【Weekly Question 20090601】Discussion on the straight pipe sections before and after the flow meter
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This post was last edited by wwxxss_1 on 2009-7-26 at 11:05. According to the specifications, strict requirements exist regarding the straight pipe sections before and after the flow meter, as well as regarding the inclination of the pressure tapping pipes used with the throttling device. Have you encountered situations during installation or maintenance where the straight pipe sections before and after the flow meter did not meet these specification requirements? Could it be that the insufficient straight pipe section affects the flow measurement?
The straight pipe section, which serves as a throttling element in production processes, does not require high precision, but when used as a measuring instrument for energy consumption assessment and in international trade, high accuracy is required. I think it was 1.0.
Instruments that have requirements regarding straight pipe sections include electromagnetic flowmeters, vortex flowmeters, Anubas meters, and orifice plates; the typical requirement is 5 units in front of and 3 units behind the straight pipe section. Most electromagnetic flowmeters today use non-uniform electromagnetic fields, which require less axial symmetry in the fluid velocity distribution; as a result, the size of electromagnetic flowmeters can be reduced accordingly ； On the other hand, vortex flow meters, Coriolis meters, and orifice plates have relatively strict requirements regarding the fluid velocity distribution; failure to meet the requirements for straight pipe sections may result in an inability to carry out measurements.
Our unit strictly adheres to the standards, ensuring that there are straight sections before and after; to date, there has been no instance where measurements were affected due to defective straight sections.
The wedge flow meter used in our facility sees its accuracy decrease when the requirements for straight pipe sections are not met.
We have also encountered situations where there wasn’t enough straight pipe section for the orifice plates used for measurement. After some consideration, we decided to take pressure readings from the middle of the pipe; this is because the flow velocity of the fluid at the top and bottom of the pipe at the bends is certainly not uniform, while the velocity in the middle is relatively uniform. By taking pressure readings from the middle, there is indeed some error, but it remains within acceptable limits.
I have used electromagnetic flowmeters; we need to adhere strictly to the requirements. I think that since there are such requirements, there must be some impact~
The boiler units we manufacture have a compact design, and it often happens that the requirements for straight pipe sections are not met.
This post was last edited by xu*ang309 on 2009-6-8 09:12. The straight pipe section requirements for velocity-type flow meters are quite strict, usually ten times the diameter in front and five times the diameter behind. However, if the on-site conditions are not sufficient, it is possible to consider installing flow straighteners in the pipeline where the sensors are placed.
This common installation problem can be seen everywhere. Although the specifications provide clear guidelines, this situation often arises due to laziness or a \"spirit of thrift\" on the part of the workers during installation, or as a result of \"negligence\" in renovation projects. In one set of installations, where only ROSEMOUNT vortex flowmeters were installed, I found that eight sets did not meet the installation requirements of 15D ahead and 5D behind the flowmeter due to insufficient straight pipe sections. Since the stability of vortex separation is affected by flow velocity, it is necessary to have adequate straight pipe sections; otherwise, accurate measurements cannot be achieved. In one sentence: \"Details determine success or failure.\"
A rectifying device can be added upstream, which can reduce the length of the straight sections before and after it.