Better Ways to Use Differential Pressure for Liquid Level Measurement

 Suggesting ways to improve liquid level measurement with differential pressure (DP) is sort of like suggesting better ways to tie your shoes. Everybody knows how it works, right? Well, maybe not. 

The basic concepts are clear, but the mechanics of making it actually work aren’t always as widely known as we might expect, especially in challenging applications. Reexamining some of these points is the main topic of my article in the October 2019 issue of Processing, Better Ways to Use Differential Pressure for Liquid Level Measurement. DP is indeed a common method for level measurement, but the way some plants apply it can create maintenance headaches or inaccurate readings.

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.

So where do users go wrong? If the concepts really are so simple, where do the problems originate? As the article discusses, understanding liquid density is important, but usually presents minimal difficulties. The more challenging problems are associated with pressurized tanks and vessels. When the interior is no longer vented to atmosphere, problems can emerge.

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.

How the connection is made to the head space is the tricky part. There are different approaches for spanning the distance from the transmitter at the bottom and the port at the top. Some work better than others, and some can create the serious maintenance headaches mentioned.

One approach that is catching on is Emerson’s RosemountTm 3051S Electronic Remote Sensor (ERSTm) System, which features best-in-class performance and measurement stability. It is engineered with flexible, digital architecture which can calculate differential pressure electronically using two pressure sensors linked together with an electrical cable. This safety-certified system is engineered for faster response time than traditional systems, providing exceptional performance in a wide range of applications.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange 365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Pressure and Level Groups and other specialty areas for suggestions and answers.