Electronic differential pressure transmitters and why we love them
Say hey! All good? We had an intense week at Hannover Messe 2017, but I’m sure you found a lot of new solutions and devices to support your processes, right? Now we’re back to the regular Visaya schedule, providing you knowledge about process automation.
Level measurement is an art
Let’s have a chat about the possibilities in level measurement. Today we can find the best level solutions for our processes from a vast range of products. However, some companies still use old ideas rather than try new technology. Of course, sometimes they have reasons to avoid applying new tech all the time. They need to keep the process running, or they need certain certifications.
A well-known choice for level measurement is the differential pressure (DP) transmitter, right? However, some processes challenge standard DP transmitters. To solve these issues, manufacturers created electronic DP transmitters.
So today we’ll talk about traditional installations and their problems. Then we’ll discuss how electronic differential pressure transmitters can avoid these problems. You should note that vendors have different names for it, though. For example, Endress+Hauser says electronic DP transmitter, Emerson calls it an Electronic Remote Sensor (ERS), and so on.
When you have a pressurized or closed vessel, you need to compensate for the pressure or vapor with the low-pressure cell of the transmitter. Depending on your needs, you can use a capillary or impulse line, but either way, you can have problems.
When do you apply a capillary or impulse line? First of all, you can start with impulse lines, but you may need the capillary to solve certain problems. Impulse lines have two segments, called wet leg and dry leg. The wet leg has the line filled with either column product, and the dry leg has a suitable dry gas instead of liquid. When the vapor or gas won’t condense into the impulse line, you can use the dry leg. If you have the possibility of condensation, then you need the wet leg.
These kinds of setups have disadvantages, though. A wet leg may leak or ice up, requiring a special installation to avoid these issues. A dry leg may create condensation in the line. These issues can make your measurements vary and demand extra maintenance. Hence, the capillary and seal system came to eliminate many of these problems. However, the solution also has its own problems!
This oil-filled diaphragm mounts directly into the high- and low-pressure cells. When the pressure changes, the diaphragm deflects it and sends the force through the oil system up to the sensor. This effect results in the measurement. Because the capillaries can flex, you don’t need to build a structure for the system, making installation easier than for an impulse line.
You have two capillaries here, one in the high-pressure cell and the other in the low. The capillary construction usually reflects a balanced system, with identical seals and equal lengths of tubing on each side of the transmitter. With the same conditions on both sides, when the ambient temperature changes, both sides will react the same. That means the measurement won’t vary. Ideally, of course.
In reality, you may still have variance. If the low side gets more heat than the high side, then the density will change on the low side and mess up the measurement. We call this a temperature-induced density effect. Kinda obvious name, isn’t it?
There are other ways to solve this problem. For example, Emerson created the “Rosemount Tuned-System,” with the capillary in a unique mounting to avoid traditional problems. But enough about traditional stuff! Let’s get into the electronic solutions out there.
Sensors and transmitters and systems, oh my!
This idea came about to solve all the issues with impulse lines and capillary systems. As I said before, different brands have different names, but they bring the same benefits to the end users.
Emerson’s Electronic Remote Sensor (ERS) System has two transmitters, one on the low-pressure side and one on the high, communicating with each other digitally. One of them calculates the level measurement, sending the data to the control system.
The Endress+Hauser electronic differential pressure system has a sensor on each side and one transmitter. Here the transmitter calculates the level, although the sensors also communicate with each other digitally. Plus, you can change a damaged sensor without replacing the whole system.
Now you won’t have to fight temperature changes, condensation, or mechanical issues. You can reduce installation costs too, because these systems go in much easier compared to an impulse line or capillary system. Electronic systems have one drawback, though – they hate high heat. Still, some vendors offer devices that can withstand 410 degrees Celsius and more.
So we have the technology, or we will, to avoid many traditional issues that come from DP transmitters. No more floating level measurement in your level measurement!