A Smooth Profile: Straight Piping Requirements for Flow Measurement
What are the straight run piping requirements for the various types of flow technologies?
Why do straight piping runs even matter?
The answer to this question is surprisingly simple. 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 can cause a flow profile to become turbulent in the pipe. If this occurs within a relative distance to instrumentation, it can lead to significant errors, up to 50% of the reading.
There are many types of obstructions 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, tees, other instrumentation, thermowells, reducers, and more. Anything that could cause the flow to act in an erratic or nonlinear way can be considered an obstruction. These need to be accounted for in creating an ideal flow profile to ensure the accuracy of your meter readings.
Different flow meter technologies have different straight run requirements, based on the inherent character of how each technology works. The requirements are usually expressed in terms of distance away from the instrument in a multiplication factor of the pipe diameter. They are also expressed as either being upstream (from the instrument) or downstream (from the instrument). It is important to consider straight run requirements to ensure that the meter you select will fit in the space and system you already have in place or to help design the process layout.
What are the requirements for each flow technology type?
While there are some general ballpark rules of thumb that can be used for each flow technology, each application may have its own special unique considerations that will need to be accounted for when choosing a suitable flow technology. If you are unsure, our expert engineers are here to help you through those considerations when choosing a flow meter technology for your application.
- Usually 5X upstream and 3X downstream
- Minimal requirements in comparison to many other technology types
- Usually 20X to 50X upstream
- Requirements are higher partially because the technology itself creates an obstruction
- Install upstream of any flow obstacles
- Provide as much space between obstructions and flow meter
- Can go as close as 5X, but it results in an extra 1% inaccuracy
- Example: KOBOLD DUK requires 10X in and out
- Require a stable and symmetrical flow profile
- Requirement depends on the obstruction
- Usually 30X downstream of control valves
- Pressure taps must be 3-4X downstream
- Temperature elements must be 5-6X downstream
Thermal Mass Meters
- Usually 5X in and out recommended
- Do not generally require any straight runs
- Technology is insensitive to disruptions in flow profile
Variable Area Meters
- No straight line requirements
- Usually 10-15X upstream and 5X downstream
- Certain obstructions change the general value
- 20x for 90 degree elbow, tee, etc.
- 25X for partially open valve
- 50X for 2 elbows or any spiraling flow
Paddle Type Meters
- Usually 5X in and out recommended
Paddle Wheel Meters
- Depends upon the meter selected
- Can be anywhere from none to 20X
Positive Displacement Meters
- No straight runs required
Differential Pressure Meters
- Difficult to give a general range
- Helpful to use impulse lines on either side
- Actual value 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