Page 251 - Chemical engineering design

PIPING AND INSTRUMENTATION
Table 5.5.
Pump Specification Sheet (Example 5.8)
Pump Specification 227
Type: Centrifugal
No. stages: 1
Single/Double suction: Single
Vertical/Horizontal mounting: Horizontal
Impeller type: Closed
Casing design press.: 600 kPa
design temp.: 20 ° C
Driver: Electric, 440 V, 50 c/s 3-phase.
Seal type: Mechanical, external flush
3
Max. flow: 7.7 m /h
Diff. press.: 600 kPa (47 m, water)
5.7. CONTROL AND INSTRUMENTATION
5.7.1. Instruments

Instruments are provided to monitor the key process variables during plant operation.
They may be incorporated in automatic control loops, or used for the manual monitoring
of the process operation. They may also be part of an automatic computer data logging
system. Instruments monitoring critical process variables will be fitted with automatic
alarms to alert the operators to critical and hazardous situations.
Comprehensive reviews of process instruments and control equipment are published
periodically in the journal Chemical Engineering. These reviews give details of all the
instruments and control hardware available commercially, including those for the on-line
analysis of stream compositions (Anon., 1969). Details of process instruments and control
equipment can also be found in various handbooks, Perry et al. (1997) and Lipak (2003).
It is desirable that the process variable to be monitored be measured directly; often,
however, this is impractical and some dependent variable, that is easier to measure, is
monitored in its place. For example, in the control of distillation columns the continuous,
on-line, analysis of the overhead product is desirable but difficult and expensive to achieve
reliably, so temperature is often monitored as an indication of composition. The temper-
ature instrument may form part of a control loop controlling, say, reflux flow; with the
composition of the overheads checked frequently by sampling and laboratory analysis.

5.7.2. Instrumentation and control objectives
The primary objectives of the designer when specifying instrumentation and control
schemes are:
1. Safe plant operation:
(a) To keep the process variables within known safe operating limits.
(b) To detect dangerous situations as they develop and to provide alarms and
automatic shut-down systems.
(c) To provide interlocks and alarms to prevent dangerous operating procedures.
2. Production rate:
To achieve the design product output.