Accurate flow measurement is crucial for process control. Before purchasing a flow meter, you should consider the installation requirements. The meter’s installation location and the length of upstream and downstream straight pipe runs directly affect the accuracy of the measurement results.
Are Straight Pipe Runs Necessary for Flow Meters?
In fact, the answer is not necessarily true. First, departmental mass flow meters and positive displacement gear flow meters do not require straight pipe runs. Other types of flow meters, such as electromagnetic flow meters and vortex flow meters, do require straight pipe runs for installation.
What is Straight Run for Flow Meters?
Straight Run for Flow Meters refer to the two rigid straight pipe sections upstream and downstream of the flow meter when it is installed.
So, Why are Straight Pipe Runs for Flow Meters Necessary?
Sufficient straight pipe length is crucial for ensuring flowmeter measurement accuracy. The ideal installation location for a flowmeter is one where the flow conditions upstream and downstream are stable, single-phase. This ensures that the effects of local resistance pipes upstream and downstream of the flowmeter on the flow state do not directly affect the flowmeter’s measurement accuracy.
If the fluid flows through obstructions (elbows, tees, valves, etc.), the velocity distribution will be distorted and vortices will be generated. This creates a non-fully developed flow, where the velocity at any section of the pipe is different and constantly changing.
Only at the end of a very long straight pipe section or after the installation of a rectifier can the velocity distribution be restored to that of a fully developed flow. This is the fundamental reason why many flowmeters require a sufficiently long straight pipe section.
Straight Run Requirements for the Different Flow Meter Types
| Types | Straight Pipe Requirements |
| Mass flowmeter | No requirements; |
| Rotameter | Upstream: not less than 5 pipe diameters, downstream: no requirements; |
| Target flowmeter | Upstream: not less than 5 pipe diameters, downstream: not less than 3 pipe diameters; |
| Turbine flowmeter | Upstream: not less than 5-20 pipe diameters, downstream: not less than 5 pipe diameters; |
| Vortex flowmeter | Upstream: not less than 10-40 pipe diameters, downstream: not less than 5 pipe diameters; |
| Electromagnetic flowmeter | Upstream: not less than 5-10 pipe diameters, downstream: not less than 3-5 pipe diameters; |
| Ultrasonic flowmeter | Upstream: not less than 10-50 pipe diameters, downstream: not less than 5 pipe diameters; |
| Positive displacement flowmeter | No requirements; |
| Orifice flowmeter | Upstream: not less than 5-80 pipe diameters, downstream: not less than 2-8 pipe diameters; |
| Nozzle | Upstream: not less than 5-80 pipe diameters, downstream: not less than 4 pipe diameters; |
| Venturi tube, elbow, wedge tube | Upstream: not less than 5-30 pipe diameters, downstream: not less than 4 pipe diameters; |
| Averaging tube | Upstream: not less than 3-25 pipe diameters, downstream: not less than 2-4 pipe diameters. |
| Thermal mass flowmeter | The upstream shall not be less than 10 times the pipe diameter, and the downstream shall not be less than 5 times the pipe diameter. |
Straight Run Requirements for Special Installation Points:
- If there is a 90° elbow or downstream connector upstream of the flowmeter installation point, a straight run of equal diameter pipe should be provided at least 20D upstream and at least 5D downstream.
- If there is a 90° elbow in the same plane upstream of the flowmeter installation point, a straight run of equal diameter pipe should be provided at least 25D upstream and at least 5D downstream.
- If there is a convergent pipe upstream of the flowmeter installation point, a straight run of equal diameter pipe should be provided at least 15D upstream and at least 5D downstream.
- If there is a divergent pipe upstream of the flowmeter installation point, a straight run of equal diameter pipe should be provided at least 18D upstream and at least 5D downstream.
Magnetic Flow Meters
Electromagnetic flow meters, also called mag meters or magnetic meters. Mag meter is ideal for wastewater applications, or any dirty liquid which is conductive or water based. Accuracy over a wide flow range can be as good as ± 0.5% of flow rate or better.
Turbine Flow Meters
Turbine flow meter is the most popular equipment to measure inline flow electronically. Turbine flow meters measure the velocity of liquids, gases and vapors in pipes. Such as hydrocarbons, chemicals, water, cryogenic liquids, air, and industrial gases.
Vortex Flow Meters
Vortex flow meters are suitable for measuring steam as well as a variety of liquids and gases. Pressure and Temperature compensation ensure measurement accuracy. When we use them to measure steam or gas.
Ultrasonic Flow Meters
Ultrasonic flowmeters measure the fluid flow rate in a pipe. With the sound waves transit time principle to determine the flow rate. Ultrasonic flow meters, with easy installation, could be clamp on, handheld, and portable.
Mass Flow Meters
Mass Flow meters for all industrial flow measurement. Coriolis mass flow meters and Thermal mass flow meters. Coriolis Mass flowmeters are ideal for liquid flow measuring applications. And Thermal Mass flowmeters are for compressed gas applications.
Oval Gear Flow Meters
Industrial Oval Gear Flow Meters are positive displacement type volume meters. Sino-Inst offers High temperature and high pressure types oval gear flow meters for special use. Oval Gear Flow Meters are suitable for chemical fluids, like: fuel oil, water…
Differential Pressure Flow Meters
Differential Pressure Flow Meters, also called DP flow meters. Differential pressure flow meters consist of flow sensors and pressure/differential transmitters. Flow sensors are the important part of differential pressure flow meters. Like: Orifice plate, Venturi tube, Wedge, V-cone and Averaging Pitot Tubes.
Reference Standards
You can follow the manufacturer’s instructions to determine the required straight pipe length for your flowmeter installation. Alternatively, you can consult industry standards such as ISA, ISO, API, AGA, ASME, and ANSI. These guidelines and standards have been tested to ensure optimal flowmeter performance.
- Coriolis – ISO 10790
- Critical Nozzle – ISO 9300
- Differential Pressure – ISO 5167
- Electromagnetic – ISO 6817
- Positive Displacement – ISO 2714
- Thermal Mass – ISO 14511
- Turbine – ISO 2715
- Ultrasonic – ISO 17089
- Variable Area – ISO 11605
- Vortex – ISO/TR 12764
- ASME MFC-3M-2004
- ASME MFC-6-2013
- ASME MFC-11-2006
- ASME MFC-5.1-2011
- ASME MFC-16-2014
How to Fix Insufficient Straight Pipe Length for a Flowmeter
When installing a flowmeter, to achieve the same accuracy as actual flow verification, specified flow conditions must be met. Currently, with the exception of a few types, such as positive displacement and Coriolis mass flowmeters, all have specific requirements for the length of straight pipe sections before and after the flowmeter.
However, in actual projects, site constraints may not guarantee an ideal installation location that guarantees the required straight pipe length. Furthermore, in some renovation projects, installing a flowmeter in existing pipelines can also lead to issues with insufficient length. In these cases, engineers often resort to the following methods to resolve the issue.
More Flow Measurement Solutions
Properly selecting the installation site and installing the flowmeter are both crucial. Maintaining high flowmeter flowmetering requirements in both upstream and downstream pipe sections ensures accurate flow measurement and extends the life of the flowmeter.
Sino-Inst has extensive experience in flow measurement and can provide customized flowmeter selection and installation solutions. If you need to purchase a flowmeter or have any technical questions, please feel free to contact us.