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Process industry environmental protection scheme(differential pressure2)
Feb 26, 2020

1.4 flow 101

Fluid theory is the study of moving fluid. Fluid can be defined as any substance that flows, so both liquid and gas are fluids. In order to accurately measure and control the fluid flowing through the pipeline, it is necessary to have a deep understanding of technical knowledge, which is very important in almost all process industries.

Key factors flowing through the pipeline:

Physical configuration of pipeline;

Fluid velocity;

Fluid friction along the pipe wall;

Fluid density;

Fluid viscosity;

Reynolds number;

Pipe configuration: the diameter and cross-sectional area of the pipe can be used to determine the volume of fluid for a specified pipe length, or to determine the Reynolds number of the application. Flow rate: depends on the pressure or vacuum that drives the fluid into the pipe.

Friction: there is no absolutely smooth pipe wall, so the fluid will produce friction when contacting with the pipe wall, so the velocity near the pipe wall is slower than that at the center. The larger, smoother or cleaner the pipe diameter, the smaller the impact on the flow.

Density: density affects flow, because the more viscous the fluid is, the greater the pressure required to reach the specified flow. Due to the incompressibility of liquid (considering practical application) and the compressibility of gas, different methods are needed to measure their flow.

Viscosity: it is defined as the molecular friction of the fluid. Generally, the higher the viscosity is, the greater the work required to reach the required flow, so the viscosity will affect the flow. Temperature affects viscosity, but not always visually. For example, a higher temperature will reduce the viscosity of most fluids, but when the temperature is higher than a certain value, the viscosity of some fluids will actually increase.

Reynolds number: the Reynolds number can be calculated to describe the type of fluid flow profile by classifying it into various factor relationships of a given system. This is important when choosing the right flow measurement method.

Three different flow profiles are defined in different Reynolds number ranges. Laminar flow is a smooth flow of fluid in parallel layers, characterized by Reynolds number less than 2000. Generally, the flow rate of the fluid is small, the mixing is very little, and sometimes the viscosity of the fluid is high. When the Reynolds number of the fluid flow profile is between 2000 and 4000, it is considered to be in the transition zone. Reynolds number above 4000 is called turbulence. It is characterized by high flow rate, low viscosity and fast and sufficient fluid mixing.

Turbulent differential pressure flow measurement is the most accurate. Because the separation point between the fluid and the edge of the throttling element is easier to predict and keep unchanged in turbulent flow. This kind of fluid separation forms a low pressure area at the downstream side of the throttling element, so the throttling element can be used as the primary element of the differential pressure flowmeter. According to the type of throttling element and the design of flowmeter, the minimum tube Reynolds number of a specific flowmeter can be much higher than 4000.

The derivation formula of fluid continuity describes the basic principle of energy conservation. In Chapter 3, Bernoulli's equation is introduced in detail, which is based on the principle of energy conservation and defines the energy conservation suitable for fluid flow.

Differential pressure flowmeter

By introducing a throttling element into the pipeline, the primary element forms a pressure drop at both ends of the flowmeter. The pressure drop is measured by the secondary element differential pressure transmitter. Third party components include all parts necessary for the operation of the instrument, including the impulse piping and connections to introduce upstream and downstream pressure into the transmitter.

Integrated differential pressure flowmeter

By setting the throttling element in the pipe, the flow can be calculated by Bernoulli equation, because the square root of the differential pressure through the throttling element is proportional to the flow.

There are some important precautions for differential pressure flow measurement, including:

Make sure that the impulse piping is not blocked by particles or deposits

Correct orientation of impulse pipes (these pipes need to be tilted to prevent gas accumulation in liquid applications or liquid accumulation in gas applications)

Ensure that regular calibration does not reduce accuracy (avoid this with high accuracy calibration equipment)

Primary element type

There are many types of primary components, such as:

Single hole and adjusting orifice plate

Single and porous pitot tubes

Venturi tube

Flow measuring nozzle

V cone

Wedge block

Transmitter options

There are two main types of pressure transmitters that use differential pressure to calculate flow. The first is the traditional differential pressure type, which only measures the differential pressure and has no other functions. The second is multi parameter transmitter. Multiparameter transmitter is a kind of multiparameter transmitter which can measure many process variables (including differential pressure, static pressure and temperature). When used as a mass flow transmitter, these independent values can be used to compensate for changes in density, viscosity, and other flow parameters.

1.5 application of differential pressure flow measurement

Product consistency: products produced in batches depend on accurate proportions of ingredients, and differential pressure flow meters help to ensure accurate delivery of liquids and gases.

Production efficiency: from batch control to by-product removal to emission monitoring, flow measurement and measurement are indispensable in a large number of process control variables related to efficiency.

Process variable control: a process usually includes multiple variable inputs. The control of these variables (including flow) is the key to high quality production.

Safety: differential pressure flow helps to prevent various threats to safety, including overfilling, reactor control, etc.

Internal material list and resource allocation: strict control of inventory and process rate will directly contribute to the improvement of profitability. For many experienced producers, the internal material list of process cost directly affects productivity.Trade handover: flow measurement for products sold by volume or weight

1.6 installation of flowmeter

Traditional installation

Traditional installation methods require three different component categories.

Primary element (differential pressure generator)

Secondary element (transmitter)

Third party components (impulse piping, connecting hardware, piping, joints, valves, etc.)

Traditional differential pressure flow installations include independent

Primary, secondary, and third-party components.

The traditional form can make the design between components meet various applications, and can be designed specifically to meet the trade handover standards.

Traditional installation has its own limitations or problems. There are many possible leakage points on the joint, single / wrong impulse pipe and valve group device; the accuracy problems caused by long impulse pipe. In addition, the installation is extremely complex, requiring long straight pipe sections (depending on the primary element used) and careful component configuration. Over the years, people have solved some of these problems through a lot of work, thus expanding the practicability and value of differential pressure flow equipment.

Integrated installation

The integrated flowmeter integrates the primary element and transmitter into a whole flowmeter. To a large extent, this is to minimize the installation problems of the old traditional flowmeter. Therefore, less parts and manpower are needed for installation than for traditional flowmeter.

The working mode of Integrated flowmeter is very similar to that of traditional flowmeter. Adopt the same measurement and calculation method, work with the same primary element, and have the same transmitter (differential pressure transmitter and multi parameter transmitter).

Advantages of Integrated flowmeter:

Do not use fittings, impulse piping, valves, adapters, manifolds, and mounting brackets

Potential leak point reduction (factory leak check)

Reduction of flow measurement error source

Ordering and installation process simplification

Reduce the possibility of freezing and blocking

More compact design

Rosemount Integrated flowmeter combines industry-leading transmitter with innovative primary element technology and connection system. A flowmeter integrates 10 traditional parts, simplifying the design, procurement and installation procedures.

The structure of traditional differential pressure flowmeter is compared with that of integrated multi parameter differential pressure flowmeter.

1. Flow computer

2. Primary element

3. heat bushing

4. Temperature sensor

5. Temperature transmitter

6. Sensor wiring

7. Pressure transmitter

8. Differential pressure transmitter

9. valve group

10. Connecting flange

1.7 other Flow Technologies

In addition to pressure based technology, there are various flow measurement technologies. These include open channel, mechanical, ultrasonic, electromagnetic, Coriolis, optical, thermal and vortex flowmeter.

The electromagnetic flowmeter requires the use of conductive fluid and the introduction of magnetic energy into the flow. The flowmeter uses electrodes to sense the current induction of magnetic flux.

According to Gu Mingsi, Coriolis flowmeter uses Coriolis effect to drive vibration tube to deform.

The optical flowmeter uses a photodetector to measure the movement of particles in the illuminated fluid.

Differential pressure flow is still the most commonly used flow measurement form in the industry.

The vortex flowmeter uses an electrical pulse generator - usually a piezoelectric crystal - to measure fluid disturbances (vortices) around a calibrated body.

Nowadays, all kinds of different flow measurement technologies have their ideal application range. However, due to its long history, ease of use and wide range of application, differential pressure flow is still the most commonly used flow measurement form in the industry.