ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 13 docx

Standard explosion-proof junction boxes and conduit fittings.Courtesy of Crouse-H/nc/s Electrical Construction Materials, a division of Cooper Industries, Inc.. Courtesy of Crouse-Hinds

Figure 17*24 Standard explosion-proof junction boxes and conduit fittings.

(Courtesy of Crouse-H/nc/s Electrical Construction Materials, a division of Cooper Industries, Inc.)

1 Confine internal explosions to explosion-proof enclosures and

con-4 Prevent "pressure piling."

Pressure piling is a phenomenon caused by the fact that ignition in an enclosure can first pre-compress gases in a conduit or other enclosure to

Figure 17-25 Standard sealing fittings (Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.]

Figure 17-26 Cutaway drawing of a property installed sealing fitting (Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.]

which It is connected When pre-compressed gases are then ignited in the second enclosure, pressures exceeding those for which it has been tested can be reached.

Receptacles and Attachment Plugs

Receptacles and attachment plugs for Class I, Division 1 and 2 areas must be approved for the area They must provide a means of connection

to the grounding conductor of a flexible cord Typical Class I receptacles and attachment plugs are shown by Figure 17-27.

Figure 17-27 Typical Class I, Division 1 and 2 receptacles and attachment plugs.

(Fop, courtesy of Crouse-Hinds Electrkal Construction Atoteria/s, a division of Cooper Industries, Inc.; bottom, courtesy ofAppleton Electric Co., a division of Emerson Electric Co.)

Seal Locations

Seals must be installed in the following locations:

1 Seals are required at entries by conduit or cable to explosion-proof enclosures containing arcing or high-temperature devices in Divi-

sion 1 and Division 2 locations It is not required to seal \1A in or

smaller conduits into explosion-proof enclosures in Division 1 areas housing switches, circuit breakers, fuses, relays, etc., if their cur- rent-interrupting contacts are hermetically sealed or under oil (hav- ing a 2-in minimum immersion for power contacts and 1-in for control contacts).

2 Seals are required where 2 in or larger conduits enter proof enclosures containing taps, splices, or terminals in Division 1 areas (but not Division 2 areas).

explosion-3 Seals are required in conduits leaving Division 1 areas or traversing from Division 2 areas into unclassified areas, on either side of the boundary No union, coupling, junction box, or fitting is allowed between the seal and the boundary Metal conduits that pass com- pletely through a Division 1 or a Division 2 location without a union, coupling, junction box, or fitting within 12 in of the Division 1-Division 2 or Division 2-unclassified boundary do not require a sealing fitting at the boundary.

4 Except for conduit or cable entries into explosion-proof enclosures containing arcing or high-temperature devices (as described in Item

1 above), cables that will leak gas through the core at a rate of less than 0.007 ft3/hr at 6 in of water pressure need not be sealed if they are provided with a continuous gas/vapor-tight sheath Cables with such a sheath that will transmit gas at or above this rate must be sealed if connected to process equipment that may cause a pressure

of 6 in of water at the cable end.

5 Cables without a continuous gas/vapor-tight sheath must be sealed

at classified-unclassified area boundaries.

6 All cable terminations in Division 1 areas must be sealed This

requirement is imposed by API RP 14F, when specific cables are allowed in Division 1 areas.

7 Special sealing fittings (not yet commercially available) are required for cables and conduits connected to process connections that depend on a single seal, diaphragm, or tube to prevent process fluid

from entering the conduit or cable system Single barrier devices probably are best avoided if multiple barrier devices are available Sealing fittings must be installed as close as practicable to explosion- proof enclosures, but in no case more than 18 inches from the enclosures, Although junction boxes and other devices which materially increase the cross-sectional area of the conduit system connecting the enclosure and the seal may not be installed between an enclosure and a seal, explosion- proof unions, couplings, elbows, capped elbows, and conduit bodies such

as an "L," "T," or "cross" are allowed if the conduit bodies are not larger than the trade size of the connecting conduit, A single seal may suffice for two enclosures if it is installed no more than 18 in from either enclosure Certain devices may be obtained which are "factory-sealed*9—that is, interconnecting wiring is sealed by the manufacturer where it enters/exits enclosures These devices do not require an additional (external) seal, and often can be utilized to advantage in lessening installation time and reducing space requirements (for an external seal).

These devices are tested only for internal explosions and not for

exter-nal explosions pressurizing the devices from the outside As an example,

a factory-sealed push-button start/stop station connected to an

explosion-proof motor starter cannot suffice as a seal for the motor starter conduit

entry A separate seal must be installed at the point of conduit entry Cable termination fittings are available which also are approved as sealing fittings, and often incorporate a union in their design Particularly for space-limited installations, the difference in length requirements for such dual-purpose devices, as compared to standard sealing fitting/cable terminator/union combinations, can be consequential.

Seal Fittings Installation

When seal fittings are installed, certain mechanical practices must be followed The following requirements are often overlooked by both installers and inspectors:

1 Accessibility Sealing fittings should not be installed behind walls or

in other inaccessible locations.

2 Orientation Certain fittings are designed specifically for either zontal or vertical mounting; others may be installed either horizon- tally or vertically, or even at oblique angles.

hori-3, Approved compound Only approved compound and damming fiber may be used Rags or putty materials may not be used to construct dams Some cable seals use a self-hardening putty-like material and

do not require damming fiber.

4, Splices Splices and taps may not be made in sealing fittings Most sealing compounds are poor insulators, and electrical shorts could occur.

5 Drains Drains or drain seals must be provided in locations sary to prevent water accumulation.

neces-6 Thickness Completed conduit seals must have a seal which is at least as thick as the trade size of the conduit In no case can the seal

be less than %-in thick.

Specific Equipment Considerations

Transformers

Although transformers suitable for other industrial installations are generally suitable for producing applications, certain options may be desirable—primarily due to environmental considerations At locations subject to harsh environmental conditions, and particularly at locations subject to washdown with high-pressure hoses, non-ventilated enclosures are desirable, if not necessary Likewise, at locations subjected to salt water and salt-laden air, it often is desirable to specify copper windings and lead wires Most manufacturers provide standard units with alu- minum windings and lead wires Even if aluminum coils are used, it is almosl always desirable to require stranded copper lead wires This will lessen corrosion and loose terminal problems when transformers are interconnected to the facility electrical system with copper conductors If the transformers are to be installed outdoors in corrosive environments, cases should be of corrosion-resistant material (e.g., stainless steel) or be provided with an exterior coating suitable for the location.

Many producing facilities are located offshore or in other tally sensitive areas In these areas, the use of dry (versus liquid-filled) transformers will eliminate the necessity of providing curbing and other containment systems to prevent pollution Dry transformers are normally preferred for most production facility applications Liquid-filled trans- formers should be considered, however, for high voltage and large units (particularly over several hundred kVA).

environmen-Electric Motors

Apart from considerations given to corrosion resistance and suitability for hazardous (classified) areas, the selection of electric motors for oil field applications is the same as the selection of electric motors for other industrial applications One exception may be the selection of motors for areas where electric power is self-generated Frequency and voltage vari- ations may occasionally occur at such locations For such locations, con- sideration should be given to specifying motors which are tolerant to at least 10% voltage variations and 5% frequency variations.

It is cautioned that NEMA Design B motors (normal starting torque) may not be suitable for applications requiring high starting torque such

as positive displacement pumps NEMA Design C motors should be used

in this service.

Most standard motors are manufactured using non-hygroscopic NEMA Class B insulation For added protection in an offshore environment, open drip-proof or weather protected motors should be specified with a sealed insulation system NEMA Class F insulation is also available in most motor sizes and is advisable to provide an improved service factor.

A motor used in standby operation mode should be equipped with a space heater to keep the motor windings dry In classified areas these space heaters must meet the surface temperature requirement of the spe- cific hazardous area.

Lighting Systems

Lighting systems are installed both to provide safety to operating sonnel and to allow efficient operations where natural light is insufficient The lighting required for safety to personnel depends on the degree of the hazard requiring visual detection and the normal activity level It typically varies from 0.5 to 5.0 footcandles Lighting levels required for efficient operations vary from as low as 5 footcandles to as high as 100 footcan- dles, or more Table 17-4 contains some general lighting guidelines The first step in the design of a lighting system is the determination of the various lighting levels required for the specific areas of the facility Typically, the majority of the fixtures are high intensity discharge (HID) fixtures and fluorescent fixtures Certain applications may require incan- descent fixtures as well.

per-HID fixtures include those using mercury vapor and sodium vapor lamps Mercury vapor fixtures are usually less expensive than sodium

Table 17-4A Minimum Recommended Levels of Illumination

for Efficient Visual Tasks

Minimum Lighting Level Area (Footcandles)

Offices, General 50

Offices, Desk Area 70

Recreation Rooms 30

Bedrooms, General 20

Bedrooms, Individual Bunk Lights 70

Hallways, Stairways, Interior 10

Walkwavs, Stairways, Exterior 2

Baths, General 10

Baths, Mirror 50

Mess Halls 30

Galleys, General 50

Galleys, Sink & Counter Areas 100

Electrical Control Rooms 30

Storerooms, Utility Closets 5

Walk-in Freezers, Refrigerators 5

TV Rooms (lights equipped with dimmers) Off to 30Work Shops, General 70

Work Shops, Difficult Seeing Task Areas 100

Compressor, Pump and Generator Buildings, General 30

Entrance Door Stoops 5

Open Deck Areas 5

Panel Fronts 10

Wellhead Areas 5

fixtures initially, and are readily available in most styles However,

sodi-um vapor fixtures are more efficient in the use of electricity.

Because of quite poor color rendition and difficulty in safe disposal of expended lamps, low pressure sodium fixtures are less desirable than high pressure sodium fixtures and are seldom recommended for produc- tion facilities High pressure sodium fixtures are particularly attractive for illuminating large open areas At locations where power cost is low and where many fixtures are required due to equipment shadowing, mer- cury vapor fixtures often are preferred because of their lower initial cost, lower replacement lamp cost, and better color rendition.

The low profile of fluorescent fixtures often dictates their use in areas with low headroom, such as in wellbays on offshore platforms and in

Table 17-4B Minimum Recommended Levels of Illumination

Compressor and Generator Rooms 5.0

Electrical Control Rooms 5.0

Open Deck Areas 0.5

Lower Catwalks 2.0

buildings with conventional ceiling heights The relatively short life, low efficiency, and susceptibility to vibration exclude incandescent lamps from serious consideration for many applications, particularly for general area lighting In areas free from vibration and easily accessible for main- tenance, however, incandescent fixtures may be quite acceptable.

When designing lighting systems, particular attention should be given

to locating fixtures where relamping can be performed safely and ciently Poles which can be laid down, as opposed to climbed, are often preferred—particularly at offshore locations This feature offers less advantage, of course, at land locations where bucket trucks or the like can be used for relamping In locations subject to vibration, it normally is prudent to install lighting fixtures with flexible cushion hangers or flexi- ble fixture supports (hanger couplings) to increase lamp life Remotely mounted ballasts for HID fixtures are frequently desirable, particularly when the fixtures themselves must be installed in locations of high tem- perature and locations difficult to access for maintenance The ceilings of large compressor and pump buildings are examples of locations where remote ballasts often are attractive.

effi-Motor Control Center

The engineer providing the initial design of major facilities is faced with the decision of providing a motor control center building or individ- ual (usually rack-mounted) motor starters and corresponding branch cir- cuit protection devices For installations using only several motors it fre-

quently is more economical to provide individual (usually proof) motor starters and circuit protection devices.

explosion-For facilities that include large numbers of motors and other electrical equipment, it normally is both more economical and more convenient to furnish a building to enclose the required motor starters and distribution panels This building is normally referred to as a motor control center (MCC) In addition to typically allowing less expensive non-explosion- proof equipment, these buildings are frequently environmentally con- trolled (air conditioned, and possibly heated in colder climates) to reduce equipment corrosion and enhance reliability Maintenance is more easily performed indoors than if the equipment were installed outside and main- tenance personnel were subject to extreme cold, rain, snow, or other adverse weather conditions.

If air conditioning systems are designed for buildings housing cal equipment, the heat generated by the electrical equipment must be considered when sizing the air conditioning equipment Artificial heat is seldom required in all but the coldest of climates.

electri-Enclosures

The selection of equipment enclosures involves consideration of ronmental conditions as well as the possibility of exposure to flammable gases and vapors The National Electrical Manufacturers Association (NEMA) provides a list of designations for enclosures that is adequate to specify many enclosure requirements As an example, enclosures desig- nated as NEMA 7 are explosion-proof, suitable for Class I areas for the gas groups labeled NEMA 7 enclosures may be labeled for only one group (such as Group D) or for several groups (such as Groups B, C, and D) NEMA 1 enclosures are designed to perform little other purpose than

envi-to prevent accidental personnel contact with enclosed energized nents, but are suitable for most unclassified areas NEMA 4X enclosures, watertight, and constructed of corrosion-resistant material, are often pre- ferred for outdoor non-explosion-proof applications in areas subjected to harsh environmental conditions or high pressure hose washdown.

compo-CORROSION CONSIDERATIONS

Even though the electrical design details of a system may be well specified, the system will not endure or continue to provide safety to per- sonnel unless proper materials are selected and certain installation proce-

dures followed Most land-based facilities are not subjected to the same harsh environmental conditions as offshore and marshland locations, but even they must be given careful consideration in material selection and installation procedures.

At locations where salt-laden air is present, aluminum should be fied as containing 0.4% or less copper Such aluminum is often referred

speci-to as "copper-free" or "marine grade." Also, galvanic action will occur if aluminum and steel (or other dissimilar metals) are in direct contact Gal- vanic action is accelerated in the presence of salt and moisture Rapid corrosion of uncoated aluminum will occur if it is exposed to materials of high or low pH (less than 4.5 or greater than 8.5) Drilling fluids may fall into the class of high pH materials Additionally, if aluminum is allowed

to contact common fireproof ing materials containing magnesium chloride, rapid corrosion will occur in the presence of moisture.

oxy-To prevent the accumulation of moisture in conduits and enclosures, drains should be installed at all low points In classified areas, breathers and drains must be explosion-proof Figure 17-28 shows typical explo- sion-proof breathers and drains.

Space heaters, particularly in electrical motors and generators which may be idle for significant periods of time, can also help prevent the accumulation of moisture Space heaters installed in classified areas must operate at temperatures below "high temperature" devices.

To retard corrosion and to facilitate future maintenance (e.g., allow the non-destructive removal of threaded junction box covers), all threaded connections should be lubricated with an antiseize compound which will not dry out in the environment If lubricant is applied to the threaded (or flanged) portion of covers of explosion-proof enclosures, the lubricant must have been tested and approved as suitable for flame path use It is cautioned that some lubricants contain silicone, which will poison most catalytic gas detector sensors and should not be used near gas detectors.

Figure 17-28 Standard explosion-proof drains and breathers (Courtesy of

Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.]

Many materials are subject to deterioration by ultraviolet light (UV), particularly many of the "plastics" and fiberglass materials Fiberglass materials for outside use should be specified as UV-stabilized, and most plastics installed outdoors should be carbon-impregnated (black in color) It is particularly recommended that plastic cable ties, which secure cables in cable trays, be carbon-impregnated if installed outdoors.

In areas where electrical equipment is exposed to contaminants, the selection of equipment whose contacts are oil-immersed or hermetically sealed can increase reliability and equipment life Similarly, providing environmentally-controlled equipment rooms can greatly increase equip- ment life at locations where contaminants are prevalent In offshore and other areas exposed to salt, type 316 stainless steel is often preferred over types 303 and 304, which will pit with time Likewise, in similar loca- tions, equipment; fabricated from galvanized steel will corrode much more rapidly than equipment hot-dip galvanized after fabrication.

ELECTRICAL STANDARDS AND CODES

American National Standards Institute (ANSI)

1430 Broadway

New York, NY 10018

C84.1 Voltage Ratings for Electrical Power Systems and Equipment

(60 Hz)

Y 14.15 Electrical and Electronics Diagrams

American Petroleum Institute (API)

2101 L Street, NW

Washington, DC 20037

RP 14F Recommended Practice for Design and Installation of

Elec-trical Systems for Offshore Production Platforms

RP 500 Recommended Practice for Classification of Locations for

Electrical Installations at Petroleum Facilities Classified As Class I, Division 1 and Division 2,

American Society of Heating, Refrigerating and Air Conditioning neers, Inc (ASHRAE)

Engi-1791 Tullie Circle, NE

Atlanta, GA 30329

ASHRAE Fundamentals Handbook