Page 571 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Applied Process Design 537

• increasing the surface conductivity through the rais- d = diameter, inches (usually of pipe)
ing of the relative humidity or through surface treat- d' = flare rip diameter, fr
ment D = d, = flare tip diameter, in.
• increasing the conductivity of the air DF = minimum distance from midpoint of flame to
the object, ft
• relative low working speeds dp/dt = rate of pressure rise, bar/sec or psi/sec
• proper choice of contact materials E = joint efficiency in cylindrical or spherical shells
• proper control of the contact temperatures of the or ligaments between openings (see ASME
surfaces Code Par.lJW-12 or UG-53)
e = natural logarithm base, e = 2.718
• or a combination of the above e, = TNT equivalent (explosion) (see Table 7-26)
F = environment factor for Table 7-8
Note that charges can be transported by persons or F'gs = relief valve factor for non-insulated vessels in
containers from a nonhazardous area into an unsuspect- gas service exposed to open fires
ing unsafe ( or hazardous) area and ignition could then F = F h = fraction of heat radiated
]7' = operating environment factor for safety relief of
take place [63]. gas only vessels (see pg. 446)
It is essential that the process hazardous atmosphere F = Flow gas/vapor, cubic feet per minute at 14.7
and the process system and handling of combustible psia and 60°F
hydrocarbons/ chemicals be recognized in the physical Fu = The ratio of the ultimate stress of the vessel to
the allowable stress of the vessel
designs by conforming to the appropriate class of atmos- F, = ratio of the yield stress of the vessel to the allow-
phere/ environment codes specified by the National Elec- able stress of the vessel
trical Code [71, 83, and 84]. F1 or F2 = relief area for vessels 1 or 2 resp., sq ft
F2 = coefficient of subcritical flow (see Figure 7-29)
Nomenclature °F = temperature, "Fahrenheit
f = specific relief area, sq meter I cubic meter, or
area/unit volume
a = area, sq in. fq = steam quality, dryness fraction
ap = cross-sectional area of the inlet pipe, sq ft G = specific gravity' of gas (air = 1), or specific gravi-
A = area, sq meters, sq ft, or sq in.; consistent with t:y of liquid (water = I) at actual discharge tem-
equation unit, perature
or A = nozzle throat area, or orifice flow area, effective GPM = gallons per minute flow
discharge area (calculations required), or from g = acceleration of gravity, 32.0 ft/sec/sec
manufacturer's standard orifice areas, sq in. He = heat of combustion of gas/vapor, Btu/lb
A1 = initial vessel relief area, sq in. or sq meters H = total heat transfer rate, Btu/hr
A2 = second vessel relief area, sq in. or sq meters h1 = seal, submerged, ft (Figure 7-70)
A3 = exposed surface area of vessel, sq ft hL = seal pipe clearance, (Figure 7-70)
A. = internal surface area of enclosure, sq ft or sq h = head of liquid, ft
meters he = net calorific heat value, Btu/SCF
Av = vent area, sq meters or sq ft j = number of purge cycles (pressurizing and
A,,, = total wetted surface area, sq ft relief)
1-VT = auto-ignition temperature K = permissible design level for flare radiation
B = cubical expansion coefficient per °F of liquid at (including solar radiation), Btu/hr I sq ft, Table
expected temperature (see tabulation in text) 7-32
0
BP = boiling point, °C or ]7 KP = liquid capacity correction factor for overpres-
B.P. = burst pressure, either psig or psi abs sures lower than 25% from Figure 7-22. Non-
Bar = 14.5 psi= 0.987 atmosphere= 100 k Pa atmos- code equations only.
phere; 14.7 psia = 1.01 bars I<t, = vapor or gas flow correction factor for constant
C1 = c = C = gas/vapor flow constant depending on ratio of back pressures above critical pressure from
specific heats cp/Cv (see Figure 7-25 sonic) curve on Figure 7-26
cp/c,. = ratio of specific heats .K,. = vapor or gas flow factor for variable back pres-
C,, = stoichiometric composition of combustible sures from Figure 7-27A or 27B. Applies to Bal-
vapor in air expressed as a volume percent anced Seal valves only.
C 0 = sonic flow discharge orifice constant, varying Kw = liquid correction factor for variable back pres-
with Reynolds number sures from Figure 7-28. Applies Lo balanced seal
C(psi) 12 = venting equation constant, fuel characteristic valves only. Conventional valves require no cor-
1
constant for explosion venting equation, Table rection.
7-27 (constant characterizes the fuel and clears Ku = liquid viscosity correction factor from chart Fig-
the dimensional units (gases and vapors) ) ure 7-24
Ch = specific heat of trapped fluid, Bm/lb/°F K,1t = steam superheat correction factor from Table 7-7
C2 = subsonic flow constant for gas or vapor, func- K = Napier steam correction factor for set pressures
0
tion of k = c 11/ c., Table 7-11 between 1300 and 2900 psig from Table 7-6
c = orifice coefficients for liquids K = Ker = coefficient of discharge:"