Chemical Engineering August 2020 - 45

TABLE 1. THE EN STANDARD FOR THE THREE TYPES OF PRESSURE GAGE
Reference of the
Standard
Type of pressure
gage
Pressure
range
Gas or
Steam
EN 837-1
EN 837-3
EN 827-3
Bourdon tube
Diaphragm
Capsule
0.6-1,600 bars
2.5-25 bars
1-600 mbar
X
X
X
Note 1) Separators should be used
Note 2) The capsule and the pipe shall be fully filled with the liquid
as a blow-out back. In the event of
damage (Bourdon tube bursting),
the baffle wall ensures that any energy
arising is dissipated through
the back of the unit. The front window,
in most cases made from
safety glass anyway, remains intact,
and any personnel who happen to
be checking the pressure at that instant
are protected.
Safety in critical situations
Where strong vibrations could damage
or destroy a measuring instrument,
a pressure gage with liquid filling
is recommended. The liquid (usually
glycerine) absorbs the vibration acting
on the instrument and thus also
the oscillations of the pointer, so that
the measured value can still be read
correctly. Moreover, the liquid acts
as a lubricant between the mechanical
components, which increases the
durability of the pressure gage.
For
other
typical
applications,
measuring instruments can be exposed
briefly to elevated pressures,
such as when switching on a pump
or opening/closing a valve. For this,
diaphragm pressure gages are better
suited than Bourdon-tube pressure
gages due to the diaphragm
being attached to the upper flange.
Some diaphragm pressure gages,
for example, come as standard with
an overpressure safety of five times
the full scale value. For the lowest
measuring ranges (from 16 mbar)
there are special versions that can
withstand an overpressure of up to
400 bars. Bourdon-tube pressure
gages can only be protected through
a high design input to prevent plastic
deformation of the measuring element.
This is achieved by means of
an additional link and an overpressure
bracket. For pressures above
the nominal value both components
interlock, tube resistance increases
and the motion of the measuring element
is limited.
Final remarks
Due to their advantages, mechanical
pressure-measuring instruments will
continue to be indispensable for instrumentation
in the process industries.
Their only limitation: they cannot
perform any control or regulation
tasks. Their electronic " cousins " are
necessary for this. Anyone who, for
safety reasons, still needs an additional
local display, free from external
power, does not automatically
TABLE 2. ASSIGNMENT OF ACCURACY CLASSES TO NOMINAL SIZES
Nominal size Accuracy class
NS
0.1
40 and 50
63
80
100
160
250
X
X
X
X
X
0.25
0.6
1
X
X
X
X
X
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
Process fluid
Low
viscosity
X
X
X2)
Liquid
High
viscosity
1)
X
Polluted
1)
X
have to operate with two measuring
points. The dual function needed
may be fulfilled by a mechatronic instrument;
that is, a combination of
pressure gage and electrical output
signal or switch contact. Such twoin-one
solutions save space and expense
- which really assists in the
cost-effectiveness of the process. ■
Edited by Gerald Ondrey
Author
Silvia Weber is a product manager
at WIKA Alexander Wiegand
SE & Co. KG (Alexander-WiegandStraße
Germany;
Phone: +49 9372 1322862;
Email: silvia.weber@wika.
com). She has over 10 years of
experience in the field of pressure
measurement technology.
Her
area of expertise includes mechanical
and mechatronic pressure measuring instruments,
particularly in the chemical, petrochemical, oiland-gas
industries, as well as the food and
pharmaceutical industries. Based on her train-thetrainer
education, a further focus is the development of
concepts for web-based training as well as further inhouse
education in the field of pressure measurement
technology. In various publications she has already reported
on the application, strengths and innovations of
pressure gages. She holds a B.A. degree from the Duale
Hochschule Baden-Wüttemberg in Germany.
30, 63911 Klingenberg,
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Chemical Engineering August 2020

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