Module 9 : Process Hazards and Safety Measures in Equipment Design

Lecture 3 : Safety measures in equipment design and Pressure Relief Devices

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6.4 Heat transfer equipments

The heat transfer equipment such as evaporator, reactors, furnaces, heat exchangers require some type of heating which may be directly fired with the help of fuel, electric heating, or using heat transfer media like steam or heating fluids. While designing such equipment special precaution should be taken which would not only prevent over heating but protect from fire and explosions this can be accomplished by different ways.

 

  1. i.  Provide sufficient heating surface so that excessive rate of heat input per unit area can be avoided

    2. ii.  In such equipments the heat absorbed by the tubes must be continuously removed by circulating the fluids and to prevent excess temperature rise through the liquid film heat transfer coefficient should be sufficiently high.

    iii.  Periodic inspection of the equipment is necessary and for that reason sufficient numbers of inspection opening must be provided, if applicable.

    iv.  Provision of vent valves at all high spots in the equipment is necessary.

    v.  An expansion or surge tank may be provided in case of liquid phase systems.

    vi.  In case of heaters the tubes must be tightly secured to headers and vapour drums.

    vii.  For high temperature equipments pressure relief devices must be considered in design.

    viii.  While designing precaution should be taken to keep allowance for the stresses due to thermal expansion.

    ix.  Choice of the insulation should be appropriate and consistent with the material handled by the equipment.

 

6.5 Equipment involving electrical energy

All the electrical installations are inherent source for ignition. Special design features are required to prevent the ignition of flammable vapors and dusts from electrical devices. The fire and explosion hazard is directly proportional to the number and types of electrically powered devices in a process area. The process areas are divided into two major types of environment explosion proof and non explosion proof. Explosion proof means flammable material may be present at certain times, and non explosion proof means that flammable materials are not present even under abnormal conditions (for example: the areas like open flames, heated elements and other sources of ignition may be present). The explosion proof design should include the use of conduit with special sealed connections around all junction boxes.

The design of electrical equipment and instrumentations is based on the nature of the process hazards or specific process classifications. It is a function of the nature and degree of the process hazards within a particular area. For example in petroleum industries we always have a classification of hazardous areas of electrical work also.

Class 1: Location where flammable gases or vapors are present.

Class 2: In normal operation explosive mixtures is most likely to occur.

Class 3: Hazard locations where combustible fibers or dusts are present but not likely to be in suspension.

 

6.6 Equipment involving rotary motion

The rotary equipment involves a mechanical drive system that should be protected by guards. The bearings used in the system should be well lubricated and cooled, if necessary, to reduce temperature.