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Process industry environmental protection scheme(differential pressure4)
Feb 28, 2020

2.5 pressure

Pressure is the force acting vertically on the area

Pressure refers to the force acting on the surface in normal direction (i.e. vertical direction) per unit area

The U.S. unit of pressure is pounds per square inch or PSI, where pounds are forces rather than masses.

The SI unit of pressure is Pascal, equal to 1n / m2. Pascal is a very small unit of pressure, usually expressed in kPa (kilopascal) or MPa (megapascal). Other common SI units of pressure are bar, which is equal to 100 kPa.

Absolute pressure and gauge pressure

Absolute pressure PAbs is the pressure relative to the ideal vacuum. Gauge pressure, or pgage, is the pressure relative to atmospheric pressure PATM. So:

Absolute pressure is used to calculate gas density. Because gauge pressure is related to atmospheric pressure, it is used to ensure that the pressure retaining parts (i.e. the pipe or flowmeter parts that maintain pressure when installed on the pipe) are kept within the safe working limit.

Standard atmospheric pressure is usually defined as 1 atm, 14.69595 psi, or 101.325 kPa. The actual atmospheric pressure at a given location depends on the altitude of the location and the weather conditions of the day. Atmospheric pressure changes due to weather are usually not used in the calculation of absolute pressure. However, in order to measure the flow, the atmospheric pressure is determined using the local standard atmospheric pressure - adjusted for altitude.

The pressure values in flowmeter applications are used to provide information for two important independent engineering tasks:

Calculation of fluid parameters - especially gas or vapor density and gas expansion coefficient

Check the compatibility and safety factor of the installation hardware

differential pressure

When differential pressure flow calculation requires to determine the difference between two pressures, it is called differential pressure. The SI unit of differential pressure is PA or kPa. The U.S. unit of differential pressure at the specified temperature is psi or inh2o.

An inch water column unit is a legacy of history when a piezometer was used to measure flow, indicating the pressure at the bottom of the water column at a specified height at a specific temperature. For example, a differential pressure of 25 inches of water at 68 ° f indicates that this is the pressure at the bottom of the 25 inches of water at a uniform 68 ° f temperature. There are two common versions of the unit: inches of water at 68 ° f (for the U.S. process control industry) and inches of water at 60 ° f (for the U.S. natural gas industry). The conversion factors in psi are:

The difference between the two reference temperatures is 0.08%, so it is important to understand the reference temperature.

Piezometers are the simplest instruments for measuring small pressure differences. The piezometer adopts a U-shaped pipe or two vertical pipes connected at the bottom, and the pipe is filled with some liquid. When two kinds of pressure are applied on the top of each pipe, the liquid height in the two pipes changes, and the scale on the manometer is used to measure the height or height difference H

The standard manometer fluid used for gas flow differential pressure measurement is water, with height in inches and differential pressure in inches of water. Of course, if you measure water or other liquids, you need to use a denser piezometer fluid. Mercury (S.G. = 13.5) or brominated liquids (S.G. = 2.5 to 3.0) are commonly used. All fluids heavier than water can be used, but must be "insoluble" or mixed with fluids that cannot be contacted by the manometer.

At present, the best technology is to use electronic differential pressure transmitter and static pressure transmitter. These transmitters can provide extremely accurate readings in a large range of pressure or differential pressure, and can work in a large range of ambient temperature without external correction. The electronic signal output can be easily input into the microprocessor for calculating flow or recording data.

2.6 temperature

Industrial methods of temperature measurement are based on substances whose resistance varies with temperature, such as RTDS (resistance temperature detectors) or thermocouples that produce voltage varying with temperature at different metal joints.

In addition to engineering, temperature measurements are usually performed using the relative Fahrenheit and Celsius scales, which were originally designed to measure the temperature range of the earth. However, flow engineering problems require a different temperature scale representing absolute temperature. The SI unit for absolute temperature is Kelvin, and the US unit is Rankin.

Absolute temperature is required to calculate fluid properties such as density, viscosity, and isentropic index. The calculation of thermal expansion effect involves temperature difference, so ° f or ° C is often used.

Absolutely zero Rankin is absolutely zero Kelvin. The Rankine scale increments are Fahrenheit, while the Kelvin scale increments are Celsius.

All real fluids have viscosity, which mainly changes with temperature. Therefore, it is usually necessary to draw the curve of fluid viscosity and temperature, establish the equation, and calculate the viscosity after knowing the temperature. The viscosity of liquid decreases with the increase of temperature; the viscosity of gas increases with the increase of temperature.

Kinematic viscosity is the absolute viscosity divided by the density of the fluid at the same temperature, or the fluid is classified according to the relationship between the fluid stress (the force required to overcome the viscosity) and the strain (fluid flow rate). Differential pressure flow meters are limited to "Newtonian" type fluids or fluids with constant slope (μ) of the fluid stress / strain curve. The viscosity of shear thinning fluid decreases with the increase of shear stress, such as ketchup, volcanic magma or polymer solution and melt polymer. Shear thickening (viscosity increases with shear stress) fluids include suspensions such as corn starch in water. Bingham plastic fluids do not flow until they yield beyond the critical stress; this type of fluid includes toothpaste.

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