Flow Instruments

DOE-HDBK-1132-99

Flow Instruments .

4.9.5

Differential pressure sensing lines should be run together to the extent

practical to keep both lines at the same temperature and with no hydraulic

head difference between the lines. If they are insulated, they should be

insulated together.

Differential pressure transmitters vented to the atmosphere should have

the vent connection hard-tubed without a valve and turned down. For

outdoor applications, an insect screen should be considered.

Liquid Level Instruments .

4.9.6

Liquid level instrument connections should be made directly to the vessel

and not on flow lines.

The lower instrument connection should be on the side of the vessel.

Bottom connections are not recommended because of the possibility of

trapping solids in the sensing lines.

A stilling well is preferred where displacement or float-type elements are

located inside a vessel subject to turbulence.

Differential pressure-level instruments should be located below the

process connection on the vessel unless a gas purge or compensated

isolation chamber is used.

Liquid level devices should be placed away from areas of turbulence to

prevent inaccurate level measurement.

4.9.7

Leak Detection . Conductivity probes should be protected from freezing

conditions. A film of ice over the probe could render the leak detection system

inoperable.

4.9.8

Freeze Protection . Consider the need for freeze protection during design to

prevent instrumentation systems from freezing, to ensure reliable performance

II-52