Process Instrumentation’s ‘Utility Infielder’

This is the final installment of a five-part series on Process Sensing. Pressure sensing appeared in March. Smart sensors appeared in May. Other articles included temperature sensing (June) and flow measurement (September).It is probably safe to say that process automation has been applied in some way or another to every industry.

By Dick Johnson, CONTROL ENGINEERING November 1, 1998


Process control & instrumentation

Level sensing


Sidebars: No sweat retrofit

This is the final installment of a five-part series on Process Sensing. Pressure sensing appeared in March. Smart sensors appeared in May. Other articles included temperature sensing (June) and flow measurement (September).

It is probably safe to say that process automation has been applied in some way or another to every industry. In applications that call for detection or measurement of bulk materials in some form of vessel or storage bin, level instrumention has been called on to sense everything from chicken feathers to chicken feed.

Level instrumentation can be used several ways. Point level sensing provides a signal when material reaches or falls below a predetermined height in a bin or silo. Continuous level sensing, however, furnishes an output that gives a control system an indication of level anywhere between two predetermined points. Both types now leverage a wide range of sensing technologies letting them handle a wider range of product.

Delevan Process Instrumentation’s (Naugatuck, Conn.) Captrol/500 is a microprocessor-based capacitance level switch is intended for use with bulk solids with a dielectric constant greater than 2.0 and bulk density greater than 10 lb/cu ft, which includes most common materials such as grain, flour, sand, corn, etc. The device’s electronics can eliminate the effect of material build up that causes false signals, as well as provide calibration capability with the unit either exposed or covered with process material. Captrol/500 also features built-in diagnostics, built-in static suppression, and field-selectable failsafe modes. Operating temperature range is–40 to 450 °F.

Take a powder!

Sensing point level in granular and powdered materials of medium to high bulk density has long been the domain of mechanical sensors. Because devices such as rotary paddles can be damaged by rapid filling or contact with tramp materials, reducing sensor protrusion into bins has some real advantages. Scientific Technologies Inc.’s (Fremont, Ca.) VP10 Series piezoelectric proximity sensor operates by creating a specific vibration frequency. When material surrounds the sensor, a change in vibration frequency is detected and processed through a comparator circuit. If the sensor is sufficiently dampened, the state of output changes.

The device works on solids with bulk density greater than 12.4 lb/cu ft and features standard transistor output. Relay output is an optional feature. According to its manufacturer, VP10 is unaffected by moisture, humidity, or temperature variation, but must not be exposed to temperatures over 140 °F.

If the sensing device must protrude into a bin it requires rugged construction. Omega Engineering’s (Stamford, Conn.) LV750 Series radio frequency (RF)-based level switch features a sturdy no-moving-parts approach to solids level sensing. The RF switch is said to tolerate reasonable amounts of buildup. When intended for use with high dielectric constant materials that have a tendency to “bridge” between the probe and bin wall, an optional Teflon probe is recommended.

LV750 Series can be fitted with various probe lengths (12 in. minimum to 72 in.) depending on application. Other features include vibration-resistant potted electronics construction, a NEMA 4 housing, adjustable time delay, and high/low failsafe capability. The unit’s electronics operate in a temperature range of [-40] to 185°F.

Hang loose

Capacitance-based point level detection has broad application in bulk-solids applications. Milltronics Inc.’s (Arlington, Tex.) Pointek CLS capacitance level switch is said to operate independently of tank wall’s influence. This means that the device does not require an external reference electrode for level detection in a nonconductive vessel. Keep in mind, concrete and wood, to a lesser extent, are still seen frequently in silo construction for both building materials (stone, sand, etc.) and food stuffs (grain, corn, etc.). This feature also makes it easier to adapt the CLS to existing nonmetallic structures.

The device has a fixed detection trip point unaffected by temperature changes and shifts in dielectric values. The CLS comes in a wide variety of configurations including rigid, sanitary, and extended reach (up to 115 ft). To ensure wide compatibility, probes are available in either Kynar or stainless steel, with a temperature rating of 257 °F on the process wetted portion of the probe. The device’s housing is rated NEMA 4X or NEMA 7/9.

Fall into the gap

Ultrasonic gap sensors have long been used in the process industries for measuring the presence of liquid. According to Cosense Inc. (Hauppauge, N.Y.), this technology has been overlooked for dry bulk sensing. Cosense’s Model DL-709 is said to be ideal for dry materials (carbon black, wood chips, polystyrene flakes, etc.) which are difficult to detect because of low bulk densities and/or low dielectric constants. The device works by transmitting a high frequency in air through the gap in the probe that protrudes into the storage structure. Presence of dry material blocks ultrasonic energy, indicating material level. The 316 stainless steel electrically grounded sensor dissipates the product’s static charge, allowing it to be used in potentially hazardous environments.

DL-709 has self-testing abilities which include failsafe electronics and crystal bonding checks. Output is 10 amp DPDT (standard) and 4-20 mA loop-powered two wire (optional). The housing is a NEMA 4/7 watertight explosion-proof cast aluminum enclosure, Class 1, Group C & D, Class II, Group E, F, G, and Class III, Div. 1 & 2. Operating temperature range for the sensor is–30 to 150 °F, up to 250 psig.

Widening the range

Drexelbrook Engineering Co.’s (Horsham, Pa.) NoCal LCS RF level switch handles a wide range of process materials including water and other liquids, slurries, liquid/liquid interfaces, and all types of granular materials up to 2-in. diameter chunks. The advantage of this broad adaptability is that less spares inventory may be required multiprocess stream industries.

The device also features simple setup, regardless of product. According to the company, the unit need only be mounted, wired, and initiated using a magnetic key. No calibration is required. In addition, the unit’s tamper-resistant design allows the control engineer or technician to lock out unauthorized personnel by setting it to the “supervisor” mode. NoCal LCS features an explosion-proof housing and has an electonics’ operating range of–40 to 140 °F. The sensor’s rating is 200 psi at 450 °F. RFI protection is built in.

Continuous measurement

Not all dry bulk-solids are created equal. In fact, certain dry powders can be very difficult to measure. According to Chuck Cable, product manager, Bindicator Inc.’s (Spartanburg, S.C.) MS-2010 Series ultrasonic level measurement system features unique technologies for just that purpose. The device features its developer’s proprietary digital signal processing (DSP) technology, which digitizes and processes the sensor’s return signal, improving accuracy over traditional analog systems. The system’s firmware is also programmed with various algorithms which automatically evaluate each return signal and make adjustments as need.

The MS-2010 also features a high-energy (3,200 V), high-mass transducer that penetrates through areas of product which exhibit loss of bulk density caused by the pneumatic filling cycle (an earmark of “problem” powders). It also penetrates any ambient noise to help ensure a good return echo. Some of the materials handled include plastic resin powder (fillers and additives), fly ash, talc, powdered sugar, and magnesium oxide. Besides powders, MS-2010 also handles loose granular and coarse/aggregate materials.

LMI Selcom (Southfield, Mich.) has introduced two noncontact laser-based sensors intended for high accuracy, continuous measurement of bulk dry solids. The SPR 02 and SLS 6000 have a measurement range of 168 in. and 40 in., respectively. According to David Allen, manager, Industrial Sensors Division, both sensors are suited for measuring all bulk materials including gypsum.

Both come with full duplex communication, allowing them to be addressed individually. Additionally, the communication system allows quick change of sensor parameters and sensitivity and accelerated data exchange.

Combating filling problems

When solid materials are transferred into a storage vessel or agitated in a mixing cycle, movement of the material can make sensing both difficult and often inaccurate. To overcome loss of accuracy due to heavy dusting, vapor formation, and varying temperature layers, HiTech Technologies, Inc. (Newtown, Pa.) uses methods to greatly increase the penetrating power of the outgoing acoustical beam. The resulting ultransonic signal is said to make reliable measurements in very difficult applications.

The noncontact system is intended for free-flowing materials (coal, fly ash, cement, cocoa nibs, etc.) that do not bridge or “rat hole” in the vessel. Angle of repose problems are eliminated by use of a proprietary aiming device and application-specific software. The instrumentation provides a 4-20 mA output and is rated NEMA 4. Operating temperature limits are–13 to 140 °F.

Endress + Hauser’s (Greenwood, Ind.) Levelflex FMP 232/332 is a top-mounted, compact level transmitter for both process control and storage inventory applications. The device operates with micro-impulse radar on the guided time-of-flight principle. Intended for “no calibration” setup with all fine to medium grain bulk solids with a dielectric constant of 1.8 or greater. However, it can be configured for lower dielectric constants. Levelflex is said to perform well when measuring materials such as flour, talc, grains, sand, animal food, and flash.

The system’s micro-impulse time domain reflectometry (MTDR) technology uses a single conductor cable. The single cable eliminates the risk of bridging or twisting present ith dual conductor systems. Additionally, the cable (with weight or tie-down) on which the radar impulse travels can be shortened in the field as necessary. MTDR technology provides measurement independent of material properties, bin construction, and operating conditions such as dust and angled materials surfaces caused during bin filling. Output of the device is 4-20 mA. Normal temperature operating range is–40 to 250 °F.

Don’t crush that probe

Bulk solids can be rough on whatever is in the bin with them. Material weight, forces caused by material movement, abrasiveness, and moisture entrapment can affect the integrity and/or service life of in-tank probes required by contact instrumentation. These effects can be controlled, however, by matching the application to the device and careful probe placement.

Krohne Inc.’s (Peabody, Mass.) BM100 Reflex Radar Level Gauge uses Time Domain Reflectometry (TDR) which measures the time-of-flight of an electromagnetic pulse generated by a converter which travels to the surface of the product and is reflected back. Level is proportional to pulse travel time divided by two. TDR’s accuracy allows level to be measured toime, wheat bran, etc.) as long as the material “chuck size” will not crush the required probes.

Something different

BinMaster’s (Lincoln, Neb.) BBR II system uses a microprocessor-driven “captive pulley” plumbob unit, capable of measuring a broad range of products in vessels up to 150 ft. The device takes separate measurements in both descend and retract modes, ensuring it travels the same distance in both directions, verifying the level, and making sure the bob is in its fully retracted standby position. The unit can be set to override the command to take a measurement during a fill cycle, reducing the likelihood of burying the bob in the material. The system is available in explosion-proof models. Remote sensing units will interface with PLCs via a direct serial port, or an RTU interface with 4-20 mA output. The device is accurate to 1/10 ft.

BBR II is suitable for use in dusty powders and granules; signal absorbing materials (plastic pellets or resins); heavy, corrosive, conductive, or abrasive materials; slurries, and foaming/aromatic liquids. In fact, the its mechanical nature makes it useful in some rather unusual applications. According to Rich Travis, application specialist, “The system has been installed at a feed mill which stores 16 different raw materials, including molasses, feathers, clay, urea, and grain.”

Can bulk-solids level sensors really measure anything? Maybe not, but even to a casual observer their versatility and longevity seem to be in a league with the Baltimore Orioles’ Cal Ripkin Jr.

No sweat retrofit

The word “retrofit” can strike fear into the heart of the most stalwart process control engineer. Thoughts of shutting down processes; emptying appropriate conveyors, bins, and silos; removing old instruments; rewiring; mounting new instruments and integrating them into the existing control system, and restarting the process can keep them up nights. Now, place the retrofit equipment high off the plant floor, like most level-sensing devices, and a real nightmare can take shape.

As Texas billionaire, H. Ross Perot, is fond of saying, “The devil’s in the details.” And, as any engineer or technician who has had to add or upgrade equipment in an existing plant situation knows, this statement is true . Level sensing instrumentation developers have, however, taken time to provide designs that help with those “details.”

A high percentage of continuous-reading bulk-solid level sensing products are available either precalibrated or easily calibrated in the field. Although this greatly speeds up the commissioning process, one of its biggest advantages is that vessels need not be emptied of product for setup. Even with scheduled downtime, the logistical details of emptying, relocating, storing, and refilling silos can be complicated and costly.

Rewiring, a perennial retrofit problem, has been addressed by Bindicator Inc.’s use of LonWorks communication protocol with its MS-2010 device. Use of only a single twisted pair for multiple remote transducer with PC/PLCs eliminates the need for multiple pair or adding conduit runs.

According to Glenn Estes, applications specialist, at Milltronics Inc., offering common mounting options for bulk-solids level instruments is not as straightforward as it seems. Depending on the type of device being replaced, existing openings and mounting holes may not accommodate the newer instrument. Existing hole location may also be a poor choice for the new sensing technology chosen. For example, Milltronics provides several standard kits for ultrasonic devices that allow them to be aimed at the tank center, ensuring accurate measurement.

Actual mountings, however, can vary from custom designed to standard ANSI flanges. Choice of flange usually depends on where it has to be fabricated (in the field or in a shop), Mr. Estes continued. When asked if any part of field adaptation was easy, Estes replied, “If it were easy to put holes in an existing tank, our engineers would have not taken the time to combine the unit’s temperature sensor in a single mounting hole configuration.”

Because mounting to existing vessels is a problem, level sensing vendors are careful to supply a range of compatible instrumentation. According to Paula Hollywood, level product specialist at Krohne Inc., “To make installation easier, our instruments are offered in various flange sizes and ratings and probes (when required) can also utilize existing chambers.” In some cases, probe-equipped instruments can be offered with either rigid rods or flexible cable. Ms. Hollywood continued, “In the case of loosely packed product, both can be installed with product in place. A counterweight at the end of a cable version will fall through the product and cause the cable to stretch to its full length so operation can commence.”

All things considered, bulk solids level sensing manufacturers have taken time to make retrofits easier. Help is well on the way.

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