Enclosure Selection

The selection of the motor enclosure type involves not only economics but also assessment of two factors: area hazard classification and other area operation conditions.

The determination of area hazard classifications has been aided greatly by publication of API Standard RP 500. This standard provides an engineering guide for assessing the degree of hazard. The standard should be used as a guide by anyone responsible for determining area classification or applying equipment in a hazardous area. It is not a substitute for sound engineering judgment.

Operating conditions that also must be considered in selecting the enclosure type are exposure to airborne dust, dirt and moisture, the possibility of corrosive dust or vapors in the area, and the expected maximum ambient temperature.

The compressor or blower installation in a typical refinery or chemical process unit is not out-of-doors completely. Some form of shelter often is provided, ranging from only a roof to a completely closed building. When process equipment such as a centrifugal gas compressor, which is not hazardous in normal operation, is present in the shelter, the hazard classification depends on the extent to that which the shelter restricts ventilation. The extent of the shelter provided determines the area classification and the type of motor enclosure that should be applied.

It is generally agreed that a shelter with a roof having ridge ventilation and with curtain walls not extending lower than 8 feet above the operating platform would be freely ventilated. Because a gas compressor would not be a source of hazard, except under abnormal conditions such as an equipment failure, this type of compressor shelter is usually classified as a Di vision 2 area.

In accordance with the RP 500 definition, any shelter having more obstruction to air passage than a "roof and one wall closed" limit is considered to have restricted ventilation and is classified Division 1.

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