Differential Pressure Transmitter-FAQ

 

A differential pressure transmitter is a device to measures two opposite pressure in the pipe and is the most versatile and commonly used measuring instrument in various applications. Differential pressure transmitter also measures flow in various industries, for different fluids at high temperature.

Before purchasing the differential pressure transmitter, it is vital to know the basics of differential pressure transmitter, its working, installation, and applications.

In this guide, we will answer the frequently asked questions about the differential pressure transmitter. We hope that this information will help anyone interested in purchasing a differential pressure transmitter.

1. What is a differential pressure transmitter?

Differential pressure transmitter is the most common measuring instrument in industries. It measures the pressure difference between two ports and sends the signal that shows the pressure compared to the calibration range. As a result, various pressure-sensing technologies have adopted the differential pressure transmitter.

The measurement of the differential pressure transmitter is accurate and stable. For those organizations where pressure has a lot of influence on the production and product quality, a differential pressure transmitter is the best choice. A high-performance differential pressure transmitter is suitable for a wide range of applications with extreme pressure and temperature.

2. How does a differential pressure transmitter work?

A differential pressure transmitter consists of a sensing cell and a transmitter. The sensing cell has two pressure chambers; one chamber is a high-pressure side chamber while the other is low-pressure. The diaphragm separates these chambers. The differential pressure transmitter is suitable for various industrial applications, with a strong body for sensing elements. In addition, it has a compartment with mechanical or electronic components to convert pressure into a signal.

The pressure sensing element in the differential pressure transmitter is part of the bottom half of the device. The electronics are in the top half. Therefore, regardless of different models or designs of the differential pressure transmitter, each has pressure ports to measure other fluid pressures.

3. What is the working principle of a differential pressure transmitter?

The differential pressure transmitter consists of the electrical pressure sensor. The sensing element has a metal diaphragm that has two metal surfaces. An electrical fill fluid (liquid or silicon) transfers motion from isolating diaphragm to isolating diaphragm. If there is any difference of pressure across the cell, it causes the diaphragm towards lower pressure.

The fluid pressures cause two forces by two fluid pressures to work against one another. The force is related to the diaphragm, where the diaphragm is constant. The secondary function of the diaphragm acts as a method for measuring displacement. The distance and the capacitance between conductors are inversely proportional to one another.

The capacitance on the low-pressure will increase, while capacitance on the high-pressure side will decrease. This is because the capacitor circuit uses high-frequency AC to measure the difference in capacitance between two halves.

4. How to install a differential pressure transmitter?

Installation of the differential pressure transmitter is a simple process; however, you need to follow certain practices. One fundamental aspect to consider in installation is impulse piping between the transmitter and the process. The transmitter transfers the pressure for accurate measurement. The common errors that occur during installing differential pressure transmitters are friction loss, pressure transfer leaks, and trapped gas in a liquid gas or density variations between low pressure and high-pressure impulse lines.

To ensure the proper installation and accuracy of differential pressure transmitter, the following best practices should be followed:

• Keep impulse piping short
• Make sure that impulse lines are at the same temperature
• Fill the piping legs to the same level while using a sealing fluid
• Don’t get corrosive material in direct contact with the sensor module
• Don’t allow conditions that might allow process fluid to freeze

5. How to calibrate pressure transmitter?

For calibrating differential pressure transmitter, set up the transmitter, communicator, power supply, and multimeter. Close the equalizing valve, and apply the pressure to the transmitter equal to lower range pressure. For example, in the calibrated range of 0-10, the lower range pressure should be 0. Read the pressure in HART communicator. Check whether the output of the transmitter is the same with applied pressure by using a multimeter.

Check the output through the HART communicator, and adjust the output of the transmitter to 4 mA. Next, apply the pressure to the upper range pressure, which should not exceed 20 mA. Again, read the HART communicator’s pressure, adjust the output, and apply pressure on the same level.

6. How to measure level with differential pressure transmitter?

We use level measurement in open tanks on the Pascal equation for hydrostatic pressure to calculate using differential pressure transmitter. The differential pressure transmitter has two cells, one for high pressure and one for low. For measuring hydrostatic pressure, the high-pressure cell reaches the bottom of the tank. For measurement calculation in an open tank, the low-pressure cell should be open to the air.

With the installation of the transmitter at zero levels, the minimum and maximum measurements are considered. Minimum at level 0 %( HP-LP), and maximum at 100%. HP and LP stand for low pressure and high pressure.

With the transmitter below the zero levels, the installation is different. This is because the value of the transmitter is higher than the actual value, so we call this suppressed zero, where the minimum level is 0% and the maximum level is 100%.

7. How does differential pressure transmitter measures gauge pressure?

Differential pressure transmitter is the pressure measuring device, which measures pressure difference, positive and negative pressures, and absolute pressures. This technique measures the pressure by connecting the high and low sensing ports. In these applications, metal or plastic tubes connect the transmitter sensing ports to the impulse lines. Thus, the differential pressure transmitter acts as a simple pressure gauge by measuring the pressure over atmospheric pressure.

DP transmitter measures the pressure difference between vessel and atmosphere by connecting the high side of the differential pressure transmitter to the pressure vessel while leaving the low side vented to the atmosphere.

8. How do a differential pressure transmitter measures vacuum and absolute pressure?

The differential pressure transmitter also measures the absolute pressure in the system. Absolute pressure is the difference between a perfect vacuum and current fluid pressure in the system. Differential pressure transmitter measures the absolute pressure by connecting the low side of the pressure sensing element with the vacuum while connecting the high side to the process vessel. If the pressure is more than a perfect vacuum, it is the positive pressure difference.

The same principle is used for measuring vacuum. You connect the differential pressure transmitter to the process and other ports to the atmosphere for measuring the vacuum. Make sure to connect the low side to the vacuum process while venting the high side to the atmosphere. If the pressure is more than the atmospheric pressure, it is a positive pressure difference. This is because the signal output of the differential pressure transmitter is greater with a stronger vacuum in the process.

9. How to measure liquid and gas flow using a DP transmitter?

Differential pressure transmitter also measures process variables. DP transmitter measures the fluid flow using a pipe. Pressure in the constriction in the pipe varies with changes in flow rate and fluid density. Keeping fluid density constant, you can measure the pressure drop in the piping constriction. In various industrial applications, the orifice plate is the most common form of constriction. It has the material of metal plate with précised hole in the center. When fluid passes through this hole, it changes the velocity, leading to pressure drop, which infers the flow rate in the pipe.

One part of the DP transmitter is connected to the upstream side of the pipe and the other side with the downstream of the pipe. Differential pressure transmitter gives a signal after identifying the pressure difference between upstream and downstream sides of constriction.

 

10. What causes DP transmitter malfunction?

If the maintenance of the differential pressure transmitter is not done, it can cause malfunction of the transmitter. Once the problem with the transmitter is detected, you need to locate the possible cause of the issue to resolve it accordingly. It is better to contact the manual that will help you identify the malfunctioning parts of the transmitter. In addition, you need to follow the following precautions and guidelines for the proper functioning of your DP transmitter.

If the transmitter is disassembled, the pressure present in it can cause serious injury or even the deaths of the technicians. Isolate the malfunction transmitter from its pressure source. Check the differential pressure transmitter loop on the specified voltage. If you exceed the limit, it can cause damage to electronics. If there is an issue with the transmitter and you need to open it, follow your manufacturer’s specific guidelines to avoid any injury.

11. What are the major parts of differential pressure transmitters?

There are two essential parts of a differential pressure transmitter, the pressure sensing capsule, and the electronics. The electronics convert the differential pressure into a 4 – 20mA DC signal. The differential pressure transmitter has pressure sensing ports with threads that accept the connection to the process medium. These ports are labeled as high and low. These labels show the effect of pressure on the output level.

Another important part of the differential pressure transmitter is the diaphragm. The fluid pressure enters the diaphragm from the port labeled H while process fluid pressure enters from label L. Suppose any pressure difference between the two ports causes the diaphragm to move from its original position, which is translated into a signal. For example, if the pressure at port H is more than the pressure on port L, the transmitter’s output signal reaches 20 mA. At the same time, it falls to 4 mA if the pressure at L is more than port H.

12. What are the main advantages of a differential pressure transmitter?

Differential pressure transmitter offers a lot of benefits. The primary advantage of a differential pressure transmitter is that it can be installed easily on the vessel. In addition, the level sensors of differential pressure can be mounted to the surface of the vessel. These sensors can also provide block valves for isolating them from process liquid during maintenance and testing of differential pressure transmitters.

The differential pressure transmitter has wide applications in level measurement, such as total level in separator vessels. Differential pressure transmitter is inexpensive and has a compact sensor. Therefore, you can use them for a wide range of sizes with high accuracy and durability. The differential pressure transmitter measures the pressure by connecting the higher side to the large diameter and the lower side to the smaller diameter, which is an accurate solution at a low cost.

13. What are the disadvantages of a DP transmitter?

There are some drawbacks of differential pressure transmitters. If the density of the process fluid changes at any stage because of factors like temperature variations, it can cause errors in the system. In addition, you can only measure clean liquids using a differential pressure transmitter. Another problem with the DP transmitter is that one of its vessel penetrations is installed near the vessel that causes leakage.

The differential pressure transmitter can cause pressure drop that results in limited accuracy and turndown range. While installing the differential pressure transmitter, you have to consider the location of the transmitter, the length of the meter run, and several other items as they directly affect the DP transmitter.

14. What are the applications of differential pressure transmitters?

The differential pressure transmitter provides exceptional accuracy and stability in measurement. Because of their versatility, these transmitters are suitable for many industries on a variety of fluids. Differential pressure transmitter is different from another transmitter because they have reference pressure systems dependent on the pressure type that the application needs.

DP transmitters also measure gauge pressure. It measures the gauge pressure by connecting the high end of the sampling to the pipe or vessel and the low end to the atmospheric pressure.

Differential pressure transmitter also plays a vital role in absolute pressure measurement, where the low end of the transmitter is in a vacuum. The differential pressure transmitter measures the atmospheric pressure at atmospheric conditions.

15. What are the elements of differential pressure transmitters?

There are two elements for differential pressure transmitters. The first one is the primary element, which produces a pressure difference called the high side. The secondary element measures the difference produced by the primary element and is called the low side. Examples of the primary elements include orifice plates, flow nozzles, and laminar flow elements. Secondary elements include sensors present in the differential pressure transmitter.

The most commonly used sensors in differential pressure transmitters are strain gauges and vibrating wires. These sensors send electronic signals to the housing. The electronic housing protects the sensor. It interprets the signal into the voltage or current, which can be read easily by the control system.

16. How to select the differential pressure transmitter for specific applications?

The differential pressure transmitter uses to measures the difference between the low side and high side pressures. DP transmitter then converts these measurements into a signal. With the development of technology, new designs and types of differential pressure transmitters have made selection difficult. Therefore, you need to consider specific and industrial factors while selecting the right kind of DP transmitter. Differential pressure transmitters are used as pressure sensors for maintenance in high-pressure fluid lines and mining industries.

The differential pressure transmitter measures pressure, so you need to consider the pressure conditions while choosing the transmitter. The differential pressure transmitters are designed for a specific pressure range that can be different for different applications. So they should be selected with a suitable pressure range. In applications where pressure rises and falls suddenly, the operator should know the highest and lowest pressure spikes during operation. High pressure can cause damage to the system, while low pressure can affect the accuracy of results.

17. What is the process connectivity for DP transmitter selection?

Differential pressure transmitters are part of the integrated system supported by various components, such as orifice plates, pressure tanks, etc. Therefore, various mountings are available depending on the versatility of the connection. These mounting includes inline mounting, coplanar mounting, and bi-planar mounting.

When the differential pressure transmitter connects in a single connection at the end of the pipe, it is known as inline mounting of DP pressure transmitters. This type of mounting is suitable for the direct measurement of pressure.

Coplanar pressure transmitter is made for differential pressure sensing. These are suitable for operations with absolute or differential pressure measurements. In bi-planar mounting, the differential pressure transmitter is connected with two ports in the measurement line.

18. What is a remote seal differential pressure transmitter?

The pressure sensors in differential pressure transmitters are integrated with various mechanical components, which cause exposure to oil, chemicals, corrosion, etc. To overcome this issue, remote seals are used. Remote seals provide protection to differential pressure transmitters from any damage. When there is a variation in pressure, these pressure differences are sent to the differential pressure transmitter sensor.

The seals are filled with oil that is compatible with temperature and pressure. The transmitter is in close connection to the high-pressure process connection. All cavities have oil filling, including the transmitter, diaphragm, and capillary. The remote seal help ensure high quality and easy cleaning options.

19. What is a differential pressure transmitter 3-valve manifold?

The valve manifold is an essential accessory in the differential pressure transmitter. DP transmitters come with a 3-valve or 5-valve manifold, depending on the application. The purpose of these valve manifolds is to ensure that the differential pressure transmitter is not over ranged and to separate the maintenance process from the DP transmitter. 3- The valve manifold has three valves that isolate the differential pressure transmitter from maintenance and calibration. It has high pressure, low-pressure back valve, and an equalizing valve.

While operating the differential pressure transmitter, HP and LP block valves are open while the equalizing valve is closed. For starting the 3-valve manifold, first close the LP, HP, and equalizing valve. Then, make sure that the differential pressure is zero on both sides by opening the equalizing valve.

Close the equalizing valve, and open the low-pressure block to apply process pressure on the low-pressure side of the transmitter. You can measure the differential pressure as the differential pressure transmitter is in service. You can remove the 3-valve manifold by closing the low-pressure block, opening the equalizing valve, and closing the high-pressure block.

20. How 5-valve manifold works in the DP transmitter?

During the operation of the 5-valve manifold, the HP and LP valves are open while the equalizing valve is closed. The equalizing valve must be completed when LP and HP valves are open. If both are open, it will let dangerous fluid reach the transmitter and causes damage. Therefore, the operational sequence of the 5-valve manifold is as follows:

• Check HP and LP valves, open the equalizing valve, and ensure there is zero differential pressure on both sides.
• After opening the high-pressure valve, check if there is any leakage on either side of the transmitter.
• Close the equalizing valve as it will lock pressure on both sides of the transmitter.
• Open the low-pressure block to establish differential pressure.

Similarly, remove the 5-valve manifold from service by closing HP and LP valves, open the equalizing valve, and measuring the process pressure.

Conclusion

Differential pressure transmitter is a beneficial and reliable pressure measuring instrument used in various industries. It measures the pressure by sensing the difference between two ports and then sending the output regarding the calibrated range. They are easy to install at a cheap installation cost.

At Instruthink, we offer high-quality and durable differential pressure transmitters at very affordable rates. You can contact us to install the DP transmitter, and our team of professional experts will install the differential pressure transmitter based on your needs. You can contact us at sales@instruthink.com or directly call us on +44 7700 307784. We are here to assist you in every possible way.