Selection of orifice flowmeter is depending on the type of fluid and its purity of the process fluid. Generally, the impulse pipes should depart from the process pipe at the same height and should end at the transmitter at the same height. Therefore the fluid columns above the transmitter diaphragms have exactly the same weight and do not bias the DP measurement.
To avoid permanent bubbles in liquid applications and liquid stoppers in gas applications, impulse pipes must not run horizontally—although a minimum of 4° angle is required, 10◦ is recommended.
Selection for different fluids are considered here:
With steam, the impulse pipe intakes should be at exactly the same height. The transmitter must be installed below the process piping. This also ensures that enough steam can be condensed and that the impulse pipes and the transmitter are filled only with condensate.
With the gases, the impulse pipes must exit above the process pipe. The transmitter is installed above. This prevents the impulse pipes and the transmitter from being filled with condensate, which would result in a zero DP error.
For gas temperatures up to 175 ° F, the collector block and transmitter can be mounted directly on the orifice intakes. These compact versions significantly reduce the installation cost.
When a wet gas is measured, an installation according to Fig. 73 is recommended. Deposition
chambers can be installed, which can be emptied regularly to prevent the impulse pipes from
filling with condensate.
Liquids Carrying Gas:
Liquids carrying gas and cryogenic gases should have an installation as represented below:
The impulse pipes are now filled with gas, and liquid cannot enter them. In cryogenic installations, natural or forced heating of the impulse lines must be guaranteed to avoid condensation