Cutting cost of pressure

Smart measurements and integrated manifolds can save process operators a great deal of money BY ALAN REEVE

Pressure transmitters are one of the most common measurement devices in the process industry. Their versatility and overall value make them the technology of choice for measuring process flow, level and pressure.

As users confront today’s demands of reducing cost, complying with tightening environmental and safety regulations and accelerating their time-to-market, pressure transmitters are being looked upon to deliver new solutions to meet these needs.

Starting with Druck, it is now celebrating 25 years specialising in pressure measurement. It is now a £50m international group with some 800 staff world wide, backed by representatives in 50 countries. Latest products are the low pressure LPX/LPM ranges (see panel), and the PMP 4000 high performance (0.04% accuracy) transducer for automotive use.

Next is the I/A Series pressure transmitters from Foxboro, which use silicon strain gauge technology (see panel).

Coplanar technology

Further innovation comes with details of the coplanar design, from Fisher Rosemount.

These transmitters provide good performance in a flexible, compact package that can be half the size and weight of some conventional designs.

Conventional design pressure transmitters orient the process isolating diaphragms parallel to one another on either side of a sensor module, requiring two process flanges. Various process connections and instrument manifolds bolt up to these two separate transmitter flanges.

With the coplanar design, the process isolating diaphragms are orientated in the same plane on the bottom of the sensor module, creating a ‘flat’ interface requiring a single process flange.

This type of orientation allows the sensor module to mate directly to numerous types of process connections, often eliminating the need for the transmitter flange to be supplied.

Instrument manifolds

Instrument manifolds are utilised in approximately 40% of all pressure transmitter applications.

These manifolds are used to block off the process during start-up, to isolate the transmitter when calibrating or testing, and to remove the transmitter without having to shut the process down. Manifolds are typically bolted directly to the transmitter, but can also be installed separately in the piping.

Many instrument manifold suppliers have enhanced the design of their manifold valve bodies such that they now mount directly to the sensor module of the coplanar transmitters.

The first benefit is a reduction in the installed cost of the integrated package. When the instrument manifold is integrated into the pressure transmitter, some users report that savings can exceed £150 per installation.

This is based on arrival at the job site fully assembled, process seal tested, and calibrated for direct installation. Another £100 can be reportedly saved by eliminating the bracketing, pipe stand, impulse piping, and civil work required to mount everything into the ground.

Environmental and safety regulations are also helped. Integration of manifold functionality into a pressure transmitter helps ease compliance in a number of ways. In addition, the entire manifold and transmitter assembly can be high-pressure seal tested as one device before leaving the factory.

Lastly, all conventional transmitters require drain/vent valves to purge gases or liquids that build up in the flanges and impulse lines, which can further cause measurement error.

When used, these vent valves release process fluids and gases to the atmosphere, creating potential safety and environmental hazards. The coplanar transmitter/manifold assembly provides the ability to eliminate drain/vent valves and replace them with controlled vent ports, which allow 100% capture of vented product.

These capabilities result in reduced risk, maintenance, and better compliance with environmental and safety regulations.

As a result of the coplanar design, virtually every component of the pressure transmitter installation can be redesigned to reduce operating costs.

Capable of precise differential or absolute measurement at very low pressures, the LPX/LPM Series of sensors from Druck are based on proven eddy-current sensor technology.

Ranges are from 0.1mb diff. and 2.0mb absolute, with accuracies to 0.1% full scale. LPX provides a 4-20mA output, while the LPM follows the three-wire format.

Foxboro’s I/A series pressure transmitters conform to all EC Directives and come with a choice of communications.