Page 562 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

528 Applied Process Design for Chemical and Petrochemical Plants

used. In freezing conditions the unit should receive fires creating the need to empty or blowdown all or parts
personal inspection for condition of liquid. of a system.
• Provide overflow anti-siphon seal drains.
• Provide inlet vacuum seal legs. Sizing
• Some hydrocarbons may form gel clusters or layers Diameter: sizing based on stack velocity [33c], solve for "d."
with some sealant fluids; therefore, providing for
cold weather heating and/ or cleaning of the unit is
,/
necessary. Mach = (1. 702) ( 10 - 5 ) (WI P, d 2 ) T / ( kM ) (7 - 78)
• Ref. [33] suggests minimum design pressure for such
a seal vessel of 50 psig, ASME Code stamped (this
where Mach = ratio of vapor velocity to sonic velocity in vapor,
author). Most flare seal drums operate al 0-5 psig dimensionless. Mach = 0.5 for peak for short-
pressure. term flow, and 0.2 for more normal and fre-
• Be extremely cautious and do not install light weight quent conditions [ 33c].
gauge glass liquid level columns. Rather use the heav- W = vapor relief rate to stack, lbs/hr
ier shatterproof style. Pt = pressure of the vapor just inside flare tips (at
• Provide reliable seal liquid makeup, using liquid level top), psia (For atmospheric release, Pt=
gauging and monitoring with recording to ensure 14.7 psia)
good records of performance. The liquid level must d = flare tip diameter, ft (end, or smallest diameter)
0
be maintained; otherwise, the hazards of a T = temperatures of vapors just inside flare tip, R
bleedthrough or backflow can become serious. k = ratio of specific heats, cp/ cv for vapor being
relieved
M = molecular weight of vapor
Flares
A peak velocity through the flare end (tip) of as much
Flares are an attempt to deliberately burn the flamma- as 0.5 mach is generally considered a peak, short term. A
ble safety relief and/ or process vents from a plant. The more normal steady state velocity of 0.2 mach is for nor-
height of the stack is important to the safety of the sur- mal conditions and prevents flare/lift off [58]. Smokeless
roundings and personnel, and the diameter is important (with steam injection) flare should be sized for conditions
to provide sufficient flow velocity to allow the vapors/ of operating smokelessly, which means vapor flow plus
gases lo leave the top of the stack at sufficient velocities to steam flow [33c]. Pressure drops across the tip of the flare
provide good mixing and dilution after ignition at the have been used satisfactorily up lo 2 psi. It is important
flare tip by pilot flames. not to be too low and get flashback (without a molecular
API [33] discusses factors influencing flare design, seal) or blowoffwhere the flame blows off the tip (see Ref.
including the importance of proper stack velocity to allow 57), Figure 7-71.
jet mixing. Stack gases must not be diluted below the flam- Another similar equation yielding close results [59]:
mable limit. The exit velocity must not be too low to allow
flammable gases to fall to the ground and become ignited.
2
The atmospheric dispersion calculations are important for d , = (W/1370) � T/M , (generally for
the safety of the plant. Computer models can be used to smokeless flares) ( 7 - 79)
evaluate the plume position when the flare leaves the stack
under various atmospheric wind conditions. This should based on mach 0.2 limitation velocity, k cp/ cv = 0.2
be examined under alternate possibilities of summer and gas constant R = 1546 (Ft-lb force z r'R) (mole)
through winter conditions. (Also see Ref. [78]).
The velocities of the discharge of relief devices d, = flare tip diameter, in.
through a stack usually exceed 500 feel/ second. Because W = gas vent rate, lb/hr
this stream exits as a jet into the air, it is sufficient to cause T = gas temperature in stack, R
0
turbulent mixing [33]. M = molecular weight of gas/vapor
For a flare stack to function properly and lo handle the
capacity that may be required, the flows under emergency For non-smokeless flares (no steam injection) about 30%
conditions from each of the potential sources must be higher capacity can be allowed [59]. Therefore, the diam-
carefully evaluated. These include, but may not be limited eter of a non-smokeless flare slack is approximately (0.85)
lo, pressure relief valves and rupture disks, process blow- ( diameter of the smokeless flare stack).
down for startup, shutdown, upset conditions, and plant The amount of steam injection required for smokeless