Naturally, we must safely accomplish measurement, control calculations, and process modulation through adjusting the final element. Control systems contribute to safe process operation through basic control design, valve failure positions, alarms, safety interlock systems, and pressure relief systems (e.g., Marlin, 2000; AIChE, 1993; Lees, 1996). This section addresses one important hazardous condition, fire and explosion, that is affected by the design and implementation of control and transmission equipment. The material in this section is applicable to a wide range of processes and industries using either analog or digital transmission.
The information presented here provides an introduction to safety through the use of proper control equipment. This section gives a simplified discussion that is not adequate for engineering practice. The reader is cautioned to · Refer to up-to-date safety specifications for control equipment, · Ensure that the appropriate regulations are used for the location where the equipment will be installed, and · Engage an experienced, registered engineer to review all designs. |
All control equipment outside of a protected control room is in an environment with air and possibly, combustible materials; hydrocarbons, dust, or other materials. Note that these combustibles might not normally be present, but they are present in the process (e.g., within vessels and pipes) and could be in proximity to control equipment during unusual situations. The electrical power provided for the instrument introduces the third of the three components required for combustion or explosion, as shown in Figure 5.1. Naturally, combustion and explosion must be prevented, and two commonly employed approaches to prevent hazards are summarized in this section.
Figure 5.1. Triangle showing the three key elements leading to fire and explosions.
Safety can be achieved by removing at least one of the elements in the environment around instrumentation. An additional safety measure could contain the effects of any fire or explosion in a small region, which would prevent it from propagating and creating a major hazard. An approach for achieving safety by influencing each approach is introduced in the following.
· Fuel - A controlled environment can be continuously purged with air or an inert to remove fuel.
· Oxygen - The environment around an instrument can be immersed in a liquid or granular solid that will prevent oxygen (and fuel) from being affected by the source of ignition.
· Ignition - The power source can be maintained below the critical value that could initiate fire or explosion.
· Containment - An instrument can be surrounded with an enclosure that can contain a fire or explosion within the small region, where it will extinguish quickly because of lack of fuel and oxygen. This approach is termed "explosionproofing" in the United States and Canada and "flameproofing" in Europe ; note that the term “proof” here does not mean “no explosion or flame”, it means the combustion is contained and will not propagate to other areas in the process.
Generally, a process has a centralized control building that has an environment free from combustibles. The computers performing control calculations, safety controllers, historical data storage and other higher-level computing are located in this building, as are operations personnel. Sensors and final elements are located at the process, which can have oxygen and fuel present. We note that the fuel should not be present in high concentrations, except within process vessels and pipes. Instrumentation must be designed and operated to be safe, and instrumentation located in areas where a fuel source is not normally present must be safe even during the occurrence of very infrequent fuel releases due to small leaks or spills.
Hazardous Area Classification and additional specifications
The proper instrumentation design and installation depends on the likelihood of fuel being present and the type of fuel that could be present. The engineer must select the area classification from several categories and ensure that the instrumentation is compatible with safe operation. The appropriate local regulatory agency defines the categories, and the instrumentation manufacturer defines the set of specifications appropriate for each equipment. In most countries, the instrumentation equipment must be tested by an independent agency, such as Factory Mutual or Underwriters Laboratory, to verify the specifications given by its manufacturer.
Hazardous Area
The hazardous area classifications differ from country to country; for example, the classifications are different between North America and Europe , although efforts are being made to make them consistent. The classifications presented here are for North America , although since the classifications are in a state of change, the practicing engineer should check with the relevant agency for up-to-date information. Then, references are given for comparisons between the North American and European standards. Area classifications for combustible vapors and dusts are given in Table 5.1 (Ode, 2000).
Area Designation | Area Description |
Zone 0 | Ignitable concentrations of flammable gases or vapors are present continuously or present for long periods of time. Examples include, · Interior of tanks · Locations near vents |
Zone 1 | There may be ignitable concentrations during normal operating conditions or concentrations exist frequently from repair or maintenance of the equipment. Examples include, · An area where the breakdown of equipment could lead to a release · Remember that pumps and compressors can have small leaks |
Zone 2 | There may be ignitable concentrations during temporary situations. Examples include, · Storage where hazardous materials are in containers. · Areas adjacent to Zone 1 with no hazards of its own · Ventilation could prevent the hazard, but it could fail during a leak |
Combustible material specification
In addition to a general quantity and likelihood of hazardous materials, the specific material is important. To simplify classification, several groups shown in Table 5.2 have been defined (Ode, 2000).
Material Group | Description |
Group A | Contains acetylene |
Group B | Contains hydrogen |
Group C | Contains ethylene |
Group E | Contains metal dust |
Group F | Contains coal dust |
Group G | Contains grain dust |
A key difference between the groups is the amount of energy required to cause ignition. For the gases, the most restrictive is Group A (lowest energy for ignition) and least restrictive is Group C.
Temperature Specification
Additional specifications are given for other performance variables, such as the operating temperature; categories for the maximum temperature are given in Table 5.3 Ode, 2000).
Category | Maximum temperature °C (with 40 °C as ambient) |
T1 | 450 |
T2 | 300 |
T3 | 200 |
T4 | 135 |
T5 | 100 |
T6 | 85 |
Note that some categories have sub-categories.
The specifications just described apply to above ground manufacturing and address fire and explosions, and they do not apply to special conditions, such as the following.
· Highly oxygenated atmospheres (oxygen greater than 20 mole %)
· Pyrophoric materials
· Underground, mines
· Any other hazards, e.g., hygiene or toxicology in food and pharmaceuticals
The remainder of this section presents two of the most important approaches for achieving safe instrumentation in the process industries, intrinsic safety and explosion proofing.
Intrinsic safety
Intrinsic safety influences the potential source of ignition without affecting the other two key elements in the safety triangle in Figure 5.1.
Intrinsic Safety: “A technique that achieves safety by limiting the ignition energy and surface temperature that can arise in normal operation, or under certain foreseeable fault conditions, to levels that are insufficient to ignite an explosive atmosphere” (Bentley Nevada, 2006). |
If safety is to be ensured by preventing sources of ignition, excessive power must be prevented for normal and foreseeable fault conditions. For example, low electrical power could be used during normal operation, but reliable safety must also ensure that an electrical fault, which would provide higher voltage or current, must not propagate to the areas in contact with the combustibles.
The concept is shown in Figure 5.2. Note that the intrinsic safety barriers, wiring and the field instrumentation in the process area must be designed and installed as an integrated system.
Figure 5.2. Concept of intrinsic safety in a process plant. Intrinsic safety includes the barriers, field instruments and wiring that must combine to prevent a source of ignition.
The fuel sources can vary widely in the process industries or even within different sections of a large plant. Therefore, several hazardous area classifications are defined that depend on the types of materials. The definitions and equipment performances are provided by national and international professional organizations and each country defines the requirements that must be satisfied within their jurisdiction. To satisfy these requirements, the equipment must be tested and certified by a body accepted by the relevant governmental agency. While the concepts and general goals for intrinsic safety are the same throughout the world, the numerous agencies can define different specifications, so that the engineer must be aware of and abide by the local regulations. In addition, insurance providers may define additional or more restrictive designs.
Explosion proofing
This approach reduces the possibility of a combustible mixture near the source of power combined with limits to the damage that could be caused by an explosion. Again, national and international regulations and standards are available.
Table 5.4 provides a quick comparison of intrinsic safety and explosion proofing, which is paraphrased from Honeywell (2006).
Advantages | Disadvantages | |
Intrinsic Safety | · Allows all three components of ignition triangle to coexist · No limit to power consumption | · Enclosures can be bulky and costly · Any failure can compromise the entire system · Requires periodic inspection |
Explosion Proofing | · Safest · Inexpensive · Periodic inspection not required · Also prevents electrical hazards (shock) to workers | · Applicable when less than 1 Watt required · Does not protect against ignition from other sources, e.g., lightening · Requires all elements of circuit to be intrinsically safe |
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