How to choose between orifice plate flange, corner joint, and pitch for pressure tapping?
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1 Pressure tapping at the flange: The pressure tapping at the flange is as shown in Figure 1. The distance between the upstream pressure taps is 25.4 mm, which is the length from the upstream end face to the orifice plate; the distance between the downstream pressure taps is also 25.4 mm, which is the length from the downstream end face to the orifice plate. Figure 1: Pressure tapping using a flange. When pressure tapping is done via a flange, the pressure tapping flange is also provided as part of the throttling unit, and it is welded to the pipeline. The flange-based pressure tapping method has a relatively simple structure and is easy to manufacture. However, it should be noted that this is a specialized orifice plate flange, not an ordinary flange; its pressure rating is 300LB (the thickness of the flange is related to its pressure rating, and for flange-based pressure tapping, the distance from the pressure tap to the flange must be 25.4 mm). In situations where the operating pressure is low, this approach ends up increasing costs unnecessarily. The pressure tapping flange is as shown in the figure below: There are two types of corner-based pressure tapping, namely pressure tapping through the annular chamber and pressure tapping through individual brick holes. As shown in Figure 2, the upper part of the figure represents the method of pressure tapping through the annular chamber, while the lower part represents pressure tapping through individual brick holes. Figure 2 shows the actual images of pressure tapping at corners: (a) Pressure tapping at the corners of the ring chamber; (b) Pressure tapping at individual brick holes. The method of tapping at the ring chamber can also help stabilize pressure and improve measurement accuracy, but it requires strict standards for manufacturing and installation. Taking pressure readings from annular chambers with large diameters is costly; such chambers are suitable for diameters below DN400, while pressure readings taken from brick holes are appropriate for diameters above DN400. 3. Pressure tapping at different radii: The distance between the pressure taps is the distance between the centerlines of these taps and a specified end face of the orifice plate. The distance between the upstream pressure taps is equal to the pipe diameter D, while the distance between the downstream pressure taps is D/2; hence this approach is also referred to as pressure tapping at D and D/2, as shown in Figure 3 below. To avoid installation errors on site, when using radial distance for pressure measurement, the throttling device is usually supplied complete with straight pipe sections. Figure 3: Pressure tapping at the diameter. According to clause 6.2.5 of the \"Specifications for the Selection and Design of Automated Instruments\" (HG/T20507-2014), corner-type pressure tapping is recommended for pipes with a diameter of DN50 or less ； For DN50～DN300, flange-type or corner-connected type pressure tapping can be used. In practical applications, orifice plates are commonly used in this range; for low-pressure media, corner-connected pressure tapping can be employed, while for high-pressure media, flange-type pressure tapping is suitable ； For DN300 and above, diametral pressure tapping is recommended.
Saved, saved – I finally understand. It seems that flange pressure taps are used more frequently on-site.
Very useful material; thanks for sharing, I’ve learned something from it
Intuitive and detailed, thanks for sharing. In conditions where the medium contains a high amount of water (such as air), using the radial zone pressure at the opening in a vertical pipe yields excellent results, with the transmitter being positioned above the zone pressure hole.
Do all flanged orifice plates come equipped with pressure tapping valves at the front and back?
Pressure tap pipes for instruments generally require a pressure tapping valve at their base, whether the tapping is done via flanges or elbows, so that in the event of problems with the pressure tap pipe system, the valve can be closed to address the issue without affecting the operation of the process. This pressure tapping valve only shuts off the pressure guide tube of the flow meter, not the process pipeline. I wonder if the super edition is asking about this aspect?
I didn’t express myself clearly; what I mean is, are there pressure tapping points on both the front and back of the orifice plate?
Yes, because orifice flow meters calculate flow rate based on the pressure difference before and after the orifice, there are pressure tapping ports on both sides of the orifice, along with valve seats at those tapping points; a differential pressure transmitter is then used to measure this pressure difference.