Emerson Process Experts

Safety Instrumented System Diagnostics to Prevent, Not Initiate Safety Trips

Jim.Cahill@emerson.com (Jim Cahill) — Thu, 24 May 2012 13:53:07 +0000

Emerson’s Mike Boudreaux pointed me to a great blog post by exida’s Principal Partner, Dr. William Goble. The post, I Don’t Want No D*** Diagnostics! describes how automatic diagnostics in a safety-rated transmitter led to a false trip, much to the displeasure of the impacted control engineer and plant staff.

Dr. Goble describes the incident:

The transmitter diagnostics were annunciated by sending the analog current out of range. In this case, the current went to 3.6 milliamps. The problem was that the logic solver was configured for a low trip and did not recognize out of range signals as a diagnostic alarm. It interpreted the signal as a trip condition. The safety instrumented function (SIF) worked perfectly. It did the job for which it was programmed.

He asks:

…how could anyone not want a product that can tell them when it fails? The answer: When the system is programmed to trip on a diagnostic and trips are NOT WANTED!

He also points out that:

…there are those who demand that every diagnostic alarm cause a trip. “It is the safest thing to do,” I have heard. But is it?

Dr. Goble concludes:

Please be careful when you design a SIF. Take full advantage of new product features like diagnostics that significantly improve safety. But be careful that you understand how the products work and design the parts of the system to play well with other.

In an earlier post, Using HART Diagnostics in Safety Instrumented Systems, we discussed how these diagnostics can be used to provide early warning before a safety trip is initiated.

Mike provided his thoughts in a comment on Dr. Goble’s post [hyperlinks added]:

Regarding your point about tripping on diagnostic detection of a failed device, IEC 61511-1, section 11.3 provides guidance for providing “continued safe operation of the process while the faulty part is repaired.” Why shut down the process if you can detect a failure and safely repair it online? False trips can actually negatively impact safety and tripping the process very often is not “the safest thing to do” because you go from normal operation to transient operating states.

According to a study referenced in a Chemical Processing article, Tame Your Transient Operations, “A typical refining or petrochemical facility will spend less than 10% of its time in transient operations — yet 50+% of process safety incidents occur during these operations. Deficiencies in procedures and employee training often are cited as root causes of these incidents. The increased reliability and extended turnaround intervals of plants result in less familiarity with tasks outside of normal operations. So, while it’s critically important to follow procedures during transient operations, a high percentage of procedural violations are found to occur during them.”

Are you using diagnostics in your safety instrumented functions and if so, are they configured to trip or inform?

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Lambda Tuning-Yeah or Neah?

Jim.Cahill@emerson.com (Jim Cahill) — Wed, 23 May 2012 13:30:42 +0000

I saw a recent ControlGlobal.com article by process control legend, Greg Shinskey, The Case Against Lambda Tuning. He writes:

In recent years, this practice of emulating model-based response using PID controllers has been simplified with the introduction of “Lambda” (λ) tuning, wherein λ represents the closed-loop time constant of the loop’s response to setpoint changes. Proponents suggest values for λ ranging from one to three times the dominant process lag. With λ = 1, setpoint response would be essentially the same as stepping the controller output in manual; higher values will only make it slower. When controlling flow, whose response time may be 5 seconds or thereabouts, this approach is reasonable.

I have addressed the use of Lambda tuning in several posts on this blog. I asked another process control legend, Control Talk Blog‘s Greg McMillan, for his thoughts on this article.

Lambda tuning allows you to specify a closed loop time constant (Lambda) for self-regulating processes and an arrest time for integrating processes. While Lambda may have originally started as 1 to 3 times the dominant process lag for self-regulating processes, Lambda today can be as small as one percent of this lag for well-mixed reactors. Also, the Lambda tuning rules have been considerably improved compared to the rules documented in Advanced Control Unleashed.

There are many uses of Lambda. The Lambda specification helps provide a consistent model for loops whose remote cascade setpoint is coming from model predictive control (MPC). The Lambda specification helps in the coordination of loops, particularly important for blend control. The Lambda specification helps to decrease interactions by reducing the transfer of variability from the process variable to the manipulated variability for the least important loop or by making the closed loop response of one loop 4x faster than the other loop.

For cascade control, the Lambda specification can make sure the secondary loop is 4x faster than the primary loop (cascade rule). Thus, Lambda is useful for achieving various process objectives beyond the view of just best disturbance rejection of a single loop.

For bioreactors, temperature and pH setpoint overshoot is critical due to extreme sensitivity of cell culture kinetics. Time to reach setpoint is less important and disturbance rejection is often a non-issue because biological disturbances are so slow. The batch time of 10 to 14 days give you an indication of the time frame.

For control loops on self-regulating processes where the process time constant is much larger than the process deadtime, setting the reset time equal to the process time constant by Lambda tuning rather than proportional to the process deadtime causes the reset time for Lambda tuning to be significantly larger. This slows the return to setpoint for unmeasured disturbances increasing the integrated error.

If you treat these large time constant processes as “near-integrators” since they look like a ramp in the frame for closed loop correction, the revised Lambda tuning equations for integrating processes provides a vastly improved reset time setting that is coordinated with the controller gain.

If you set Lambda equal to the total loop deadtime, the PID gain from the revised Lambda tuning rules for integrating and self-regulating processes is comparable to the controller gain used for maximum disturbance rejection. In fact, I have shown that all major tuning methods converge to the same expression for PID gain when the objective is maximum disturbance rejection.

The peak error will largely be the same since for large time constant and integrating processes the peak error mostly depends upon the controller gain. Finally, by using this higher PID gain with the revised integrator Lambda tuning rules, you end up with a much smaller reset time comparable to what is used for maximum disturbance rejection.

For self-regulating processes where the process time constant is comparable or less than the total loop deadtime, Lambda tuning rules where the reset time is set equal to the process time constant will provide a reset time that is smaller than what is suggested by disturbance rejection tuning. However, the controller gain being proportional to the reset time also decreases providing stability. Lambdas larger than the deadtime are used to prevent overly aggressive control.

Part of what goes on with tuning studies is gamesmanship where the focus is on metrics for step disturbances without significant consideration of disturbance lags, controller robustness, and application difficulties and objectives. The tuning settings for minimum integrated and peak error are rarely used. Most loops could benefit from better tuning but nonlinearities and unknowns cause one to be cautious. The setting of Lambda to meet application requirements has proven to be effective and flexible.

The main constraint is Lambda must not be less than the total loop deadtime or the response becomes oscillatory. The use of a Lambda factor (factor of process time constant) rather than Lambda (closed loop time constant) can confuse the issue for the wide variety of unit operations encountered in the process industry.

For example, to have a Lambda approaching the loop deadtime for a reactor temperature control loop, the Lambda factor might range from 0.01 to 0.05. For people accustomed to Lambda factors of 1 to 5, this two order of magnitude decrease in Lambda factor can seem strange and extreme.

The use of dynamic reset limiting (external-reset feedback) and velocity limits or filters on setpoints may accomplish many of the objectives achieved by Lambda tuning enabling the use of faster tuning designed for unmeasured disturbance rejection. Also, adaptive reset can help prevent overshoot and undershoot. These are my areas of investigation.

Do you have thoughts to share on the use of Lambda tuning for your PID control loops?

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Expanding Level Measurement Technologies and Applications

Jim.Cahill@emerson.com (Jim Cahill) — Tue, 22 May 2012 20:08:18 +0000

Level measurement plays a large role in most process manufacturing facilities. It’s a subject we address here on the blog from time to time in the level category. I mention this in case you didn’t see the recent news—Emerson acquires ISE Magtech. ISE Magtech joins the Rosemount Measurement portion of the business and the press release highlights the importance for process manufacturers in:

…enabling Emerson Process Management to strengthen its level measurement solutions in the oil and gas, refining, chemical, and power generation industries…

ISE Magtech designs and manufactures level gages and associated equipment for use in industrial applications. This acquisition will allow Emerson Process Management to leverage the strength of its existing portfolio of level measurement products and provide complete level measurement solutions across the process industry.

I asked some of my friends who manage the Rosemount brand for more background that I could share with you. ISE Magtech provides a range of level measurement products including magnetic level gauges, magnetostrictive level transmitters, reed chain analog level transmitters, radar solutions, and more.

To highlight one application, a natural gas pipeline company uses magnetic level gauges (MLG) for increased safety, fugitive emission monitoring, and reduction in maintenance costs associated with conventional sight-glass level indication for their gas plants. The MLG technology provides a much safer design than conventional level indication, releases fewer fugitive emissions, and requires very low levels of maintenance. The transmitters and switches are also non-invasive to the process. This also means that the process does not have to be shutdown to be installed.

The pipeline company noted that the conventional site glass gauges required spare parts and a schedule of general maintenance, both which were eliminated with the switch to the MLG technology. The engineering team also noted that the transmitters came pre-calibrated from the factory, which streamlined the installation and commissioning process. Over the lifecycle of operation, the engineers shared that MLG technology is one of their most reliable standards for level measurement.

For those unfamiliar with this technology, here’s the principle of operation:

ISE-MagTech Magnetic Level Indicators
Inside the indicator tube is a lightweight magnetized indicator or series of metallic flags. The indicator is magnetically coupled to the float and moves up and down with the liquid level. The indicator allows the operator to read the level from more than 100 feet away. The only moving parts are the float and the indicator.

The float chamber is basically a column of 2-1/2 inch pipe with process connections to match those of the storage tank, reactor, or other equipment where level is to be measured. These connections may be side couplings or flanges, or top and bottom flanges.

The magnetic float moves up and down inside the chamber as the process level changes. Float type is determined by the process fluid, light enough to maintain buoyancy and have properties that allow it to withstand the temperature and pressure combination.

It’s great to have the ISE Magtech team join the Emerson team to provide a broader set of level measurement technologies to process manufacturers across many process industries and world areas. You can find out more in the Rosemount area of the website including a Live Chat capability directly with the team.

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China-Mining Resource Consumer to Investor

Jim.Cahill@emerson.com (Jim Cahill) — Mon, 21 May 2012 19:41:27 +0000

Emerson’s Juan Carlos Bravo of the metals and mining industry team describes the changes occurring in Latin America through Chinese investments.

In today’s world, we are used to seeing China as the big consumer of global raw materials, with the demand helping to set the prices in commodities such as iron ore, gold, copper, nickel and other raw materials needed for continued industrialization. But few of us are used to seeing China as a provider of big mining equipment or a big capital investor in mining projects.

While attending EXPOMIN in Santiago, Chile, I saw how large the China presence was—95 different equipment suppliers and a half-day session to promote Chinese investments in mining in Latin America. Here I learned that Asian companies continue to lead the top 500 machinery rankings, and an increasing number of Chinese manufacturers are emerging as the global leaders in their respective segments.
From EXPOMIN: www.expomin.cl/marketing/pdf/2012/presentacion_javier_cunat.pdf

From EXPOMIN: www.expomin.cl/marketing/pdf/2012/presentacion_javier_cunat.pdf

With the allure of lowering procurement costs, I learned that large companies like CODELCO are procuring most of it tiers and refractory brick from Chinese suppliers and looking to expand it to other products made in that country. In addition, Chinese companies are leveraging their relationships with their suppliers in order to exchange copper for equipment.

China is now moving beyond just supplying equipment to establishing mine ownership stakes. I learned that Chinese capital is behind the investment in two mines in Ecuador, two mines in Peru, and have planned a lot more investments in key sectors like power and manufacturing.
From EXPOMIN: www.expomin.cl/marketing/pdf/2012/presentacion_javier_cunat.pdf

I believe we will see more Chinese companies entering the mining equipment market space since they have proven to be successful in developing technology for several other industries. By offering cost benefits when procuring equipment when building a plant, helps the project’s return on investment. But I also believe Chinese companies will face similar challenges when entering the mining industry—energy prices, scarce water, environmental restrictions, relationship with local communities, and lack of local, qualified workers.

There is no doubt that a new competitive landscape is unfolding and it is apparent that China has an aggressive investment plan for the Latin-American and African markets. The industry will continue to adjust to the changes introduced by these plans as they continue to impact suppliers and mining companies.

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Chilean Copper Producer Need for Improved Raw Material Flow Measurement

Jim.Cahill@emerson.com (Jim Cahill) — Fri, 18 May 2012 06:33:07 +0000

Emerson’s Michael Pearson, part of the metals and mining industry team from a Rosemount measurement perspective, shares a story of a Chilean copper producer. This producer needed a way to improve monitoring the flow of raw material inputs in their production process.

With copper prices fluctuating regularly, many downstream metal processing facilities are faced with a growing number of challenges. Pressure to increase throughput and reduce costs is growing, and many plants lack the measurement infrastructure needed to effectively achieve these demands.

In many cases, measurement points were never engineered into the original plant. In other cases, measurements have been added, but without real-time monitoring or any effective way to get the measurements back to the control room. These non-existent and “stranded” measurements can often cost companies thousands of dollars in non-optimized processes and wasted resources.

Advances in wireless technology have enabled many operations to gain the additional insight needed to track their resources better. The best part is that use of wireless drastically cuts. In a previous post, I mentioned the story of Barrick Zaldivar, who added wireless flow measurements to their leaching pads to improve operational performance and business results. Now, I’d like to tell you about another producer who is incorporating wireless flow technology.

This Chilean copper producer operates a facility, which produces over 300,000 tons of cathode copper and 200,000 tons of precious metals, in addition to copper concentrate and acid. In an effort to reduce raw inputs to their processes they desired to monitor flow of a variety of resources, namely, diesel, air, oxygen, nitrogen, natural gas, water and purified water.

The company had many challenges in these flow measurements. Either the measurement points did not exist or they had power in place but no way to integrate the measurement data into their control system. To overcome these issues, the company worked with the Emerson team on ways to incorporate wireless measurement devices. The copper producer’s project team worked with the Emerson wireless application team to incorporate these IEC 62591 WirelessHART devices:

By incorporating these additional measurements, the copper producer now has a real-time view into the use of the raw material inputs. This view provides their engineers an opportunity to optimize their use by reducing their consumption. This helps to increase the efficiency of the overall production process.

Now that wireless technology has proven itself effective and positively impacting efficiency, I think we will see more and more mining companies looking to this technology to help optimize operations and solve real problems.
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