ARNOLD, K. (1999). Design of Gas-Handling Systems and Facilities (2nd ed.) Episode 2 Part 12 pps

Electrical Systems 511 Figure 17-14. Open sump in a nonenclosed, adequately ventilated area. (Reprinted with permission from API RP 500.) Figure 17-15. Hazardous area location diagram for a typical offshore production platform. 512 Design ofGAS-HANDLlNG Systems and Facilities For buildings of 1,000 ft 3 or less (such as a typical meter house), API RP 500 defines the building as being adequately ventilated if it has sufficient openings to provide twelve air changes per hour due to natural thermal effects. Assuming the building has no significant internal resistance and that the inlet and outlet openings are the same size and are vertically sepa- rated and on opposite walls, the required free area of the inlet or outlet is; where A = free area of inlet (or outlet) openings (includes a 50 percent effectiveness factor), ft 2 Vol = volume of building to be ventilated, ft 3 Tj = temperature of indoor air, °R T 2 = temperature of outdoor air, °R H' = height from the center of the lower opening to the Neutral Pressure Level (NPL), ft NPL is the point on the vertical surface of a building where the interior and exterior pressures are equal It is given by: where H = vertical (center-to-center) distance between A] and A 2 , ft Aj = free area of lower opening, ft 2 A 2 = free area of upper opening, ft 2 For example, assume a building with inside dimensions of 8 ft wide, 10 ft long, and 8 ft high, an outside temperature of 70°F, inside temperature of 80°F, AI = A 2 and the vertical (center-to-center) distance between A } and A 2 of 6 ft. The height from the center of the lower opening to the NPL is: *Equation derived from 1985 ASHRAE Handbook of Fundamentals, Chapter 22, assuming an air change every five (5) minutes. Refer to the ASHRAE Handbook, Chapter 22, for additional information on naturally ventilated buildings. Electrical Systems 513 Therefore, the minimum area required is: A = (8X10X8) 1,200 [2.97(10/54Q)] 1/2 = 2.27 ft 2 for both the inlet and the outlet GAS DETECTION SYSTEMS Combustible gas detection systems are frequently used in areas of poor ventilation. By the early detection of combustible gas releases before ignitible concentration levels occur, corrective procedures such as shut- ting down equipment, deactivating electrical circuits and activating ventilation fans can be implemented prior to fire or explosion. Combustible gas detectors are also used to substantiate adequate ventilation. Most combustible gas detection systems, although responsive to a wide range of combustible gases and vapors, are normally calibrated specifically to indicate concentrations of methane since most natural gas is comprised primarily of methane. Gas detectors are also used to sense the presence of toxic gases—primarily hydrogen sulfide (H 2 S). These detectors often activate warning alarms and signals at low levels to ensure that personnel are aware of potential hazards before entering buildings or are alerted to don protec- tive breathing apparatus if they are already inside the buildings. At higher levels, shut-downs are activated. Consensus performance standards and guidance for installation are provided for combustible gas detectors by ISA SI2.13 and RP 12.13 and for hydrogen sulfide gas detectors by ISA S12.15 and RP 12.15. Required locations of gas detectors (sensors) are often specified by the authority having jurisdiction. For example, API RP 14C recommends certain locations for combustible detectors. These recommendations have been legislated into requirements in U.S. Federal waters by the Minerals Management Service. RP 14C should be referred to for specific details, but, basically, combustible gas detectors are required offshore in all inad- equately ventilated, classified, enclosed areas. The installation of sensors in nonenclosed areas is seldom either required or necessary. Ignitible or high toxic levels of gas seldom accumulate and remain for significant periods of time in such locations. S14 Design of GAS-HANDLING Systems and Facilities When specifying locations for gas detector sensors, consideration should be given to whether the gases being detected are heavier than air or lighter than air. Hydrogen sulfide is heavier than air and therefore, hydrogen sulfide detectors are normally installed near the floor. Since sensors may be adversely affected (even rendered ineffective) if coated with water, they normally should be installed 18 to 36 inches above the floor if they may be subjected to flooding or washdown. Most combustible gas detector sensors are installed in the upper por- tions of buildings for the detection of natural gas. However, in many cases the vapor which flashes off oil in storage tanks can be heavier than air. Below grade areas should be considered for sensor installations where heavier-than-air vapors might collect. Sensing heads should be located in draft-free areas where possible, as air flowing past the sensors normally increases drift of calibration, short- ens head life, and decreases sensitivity. Air deflectors are available from sensor manufacturers and should be utilized in any areas where significant air flow is anticipated (such as air conditioner plenum applications). Additionally, sensors should be located, whenever possible, in locations which are relatively free from vibration and easily accessed for calibration and maintenance. Obviously, this cannot always be accomplished. It usually is difficult, for example, to locate sensors in the tops of compres- sor buildings at locations which are accessible and which do not vibrate. It generally is recommended, and often required, that gas detection systems be installed in a fail-safe manner. That is, if power is disconnected or otherwise interrupted, alarm and/or process equipment shutdown (or other corrective action) should occur. All specific systems should be carefully reviewed, however, to ensure that non-anticipated equipment shutdowns would not result in a more hazardous condition than the lack of shutdown of the equipment. If a more hazardous situation would occur with shutdown, only a warning should be provided. As an example, a more hazardous situation might occur if blowout preventers were automatically actuated during drilling operations upon detection of low levels of gas concentrations than if drilling personnel were only warned. Concentration levels where alarm and corrective action should occur vary. If no levels are specified by the authority having jurisdiction, most recommend alarming (and/or actuating ventilation equipment) if combustible gas concentrations of 20 percent LEL (lower explosive limit) or more are detected. Equipment shutdowns, the disconnecting of electrical power, production shut-in, or other corrective actions usually are recommended if 60 percent LEL concentrations of combustible gas are detect- Electrical Systems 515 ed. Hydrogen sulfide concentrations of 5 ppm usually require alarms and actuation of ventilation equipment and levels of 15 ppm usually dictate corrective action. Special attention should be given to grounding the sheathes and shields of cables interconnecting sensing heads to associated electronic controllers. To avoid ground loops, care should be taken to ground shields only at one end, usually at the controller. If cables are not proper- ly grounded, they may act as receiving antennas for radio equipment and other RF generators at the location, transmitting RF energy to the electronic controller. This RF energy can cause the units to react as if combustible or toxic gas were detected, causing false alarms or unwarranted corrective action. The use of RF-shielded enclosures is recommended where RF problems are experienced or anticipated. GROUNDING A ground, as defined by the National Electrical Code, is a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth. Proper grounding of electrical equipment and systems in production facilities is important for safety of oper- ating personnel and prevention of equipment damage. The term "grounding" includes both electrical supply system grounding and equipment grounding. The basic reasons for grounding an electrical supply system are to limit the electrical potential difference (voltage) between all uninsulated conductive equipment in the area; to provide isolation of faults in the system; and to limit overvoltage on the system under various conditions. In the case of a grounded system it is essential to ground at each sepa- rately derived voltage level. Electrical Supply System Grounding The electrical supply system neutral can be grounded or ungrounded, but there is an increasing trend in the industry toward grounded systems. Ungrounded power systems are vulnerable to insulation failures and increased shock hazards from transient and steady state overvoltage conditions. Grounding of an electrical supply system is accomplished by connecting one point of the system (usually the neutral) to a grounding electrode. The system can be solidly grounded, or the ground can be 516 Design of GAS-HANDLING Systems and Facilities through a high or low resistance. A resistance ground is more suited for certain systems—particularly when process continuity is important. Equipment Grounding Equipment grounding is the grounding of non-current carrying conductive parts of electrical equipment or enclosures containing electrical components. This provides a means of carrying currents caused by insulation failure or loose connections safely to ground to minimize the dan- ger of shock to personnel. The following equipment (not all inclusive) requires adequate equipment grounding: 1. Housings for motors and generators 2. Enclosures for switchgear and motor control centers 3. Enclosures for switches, breakers, transformers, etc. 4. Metal frames of buildings 5. Cable and conduit systems 6. Conductive cable tray systems 7. Metal storage tanks Groundling for Static Electricity A discharge to ground of static electricity accumulated on an object can cause a fire or explosion. A static charge can have a potential of 10,000 volts, but because it has a very small current potential, it can be safely dissipated through proper bonding and grounding. Bonding two objects together (connecting them electrically) keeps them at the same potential (voltage), minimizing spark discharge between them. Generally, equipment bonded to nearby conducting objects is adequate for static grounding. The equipment grounding conductor carries static charges to ground as they are produced. Grounding for Lightning Elevated structures such as vent stacks, buildings, tanks, and overhead lines must be protected against direct lightning strikes and induced lightning voltages. Lightning arrestors or rods are installed on such objects and connected to ground to safely dissipate the lightning charges. Electrical Systems 517 Grounding Methods Onshore, grounding is generally provided by installing a ground loop, made of bare copper conductors, below the finished grade of the facility. Individual equipment grounding conductors and system grounding conductors are then connected to this ground loop, usually by a thermow- eld process. A number of grounding electrodes, generally %-in. to %-in. diameter and 8-10 ft long copper or copper-clad steel rods, are driven into the earth and connected to the ground loop. The number of ground rods required and the depth to which they should be driven are calculated based on the resistivity of the soil and the minimum required resistance of the grounding system. Most grounding systems are designed for less than 5 ohms resistance to ground. A continuous underground metallic water piping system can provide a satisfactory grounding electrode. The National Electrical Code, Article 250, covers requirements for sizing ground loops and equipment/ system grounding conductors. Offshore, the equipment and system ground conductors are connected to the facility's metal deck, usually by welding. The metal deck serves the function of the ground loop and is connected to ground by virtue of solid metal-to-metal contact with the platform jacket. D.C POWER SUPPLY Generally, electrical control systems are designed "Fail-Safe." If power is temporarily lost, unnecessary shutdown of the process may occur. Thus, most safety systems such as fire and gas detectors, Nav-Aids, communi- cations, and emergency lighting require standby D.C. power. Most D.C. power systems include rechargeable batteries and a battery charger system which automatically keeps the batteries charged when A.C. power is available. In some systems, a D.C to-A.C. inverter is provided to power some A.C. emergency equipment such as lighting. Solar cells can also be used for charging batteries. Solar cells are frequently used at unmanned installations without on-site power generation. Some- times non-rechargeable batteries are also used at such locations. 518 Design of GAS-HANDLING Systems and Facilities Batteries Numerous types of batteries are available. A comparison of batteries by cell type is shown in Table 17-1. Rechargeable batteries emit hydrogen to the atmosphere, and hence must be installed such that hydrogen does not accumulate to create an explosion hazard. Ventilation should be provided for battery compartments. Batteries should normally be installed in an unclassified area. Howev- er, if installed in Division 2 areas, a suitable disconnect switch must be installed to disconnect the load prior to removing the battery leads and thus avoid a spark if the battery leads are disconnected under load conditions. Batteries should not be installed in Division 1 areas. Battery Chargers Battery chargers are selected based on cell type and design ambient conditions. Chargers connected to self-generated power should be capa- ble of tolerating a 5% frequency variation and a 10% voltage variation. Standard accessories of chargers include equalizing timers, A.C. and D.C. fuses or circuit breakers, current-limiting features, and A.C. and D.C. ammeters and voltmeters. Optional accessories such as low D.C. voltage alarms, ground fault indications, and A.C. power failure alarms are usually available. Chargers are normally installed in unclassified areas. However, it is possible to purchase a charger suitable for installation in a classified area. CATEGORIES OF DEVICES Electrical switches, relays, and other devices are described for safety reasons by several general categories. Since these devices are potential sources of ignition during normal operation (for example, arcing contacts) or due to malfunction, the area classification limits the types of devices which can be used. High-Temperature Devices High-temperature devices are defined as those devices that operate at a temperature exceeding 80 percent of the ignition temperature (expressed in Celsius) of the gas or vapor involved. The ignition temperature of natural gas usually is considered to be 900°F (482°C). Therefore, a device is Table 17-1 Comparison of Batteries by Cell Type Projected Projected Wet Shelf Useful life Cycb Life 1 Life** Type (Years) (Number of Cycles) (Months) Primary 1-3 1 12 SLI (Starting, %-2 400-500 2-3 Lighting & Ignition) (Automotive Type) Lead Antimony 8-15 600-800 4 Lead Calcium 8-15 40-60 6 Comments*** Least maintenance. Periodic replacement. Cannot be recharged. High hydrogen emission. High maintenance. Not recommended for float service or deep discharge. Low shock tolerance. Susceptible to damage from high temperature. High hydrogen emission. Periodic equalizing is required for float service and full recharging. Low shock tolerance. Susceptible to damage from high temperature. Low hydrogen emission if floated at 2.17 volts per cell. Periodic equalizing charge is not required for float service if floated at 2.25 volts per cell. However, equalizing is required for recharging to full capacity. When floated below 2.25 volts per cell, equalizing is required. Susceptible to damage from deep discharge and high temperature. Low shock tolerance. (table continued on next page) Table 17-1 (Continued) Comparison of Batteries by Cell Type Projected Projected Wet Shelf Useful Life Cycle Life* Life** Type (Years) {Number erf Cycles) (Months) Comments*** Lead Selenium 20+ 600-800 6 Low hydrogen emission if floated at 2.17 volts per cell. Periodic equalizing charge is not required for float service if floated at 2.25 volts per cell. However, equalizing is required for recharging to full capacity. When floated below 2.25 volts per cell, equalizing is required. Low shock tolerance. Susceptible to damage from high temperature. LeadPlante 20+ 600-700 4 Moderate hydrogen emission. (Pure Lead) Periodic equalizing charge is required for float service and full recharging. Low shock tolerance. Susceptible to damage from high temperature. Nickel Cadmium 25+ 1000+ 120+ Low hydrogen emission. (Ni-Cad) Periodic equalizing charge is not required for float service, but is required for recharging to full capacity. High shock tolerance. Can be deep cycled. Least susceptible to temperature. Can remain discharged without damage. C»wtt">\ of \P1 RP 14F *f \ ( le life n the number, /, u/<n atnhit.fi nnu « • si hatred ba'w- ^ "*' 't-raif r»-l<. %>'f 'rfi" oriqiml ampere-hour capacity. A cycle is defined as the removal of 15% oj the lated batten ampei e hour capacity "~l\(t \htflttije ii defined ds the time that m initial* nlh i hatst i hatit'n tan he >/.>»«* at 7 ~"f until permanent cell damage wcurs. ' "*Float ivltage^ listed aie tor 77 f [...]... and gas producing facility applications Hermetically sealed devices are often desirable to protect electrical contacts from exposure to salt air and other contaminants Table 17 -2 Temperature Ratings of Explosion-Proof Enclosures Maximum Temperature °C °F 450 300 28 0 26 0 23 0 21 5 20 0 180 165 160 135 120 100 J*5 8 42 5 72 536 500 446 419 3 92 356 329 320 27 5 24 8 21 2 J85 Identification Number Tl T2 T2A T2B... that the design of intrinsical- 524 Design of GAS-HANDLING Systems and Facilities ly safe equipment is a highly specialized skill and normally best left to those specifically trained in that art The installation of equipment which has been rated by a testing organization as intrinsically safe should follow the guidelines of ISA RP 12. 6, "Recommended Practice for Installation of Intrinsically Safe Systems. .. inside the enclosure and damage enclosed electrical equipment The surface temperature of explosion-proof enclosures cannot exceed that of high-temperature devices Equipment can be tested by nationally recognized testing laboratories and given one of 14 "T" ratings, as indicated in Table 17 -2 This equipment may exceed the "80 percent rule," 522 Design of GAS-HANDLING Systems and Facilities but the "T"... usually contain arcing devices and are not acceptable (text continued on page 529 ) figure 17-16 Typical devices containing arcing contacts in explosion-proof enclosures, {Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.] 528 Design of GAS-HANDLING Systems and Facilities Figure 17-17 Standard explosion-proof alarm devices (Courtesy of Crouse-Hinds Electrical... stems and provided with set screws to prevent loosening Stems over 12 inches in length must be laterally braced within 12 inches of fixtures All portable lamps in Division 1 areas must be explosion-proof Figures 17-19, 17 -20 , and 17 -21 show typical explosion-proof lighting fixtures Lighting fixtures for Division 2 locations must be either explosionproof or labeled as suitable for Division 2 for the particular... metal conduit 2 IMC (Intermediate Metal Conduit) 3 MI cable (Mineral Insulted Cable) 4 Explosion-proof (XP) flexible connections Threaded rigid metal conduit must be threaded with an NPT standard conduit cutting die that provides %-in taper per foot, must be made up 5 32 Design of GAS-HANDLING Systems and Facilities Figure ! 7 -22 Standard lighting fixtures suitable for Class I, Division 2, Group D areas... philoso- 530 Design of GAS-HANDLING Systems and Facilities Figure 17-19 Typical Class I, Division 1 lighting fixures (Courtesy of Grouse-Minds Electrical Construction Materials, a division of Cooper Industries, Inc.] Figure 17 -20 Typical explosion-proof fluorescent lighting fixtures (Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.] Electrical Systems 531... majority of new systems are 534 Design of GAS-HANDLING Systems and Facilities Figure 17 -23 Liquidtight and flexible cord connectors (Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.] cable—particularly a cable with a gas/vapor-tight continuous corrugated aluminum sheath, rated Type MC, and with an overall jacket (usually PVC) and normally installed in cable... liquidtight and flexible cord connectors and an explosion-proof flexible connection are shown in Figure 17 -23 Wiring System Selection The designer must decide at inception whether to provide a cable system or a conduit system Although both systems have specific advantages, the present trend is toward the installation of cable systems rather than conduit systems Offshore, the vast majority of new systems. .. Materials, a division of Cooper Industries, Inc.) Figure 17-18 Typical telephone instrument suitable for Class I, Divisions 1 and 2, Group D areas (Courtesy of Crouse-Hinds Electrical Construction Materials, a division of Cooper Industries, Inc.] Electrical Systems 529 (text continued from page 525 ) unless the arcing devices are installed in explosion-proof enclosures D.C motors contain brushes and are not acceptable . 17 -2 Temperature Ratings of Explosion-Proof Enclosures Maximum Temperature Identification °C °F Number 450 8 42 Tl 300 5 72 T2 28 0 536 T2A 26 0 500 T2B 23 0 446 T2C 21 5 419 T2D 20 0 3 92. 356 T3A 165 329 T3B 160 320 T3C 135 27 5 T4 120 24 8 T4A 100 21 2 T5 J*5 J85 T6 Electrical Systems 523 Purged Enclosures Purged enclosures are those enclosures provided with a purge. "T" ratings, as indicated in Table 17 -2. This equipment may exceed the "80 percent rule," 522 Design of GAS-HANDLING Systems and Facilities but the "T"

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