Innovation in Global Power - Parsons Brinckerhoff

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Thermal – Achieving New Efficiencies, Reducing Carbon Emissions
PB Inspections Help to Ensure Power Plant Safety
By Stewart Gray, Bangkok, Thailand, 66 (0) 2343 8866, gray.stewart@pbworld.com
The author provides some
insight into the application
of engineering to prevent
explosions and fire in highly
hazardous areas of power
plants fuelled by oil or gas.
Acronyms/Abbreviations
IEC:
International
Electrotechnical
Commission
Figure 1: Schematic of the
principle of a flameproof
enclosure.
In modern thermal power plants fuelled by either oil or gas, fuel handling processes give rise
to situations where electrical equipment could cause an explosion due to a hot surface or a
spark. Indeed, there have been several incidents in the past where lives have been lost and
plant destroyed. Places where these situations arise are termed “hazardous” or “classified areas.”
Special engineering practices designed to prevent explosions in these areas are available.
These practices are often misunderstood and applied incorrectly, however, expert supervision
should always be used at a project start-up to ensure such engineering practices are implemented
properly. The following information is based on the experiences of some of PB’s
workers in this field, particularly our assessments of power plant installations and our ensuring
that relevant codes and practices, local statute and insurance requirements are adhered to.
Applicable Codes or Practice
The code or practice applicable to each installation is normally determined by its locality,
although the several different practices applied worldwide have many similarities. The most
commonly applied codes are International Electrotechnical Commission (IEC) 60079 “Electrical
apparatus for explosive gas atmospheres” and National Fire Protection Association (NFPA) 70
“National Electrical Code.” Both define sets of special precautions (types of protection) required
for electrical equipment in hazardous/classified areas using some very definite vocabulary.
Choice of Types of Explosion Protection
Figure 2: Schematic of the
principle of increased safety.
Figure 3: Schematic of the
principle of intrinsic safety.
It is important to establish the extent of hazardous areas that exist at an early stage of any
plant’s design. These areas are customarily delineated using a plan called a “hazardous areas
layout drawing.” While it is always best to install the electrical equipment elsewhere, doing so
is often unavoidable.
All electrical equipment installed in a hazardous area requires explosion protection. IEC
60079 defines nine types of such protection. Of these, the three types of protection most
commonly found in modern power plant are:
• Flame proof enclosure (type d). This technique limits the effect of an explosion. Parts
that could cause an explosion are placed inside a special enclosure that is strong enough to
contain an internal explosion (Figure 1). The resulting hot gasses exit through a specially
machined path that is relatively long and narrow. As they exit they are cooled sufficiently
to avoid spreading the explosion outside.
The main uses for this type of protection are electrical power equipment, switches, etc.
While this is a well known technique, it is somewhat less readily available than others. It
is also expensive and requires special installation rules.
• Increased safety (type e). This technique (Figure 2) prevents explosions. Parts that could
cause an explosion are made with a superior degree of safety, including long creepages and
clearances, and temperature limitations. Its main use is for junction boxes. This technique is
well known, readily available, and inexpensive. Its use requires observation of special design
and installation rules.
• Intrinsic safety (type i). This technique (Figure 3) also prevents explosions. The circuit
is arranged so the amount of energy that can flow into the hazardous area is limited and
incapable of causing an ignition. Normally, energy limiting “barrier” devices used in the safe
area contain zenner diodes or optical isolators to achieve the energy limitation. Care needs
to be taken to ensure that the hazardous area part of the circuit cannot store large amounts
of energy (i.e., use of low capacitance cables).
11 PB Network #68 / August 2008