Solartron Mobrey 7828, Technical Manual

Page: 29 / 110

7828 Density Transmitter Technical Manual Mechanical Installation
78285000_AB
2-15
provided it is thermostatically controlled and the thermostat is set to operate below the
minimum operating temperature of the system.
In cases where it is necessary to heat or cool the fluid - to bring it within the temperature
range of the transmitter, for example - heat exchangers can be installed in the fluid flow.
Solartron can provide more details on this, or provide a complete system if required.
2.4.3 FLOW RATE
Flow rates and velocities should be maintained relatively constant within the limits given.
The fluid flow provides a steady heat flow into the transmitter, and the flow rate influences
the self-cleaning of the sensor and the dissipation of bubbles and solid contaminants.
Where it is necessary to install the transmitter in a by-pass (either using the free stream
installation in a 4” diameter horizontal by-pass, or a flow-through chamber), flow may be
maintained using pressure drop, pitot scoop, or by a sample pump. Where a pump is
used, the pump should be upstream of the transmitter.
2.4.4 ENTRAINED GAS
Gas pockets can disrupt the measurement. A brief disruption in the signal caused by
transient gas pockets can be negated in the internal signal conditioning software, but
more frequent disruptions or serious gas entrainment must be avoided. This can be
achieved by observing the following conditions:
•
=
Keep pipe lines fully flooded at all times;
•
=
Vent any gas prior to the density transmitter;
•
=
Avoid sudden pressure drops or temperature changes which may cause dissolved
gases to break out of the fluid;
•
=
Maintain a back pressure on the system sufficient to prevent gas break out;
(e.g. back pressure equivalent to twice the headloss plus twice vapour pressure);
•
=
Maintain flow velocity at the sensor within the specified limits.
2.4.5 SOLIDS CONTAMINATION
•
=
Avoid sudden changes of velocity that may cause sedimentation.
•
=
Install the transmitter far enough downstream from any pipework configuration which
may cause centrifuging of solids (e.g. bends).
•
=
Maintain flow velocity at the sensor within the specified limits.
•
=
Use filtration if necessary.
•
=
Specify the 7828 transmitter with a non-stick PTFE protective layer.
2.4.6 EXAMPLE INSTALLATION
The diagram below illustrates some of the principles outlined in this section. It shows a
free-stream density transmitter installation with an additional sample take off. The position
of both is such that the static mixing (which could be caused by pump discharge or
partially closed valve), has negated the adverse effects of bends and established laminar
flow, and has ensured that the fluid is thoroughly mixed and thus of uniform composition
and temperature. The ideal place for a free stream or “T” piece installation, or for the by-
pass take off point is where the flow has just begun to be laminar.