The issue of pipeline diameter at the control valves at both ends of the flow meter
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A metal rotameter is installed on the pipeline; the pipe diameter is DN50, while the flange diameter of the meter is DN25. Question: Should valves with a diameter of DN50, matching that of the pipeline, be used for controlling both ends of the meter, or should valves with a diameter of DN25, the same as that of the meter, be used?
The valves match the dimensions of the main pipes.
Valves with the same diameter as the pipe should be used to minimize the flow-limiting effect caused by the valves
Use valves with a pipe diameter of DN50
The valves must have a large diameter; the flow meter is only used to ensure full pipe flow and to control costs
The valve should have the same diameter as the flow meter
In fact, by referring to the SH/T3104 Standard for the Installation and Design of Petrochemical Instruments, it becomes clear that rotameters do not require straight pipe sections before and after them; therefore, the diameter can be changed directly before or after the flow meter. To reduce the pressure drop in the pipeline, the upstream and downstream shut-off valves (note: not control valves) have the same diameter as the main pipe. Bypass valve, which can be selected to have the same diameter as the flow meter.
Appendix A of SH/T 3005-2016 specifies the straight pipe section requirements for various instruments; for rotameters, a straight pipe section of 5 D in front and 2 D behind is required
Standards are updated too quickly; the original SH3005 and 3104 models had no such requirements. It’s true that one must keep learning throughout life! Thanks for the reminder from above. However, if that’s the case, the diameter change must occur before the straight pipe section; it is recommended to use the same type of valve as before the diameter change, in order to reduce resistance.
It depends on the type of valve; if its KV value meets the requirements, there’s no problem at all, and small cost savings can be considered. The requirements for 3D piping also need to be taken into account.