Many industries must routinely deal with materials that explode, burn, or explode and burn. Actually, explosions in gas, vapors, or dusts are not detonations but very rapid burning of the media best described as deflagrations. Handling 'media from hell' often requires many specialized equipment and disciplines (everything from mechanical system design to satisfying numerous industry associations and regulatory agencies, such as API, AGA, OSHA, the USEPA, UL, FM, etc., to name a few).

All processes that make 'stuff' require instrumentation to monitor the media as it undergoes chemical change or tracks conditions during ingredient mixing or blending. Thus, pressure, temperature, level, flow, and other specialty instruments must come in contact with some very combustible and highly explosive materials, 'genies' that are a far cry from Major Nelson's mythological, impossible-to-understand friend.

Although the type of media present in the processing plant determines the amount of 'bang for the buck' in potentially explosive applications (See Classification of hazardous areas), the hazardous area classification for Class I determines the National Electrical Code (NEC) protection method that must be used.

Because gases and vapors are always present, Division I applications require 'heroic' steps to avoid possible disaster. Sensors in these areas must be protected by one of several methods. These include the use of explosion-proof designs, intrinsically safe construction, or use of purge or pressurized housings.

On the other hand, Division 2 areas require protection methods often confined to the design of the sensors and their associated electronics. These include such techniques as encapsulation/hermetic sealing, nonincendive design, nonsparking design, and use of oil immersion. These basic design considerations simply isolate potential sources of ignition (heat, sparks, or flame) from explosive or flammable media. However, because Division 2 areas have a very low probability of actually seeing gases and vapors, these methods have been deemed sufficient for these infrequent and usually short-lived exposures.

Even deceptively simple applications, such as two-wire thermocouples used in high-temperature furnaces or kilns can present a danger to personnel. According to Dennis Hablewitz, senior application engineer at Eurotherm/Barber-Colman's Loves Park, Ill., facility, type K thermocouple can pick up common mode noise from high-voltage electric heaters used in these applications, allowing dangerously high voltages between the thermocouple leads and ground. Stray high voltages like this can cause arcing-dangerous in flammable situations-and an electrocution hazard to workers.

'It is not uncommon to see as much as 380 volts on either lead. The problem is caused by the fact that both a thermocouple's ceramic protection tube and the furnace insulation do not act as insulators at high temperatures (see accompanying figure). Both start to conduct at approximately 700 °C. Once the elevated temperature defeats their insulating ability, heater load faults can be conducted through the metal furnace wall to the protection tube. From this point, the heater potential has a straight path to the measuring instruments-a dangerous, and not uncommon, situation,' Mr. Hablewitz says.

'In this case, the situation was resolved by using three-wire thermocouples. The third lead from the measuring junction was tied to earth ground at the furnace wall,' Mr. Hablewitz adds.

Monitoring digester gas flow in the wastewater industry is another example of instrumentation working in a potentially dangerous situation. Methane gas, a byproduct of digester operation, is classified as Group D. A digester is basically a 'cooker' that heats sludge under pressure to produce a mixture of CO₂ and methane. A flowmeter mounted in the 48-in. output line provides an indication of how well the microorganism-prompted process is working; high flow indicates an efficient reaction. The methane gas is cleaned and used either internally to power other equipment or sold to cogeneration or independent power producers.

Fluid Components International (FCI, San Marcos, Calif.) supplies flowmeters internationally for these applications, specifically Model GF90. The sensor uses low wattage heaters and encapsulates the RTDs in a stainless steel thermowell. This non-incendive design allows the sensing head to be placed directly in the gas flow. According to Glen Fishman, senior application engineer, 'Because the encapsulated sensors cannot be damaged by the media flow, sparks cannot be introduced into the process. Additionally, low-wattage heaters add to the safety of this design.'

What makes an instrument itself explosion-proof? De-pending on device size, any number of design refinements can be incorporated so the sensor cannot ignite an explosive atmosphere by supplying spark or open flame. According to Charles Isaac, product manager, pressure and temperature switches, Barksdale Inc. (Los Angeles), 'Smaller instrumentation of all types can be designed to get explosion-proof status. For example, Barksdale has recently introduced a line of compact pressure switches that are UL-, CSA-, and Cenelec-approved as explosion proof.'

From a design standpoint, smaller devices are often easier to make explosion proof. Housings, gaskets, fasteners, and covers must retain their integrity in case of operational failure. They must also handle high pressure, shock, and vibration. Covers and access plate must be tamper proof. Sensors are often hermetically sealed to exclude the surrounding hostile atmosphere from coming in contact with any source of spark. Additionally, mating surfaces are thoroughly gasketed or permanently sealed to prevent leakage.

Keeping instrumentation designs safe cannot always be done mechanically. And it is one thing to keep a malfunctioning instrument from causing a fire and explosion when it is buried in a thermowell or wrapped in a custom enclosure much like explosion-proof devices are. However, an instrument that 'hangs' on the top of a vessel full of volatile liquid to measure level is often neither buried or enclosed. An intrinsic safety (IS) rating may be the only way out because IS devices cannot produce a spark to ignite an explosive atmosphere. See Protecting against tragedy sidebar.

Ametek Drexelbrook (Horsham, Pa.) provides level instruments in explosion-proof housings, and many are designed as intrinsically safe for hazardous areas. According to Bill Sholette, product support manager, the benefits of instrumentation level intrinsic safety are:

• The instrument/transmitter enclosure can be opened in a hazardous area without the danger of an explosion.
• There is no need to 'sniff' the area using a handheld monitor prior to opening a protective enclosure.
• There is typically a reduced installation cost because conduit and explosion-proof enclosures are not required.

In the process instrumentation field, use of more than one protection method applied to the same device is a common practice, Even though it may seem like 'wearing pants with both a belt and suspenders,' circuits with intrinsically safe inputs can be mounted in segregated or explosion-proof enclosures. Generally, mixed systems are not difficult to install if the single protection methods are appropriately used and are according to relative standards.

'Many of Ametek Drexelbrook's level devices are available as both intrinsically safe and explosion proof,' Mr. Sholette continues. 'Despite the added cost and installation time, many process industry users specify these types of instruments.'

Out of sight doesn't necessarily mean out of mind. Just because sensors can be remotely mounted does not mean they are out of harms way when it comes to being a possible ignition source in a potentially hostile environment. In the case of GE Silicones (Waterford, N.Y.), moving the source of temperature measurement in a mixing operation brought about another problem.

Silicone powder and other raw materials are blended and heated to a temperature between 100 and 200 °C. The idea was to remove high-maintenance thermocouples located in the base of the kettle and relocate them as four noncontact devices on the mixer's lid. In their new position, however, explosive gases that were sometimes liberated during the mixing process proved a definite safety hazard. Leveraging the accuracy and robustness of the infrared sensors needed safety backup, which came in the form of an intrinsically safe unit from Raytek Inc. (Santa Cruz, Calif.).

At the time of installation, Raytek's Thermalert TX was the only intrinsically safe device available, the company said. Installation was a success. According to Bob Secreti, control systems craftsman at GE, 'The number one benefit by far is product quality consistency. We also have less maintenance problems; the sensors just work.'

Ensuring worker safety in today's process plants often requires the control engineer to have first-hand knowledge of many safety technologies. These can vary from exotic software-enabled plant safety shutdown sequences to the basics of explosion control technology. Although instrument level safety seems pretty basic in the overall scheme of things, it often provides the first line of defense against unthinkable tragedy, a genie no one wants to uncork.

For related information, visit the 'Safety Systems' area of Control Engineering Online's Process and Advanced Control channel at www.controleng.com.