There are two types of on-off control systems. First, there is on-off control to run
the normal operation of the plant. There are motors to start and stop and equipment
to run. Failure of these control schemes is certainly undesirable. A failed
on-off control scheme might result in a troublesome spill or produce off-specification
product. These results are annoying and expensive, but no one is hurt or at
risk. No equipment is destroyed. To illustrate, we will assume that we can implement
this type of on-off control by use of a basic process control system (BPCS).
The second category introduces a different responsibility for on-off control systems—
those that are safety related. The documentation may appear quite the
same, but importance of these systems is much greater. Unfortunately, the distinction
between normal operating on-off control systems and safety systems
can be fuzzy. One approach might be to use the regulatory documents to
define which systems are truly safety systems and which are normal operating
systems. There are many codes and standards that address on-off control. For
instance, the National Fire Protection Association (NFPA) has published standards
for the operation of many different kinds of burners. The standards
describe in detail how the on-off control is to work. To illustrate, we will
assume that we can implement a similar but not identical process using a safety
instrumented system (SIS).
The ISA Dictionary defines an SIS as: A system that is composed of sensors,
logic solvers, and final control elements whose purpose is to take the process to
a safe state when predetermined conditions are violated…other terms commonly
used include emergency shutdown system (ESS), safety shutdown
system (SSD), and safety interlock system.”
From the P&ID we can see there is some sort of SIS involving a low level in
01-D-001, the condensate pump 01-G-005, and the on-off valve HV-400.