Differential Pressure Transmitter Inferential Applications

Differential pressure transmitter for industrial
process control applications.
Image Courtesy Azbil North America

Differential pressure transmitters are utilized in the process control industry to represent the difference between two pressure measurements. One of the ways in which differential pressure (DP) transmitters accomplish this goal of evaluating and communicating differential pressure is by a process called inferential measurement. Inferential measurement calculates the value of a particular process variable through measurement of other variables which may be easier to evaluate. Pressure itself is technically measured inferentially. Thanks to the fact numerous variables can be related to pressure measurements, there are multiple ways for DP transmitters to be useful in processes not solely related to pressure and vacuum.

An example of inferential measurement via DP transmitter is the way in which the height of a vertical liquid column will be proportional to the pressure generated by gravitational force on the vertical column. The differential pressure transmitter measures the pressure exerted by the contained liquid. That pressure is related to the height of the liquid in the vessel and can be used to calculate the liquid depth, mass, and volume. The gravitational constant allows the pressure transmitter to serve as a liquid level sensor for liquids with a known density. A true differential pressure transmitter also enables liquid level calculations in vessels that may be pressurized.

Gas and liquid flow are two common elements maintained and measured in process control. Fluid flow rate through a pipe can be measured with a differential pressure transmitter and the inclusion of a restricting device that creates a change in fluid static pressure. In this case, the pressure in the pipe is directly related to the flow rate when fluid density is constant. A carefully machined metal plate called an orifice plate serves as the restricting device in the pipe. The fluid in the pipe flows through the opening in the orifice plate and experiences an increase in velocity and decrease in pressure. The two input ports of the DP transmitter measure static pressure upstream and downstream of the orifice plate. The change in pressure across the orifice plate, combined with other fluid characteristics, can be used to calculate the flow rate.

Process environments use pressure measurement to inferentially determine level, volume, mass, and flow rate. Using one measurable element as a surrogate for another is a useful application, so long as the relationship between the measured property (differential pressure) and the inferred measurement (flow rate, liquid level) is not disrupted by changes in process conditions or by unmeasured disturbances. Industries with suitably stable processes – food and beverage, chemical, water treatment – are able to apply inferential measurement related to pressure and a variable such as flow rate with no detectable impact on the ability to measure important process variables.

Share your process measurement challenges with instrumentation specialists, leveraging your own process knowledge and experience with their product application expertise to develop an effective solution.

Industrial Transmitters for Pressure and Differential Pressure from CTi Controltech

Filled Impulse Lines With Pressure Sensors

Pressure sensors or transmitters are installed directly to
process lines or vessels, or remotely using impulse lines.
Image courtesy Azbil NA, Inc.

Pressure sensors intended for use in industrial process measurement and control applications are designed to be robust, dependable, and precise. Sometimes, though, it is necessary or beneficial to incorporate accessories in an installation which augment the performance of pressure sensors in difficult or hazardous environments. There are some scenarios where the sensor must be isolated from the process fluid, such as when the substance is highly corrosive.

A way to aid pressure sensing instruments in situations where direct contact must be avoided is by using a filled impulse line. An impulse line extends from a process pipe of vessel to a pressure measurement instrument or sensor. The line can have a diaphragm barrier that isolates the process fluid from the line, or the line can be open to the process. There are best practices that should be followed in the design and installation of an impulse line to assure that the line provides a useful transmission of the process pressure to the sensor and whatever degree of isolation or protection is needed remains in effect.

The filled impulse line functions via the addition of a non-harmful, neutral fluid to the impulse line. The neutral fluid acts as a barrier and a bridge, allowing the pressure sensing instrument to measure the pressure of the potentially harmful process fluid without direct contact. An example of this technique being employed is adding glycerin as a neutral fluid to an impulse line below a water pipe.

Glycerin’s freeze point is lower than water’s, meaning glycerin can withstand lower temperatures before freezing. The impulse line connected to the water pipe may freeze in process environments where the weather is exceptionally cold, since the impulse line will not be flowing in the same way as the water pipe. Since glycerin has a greater density and a lower freezing point, the glycerin will remain static inside the impulse line and protect the line from hazardous conditions.

The use of an isolating diaphragm negates the need for certain considerations of fill fluid density, piping layout, and the need to create an arrangement that holds the fill fluid in place within the impulse line. System pressure will be transferred across the diaphragm from the process fluid to the fill fluid, then to the pressure sensor. It is important to utilize fluids and piping arrangements that do not affect the accurate transference of the process pressure. Any impact related to the impulse line assembly must be determined, and appropriate calibration offset applied to the pressure sensor reading.

An essential design element of a filled impulse line without an isolating diaphragm is that the fill fluid must be compatible with the process fluid, meaning there can be no chemical reactivity between the two. Additionally, the two fluids should be incapable of mixing no matter how much of each fluid is involved in the combination. Even with isolating diaphragms employed, fluid harmony should still be considered because a diaphragm could potentially loose its seal. If such a break were to occur, the fluids used in filled impulse lines may contact the process fluid, with an impact that should be clearly understood through a careful evaluation.

Share your pressure measurement requirements and challenges with experienced application specialists, combining your own process knowledge and experience with their technical expertise to develop an effective solution.

Advanced Pressure Transmitters for Process Measurement

Direct mount pressure transmitter
Courtesy Azbil

The measurement of pressure is a common task throughout many industrial spheres. Depending on the application, a wide range of process or machinery operation status can be derived from a pressure reading. Accuracy, ruggedness, and flexibility in application are hallmarks of a useful pressure transmitter.

Azbil North America advanced pressure transmitters offer a combination of features that can make them an advantageous selection for almost any application.

• Stability of +/-0.1% for 10 years
• Little to no downtime for calibration
• Sensor technology that provides day-one accuracy for the life of the transmitter
• Customizable display
• Alarm outputs
• Fast response
• International standard certifications

The advanced pressure transmitter is available in variety of mounting configurations to suit most applications. More information is contained in the document included below. Share your process measurement challenges with application experts, combining your own process knowledge and experience with their product application expertise to develop effective solutions.

Advanced Direct Mount Pressure Transmitter from CTi Controltech

Liquid Level Measurement With Differential Pressure

Flange mount differential pressure transmitter
Courtesy Azbil North America

Industrial process operations frequently involve the use of liquid storage tanks for raw materials, in-process liquids, or finished product. Regardless of the tank usage, the data point indicating the liquid level in the tank is useful for effective process operation and safety.

Deriving tank liquid level can be accomplished utilizing a number of technologies, classed as direct or inferred. Direct level readings indicate level by interacting with the interface of the liquid surface and the unfilled area of the tank. A simple example of this would be a float, the position of which is read on a scale or transmitted to a controller. Inferential measurements are derived by directly measuring a condition which is affected in a predictable manner by the tank liquid level.

Differential pressure is an inferential methodology for determining liquid level. A differential pressure transmitter measures the total pressure of the fluid column and subtracts the pressure exerted on the surface of the liquid, delivering a signal indicating the net pressure exerted by the liquid column in the vessel. This value can be used to calculate the height of the liquid column, weight of the fluid, percentage of tank fill, or other useful values. The differential pressure method of liquid level measurement has some key features:

• Cost effective
• Limited moving parts in a complete installation, generally consisting of isolation valves for transmitter servicing
• Best applied to clean liquids with known uniform density
• Provides good accuracy over a wide operating range
• Easily mounted or retrofitted to vessel surfaces
• Low maintenance requirements
• Specifically designed seals can be utilized to provide hygienic vessel connections, or device protection from high temperature or corrosive process media

Like any measurement technology, the differential pressure method of liquid level measurement provides the best results when properly installed and maintained. There are numerous installation configurations and options to fulfill a wide variety of applications. A product application specialist can help you specify the best arrangement for your application.

Azbil Industrial Pressure Transmitters from CTi Controltech