What are Straight Runs for Flow Meters?

Why do straight piping requirements for flow meters even matter? The answer to this question is surprisingly simple. For many flow technologies, turbulent and swirling media within a pipe is hard to measure in a reliable and accurate manner. There are many obstructions and details within a piping system that case cause a flow profile to become turbulent. If this occurs relatively close to flow meters, it can lead to significant errors as high as 50% of the reading.

There are many types of variables and configurations that can lead to an undesirable flow profile. Some common examples are bends in the pipes from elbow joints, control valves, pressure taps, T-joints, other instrumentation, thermowells, reducers, and more. Anything that can cause the flow to move in a non-linear way needs to be properly accommodated for to ensure the accuracy of your flow meter readings.

Different flow meter technologies have different straight run requirements, based on the inherent principle of operation being used. The requirements are usually expressed in terms of the distance away from the flow meter in a multiplication factor of the pipe diameter. They are also expressed as being either upstream (from the instrument) or downstream (from the instrument). It is important to consider straight run requirements ahead of time to ensure that the flow meter you select will fit in the space that the current system already has to offer.

While there are some general ballpark rules of thumb that can be used in reference to the straight run requirement for the flow meter technology type, each application may have its own special unique considerations that will need to be accounted for when choosing a flow meter.

Always consult the manual that came with your flow meter for exact straight run requirements as the following information below is only a general reference. If you are unsure, you can always contact our expert engineers for fast and free flow meter selection help.

• Usually, 5x upstream and 3x downstream straight run requirement
• Minimal requirements compared to many other technology types

• Difficult to give a general range for DP flow meters
• Helpful to use impulse lines on either side
• Actual requirement depends on the type of upstream elements and the beta ratio
• Beta ratio is defined as the diameter of the pipe orifice divided by the pipe diameter

• Require a stable and symmetrical flow profile
• Requirement depends on the obstruction
• Usually, 30x downstream of control valves
• Usually, pressure taps must be 3x-4x downstream
• Any temperature elements must be 5x-6x downstream

• Generally, do not have any straight run requirements
• Operating principle is insensitive to disruptions in the flow profile

• Install upstream of any flow obstacles
• Provide as much space between obstructions and the flow meter
• Can go as close as 5x, but it results in an additional 1% inaccuracy in the reading
• Example: KOBOLD DUK requires 10x upstream and downstream straight run requirement

• Usually, 5x upstream and downstream straight run requirement recommended

• Due to the nature of the measuring probe protruding into the media and creating its own obstacle of sorts, the straight run requirements for insertion flow meters is high
• Usually, 20x to 50X upstream straight run requirement recommended

• No straight run requirements necessary

• Usually 10x-15x upstream and 5x downstream straight run requirements
• Certain obstructions can increase the requirements
• 20x for 90-degree elbow or T, etc.
• 25x for partially open valves
• 50x for 2 elbows or any spiraling flow

• Usually, 5x upstream and downstream straight run recommended

• Depends on the flow meter
• Can be anywhere from no requirement to 20x

• No straight run requirements