Page 90 - Plant design and economics for chemical engineers

68

Since a majority of the chemical process industry now uses some version of
HAZOP for all new facilities and selectively uses it on existing ones, an example
of this technique, as originally described by Ozog,? is given in the following
paragraphs.
Assume that a HAZOP study is to be conducted on a new flammable-
reagent storage tank and feed pump as presented by the piping and instrument
diagram shown in Fig. 3-1. In this scheme, the reagent is unloaded from tank
trucks into a storage tank maintained under a slight positive pressure until it is
transferred to the reactor in the process. For simplification, the system is
divided into two elements-the tank T-l and the pump P-l and the feedline.
Application of the guide words to these two elements is shown in Table 4 along
with a listing of the consequences that result from the process deviations. Note
that not all guide words are applicable to the process deviations listed. Also,
some of the consequences identified with these process deviations have raised
additional questions that need resolution to determine whether or not a hazard
exists. This will require either more detailed process information or an estima-
tion of release rates. For example, similar release rates could be the conse-
quence of either Event 3 07-3 open or broken) or Event 4 (V-l open or broken);
however, the total quantity released through V-3 could be substantially reduced
over that with V-l open or broken by closing V-2. Of the 41 events listed in
Table 4, Event 5 (tank rupture) and Event 24 (external fire) would provide the
worst consequences since both would result in instantaneous spills of the entire
tank contents.
Hazard assessment is a vital tool in loss prevention throughout the life of
the facility. Ideally, the assessment should be conducted during the conceptual-
design phase, final design stage, and prestartup period as well as when the plant
is in full operation. In the conceptual-design phase many potential hazards can
be identified and significant changes or corrections made at minimal cost.
Results of these assessments are key inputs to both site-selection and plant-
layout decisions. The major hazards usually include toxicity, fire, and explosions;
however, thermal radiation, noise, asphyxiation, and various environmental
concerns also need to be considered.
A thorough hazard and risk assessment of a new facility is essential during
the final design stage. At this stage, the piping and instrument diagrams,
equipment details, and maintenance procedures are finalized. However, since
equipment often has not been ordered, it is still possible to make changes
without incurring major penalties or delays.
A hazard assessment during the prestartup period should be a final check
rather than an initial assessment. This review should include the status of
recommended changes from previous hazard studies and any significant design
changes made after the final design. If serious hazards are identified at this
time, it is unlikely that they can be eliminated without significant cost or startup
delay.

tH. Ozog, Chem. Eng., 92(4):161 (Feb. 18, 1985).