7. Disposal System Design Criteria for Vapor Discharge
7.1 Direct Vapor Discharge to Atmosphere
7.1.1 Discharges to the atmosphere shall be in the vapor state and shall be below their auto-ignition
temperature. The discharges shall also meet all applicable air quality control requirements and at least
one of the following physical property requirements:
a. Flammable vapors are lighter than air
b. Flammable vapors are equal to or heavier than air, molecular weight is less than 80 and
minimum velocity at discharge is 152 m/s (500 ft/s) based on the maximum capacity of the relief
valve. Maximum velocity shall not exceed 80 percent of sonic velocity (see section 9.1).
c. Flammable vapors have a molecular weight greater than 80. Velocity at discharge is as specified
in section 9.1. A vapor dispersion analysis shall be conducted to verify that ground level vapor
concentrations do not pose a flammability or health hazard.
d. Toxic vapors shall have a vapor dispersion analysis conducted to verify that ground level vapor
concentrations do not pose a health hazard
e. Vapors, of any molecular weight, that are nonflammable, nontoxic, noncondensable and
noncorrosive
7.1.2 A typical direct atmospheric discharge relief system shall include the following components:
a. One or more pressure relief devices
b. Discharge piping
7.2 Vapor Discharge to Closed Pressure Relief System.
a. Where discharge of vapor to the atmosphere or to a lower pressure system is not permissible or
practical, vapor shall discharge to a closed pressure relief system that terminates in a flare. Vapors
from volatile liquids and non-condensable gases shall also discharge to a closed pressure relief
system. The design of the closed pressure relief system shall meet applicable air quality requirements
and combustion products shall be released at a safe location. Continuously burning pilots are
recommended to ensure the combustion of all gas and vapors.
b. Discharge of vapors into a closed system terminating in a vent stack, which releases the vapors
to the atmosphere, is acceptable when allowed by regulations
c. Disposal of toxic materials may require special design considerations. Guidance shall be sought
from a specialist for review of these situations.
7.2.1 Special Considerations
Special considerations are necessary when closed pressure relief systems are required to handle either
extremely hot, cold or heavy vapors or reactive chemicals for example, pyrophoric or corrosive materials.
7.2.2 Pressure Relief Discharges
Pressure relief discharges to a flare shall not exceed the maximum allowed percentage of oxygen by
volume as determinded by the particular process and local conditions. If the potential exists during
non-routine operating activities, for example during a unit regeneration, to exceed the maximum allowed
percentage of oxygen, the pressure relief valves that normally discharge into a closed pressure relief
system shall be arranged so that the discharge can be readily diverted to a combuster that is designed to
handle oxygen-rich hydrocarbon mixtures within the flammable range. The maximum allowed percentage
of oxygen volume for a typical hydrocarbon mixture in 6 percent.
7.2.3 Typical by Closed System
A typical closed pressure relief system (see Figures 1 through 4) shall include the following components:
a. Pressure relief devices; see Section 9.1
b. Disposal piping (leads, laterals, unit and main headers); see Section 9.2
c. Knockout drums (scrubbers) for accumulation and removal of liquids that have condensed or
settled out; see Section 9.3
d. Seals or purge gas or both, for flashback protection; see Section 9.4
e. Flare stacks and tips or a vent stack; see Section 9.5 or 9.10
f. Igniter(s) and pilots (for flare only); see Section 9.6
g. Steam or air for smokeless burning (for flare only); see Section 9.7
h. Figure-eight blinds and gate valves located in unit headers near battery limits, with suitable
platforms, to safely isolate the unit pressure relief system when units are shut down; see Figure 1
Figure 1
Pressure Relief System–Pressure Relief Valves, Leads, Laterals, Headers and Purge Gas System
Figure 1
Pressure Relief System–Pressure Relief Valves, Leads, Laterals, Headers and Purge Gas System
Figure 3
Closed Pressure Relief System Using Combination Igniter/Pilot
Figure 4
Closed Pressure Relief System Using Flame-Front Generator
7.3 Vapor Discharge to Lower Pressure Process System or Vessel.
7.3.1 Individual pressure relief valves may discharge to a lower pressure process system or to a vessel
capable of handling the discharge. Although this type of disposal system is infrequently used for pressure
relief in the vapor state, it is recommended for discharges that contain corrosive materials (which may
damage a closed pressure relief system) or for materials that shall be recovered. Care shall be taken to
ensure that a high-velocity stream from a pressure relief valve does not impinge on internal vessel fittings
or displace trays or other internal equipment.
7.3.2 Pressure relief valves that discharge vapors to a lower pressure process system or to a vessel may
require a design with high backpressure. The backpressure is determined by adding the
lower-pressure-system relief pressure and the pressure drop in the piping from the pressure relief valve to
the lower pressure system. Pressure relief valves that discharge to a lower pressure system shall meet the
requirements of Section 9.1.
7.3.3 A typical vapor discharge system to a lower pressure process system or vessel shall include the
following components:
a. Pressure relief devices see Section 9.1
b. Discharge piping to the lower pressure system or vessel see Section 9.2
7.4 H
2
S Relief
Pressure relief for H
2
S bearing streams from sulfur recovery units (SRUs) and other sources shall be
discharged to either a dedicated “acid gas” flare or a common flare with hydrocarbons, as described
below.
7.4.1 H
S Flare Systems
a. Recovery units typically employ a dedicated flare for disposal of gas during process upsets or
2
emergency conditions
b. Flare Systems shall be sited in accordance with the Safety Considerations For Plant Layout
SES S01-G01
c. H
S disposal facilities which use a dedicated flare shall be arranged as follows:
(i) Facilities shall be provided to inject a hydrocarbon gas stream, for example, fuel gas, into the
2
flare header to assure the minimum required heating value for the flare is met. The hydrocarbon gas
shall be automatically added in any upset conditions that are likely to send the ammonia/acid gas
stream to the disposal system. (See ASME B31.4.)
(ii) A forced air pilot ignitor system shall be provided to handle all regenerator and ammonia acid
gas streams. The stack can be adjacent to a hydrocarbon flare stack and supported within the
hydrocarbon stack support structure. This minimizes acid corrosion and improves control of H2S/NH3
combustion.
(iii) Steam shall not be added to the H2S flare stack since that will reduce the combustion
temperature and reduce H2S combustion to SO2/SO3
(iv) Stack diameter shall be sized to keep gas exit velocity below 46 m/s to avoid flame instability.
7.4.2 Discharge to Hydrocarbon Flare
a. Where gas containing H2S is discharged to a hydrocarbon flare, the metallurgy of the flare
system piping, scrubber, flare stack, pilots, burners and other system components shall be suitable for exposure to the expected concentration of H2S
S. The requirements for minimum heating value in
section 7.4 above shall also be met in this case.
b. The presence of H2S in combination with steam and water typically found in a flare system
results in the formation of highly-corrosive sulfuric acid. The flare system components shall resist this
corrosion for their service life.