November 14, 2010 (One Question per Day)

The orifice plate flow meter, also known as a differential pressure flow meter, consists of a primary sensing element (throttle element) and secondary components (differential pressure transmitter and flow display unit), and is widely used for measuring the flow rate of gases, steam, and liquids. It features a simple structure, easy maintenance, stable performance, and reliable operation. 　　I. Overview The orifice plate flow meter, also known as a differential pressure flow meter, consists of a primary sensing element (the orifice element) and secondary devices (a differential pressure transmitter and a flow display unit). It is widely used for measuring the flow rate of gases, steam, and liquids. It features a simple structure, easy maintenance, stable performance, and reliable operation. The orifice plate throttling device is a standard throttling element that can be manufactured in accordance with **standards without the need for calibration. 1. **Standard GB2624-81

Basic principle of orifice plate flow meters: An orifice plate throttling element is installed inside the pipe. Since the diameter of this orifice is smaller than the inner diameter of the pipe, when the fluid flows through it, the cross-sectional area of the flow stream narrows suddenly, causing the flow velocity to increase. The static pressure of the fluid behind the throttle element decreases, thereby creating a static pressure difference between the front and back of the throttle element. This static pressure difference has a definite numerical relationship with the flow rate of the fluid passing through it. A differential pressure transmitter (or differential gauge) is used to measure the differential pressure before and after the throttling element, thereby enabling the measurement of flow rate. The structure generally consists of a pressure tapping flange, an orifice plate, pressure guide tubes, gaskets, nuts, and bolts.

The orifice plate flow meter, also known as a differential pressure flow meter, consists of a primary sensing element (throttle element) and secondary components (differential pressure transmitter and flow display instrument), and is widely used for measuring the flow rate of gases, steam, and liquids. It features a simple structure, easy maintenance, stable performance, and reliable operation. A orifice plate is installed in the pipeline, with pressure measurement tubes connected to both sides of the orifice plate, which are in turn connected to a U-tube manometer. A orifice flow meter utilizes the throttling effect of fluid passing through a sharp orifice, which increases the flow velocity and decreases the pressure, thereby creating a pressure difference before and after the orifice that is used as the basis for measurement.

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A orifice plate is installed in the pipeline, with pressure measurement tubes connected to both sides of the orifice plate, which are in turn connected to a U-tube manometer. A orifice flow meter utilizes the throttling effect of fluid passing through a sharp orifice, which increases the flow velocity and decreases the pressure, thereby creating a pressure difference before and after the orifice that is used as the basis for measurement. If the diameter of the pipeline is d1, the diameter of the sharp orifice in the orifice plate is d0, the diameter of the constriction formed as the fluid passes through the orifice plate is d2, and the density of the fluid is ρ. At points I and II on the interface, namely at the pressure measurement probe in front of the orifice plate and at the constriction, the velocities are u1 and u2 respectively, and the pressures are p1 and p2. According to Bernoulli’s equation, ignoring energy losses, we have: (u2² – u1²)/2 = (p1 – p2)/ρ = gh, or (u2² – u1²)¹/² = (2gh)¹/². Since the position of the constriction changes with the flow velocity, and the cross-sectional area S2 is difficult to determine, whereas the area of the orifice is known, and the position of the pressure measurement point remains unchanged after the device is manufactured, therefore u2 is replaced by u0, which corresponds to the velocity at the orifice diameter. Additionally, energy losses due to local resistance in the fluid are taken into account, and a correction factor C is used for correction. Then we have: (u02 – u12)1/2 = C(2gh)1/2. For incompressible fluids, according to the continuity equation, we also have: u1 = u0S0/S1. After some rearrangements, it can be shown that u0 = C*(2gh)1/2 / 1/2. If we define C0 as C/1/2, then the expression can be simplified further to: u0 = C0(2gh)1/2. Using u0 and S2, it is possible to calculate the volumetric flow rate of the fluid: Vs = u0 * S0 = C0 * S0 * (2gh)1/2, or Vs = C0 * S0 * 1/2. Here, R represents the reading of the U-tube manometer ； ρr—is the density of the liquid in the manometer, ； C0—is the orifice flow coefficient, which is determined by the shape of the sharp orifice in the orifice plate, the location of the pressure tap, the ratio of the orifice diameter to the pipe diameter, and the Reynolds number; its specific value is determined through experiments. When d1/d2 is constant, once the Re value exceeds a certain threshold, C0 approaches a constant value. Generally, in industry, orifice plate flow meters are specified to be used under flow conditions with a constant C0.

It is by installing an orifice plate to create a pressure difference, and the flow rate is calculated based on this pressure difference

The fluid filling the pipe flows through a throttling device within the pipe, causing a local contraction near the throttling element, which increases the flow velocity and creates a static pressure difference on the upstream and downstream sides. Installation requirements for orifice plate flowmeters: The requirement for straight pipe sections is generally 10D ahead and 5D behind

　Structure of orifice plate flow meters Components of the throttling device Throttling elements: standard orifice plates, standard nozzles, long-diameter nozzles, 1/4 circular orifice plates, double orifice plates, eccentric orifice plates, notched orifice plates, conical inlet orifice plates, etc. Pressure extraction devices: annular chambers, pressure extraction flanges, clamping rings, pressure guide tubes, etc. Measurement tube Installation requirements for orifice plate flow meters: The required straight pipe sections are generally 10D in front and 5D behind the orifice plate; therefore, when selecting an orifice plate flow meter, it is essential to choose one that is suitable for the specific conditions at the installation site. Working principle of orifice plate flow meters: The fluid filling the pipeline flows through a throttling device located within it, causing a local contraction near this throttling element, which increases the flow velocity and creates a static pressure difference on the upstream and downstream sides. Given the known parameters, the relationship between differential pressure and flow rate can be derived using the principle of fluid continuity and Bernoulli’s equation, thereby allowing the flow rate to be determined. Its basic formulas are as follows: c – discharge coefficient, dimensionless; d – diameter of the orifice or throat of the throttling element under operating conditions; D – inner diameter of the upstream pipe under operating conditions; qm – mass flow rate, Kg/s; qv – volume flow rate, m/s; d/D – diameter ratio, dimensionless; density of the fluid, Kg/m; coefficient of expandability, dimensionless