Instrumentation: [Weekly Question] Week 25 of 2011: Issues related to the installation of flow orifice plates

This post was last edited by denghl on 2012-12-21 at 21:16. When throttling devices such as orifice plates, as well as flow measurement instruments and control valves, are installed on the same pipeline, it is generally required that they be placed upstream of the control valve. This is because most flow measurement instruments require a certain length of straight pipe section; the longer this section, the higher the measurement accuracy. The flow state of the fluid changes after passing through the control valve, which affects the measurements taken by the flow meter. Orifice plates belong to differential pressure flow meters; the pressure drop is very small, so they have little impact on the control valve downstream. The pressure drop of control valves is much greater than that of orifice plates (typically accounting for 30% of the pipeline system’s pressure loss), and this leads to significant pressure fluctuations downstream during regulation, affecting the accuracy of the orifice plate. Of course, there are also issues with straight pipe sections. The gas pipeline issue is the biggest, especially. For orifice plates used for flow restriction, the orifice diameter is calculated using a formula; thereafter, based on this value along with the k value, the critical flow rate pressure ratio (γc) is determined by referring to the “γc-k-d0/D relationship table” in this appendix. When the pressure ratio P2/P1 before and after each orifice plate is ≤ γc, the liquid flow rate can be restricted to a certain value. See the calculations for orifice plates used for flow restriction. Throttle orifice plates are always installed upstream of the control valve. Otherwise, it will not serve as a current limiter.

This post was last edited by denghl on 2012-12-21 at 21:17. In the theoretical derivation for measuring flow rate using throttling devices, there are two assumptions: namely, that the fluid flow is continuous, and that the law of conservation of energy applies during the fluid flow process. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, in order to avoid increasing measurement errors. Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable.

This post was last edited by denghl on 2012-12-21 at 21:18, as fluid flow is continuous and the law of conservation of energy applies during fluid flow. Therefore, orifice plates cannot be installed in areas where vapor and liquid coexist, to avoid increasing measurement errors. After the pressure of some liquids is reduced by a control valve, they sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve

The flow velocity before the control valve should be stable compared to that after it; therefore, the requirements for straight pipe sections are not as strict as those after the valve. That’s probably how it is!

This post was last edited by denghl on 2012-12-21 at 21:20. Reply 1# The fluid fluctuations behind the valve in Baghdad are significant, which affects flow stability. Control valves are used to regulate flow rate and require feedforward control; they fail to meet the control requirements after being installed

This post was last edited by denghl on 2012-12-21 21:21. Why is it reasonable to install the orifice plate in front of the control valve? Answer: For safety reasons, it is advisable to consider the scenario where there is a leak in the pipeline directly connecting the control valve and the orifice plate; this will certainly result in a decrease in flow rate. However, the control valve will open wider, which could lead to an further escalation of the accident.

Flow orifice plates are measured using the pressure difference before and after. Installing the control valve at the front affects measurement accuracy.

This post was last edited by denghl on 2012-12-21 at 21:23. Reply to 1# Baghdad: In the theoretical derivation for measuring flow rate using throttling devices, there are two assumptions: namely, that the fluid flow is continuous, and that the law of conservation of energy applies during the fluid flow process. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, in order to avoid increasing measurement errors. After passing through the control valve and having its pressure reduced, some liquids may partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable.

This post was last edited by denghl on 2012-12-21 at 21:26. Reply to 1# Baghdad: In the theoretical derivation for measuring flow rate using throttling devices, there are two assumptions: namely, that the fluid flow is continuous, and that the law of conservation of energy applies during the fluid flow process. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, in order to avoid increasing measurement errors. Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases. Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable.

This post was last edited by denghl on 2012-12-21 at 21:28. ①In the theoretical derivation for measuring flow rate using a throttling device, there are two assumptions: namely, that the fluid flow is continuous, and that the law of conservation of energy applies during the fluid flow process. This tells us that orifice plates cannot be installed in areas where gas and liquid coexist, to avoid increasing measurement errors ； ②Some liquids (such as liquefied hydrocarbons), after having their pressure reduced by a control valve, sometimes partially vaporize, resulting in a coexistence of two phases ； ③Therefore, the flow orifice plate should be installed as close as possible in front of the control valve; this installation method is reasonable. When throttling devices such as orifice plates, flow measurement instruments, and control valves are installed on the same pipeline, it is generally required that they be placed upstream of the control valve, as most flow measurement instruments need a certain length of straight pipe section; the longer this straight pipe section, the higher the measurement accuracy. The flow state of the fluid changes after passing through the control valve, which affects the measurements taken by the flow meter. Orifice plates belong to differential pressure flow meters; the pressure drop is very small, so they have little impact on the control valve downstream. The pressure drop of control valves is much greater than that of orifice plates (typically accounting for 30% of the pipeline system’s pressure loss), and this leads to significant pressure fluctuations downstream during regulation, affecting the accuracy of the orifice plate. Of course, there are also issues with straight pipe sections. The gas pipeline issue is the biggest, especially. For orifice plates used for flow restriction, the orifice diameter is calculated using a formula; thereafter, based on this value along with the k value, the critical flow rate pressure ratio (γc) is determined by referring to the “γc-k-d0/D relationship table” in this appendix. When the pressure ratio P2/P1 before and after each orifice plate is ≤ γc, the liquid flow rate can be restricted to a certain value. See the calculations for orifice plates used for flow restriction. Throttle orifice plates are always installed upstream of the control valve. Otherwise, it will not serve as a current limiter.