Assuming the building has no significant internal resistance andthat the inlet and outlet openings are the same size and are vertically sepa-rated and on opposite walls, the required fre
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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.
Trang 2512 Design ofGAS-HANDLlNG Systems and Facilities
For buildings of 1,000 ft3 or less (such as a typical meter house), API RP
500 defines the building as being adequately ventilated if it has sufficientopenings to provide twelve air changes per hour due to natural thermaleffects Assuming the building has no significant internal resistance andthat 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), ft2Vol = volume of building to be ventilated, ft3
Tj = temperature of indoor air, °R
T2 = temperature of outdoor air, °R
H' = height from the center of the lower opening to the NeutralPressure Level (NPL), ft
NPL is the point on the vertical surface of a building where the interiorand exterior pressures are equal It is given by:
where H = vertical (center-to-center) distance between A] and A2, ft
Aj = free area of lower opening, ft2
A2 = free area of upper opening, ft2
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 = A2 and the vertical (center-to-center) distance between A}and A2 of 6 ft The height from the center of the lower opening to theNPL is:
*Equation derived from 1985 ASHRAE Handbook of Fundamentals, Chapter 22,
assum-ing an air change every five (5) minutes Refer to the ASHRAE Handbook, Chapter 22, for additional information on naturally ventilated buildings.
Trang 3Electrical Systems 513
Therefore, the minimum area required is:
A = (8X10X8)
1,200 [2.97(10/54Q)]1/2
= 2.27 ft2 for both the inlet and the outlet
GAS DETECTION SYSTEMS
Combustible gas detection systems are frequently used in areas of poorventilation By the early detection of combustible gas releases beforeignitible concentration levels occur, corrective procedures such as shut-ting down equipment, deactivating electrical circuits and activating ven-tilation fans can be implemented prior to fire or explosion Combustiblegas detectors are also used to substantiate adequate ventilation Mostcombustible gas detection systems, although responsive to a wide range
of combustible gases and vapors, are normally calibrated specifically toindicate concentrations of methane since most natural gas is comprisedprimarily of methane
Gas detectors are also used to sense the presence of toxic marily hydrogen sulfide (H2S) These detectors often activate warningalarms and signals at low levels to ensure that personnel are aware ofpotential hazards before entering buildings or are alerted to don protec-tive breathing apparatus if they are already inside the buildings At higherlevels, shut-downs are activated
gases—pri-Consensus performance standards and guidance for installation areprovided for combustible gas detectors by ISA SI2.13 and RP 12.13 andfor hydrogen sulfide gas detectors by ISA S12.15 and RP 12.15
Required locations of gas detectors (sensors) are often specified by theauthority having jurisdiction For example, API RP 14C recommendscertain locations for combustible detectors These recommendations havebeen legislated into requirements in U.S Federal waters by the MineralsManagement 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 orhigh toxic levels of gas seldom accumulate and remain for significantperiods of time in such locations
Trang 4S14 Design of GAS-HANDLING Systems and Facilities
When specifying locations for gas detector sensors, considerationshould 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 Sincesensors may be adversely affected (even rendered ineffective) if coatedwith water, they normally should be installed 18 to 36 inches above thefloor if they may be subjected to flooding or washdown
Most combustible gas detector sensors are installed in the upper tions of buildings for the detection of natural gas However, in manycases the vapor which flashes off oil in storage tanks can be heavier thanair Below grade areas should be considered for sensor installationswhere heavier-than-air vapors might collect
por-Sensing heads should be located in draft-free areas where possible, asair flowing past the sensors normally increases drift of calibration, short-ens head life, and decreases sensitivity Air deflectors are available fromsensor manufacturers and should be utilized in any areas where signifi-cant air flow is anticipated (such as air conditioner plenum applications).Additionally, sensors should be located, whenever possible, in locationswhich are relatively free from vibration and easily accessed for calibra-tion and maintenance Obviously, this cannot always be accomplished Itusually 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 detectionsystems be installed in a fail-safe manner That is, if power is disconnect-
ed or otherwise interrupted, alarm and/or process equipment shutdown(or other corrective action) should occur All specific systems should becarefully reviewed, however, to ensure that non-anticipated equipmentshutdowns would not result in a more hazardous condition than the lack
of shutdown of the equipment If a more hazardous situation would occurwith shutdown, only a warning should be provided As an example, amore hazardous situation might occur if blowout preventers were auto-matically 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 occurvary If no levels are specified by the authority having jurisdiction, mostrecommend alarming (and/or actuating ventilation equipment) if com-bustible gas concentrations of 20 percent LEL (lower explosive limit) ormore are detected Equipment shutdowns, the disconnecting of electricalpower, production shut-in, or other corrective actions usually are recom-mended if 60 percent LEL concentrations of combustible gas are detect-
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ed Hydrogen sulfide concentrations of 5 ppm usually require alarms andactuation of ventilation equipment and levels of 15 ppm usually dictatecorrective action
Special attention should be given to grounding the sheathes andshields of cables interconnecting sensing heads to associated electroniccontrollers To avoid ground loops, care should be taken to groundshields only at one end, usually at the controller If cables are not proper-
ly grounded, they may act as receiving antennas for radio equipment andother RF generators at the location, transmitting RF energy to the elec-tronic controller This RF energy can cause the units to react as if com-bustible or toxic gas were detected, causing false alarms or unwarrantedcorrective action The use of RF-shielded enclosures is recommendedwhere RF problems are experienced or anticipated
GROUNDING
A ground, as defined by the National Electrical Code, is a conductingconnection, whether intentional or accidental, between an electrical cir-cuit or equipment and the earth Proper grounding of electrical equip-ment 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 ment grounding
equip-The basic reasons for grounding an electrical supply system are tolimit the electrical potential difference (voltage) between all uninsulatedconductive equipment in the area; to provide isolation of faults in thesystem; and to limit overvoltage on the system under various conditions
In the case of a grounded system it is essential to ground at each rately derived voltage level
sepa-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 andincreased shock hazards from transient and steady state overvoltage con-ditions Grounding of an electrical supply system is accomplished byconnecting one point of the system (usually the neutral) to a groundingelectrode The system can be solidly grounded, or the ground can be
Trang 6516 Design of GAS-HANDLING Systems and Facilities
through a high or low resistance A resistance ground is more suited forcertain systems—particularly when process continuity is important
Equipment Grounding
Equipment grounding is the grounding of non-current carrying ductive parts of electrical equipment or enclosures containing electricalcomponents This provides a means of carrying currents caused by insu-lation failure or loose connections safely to ground to minimize the dan-ger of shock to personnel
con-The following equipment (not all inclusive) requires adequate ment grounding:
equip-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 objectcan cause a fire or explosion A static charge can have a potential of10,000 volts, but because it has a very small current potential, it can besafely dissipated through proper bonding and grounding Bonding twoobjects together (connecting them electrically) keeps them at the samepotential (voltage), minimizing spark discharge between them Generally,equipment bonded to nearby conducting objects is adequate for staticgrounding The equipment grounding conductor carries static charges toground as they are produced
Grounding for Lightning
Elevated structures such as vent stacks, buildings, tanks, and overheadlines must be protected against direct lightning strikes and induced light-ning voltages Lightning arrestors or rods are installed on such objectsand connected to ground to safely dissipate the lightning charges
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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 groundingconductors 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 driveninto the earth and connected to the ground loop The number of groundrods required and the depth to which they should be driven are calculatedbased 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 canprovide 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 servesthe function of the ground loop and is connected to ground by virtue ofsolid 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 batterycharger system which automatically keeps the batteries charged whenA.C power is available In some systems, a D.C.-to-A.C inverter is pro-vided to power some A.C emergency equipment such as lighting Solarcells can also be used for charging batteries Solar cells are frequentlyused at unmanned installations without on-site power generation Some-times non-rechargeable batteries are also used at such locations
Trang 8518 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 gen to the atmosphere, and hence must be installed such that hydrogendoes not accumulate to create an explosion hazard Ventilation should beprovided for battery compartments
hydro-Batteries should normally be installed in an unclassified area
Howev-er, if installed in Division 2 areas, a suitable disconnect switch must beinstalled to disconnect the load prior to removing the battery leads andthus avoid a spark if the battery leads are disconnected under load condi-tions Batteries should not be installed in Division 1 areas
Battery Chargers
Battery chargers are selected based on cell type and design ambientconditions 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 andD.C fuses or circuit breakers, current-limiting features, and A.C andD.C ammeters and voltmeters Optional accessories such as low D.C.voltage alarms, ground fault indications, and A.C power failure alarmsare usually available
Chargers are normally installed in unclassified areas However, it ispossible to purchase a charger suitable for installation in a classified area
CATEGORIES OF DEVICES
Electrical switches, relays, and other devices are described for safetyreasons by several general categories Since these devices are potentialsources of ignition during normal operation (for example, arcing con-tacts) or due to malfunction, the area classification limits the types ofdevices which can be used
Trang 9nat-Table 17-1 Comparison of Batteries by Cell Type
Projected Projected Wet Shelf Useful life Cycb Life1 Life**
Type (Years) (Number of Cycles) (Months)
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)
Trang 10Table 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
Trang 11Electrical Systems 521
considered a high-temperature device in a natural gas environment if thetemperature of the device exceeds 726°F (385°C) The ignition tempera-ture of hydrogen sulfide is usually considered to be 518°F (270°C), Inclassified areas, high-temperature devices must be installed in explosion-proof enclosures unless the devices are approved for the specific area by
a nationally recognized testing laboratory (NRTL)
Weather-Tight Enclosures
Electrical equipment can be mounted in various types of enclosures Aweather-tight enclosure normally has a gasket and does not allow air (andthe moisture contained in the air) to enter the enclosure Offshore, such
an enclosure, if properly closed, will help protect the enclosed electricalequipment from corrosion due to salt water spray These types of enclo-sures can be used in Division 2 areas provided they do not enclose arc-ing, sparking or high temperature devices
Explosion-Proof
Equipment described as "explosion-proof" is equipment installed inenclosures that will withstand internal explosions and also prevent thepropagation of flame to the external atmosphere As the gases generated
by the explosion expand, they must be cooled before reaching the rounding atmosphere
sur-Equipment may be rated explosion-proof for certain gases but not ers For example, an enclosure may be rated as suitable for Group Dgases, but not for Group B gases Therefore, it is not satisfactory to mere-
oth-ly state that equipment must be "explosion-proof"; one must specify
"Explosion-proof for Class I, Group D," as an example Because sion-proof enclosures must have a path to vent the expanding gases cre-ated by the explosion, explosion-proof enclosures "breathe" when thetemperature inside the enclosure is different from that outside That is,they cannot be weather tight As a result, moisture frequently is intro-duced into explosion-proof enclosures Unless suitable drains are provid-
explo-ed in low spots, water can accumulate inside the enclosure and damageenclosed electrical equipment
The surface temperature of explosion-proof enclosures cannot exceedthat of high-temperature devices Equipment can be tested by nationallyrecognized testing laboratories and given one of 14 "T" ratings, as indi-cated in Table 17-2 This equipment may exceed the "80 percent rule,"
Trang 12522 Design of GAS-HANDLING Systems and Facilities
but the "T" rating must be below the ignition temperature of the specificgas or vapor involved As an example, equipment rated Tl has been veri-fied not to exceed 842°F and, therefore, is suitable for most natural gasapplications
Hermetically Sealed Devices
Hermetically sealed devices are devices sealed to prevent flammablegases from reaching enclosed sources of ignition These devices are suit-able for use in Division 2 and unclassified areas
Hermetically sealed electrical devices must be verified by a testinglaboratory to meet mechanical abuse and to withstand aging and expo-sure to expected chemicals Devices "potted" with common silicones andsimilar materials by an end user or even a manufacturer, without testing,and devices merely provided with O-rings seldom meet acceptable crite-ria Normally, hermetically sealed devices must be sealed through metal-to-metal or glass-to-metal fusion Many electrical relays, switches, andsensors are available as hermetically sealed devices for common oil andgas producing facility applications Hermetically sealed devices are oftendesirable to protect electrical contacts from exposure to salt air and othercontaminants
Table 17-2 Temperature Ratings of Explosion-Proof Enclosures
Maximum Temperature Identification