Basic Knowledge for Instrument Technicians (18) — Plug Flow 4 — Do You Know About Sonic Nozzles?

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This post was last edited by feng*aosa on 2015-8-2 at 15:38. Previous section: Basic Knowledge for Instrumentation Technicians (17) – Plug Flow 3 – Regarding Control Valves 2. Basic Knowledge for Instrumentation Technicians (18) – Plug Flow 4 – Do you know what a sonic nozzle is? 　　The causes of choked flow are explained in \"Basic Knowledge for Instrumentation Technicians (15) – Choked Flow 1\". One theory is that when the flow velocity of the fluid approaches the speed at which pressure is transmitted within that fluid, the pressure in the middle increases because it cannot be transmitted to the rear in time. This increased pressure prevents the fluid ahead from flowing backward, thus creating a blocked flow. 　　The speed of sound is the speed at which slight pressure disturbances propagate through a medium, and its value varies depending on the properties and state of the medium. The calculation formula is: C = (K/ρ)^(1/2), where C represents the speed of sound ； ρ is the density of the medium ； K is the volume elastic modulus; for liquids and solids, it remains close to a constant in the same medium. At this time, the speed of sound changes with the density of the medium. 　　If a throttle is used to cause the flow velocity of the liquid medium passing through it to reach a choked flow state (with a flow velocity close to the speed of sound), then the flow velocity of the liquid through the throttle will be directly related to its density, and no longer related to parameters such as temperature and pressure. At this point, the mass flow rate can be obtained by measuring the flow velocity through the differential pressure. 　　(For gases, the volumetric elastic modulus K is affected by temperature and pressure, and the situation is relatively complex; I don’t quite understand it either, so it’s not discussed here. Those who are interested can look up information on their own.) ) With this measurement method, due to the high pressure upstream, using an orifice plate as a throttling element is not sufficient; therefore, nozzles or venturi tubes are typically used as throttling elements. 　　Its advantage is that it can be used as a simple mass flow meter ； The problem is that a higher pressure source is required, which results in significant pressure losses in the pipes; moreover, the measurement range is narrow, requiring little variation in flow rate. 　　A flow meter with such obvious advantages and disadvantages. Where is the best place to use it? 　　In natural gas wells and oil wells where oil flows naturally, the liquid hydrocarbons produced can be ejected from underground under high pressure; therefore, flow control is necessary to regulate the output volume… Next section: Basic Knowledge for Instrumentation Technicians (19) – Actuators 1 – Pneumatic Diaphragm Control Valves