October 10, 2012

Remote Sensors for Differential Pressure Measurement

Distillation uses enormous amounts of energy, consuming up to 50 percent of a refinery’s operating costs due to intense heating and cooling cycles. Proper distillation tower operation can reduce energy consumption, but plant personnel need the right information in order to improve operation.
An electronic remote sensor system uses two direct mount pressure sensors connected with a single electrical wire.

Problems can occur when operators and engineers have insufficient information about operating conditions. Failing to properly monitor and control process variables can result in decreased product quality and throughput, increased energy costs, and unsafe operations that put employees and capital equipment at risk.

Many distillation processes use impulse piping to measure differential pressure (DP) across sections of the tower or column. In an impulse piping configuration, the reference leg (low pressure side) is filled with either a column of liquid (wet leg), or a suitable non-reactive gas (dry leg). But impulse piping can be problematic when used on distillation towers. For example, when impulse piping plugs or a wet leg freezes, the pressure measurement is lost. Many process plants have installed complicated flushing systems onto impulse piping systems to clear plugged lines. These flushing systems can be expensive, and often require a control system to operate correctly.

Electronic remote sensor technology solves many of the problems in making a DP measurement. Rather than using a single DP transmitter with mechanical impulse piping or capillary technology, an electronic remote sensor system uses two direct-mount gage or absolute pressure transmitters connected with a single electrical wire.

One of the two transmitters calculates the DP using internal software, and transmits the DP measurement back to the control system using a standard 4-20mA HART signal. Alternately, two transmitters can communicate independently to the control system using the Foundation Fieldbus digital protocol. In this implementation, the DP is calculated within the control system.

The electronic remote sensor system does not require heat tracing, never plugs, and is immune to temperature-induced drifting. This means that plant personnel will be able to get accurate pressure measurements over a large range of ambient temperatures. An electronic remote sensor system also provides process information that can be used for optimized control, such as pressure measurements from each pressure sensor and a scaled variable output that can be used to calculate level or volume.