The following discussion is part of an occasional series, “Ask the Automation Pros,” authored by Greg McMillan, industry consultant, author of numerous process control books and 2010 ISA Life Achievement Award recipient. Program administrators will collect submitted questions and solicit responses from automation professionals. Past Q&A videos are available on the ISA YouTube channel; you can view the playlist here. You can read posts from this series here.
What can be done to improve the accuracy of the totalized liquid mass charge to a unit operation considering material potentially left in pipeline, sensor accuracy and totalization method? What are considerations about using the totalizer within the flow transmitter versus performing the calculation externally (e.g., in the PLC/DCS)?
Assuming the liquid is pumped from a storage tank or receiver, install the meter in a recirculation line so it is always full and reading. Piping to the vessel should be drained or purged before charge piping arrangement to permit draining at charge completion. The meter is either in recirculation or feed line, so the exact charge is made.
For example, the mass flow meter is very close to pipeline tee with on-off valves for vertical entry to destination vessel and return storage. The pipeline volume is extremely small between mass flow meter and valve. There ideally is a dribble flow when total approaches setpoint. The charging sequence is:
If meter is a pulse signal, turbine, PD, mass, etc., it is best to totalize on the pulse reading rather than have several transducers, convert to analog, then totalize. Each of these adds inaccuracy.
The short answer to the original question is “it depends.” It depends on the piping, valve location, basic accuracy requirements and several other things which could be very unique to the needs and situation. Bob Heider’s recommendation is the ideal; however, in the world of brownfield, multi-product and multi-recipe process production facilities, it is rarely available without spending significant money. Implementing Bob’s ideal solution becomes tricky on glass lined tanks where nozzle quantity and location are limited, and where it is common for each nozzle to serve as the inlet for multiple materials. The result is that you end up with a tree of valves connected some/most inlet nozzles. As a result space limitations sometimes get in the way of piping modifications. Dead-leg concerns also come into play if the valve tree wasn’t designed well from the start.
My guess is that the use of empirically determined pre-acts (i.e., measure the amount of material that falls into the tank after the valve is commanded to close — maybe that means collecting it in a bucket and weighing the bucket) is the most common way to deal with that problem. For a variety of reasons, the pre-act amount can change over time, so it should be re-measured/re-tested on some period.
The other half of the problem is related to whether the piping between the flow meter and the tank is full or empty at the start of the transfer, and whether you can and do consistently leave it that way at the end of the transfer. Bob’s pre-transfer re-circulation idea addresses that because it ensures the line is full at the start of a transfer, but again, it is difficult to get that in a brownfield plant unless the accuracy it provides pays for the extra cost of the valving and piping changes plus the associated engineering. In my experience, most of the time it doesn’t, but there are cases where it does.
Sometimes head/charge tanks on scales are a good alternative to mass flow meters for measuring/regulating charge amounts in batch processes.
This is a question that gets asked on every capital project — some operation needs a very exact charge of something and it is not easy.
I always say start with the instrument. This is not the time to go cheap. Reaction chemistry happens in moles — which are directly related to MASS, not volume. Get yourself the best mass meter from the start. Don't wait two years while you try to even up the chair legs on a buffer solution using volumetric meters.
While talking about the instrument, ask yourself and your process engineers, "does this add have to go in as quickly as you have it on the PFD?" Does the buffer solution need to be made up in an hour when the reaction mass that goes into the buffer solution 12 hours to get through the previous step? Worrying about accuracy and pipe pounds is much easier if your flow goes in slowly and that little pipe can't hold that much material.
Now let's talk piping. Your mass meter needs to be kept full so you are not measuring air at the start of the next batch. Put your tight shutoff valve after the flow meter (and as close as you can put it without causing problems with the flow meter reading). Then aim your pipe vertically into your reactor so everything left in the line dribbles into the reactor. Still no rocket science here, but it does get done wrong routinely.
You can guess the pipe pounds at the startup of the process and then have a routine in the code that adjusts that number as you run more batches. Oh yeah, don't skimp on what you pay your programmers. The code is complex, and sometime in the future, someone is going to need to follow that code at 2 a.m. on a Saturday morning. It needs to work AND be well structured.
Have a backup plan if things are really critical. While my employer for 30 out of 45 years of my career did not believe in day tanks or drop tanks, lots of people use them. Early on, I spent four years with a design firm around the time design firms figured out the words turnkey design build was like owning a money press. I worked on a liquid detergent facility. The two most expensive ingredients in the detergent were dye and perfume. Worse, if you missed the perfume setpoint you not only wasted good money, but you also made a detergent that would make your eyes water and your entire house smell. This measurement had to be "dead-on balls accurate." (Thank you, Mona Lisa Vito from the movie "My Cousin Vinny" for my favorite industry term for accuracy.) They had a series of small drop tanks for all this low-volume expensive stuff. They used a mass flow meter with all the things I described above to fill the drop tank. As a double check, they had load cells on the drop tank and the mix tank below (three legs, never four) as a double check on the drop. If the weight on the drop tank did not match the mass of the material added to the tank, it did not get dropped into the mix tank until the issue was reconciled and resolved.
My final comment will come from the late, great Dr. Charles Wicks, head of the department of chemical engineering at Oregon State University and my first mentor in this great field. When you find your solution, ask yourself, "does this look reasonable?" If the answer is no, challenge all your assumptions, all that has been given you. You may not win every challenge (if we did, there would be no war stories), but at least you have given it everything you can to get a working solution.
Much of this echoes what others have said. There are a good number of first principals that apply to getting the additions accurate enough. It should go without saying, but defining what range of addition volumes is needed and the tolerance of the additions comes first. Then the mechanical design to reduce hold up, back flow and supply pressure variability or at least its influence. Flow transmitters need expected error that is less than the required tolerances with room for the other sources of error and loss. Shut-off devices need to be designed to have consistent shut-off performance. For example, the vent system to relieve pressure applied to a fail closed valve needs to be thoughtful if it is not as simple as venting to the room. The resistance of the downstream transfer lines needs to be high enough to have minimal effect on supply header pressure. Pre-act values need to be trialed several times. They can be auto-tuned fairly easily in many cases using a exponentially weighted moving average algorithm.
While headers kept full and transfer lines that are short and drain completely under just gravity are preferred, that isn’t always possible. Whether or not blowdowns or flushes are used to get the remaining material, it will be necessary to determine during startup how much gets left behind. The control sequencing should have this as a parameter that can be adjusted over time or by product as necessary instead of just including in the target addition so that totalized value truly indicates what was totalized — estimated actual addition volume should be a separate value reported or historized. Hold up volumes can be calculated from engineering data, but they need to be verified through physical testing.
When a transfer line needs to be drained after each addition and then flooded at the start of the next addition sequence, you may need to live with the loss of material. A mistake designers sometimes make is having the flow transmitter in the supply header so it can be shared for many users and planning to flood the transfer line from the header to the block valve at the tank by flowing through a third valve to let the gas vent out. I’m not aware of a practical way to do this and retain the material that goes past this third valve unless it can make it back to the supply system or header — which is typically too far away for this to be practical unless its turned into a distribution loop. Attempts to let the material drain back into the transfer line require some ability to know the precise volume of what went past the valve — or you need to somehow make it a very consistent amount every time and measure it during startup so you know it’s the same offset every time. If the transmitter needs to be near the tank, then it needs to be near the tank and not shared.
For tight tolerance applications or often for applications with very small volumes other devices are needed. Pumps with a fixed volume per stroke or discrete valves with a fixed volume per cycle may be needed. Some valves are available that have separate “open” and “fixed volume” actuators so high flow can be used to get close to target and then small amounts can be added until the target is reached. In some cases, it may be better to do additions by weight of the receiving unit or container instead of totalized flow. The rate of approach may be used to dynamically adjust the pre-act on each addition depending on how noisy the measurement is and how small a filter makes it workable. While weight measurements come with their own issues (don’t lean on the tank), they do tell you what actually made it into the container and stayed there — a flow transmitter tells you what went by the transmitter and doesn’t account for leaks or pipe flanges that didn’t get connected after a plant turnaround. Weight can provide a useful warning that something may not be right with a batch if they disagree with the flow totalizer.
Brownfield applications can certainly be challenging. Ideally, there is a way to use cost of failed batches and production delays due to manual intervention to justify extra piping, better devices, etc.
Using the totalizers in the instruments would be great. It would eliminate some latency and bypass some additional sources of error such as D/A and A/D converters and repeatability of IO cards. It could literally use every value the sensor/transmitter sees. The biggest barrier has been the limited options to trigger stop, start and the reset of the totalizer from control logic and the HMI. This topic comes up very often on certain automation forums. Depending on the application and what the device offers, there could be issues with handling of uncertain or bad values and seeing a diagnostic status for the total in the transmitter.
I am onboard with the idea of using a recirculation line and fast on-off valve very close to the mass flow meter and the injection point and a dribble flow rate near endpoint. There could be some prediction of reaching endpoint based on valve stroking time and some correction based on volume between valve and mass flow meter. If the destination vessel has a recirculation line, the feed could be injected into the vessel recirculation line which would promote mixing of feed and eliminates any deed holdup in downcomers. I do this for pH control since the very low reagent flow rate creates exceptional problems with downcomers. I greatly favor Coriolis flow meters because they are the only true mass flow measurement, do not require straight runs, an accuracy in percent of reading instead of percent of span, and have an accuracy and rangeability an order of magnitude better than differential head and vortex meters. They also offer a highly accurate density measurement that could be used as an inferential composition measurement. My next best choice are magmeters, which also have minimal straight run requirements and an accuracy in percent of reading with an accuracy and rangeability almost as good (e.g., half) as a Coriolis meter assuming constant concentration and operating conditions.
I’ve had a couple of interesting experiences with that in a few projects — some related to pipeline leak detection and others based on fabrication of manufactured products.
My library grew somewhat while thinking about the insecurity of what was measured vs. what conventional technology might be able to see and keep track of. Depending on the involved fluids the conditions affecting their state, and commonly available instrumentation, it can get challenging and require some application-specific thought.
A key to dealing with a lot of the inherent difficulties came from Coriolis meters and related techniques in which the mass flow rate at the appropriate time, place and conditions could be determined in real time. The basic “Coriolis” concept could also be used to monitor the condition of the in-pipe flowing manufactured product to assess the composition of the results in a way that provided a feedback parameter for a control system. This might derive from measuring Coriolis torque at a piping fixture, such as an elbow.
Deciding exactly how to approach such applications benefits from applying some “operations research” thinking to the specifics of the process and considering the benefits from knowing changes in mass flow rate.
Bob Heider has over 50 years as a process control engineer with an emphasis on the design of advanced process controls and process development. He spent 33 years with Monsanto in various plant and corporate engineering roles and worked with Greg on first PROVOX system installation.
Bob has 16 years as an adjunct professor at Washington University's department of chemical engineering.
Bob also worked for five years at Confluence Solar, providing control expertise to support the company mission to develop premium quality single crystal silicon substrates for solar applications. Presently, he is an independent engineering consultant for various confidential clients. He is a Fellow of the International Society of Automation.
Mike LaRocca is a chemical engineer who has spent 45+ enjoyable years working at manufacturing sites in the industrial chemical and pharmaceutical industry. Early in his career he found he was drawn to and had a knack for process control and automation technology, and process troubleshooting. He has held roles in project engineering, manufacturing, engineering technical services, and E&I maintenance. Mike is an ISA St. Louis Section board member focused on student outreach and membership.
Lucinda Weaver has worked on capital projects for over 45 years. She started as a programmer doing batch control on the Fisher DC2 and the last 10 years of her career she has been a project technical lead supporting of instruments, controls and everything else with a wire attached to it. Projects have been mostly pharmaceutical all over the US, Maritime Canada and some in Ireland. Retirement comes at the end of 2025 unless someone mentions the words "greenfield facility" and "Dublin, Ireland" in the same sentence; then she will reconsider. Lucinda is a proud Oregon State Chemical Engineer.
Brian Hrankowsky is a senior advisor of engineering with 23+ years process control experience in the pharmaceutical and animal health industries. Brian has experience in large and small molecule synthesis and purification, continuous utility, discrete assembly and packaging automation with various DCS, PLC, vision and single loop control platforms.
Gregory K. McMillan retired as a Senior Fellow from Solutia Inc. in 2002 and retired as a senior principal software engineer in Emerson Process Systems and Solutions simulation R&D in 2023. Greg is an ISA Fellow and the author of more than 200 articles and papers, 100 Q&A posts, 80 blogs, 200 columns and 20 books. He was one of the first inductees into the Control Global Process Automation Hall of Fame in 2001, and received the ISA Lifetime Achievement Award in 2010, ISA Mentor Award in 2020 and ISA Standards Achievement Award in 2023. His LinkedIn profile is: https://www.linkedin.com/in/greg-mcmillan-5b256514/
Ed Farmer completed a BSEE and a Physics Master degree at California State University - Chico. He retired in 2018 after 50 years of electrical and control systems engineering. Much of his work involved oil industry automation projects around the world and application of the LeakNet pipeline leak detection and location system he patented. His publications include three ISA books, short courses, numerous periodical articles and blogs. He is an ISA Fellow and Mentor.