Flow Meter Straight Length Requirements Guide

Flow Meter Straight Length Requirements are fundamental rules in flow measurement. These requirements ensure that flow meters, devices that measure the amount of liquid or gas that passes through them, work accurately. The installation of commonly used flow meters will require upstream and downstream straight pipe sections. Such as turbine flowmeter, electromagnetic flowmeter, vortex flowmeter, orifice flowmeter, etc.
Simply put, they ensure the fluid flows smoothly, minimizing disturbances that can skew readings. Adhering to these guidelines is crucial, as it guarantees precise results, allowing industries to operate efficiently and safely.

RFQ

Flow Meter Straight Length Requirements Guide

I’m not sure about you, but for me, my first question is, are there flow meters that are installed without straight pipe requirements?

The answer is yes. What I know so far are Gear flow meters (Oval gear flowmeter and circular gear flowmeter) and Coriolis mass flow meters. These two flowmeters do not require straight pipe sections during installation.

So, if your installation conditions are limited. Then you can consider these two flowmeters that do not require straight pipe sections.

Alternatively, if you know of other types of flow meters that do not require a straight pipe section during installation, please leave a comment.

Different Flow Meter Types and Their Straight Length Needs

Flowmeter type	General requirements
Orifice plate:	The upstream shall not be less than 5 to 80 times the pipe diameter, and the downstream shall not be less than 2 to 8 times the pipe diameter;
Mass flowmeter:	No request;
Nozzle:	The upstream shall not be less than 5 to 80 times the pipe diameter, and the downstream shall not be less than 4 times the pipe diameter;
Venturi tube, elbow tube, wedge tube:	The upstream shall not be less than 5 to 30 times the pipe diameter, and the downstream shall not be less than 4 times the pipe diameter;
Equalizing tube:	The upstream shall not be less than 3 to 25 times the pipe diameter, and the downstream shall not be less than 2 to 4 times the pipe diameter;
Rotameter:	The upstream shall not be less than 0 to 5 times the pipe diameter, and there is no requirement for the downstream;
Target flow meter:	The upstream shall not be less than 5 times the pipe diameter, and the downstream shall not be less than 3 times the pipe diameter;
Turbine flowmeter:	The upstream shall not be less than 5 to 20 times the pipe diameter, and the downstream shall not be less than 3 to 10 times the pipe diameter;
Vortex flowmeter:	The upstream shall not be less than 10 to 40 times the pipe diameter, and the downstream shall not be less than 5 times the pipe diameter;
Electromagnetic Flowmeter:	The upstream shall not be less than 5 to 10 times the pipe diameter, and the downstream shall not be less than 0 to 5 times the pipe diameter;
Ultrasonic flow meter:	The upstream shall not be less than 10 to 50 times the pipe diameter, and the downstream shall not be less than 5 times the pipe diameter;
Positive displacement flow meter:	No request.
These requirements apply to our Sino-Inst brand flowmeters. It does not limit other brands of flowmeters.

More details about: Different Flow Meter Installation Rule of Thumb

Straight Run Requirements for Flow Meters

Certain flow meter technology types require that the flow profile within the piping system near the flow meter be linear , not turbulent , to eliminate flow meter malfunction or inaccuracy. Variables within the process system can cause turbulence.

Straight run requirements are expressed as a distance away from the flow meter, in a multiplication factor of the pipe diameter.
Requirements for the upstream value and the downstream value can be the same or they can be different, .

Read more about: what is upstream and downstream flow?

Why are Flow Meter Straight Length Requirements Necessary?

Straight pipe runs for flow meters aren’t just a recommendation; they’re a requirement for a vital reason. At the core, flow meters aim to measure fluid or gas flow accurately. However, the journey of these substances within pipes isn’t always a straight path. Turbulence, swirls, and chaotic flow patterns can develop, especially when the fluid encounters obstructions like bends, valves, and other instruments.

So, why do these flow disturbances pose a challenge? The crux of the matter is that turbulent flows make it tough for many flow meters to capture reliable readings. In scenarios where the flow becomes disorderly near the flow meters, the readings can be drastically off – sometimes deviating by as much as 50%!

Various elements in a piping system can cause these disturbances. Common culprits include pipe bends, control valves, T-joints, instrumentation installations, and even factors like pressure taps or reducers. All these elements can redirect the flow in unpredictable ways, hindering the meter’s ability to gauge accurately.

Therefore, to safeguard the integrity of flow measurements, ensuring a smooth, undisturbed flow profile through straight pipe runs is paramount. It’s not just about adhering to guidelines; it’s about ensuring that industries get precise data to make informed decisions.

FAQ

A straight run ensures that the flow profile of the fluid or gas is smooth and undisturbed. Disturbances can result from bends, valves, and other obstructions, leading to turbulent flows. A consistent flow profile ensures that the flow meter can take accurate readings, minimizing potential errors.

Straight pipe lengths, both before (upstream) and after (downstream) the flow meter, ensure that any disturbances in the flow profile are stabilized. Upstream straight lengths help settle the flow into a consistent profile, while downstream lengths aid in maintaining that consistency after the fluid or gas passes through the meter, ensuring that backflows or reflections don’t impact the meter’s performance.

Coriolis flow meters measure fluid mass directly and are generally less susceptible to flow disturbances. While they often have reduced straight run requirements compared to other meter types, some straight length is still recommended for optimal performance. However, the exact requirements can vary, so it’s best to consult the manufacturer’s guidelines.

The requirements for the straight pipe section when installing a vortex flowmeter are very important. Its detailed requirements are as follows:

The flow meter should be installed on a horizontal, vertical, or inclined (liquid flow direction is from bottom to top) pipe with the same diameter. A certain length of straight pipe section should be configured upstream and downstream of the sensor. Its length should meet the requirements of the front straight pipe section 15 to 20D and the rear straight pipe section 5 to 1OD.
If there is a reducer pipe upstream of the flow meter installation point. There should be a straight pipe section of equal diameter not less than 15D (D is the pipe diameter) upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream.
If there is a gradually expanding pipe upstream of the installation point of the flow meter. There should be a straight pipe section of equal diameter not less than 25D (D is the pipe diameter) upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream
If there is a 90° elbow or downstream joint upstream of the flow meter installation point. There should be a straight pipe section of equal diameter not less than 20D upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream.
If there are two 90° elbows on the same plane upstream of the flow meter installation point. There should be a straight pipe section of equal diameter not less than 25D upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream.
If there are two 90° elbows on different planes upstream of the flow meter installation point. There should be a straight pipe section of equal diameter not less than 40D upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream.
The flow regulating valve or pressure regulating valve should be installed as far as 5D downstream of the flow meter. If it must be installed upstream of the flow meter, when the valve can meet the full opening requirement, there should be a straight pipe section of equal diameter not less than 25D upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream. When the valve can only meet the semi-open requirement, there should be a straight pipe section of equal diameter not less than 50D upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream.
If there are piston or plunger pumps, piston or roots fans and compressors upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 50D upstream of the flow meter. There should be a straight pipe section of equal diameter not less than 5D downstream.

Special note: If a valve is installed close to the upstream of the installation point of the vortex flowmeter, constantly opening and closing the valve will have a great impact on the service life of the flowmeter, and it is very easy to cause permanent damage to the flowmeter. Try to avoid installing the flow meter on very long overhead pipes. After a long time, the sagging of the flow meter can easily cause the seal of the flow meter to the flange to leak. If installation is unavoidable, pipe fastening devices must be installed 2D upstream and downstream of the flow meter.