--- title: Protecting the Compressor and the Process description: protecting the compressor and the process overview --- [![26TRIS065-Tri-Sen 50 years logo-R2](https://blog.tri-sen.com/hs-fs/hubfs/Assets/26TRIS065-Tri-Sen%2050%20years%20logo-R2.png?width=1476&height=573&name=26TRIS065-Tri-Sen%2050%20years%20logo-R2.png "26TRIS065-Tri-Sen 50 years logo-R2")](https://tri-sen.com/) # Tri-Sen Blog # Protecting the Compressor and the Process By [Jim Jacoby](https://blog.tri-sen.com/author/jim-jacoby) on Oct 8, 2019 10:58:17 AM ![Jim Jacoby](https://blog.tri-sen.com/hubfs/Jim%20blog%20author.jpg) Share this post - [Tweet](https://twitter.com/share) ##### Properly designed compressor controls should not only protect the compressor, they should protect the process. Centrifugal compressors are common-place in many process plants due to their small footprint, lower installation cost and low maintenance cost, relative to positive displacement compressors.  One significant shortcoming of a centrifugal compressor is its need for surge protection. Centrifugal compressors raise the pressure of a gas by accelerating the gas in an impeller, then, by Bernoulli’s principle, slowing the gas down in the diffuser section to convert the velocity energy into pressure energy. The velocity energy that is imparted to the gas is a function of the density of the gas and tangential velocity (tip speed) of the impeller. Therefore, with a given gas and impeller speed, there is a finite pressure ratio (discharge pressure vs. suction pressure) that can be supported by a centrifugal compressor. **What causes surge?** When the pressure ratio across a centrifugal compressor exceeds the maximum pressure ratio that can be produced for that gas and impeller speed, the flow will suddenly reverse through the compressor. The flow reversal allows the discharge pressure to drop and at the same time will cause the suction pressure to rise, resulting in a sudden drop in the pressure ratio. The flow reversal essentially relieves the condition that caused the flow reversal to occur. Within about one second after the flow reversal started, the flow will start moving forward again through the compressor. If the condition that caused the high pressure ratio across the compressor in the first place has not been corrected, the flow reversal will repeat. At this point, the compressor is in surge. **Preventing surge** To prevent surge from occurring, a minimum flow controller must be implemented. Unfortunately, a minimum flow controller that is configured to protect against the worst case surge conditions will severely limit the turndown of the compressor. For processes that do not use the full capacity of the compressor at all times, a simple minimum flow controller will waste a lot of driver power. Figure 1 shows the surge limit line for a typical centrifugal compressor. As the pressure ratio increases, the surge limit flow increases. For many compressor applications, operating at lower flows also requires less pressure. To take advantage of this flow/pressure relationship, compressor control engineers sought techniques that modified the surge limit of the controller to match the actual surge limit of the compressor under all operating conditions.   ![typical compressor surge limit curve](https://blog.tri-sen.com/hs-fs/hubfs/blog/typical%20compressor%20surge%20limit%20%20curve.png?width=503&name=typical%20compressor%20surge%20limit%20%20curve.png) *Figure 1 - Typical compressor surge limit curve* The earliest implementation of surge control based on surge line prediction that was invariant to gas conditions was introduced by the Foxboro® Company in the mid-sixties. Using a single loop pneumatic controller, the surge line was characterized according to a ΔP vs. h algorithm where ΔP was the differential pressure across the compressor stage and h was the differential pressure across the suction flow meter. This technique pioneered by the Foxboro Company was the industry standard for a couple of decades and as long as the suction pressure was fairly constant, this algorithm and hardware setup was quite effective for compressor protection allowing significantly better turndown than was available from a simple minimum flow controller. **The advent of digital anti-surge systems** In the eighties, with the growing popularity of digital controls, other manufactures introduced digital surge controllers. These controllers used the same algorithm as the one pioneered by the Foxboro Company twenty years earlier. These new digital controls used a sloped line that defined the surge line and characterized ΔPc (compressor differential pressure) to the Y-axis and ΔPo (orifice differential pressure) to the X-axis. As with the earlier Foxboro pneumatic solution, the digital version worked fine as long as the suction pressure did not deviate significantly from the design pressure that was the basis for the surge curve configured in the controller. These digital anti-surge controllers were quite successful in the eighties and nineties because they allowed significantly greater turndown without recycling and were good at  protecting the compressor against surge. A major complaint about these systems was the abruptness with which they would take action to protect the compressor. Since an anti-surge controller is usually not actively controlling (when the flow is well to the right of the surge line), the controller will be wound down until the operating point approaches the setpoint of the surge controller. The tuning parameters that are appropriate for this type of controller tend to be “soft,” which leads to a controller that is slow to react. And since flow signals are noisy, the derivative term of the PID can’t be used. All these issues result in a protective controller that is slow to react and ultimately over-reaction when the controller error is severe. When an upset occurs in a process plant that causes the flow to drop suddenly or the pressure ratio to rise quickly, these digital anti-surge controllers will open the anti-surge valve(s) suddenly in order to prevent surge of the compressor. This protects the compressor, but can create disruptions to the process that are severe enough to trip the process unit. Some processes can experience damage to process equipment during such events. **Surge controls are not enough** While it is important to protect a process compressor from surge, it is also important to protect the process from the shock and disruption of an overly aggressive anti-surge system response. A properly designed and implemented compressor control system can not only protect the compressor, it can protect the process, as well. The key to providing both compressor and process protection is to integrate the compressor performance control with the anti-surge control. For turbine-driven (or variable speed) compressors, this means integrating the speed control setpoint with the anti-surge system. For fixed speed compressors, the positioning of the throttle valves should be managed by the anti-surge system. In general, there are very few process changes (aside from the closing of a block valve) that will drive a compressor into surge at a rate that cannot be gracefully managed by the surge controller. But a quick drop in speed will send the operating point into the surge line very quickly. The most efficient technique for matching the compressor performance to the process demand is to vary the speed. A performance controller that manages the speed without coordination with the anti-surge controller can drive the operating point into the surge controller operating region. An interaction can occur between the performance controller and the anti-surge controller that results in a low frequency oscillation of the process. A sudden process change can drive the operating point into the surge limit, resulting in extreme actions of the surge controller and severe process disruptions. **Protecting the process** Ultimately, the process needs the compressor to always provide a pressure and flow that matches the demand of the process. A properly integrated compressor control system not only integrates the performance control and surge control, it incorporates other control features to soften or eliminate interactions and abrupt controller actions. **Override of surge controller** If the operating point moves to the left of the surge controller setpoint, the normal proportional and integral (PI) action of the controller will start opening the anti-surge valve to correct the error (move the operating point back to the right). But during severe upsets, the tuning constants used in the surge controller might not be aggressive enough to prevent the operating point from crossing the surge limit line. In this case, logic is needed that will override the surge controller and start opening the valve open-loop in response to the magnitude of the controller error. **Adaptive tuning** Aggressive gain is useful for pushing the operating point away from the surge limit line when the operating point gets close the surge limit. But the aggressive gain can cause the controller operation to be nervous when operating at the normal setpoint of the controller. And as the operating point moves away from the control line, a reduced gain will keep a noisy flow signal from unnecessarily provoking controller action. Likewise, a high integral term is useful for pushing the operating point back to the setpoint and helping to drive the anti-surge valve closed more quickly if the process load increases suddenly. **Anti-surge valve slew rate coordination** An anti-surge controller that can act in 100 msec can get way-ahead of an anti-surge valve that takes 3 seconds to open. The valve slew rate needs to be incorporated into the output logic of the controller to assure that the controller does not get too far ahead of the valve. If the controller gets wound up, by the time the valve catches up, it will have opened too far and process will be spoiled. **Performance controller de-coupling** By integrating the performance controller with the anti-surge controller, it is possible to de-couple the performance controller from the speed controller when the operating point approaches the surge controller setpoint. When the operating point reaches the surge controller setpoint (control line), the output of the performance controller can be redirected to open the anti-surge valve in response to a further reduction in the process demand instead of dropping the speed (or closing the throttling valve further). This assures that the surge controller does not get provoked by the performance controller.  This will also give the performance controller the ability to precisely modulate the compressor output from zero to full load. **Surge anticipation** Algorithms that can anticipate surge or that limit the rate at which the operating point is allowed to drop can provide the performance controller a way of limiting the demand that is placed on the surge controller in the first place. To effectively manage the process demand, this type of feature works best when it is integrated with a performance controller decoupling feature. The use of this feature will not only reduce the demand on the surge controller, it will significantly soften the effect of the compressor control system actions on the process. Digital controls have provided intelligent protection systems for the problem of compressor surge that can improve the operating efficiency of the unit. But if the additional features are not incorporated that provide protection to the process, the gains in efficiency will be offset by the process disruptions. And worse yet, if the disruptions cause the operators to distrust the anti-surge system, they are more likely to defeat the system altogether. Integration of the compressor performance controls with the anti-surge controls along with incorporation of good compressor control features will allow the benefits of improved compressor protection to be realized without sacrificing the smooth operation of the process. Topics [ compressor control ](https://blog.tri-sen.com/topic/compressor-control) , [ surge ](https://blog.tri-sen.com/topic/surge) , [ process protection ](https://blog.tri-sen.com/topic/process-protection) ## The Tri-Sen Blog... About once every two weeks, we'll discuss a turbomachinery controls topic that we were either asked about recently, or a topic that we think you might be interested in learning more about.  Topics that are included or that will be: - Compressor Controls - Turbine Controls - Turbine Safety - Relevant API directives - General Industry Happenings ### Subscribe Here! ### Recent Posts ### Posts by Tag - [compressor control (10)](https://blog.tri-sen.com/tag/compressor-control) - [surge (9)](https://blog.tri-sen.com/tag/surge) - [recycle (4)](https://blog.tri-sen.com/tag/recycle) - [process protection (3)](https://blog.tri-sen.com/tag/process-protection) - [quench (3)](https://blog.tri-sen.com/tag/quench) - [refrigeration (3)](https://blog.tri-sen.com/tag/refrigeration) - ["Jim Jacoby (2)](https://blog.tri-sen.com/tag/jim-jacoby) - [PFD average (2)](https://blog.tri-sen.com/tag/pfd-average) - [PID (2)](https://blog.tri-sen.com/tag/pid) - [PID control (2)](https://blog.tri-sen.com/tag/pid-control) - [TMC (2)](https://blog.tri-sen.com/tag/tmc) - [process control (2)](https://blog.tri-sen.com/tag/process-control) - [safety systems (2)](https://blog.tri-sen.com/tag/safety-systems) - [turbine safety (2)](https://blog.tri-sen.com/tag/turbine-safety) - [turbomachinery controls (2)](https://blog.tri-sen.com/tag/turbomachinery-controls) - [API612 (1)](https://blog.tri-sen.com/tag/api612) - [API670 (1)](https://blog.tri-sen.com/tag/api670) - [MTTF (1)](https://blog.tri-sen.com/tag/mttf) - [PTI (1)](https://blog.tri-sen.com/tag/pti) - [TSODS (1)](https://blog.tri-sen.com/tag/tsods) - [axial compressor (1)](https://blog.tri-sen.com/tag/axial-compressor) - [compressor efficiency (1)](https://blog.tri-sen.com/tag/compressor-efficiency) - [hydraulic trip (1)](https://blog.tri-sen.com/tag/hydraulic-trip) - [incipient surge (1)](https://blog.tri-sen.com/tag/incipient-surge) - [overspeed detection (1)](https://blog.tri-sen.com/tag/overspeed-detection) - [polytropic (1)](https://blog.tri-sen.com/tag/polytropic) - [proof test interval (1)](https://blog.tri-sen.com/tag/proof-test-interval) - [stonewall (1)](https://blog.tri-sen.com/tag/stonewall) - [surge curve (1)](https://blog.tri-sen.com/tag/surge-curve) - [surge detection (1)](https://blog.tri-sen.com/tag/surge-detection) - [surge margin (1)](https://blog.tri-sen.com/tag/surge-margin) - [temperature control (1)](https://blog.tri-sen.com/tag/temperature-control) - [trip block (1)](https://blog.tri-sen.com/tag/trip-block) [See all](https://blog.tri-sen.com/compressor-control-surge-margin-0#) ** HELP ###### HeadQuarters - North America  Engineering 109 Magellan Circle Webster, TX 77598 Tel:  +1-832-632-1211 Fax: +1 281-724-1226 ###### Netherlands Verlengde Zuiderloswal 4 Hilversum, NH 1216 BX Tel: [+31 88 011 9292](tel://+31%2088%20011%209292) ###### Singapore 56 Kallang Pudding Road #06-10 HH@Kallang  Singapore 349328 Tel: +65 6381 9188 Fax: +65 6381 9199 ### Search ** ### Stay in Touch - [** ** ](https://www.linkedin.com/company/tri-sen-systems-corporation/) [Privacy Policy](https://tri-sen.com/privacy) | [Terms and Conditions](https://tri-sen.com/terms) Copyright © 2019. 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