Better ways to use differential pressure for liquid level measurement

Most engineers understand how the technology works, but here are practical techniques to improve measurement performance and reduce maintenance.

Figure 1. Pressure transmitters, such as those in Emerson’s Rosemount 3051 pressure transmitter family, provide a high degree of precision and versatility for DP level applications. All images courtesy of Emerson
Figure 1. Pressure transmitters, such as those in Emerson’s Rosemount 3051 pressure transmitter family, provide a high degree of precision and versatility for DP level applications. All images courtesy of Emerson

Figure 2. A diaphragm sensor mount minimizes the potential for process containment loss while also protecting the transmitter.Figure 2. A diaphragm sensor mount minimizes the potential for process containment loss while also protecting the transmitter.Using differential pressure as a technology to measure level in tanks and vessels is a common technique, but many users apply it in ways that decrease reading accuracy or create maintenance headaches. In this article, we’ll look at a few basic concepts of the technology and then analyze an example application to provide ideas for improving and troubleshooting new and existing installations.

Figure 3. Here are three approaches to solve pressure measurement errors caused by wet legs. “A” uses a balanced approach which can vary due to temperature induced density changes. “B” is a tuned approach which improves response time. “C” illustrates Emerson’s Rosemount 3051 ERS System which eliminates the impulse line entirely.Figure 3. Here are three approaches to solve pressure measurement errors caused by wet legs. “A” uses a balanced approach which can vary due to temperature induced density changes. “B” is a tuned approach which improves response time. “C” illustrates Emerson’s Rosemount 3051 ERS System which eliminates the impulse line entirely.

Liquid residing in any type of vessel or tank develops pressure caused by its own weight, allowing pressure to be measured in inches of water column. If one were to drill into the side of a tank and insert a gauge calibrated to sense the proper range pressure and it indicated 120 inches, and if the tank is holding water and is open to the atmosphere, one can rightly conclude there is 10 feet of water above the gauge. The underlying principle is simple to understand but implementing it can get complicated quickly.

First, density is a factor. The specific gravity of the liquid has to be included in the calculation to turn pressure into depth. If the liquid and its characteristics are well understood, including the effects of temperature, the level measurement will have the same degree of precision as the pressure instrument.  

Second, if the tank is not vented to atmosphere, the interior could be above or below atmospheric pressure. If the instrument measuring level is reading against atmosphere, the interior pressure of the system adds to the pressure of the liquid and can change the calculated level reading drastically because it is reading the pressure of the liquid plus the system pressure. Under these conditions, the simple pressure gauge has to be replaced with a differential pressure (DP) gauge with the high side connected to the liquid contents and the low side to the head space above the liquid. With this approach, the reading self-compensates and provides a correct indication of liquid level.

These are basic enough concepts. Usually a company understands its process and products sufficiently to overcome the density question, but implementing DP measuring techniques can be more challenging.

Figure 4. Distillation towers come in a variety of sizes and shapes but include many common elements.Figure 4. Distillation towers come in a variety of sizes and shapes but include many common elements.

Basic implementations

The simplest implementation for the two connections of the DP reading using a digital transmitter (Figure 1) is to put a direct impulse line to the high side from the liquid tap, and then run a line from a tap at the top of the tank to the low side. The first connection is wet and the second is dry. The problem with this approach is that the impulse lines and the transmitter itself are now part of the process containment. A loose tube fitting or breach of any kind in the lines or transmitter is a loss of containment. If the process is benign and the operating temperatures and pressures are not excessive, this might be perfectly adequate. 

Where conditions are more critical, the process connections on the tank typically use a flanged spud and the transmitter connection uses a diaphragm (Figure 2), which keeps the process medium inside because the pressure is transmitted via an incompressible fill fluid inside the impulse lines on both sides. While this is very effective at protecting the process and the transmitter, once the lines containing fluid come into the picture, they introduce several side effects that can affect reading accuracy.

To address this issue, there are self-contained diaphragm and transmitter assemblies (Figure 5) that have internal fluid-filled sections designed to minimize heat transfer between the process and transmitter, while avoiding the usual drawbacks. These assemblies use two fluids: one selected to withstand the full process heat and a second suited for ambient conditions. 

The high temperature fluid is contained in a capillary tube between the main diaphragm and a second internal diaphragm, and the tube transmits pressure to the second fluid, which contacts the actual measuring diaphragm. The assembly is fully sealed and protected to avoid any physical damage to the mechanism. This type of assembly increases the thermal range possible for the transmitter, providing greater application flexibility.

Figure 5. This cutaway view of Emerson’s Rosemount 3051S Thermal Range Expander shows how it uses two fill fluids in sealed capillaries to carry pressure but not heat to the transmitter, ensuring fast response time and low maintenance.Figure 5. This cutaway view of Emerson’s Rosemount 3051S Thermal Range Expander shows how it uses two fill fluids in sealed capillaries to carry pressure but not heat to the transmitter, ensuring fast response time and low maintenance.

Solutions simplify complexity

DP can serve applications well whether the primary objective is inventory management, process control or improved safety. DP can provide a high degree of precision and repeatability but is still economical. If new technological adaptations are used, it can also be low-maintenance, providing critical process data continuously for years at a time. 

References

1. U.S. Chemical Safety Hazard and Investigation Board, Mar. 20, 2007 https://www.csb.gov/bp-america-refinery-explosion/


Nicole Meidl is a product manager for Emerson in Shakopee, Minnesota. She specializes in Rosemount DP level products, but in her five years with Emerson she has managed Rosemount pressure transmitters, multi-variable transmitters and electronic remote sensor systems. Nicole has a bachelor’s degree in mechanical engineering and is currently pursuing an Master’s in Business Administration from the University of St. Thomas in St. Paul.


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