The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #14, and was written by Hunter.

To master engineering design, you must master the art of trade-offs. An engineer is constantly balancing one criterion against another, gaining something here but giving up something else there. There will often be several factors to consider, all of which may counter or offset each other to varying degrees. Picking the right combination of features to suit the application can be challenging.

Specifying a capillary seal assembly is a perfect example of this.

Concept: Choosing the correct capillary seals for a particular transmitter installation seems like a minor thing, until you begin to understand the multitude of design decisions involved. Many an engineer has failed to grasp this and has gone through several meters until they got one that worked.

Details: Capillary seals are used to isolate a pressure or differential pressure transmitter from the process by transferring pressure from the process to a remote mounted transmitter. Some processes are prone to plugging of the impulse line, and the installation of a 2” or 3” seal in a full size line is much less likely to result in plugging than a typical ½” piece of tubing would be. In addition, sanitary applications use a lot of capillary seals because they are easier to clean. A capillary seal consists of the seal itself (which is a flexible diaphragm), a piece of capillary tubing, and a standard pressure or DP transmitter, all carefully filled with a hydraulic fluid that has had all vapor removed. When pressure is applied to the seal, it is transmitted via the hydraulic fluid to the transmitter. Differential pressure transmitters will often, (but not always), have two seals, one on each side.

Here is a brief list of items that can cause an engineer serious problems:

• If two seals are installed on a differential transmitter, make the seals the same size and the capillaries the same length if possible. (This may require coiling up the unused length of capillary on one side.) The problem is that all hydraulic fluids expand with temperature, and the overall expansion is a function of volume. If the seals are the same size and the capillaries are the same length, the hydraulic expansion from one side will cancel the other, and the overall zero shift will be minimized. If one leg is longer or one seal is bigger, the hydraulic expansion will be greater on that side, and the zero shift can be significant.

• Seals with a bigger diaphragm are more sensitive and can measure lower pressures. However, bigger diaphragm seals have a larger volume and tend to show a larger zero shift due to process temperature changes. Smaller diaphragm seals have less volume and tend to have reduced temperature-related zero shift problems, but they are not as sensitive and cannot detect low ranges of pressure.

• Larger capillary tubing provides a faster response, but the increased volume results in increased zero shift due to ambient temperature changes. Smaller diameter tubing has reduced volume and tends to cause less zero shift, but the smaller cross-sectional area increases the lag time considerably. This can be a big problem if the seal fluid has a high viscosity.

• Vacuum conditions in the process can ruin a seal, unless special hydraulic seal fluids are used. (Vacuum lowers the boiling point of the fluid and if the hydraulic fluid boils, the resulting vapor usually ruins the seal.) Some hydraulic fluids are designed to handle vacuum, but they tend to be viscous and may create other problems (see below).

• Choosing the proper seal fluid can be difficult. Trade-offs abound. Here is a quick list of things to consider:

> Some processes prohibit certain fluids (such as silicone, etc.) from being used because any leakage into the process would have undesirable consequences. Check with the plant to make sure this is not a concern.

> Low viscosity fluids provide much faster response and are usually suitable for lower temperatures, but they usually cannot handle vacuum or high temperature conditions.

> High viscosity fluids can handle higher temperature and vacuum, but they tend to have much slower response, and this response can get dramatically worse during low ambient temperature conditions.

• Be careful when trying to measure a low differential pressure between seals that are vertically far apart. (A common scenario is trying to measure the differential pressure across a distillation column.) In this scenario, the weight of the capillary fluid in the legs shifts the zero dramatically. Most transmitters will only allow a zero shift of four to five times the maximum range. If you are trying to measure 0-25” wc across two taps that are 100’ apart vertically, the required zero shift will be approximately (100’ × 12” × SG of fluid), which will be well beyond the zero shift allowed for most transmitters. A higher range transmitter can be used, but sensitivity will be lost.

Watch-Outs: Be extremely careful to select knowledgeable technicians to install capillary seals. Many a pipefitter has pulled them out of the box and bolted them up without the proper gaskets and spacers. If this happens, the seals will almost certainly be ruined.

Exceptions: If your process could encounter high vacuums at high temperatures, evaluate your options carefully. There may not be a fluid available that will suit your application.

Insight: Never use a single seal, pad type tank level DP transmitter on a tank whose temperature varies. This type has a single 3″ or 4″ seal on the high side and is vented to atmosphere on the other side. Because this arrangement has a large seal on only one side, the unit will be prone to significant zero shifts due to process temperature changes. If the process temperature fluctuates, the level reading will fluctuate as well.

Rule of Thumb: When faced with specifying this type of meter, you would be wise to seek out help from an expert until you fully understand all of the options and design trade-offs. These meters are NOT cheap and the specifying engineer can ill afford a couple of iterations to get it right.

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.

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