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

1. 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.

 

2. 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

 

3. 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.

 

4. 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)

 

5. 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

 

6. 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

 

1. 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.

 

2. 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 273 C (460 F) 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.

 

3. 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

 

4. 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