Rethink Distillation Column Pressure Measurement

Inadequate sensor systems can undermine efforts to optimize tower performance.

By Daniel Siddiqui and Ehren Kiker, Endress+Hauser

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Process requirements dictate the desired pressure profile within a distillation column. Not maintaining the correct pressure at all points can lead to energy waste, inefficient operation that can cause product loss and quality problems and, in some cases, even safety issues. That’s why pressure measurement is one of the most critical parameters for control of distillation processes — and why a column typically has multiple, and sometimes redundant, sensors to measure pressure in various zones (Figure 1).

To maintain the desired pressure profile, the pressure instruments must work safely and reliably. This requires proper selection, calibration and maintenance. So, in this article, we’ll discuss issues found when measuring pressure in distillation columns and review options, selection criteria and safety.

First, let’s set the stage by providing some background about common challenges.

The output from a pressure sensor in a column normally goes to a ground-level pressure transmitter via an impulse line or a remote diaphragm seal with capillary tube. This impulse line or capillary tube may run a long distance — say, from the top of a 100-ft tower to ground level — to get to the transmitter.

The transmitter usually is located in an enclosure to protect it from ambient conditions. However, the capillary tube often is exposed to the environment and, therefore, faces temperature variations.

Ambient temperature changes from day to night and from one season to the next affect the density of the fill fluid in an impulse tube or capillary. These fluctuations, in turn, negatively impact the reliability of the pressure measurement because pressure transmitters by design assume the density of the fill fluid in a remote capillary leg is constant.

The transmitter interprets these variations not as changes in fill fluid density but rather as changes in pressure. If not properly accounted for, these fluctuations can result in significant measurement deviations, posing problems to the reliability and safety of the process.

Typical ways to counter these fluctuations with traditional remote diaphragm seal configurations range from insulating or heat-tracing the tubes to installing critical distillation columns in climate-controlled buildings.

These options are costly to implement and maintain and, therefore, typically aren’t considered acceptable methods for handling ambient-temperature-related drift. Fortunately, new manufacturing methods for remote diaphragm seals address these ambient-temperature-related issues, as we’ll discuss below.

Tough Environment

Most distillation columns operate under vacuum conditions for greater efficiency and lower energy consumption. Some processes require high temperature as well. Vacuum service combined with high temperatures can present many challenges to pressure instrumentation.

Deformation of diaphragm seals and flanges may let atmospheric air enter the vacuum column. Extreme expansion of the fill fluid in remote seal systems can cause seal failure and, in severe cases, allow release of chemicals through damaged flanges and seals, posing potential plant safety issues. Seal damage also can result in fill fluid leaking into the process, which can keep products from meeting specifications.

Vibration of the column can cause damage, intensified by mounting issues such as close-coupling to the vessel. Thermal shock poses another potential issue, particularly when the transmitter is mounted too close to the column due to short capillary lengths.

To counter these problems, pressure instrumentation suppliers have developed three improvements: • asymmetrically flexing diaphragms in remote seals; • better seal fluid filling techniques; and • elimination of impulse tubes and capillaries.

Asymmetrically flexing diaphragms. A conventional diaphragm on a remote seal moves symmetrically when pressure is applied and has a high degree of stiffness. This requires thinner or larger diameter diaphragms to provide adequate measurement sensitivity — making the diaphragm seal more susceptible to impact from process or ambient temperature changes.

When using remote diaphragm seals with oil-filled capillaries, the common practice is to specify the largest process connection possible and allowable per process requirements to reduce the impact of temperature-induced errors on the measurement. With conventional diaphragms, the thermal error for a 2-in. flange is three times greater than that of a 3-in. one. Often, facilities install concentric reducers or other such adapters to increase the size of remote seals to improve measurement reliability.

Some manufacturers of diaphragm seal systems now offer seals that flex asymmetrically in opposite directions around the center point. This results in a seal that flexes predictably with good repeatability while using less fill fluid. It allows for thicker diaphragm seals that provide greater durability but still maintain the sensitivity required for adequate turndown.

This technology significantly reduces thermal errors, even while using smaller remote seals. For example, a 2-in. flange with an asymmetric membrane provides better measurement stability and faster recovery from thermal shocks than a 3-in. flange with conventional diaphragms.

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