Analytical Instrumentation

What is intrinsically safe system and what is its importance?

Importance of an Intrinsically safe system 1

What is Intrinsic Safety?

Intrinsic safety (IS)

  • It is an idea of a shielding or safeguarding technique for the secure operation of electrical equipment located in hazardous areas by avoiding or reducing the source of available electrical and thermal energy to cause ignition.
  • It is a low-energy signalling technique to prevent firing events by verifying that the energy conveyed to the explosive area is below the required energy to initiate an explosion.
  • The energy levels available for signalling are minute, convenient, and adequate for the majority of instrumentation systems.
  • Firing is a hazardous event in most power plants and process industries.
  • But in some cases, the risk factor is quite conspicuous when flammable gases such as hydrogen gas and propane are being produced or operated, and in other situations, the risk may be less obvious.
  • The two mechanisms that may initiate an explosion.
  1. A spark
  2. A hot surface.
  • The intrinsic safety approach simplifies the circuits and minimizes the cost of installation over other protection methods.
  • During the installation of plant equipment in a location where a fire is a risk, an engineer has two options. He must
  1. Employ explosion-proof techniques.
  2. Utilize an intrinsically safe design approach.
  • Generally, equipment certified with intrinsic safety is slightly more expensive than non-intrinsic equipment.
  • In the design of an intrinsic safety barrier, a Zener diode is used to limit voltage, resistors are used to restrict current and a fuse, none of which are expensive.
  • Being intrinsically safe in terms of electrical means limiting the flow of current or preventing sparks in a hazardous environment.
What is intrinsically safe system and what is its importance 1

Definition of Intrinsic Safety:

  1. The definition of intrinsic safety used in the suitable IEC apparatus standard IEC 60079-11.
  2. It is a kind of protection based on the restriction of electrical energy with required apparatus and of interconnecting wiring exposed to the potentially explosive atmosphere to a level below that may cause ignition by the effect of spark or heat.

Equipment considered for use in hazardous areas with intrinsically safe barriers includes:

  1. 4-20 mA DC Two Wire Transmitters.
  2. Thermocouples.
  3. RTDs.
  4. Strain Gages.
  5. Pressure, Flow, & Level Switches.
  6. I/P Converters.
  7. Solenoid Valves.
  8. Proximity Switches.
  9. Infrared Temperature Sensors.
  10. Potentiometers.
  11. LED Indicating Lights.
  12. Magnetic Pickup Flow meters.

Describe the types of intrinsically safe levels:

NEC 70 intrinsically safe levels are classified as:

Class I: The Class I environment consists of flammable gasses and vapor such as gasoline, hydrogen, natural gas propane, ethanol, or methane.

Class II: The Class II environment consists of combustible dust such as the dust of metal and its alloys, charcoal, carbon black, flour, plastics, grains, and wood. These dust exist under normal and abnormal operating conditions.

Class III: Class III consists of flying objects or fibers.

NEC: National Electrical Code

How does an intrinsic safety system work?

  • An intrinsic safety system behaves as an energy barrier by restricting the voltage and current signals in a hazardous environment.
  • These systems also check the total amount of energy in the hazardous environment.
  • Ensuring lower energy to ignite flammable gasses or particles.
  • Avoiding ignition assumes reducing both the available power and the maximum temperatures.
  • The supplied voltage and current must be less than 29V and 300 mA.
  • In a simpler view is to say that power must be less than 1.3 W.
  • The equipment installed must be certified as intrinsically safe before using in a hazardous area.
  • These safety barriers protect the system. But these must be mounted outside of the hazardous area designated as safe where the hazard is not present.

Note that most of the instrumentation devices used to consume only 24V and can often be designed to draw a current of less than 500 mA which is reasonably sufficient to meet IS certification in many situations.

What are the benefits of an Intrinsic Safe system?

Intrinsic safety system or intrinsic safety barriers provides several advantages when electrical or instrumentation equipment must be placed in a hazardous environment.

  1. Provides a safe working environment and protects from the risk of explosion
  2. The cost of explosion-proof enclosures is reduced.
  3. Saves cost by using standard instrumentation cables.
  4. Insurance premiums can be reduced when risks are minimized

What voltage is considered intrinsically safe?

Intrinsic safety is considered the technique of protection for control and instrumentation circuits.

For intrinsically safe

  1. The nominal voltage is considered to be 24 VDC or less.
  2. The current must be less than 100 mA.
  3. Power must be less than 1.3 W.

Where is intrinsic safety used?

Intrinsic safety accessories are used in hazardous areas that have a dangerous and heavy concentration of combustible gases that may be highly flammable in petrochemical refineries and mining industries.

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