Technical
Information
Diaphragm Seals
Applications - Operating Principle - Designs
WIKA Data Sheet IN 00.06
Definition
Diaphragm seals, also known as chemical seals or remote Pressure measuring instrument with diaphragm seal
seals, are used when the pressure measuring instruments
sensing element should not come in contact with the
process medium.
A diaphragm seal has two primary tasks:
1. Separate the pressure measuring instrument from the Pressure measuring
process medium instrument
2. Transfer the pressure to the pressure measuring Pressure gauge
instrument Pressure transmitter
Pressure switch
Operating principle of diaphragm seals System fill fluid
The operating principle of a diaphragm seal (using the
example of a diaphragm seal) is shown in the picture on the
right.
Capillary extension/
cooling tower
Operating principle
A diaphragm made of the appropriate material separates
the pressure medium from the pressure instrument, while Diaphragm seal
a suitable liquid transmits the pressure to the instruments Upper body
sensing element. The inner cavity between the diaphragm of diaphragm seal
and the pressure measuring instrument is completely filled
Diaphragm
with a system fill fluid. The process pressure is transmitted
Bild_1.01
by the elastic diaphragm via the transmitting fluid and from Process connection
there to the measuring element, i.e. to a pressure gauge or
transmitter.
In many cases, a capillary between diaphragm seal and
pressure measuring instrument is used, for example to
eliminate or minimize the hot temperature effects on the
pressure measuring instrument. The capillary affects the
response time of the complete assembly. must meet the corresponding temperature and corrosion
resistance requirements.
Diaphragm seal, capillary and pressure measuring instru-
ment together form a hermetically closed system. For this If the diaphragm is punctured, the system fill fluid can enter
reason, the sealed filling screws on the diaphragm seal and/ the process media. For food processing applications, this
or pressure measuring instrument should not be opened, system fill fluid must be approved for food contact.
since loss of fill fluid will impair the systems performance. When selecting a system fill fluid, factors such as compa-
tibility and temperature are of extreme importance. There
The diaphragm and the mounting flange are part of the are many different system fill fluids available, suitable for
system which comes in contact with the process medium. temperature ranges from -90 C up to +400 C (see table
Therefore, the material from which they are manufactured, Standard system fill fluids).
WIKA Data Sheet IN 00.06 08/2008 Page 1 of 6
When to use diaphragm seals Combination possibilities
Diaphragm seals are used to protect pressure measuring Assembly of the diaphragm seal and measuring instrument
instruments from the harshest of conditions. may be made via a rigid direct connection or via a capil-
lary. The rigid assembly is made either by directly screw-
Examples of typical applications ing or welding the measuring instruments to the diaphragm
The pressure medium is corrosive and would damage seal or through a gauge adapter. For high temperatures a
the measuring element. A diaphragm seal allows the cooling tower can be fitted between seal and instrument.
use of different materials to either eliminate or reduce The configuration of the combination of pressure measuring
damage caused by corrosive media. instruments and diaphragm seals depends, among other
things, on the application conditions in which the assembly
The pressure medium is highly viscous or contains must work.
suspended matter which may obstruct the pressure
entry or deposit inside the pressure sensing element.
The pressure medium tends to crystallise or polymerise.
The pressure medium temperature is very high and
exceeds the temperature limit of the pressure measuring
instrument or the temperature effect may cause inaccu-
rate measurement.
The pressure measuring point does not allow direct
installation of a pressure measuring instrument.
A diaphragm seal with capillary (remote mounting) may
facilitate easy observation of the instrument.
The application requires sanitary connections. A flush
mounted diaphragm seal avoids cavities and dead Direct assembly
volumes.
The pressure medium is toxic or the escaping medium
may pollute the environment. A suitably designed
diaphragm seal will provide protection.
The WIKA diaphragm seal system combines innovative of
materials and production methods with expertise gained Capillary
from countless practical solutions of problems in pressure
measurement.
Diaphragm seals improve the performance of the opera-
tions and processes through:
extended service life of the instrument
reduced cost of installation
reduced or even eliminated costs of maintenance
Cooling tower
Page of 6 WIKA Data Sheet IN 00.06 08/2008
Designs
Since diaphragm seals are used under many different conditions, one model can not fulfill all requirements. Over time,
different versions and designs have proved to be advantageous for specific applications.
There are three standard versions of
diaphragm seals:
Diaphragm Seal
Diaphragm In-line Seal
Diaphragm Probe Seal
Diaphragm Diaphragm Diaphragm
Bild_2.01
Seal In-line Seal Probe Seal
The selection of a diaphragm seal depends on the technical specification as well as the mounting connection and require-
ments of the pressure measurement situation.
Diaphragm seals
Diaphragm seals are mounted to existing fittings. Gener-
ally the fittings are T-pieces (which are integrated into a
pipeline) or welding sockets (which are then welded onto a
pipe, process vessel or tank).
This diaphragm seal type offers the advantage that
the contact surface between pressure medium and
diaphragm is relatively large, thus ensuring accurate
pressure measurement. The fact that they can be easily
990_10.01
dismounted (e.g. for cleaning or calibration purposes) is a
further advantage.
WIKA Data Sheet IN 00.06 08/2008 Page of 6
Flange-type Cell-type (sandwich)
The diaphragm is an integral part of a standard flange and The diaphragm body corresponds to the size of the sealing
is flush with the flange sealing face. face of a standard flange.
This enables simple installation while the diaphragm is fully Upon installation the diaphragm seal is sandwiched
exposed to the pressure medium. between the process flange and standard blind flange.
The blind flange may be of inferior material since it is not
exposed to the pressure medium.
Diaphragm seal
Blind flange
(Flange-type)
Diaphragm seal
(Cell-type)
Gasket
Gasket
Process flange
990_27.01
990_28.01
Process flange
Diaphragm seal with extended-diaphragm
The tubular extension provides a smooth inner surface with
thick walled or heavily insulated pressure vessels.
Both flange and cell-type are available.
Diaphragm seal Blind flange
(Flange-type
with extended Diaphragm seal
diaphragm) (Cell-type with
extended diaphragm)
Gasket Gasket
Process flange Process flange
Insulation Insulation
990_29.01
990_35.01
Inner wall Inner wall
of vessel of vessel
Diaphragm seals are available up to 600 bar pressure rating
and +400 C temperature rating.
Page of 6 WIKA Data Sheet IN 00.06 08/2008
Diaphragm in-line seals
In-line seals are particularly suited for rapidly flowing
pressure media and high viscosity. The seal is integrally
mounted into the pipeline for use with flowing process
media.
This seal contains a patented cylindrical diaphragm that
avoids any interuptions to the process flow. In-line seals are Diaphragm
available with various process connections to meet specific in-line seal
industry requirements.
Upon installation the in-line seal is sandwiched between Gasket
two standard process flanges.
Diaphragm in-line seals are available up to 400 bar pressure
Process
rating and +400 C temperature rating. flange
981_10.01
Diaphragm probe seals
This type is suitable for flowing, heterogeneous measur- Diaphragm
ing media, since it is a immersed directly in the medium. probe seal
Compared with other diaphragm seals it takes up extremely Sealing washer
little space. The pressure is registered punctual.
Process
connection
The diaphragm seal comprises of an oval tube as pressure
sensor enclosed on the one side and the connection piece
welded to it. A support is fitted in the sensor for stabiliza-
tion. Adaption to the measuring point is with inner and outer
thread.
The maximum pressure range is 1,600 bar, the normal
temperature limit +400 C.
970_10.01
WIKA Data Sheet IN 00.06 08/2008 Page of 6
The standard material for diaphragm seals is stainless steel 316 L. Numerous wetted parts materials are available for nearly
all diaphragms.
Standard wetted parts materials
Material Code designation Material Code designation
Stainless steel Mat nos. 316L, 1.4571, 1.4404, Nickel Material no. 2.4066 / 2.4068
1.4435, 1.4541, 1.4542, 1.4539 Platinum Pt
Duplex 2205 Material no. 1.4462 Tantalum Ta
Hastelloy B2 Material no. 2.4617 Titanium Material no. 3.7035
Hastelloy C4 Material no. 2.4610 Zirconium Zr
Hastelloy C22 Material no. 2.4602 Ceramic wikaramic
Hastelloy C276 Material no. 2.4819 Polytetrafluorethylene PTFE
Incoloy alloy 825 Material no. 2.4858 Perfluoralkoxy PFA
Inconel alloy 600 Material no. 2.4816 Copolymer of Ethene and ECTFE (Halar)
Monel alloy 400 Material no. 2.4360 Chlortrifluorethene
Standard system fill fluids for diaphragm seals (others on request)
Common WIKA Suitable temperature range S.G. at Viscosity Notes
designation code-no. p 1000 mbar p < 1000 mbar 1) temperature at temperature
KN abs abs [g/cm] [C] [m/s10-6] [C]
Silicone oil KN 2 -20 ... +200 C - 0.96 +25 50 +25 Standard
Silicone oil KN 2.2 -40 ... +300 C -40 ... +150 C 0.96 +25 55 +20
Silicone oil KN 17 -90 ... +180 C -90 ... +80 C 0.914 +20 4 +20
2)
High temperature oil KN 3.1 -20 ... +300 C -10 ... +100 C 1.07 +20 39 +20
High temperature oil KN 3.2 -20 2) ... +400 C -10 ... +200 C 1.07 +20 39 +20
Halocarbon liquid KN 21 -40 ... +175 C -40 ... +80 C 1.968 +20 14 +20 for oxygen and
(max. 160 bar) chlorine, BAM 4) tested
Glycerine KN 7 +17 3) ... +230 C - 1.26 +20 1110 +20 Food and beverage
Neobee M-20 KN 59 -20 ... +200 C -20 ... +160 C 0.92 +20 10.1 +25 Food and beverage
Medicinal white KN 92 -10 ... +260 C -10 ... +160 C 0.85 +20 23 +40 Food and beverage
mineral oil
1) Vacuum service required
3) From -10 C with capillary extension
2) From 0 C with capillary extension
4) Federal Institute for Materials Research and Testing
9099980 08/2008 GB
Page of 6 WIKA Data Sheet IN 00.06 08/2008
WIKA Alexander Wiegand GmbH & Co. KG
Alexander-Wiegand-Strae 30
63911 Klingenberg/Germany
Tel. (+49) 9372/132-0
Fax (+49) 9372/132-406
E-mail info@wika.de
www.wika.de
