The DP transmitter is a very versatile pressure-measuring device. This one instrument may be used to measure pressure differences, positive (gauge) pressures, negative (vacuum) pressures, and even absolute pressures, just by connecting the “high” and “low” sensing ports differently.
In every DP transmitter application, there are means of connecting the transmitter’s pressure-sensing ports to the points in a process. Metal or plastic tubes (or pipes) are the means used for this purpose, and are commonly called impulse lines or sensing lines.
Let us now look at a few of the several applications using the versatile DP transmitter:
Measuring Positive Gauge Pressure:
The DP transmitter can be used as a simple pressure gauge if needed. It does this by simply responding to pressures in excess of atmospheric pressure. If we simply connect the “high” side of the DP transmitter to a process vessel using an impulse tube, while leaving the “low” side vented to atmosphere, the DP transmitter will interpret any positive pressure in the vessel as a positive pressure difference between the vessel and atmosphere as shown below:
Measuring Absolute Pressure:
The DP transmitter could also be adapted to measure absolute pressure. Absolute pressure is defined as the difference between a given fluid pressure and a perfect vacuum. In an absolute pressure sensing DP transmitter, the “low” side of its pressure-sensing element is connected to a vacuum chamber, while the “high side” is connected to the process vessel. This way, any pressure greater than a perfect vacuum will register as a positive pressure difference:
We can use the same principle of connecting one port of a DP transmitter to a process and venting the other port to the atmosphere to measure vacuum (pressures below that of atmosphere). All we need to do is connect the “low” side to the vacuum process and vent the “high” side to atmosphere. Any pressure in the process vessel less than atmospheric pressure will register to the DP transmitter as a positive pressure difference (with PH>PL). Thus the stronger the vacuum in the process vessel, the greater the signal output by the DP transmitter:
Measuring Liquid Level:
One of the greatest versatility of DP transmitters is their use in inferring many other process variables. One of such variable is level measurement.
From our elementary physics, we know that Liquids generate pressure proportional to height (depth) due to their weight. The pressure generated by a vertical column of liquid is proportional to the column height (h), and liquid’s mass density(ρ), and the acceleration of gravity (g). Mathematically, this can be put as:
P = ρgh
With this simple relation above, we may use a DP transmitter as a liquid level-sensing device if the density of the liquid remains fairly constant. As the liquid level in the vessel increases, the amount of hydrostatic pressure applied to the DP transmitter’s “high” port increases in direct proportion. Thus, the DP transmitter’s increasing signal represents the height of liquid inside the vessel since:
h = (P/ρg)
The diagrams below represent two variants of the application of DP transmitters in level measurement applications:
Please see Level Measurement for more on measuring liquid level
Measuring Gas and Liquid Flow:
Another very common inferential measurement using DP transmitters is the measurement of fluid flow through a pipe. Pressure dropped across a constriction in the pipe varies in relation to flow rate and fluid density. If the fluid density remains fairly constant, we can measure pressure drop across a piping constriction and use that measurement to infer flow rate.
The most common form of constriction used for inferring flow in most industrial applications is the orifice plate. The orifice plate is a metal plate with a precisely machined hole in the center. As fluid passes through this hole, its velocity changes, causing a pressure drop. It is this pressure drop across the orifice plate that is then used to infer the flow rate in the pipe:
As seen in the diagram above, in using the orifice plate to measure flow, one port of the DP transmitter is connected to the upstream side of the pipe while the other port is connected to the downstream side of the pipe. Differences of pressure between the upstream and downstream sides of the constriction (orifice plate) will cause the DP transmitter to register flow.
Measuring Vessel Clogging:
In many industrial plants, it is common to find DP transmitters connected across process vessels especially those with the tendency to be clogged or become dirty. The DP transmitter is used here to indicate when the process vessel will become dirty and be due for cleaning. In these applications, the DP transmitter is used to measure the difference in pressure across the process vessel e.g a filter, a heat exchanger, or a chemical reactor. The diagram below shows a common scheme for this kind of application: