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Getting it right – remote seals for pressure measurement

06 October 2010

Remote seals provide extra protection for pressure instrumentation in arduous conditions but they also introduce an added layer of complexity to the system, which places extra responsibilities on the user to choose and install remote seals correctly. Trevor Dunger offers some pointers

Some pressure measurement applications and process fluids are simply too hostile or hazardous to come into direct contact with electronic instrumentation. Fluids might be corrosive or dirty, for instance, or the process might operate at an extreme temperature that would damage the transmitter.

This kind of demanding application calls for the added protection of remote seal technology. But specifying the right remote seal installation is not always straightforward, and getting it wrong can seriously impair the speed and accuracy of pressure measurements.

Over 50% of all installed remote seal pressure are not optimised, because of factors such as the capillary being too long or the measuring cell has an excessively high turndown ratio. Such factors can all combine to reduce the overall accuracy of the installation.

Standard electronic pressure transmitters are generally connected directly to the process via impulse lines. The process fluid fills the impulse lines and enters the body of the transmitter. Where this is not practical for any reason, the alternative is to use a remote diaphragm seal to isolate the pressure transmitter from the process fluid.

In this case, the process fluid does not leave the process pipe work. Instead, a diaphragm passes on any changes in process pressure to a capillary filled with a suitable incompressible fluid, which then transfers those changes indirectly to the remotely-mounted transmitter.
The suitability of a remote seal system for a particular application is influenced by a number of factors, including the design of the overall system, the characteristics of the fill fluid and the ambient operating conditions. Any of these can have an impact on the two crucial operating parameters associated with pressure measurement – response time and accuracy. The situation is further complicated by the fact that optimising the system for both speed and accuracy will often involve a direct trade-off.

Of course, other considerations often come into play in these decisions. For example, a shorter capillary means that the transmitter must be located closer to the process line, which may not be practical in high-temperature applications or in locations that would restrict access and make it difficult to access the instrument for future maintenance.

Slippery customers
The viscosity of the fill fluid in the capillary line is another factor affecting the speed of response, with high-viscosity fluids taking longer to transmit an applied force through the system.

Around 80% of non-food applications are catered for by standard, silicon-based products such as Dow Corning’s popular DC200 fluid, but the choice of fill fluid may not be so easy in some applications, especially those at each end of the temperature scale. Applications that are hotter than approximately 200oC require specialist high-temperature fill fluids to prevent them boiling in the capillaries.

On the other hand, these high-temperature fluids become too viscous if the temperature drops to room ambient. In the same way, cryogenic fluids remain free-flowing for applications as low as -100oC, but will tend to boil if the temperature rises too high.

Other considerations include the compatibility of the fill fluid and the process medium, in case the diaphragm ruptures and the two come into direct contact. For example, only non-toxic fluids will make the grade in food applications, while it takes an inert fluid to measure the pressure in an oxygen line safely.

Minimising errors
Remote seal pressure transmitters introduce new sources of possible error when compared with the performance of standard transmitters. Operators, for example, can easily cause a failure of a diaphragm seal simply by unscrewing the capillary.
Another source of error is temperature effects. Although fill fluids are typically designed to minimise thermal expansion as much as possible, all fluids tend to expand as the temperature rises, increasing the pressure in the capillary and introducing an error.

This is not such a problem for differential pressure instruments with two process connections, because both legs will be subject to the same errors and these will cancel each other out. This will only work if both legs are subject to the same temperature conditions, however, so it is a bad idea to have one leg in the sun while the other is in shade, for example.
Of course, this is no help for instruments with a single leg. In this case, such temperature effects should be ‘designed out’ as far as possible. For example, the volume of fill fluid should be minimised by using a capillary with a small bore and careful profiling of the seat behind the diaphragm.

The specific gravity of the fill fluid itself can also introduce an error if the capillary extends vertically. This is a common problem in applications such as liquid level measurement, where it is not unusual for the diaphragm to be positioned, say, 10m above the transmitter. In this case, it is important that the measuring range of the transmitter is sufficiently wide to allow the system to be zeroed to take any changes in level into account. Temperature variations will also have an effect here, because they change the density of the fill fluid.
Getting it right
Remote seals offer a much greater range of materials for a variety of applications compared with standard transmitters, including oil and gas specific Superduplex, Hastelloy C2000, Inconel 625 and proprietary coatings. As such, they offer the better alternative for corrosive processes, significantly extending the service life of the transmitter over and above the materials used for standard transmitters.

Users need to ensure they select a metal that suits their application, weighing up the advantages and limitations of each. For example, Hastelloy is a nickel based alloy that offers the best corrosion resistance, making it ideal for even the most arduous of environments. Monel varieties, however, are particularly suitable for applications that are exposed to seawater, whilst gold-plated diaphragms are efficient at stopping the penetration of hydrogen from processes containing free hydrogen.

So, remote seals add an extra layer of complexity to pressure measurement installations. Making the best choice is by no means straightforward. Whether the user opts for an integrated system, or an add-on seal combined with a conventional pressure transmitter, there are a number of competing factors that must be taken into account during selection and installation.

Reputable suppliers provide a wealth of selection data to help get it right, but they should also be able to provide the skilled staff needed to support customers as they make their choice.

Trevor Dunger is with ABB

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