March 2009

A matter of gravity

By Donald R. Gillum

The chemical and petroleum industries especially see applications where interface control is more important than measurement. However, for feedback control, you need to provide measurement. If you control the water distillate interface level in a separator at some point around the midlevel value, the water can draw off through a valve whose position is a function of the interface level. The main concern in such an application is the valve should be closed when the distillate level is low to prevent losing product, and open when the level is high to permit removing water from the distillate. Some field-mounted liquid-level controllers can see use for liquid interface applications.

A level controller used for interface service should be able to respond over a wide range of adjustments of specific gravity differences. The operation of a displacer system is a function of the difference of the specific gravity in the two liquids forming the interface. A standard controller and displacer may apply where there are very small differences in specific gravity values that would otherwise be beyond the adjustable limits of the devices. A controller with a displacer suited for a limit of 0.5 for a specific gravity difference can see use for a difference as low as 0.1. In such applications, you would get the specific gravity index on 0.5 and the proportional-band setting of the controller on 20% if the actual desired proportional band were 100%. The gain of the level controller must be five times as great for a specific gravity of 0.1 as for a specific gravity of 0.5 because the transmitter gain varies directly with the specific gravity of the fluid. When the desired gain of the controller is one or proportional band is 100%, you can multiply the controller gain by five to account for the related decrease in the level instrument response.

When you want to measure as well as control a liquid level, a duplex-type controller-transmitter is available. This type of device operates from a single displace and torque-tube assembly to generate two completely independent signals. One is used by a control segment that positions a final control element for control purposes, and the other is used for the remote indication or recording of the level value.

A pneumatic type controller incorporates a four-arm reversing arc to transmit angular motion of the torque tube to the pneumatic mechanism that generates the outputs. By adjusting a linkage mechanism along these arcs, you can set specific gravity values. When the linage moves from one side of the arc to the other, the controller action changes from direct to reverse or vice versa.

The pneumatic displacer level transmitter-controller has seen use in applications predominantly in the oil and gas industries, and the industry is still manufacturing pneumatic displacer instruments. As is true with other modern field instruments, microprocessor-based displace transmitters and controllers are available.

A digital displacer level controller and displacer can see use with caged or uncaged displacer sensors. You can use it in any displacer applications that include liquid level, interface level, and density measurement. This type of controller and compatible displacer can connect to industry standard or custom process vessel connections.

The operation of the digital transmitter is such that the linear motion of the displacer converts to rotary motion by the torque tube arrangement. 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 sensor converts the magnetic field signal to an electronic signal that sees conditioning and converts to a 4-20 mA output signal.

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.

Field wiring connections are in a compartment separate from the electronics. This makes the electronic section immune to moisture introduced in the field wiring. Should you ever need to remove the digital level controller for in-shop maintenance, you do not need to disconnect the field wiring.

A guided wave radar transmitter is available to replace displacer-operated transmitters and often replaces the displacer and transmitter when mounted on the displacer chamber. In other cases, the displacer chamber may be replaced with the guided wave radar transmitter and cage assembly, completely replacing the displacer assembly.

SOURCE: Industrial Pressure, Level, and Density Measurement, 2nd Edition, by Donald R. Gillum, ISA, 2009.