The intent of meter station piping design is to create a layout that is affordable to build while maximizing capacity and flow meter performance. Of the various piping elements used to construct these designs, elbows and tees are critical components that are required to allow for fluids to change directions. However, use of elbows and tees in pipe design can add to measurement errors if they are not used properly and not optimally placed. Given the option between an elbow or a tee for your piping design, an elbow should always be the preferred choice.
Elbows and tees typically cause plenty of discussion and controversy when these fittings are used because they present two different solutions to the same problem, each with their own pros and cons. Why do we need elbows and tees? Because fluid flow always needs to change direction at some point, in order to get the fluid from the pipeline to a meter station or facility.
As simple as this seems, there are two different methods of accomplishing this. Both elbows and tees allow a 90 degree change in direction. Elbows do this with a gradual, smooth change. The turning radius of an elbow is typically 1.5x the nominal pipe diameter, which results in lower amounts of swirl and minimal amounts of turbulence (which, in turn, causes less measurement errors).
In contrast, a tee is an immediate hard 90 degree turn. The turning radius of a tee is typically less than the pipe nominal diameter, which creates higher amounts of swirl. Elbows typically produce only 10 – 15 degrees of swirl, while tees can easily double this, creating 20 – 30 degrees of swirl. The swirl can even reach as high as 45 degrees in installations where the upstream piping is particularly bad.
In fluid flow, nothing likes to happen suddenly or abruptly. Due to the sharp turning radius of the tee, the fluid cannot physically make the turn as quickly. Above certain velocities, the flow separations from the inner surface of the tee, can cause the tee to ‘stall’ like an aircraft wing. As a result, Tees create very large amounts of turbulence. This produces vortex shedding and detrimental harmonics within the meter tube. This can result in significant pulsation in the meter tube, which can cause significant measurement errors or outright meter malfunctions.
Building a meter station without first studying and gaining familiarity with the fluid behaviour within the piping design can be costly, due to the measurement inaccuracies and the resulting expenses required to diagnose and fix any significant issues. Before implementing any meter station design or modification, and particularly when you are choosing placement of an elbow or a tee in your design, it can be very helpful to study the fluid performance beforehand using CFD simulations. Simulations allow the piping and components to be built virtually and studied in advance, thus allowing problematic causes of flow distortion to be caught and fixed before construction begins.
Although tees are often used in meter station design, and are liked due to (1) their compactness, (2) their ease of access for inspecting the meter tube, and (3) their ability to isolate valve noise, it must be acknowledged that they have significant issues that must be accounted for in the piping design. Tees create large varying behaviours in the flow that are not ideal for optimal fluid flow and measurement. Elbows, in comparison, have more consistent behaviours and will have less impact on potential measurement errors. Therefore, elbows should always be the preferred choice for making turns with your piping design. When making your choice, consider studying your design with CFD simulations to verify the fluid flow behaviour in advance.
Canada Pipeline Accessories (CPA) can assist customers and users with designing the most effective measurement stations and help pinpoint flow disturbances to eliminate measurement errors. CPA’s engineering team combines extensive experience in engineering, fluid dynamics, flow measurement, CFD, and finite element analysis with cutting-edge software to provide innovative solutions to any flow-related problem. We invite you to check out our website for more information regarding flow measurement. Or call us today to speak to one of our experts: (403) 236-4480
