Keeping the Pressure On Pressure Instrumentation

This is the first installment of a five-part series on Process Sensing. Smart sensors will appear in May. Others include temperature sensing (June), flow measurement (September), and level sensing (November).Temperature may be the most measured process variable, but pressure sensing devices may be the most ubiquitous.

03/01/1998


KEY WORDS

 

  • Process control & instrumentation

  • Pressure sensing/measurement

  • Transmitters

Sidebars:
Try a Little Friendliness—Environmental Friendliness, That Is

This is the first installment of a five-part series on Process Sensing. Smart sensors will appear in May. Others include temperature sensing (June), flow measurement (September), and level sensing (November).

Temperature may be the most measured process variable, but pressure sensing devices may be the most ubiquitous. These instruments, besides their intended pressure measurement duties, get "into the act" on flow and level measurement. A large process plant may require thousands of these hardworking instruments to keep processes under control.

The fact that pressure sensing devices must penetrate the process to do their job only complicates matters. Hazardous materials often make up the process media being monitored. If these materials are corrosive, they shorten instrument life and raise associated maintenance costs. They can also add to a plant's fugitive emission burden. Therefore, minimizing potential leak sites in these widespread devices is a requisite for good instrument design. (See accompanying sidebar).

Overall, pressure instrumentation design has kept up with demands. Yokogawa Industrial Automation (YIA, Newnan, Ga.) now offers the A-Series DPharp digital pressure transmitter that is said to offer a 40% improvement in general specification for ambient temperature and static pressure effects. The device (available in eight models) meets a 3-sigma conformance criteria on every model. On dp models, reference accuracy in rated at 0.075% of span plus 0.02% of URL per 50 °F temperature change. Long term stability is 2

In the real-world example, YIA's DPharp device is used to accurately control the steam/water interface in utility boiler drums. Fluctuating pressures within the boiler during startup and shutdown (times when level is most critical) require the device's stability in the face of pressure effects if accurate level is to be maintained. The silicon sensor's hysteresis-free capsule design also provides protection against overpressure conditions (and subsequent calibration requirements) that can occur during operation.

Under pressure

Ceramic sensors, known for corrosion and wear resistance, also provide excellent measurement stability and protect against process overpressure. Endress + Hauser (Greenwood, Ind.) now offers the compactly designed Cerabar T ceramic-sensor-based pressure measurement system with its sensor line that leverages these features. The device, which withstands up to 100 times overpressure, is said to have virtually no maintenance requirements, is easy to install, and requires no calibration.

Adaptable to a wide range of industrial application, the Cerabar T can be used for absolute or gage pressure in gases, steam, liquids, or vapors. The transmitter has a measuring range of up to 580 psi at temperature ranges from–4 to 185 °F (–20 to 85 °C). Overall accuracy is 0.5%. Chosen for its ceramic sensor specifications and competitive price ($175 ea.), a current application is in marine and stationary diesel engines manufactured by a European-based OEM.

High overpressure capability at high temperatures is also a feature of Omega Engineering's (Stamford, Conn.) PX82 Series wet/wet dp transmitters. The device, which can measure inches of water at line pressures up to 1,500 psi, has found application in flow situations where the pressure drop across inline filters must be closely monitored.

The device is FM/CSA approved and intrinsically safe. FM hazardous location capability is optional. Fabricated from stainless steel and Inconel for use in harsh environments, the transmitter provides a 4-20 mA output.

Taking the heat

Exposure to high temperatures can be troublesome for many pressure sensors. To overcome this problem, ABB Instrumentation (Rochester, N.Y.) developed the 614P remote-seal gage pressure transmitter specifically for use in extruders and homogenizers. The device's process diaphragm is made of a series of convoluted rings, similar to those used on nonremote seal isolation diaphragms for industrial transmitters. The convolutions are said to provide the flexibility necessary to increase performance, reliability, and instrument life while reducing drift in the face of elevated temperatures.

The 614P, which is "HART Smart" and withstands temperatures—up to 645 °F (340 °C)—found in equipment such as extruders and homogenizers, has minimum and maximum spans of 500 and 6,000 psi. The 614P is currently used in a plastic-fiber extruder line to control premature failure and unacceptable drift, caused by wide temperature changes experienced in everyday operations.

Standing up to high temperature during the pressure sensing process is also the forte of Honeywell IAC's (Phoenix, Ariz.) Smart Gauge Pressure Temperature for High Temperature. True to its name, the device withstands higher process temperatures than Honeywell's standard product (150 vs. 110 °C). which allows the elimination of remote seals and filled capillary systems for many applications. Key uses include applications which must comply with strict sanitary requirements, such as ice cream manufacture.

Because of its raised operating temperature spec, the transmitter can now survive over-temperature conditions caused by steaming instrument lines after a "freeze." Additionally, because the process equipment must be cleaned regularly with steam and a caustic solution, elimination of remote seals and filled capillary systems speeds the cleaning process, thus lowering sanitation costs.

Following the digital path

The original I/A Series pressure transmitters were introduced by The Foxboro Co., (Foxboro, Mass.) in October 1987. The device, designed with a field-swappable electronics module (either basic 4-20 mA or Foxcom digital), can incorporate either newly introduced enhanced analog or HART (Highway Addressable Remote Transducer) communication modules. According to the company, a Foundation fieldbus module will soon be available.

This innovation provides smooth migration from analog to digital communication during a control system upgrade, allowing original plumbing and hardware to remain in place. The flexibility preserves much of the original hardware investment. Additionally, reranging capabilities (previously only in the domain of digital communication) are now available in the enhanced analog electronics. Users can now adapt to changing process pressure requirements, using its built-in display and keypad to rerange in the field. Because an analog transmitter can now be reranged to meet different process applications, users can reduce spare parts inventories.

Controlling installed costs

Approximately 40% of all pressure transmitters are used in conjunction with instrument manifolds. These manifolds are used to block off the process during start-up, to isolate the transmitter when calibrating or testing, and to allow removal of the device without having to shut down the process. To help control both installation and hardware costs, the recently introduced Fisher-Rosemount (Eden Prairie, Minn.) Model 3051 Coplanar pressure transmitter integrates manifold functionality directly into the transmitter itself.

The integrated device now arrives at the job site fully assembled, seal tested, and calibrated, greatly simplifying installation and speeding commissioning. The unit, which weighs less than 8 lb (3.5 kg) can be directly mounted to the process. According to its developer, eliminating brackets, pipe stands, impulse piping, and installation labor can save as much as $150 per installation. Its integral transmitter/manifold construction also helps users meet environmental and safety regulations.

Look ma, no hands!

Keeping track of handheld devices needed for instrumentation configuration or calibration can be a real headache, especially on a large plant site. Sitrans P pressure transmitter from Siemens Energy & Automation (Alpharetta, Ga.) addresses the problem by allowing instrument configuration via pushbuttons located on the unit. It is also remotely configurable via the HART protocol. According to its manufacturer, it is the only transmitter on the market that does not require a handheld calibration device.

The Sitrans P, which is one of the most stable devices available, can measure pressure between 0.4 in. H 2 O and 5,800 psi. The device can be used to measure differential, gage, and absolute pressure, as well as flow and level.

Protecting the bridge

SOR Inc's (Lenexa, Kan.) S530 Series Mini Hermet is an environmentally sealed, analog-based pressure transmitter. The device uses alumina-based (Al 2 O 3 ) ceramic sensor technology. Alumina is almost perfectly elastic within its load range, as strong as steel in tension and many times stronger in compression, and has an elastic modulus 1.6 times that of steel. It is also inert, extremely hard, and an electrical insulator. The device's strain gage bridge is fired into the ceramic diaphragm at 1,000 °C, ensuring linearity and stability, necessitating fewer calibrations than standard analog devices.

530 Series devices are field adjustable using external, magnetically coupled adjustments, and are designed for use in explosion proof and intrinsically safe operations. The units are serialized for traceability and handle pressures from 10 to 2,000 psi, with agency approvals from FM, CSA, and CENELEC.

Turning it on (or off)

PN7 Series analog pressure sensors developed by "efector inc" (Exton, Pa.) incorporate microprocessor-based electronics in a rugged compact housing. This series of sensors is field programmable and requires no bench calibration, substantially reducing installation cost. The switch features 4-20 mA output, single switch setpoint, and a 3-digit LED display. There are two independent switches in each housing, allowing the device to handle applications that require two independent output with adjustable hysteresis. One unit can replace a gage, transmitter, and switch. Pressure ranges of 30 to 6,000 psi are available.

PN7s have been used by the paper industry to detect clogged starch filters by monitoring pressure on either side of filter screens. As the screens clog, restricted flow raises the differential pressure (dp). The two analog outputs are sent to a PLC to determine the dp. When the specified dp is reached, flow is diverted to a parallel line and the switch output is used to signal a local high-pressure alarm. The device's display verifies functionality.

Silicon-based accuracy

MT10 Mini-manometer from Yokogawa Corp. of America (Newnan, Ga.) is a high-reliability, high-accuracy instrument. Small and lightweight, the device is suitable for use at a process site and for diagnosing factory equipment, as well as for research and development.

The MT10 employs a microformed silicon-resonant sensor. The sensor, which uses a single-crystal silicon resonator embedded in a silicon diaphragm, is said to provide ultrastable performance with an accuracy of 0.01% of reading. The device features a data-hold function and an RS-232 communication interface.

Evolving technologies in both sensor and materials technology have kept the ubiquitous pressure sensor ahead of the process requirements game. Can it continue? As long as the pressure is on!




Try a Little Friendliness—Environmental Friendliness, That Is

Monitoring of fugitive emissions is required by the maximum achievable control technology (MACT) standards under Clean Air Act Amendments (CAAA) Title III. This applies to synthetic organic chemical manufacturers, pharmaceutical manufacturers, and petroleum refiners. A facility's fugitive emissions also must be included when determining its potential to emit hazardous air pollutants under CAAA Title III and V. However, the U.S. Environmental Protection Agency has been known to waive some measurement requirements if satisfied with a process' containment measures.

Increasing instrument accuracy, while pushing design and materials limits, has enabled manufacturers to help users minimize point emissions, process waste, and nonpoint (fugitive) emissions. High-accuracy instrumentation allows operators to run processes as near to optimum as possible, accurately controlling energy requirements and subsequent stack emissions. Close process control also eliminates out-of-spec product or waste that can lead to disposal problems and related concerns.

In the case of Honeywell IAC's ST 3000 and SMV 3000 DCS products, digital integration with their DE protocol control systems offers real-time, online diagnostics and error checking that are said to improve reading reliability and confidence in measurements sent to the control room. According to a spokesperson for Honeywell, a high degree of confidence in field signals allows operators to "push" the plant harder and closer to design limits, resulting in higher efficiencies with less pollution and waste.

Help from the design

Removing potential leak sites can also eliminate fugitive emissions. In the case of pressure transmitters, drain vent valves are used to purge gases' or liquids' error-causing buildup in the flanges and impulse lines materials. Also, the valves vent product to atmosphere prior to calibration or removal of the instrument from the process. With the Fisher-Rosemount's Model 3051 Coplanar device, these drain vent valves are replaced with controller vent ports which allow capture of all vented product into collection lines.

Design of instrumentation to eliminate potential "plumbing" leak points also helps to eliminate nonpoint emissions. Use of integral manifolding, all-welded gasketless connections, and transmitter designs that eliminate the need for dp equalization 3-valve manifolds similar to the design used in Yokogawa Industrial Automation's DPharp pressure transmitter line offer this solution.

Finally, use of engineered gasket materials offer additional help in controlling nonpoint toxic releases. For example, Foxboro uses Teflon as standard gasket material in its I/A Series dp pressure transmitters. This material provides superior corrosion resistance across virtually all process applications, minimizing the chance of gasket breakdown and resulting toxic releases. In most cases, pressure instrumentation makers now allow users to specify gasket materials that meet process requirements.

As pressure instrumentation design evolves, manufacturers continue to meet pollution challanges through innovative design. Even in a situation where sheer numbers are not in its favor, these devices have shown they can "keep a lid on it."



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