Flow Measurement and Instrumentation: Critical Components for Sustainable Processes

Flow Measurement and Instrumentation: Critical Components for Sustainable Processes

This post was written by Gregory M. Gomez, vice president of flow instrumentation for Badger Meter.

Consumers, industry, and society in general benefit from a commitment to sustainability. This concept is not purely about conservation or “being green.” Sustainability is not about using less, but about using the right or necessary amount—reducing waste in the process. The value of sustainability as a business opportunity is balanced between altruistic ideas related to protecting the environment and the very practical economic drivers of maximizing process efficiency and revenue. In fact, research indicates that both business and consumers in the mainstream prioritize the related economic benefits of sustainability and view leaving a better environmental footprint as an important bonus. With process efficiency as the goal, cost savings, increased profits, customer goodwill, and yes, environmental benefits, result.

 

Flow measurement and instrumentation are critical components for making usage visible to manage sustainable processes and operations. The hospitality industry is a great example. It is common to see a card on the bathroom sink that asks us to conserve resources by hanging our towel back on the bar. Intuitively it is clear to us that less laundry is less water used by the hotel, but by how much exactly? It is difficult to manage what is not measured. Without an understanding of the true benefits of making the change, actually significantly decreasing water use is difficult.

Using flowmeters and analytics platforms available today, hotel management can make water use much more visible than a single line item on the water bill. Employing near-real-time data and analytics dashboards, management is able to truly see and manage water use across a variety of aspects of its operations (e.g., guest rooms, laundry, bars/restaurants, golf courses, other irrigation, industrial, process operations). The sign will be much more effective at influencing our behavior when the hotel can associate a specific percentage of water actually saved with hanging up the towel. This also produces a related savings in energy, supplies, and labor hours to the hotel. Plus, there is a public relations benefit to the guest who is interested in conserving water.

Accurate and reliable flow measurement and instrumentation directly lead to more efficient use of our precious, often expensive resources, and in turn, benefit the bottom line. More and more corporations see sustainability as a business opportunity, both in terms of maximizing efficiency and attracting “green-minded” customers.

The concept of sustainability has obvious ties to the efficient use of potable water, and it also relates to virtually any commercial or industrial process, including oil and gas production. The increasing use of water in unconventional production processes, like hydraulic fracturing, competes with the need for drinking water and irrigation for agriculture. For some, water use is also an environmental concern. The measurement and efficient use of water in these operations is important to managing costs and environmental impact.

Investors across any number of industries are also increasingly asking corporations to manage and report on their exposure to resource risks, including water, fossil fuels, and other key raw materials. Many publicly traded companies are including sustainability reporting in their annual reports. The interest of investors highlights that sustainability appeals to a variety of corporate stakeholders, including employees. Increasingly, younger generations want to work for a company that operates as a good corporate citizen, pursuing ideals beyond just maximizing profit. Sustainability can help employers attract and retain employees, helping manage critical human resources, an important component of an effective operation.

Finally, population growth and economic development are leading to a water-energy-food nexus. Each facet is critically important to society; each is facing increasing demands over the next several decades; and each competes with the other for use of finite global resources. Resource scarcity makes it important to long-term business success to use only the necessary amounts of limited resources, like water, petroleum, and other raw materials. The widespread understanding of this fact makes practicing sustainability in commercial and industrial processes a powerful opportunity to not only benefit the bottom line, but future generations as well.

About the Author
Gregory M. Gomez is the VP of flow instrumentation for Badger Meter. He joined Badger Meter in 1984 as an intern with the Inroads program while a student at Marquette University. He held several engineering positions at Badger Meter from 1987 to 2009. In 2009, Gomez became VP of business development, where he was responsible for the execution of five acquisitions between 2010 and 2014. Gomez is or has been a member of the following boards: Discover World, Wisconsin Manufacturer’s Extension Partnership, Inroads: Milwaukee Advisory Board, Great Lake Regional Board, and National Board.

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A version of this article originally was published at InTech magazine.

IoT Is Ready for the Process Industry

IoT Is Ready for the Process Industry

This post was written by Tom Moser, president and CEO of Phillips and Temro Industries.

In today’s competitive business climate, process plants are looking for ways to quickly reduce costs, improve operations, and comply with regulatory requirements. Addressing these challenges with yesterday’s technologies, while practical, often has unsatisfactory results and does not yield the desired competitive advantage. But there is a technology ready for deployment right now that can address many operational challenges, and it is proven in use with more than 5 billion operating hours. This innovation consists of adding wireless sensors to process plants, and then connecting these sensors to internal intranets or to the Internet to create an industrial Internet of Things (IoT) infrastructure. The data from these sensors can then be interpreted and analyzed to help users save energy, improve reliability, and increase throughput. This is not some futuristic vision of the IoT, but is a current reality at many process plants around the globe.

The first step is to identify plant processes or equipment where substantial savings can occur with real-time, continuous information and interpretation. Plant personnel typically have a long wish list of such areas, and suppliers can assist with a study to uncover more. In most cases, countless areas for improvement have been neglected or forgotten because of the high cost, long installation time, and required downtime for wired sensor installations. Now that these negating factors have been eliminated, rapid improvements to plant operations can be made at costs often low enough to be funded by operating budgets.

Installing wireless sensing technologies is quick and low cost, because battery-powered wireless sensors can be brought online in a fraction of the time of a traditional sensor. No wiring is required either for power or for delivery of actionable information to plant personnel. An increasing number of today’s sensors are nonintrusive, further easing installation.

Once this sensing network is established, a combination of smart software and personnel with domain expertise can interpret sensor data. This can be done on site if a plant has the right people, or off site by either corporate engineering or supplier personnel.

For both on-site and off-site analysis, the industrial IoT delivers data to the right personnel without requiring connection to the plant’s real-time control system. This is important, because control system connections require careful vetting and strict procedures, lest plant operation be compromised.

Wireless sensors can be connected directly to plant maintenance management systems or historian databases, and from there to the cloud via secure one-way Ethernet or Internet connections. These clouds can be public, private, or hybrid—in each case providing the required level of data security.

Once the data is received, actionable information can be ascertained and securely delivered to the right people. With this information, plant personnel are empowered to make decisions to immediately improve plant operations.

In some instances, the most pressing need for a process plant is not the bottom line, but rather compliance with health, safety, and environmental regulations. In these instances, the cost of noncompliance can be extremely high, ranging from daily fines to plant shutdowns. As an example, a refinery needed to prevent vapor cloud releases associated with pump failures. It installed wireless, nonintrusive vibration sensors to tell operators which pumps needed service. They now monitor more than 100 pumps at less than the cost of manually checking just a few pumps, and the wireless system has given early warning of three impending pump failures in its first year of operation.

The industrial IoT is here today, being used by hundreds of process plants in thousands of unique applications globally. Wireless sensors have opened up countless new opportunities, and are now able to quickly deliver bottom-line benefits. The question to be asked is this, “What if there was a way to . . . ?”

About the Author
Tom Moser is president and CEO of Phillips and Temro Industries. Previously, Tom worked for 26 years with Emerson Process Management, and he has held several positions, including president of Rosemount, president of Micro Motion, VP of Rosemount Asia Pacific, and VP of Rosemount Europe, Middle East, and Africa. He holds bachelor’s and master’s degrees in mechanical engineering from the University of Minnesota, and an MBA from Duke University.

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A version of this article originally was published at InTech magazine.

New Applications for Pressure Measurement Technologies

New Applications for Pressure Measurement Technologies

This post was written by Wally Baker is a global pressure content marketing manager with Emerson Automation Solutions.

Accurately measuring liquid, gas, and steam pressure is a basic requirement for many industrial processes to operate safely, efficiently, and with optimum quality control. In addition to directly measuring pressure values, pressure measurements can be used to determine or infer flow rates, fluid levels, product density, and other parameters. As a result, many plants rely on pressure-measurement devices to get required field measurements.

 

Some example applications using pressure or differential measurements are:

  • flow rate through a pipe
  • level of fluid in a tank
  • density of a substance
  • interface between two or more liquids in a tank

For example, if a constriction, such as an orifice plate, is placed in a pipe, pressure will drop in a predictable way. By measuring the pressure in the pipe both before and after the orifice plate, the rate of flow through the pipe can be calculated.

This article examines the elements of pressure measurement and looks at some recent advances in the technology for better pressure measurements.

Absolute, gauge, and differential pressure

Pressure-measurement devices can be categorized according to the measured reference pressure. The three reference pressures are:

  • Absolute: Absolute pressure measurements compare measured pressure to a perfect vacuum (0 pounds per square inch, absolute [psia]).
  • Gauge: Gauge pressure measurements compare measured pressure to the pressure of the surrounding atmosphere (approximately 14.7 psia) as a reference pressure. Changes in atmospheric pressure (such as those due to changes in the weather) are taken into account for the output of a gauge transmitter. Gauge devices are often used to measure level in holding tanks that are open to the atmosphere.
  • Differential: A differential pressure measurement uses a second process pressure as a reference pressure. Differential pressure measurements are often used to infer the rate of flow through a pipe by determining the pressure drop that occurs from one point in a system to another, such as the drop that occurs across an orifice plate in a pipe.

Pressure transmitters can be used to measure pressure, flow, level, and density.

Modern transmitter basics

Pressure sensors and transmitters have been used in the process industry for decades, starting in the 1940s with pneumatic transmitters that supplied a pressure signal to pneumatic controllers. In the 1950s, electronic transmitters appeared, which converted the pressure signal to a 4–20 mA signal that could be used by electronic controllers and, later, by computers. HART was added to the 4–20 mA signal in the 1980s, creating the smart pressure transmitter. Fieldbus digital signals came later, and now wireless is available in both types of smart transmitters.

Smart transmitters allow more than just the pressure value to be reported from the field. Now a significant amount of information can be reported from a single pressure transmitter. One of the most important advances of smart transmitters is in the area of diagnostics.

Most smart pressure transmitters include a basic set of diagnostics that notifies the operator when the device is broken or needs to be serviced. Today’s more advanced smart transmitters provide additional diagnostic insights not only into the state of the transmitter, but also into the electrical loop and process itself, issuing proactive alerts so operators can respond immediately and avoid downtime. New advances in diagnostics include:

  • Process monitoring: Pressure transmitters can listen to the background noise of a process and detect deviations from normal operation that could signify plugged impulse lines or more serious issues with the process itself, such as distillation column flooding, flame instability, or pump cavitation.
  • Loop monitoring: Diagnostics can monitor the integrity of the electrical loop that connects a field device to the control room to notify operators of any irregularities such as water in housings and junction boxes, wire corrosion, or unstable power supplies.

Wireless pressure transmitters can be quickly installed anywhere at a very low cost compared to wired counterparts.

Going wireless

Traditional wired pressure transmitters require a supporting infrastructure including wire, cabling, conduit, junction boxes, marshalling cabinets, and I/O to transmit the pressure signal back to the control system. This infrastructure sometimes makes it too difficult or expensive to install pressure transmitters in certain locations.

Installing and implementing wireless devices can take up to 75 percent less time than traditional wired devices with the elimination of wiring and construction labor, and the capital costs associated with wireless technology can be up to 40 percent less. In addition, the insights gained from added pressure monitoring points can help extend the life of assets, creating an even greater return on investment. With a battery-powered wireless pressure transmitter, measurements can be made and transmitted anywhere a pressure transmitter can be installed in a pipe, tank, or steam line for flexibility in a variety of applications.

Improved process connections

The way a pressure transmitter is physically connected to the process can greatly affect the overall accuracy and reliability of the measurement. Simple installations often use impulse tubing that is prone to leaking, plugging, freezing, and other issues to connect sensors. More complex applications, including those on tanks and meter runs, can require multiple components, pipe penetrations, and connections that can increase the amount of impulse tubing required.

Newer pressure transmitters are available in compact assemblies that use fewer components, are simpler to install, and require less ongoing maintenance. These solutions permit close coupling, a best-practice installation procedure that results in more accurate measurements.

In traditional differential pressure (dP) flow applications, for example, integrated flowmeter assemblies eliminate a significant amount of impulse tubing. These flowmeters can replace multiple transmitters that formerly required many mechanical parts and multiple pipe penetrations, which contributed to leaking, plugging, freezing, and inaccurate process measurements.

In tank level applications, an electronic dP remote sensor system can even be used to replace impulse tubing with two pressure transmitters installed on the top and bottom of the vessel

Eliminating impulse piping components greatly reduces maintenance costs and installation time.

and connected electronically. The remote sensors eliminate impulse tubing and associated environmental effects. The system also eliminates the need to heat trace the impulse lines and protect them from freezing.

Heat tracing can add considerable cost and complexity to any process and requires a great deal of maintenance to keep it operational, because applications using heat tracing suffer from frequent failures. These measurement challenges and suboptimal performance can lead to lower throughput, degraded product quality, and cost overruns.

Some dP level transmitters can address these challenges with specialized remote seals to expand the temperature operating range.

The remote seals can operate in hotter or colder temperatures than traditional transmitters can. With this solution, an intermediate diaphragm seal in the transmitter separates two different fill fluids with different optimal operating temperatures.

For example, on a high-temperature application, the high-temperature fill fluid is only used immediately next to the hot process, and a traditional fill fluid is used for the remainder of the

Remote seals and fill fluids allow pressure transmitters to operate in hot or freezing cold temperatures.

connected capillary. The solution is not dependent on heat tracing and produces more reliable, accurate, and faster process measurements, particularly in extreme conditions.

Liquid versus gas pressure

The factors that influence the pressure of a liquid are different from the factors that influence the pressure of a gas. When measuring pressure, it is important to understand the pressure properties of liquids and gases. The hydrostatic pressure exerted by a liquid is influenced by three factors:

  • height of liquid in a column
  • density of the liquid
  • pressure on the surface of the liquid (vapor space)

The pressure at the bottom of a column of liquid increases as the height of the liquid in the column increases. Pressure is affected by the height rather than the volume of a liquid.

Unlike a liquid, a gas exerts equal pressure on all parts of the container in which it is held. Two factors affect the pressure exerted by a gas:

An electronic/digital connection between two pressure transmitters in a dP level system helps eliminate problems with impulse lines, such as temperature effects, clogging, or freezing in the winter.

  • volume of the container in which the gas is held
  • temperature of the gas

The relationship between pressure, temperature, and volume of a gas can be determined by applying the ideal gas law.

Ideal gas law: PV = nRT

Gas pressure is affected by changes in temperature. If the volume of the vessel holding a gas and the amount of gas are unchanged, the pressure exerted by the gas on the vessel walls will change in proportion to changes in the temperature of the gas. Or simply stated, by measuring the pressure and temperature in the tank, you can measure the gas.

Measuring flow

A common use of a pressure measurement is to infer a fluid’s flow rate through a pipe. As a fluid flows through a pipe restriction, the fluid pressure drops. The pressure of the fluid flowing through a pipe is greater on the upstream side of the restriction and lower on the downstream side.

Because hydrostatic pressure is directly proportional to the height of the liquid, differential pressure measurements can be used to report levels.

If pressure is measured before and after the restriction in the pipe (e.g., a flow element such as an orifice plate, venturi tube, flow nozzle, wedge, or annubar), the square root of the pressure drop is proportional to the flow rate of the fluid through the pipe.

Measuring level

The level of a liquid in a tank or vessel can be determined from a pressure measurement by this equation:

height = pressure/liquids specific gravity

Closed tanks or vessels need a dP transmitter to account for the vapor space pressure. Open tanks or vessels need an absolute pressure transmitter or a dP transmitter with the low-pressure side vented to the atmosphere.

Measuring density

Pressure is equal to the height of the column of liquid being measured multiplied by the specific gravity of the liquid. Therefore, if the height of the column is a known constant, as in the case of the distance between two pressure-measurement points on a vessel, the density can be inferred from the pressure reading using the following equation:

Approximately half of all flow measurements are made by inferring the flow rate from a differential pressure measurement.

specific gravity = pressure/height of liquid (level)

Specific gravity values can then be converted to density or mass-per-unit-of-volume units such as grams per cubic centimeter. Density measurements are often used in the brewing industry to determine stages of fermentation.

Even after decades of use in industrial applications, pressure measurement technologies continue to advance. These advancements make traditional measurements easier, and often allow measurements in areas where they were not possible before.

About the Author
Wally Baker is a global pressure content marketing manager with Emerson Automation Solutions. He manages Rosemount’s pressure product launches and works with users on how to better understand their pressure uses and product needs. He has been in the process control industry since 1999 in various roles, including pressure engineering, temperature global marketing, Singapore wireless marketing, and oil and gas business development.ciation.

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A version of this article originally was published at InTech magazine.

PID Tuning for Near and True Integrating Processes

PID Tuning for Near and True Integrating Processes

The following technical discussion is part of an occasional series showcasing the ISA Mentor Program, authored by Greg McMillan, industry consultant, author of numerous process control books, 2010 ISA Life Achievement Award recipient and retired Senior Fellow from Solutia Inc (now Eastman Chemical). Greg will be posting occasional questions and responses from the ISA Mentor Program, with contributions from program participants.

The most important loops on vessels and columns typically have a near or true integrating response. A self-regulating process is classified as near integrating if the process time constant is larger than four times the process dead time. The composition, pH, and temperature response of continuous columns tend to have a time constant to dead time ratio of about 6:1 for changes in the liquid material balance.

The composition, pH, and temperature response of continuous well-mixed vessels tend to have a time constant to dead time ratio of 50:1 or larger. Level and gas pressure have a true integrating response since changes in level or pressure have a negligible effect on the discharge flow notable exceptions occurring for gravity flow for liquid level control and relatively high but non critical pressure drops for gas pressure control. The composition, pH, and temperature of batch columns and batch vessels have a true integrating response in the normal operating range.

near-integrating-process-response

A notable exception is the composition control of a batch reaction when there is no deficiency of any reactant concentration. Here the response is near-integrating with a very large time constant to dead time ratio making differentiation between true and near integrating inconsequential. Reactors with a potentially runaway response are treated as true integrators with the intent being that control action is sufficient to prevent the loop from seeing the acceleration from a runaway response.

The designation of having an integrating response is critical in terms of tuning and recognizing there is a window of allowable controller gains, where too low of a PID gain as well too high of a PID gain will cause excessive oscillations. For a PID gain that is too low, the oscillations tend to be much larger and 10 times slower (e.g., period is 40 dead times for low PID gain and four dead times for high PID gain). For a PID gain greater than the ultimate gain, the oscillations can grow and the loop becomes unstable. For a PID gain that is too low, the oscillations will always decay but the decay rate becomes incredibly slow as the PID gain is decreased. For a runaway reaction, too low of a PID gain is disastrous in that the process can runaway reaching a point of no return.

The Lambda tuning rules switch from a Lambda being the closed loop time constant for a setpoint change for self-regulating processes to Lambda being an arrest time for a load disturbance with the objective of stopping the ramping effect of integrating processes and potential acceleration of runaway processes.

Questions from ISA Mentor Program Participant Hector Torres

  1. How do you calculate Lambda for near-integrating processes? I understand we should follow the Integrating Process rules but I am not clear as of how to determine the desired arrest time. You mention that for maximum unmeasured disturbance rejection a Lambda equal to the dead time is used. Also it is stated that a Lambda equal three dead times minimizes consequences of nonlinearities, inverse response and resonance.
  2. Why should we consider Lambda of one or three times dead time in these rules? To be identified as a near integrating process the time constant should be four times greater than the dead time. Why should we make Lambda a factor of dead time here? I remember it was mentioned that Lambda should be set at three or four times the largest of the dead time or the time constant. Would this apply here? Am I mixing in my mind the rules for self-regulating and integrating processes?
  3. I understand that integrating processes can have an inverse response that is problematic. What could be an example of inverse response? What do you mean by this?
The ISA Mentor Program enables young professionals to access the wisdom and expertise of seasoned ISA members, and offers veteran ISA professionals the chance to share their wisdom and make a difference in someone’s career. Click this link to learn more about how you can join the ISA Mentor Program.

 

Greg McMillan’s Answers

1) In integrating tuning rules, Lambda is the arrest time, which means for a step disturbance or step change in PID output, how long does it take for the PV to halt its excursion and start its return to setpoint. If you multiply the integrating process gain (%PV/sec/%CO) by a the change in controller output (CO%) required to get a ramp rate (%PV/sec) and then Lambda, you have the peak error (maximum excursion in %PV). For level and pressure it is easier to visualize in that the maximum PV excursion (peak error) for a maximum expected change in controller output added to the setpoint must not hit an alarm or trip point. The integrated error (% sec area between the PV and SP on trend chart) is the peak error (%) multiplied by Lambda (sec). Thus, the peak error is proportional to Lambda and integrated error is proportional to Lambda squared where Lambda is the arrest time set relative to the dead time.

2) The ability of a loop to handle changes in gain and dynamics is expressed by the gain margin and phase margin, which are both a function of Lambda relative to dead time. The Lambda tuning rules reduce to tuning rules commonly used for the last six decades if you realize Lambda should always be thought of and set relative to dead time and not a time constant or an integrating process gain (as mistakenly shown in various publications). Also for large time constants you have a near-integrating process and must switch to integrating tuning rules.

In the 5th edition of the Process/Industrial Instruments and Controls Handbook (1999 edition for which I became chief editor), Bialkowski on pages 10.52 and 10.53 shows how the gain margin and phase margin are a function of Lambda varying from one to five dead times. Elsewhere he talks about the concept of near-integrating processes. I think the rule sometimes states of choosing the largest of three times the dead time or three times the time constant are not in tune with advancement in understanding of Lambda always being thought of as a value relative to dead time. While Bialkowski did not say when to switch integrating tuning rules, I estimated that the switch point of when the time constant to dead time ratio was greater than four would result in tuning rules similar to what has been practices for the last six decades. This rule plus realizing that Shinskey essentially was using a Lambda of 0.6 x dead time to give the impressive maximum disturbance rejection results he has in his articles and books. His response is oscillatory. The acknowledged practical limit for a smooth response even if you exactly know the process dynamics and they never change is a Lambda equal to the dead time.

Thinking of Lambda as being three dead times is a good rule for both self-regulating and integrating tuning rules. For self-regulating processes with a time constant to dead time ratio between one and two , there might be some advantage of using three times constants instead of three dead times for PI control but I think the advantage is minimal and is negligible compared to other issues. If you have a nonlinear valve or process, you may need to increase Lambda to be five or six dead times unless you do signal characterization, gain scheduling or adaptive control.

To summarize, for all Lambda tuning rules (self-regulating and integrating), the most aggressive tuning if the process dynamics are fixed and exactly identified (rare case), is a Lambda of 0.6 dead times. This is case is normally only used to show how well Lambda tuning can do compared to other tuning methods (showcase test for gamesmanship). Normally, to deal with unknowns and nonlinearities a Lambda of three dead times is used but may be increased for more uncertain applications and greater changes in dynamics whether due to the process or valve. For bioreactors where the disturbances are extremely slow, rise time is inconsequential for setpoint changes, and process gains can change dramatically from the pre-exponential to the exponential growth phases, Lambda may be as large as 10 dead times.

To better understand different process responses and tuning objectives, watch the three-part ISA Mentor Program webinar on PID options and solutions:

3) Inverse response is where the initial response of the PV is in the opposite direction of the final response.  If a feedforward correction arrives too soon it can cause inverse response. For feedback control, inverse response originates typically occurs when the feed stream throttled that has a temperature less than the operating temperature of the equipment. The classic case of inverse response is boiler drum level. An increase in feed water flow being colder than the boiling water in down comers will cause bubbles to collapse which will cause fluid to go down from the drum into the down comers causing shrink (decrease in drum level). Eventually the increase in feed water is heated enough in the down comers to increase the drum inventory (increase drum level). For a decrease in feed water flow, the bubbles in the down comers increase in number and size pushing fluid up into the drum causing swell (increase in drum level). This shrink and swell is quite common and can be reduced by feed water preheaters. For furnace and reactor temperature an increase in air flow or reactant flow that is colder than furnace or reactor temperature will cause the equipment temperature to decrease until the firing rate and reaction rate generates enough heat to increase the equipment temperature. This is the main reason plus the time constant of the concentration response why reactor temperature should not be controlled by manipulating reactant flow. This is true for liquids and polymers. For gases in fluidized bed reactors, the reaction rate and concentration time constant are so fast, inverse response is imperceptible.

See the ISA book 101 Tips for a Successful Automation Career that grew out of this Mentor Program to gain concise and practical advice. See the InTech magazine January/February 2013 feature article “Enabling new automation engineers” for the candid comments of some of the original program participants. See the May 2015 Control Talk column “How to effectively get engineering knowledge” with the 2014 addition to Mentor Program Keneisha Williams on the challenges faced by young engineers today. Providing discussion and answers besides Greg McMillan and co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) are resources Brain Hrankowsky (consultant engineer at a major pharmaceutical company), Michel Ruel (executive director, engineering practice at BBA Inc.), Leah Ruder (process systems automation group manager at the Midwest Engineering Center of Emerson Process Management) and Nick Sands (ISA Fellow and Manufacturing Technology Fellow at DuPont).

What is the proper back pressure control valve type?

This guest post is authored by Greg McMillan.

In the ISA Automation Week Mentor Program, I am providing guidance for extremely talented individuals from Argentina, Brazil, Malaysia, Mexico, Saudi Arabia, and the USA. We will be sharing a question and the answers each week. If you would like to provide additional answers, please send them to Susan Colwell at ISA. The fourteenth question is from Muhammad Al-Khalifah in Saudi Arabia:

“What is the proper back pressure control valve type used for high turndown ratio at the same given pressure drop (minimum & maximum flow conditions)? What will be the effect on valve seat if case valve is operating most of the time at minimum flow condition?”

Answers from Hunter Vegas (Avid Solutions, Inc.):
I have never seen a back pressure control valve that had the same upstream and downstream conditions regardless of flow…but I suppose it could occur.  The short answer to your question would be:

  • Use a sliding stem control valve
  • Use an equal percentage trim to get the maximum turndown
  • Use a digital positioner with an oversized actuator to get the best control through the range.
  • If your pressure drop is very high, you may need anti-cavitation and hardened trim to handle the flashing and cavitation conditions.
  • If you need a HUGE control range, you might use two control valves – one about 10 times bigger than the other in parallel. Use pressure to control the small valve and use a position PID gap gain controller that will move the big valve when the small valve is > 75% open or < 25% open.

If the valve is routinely operating nearly closed at high pressure drops then you’ll likely encounter cavitation/flashing and the seat and valve body will erode. If continuous operation will be required under these conditions, you’ll want to consult your valve manufacturer and make sure they understand the situation. They may go with a specialized valve body style or particularly hardened trim to provide the long life you need or you’ll be replacing control valves often.

Thoughts by Greg McMillan (CDI Process & Industrial):
Equations and Figures noted below are in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industry.

For back pressure and feed valves, I would use an equal percentage characteristic with a ratio of valve pressure drop to system pressure drop greater than 0.25 so the installed characteristic is not too flat near the closed and near the open position (see figures 7-47a-c and equations 7-19a-d on pages 407-411 for the effect of this ratio on rangeability). For the recycle valve discussed in the post last week, a linear characteristic might be better since the pressure drop across the valve is fixed and the ratio of valve drop to system drop approaches 1.0 yielding an installed characteristic that is close to the inherent characteristic and thus a nearly constant valve gain (constant slope of installed valve characteristic).

For the back pressure valve to minimize stick-slip and backlash and thus oscillations near the seat, I would use a rugged sliding stem double port globe valve as shown in Figure 7-4 on page 327 (e.g., Fisher E body) with a sensitive actuator (e.g., diaphragm) sized for at least twice the maximum expected pressure drop at shutoff, and a smart digital positioner. The valve should be installed in the flow-to-open direction to prevent the bathtub stopper effect at low flow. For the feed valves, I suggest a single port globe valve as shown in Figure 7-3 on page 326 with the same actuator and positioner design considerations. The trim is easier to replace in single port valves and the shutoff is tighter, which could be important for isolating downstream users. A new high pressure design diaphragm actuator as shown in Figure 7-15c has been developed providing much greater thrust/torque. For very high pressure drops, you may need to go to a piston actuator. For large valves you may need to go to a v-notch ball valve as shown in Figure 7-9 or contoured butterfly valve as shown in Figure 7-11. You may need to add a volume booster as shown in Figures 7-29 and 7-30 on the positioner outlet(s) of the pressure control valve if the actuator volume is so big the
pre-stroke time delay is greater than about 0.1 seconds.

To minimize interaction between a back pressure loop and a flow loop in series, the pressure drop should be much greater across the flow control valve. The pressure loop should be tuned for fast maximum disturbance rejection. The pressure controller module execution time (0.1 to 0.2 seconds) should be 5 times faster than the flow controller execution time (0.5 to 1.0 seconds). To save energy, a valve position controller (VPC) could be used to slowly reduce the pump pressure to keep the largest of the flow control valve positions at a maximum in the suggested throttle range of the valve. A VPC can also be used to for the simultaneous throttling of a big and small valve in parallel. The VPC process variable is small valve position, the VPC setpoint is mid throttle position (e.g., 50%), and the VPC output is the big valve signal. For more details on VPC use, see my November article “Don’t Over Look PID for APC” in Control magazine.

A field pressure regulator is faster than a pressure control loop but you lose visibility and adjustability of pressure. A variable frequency drive (VFD) would be faster than a control valve if rate limiting is not excessively used in the setup of the speed control but rangeability is a problem if the system frictional pressure drop is low compared to the static head at minimum flow. The Chemical Processing article by Cecil L. Smith “Watch out for variable speed pumping” provides considerable insight as to the effect of static head on VFD rangeability. The deadband and slip in a VFD is small if properly designed and setup (deadband is often introduced in the drive setup to eliminate reaction to noise but this deadband may be too large causing poor control).

 

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