7.1 Introduction:
Measurement of pressure, flow and temperature are the key process parameters
in cement industry. The quality of the process entirely depends upon accuracy of
the process operation and controls. The accurate process operation and control
may not be achieved without accurate measurement of process parameters as
mentioned above. There are four types of devices used for measurement of
process parameters as below:
a) Electro Mechanical Switches:
An electromechanical switch provides only single point information of the
process parameter hence used for higher limits and lower limits only. The
switches have been phased out from process control functions. The switches
are mainly used to get final limit information only despite the lower equipment
cost. The key features of mechanical switches are as below:
a. Mechanical switches are cheaper than any process device and their cost
varies from Rs. 4,000/- to Rs. 6,000/- depending upon manufactures and
principle of working.
b. The installation of these switches are very simple using ¼”, ½”, ¾” NBS sockets,
c. The interface of the switches is very simple and they can be connected with
DCS using ordinary 2 core signal cable only.
d. The switches give only digital output to the system and interfacing is easy
with hardware as well as software,
e. Accuracy of the switches can be available up to 97% only, hence for precision
process application of the switch is not possible
f. No skill is required for installation, commissioning and repairing of the switches,
g. The calibration of the switches is only possible under off line conditions
using suitable tool kit.
h. The performance of the switches is affected by aging factor. Hence they are
required to be replaced after definite periods due to change of stiffness of
the spring.
i. Easy availability and spot delivery is possible being common item and widely
used in small scale industries.
j. Switches are very useful for end limit and positions applications.
b) Analogue Transmitters:
The new generation devices for measurement of temperature, flow and pressure
parameters are available as process transmitters. The transmitter measures
the process parameters (mechanical signal) in efficient manner and converts
the same into electronic signal in desired output to interface with DCS system.