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INSTRUMENTATION

MECHANICAL

SIVIL

ELECTRICAL

Introduction : Orifice plate is a device which measures the rate of fluid flow. It uses the same principle as a Venturi nozzle, namely Bernoulli's principle which says that there is a relationship between the pressure of the fluid and the velocity of the fluid. When the velocity increases, the pressure decreases and vice versa.

Theory : An orifice plate is basically a thin plate with a hole in the middle. It is usually placed in a pipe in which fluid flows. As fluid flows through the pipe, it has a certain velocity and a certain pressure. When the fluid reaches the orifice plate, with the hole in the middle, the fluid is forced to converge to go through the small hole; the point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called "vena contracta" point (see drawing to the right). As it does so, the velocity and the pressure changes. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation

Intro : Flow orifices and flow restrictors contain precision-machined holes and filters or screens to restrict flow and reduce pressure. They are available as stand-alone devices and in kits, assemblies, restrictors, and restrictor valves. Flow orifices and flow restrictors vary in terms of both specifications and features. Specifications include orifice diameter, maximum pressure, maximum liquid or gas flow rate, flow tolerance, and media temperature. Typically, flow tolerance is expressed as a percentage. In part, media temperature depends upon whether devices are rated for pneumatic air, hydraulic fluids, ink, chemicals, or gases. Optional features include multiple openings for increased control. Some devices are bi-directional. Others are constructed for high-purity applications such as semiconductor manufacturing.
Devices traceable to the National Institute of Standards and Technology (NIST) and the American Society of Mechanical Engineers (ASME) are often available. Selecting flow orifices and flow restrictors requires an analysis of connection methods. Some devices use male or female national pipe thread (NPT) ports. Others use male or female, tapered or straight British standard pipe (BSP) measurements. Connection standards include unified national course (UNC), unified national fine (UNF), and other English thread series. Flow orifices and flow restrictors with metric measurements are commonly available. Devices with plain ends fit into bells and sockets. Fittings with groves are well-suited for use with coupling features such as O-rings and elastomeric seals. Flares are designed to mate with connection nuts or ferrules with a complementary geometry. For rigid metallic connections, fitting ends are typically welded or brazed. Other connection methods include flanges, compression fittings, pipe clamp ends, push-to-connect collars, and hardware with barbs and ridges.

Design : Flow orifices and flow restrictors are made from a variety of metals, metal alloys, and thermoplastics. Aluminum is a metallic element with good electrical and thermal conductivity, high reflectivity, and resistance to oxidation. Copper, another metallic element, is one of the best conductors of heat and electricity. Brass is a copper alloy with excellent corrosion resistance and low magnetic permeability. Other commonly used alloys include bronze and nickel. Typically, “black pipe” is made of general-purpose carbon steel that contains alloys that improve toughness and hardness. Stainless steel resists chemicals and corrosion and can have relatively high-pressure ratings. In terms of thermoplastics, some flow orifices and flow restrictors are made from acrylonitrile butadiene styrene (ABS), a rigid pipe used for drain lines. Others are made from nylon or fluoropolymers.

Application : Flow orifices and flow restrictors are used in wire guides, flow measurement, ink jet printing, nozzles, and pneumatic controls. They are also used in a variety of biological and medical instrumentation applications. Devices that are used in particle counting as an inactive control method use a fixed restriction and rely upon a constant pressure, temperature, and viscosity. Devices that are used in water jet applications can convert energy from pressure to velocity to produce an outgoing stream of water traveling at over twice the speed of sound. Snubbers are flow orifices and flow restrictors that provide shock absorption or dampening. They are often used in connection gauges in pressure or process lines. Flow orifices and flow restrictors that are designed for use in atmospheric burners contain holes that are small enough to provide the correct gas flow, but large enough to ensure sufficient velocity and suction.

Specification : Plate thickness not exceed 1/8 of orifice bore or 1/50 of pipe, diameter Concentricity centered within 3% of pipe, Flatness within 0.01 inch per inch of radius, Upstream face smooth
Upstream Edge square and sharp (45% bevel is permissible).

Plates are chosen for its ability to resist: Erosion, Corrosion, Distortion