Hoạt động của đồng hồ lưu lượng dạng Orifice

Đồng hồ lưu lượng dạng Orifice được dùng cho chất lỏng, chất khí và hơi bão hòa, hơi quá nhiệt. Đồng hồ lưu lượng dạng Orifice được biết đến là dạng lâu đời nhất, phổ biến nhất ( chiếm khoảng 40% tổng số đồng hồ lưu lượng toàn cầu) bền bỉ và kinh tế.

Như tên gọi của nó, đồng hồ lưu lượng dạng Orifice đựa vào sự chênh áp suất trước và sau tấm lỗ Orifice. Sự chênh lệch áp suất này là tuyến tính và tỷ lệ thuận với tốc độ qua tấm lỗ Orifice của chất lỏng, khí hay hơi.

Đồng hồ đo lưu lượng Orfice DKV là dạng compact nhỏ gọn:

– Tấm lổ Orifice dạng một mảnh, Cảm biến chênh áp, áp suất (nhiệt độ) được lắp trên cùng của tấm lỗ Orifice.
– Giảm thiểu việc rò rĩ, cải thiện độ tin cậy.

Giảm chi phí lắp đặt
– Chỉ có một phần để cài đặt
– loại bỏ nhu cầu cung cấp và kết nối đường ống, bộ truyền và xung riêng biệt

An orifice in a pipeline is shown in below figure with a manometer for measuring the drop in pressure (differential) as the fluid passes thru the orifice. The minimum cross sectional area of the jet is known as the “vena contracta.”

How does it work?

As the fluid approaches the orifice the pressure increases slightly and then drops suddenly as the orifice is passed. It continues to drop until the “vena-contracta” is reached and then gradually increases until at approximately 5 to 8 diameters downstream a maximum pressure point is reached that will be lower than the pressure upstream of the orifice.

The decrease in pressure as the fluid passes thru the orifice is a result of the increased velocity of the gas passing thru the reduced area of the orifice.

When the velocity decreases as the fluid leaves the orifice the pressure increases and tends to return to its original level. All of the pressure loss is not recovered because of friction and turbulence losses in the stream.

The pressure drop across the orifice ( ΔP in Fig.) increases when the rate of flow increases. When there is no flow there is no differential.

The differential pressure is proportional to the square of the velocity, it therefore follows that if all other factors remain constant, then the differential is proportional to the square of the rate of flow.
Orifice Construction:

Orifice Construction
Inlet Section

A linearly extending section of the same diameter as the inlet pipe for an end connection for an incoming flow connection. Here we measure the inlet pressure of the fluid / steam / gas.
Orifice Plate

An Orifice Plate is inserted in between the Inlet and Outlet Sections to create a pressure drop and thus measure the flow.
Outlet Section

A linearly extending section similar to the Inlet section. Here also the diameter is the same as that of the outlet pipe for an end connection for an outgoing flow. Here we measure the Pressure of the media at this discharge.

As shown in the adjacent diagram, a gasket is used to seal the space between the Orifice Plate and the Flange surface, prevent leakage.

Sections 1 & 2 of the Orifice meter, are provided with an opening for attaching a differential pressure sensor (u-tube manometer,differential pressure indicator).
Material of construction :

The Orifice plates in the Orifice meter, in general, are made up of stainless steel of varying grades.
Shape & Size of Orifice meter :

Orifice meters are built in different forms depending upon the application specific requirement, The shape, size and location of holes on the Orifice Plate describes the Orifice Meter Specifications as per the following:Types of Orifice plates

Concentric Orifice Plate
Eccentric Orifice Plate
Segment Orifice Plate
Quadrant Edge Orifice Plate

Concentric Orifice Plate

It is made up of SS and its thickness varies from 3.175 to 12.70 mm. The plate thickness at the orifice edge should not be exceeded by any of following parameters:

1 – D/50 where, D = The pipe inside diameter
2 – d/8 where, d = orifice bore diameter
3 – (D-d)/8

*Beta Ratio(β): It is the ratio of orifice bore diameter (d) to the pipe inside diameter (D).
Eccentric Orifice Plate

It is similar to Concentric Orifice plate other than the offset hole which is bored tangential to a circle, concentric with the pipe and of a diameter equal to 98% of that of the pipe. It is generally employed for measuring fluids containing

Media having Solid particles
Oils containing water
Wet steam

Segment Orifice Plate

It has a hole which is a semi circle or a segment of circle. The diameter is customarily 98% of the diameter of the pipe.
Quadrant Edge Orifice Plate

This type of orifice plate is used for flow such as crude oil, high viscosity syrups or slurries etc.

It is conceivably used when the line Reynolds Numbers range from 100,000 or above or in between to 3,000 to 5,000 with a accuracy coefficient of roughly 0.5%.
Operation of Orifice meter

The fluid flows inside the Inlet section of the Venturi meter having a pressure P1.
As the fluid proceeds further into the Converging section, its pressure reduces gradually and it finally reaches a value of P2 at the end of the Converging section and enter the Cylindrical section.
The differential pressure sensor connected between the Inlet and the and the Cylindrical Throat section of the Venturi meter displays the difference in pressure (P1-P2). This difference in pressure is in direct proportion to the flow rate of the liquid flowing through the Venturi meter.
Further the fluid passed through the Diverging recovery cone section and the velocity reduces thereby it regains its pressures. Designing a lesser angle of the Diverging recovery section, helps more in regaining the kinetic energy of the liquid.

Advantages of Orifice meter

The Orifice meter is very cheap as compared to other types of flow meters.
Less space is required to Install and hence ideal for space constrained applications
Operational response can be designed with perfection.
Installation direction possibilities: Vertical / Horizontal / Inclined.

Limitations of Orifice meter

Easily gets clogged due to impurities in gas or in unclear liquids
The minimum pressure that can be achieved for reading the flow is sometimes difficult to achieve due to limitations in the vena-contracta length for an Orifice Plate.
Unlike Venturi meter, downstream pressure cannot be recovered in Orifice Meters. Overall head loss is around 40% to 90% of the differential pressure .
Flow straighteners are required at the inlet and the outlet to attain streamline flow thereby increasing the cost and space for installation.
Orifice Plate can get easily corroded with time thereby entails an error.
Discharge Co-efficient obtained is low.

Applications of Orifice meter

Natural Gas
Water Treatment Plants
Oil Filtration Plants
Petrochemicals and Refineries

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