How to Specify a Capillary Seal Assembly

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 ?” 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.

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