04-09-2012, 12:37 PM
Seminar on Intrinsic Safety
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INTRODUCTION
Intrinsic safety prevents instruments and other low-voltage circuits in hazardous areas from releasing sufficient energy to ignite volatile gases. Intrinsic safety is an explosion protection technique applied to electrical equipment and wiring intended for installation in hazardous locations. The technique is based upon limiting both electrical and thermal energy to levels that are incapable of igniting a hazardous mixture that is present in its most easily ignitable concentration.
All intrinsically safe circuits have three components: the field device, referred to as the intrinsically safe apparatus; the energy-limiting device, also known as a barrier or intrinsically safe associated apparatus; and the field wiring. When designing an intrinsically safe circuit, begin the analysis with the field device. This will determine the type of barrier that can be used so that the circuit functions properly under normal operating conditions but still is safe under fault conditions.
It also is important to make sure that the intrinsically safe circuit will work under normal conditions. With the current-limiting resistor, a voltage drop will occur between the input and output of the barrier. This has to be accounted for in our circuit design.
COMBUSTION THEORY
We need to start our discussion on Intrinsic Safety with some background about ignition, combustion, and explosions. Since our goal is to prevent an explosion, we first need to know how an explosion can occur. An explosion requires the proper environment in order to exist. By eliminating those conditions an explosion is impossible.
The three ingredients necessary for an explosion (fuel, oxidizer and ignition source) make up the Ignition Triangle (see Fig i). All of the triangle’s components must be present in the correct pro-portions for ignition to occur. Once ignition occurs, if the conditions are right, a combustion wave can grow from the ignition. An explosion is any uncontrolled combustion wave.
THE COMBUSTION PROCESS
An explosion starts when the fuel and oxidizer are present in the correct proportions, and an ignition source (i.e. spark) occurs. Energy then proceeds along a narrow combustion zone, and is conducted to unburnt gas ahead of the flame front as well as to burnt gas behind the flame. Initially, the spark supplies most of the energy. If more energy is supplied by the spark (through the combustion zone) than is lost to the surrounding gas, the flame sphere will grow. As the diameter of the sphere increases, the wave begins to resemble a planar wave. Eventually, the amount of energy produced by combustion is sufficient to continually supply energy to the unburnt gas ahead of the flame, thereby causing the wave to self propagate. Fig ii shows the beginning stages of combustion. The larger the initial spark is, the easier it is for the wave to propagate. If the initial spark is not large enough, (i.e. not enough energy), the combustion zone will not have enough energy to self propagate. It will just collapse upon itself, and fizzle out. This means a small spark can occur in a potentially explosive air/gas mixture, with no danger of an explosion. The maximum amount of energy of this “safe” spark varies with the specific air/gas mixture present.
INTRINSIC SAFETY
In the many industrial processes where flammable materials are handled, any leak or spillage may give rise to an explosive atmosphere. To protect both personnel and plant, precautions must be taken to ensure that this atmosphere cannot be ignited. Intrinsic safety is a protection concept employed in potentially explosive atmospheres. Intrinsic safety relies on the electrical apparatus being designed so that it is unable to release sufficient energy, by either thermal or electrical means, to cause an ignition of a flammable gas.
When intrinsically safe systems are discussed as being safe under normal or abnormal conditions it is inferred that regardless of a circuit's condition, power levels will not be of sufficient magnitude to ignite a specific hazardous mixture. When discussing an electrical measurement or control circuit in a hazardous location, abnormal conditions, commonly referred to as fault conditions, would generally be considered to be those circumstances in which the circuit has failed in an unsafe manner creating the risk of explosion. With intrinsic safety, these fault conditions are considered to be normal and expected. Fault conditions may be opening, shorting or grounding of field wiring as well as the application of higher voltages than were intended for the circuit. Each would collectively increase the potential of igniting a flammable or combustible mixture.