Flange type differential pressure transmitter in the installation of precautions

3.1 Selection of installation location

The principle of differential pressure liquid level measurement is mainly based on formula (1): △P=ρgh (1)

In the formula: △P represents the differential pressure, ρ represents the density of the medium, g represents the acceleration of gravity, and h represents the height. If the value of ρ is relatively unchanged, as long as the value of △P is known, the required height value can be obtained. The flange-type differential pressure transmitter used in this project is a double-flange differential pressure transmitter. There are three installation methods, as shown in Figure 5:

The three installation positions are the same. The measurement range and migration amount are analyzed and calculated as follows:

1) Measurement range calculation. Taking the transmitter installed in the middle of two flanges as an example, when the liquid level is low, the high-pressure side pressure of the flange transmitter is P2=-H1ρ0g+P0, so the transmitter range △P=P1-P2=(H1+ H2) ρ mediated g=H0 ρ mediated g. The same method can be used to prove that the range of the transmitter installed above and below the two flanges is also H0ρ.

2) Calculation of zero point migration amount. Still taking the transmitter installed between two flanges as an example, when the liquid level is low, the static pressures P2 (high pressure side) and P3 (low pressure side) on the high and low pressure sides of the transmitter are as follows: The transmitter is installed on At the upper position, P2=-H1ρ0g+P0; P3=-H2ρ0g+P0.

If P0=0, when the transmitter is installed under two flanges, P2 and P3 are both positive pressure; when the transmitter is installed in the middle position, P2 is negative pressure and P3 is positive Pressure; best way to install on 2 flanges.

3.2 Selection of installation parts

1) Regarding the selection of primary valves, it should be noted that butterfly valves cannot be used. Otherwise, the valve plate will touch the isolation flange diaphragm during the opening process, affecting the measurement accuracy and even damaging the diaphragm. Once the isolation flange diaphragm is damaged, the entire transmitter will be scrapped.

2) The selection of fasteners is mainly the selection of gaskets. As can be seen from Figure 3, the isolation flange is connected to the primary valve flange, and a gasket is required to fasten the connection between the primary valve flange and the storage tank flange. This gasket must be a metal-wound graphite gasket. , thinking that this type of gasket has good sealing performance under high pressure and is suitable for places such as liquid ammonia storage tanks that require high sealing levels. In addition, during the installation and tightening process of the diaphragm, the gasket must not be pressed against the diaphragm, otherwise the measurement will be affected.