The basic principle behind hydrostatic pressure-based liquid level measurements is fairly simple. Pressure and DP transmitters measure level based on the principle that pressure is proportional to the level of liquid multiplied by the specific gravity (the ratio of the fluid’s density to the density of water). Another way of looking at it is that level equals the hydrostatic head (pressure) measurement divided by the density of the liquid.
In open (vented) tanks or vessels, the DP transmitter is mounted at the bottom of the vessel. The high side or the sensor measures the hydrostatic pressure exerted by the fluid in the vessel. The low (reference) side senses atmospheric pressure.
In sealed (pressurized) tanks or vessels, a diaphragm mounted on the vessel at a point above the liquid is connected to the low side of the sensor to provide a reference leg. With conventional analog transmitters, the pressure difference between the high and low sides is converted into a pressure-proportionate output signal. With intelligent transmitters, the pressure difference is converted directly into engineering units.
“The original installation method for using a DP transmitter for level was to use impulse tubing to connect the high and low pressure sides of the DP sensor to the bottom and top of the vessel,” Graupmann explained. “The impulse tubing used for the low-side reference is called a ‘dry leg’ system when used on dry, non-condensing vapor space applications. The impulse reference leg is filled when the vapor space might condense and is called a ‘wet leg’ system. Capillary/seal systems were introduced to eliminate the measurement uncertainty and maintenance of impulse tubing installations.”
While capillary/seal systems have helped improve pressure-based level performance, they can also add a bit of complexity. “One issue we have had to deal with regarding remote seal DPs is that we have so many with different tap dimensions,” commented Motiva’s Breaux. “This requires us to stock many spare devices, with different capillary lengths. Sometimes we must replace a device with a new device from stock with excessive capillary length, because it is the shortest one that will fit. Stocking of spare parts is an issue for a long-term, run-and-maintain operation.”
Breaux adds, “We have created a short checkout procedure we perform during installation of remote seal DPs. We first check the accuracy of the device on the bench, then as installed on the vessel open to atmosphere before lining it up to the process. This resulted from our early experience, where we found a few devices reading incorrectly after they were installed. We removed them and found damaged diaphragms, probably from shipping or handling.”
Since density variations, which are commonly caused by temperature fluctuations, can result in pressure-based level measurement errors (especially in closed tanks), additional pressure and/or temperature measurements are also often required to be able to continuously calculate the density of the liquid in both the vessel and reference leg(s). These additional measurements typically are performed using separate transmitters with the density and level calculations performed in the control system.
With the wide variety of different process connections (direct-mount, flanged, remote seal, etc.), process-wetted materials (carbon steel, 316L stainless, Hastelloy C, titanium, Monel, etc.), and fill fluids available for DP transmitters, they can be adapted for the majority of liquid level measurement applications encountered in process plants, including high-temperature, high-pressure, corrosive and sanitary level measurements.
Some Recent Developments
Emerson Process Management, Endress+Hauser, Invensys Process Systems and other leading automation vendors quietly have been enhancing their pressure instrumentation families in recent years to be able to provide process industry end users with improved pressure, flow and level measurements in increasingly challenging applications. These are helping to maintain the viability of well-proven pressure-based level measurements against newer and “sexier” technologies, especially in existing plants where pressure-based level measurements are already being used.
Ceramic pressure sensors
For example, new ceramic pressure sensors from Endress+Hauser are made from 99.9% pure aluminum oxide to provide extended chemical compatibility. The physical durability of the ceramic pressure sensor makes it appropriate for use for abrasive fluids or applications requiring frequent buildup on the diaphragm. According to Endress+Hauser the dry (no fill fluid) ceramic sensors can also be used to measure deep vacuums at elevated temperatures without damage to the diaphragm.
Asymmetrical diaphragm seals
In the past, the standard practice for measuring level with DP transmitter remote seal was to specify symmetrical systems with the same diaphragm seals and identical capillary lengths for both the high and low pressure process connections. In an effort to reduce the temperature-induced errors inherent to diaphragm seal systems, Emerson has developed an asymmetrical approach, with each system specially tuned for the individual application. According to Emerson’s Graupmann, these “tuned system assemblies” combine direct-mounting of the high-side seal to the DP transmitter with the optimal seal system to improve performance, reduce response time and reduce installed costs as compared to conventional balanced-seal systems with equal seals and capillary lengths. In effect, the asymmetry of these tuned systems compensates for the temperature-induced errors.
According to Emerson product literature, “Tuned-Systems are an asymmetric configuration of a differential-pressure diaphragm seal system. The simplest form of a tuned system directly mounts the diaphragm seal to the high-pressure process connection. Eliminating the excess high-pressure capillary immediately improves response time and performance, while reducing installed costs. Total system error is compensated by leveraging diaphragm-induced temperature errors against head-effect temperature errors.”