In this post, I am interested in covering the details about how to part flow using Orifice meter. And I am beginning it from the scratch.
Basic Principle
Differential Pressure Sensor Principle
When an orifice plate is settled in a pipe carrying the fluid whose rate of flow is to be measured, the orifice plate causes a pressure drop which varies with the flow rate. This pressure drop is measured using a differential pressure sensor and when calibrated this pressure drop becomes a part flow rate.
Description
The main parts of an orifice flow meter are as follows:
* A stainless steel plate which is held in the middle of flanges of a pipe carrying the fluid whose flow rate is being measured.
* It should be noted that for a positive distance before and after the plate fitted in the middle of the flanges, the pipe carrying the fluid should be level in order to maintain laminar flow conditions.
* Openings are provided at two places 1 and 2 for attaching a differential pressure sensor (U-tube manometer, differential pressure gauge etc).
Operation
* The detail of the fluid movement inside the pipe and orifice plate has to be understood.
* The fluid having uniform cross section of flow converges into the orifice plate's opportunity in its upstream. When the fluid comes out of the orifice plate's opening, its cross section is minimum and uniform for a singular distance and then the cross section of the fluid starts diverging in the down stream.
* At the upstream of the orifice, before the converging of the fluid takes place, the pressure of he fluid (P1) is maximum. As the fluid starts converging, to enter the orifice opportunity its pressure drops. When the fluid comes out of the orifice opening, its pressure is minimum (P2) and this minimum pressure remains constant in the minimum cross section area of fluid flow at the downstream.
* This minimum cross sectional area of the fluid obtained at downstream from the orifice edge is called Vena-Contracta.
* The differential pressure sensor attached in the middle of points 1 and 2 records the pressure dissimilarity (P1 - P2) in the middle of these two points which becomes an indication of the flow rate of the fluid straight through the pipe when calibrated.
Applications
1. The concentric orifice plate is used to part flow rates of pure fluids and has a wide applicability as it has been standardized.
2. The eccentric and segmental orifice plates are used to part flow rates of fluids containing suspended materials such as solids, oil mixed with water and wet steam.
Advantages
1. It is very cheap and easy method to part flow rate.
2. It has predictable characteristics and occupies less space.
3. Can be use to part flow rates in large pipes.
Limitations
1. The vena-contracta distance depends on the roughness of the inner wall of the pipe and sharpness of the orifice plate. In positive cases it becomes difficult to tap the minimum pressure (P2) due to the above factor.
2. Pressure saving at downstream is poor, that is, unabridged loss varies from 40% to 90% of the differential pressure.
3. In the upstream straightening vanes are a must to gather laminar flow conditions.
4. Gets clogged when the suspended fluids flow.
5. The orifice plate gets corroded and due to this after sometime, inaccuracy occurs. Moreover the orifice plate has low bodily strength.
6. The coefficient of extraction is low.
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