Page 572 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

538 Applied Process Design for Chemical and Petrochemical Plants

0.975 for air, steam, vapors and gases Pd = ASME Code design pressure ( or maximum
0. 724 for ASME Code liquids** allowable working pressure), psi
0.64 for non-ASME Code liquids P do = pressure on outlet side of rupture disk, psia
0.62 for bursting /rupture disk Pe = exit or back pressure, psia, stamped burst pressure
*Where the pressure relief valve is used in Per = perimeter of a cross section, ft or meters
series with a rupture disk, a combination PH = Initial high pressure, mmHg
capacity factor of 0.8 must be applied to the P; = maximum initial pressure at which the com-
denominator of the above valve equations. bustible atmosphere exists, psig
Consult the valve manufacturer (also see spe- P\ = initial pressure of system, psia
cific section this chapter of text) for higher PL = initial low pressure or vacuum, mmHg
factors based on National Board flow test P max = maximum explosion pressure, bar, or other
results conducted with various rupture disk consistent pressure units
designs/arrangements (see Table 7-12). Puv = maximum pressure developed in an unvented
**For saturated water see ASME Code, Appen- vessel, bar (gauge) or psig
dix 11-2. Pop = Normal expected or maximum expected oper-
K 0 = discharge coefficient orifice or nozzle ating pressure, psia
Kc = deflagration index, maximum rate of pressure P;; P O = relieving pressure, psia, or sometimes upstream
rise for gases, bar-meter I second = bar-m/ sec pressure, psi abs, or initial pressure of system
K., = deflagration index, maximum rate of pressure Pr = design pressure to prevent rupture due to inter-
rise for dusts, bar-meters/ sec = bar-m/ sec nal deflagration, psig
I{... = variable or constant back pressure sizing factor, P,ed = reduced pressure termed maximum internal
balanced valves, liquids only (Figure 7-28) overpressure that can be withstood by the weak-
k = ratio of specific heats, cp/ c, est structural element, psig, or bar ga, or kPa
L = liquid flow, gallons per minute (gas/vapor), or maximum pressure actually
Lr = length of flame, ft developed during a vented deflagration
L_. = L = latent heat of vaporization, Btu/lb P,ed = maximum pressure developed during venting,
L, = distance between adjacent vents, meters or feet bar ga (dusts)
LEL = lower explosive, or lower flammable limit, per- P,ta, = vent closure release pressure, bar ga (dusts)
cent of mixture of flammable gases only in air P 11 = normal operating gas pressure, psia
L1, �' etc = lower flammability limits, vol % for each flam- 6P, = pressure drop at flare tip, inches water
mable gas in mixture P, = pressure of the vapor just inside flare tips (at
L3 = longest dimension of the enclosure, ft top), psia
L/D = length-to-diameter ratio, dimensionless P 1 = upstream relieving pressure, or set pressure at
inlet to safety relief device, psig (or psia, if con-
M = molecular weight sistent)
m = meter or percent moisture, or 100 minus steam P 2 = back pressure or downstream at outlet of safety
quality
relief device, psig, or psia, depending on usage
MAWP = maximum allowable working pressure of a pres- p = rupture pressure for disk, psig or psia
sure vessel, psi gauge (or psi absolute if so p" = overpressure (explosion), lb force/sq in.
specifically noted) p' = pressure, psi abs
MP = melting point (freezing point), °C or °F 6P = pressure differential across safety relief valve,
MR = universal gas constant = 1544 ft lb/lb sec-sec. inlet pressure minus back pressure or down
Units depend on consistency with other symbols stream pressure, psi. Also = set pressure + over-
in equation, or manufacturing range for metal pressure, psig-back pressure, psig. At 10% over-
bursting/rupture disks. pressure delta P equals 1.1 P1 - P 2. Below 30
mj = spark energy, milli-joules psig set delta P equals P1 + 3 - P2.
mn, = mass of TNT, lb 6p = differential pressure across liquid relief rupture
n = moles of specified components disk, usually equals p, psig
nH = total number moles at pressure or atmospheric 6P(dusts) = pressure differential, bar or psi
condition Q = total heat absorption from external fire (input)
nL = total number mols at atmospheric pressure or to the wetted surface of the vessel, Btu/hr
low pressure or vacuum condition Q = liquid flow, cu ft/sec
P = relieving pressure, psia = valve set pressure + Q\ = required flow, cu ft/min at actual flowing tem-
permissible overpressure, psig, + 14.7, or any perature and pressure, ACFM
pressure, bar (gauge), or a consistent set of Qr = heat released by flame, Btu/hr
pressure units. Minimum overpressure is 3 psi Qr = heat release, lower heating valve, Btu/hr
P1 = P' = pressure, psia Q., = required flow, cu ft/min at standard conditions
p" = maximum header exit pressure into seal, psig of 14. 7 psi a and 60°F, SCFM
Pb = stamped bursting pressure, plus overpressure q = Average unit heat absorption, Btu/hr/sq ft of
allowance (ASME 10% or 3 psi, whichever is wetted surface
greater) plus atmospheric pressure (14.7), psia R = ratio of the maximum deflagration pressure to
Pc= Pcrit = critical pressure of a gas system, psi abs the maximum initial pressure, as described in
Pd = design pressure of vessel or system to prevent NFPA Code-69, Par 5-3.3.1
deformation due to internal deflagration, psig also R = individual gas constant= MR/M = 1544/M