Page 318 - Offshore Electrical Engineering Manual

Explosion-Proof Ex ‘d’ Equipment 305

EXPLOSION-PROOF EX ‘D’ EQUIPMENT (OLDER ‘FLAMEPROOF’
CERTIFICATION IS SIMILAR)

The principle of protection, as with the metal gauze on the Davy lamp, is to control
the flow of and to cool any burning gas escaping from the motor casing, so that any
gas that has escaped is no longer hot enough to ignite external gas. The size of gap or
flamepath necessary to sufficiently cool the ignited gas on its way out of the enclo-
sure varies according to the gas or vapour involved. Gases and vapours are subdivided
according to experimental data which has been established to determine the maximum
experimental safe gap (MESG). In the case of metal-to-metal joints in a flameproof
motor, for example, that of the frame to the end shield, these will consist of a long metal
spigot fitting into a long recess which will normally be clamped tightly to fixing bolts.
A flamepath will always exist between the shaft and the motor interior.
For safety, all the flamepaths or gaps in the motor enclosure must never exceed
mandatory dimensions, and the casing of the motor must be strong enough to with-
stand an explosion caused by the ignition of the maximum free volume of air/gas
mixture it can contain, with all flamepaths at minimum production values. It should
be noted that this could be exceeded in very cold regions (polar latitudes) owing to
the increased density of the gas or at the elevated pressures used in diving. This type
of protection is common in low-voltage motors up to a 315 frame size; on larger sizes,
severe cost and weight penalties are likely owing to the cast method of construction.
The following criteria should be considered by the installer to ensure that certifi-
cation is not invalidated and/or the risk of explosion increased:

1. Further entries should not be drilled out on flameproof enclosures. It is advis-
able to allow some margin in the number of entries specified to the manufac-
turer. Approved threaded plugs are available to plug unused entries. All unused
entries must be plugged by such approved plugs.
2. Only tools suitable for use on flameproof equipment should be used.
3. Only flameproof cable glands should be used on flameproof enclosures.
4. The painting or obstruction of gaps between flanges must be avoided.
5. No flameproof equipment should be drilled, cut or welded.
6. Always check that all nuts, bolts, door hinges, isolators and interlocking devices
are installed and are functioning as explained in the manufacturer’s instructions.
7. All Ex ‘d’ equipment flanges should be greased with a certified grease such as
Chemodex copper grease.

It is often operators’ practice that explosion-proof equipment such as glands,
luminaires and small motors are used throughout the installation in order to ration-
alise spares and simplify maintenance procedures. If this philosophy is in operation,
the equipment sited in safe areas should be installed as if it were being installed in a
hazardous area, with the correct seals, flange gaps, etc. The reason for this is that the
equipment will be marked Ex ‘d’, and if hazardous zones are altered at some future
date so that the equipment then comes within such a zone, it will not be obvious that
internal seals or other components are missing or not correctly installed.