TABLE OF CONTENTS Emergency shut-down of a hydrocarbon processing plant may involve the depressurization of major process vessels by blowdown to the flare. On offshore production platforms blowdown is a normal part of emergency shut-down.
In many cases, blowdown will be two-phase. One aspect of the treatment of blowdown is therefore the estimation of the flow. The account of flow in relief systems given in the previous section is applicable to blowdown. The other aspect is the hazards associated with blowdown, particularly the fall in the temperature of the gas.
A treatment which covers both aspects has been given by Richardson, Saville and co-workers (S.M. Richardson, 1989; Haque et al., 1990; Haque, Richardson and Saville, 1992; S.M. Richardson and Saville, 1992).
The hazards arise essentially due to the large temperature drop associated with the sudden depressurization of a vessel containing gas or vapour. Thus, for example, if nitrogen at 150 bar and atmospheric temperature is expanded adiabatically down to atmospheric pressure the isentropic expansion gives a final gas temperature of 78 K.
The type of vessel of interest is one that retains, under pressure, avapour space, a layer of liquid hydrocarbon and beneath that a layer of water. The potential effects of the temperature drop associated with sudden depressurization are chilling of the vessel walls, condensation of vapour to form droplets with carryover of these droplets into the flare header and formation of hydrates in the hydrocarbon liquid. It is therefore of practical interest to...
