Does the flow meter require a bypass and inlet/outlet isolation valves?

The method of using upstream and downstream valves along with a bypass for flow meters allows, in the event of a malfunction that requires maintenance, the upstream and downstream valves to be closed and the bypass to be activated, without disrupting the operation of the system. However, if the operating system uses flow display for interlocked control, there is no need to install inlet and outlet valves as well as a bypass valve for the flow meter. Since the flow meter is faulty, even though interlock control can be operated by providing an assumed flow value via the DCS, the actual flow rate resulting from the size of the bypass opening remains unknown. Therefore, in this case, there is no need for a manual valve or a bypass; if the flow meter breaks down, the system is stopped to replace or repair it. If it is necessary to install hand valves, it is a common design approach to provide an additional flow meter on the bypass line as well as hand valves at both the front and back. So, your question depends on your specific operating conditions; it cannot be answered in a general manner.

We all have cut-off valves before and after the flow meter, as well as purge and bypass valves. Each type of flowmeter requires different straight pipe sections, with 5 in front and 2 behind.

With a bypass in place, isn’t there a risk of covert metering? Especially those related to public utilities

Ensuring production is of top priority; many instruments cannot be repaired or replaced when they break down due to the lack of bypass circuits, forcing operators to rely on experience in carrying out operations

There are no mandatory standards regarding the valves before and after a flow meter; it all depends on the process requirements. For example, if there is a high failure rate or a need for continuous operation, then valves before and after the flow meter as well as bypass valves are necessary. But if it is connected to important interlocks, such valves cannot be used. Different flow meters require different straight pipe sections; configurations such as five before and two after, or three before and four after have all been used

Flow meters are generally not equipped with bypasses, as the opening of a bypass valve prevents the flow meter from accurately indicating the flow rate of the material fed. The installation of valves before and after the flow meter is also not necessary; if the flow meter is damaged, it is better to replace the entire pipeline rather than just the flow meter itself

Flow meters generally do not have bypasses, but a manual valve can be added for maintenance purposes!

Mass flow meters do not require straight sections before and after them, while differential pressure flow meters and vortex flow meters do need such straight sections

According to the SH4104‑2013 Code for Installation and Design of Petrochemical Instruments, whether a bypass is used is determined by the process requirements. Bypasses should not be installed on flow meters used in trade. In fact, the 2000 version of this standard specifies that certain flowmeters must be equipped with bypasses, such as gear or paddle wheel flowmeters. However, as times have progressed, the reliability of flowmeters has increased, and the standards have been updated accordingly; the requirement to include a bypass has been removed, with the decision now left to the process design. The process will primarily consider whether to include a bypass based on control requirements and operational/maintenance needs. Based on general experience, in process applications, with few exceptions such as orifice plates, most flow meters come equipped with a bypass, whether it is necessary or not.