Page 394 - Chemical engineering design

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CHEMICAL ENGINEERING
equipment. The use of portable electrical equipment, welding, spark-producing tools and
the movement of petrol-driven vehicles would also be subject to strict control.
Exhaust gases from diesel engines are also a potential source of ignition.
9.3.5. Ionising radiation
The radiation emitted by radioactive materials is harmful to living matter. Small quantities
of radioactive isotopes are used in the process industry for various purposes; for example, in
level and density-measuring instruments, and for the non-destructive testing of equipment.
The use of radioactive isotopes in industry is covered by government legislation, see
hse.gov.uk/pubns.
A discussion of the particular hazards that arise in the chemical processing of nuclear
fuels is outside the scope of this book.

9.3.6. Pressure

Over-pressure, a pressure exceeding the system design pressure, is one of the most serious
hazards in chemical plant operation. Failure of a vessel, or the associated piping, can
precipitate a sequence of events that culminate in a disaster.
Pressure vessels are invariably fitted with some form of pressure-relief device, set at
the design pressure, so that (in theory) potential over-pressure is relieved in a controlled
manner.
Three basically different types of relief device are commonly used:
Directly actuated valves: weight or spring-loaded valves that open at a predetermined
pressure, and which normally close after the pressure has been relieved. The system
pressure provides the motive power to operate the valve.
Indirectly actuated valves: pneumatically or electrically operated valves, which are
activated by pressure-sensing instruments.
Bursting discs: thin discs of material that are designed and manufactured to fail at a
predetermined pressure, giving a full bore opening for flow.
Relief valves are normally used to regulate minor excursions of pressure; and bursting
discs as safety devices to relieve major over-pressure. Bursting discs are often used in
conjunction with relief valves to protect the valve from corrosive process fluids during
normal operation. The design and selection of relief valves is discussed by Morley
(1989a,b), and is also covered by the pressure vessel standards, see Chapter 13. Bursting
discs are discussed by Mathews (1984), Askquith and Lavery (1990) and Murphy (1993).
In the United Kingdom the use of bursting discs is covered by BS 2915. The discs are
manufactured in a variety of materials for use in corrosive conditions; such as, imper-
vious carbon, gold and silver; and suitable discs can be found for use with all process
fluids.
Bursting discs and relief valves are proprietary items and the vendors should be
consulted when selecting suitable types and sizes.
The factors to be considered in the design of relief systems are set out in a compre-
hensive paper by Parkinson (1979) and by Moore (1984); and in a book published by the
Institution of Chemical Engineers, Parry (1992).