A matter of gravity

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					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.
                         face. A
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Description: The rotary motion moves a magnet attached to the lever assembly, changing the magnetic field; this change is sensed by a Hall-effect sensor. The digital level controller is loop-powered, has high resolution, and provides input filtering and output damping for noise attenuation for mechanical disturbances or liquid turbulence at the displacer.
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