automation basics | Level
A matter of gravity
By Donald R. Gillum
he chemical and petroleum industries especially A pneumatic type controller incorporates a four-
see applications where interface control is more arm reversing arc to transmit angular motion of the
important than measurement. However, for torque tube to the pneumatic mechanism that gener-
feedback control, you need to provide measurement. ates the outputs. By adjusting a linkage mechanism
If you control the water distillate interface level in a along these arcs, you can set speciﬁc gravity values.
separator at some point around the midlevel value, When the linage moves from one side of the arc to
the water can draw off through a valve whose po- the other, the controller action changes from direct to
sition is a function of the interface level. The main reverse or vice versa.
concern in such an application is the valve should The pneumatic displacer level transmitter-control-
be closed when the distillate level is low to prevent ler has seen use in applications predominantly in
losing product, and open when the level is high to the oil and gas industries, and the industry is still
permit removing water from the distillate. Some ﬁeld- manufacturing pneumatic displacer instruments. As
mounted liquid-level controllers can see use for liquid is true with other modern ﬁeld instruments, micro-
interface applications. processor-based displace transmitters and control-
lers are available.
A level controller used for interface service should be A digital displacer level controller and displacer can
see use with caged or uncaged displacer sensors. You
able to respond over a wide range of adjustments of can use it in any displacer applications that include
speciﬁc gravity differences. liquid level, interface level, and density measurement.
This type of controller and compatible displacer can
A level controller used for interface service should connect to industry standard or custom process vessel
be able to respond over a wide range of adjustments connections.
of speciﬁc gravity differences. The operation of a dis- The operation of the digital transmitter is such
placer system is a function of the difference of the that the linear motion of the displacer converts to
speciﬁc gravity in the two liquids forming the inter- rotary motion by the torque tube arrangement.