Emerson Micro Motion 3711, Configuration And Use Manual, Page: 53 / 234

Configuration and Use Manual 45
Configuring the Core Interface
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5.5.4 Slug flow
Slugs – gas in a liquid process or liquid in a gas process – occasionally appear in some applications.
The presence of slugs can significantly affect the process density reading. The slug flow parameters
can help the transmitter suppress extreme changes in process variables, and can also be used to
identify process conditions that require correction.
Slug flow parameters are as follows:
• High slug limit – the point above which a condition of slug flow will exist. Typically, this is the
highest density point in your process’s normal density range.
• Low slug limit – the point below which a condition of slug flow will exist. Typically, this is the
lowest density point in your process’s normal density range.
• Slug duration – the number of seconds the Model 3711 waits for a slug flow condition ( outside
the slug flow limits) to return to normal ( inside the slug flow limits) before taking slug flow
actions.
If the Model 3711 detects slug flow (process density above the high slug limit or below the low slug
limit):
• A slug flow alarm is immediately displayed on the Diagnostics panel (see Section 13.4).
• During the slug duration period, the core processor holds the mass flow rate at the last
measured pre-slug value.
• If slugs are still present after the slug duration period expires, the core processor forces the
mass flow rate to 0, independent of the mass flow rate measured by the sensor.
Note: During conditions of slug flow, all internal calculations that include mass flow rate, including
volumetric flow calculations, will be affected by the slug flow functionality.
When the slug flow condition clears:
• The slug flow alarm is cleared from the Diagnostics panel.
• The mass flow rate reverts to the measured value.
The default slug limits are set so that slug flow functionality will not be activated in a typical gas
application. Raising the low slug limit or lowering the high slug limit will increase the possibility that
slug flow functionality will be activated.
Example The meter is installed and proved for the first time. The meter mass
measurement is 250.27 lb; the reference device measurement is 250
lb. A mass flow meter factor is determined as follows:
The first mass flow meter factor is 0.9989.
One year later, the meter is proved again. The meter mass
measurement is 250.07 lb; the reference device measurement is
250.25 lb. A new mass flow meter factor is determined as follows:
The new mass flow meter factor is 0.9996.
MassFlowMeterFactor 1
250
250.27
------------------
×
0.9989
= =
MassFlowMeterFactor 0.9989
250.25
250.07
------------------
×
0.9996 = =