Chemical Engineering August 2020 - 42

Feature Report
Mechanical Pressure
Gages in the CPI
Silvia Weber
WIKA Alexander
Wiegand SE & Co. KG
IN BRIEF
TYPES OF GAGES
SELECTION CRITERIA
SAFETY IN CRITICAL
SITUATIONS
W
hether at work or at home,
more and more areas of life are,
or will become, digital. Nevertheless,
the manufacturers of
mechanical pressure-measuring instruments
continue to see rising sales of their gages.
Customers in the chemical process industries
(CPI) immediately mention two reasons
for this continuing strong demand: the safety
and efficiency of these instruments.
Pressure gages always provide a reliable
measurement result that can be read at a
glance, with no external power. Even if the
power supply fails, they continue to fulfil
their measuring task and display the value
locally. The continued and widespread use
of mechanical pressure-measuring instruments
also has an economic basis. Mechatronic
and electronic solutions require a
higher investment. However, not all employees
work in the control room or have a laptop
in front of them in order to monitor the
measurement processes. In most cases,
the service and maintenance personnel are
found within the plant and can thus read the
pressure directly on site.
Types of gages
The basic decision to use pressure gages
is easily understandable. In contrast, the
answer to the question of which instrument
suits which requirement is more complex.
Technologically, we distinguish three
types of pressure gage (Figure 1): Bourdontube
pressure gages work with a Bourdon
tube as a measuring element, which expands
with increasing pressure. This travel
is transferred to the display through a link
and a movement. With diaphragm pressure
gages, the pressure acts on a diaphragm,
which is either clamped or welded around
its edge. The linear motion is transmitted
via a link directly to a movement. Due to the
large surface area of the diaphragm element,
40
low pressure
ranges can be
measured (p
= F/A). A special
form of this
type of gage
is the capsule
pressure gage.
The measuring
element is
made of two
diaphragm elements,
welded
together
around their
edges. The resulting
double
Part 2
In today's digital age, there is still a need for mechanical pressure measuring instruments for
safety, efficiency and economical advantages. Selecting the right type is described here
WIKA
tube travel enables the measurement of
even the lowest of pressures without reducing
the material thickness.
Selection criteria
All three technologies
are equally suitable
for monitoring gage, differential and
absolute pressure. In general, gage pressure
measurement is the most commonly
used method. With this, the difference is
measured to the currently prevailing ambient
pressure, which is determined by the
weather and the altitude above sea level.
Compared to the other two methods, gage
pressure measurement involves less effort
and still meets most of the requirements
within the CPI.
The EN 837 " pressure-gage standard "
therefore covers mechanical measuring instruments
with an elastic element for gage
pressure up to a maximum of 1,600 bars.
It is divided into three parts: " Bourdon tube
pressure gages - Dimensions, metrology,
requirements and testing " (EN 837 Part 1),
" Selection and installation recommendations
for pressure gages " (EN 837 Part 2)
and " Diaphragm and capsule pressure
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