”