Pressure and Temperature Measurement (Process Instrumentation)

Table of Contents: –

Pressure Measuring Instruments Page

Section 1. Introduction 03

Section 2. Pressure 03

Section 3. Absolute Pressure 06

Section 4. Units of Pressure 07

Section 5. Metric System 08

Section 6. Pressure measuring instruments 08

Section 6.1. Manometer 08

Section 6.2. Bourdon Tube 10

Section 6.3. Diaphragm Type 12

Section 6.4. Bellows Type 13

Temperature Measuring Instruments

Section 1. Introduction 14

Section 2. Units of temperature 14

Section 3. Temperature conversion 15

Section 4. Temperature measuring instruments 15

Section 4.1 Thermowell 16

Section 4.2 Capillary tube thermo meters 16

Section 4.3 Filled thermal system 17

Section 4.4 Bimetallic thermometers 18

Section 4.5 Resistance thermal detectors 19

Section 4.6 Thermocouples 20

PRESSURE MEASURING INSTRUMENTS

Introduction

Mainly, there are four process variables that we deal with in the petroleum business – pressure, temperature, level, and flow. Each of these variables uses specialized devices to measure changes in a process.

Pressure

The definition of pressure is the amount of force acting on a given area. Depending upon the substance exerting the force, different scales are used. For example, a solid object, such as the cube shown below, will exert pressure over the surface area that it is sitting upon. When discussing solids, pressure is typically described as the amount of weight exerted upon a one-inch square. A five-inch cube weighing 25 pounds would exert a pressure of 1 pound per square inch. (25 lbs. divided by 25 square inches = 1 lb./sq. in.)

What is the unit for pressure in SI (System International) now?

P = F/A . p is the pressure, F is the force in Newton (N) and A the area in m². So the unit for pressure in SI unit become N/m²( Newton per square metre). This unit of pressure have been given a special name because pressure is so commonly used and so the unit is easier to write and pronounce. This name is the Pascal and the symbol is Pa. This is the unit for pressure in SI.

1 Pa = 1 N/ m². Since this is a very small unit, 1000 Pa or 1 kPa is used for practical purpose.

P = h x r x g

P = Pressure in Pascals

h = height of the liquid in meters

r = density in kg/m3

g = gravitational constant ( 9.81m/s2)

Height of water in de-hydration tank = 12 m

Density = 1200 kg/m3

What is the pressure at the bottom?

P = h x r x g

P = 12 x 1200 x 9.81 = 141264 Pa

141264 Pa = 141264/1000 = 141. 264 kPa or 1.4 bar

The pressure exerted by a column of liquid is referred to as hydrostatic pressure and is determined by the height of a column of liquid, such as in millimeters or inches of water.

The concept of gas pressure is best illustrated by thinking of the pressure exerted by a gas in a closed vessel. Gas pressure is increased or decreased by controlling the amount of heat applied to the vessel or by compressing or decompressing the gas

ABSOLUTE PRESSURE and GAUGE PRESSURE .

The Earth’s atmosphere can be thought of as a closed vessel. The term atmospheric pressure refers to the pressure exerted by the combination of oxygen, nitrogen and other gases that make up our atmosphere. Whenever we measure any sort of gas pressure, the atmospheric pressure has to be taken into consideration. Theoretically, if there were no

pressure or air in our atmosphere, a “zero pressure” condition would exist. This “zero pressure” point is the basis for the absolute pressure scale. On this theoretical absolute pressure scale (such a zero point cannot physically be duplicated), the earth’s atmosphere exerts a pressure of 101.325 kPa (14.7 pounds per square inch) absolute. This scale is commonly referred to as absolute pressure.

Another scale that is commonly referred to is gauge scale. Gauge scale uses atmospheric pressure (101.3 kPa) as its “zero” point. Pressure measurements using this scale are commonly referred to as “kPa(g)” measurements, where the “g” stands for gauge. This “kPa(g)” scale is often referred to as simply kPa. To simplify the calculations, atmospheric pressure is often taken as 100 kPa.

ABSOLUTE PRESSURE and GAUGE PRESSURE (comparison).

Sometimes we work with pressures below atmospheric pressure (less than 101.3kPa(A)). In those instances we use a vacuum pressure scale which has its zero point at 101.3kPa(A) and decreases as we move toward absolute zero pressure. Vacuum pressure is usually measured in mm of water or mercury.

Units of pressure

Pascal: 1 Pascal = 1 N / m²( 1 N = 9.8 kg_f)
1000 Pa = 1 kPa
100 kPa = 1 Bar.
10 Bar = 1 MPa
1 Bar = 14.5 PSI,
1 PSI = 100 / 14.5 kPa = 6.896 kPa
1 Bar = 1 Kg / cm²( for normal calculations)

The Metric System

PRINCIPAL UNITS:-

Length in Metre
Capacity in Litter
Mass in Gram
Following prefixes are used for multiples.
decca d 10
hecto h 100
kilo k 1000
mega M 1000000
giga G 1000000000
tera T 1000000000000
Trillion (Brit) 10 ¹⁸
Trillion (US) 10 ¹²

Pressure Measuring Instruments

6.1 Manometer

A manometer is a device commonly used to measure pressure. In the oil and gas industry, manometers are often used to measure differential pressure. Though there are many designs , all manometers work on the same basic principle. A column of liquid of a particular height always exerts the same, specific amount of pressure.

If we take an open U-shaped container and put a fluid such as water in the container, what we find is that the level of the water in both legs of the container is the same. If we supply pressure to one side of the manometer, we find that the level changes, with one leg going up and one going down. The amount of offset in both legs is exactly the same. To determine the amount of pressure applied, we look at both legs and add together the offsets. In this example, we add together the 3.0 inch offset from zero on the left leg of the manometer to the 3.0 inch offset from zero to the right leg. The sum of those two measurements represents the total pressure applied of 6.0 inches of water .

The U-shaped manometer is a very common design, but we may also find well and inclined manometers in use. The overall appearance of these manometers differs from the U-shaped manometers, but they work on the same principle. The difference being that the pressure is read directly from a scale next to the liquid column.

6.2 Bourdon Tube

A Bourdon tube is a device that senses a change in pressure and converts this pressure change into a mechanical motion. This mechanical motion is applied to other devices such as pointers or pens via a mechanical linkage to provide a visual means for reading pressure.

A Bourdon tube consists of a hollow, curved metal tube that is closed off on one end. As pressure is applied to the open end, the tube tries to straighten itself out.

The closed off end of the Bourdon tube is called the tip. The tip end has a mechanical linkage attached to it that is attached to a gauge pointer or other indicator. As the pressure increases, the tube straightens slightly creating a small movement at the tip. This small movement of the Bourdon tube is translated by the mechanical linkage into a larger movement at the gauge pointer.

There are several types of designs of the Bourdon tube including the helical, spiral and C-type. However, all Bourdon tubes operate using the same principle.

C type Bourdon tube pressure gauge

BOURDON TUBE HELICAL

6.3 Diaphragm Type

These types of instruments are used commonly to measure and indicate very small changes in pressure. Pressure changes in a system cause a thin diaphragm to move. These diaphragms are typically made of rubber or thin metal.

As the pressure increases or decreases, the diaphragm moves back and forth and is linked mechanically to a pointer or other types of indicators.

There are pressure switches also designed to work under the diaphragm principle. Below shown is a typical example.

6.4 Bellows Type

A bellows element is a cylindrical tube, generally made of rubber or thin metal. It is pleated, giving it an accordion-like action. Bellows are used to measure either high or low pressure. In our example, pressure is applied to the bellows which is connected to a mechanical linkage, much like the Bourdon tubes or diaphragm. This linkage is then connected to a pointer or other indicator. As pressure is applied, the bellows expands or contracts and moves the linkage which translates the motion into a pressure reading. A spring on the inside of the bellows returns the bellows to a set position once the pressure is removed.

BELLOWS ELEMENT for DIFFERENTIAL PRESSURE

Temperature Measuring Instruments

Introduction.

The concept of temperature is associated with the measurement of heat exchange.

We get heat (and a rise in temperature) when there is an increase in the movement of molecules in some substance. An increase in temperature occurs:

* when there is friction in a system,

* when there is combustion, or

* when there is an increase in pressure on a vessel.

Units of Temperature

Celsius

Kelvin

Fahrenheit

Rankine

When you are working with cryogenic processes, you are still working with “heat,” although you don’t normally think of 100 degrees below zero as hot.

Like the absolute pressure scale based on a theoretical zero pressure, there is an absolute temperature scale as well. Absolute zero is the theoretical temperature at which molecules stop moving. Using our conventional Celsius scales, absolute zero is 273C (460F) below zero.

DIFFERENT TEPMERATURE SCALES (comparison)

We measure temperature by using measuring instruments, such as thermometers, which are marked off in degrees. There are many different scales used when measuring temperature, but we will be most often dealing with one of two scales – Fahrenheit or Celsius.

Temperature Conversion

To convert Use Formula

Fahrenheit to C C = 5/9 x (F-32)

Celsius to F F = 9/5 x C+32

Celsius to K K = C+ 273

Kelvin to C C = K-273

Fahrenheit to R R = F+ 460

Rankine to F F = R- 460

Kelvin to R R = 9/5 x K

Temperature Measuring Instruments

There are two major classes of temperature measuring devices used in the oil and gas industries. First one is of temperature measuring devices that use mechanical properties to operate – specifically, the property of substances to expand when heated and contract when cooled.

Second one is another class of temperature measuring devices called thermoelectrical temperature measuring devices, which operate on electrical principles.

4.1 Thermowell

Let’s look first at temperature measuring devices that use mechanical properties to operate.

4.2 Capillary Tube Thermometers. (Mechanical Properties)

In this category the most common device used for measuring temperature is the plain bulb-type thermometer. It consists of a bulb containing a fluid (usually mercury) connected to a glass tube called a capillary tube. The tube is calibrated with a scale or is attached to a mounting plate with a scale marked on it. As the fluid in the bulb is heated, it expands and moves up the capillary tube. The top of the fluid indicates the temperature in either degrees Celsius, degrees Fahrenheit, or both, or in any other scale.

A capillary tube thermometer is often found mounted inside of a thermowell. A thermowell is a type of protective sheath which is mounted on a line projecting into a process fluid. The thermowell protects the thermometer or other temperature measuring instrument from the process fluid, which may be corrosive or under high pressure. Also, a thermowell allows for easy change out of the temperature measuring instrument without disrupting the process. Often a heat transfer fluid , such as glycol or oil is put into the thermowell to assure that the heat is transferred evenly to the measuring device.

4.3 Filled Thermal Systems (Mechanical Property)

This system consists of a bulb filled with an expanding substance (usually an inert gas) connected to a capillary tube, which may be as long as 25m. The capillary tube is connected to a Bourdon tube mechanism inside of a indicator dial or gauge. The capillary tube is flexible and allows for remote installation of the gauge.

As the substance in the bulb expands the pressure in the capillary tube increases and this causes the Bourdon tube to expand and the reading on the gauge to change according to the temperature variation.

4.4 Bimetallic Thermometers (Mechanical Properties)

There are two basic principles in a bimetallic thermometer. The first principle is that metals expand when they are heated and contract when they are cooled. The second one is that different metals expand and contract at different rates.

A bimetallic thermometer is made up of a coiled, bimetallic strip connected with a rod to a pointer on a gauge. The bimetallic strip having two different metals fused together which, when heated or cooled, expand and contract at different rates.

This unequal expansion and contraction causes the coil to rotate and to move the connected pointer. The pointer indicates the change in temperature on the gauge.

BI-METALIC

SPRAL FASTENED

TO THE CASE

Bimetallic thermometers are inexpensive and can withstand in harsh environment. Because of this, the use of this type in the field is very common.

4.5 Resistance Thermal Detectors (Thermoelectrical Properties)

A resistance thermal detector (usually called an RTD) is our first example of a thermoelectrical temperature measuring device.

An RTD works on the principle that when we heat or cool a wire that has an electrical current running through it, the resistance of that wire also changes in a fixed measurable amount. Resistance, in this example, is a tendency on the part of the wire to pass or not to pass current through it. This is similar to a water hose with an adjustable sprayer on the end. When we open the sprayer, we decrease the resistance and more water flows through; if we close the sprayer, the resistance is increased and less water flows through.

RTDs are generally nickel or platinum wires that are connected to an electronic circuit. This electronic circuit is also connected to an indicator which indicates the change in temperature.

In general RTDs are in a protective stainless steel sheath that is designed to be threaded into a thermowell. This fitting also has a head end with electrical connections to connect the RTD to an indicator or any other device.

4.6 Thermocouples (Thermoelectrical Properties)

This is another type of temperature measuring device. A thermocouple is made up of two wires composed of different metals joined together at one end and connected to an electronic circuit at the other end. This electronic circuit, like that of the RTD is connected to an indicator or a to a controlling device.

Depending upon many other factors, the makeup of the thermocouple will vary, however the most common type of thermocouple has wires of iron and another metal called constantan. These wires are in a sheath that will be fitted into a thermowell.

The thermocouple principle is based on the relation between different metals insulated from each other and joined at the ends to form a simple continuos electrical circuit as shown above. If one of the junctions is maintained at a temperature higher than that of the other , an electro motive force (EMF) is set up that will produce a flow of current through the circuit. The magnitude of the net EMF depends on the difference between the temperature of the two junctions and the materials used for the conductors. The EMF is measured with a galvanometer and corresponds to temperature.

The thermocouple works because of the above mentioned fact that when heat is applied to the joined ends of the wire, a small voltage is produced in the wires. This voltage is detected by the electronic circuit which transfers the information to the indicator. The more heat applied, the greater the voltage produced and the greater the temperature measured.

Thermocouple

Pressure and Temperature Measurement (Process Instrumentation) - Jobs and Updates (2024)

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