Page 395 - Chemical engineering design

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SAFETY AND LOSS PREVENTION
Vent piping
When designing relief venting systems it is important to ensure that flammable or toxic
gases are vented to a safe location. This will normally mean venting at a sufficient
height to ensure that the gases are dispersed without creating a hazard. For highly toxic
materials it may be necessary to provide a scrubber to absorb and “kill” the material;
for instance, the provision of caustic scrubbers for chlorine and hydrochloric acid gases.
If flammable materials have to be vented at frequent intervals; as, for example, in some
refinery operations, flare stacks are used.
The rate at which material can be vented will be determined by the design of the
complete venting system: the relief device and the associated piping. The maximum
venting rate will be limited by the critical (sonic) velocity, whatever the pressure drop
(see Volume 1, Chapter 4). The design of venting systems to give adequate protection
against over-pressure is a complex and difficult subject, particularly if two-phase flow is
likely to occur. For complete protection the venting system must be capable of venting
at the same rate as the vapour is being generated. For reactors, the maximum rate of
vapour generation resulting from a loss of control can usually be estimated. Vessels must
also be protected against over-pressure caused by external fires. In these circumstances the
maximum rate of vapour generation will depend on the rate of heating. Standard formulae
are available for the estimation of the maximum rates of heat input and relief rates, see
ROSPA (1971) and NFPA (1987a,b).
For some vessels, particularly where complex vent piping systems are needed, it may be
impractical for the size of the vent to give complete protection against the worst possible
situation.
For a comprehensive discussion of the problem of vent system design, and the design
methods available, see the papers by Duxbury (1976, 1979).
The design of relief systems has been studied by the Design Institute for Emergency
Relief Systems (DIERS), established by the American Institute of Chemical Engineers;
Fisher (1985). DIERS has published recommended design methods; see Poole (1985) and
AIChemE (1992a,b). Computer programs based on the work by DIERS are also available.

Under-pressure (vacuum)
Unless designed to withstand external pressure (see Chapter 13) a vessel must be protected
against the hazard of under-pressure, as well as over-pressure. Under-pressure will normally
mean vacuum on the inside with atmospheric pressure on the outside. It requires only a slight
drop in pressure below atmospheric pressure to collapse a storage tank. Though the pressure
differential may be small, the force on the tank roof will be considerable. For example, if the
pressure in a 10-m diameter tank falls to 10 millibars below the external pressure, the total
load on the tank roof will be around 80,000 N (8 tonne). It is not an uncommon occurrence
for a storage tank to be sucked in (collapsed) by the suction pulled by the discharge pump,
due to the tank vents having become blocked. Where practical, vacuum breakers (valves that
open to atmosphere when the internal pressure drops below atmospheric) should be fitted.

9.3.7. Temperature deviations
Excessively high temperature, over and above that for which the equipment was designed,
can cause structural failure and initiate a disaster. High temperatures can arise from loss