Design Quality Improvement – Bulletin 03 4/29/19 Prepared by – Todd Sisti, PE
Orifice Plate Flow Meter Piping Requirements
Quality Concern:
Improper installation of Orifice Plate Flow Meters will cause errors in flow readings.
This document is written to address Orifice Plate type flow meter piping. There are numerous types of flow meters that all have different requirements for piping installation. The designer should always consult the Instrument Engineer or Manufactures literature on final installation requirements for any Flow Meter. With that being said; there are still some practical design applications within this document that could apply to many flow meter types.
Discussion:
With Orifice Plate Flow Meters, the objective is to deliver laminar flow to the orifice plate for minimal error in metering. Hence, the upstream piping fitting(s) or valve will have influence on the fluid behavior and the required length of straight pipe prior to the meter to deliver laminar flow. Even the relative roughness of the straight pipe can cause turbulence in the fluid flow and some manufacturers will supply the meter and the straight pipe to manage this problem and validate performance of their meters.
Up Steam and Down Stream Distances:
When a designer is planning a piping layout with an Orifice Plate flow meter, the traditional rule of thumb is between 10 and 15 pipe diameters of straight pipe upstream of the meter and 5 pipe diameters downstream of the meter. The upstream pipe diameter requirement can vary up to 50 pipe diameters or more depending the type of fitting(s) or valves leading into the straight run of pipe. The upstream pipe diameter is also affected by the Ratio of Orifice Diameter to Pipe Diameter. Generally, the smaller the ratio, the smaller the smaller the requirement for upstream pipe diameters. This ratio is usually in the range of .125 to .75.
The following Table showing Upstream and Downstream diameter-lengths is reproduced from the “Piping Guide” written by David Sherwood and Dennis Whistance and is a good reference for upstream and downstream requirements considering upstream fittings.
Flow Direction for Orifice Plate Piping:
Orifice Plates must be installed with the flow direction in the horizontal or vertical-up directions. In the horizontal position is best when the elbow after the orifice plate turns up such that the piping run is always kept full when flowing liquids. For Steam or gases this may not be as critical. With the vertical- up flow direction the line is always kept full and the line can completely drain when shut down. Flow up may be the better direction, but horizontal flow appears to be more common.
Orifice Plates should never be installed in a down flow application. Liquid separation is possible yielding poor readings. As always, there are cases when a down flow can be made to work by designing the line to maintain a flooded condition, but generally this is a last resort and not good design practice.
Orifice Plates are installed between special flanges referred to as Orifice Flanges (Weld Neck Style Flange with a tapped hole) and are Class 300 or greater flanges. These flanges have tapped holes through the flange body to the pipe center and are on either side of the orifice plate for differential pressure measurement. Class 150 flanges are not thick enough for the tapped hole. As such Class 300 flanges are used in Class 150 and Class 300 systems.
Access to Flow Meters:
Because of the long straight run requirement before and after the meter, meter runs can sometimes take up a lot of space. Hence, they are best placed along wall lines where they will not affect foot traffic in the plant. Orifice type meters typically don’t require routine maintenance or daily inspection of operation. As such they may be installed in a pipe rack or along walls or ceiling lines to accommodate the straight runs. When installing at elevated positions, it is recommended that they be installed on the outside edge of a pipe rack or along wall or ceiling lines where a ladder or lift can access them for periodic maintenance. With smaller diameter piping, the straight run requirement is not as incumbering to the plant and may be installing anywhere in the plant.
Positioning of Orifice flange taps:
When installing an Orifice plate in a vertical up-flow direction, the orifice flange taps can be positioned in any direction for gas, liquid or steam.
When installing the Orifice plate in a horizontal piping run, the orifice flange taps must be positioned as follows:
Gas – Taps up or at a 45 degree up from horizontal
Liquid – Taps horizontal to the side of the pipe, 45 degree down from horizontal, or straight down
Steam – Taps horizontal to the side of the pipe