Overview of Straight Pipe Section Requirements for Flowmeter Installation
Correctly selecting the installation location and correctly installing the flowmeter are key steps to ensure measurement accuracy and extend service life. If installed improperly, it may at best affect measurement accuracy and at worst damage the flowmeter. Different flowmeters have significantly different requirements for upstream and downstream straight pipe section lengths, and relevant specifications must be strictly followed.
Requirements for Straight Pipe Sections in GB50093-2013
According to the Code for Construction and Acceptance of Automation Instrumentation Engineering GB50093-2013, the minimum upstream and downstream straight pipe section length requirements for common flowmeters are shown in the table below. In the table, D represents the inner diameter of the pipe, and the specific value should be determined according to site conditions.
| Flowmeter Type | Upstream Straight Pipe Section Requirement | Downstream Straight Pipe Section Requirement |
|---|---|---|
| Coriolis mass flowmeter | No requirement | No requirement |
| Rotameter | Not less than 0-5D | No requirement |
| Target flowmeter | Not less than 5D | Not less than 3D |
| Turbine flowmeter | Not less than 5-20D | Not less than 3-10D |
| Vortex flowmeter | Not less than 10-40D | Not less than 5D |
| Electromagnetic flowmeter | Not less than 5-10D | Not less than 0-5D |
| Ultrasonic flowmeter | Not less than 10-50D | Not less than 5D |
| Positive displacement flowmeter | No requirement | No requirement |
| Orifice plate | Not less than 5-80D | Not less than 2-8D |
| Nozzle | Not less than 5-80D | Not less than 4D |
| Venturi tube, elbow tube, wedge tube | Not less than 5-30D | Not less than 4D |
| Averaging pitot tube | Not less than 3-25D | Not less than 2-4D |
Special Condition Requirements at the Installation Point
When specific pipe fittings exist upstream of the flowmeter installation point, the straight pipe section length must be increased accordingly to ensure fully developed flow. The specific requirements are as follows:
- If there is a 90° elbow or downward connection upstream: upstream not less than 20D, downstream not less than 5D.
- If there are 90° elbows in the same plane upstream: upstream not less than 25D, downstream not less than 5D.
- If there is a reducer upstream: upstream not less than 15D, downstream not less than 5D.
- If there is an expander upstream: upstream not less than 18D, downstream not less than 5D.
Installation Precautions and Special Requirements
In addition to straight pipe section length, the following key points should be noted during installation to avoid measurement errors and equipment damage.
General Precautions
- Vortex flowmeter: When a valve is installed relatively close upstream, frequent opening and closing of the valve will seriously affect service life and may even cause permanent damage.
- Avoid installing the flowmeter on an overhead and relatively long pipeline to prevent sagging that may cause flange seal leakage. If installation is unavoidable, pipe fastening devices should be installed at 2D upstream and downstream.
Installation Conditions for Coriolis Mass Flowmeters
Coriolis mass flowmeters have no special requirements for upstream and downstream straight pipe sections, but the following conditions must be met:
- For liquid media, the flowmeter should be located at the low point of the pipeline to avoid vaporization caused by excessively low back pressure; for gas media, it must not be located at a local low point to prevent liquid accumulation.
- The pipeline must be completely filled with liquid medium, and gas-liquid or liquid-solid two-phase flow must not exist. In vertical installation, the fluid should flow from bottom to top; if top-to-bottom flow is unavoidable, a restriction orifice can be installed after the flowmeter.
- The flowmeter and connecting flanges must be fully aligned to avoid external stress affecting measurement.
- Avoid strong electromagnetic field interference; there must be no interference sources such as large motors nearby.
- When adjacent Coriolis mass flowmeters of the same model are installed, their vibration frequencies should be staggered, and the spacing should be at least 4 times the instrument length.
- Pay attention to staggering the inherent vibration frequency of the flowmeter phase measurement from the inherent vibration frequency of the pipeline to prevent measurement fluctuations.
- Shut-off valves should be installed before and after the flowmeter to facilitate zero-point calibration.
Rotameters and Target Flowmeters
- Rotameter: Must be installed on a vibration-free vertical pipeline with medium flowing from bottom to top, and the upstream straight pipe section must be not less than 5D and not less than 300 mm.
- Target flowmeter: When measuring media containing suspended solids, horizontal installation is required; for vertical installation, the liquid flow direction should preferably be from bottom to top; the upstream straight pipe section must be not less than 5D, and the downstream not less than 3D.
Summary
The straight pipe section requirements for flowmeter installation are the basis for ensuring measurement accuracy. Engineering personnel should strictly refer to GB50093-2013, combine pipeline conditions and medium characteristics, reasonably determine the straight pipe section length, and pay attention to special installation requirements to ensure long-term stable operation of the flowmeter.