Fitting the Instrument Into the Process

The "nuts and bolts" issues that crop up in instrument integration can take a back seat to high-tech specifications. Although mounting issues seem mundane, they can figure prominently into overall success of an instrument's commissioning.General guidelines for the "big four" (pressure, temperature, level, and flow) vary little from one manufacturer to another.

By Dick Johnson, CONTROL ENGINEERING April 1, 1999


Process control & instrumentation

Pressure measurement

Temperature measurement

Level measurement

Flow measurement

The “nuts and bolts” issues that crop up in instrument integration can take a back seat to high-tech specifications. Although mounting issues seem mundane, they can figure prominently into overall success of an instrument’s commissioning.

General guidelines for the “big four” (pressure, temperature, level, and flow) vary little from one manufacturer to another. In most cases, if common sense prevails, correct placement of the sensor and transmitter is usually routine.

Placing the electronics

The sensor is the business-end of any instrument. Standing at the front line of the process, they usually bear the brunt of the process media assault. Their protection requirements can be much more application specific. Transmitters, on the other hand, can be more easily protected from harm.

According to Lawrence Vanell, Honeywell IAC’s (Phoenix, Ariz.) applications manager field measurements, general guidelines for transmitter mounting should include the following:

Transmitters should be installed in such a position that they are protected from physical damage, heat buildup, corrosion, and abrasive wear. This applies to general, sanitary, and corrosive/abrasive service situations;

With its protection assured, transmitters should be placed as close as possible to the process being measured. This shortens wiring, minimizing losses and noise; and,

Even with careful placement, materials of construction for transmitter housings must be specified to withstand the environment in which they are to exist. Absolute protection of electronics requires attention to specification of housing materials and coatings, seal materials, insulation materials (where required) and process connections.

David Brown, principle engineer for Fisher Rosemount Systems (Austin, Tex.) adds that case grounding on all transmitter cases be thoroughly checked. Although not a problem in 4-20 mA systems, poor grounds in digital systems can allow noise to degrade performance.

Smart devices that must meet CE requirements for noise suppression have EMC filtering systems that are referenced to the transmitter case. Cases must be connected to ground with the grounding lugs supplied if these systems are to work properly. What should they be grounded to? “The plant’s cable tray system is an excellent ground,” Mr. Brown answers. “When checking ground, use an earth ground tester and check impedance values in digital systems,” he adds.

When the pressure’s on

Pressure instrumentation is the most numerous type in process plants, making it a good reason to install them right the first time. According to Robert Irving, product manager at Ametek U.S.Gauge/PMT Product (Feasterville, Pa.), pressure sensors in general service should be mounted in stream or at a dead leg using the appropriate “gender” process connection.

In sanitary applications, process connections must be flush so as not trap process materials. Special attention must be given to wetted parts’ material for clean-in-place (CIP) applications where high temperatures are encountered. Polished 316 stainless steel is a good choice in this situation. Corrosive and abrasive environments also need special material for wetted process connections; 316 stainless, Monel, Hastalloy, and titanium are good choices for corrosive media. For abrasive media, Hastalloy is a good choice.

In flow situations, sensors should be positioned so that they are not influenced by piping restrictions. Keeping the sensor away from high pressure hammer effects that may exceed the instrument’s rating is paramount to its survival. Additionally, says Tim Cillessen, technical specialist at Siemens Energy & Automation (Alpharetta, Ga.), the pressure transmitter’s installation point should be easily accessible, secure, and free of vibration. Use at least two screws for wall mounting and the maximum diameter U-bolts allowable for pipe mounting. Use appropriate thread sealant on all threaded connection.

Care should be taken that ambient temperature limits for the device are not violated. Keeping transmitters out of direct sun exposure limits temperature extremes and fluctuations. Heat tracing is required where it’s possible for process fluids to freeze in measurement chambers. If the process media is gas, transmitters should be installed above the tapping point to allow condensation drainage. For liquid applications, transmitters should be mounted below the tap to allow any gas buildup to disperse.

Taking temperatures

Protecting temperature sensors from the media while maintaining their sensitivity can be a challenge. According to J.R. Madden, applications engineer for Moore Industries-International (Sepulveda, Calif.), “General service applications typically require very little protection. To produce the best measurement results (accuracy and response), the sensor normally is directly exposed to the process material, protected only with a stainless-steel outer sheath. High process flows, pressures, and temperatures should be avoided in these cases.”

For sanitary service, smooth exposed sensors and thermowells (3A finish minimum) are required to prevent foreign material from sticking. Corrosive and abrasive conditions also require the sensor to be protected. Protective tubes and thermowells should be properly coated and sealed to prevent leaking into the sensor.

Temperature sensors are tip sensitive. To make accurate measurements with optimal response time, the sensor must make constant contact with the process. Sensors perform best when spring loaded or held in constant with the thermowell’s bottom, or deeply immersed directly into the process fluid. Deeply is defined by The Foxboro Co. (Foxboro, Mass.) as when the last one inch of the sensor (sensitive portion) is properly placed in the process stream.

Jim Sulciner, national sales manager, at Burns Engineering (Minnetonka, Minn.) adds, “Always re-member the less material around the sensor, the faster the response time. Always use heat transfer materials and a 100% time response improvement will result. In the case of RTD applications, use of reduced tip sensors in thermowells improves re-sponse time over that of a tapered well.”

Temperature sensor mass should be minimized to maximize system response time. Large sensor mass results in slow response and adds or removes heat from the process. Additionally, according to RdF Corp. (Hudson, N.H.) literature, sensors should be placed as close as possible to the center of fluid flow. When penetration of the process is impossible or undesired, a surface-mounted sensor may make sense. Installation requires that the sensor be cemented to the vessel or pipe. The thickness of the cement must be keep to a minimum and the adhesive must withstand the operating temperature. To minimize errors caused by variations in ambient air temperatures, surface sensors must be well insulated.

Because temperature sensing systems are so ubiquitous, maintenance issues can be significant for most process plants. According to the Hartmann & Braun div. of Elsag Bailey Process Automation, a unit of ABB (Wickliffe, O.), a number of steps should be taken on a regular intervals to ensure circuit integrity. Besides checking for measuring element drift caused by aging and reduction of insulation resistance through humidity and pollution effects, the more “nuts and bolts” inspections should include evaluation and repair of:

Wear and tear or degradation by chemical activity of the sensor’s protecting tube;

Improper contact of wire connections; and,

Mechanical and chemical damage of the sensor and wires.

Keeping it on the level

Of all the process variables, level can provide the most challenging (read dangerous ) installation procedures. No matter which continuous reading technology is chosen, chances are a control engineer or instrument tech will have to climb to the top of a tank or bin to install the hardware and hook up the electronics. Installing point level may not require a climb to the top but “poking” accurate access and mounting holes in the side of a tank is no easy—safer, maybe—job either.

Working in an uncomfortable environment may cause installers to take shortcuts or be less careful with installation procedures. According to Roman Dziaba, buoyancy product manager at Magnatrol (Downers Grove, Ill.), “No matter what type of level control is used or where it is installed, both local and national electrical codes must be adhered to for both field power and signal cabling. Depending on service requirements, wetted parts must be match the application. As with pressure and temperature instruments, sanitary applications require the use of highly polished speciality fittings. Corrosive environments call for the use of stainless steel, Hastalloy, and Monel fittings.”

Continuous reading technologies such as RF and ultrasonic require careful placement of the sensor. According to Drexelbrook Engineering (Horsham, Pa.), sensing elements should always be mounted perpendicular to the surface of the liquid being monitored and out of the way of the fill line. Although the exact amounts vary with manufacturer and technology, the distance from the vessel side must be maintained to ensure accurate readings. In continuous RF applications, vessel grounding may be called for. Not a problem in metal vessels, grounding nonmetallic ones can be done in a number of ways. See accompanying figure.

Level instrumentation probes must not be allowed to come into contact with vessel openings or false readings will occur. Displacement of the probe can occur especially during bulk-solid product discharge. Krohne, Inc. (Peabody, Mass.) suggests that maximum nozzle height is:

H max =dL/4e

Where d=Nozzle diameter

L=Length of probe or cable

e=Nozzle offset from tank center

Intelligent pressure transmitters can be adapted to many different types of level sensing situations. The Foxboro Co. suggests that in an open vessel application, where a single device is used to sense pressure and calculate level that the minimum level never fall below centerline of the instrument. In integral diaphragm applications, the sensing diaphragm should remain fully wetted at all times.

Go with the flow

Placement of the sensor is probably the biggest issue in flow technology. However, there are other issues. According to David Wright, application engineer for Bailey, Fischer & Porter (Warminster, Pa.), guidelines for flowmeter installation in general service requires that the device be positioned so that it is kept full of the process fluid at all times. Flow should move against gravity. Additionally, if the flowmeter is to provide accurate measurements, it should be located at least 3-5 diameters downstream of elbows and tees and 10-20 diameters from pumps and valves. Flow velocities should be within 1-30 ft/sec.

Fluid Components International’s (San Marcos, Calif.) marketing manager for new products, Sam Kresh, adds that for its line of thermal-dispersion technology the instrument’s distance from flow disturbances should be expanded to at least 20 diameters upstream and 10 diameters downstream. Mr. Kresh was quick to point out that poorly defined and non-symetrical flow profiles (though correctable through the use of properly speced flow conditioners) can be caused by many things. Although fittings, valves, and pumps are the usual culprits, things as seemingly insignificant as pipe welds can disturb flow enough to adversely effect meter accuracy.

Not all flowmeter types are sensitive to flow profile and Reynold’s number. For example, mass flow and density sensors used in Micro Motion’s (Boulder, Co.) Coriolis-type flow measurement systems operate independently of these flow variables and can offer flexible installation. However, before operation, flow through the sensor must be stopped to complete the zeroing procedure. A shutoff valve should be installed downstream to facilitate a “no flow” condition when necessary.

The offset construction of Coriolis-type meters requires that they be oriented to minimize both the torque and bending load on the process connections. Additionally, using the sensor portion of the meter to support piping (though tempting) should be avoided because both sensor damage and measurement error may result.

According to Mike Dyer, applications engineer for McCrometer (Hemet, Calif.), differential pressure (dp) meters (orifice plate, venturi, etc.) still need to be isolated from flow disturbances to maintain accuracy. In most cases, adherence to ISO Standard 5167 is sufficient. However, there are dp meters that act as their own flow conditioner. In these cases isolation distances of 0-3 diameters upstream and 0-1 diameters downstream can be sufficient.

Material specifications for flowmeters in sanitary service are similar to those for other instruments. Quick disconnects for cleaning, high polish finishes, and compatibility with CIP cycles for wetted parts. Corrosive service also requires that wetted parts and gasketing materials are chemically compatible with both process temperatures and concentrations.

Careful preparation is essential to successful installation of all instruments. For instance, installing the sensor portion of vortex and magnetic flowmeters, like those available from Yokogawa Corp. of America (Newnan, Ga.), require that attention be paid to location and contamination control.

Flowmeters should not be installed near electric motors, transformers, and other power sources that may cause induction interference. Before installing a meter in new piping, flush and clean scale scale, incrustation, and sludge on the pipe wall to avoid damaging the meters.

Clamp-on flowmeters avoid many problems of wetted units. When applicable, clamp-on ultrasonic transducers can provide flow information without tapping into the process. Panametrics (Waltham, Mass.) recommends that transducers be mounted on the sides of horizontal pipe. Since the top of an horizontal run accumulates gas and the bottom sendiment that can attenuate or block the ultrasonic signal, side mounting is preferable. There are no similar restrictions on vertical pipe. Cleanliness is key because good transducer contact with the pipe is only possible if it is clean and free of debris.

If all else fails

In addition to information on operation and troubleshooting of their instruments, manufacturers provide direction for the actual mechanical installation of their equipment. A wealth of information is available from each company in written form, on the Internet, and from strategically stationed (and helpful) application engineers. Suffice to say, instrumentation manufacturers appear committed to the low-tech side of these high-tech instruments.