Keeping the 'Explosion Genie' in the Bottle
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 assoc...
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.
Defining the danger
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 CO 2 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.'
Not out of mind
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 .
Protecting against tragedy; prevention vs. containment
Unless process media are completely inert, the potential exists for fire, explosion, corrosion, and/or environmental damage in case of an alarm situation. Of these disasters, explosion and fire are often the deadliest to plant personnel.
Explosions can be prevented in several ways. One way is by limiting the amount of electrical energy available in hazardous areas. Controlling electrical parameters such as voltage and current requires the use of energy limiting devices known as intrinsically safe (IS) barriers. IS barriers limit the levels of power available in a protected barrier. If a spark or excess electrical heat cannot occur, neither can a fire or explosion. Although used in Europe for many years, intrinsic safety was not adopted as part of the U.S. National Electrical Code until 1990.
An intrinsically safe circuit contains three components: the target device, IS barrier, and wiring. Devices within the protected area can be categorized as simple (contacts, resistors, thermocouples, RTDs, etc.) or complex (transmitters, relays, solenoids, etc.). Complex devices often have complicated circuitry that can store excess electrical energy and are normally certified 'intrinsically safe' by safety testing and certification organizations, such as Underwriters Laboratories (Northbrook, Ill.) or Factory Mutual (Norwood, Mass.).
Selection of proper IS barriers requires calculation of both the open-circuit voltage and short-circuit current of simple devices. For complex devices, both allowed capacitance and inductance values must be calculated. Results are then compared to ignition curves that have been calculated for a wide variety of flammable/explosive media (gases, vapors, airborne dusts or fibers, etc.) to determine if the available energy is below the amount needed for ignition.
Brute force approach
Explosion-proof enclosures provide a brute-force method of preventing or controlling potentially explosive situations. These heavy, cast-usually but not always-devices feature sealed and securely fastened access doors. They protect the normal power level devices within them from coming into contact with an explosive atmosphere. Even under fault conditions, an explosion or fire usually cannot occur because of limited air for combustion within the sealed container. If an explosion does occur, the housing is strong enough to contain it.
Although there have been many refinements in explosion-proof enclosure design, the fact remains that they are bulky, can be difficult to mount because of their weight, and are not the handiest of housings to access. Additionally, seals, gasketing, and purging systems require inspection and maintenance if their integrity is to be trusted. The fact remains that in industries where high voltages and currents are routinely encountered and process systems are rarely reconfigured, explosion-proof enclosures remain a practical method of preventing an industrial tragedy.
Gas storage facility maximizes instrument safety with combined bus systems
The Unionville Compressor Station of Reliant Energy Pipeline Services, located outside of Dubach, La., receives raw gas from fields in eastern Texas and northern Louisiana. Recently, the company upgraded the hardware and control system for hazardous and nonhazardous areas of its Unionville Station property. The project's main goal was to add more field devices to increase accuracy in measuring flow, temperature and pressure. It included upgrading the PLC system and bus architecture at the same time. The previous system used obsolete PLCs and seriously out-of-date bus architecture. It used direct hard-wiring and was based on 4-20 mA control-loop technology and corresponding types of field devices that are used with current-loop systems. After looking at several vendor choices, as well as the pros and cons of various system scenarios, Reliant Energy decided to use Siemens Simatic PLCs and a Siemens Profibus fieldbus system.
The new system uses a total of nine Simatic 505-based PLCs. Two of these PLCs are responsible for sending and receiving data to and from the metering fields, designated as East and West Metering Runs. Central to the property is a 24-hr/day attended control station with big-screen human-machine interface (HMI) monitors. These monitors have screen displays designed with Intellution software. PLCs connect to the HMIs over an Ethernet backbone.
Safe bus choices
Profibus, used widely in discrete manufacturing operations, has also been applied in process industries, especially where a mixture of nonhazardous and hazardous environments need to be connected over a single bus system. For applications requiring fast, open communications, throughput rates are typically set at either 1.5 Mb or 12 Mb, depending on speed requirements and other factors. Profibus-DP, with 11-bit character format, is not designated as intrinsically safe for use in potentially explosive environments. Profibus-PA, on the other hand, typically operates at 31.25 Kbit/sec in accordance with IEC 1158-2. It uses an 8-bit character format and is rated as intrinsically safe according to IEC H1 and CENELEC. Using a combination of special linking and coupling modules as a simple gateway, users can link Profibus DP and Profibus PA bus systems so data transmission between the networks are 'decoupled.'
By using Profibus DP/PA couplers, up to five of the PA runs can be linked through a single linking module. A total of six linking modules were required for the East and West portions of the Unionville facility. This allowed 30 PA cable runs into the respective metering fields. For each linking module and five-coupler combination, all lines have physically isolated power supply, but constitute one bus system in terms of communication. The linking modules for each side of the facility (East and West) are daisy-chained with one DP line leading to each of the two PLCs assigned to the metering runs.
With standard Siemens Step 7 PLC software programming tools users can configure the PLC and Profibus system. From the viewpoint of the process control system, the DP/PA links are modular slaves. Individual modules of this slave are the metering devices and other field devices connected to the lower-level Profibus PA side of the system. The metering devices and other field devices are addressed indirectly via the DP/PA link. The Profibus linking module reserves one Profibus DP address, conserving the addressing capacity of the PLC system. This means from the PLC's perspective, the system makes the DP/PA couplers invisible. Couplers, which pass telegrams to linking modules, do not need a separate bus address.
Fewer hardware components, reduced installation time, and avoiding the need to enclose Profibus PA cabling in explosive-proof conduit saved money. The project management team estimated cost savings as high as $80,000, when compared to 4-20 mA systems using explosive-proof conduit and hardwiring. Open-style metal channeling attached to the ceiling and having the cabling fully exposed provided easy access and visibility to the cabling, while still meeting safety requirements.
Classification of hazardous areas
Class I: Flammable gases and vapors
Group A: Acetylene
Group B: Hydrogen, butadiene ethylene oxide, proplylene oxide
Group C: Ethylene, coke oven gas, diethyl ether, dimethyl ether
Group D: Propane, acetone, alcohols, ammnoia, benzene, butane, ethane, ethyl acetate, gasoline, heptanes, hexanes, methane, octanes, pentanes, toluene
Class II: Combustible dust
Group E: Metal dust
Group F: Coal, coke dust
Group G: Grain, plastic dust
Class III: Combustible flyings and fibers
Wood flyings, paper fibers, cotton fibers
In North America, hazardous areas are classified using two basic parameters: the type of flammable material and the probability that a hazardous material is present. U.S. National Electrical Code and the Canadian Electrical Code divide flammable materials into three classes: gases, dusts, and fibers. Gases and dusts are subdivided into groups with similar explosive potential. The following table lists some typical materials found in each category, in descending order of flammability.
In addition to classifying types of hazardous materials, the area is also defined by the probability that those materials are present. Division 1 areas are defined as ones where hazardous materials may be present under normal operating conditions. Division 2, on the other hand, is defined as an area where hazards arise only as the result of leaks, ventilation failure, or unexpected breakdowns. These areas have a low probability of danger because only an mishap, such as a spill or equipment failure, can create a hazardous situation. Probability of the presence of hazardous material must be less than 1% for an area designated Division 2.