CPCL Instrumentation - I
Compiled by
Gowthaman.A
PROCESS INSTRUMENTATION PRESSURE
PRESSURE
Pressure can be defined as,
“The normal force per unit area exerted on a imaginary or real plane surface in afluid or a gas”.
Pressure = Force /Area OR Force = Pressure x Area
ATMOSPHERIC PRESSURE
The pressure exerted by the atmosphere is defined as the atmospheric pressure. This pressure varies with the location. The standard atmospheric pressure is taken at average sea level and is 101.325 kPa A.
GAUGE PRESSURE
Gauge pressure is the pressure measured above the atmospheric pressure. An ordinary pressure gauge measures the difference between the pressure inside and outside the pressure-measuring element.
ABSOLUTE PRESSURE
Absolute pressure is the sum of gauge pressure and atmospheric pressure.
Absolute pressure = Gauge pressure + Atmospheric pressure
DIFFERENTIAL PRESSURE
The differential pressure is the pressure between two pressures. It is measured by separating the two pressures by a diaphragm and measuring the net force or motion of the diaphragm, or by observing the height of a column of liquid in a manometer.
VACUUM
Vacuum is defined as the pressure below the atmospheric pressure and is usually expressed in mm of mercury or mm of water. A full vacuum represents -760mm of Hg or -407.2 inches of H2O or -101.325 kPa.
GRAPHICAL REPRESENTATION OF COMMON PRESSURE TERMS
Pressure
Pascal ( Absolute Pressure ) ( Gauge Pressure )
Absolute Zero Pressure ( Perfect Vacuum ) Atmospheric pressure
( suction ) Pascal
Pascal Pascal Pascal
( Absolute Pressure ) ( Pressure Difference )
( 101. 325 kPa A ) Zero Gauge pressure
STANDARD ENGINEERING UNITS AND THEIR INTER-CONVERSIONS
PSI (LB / in2) Kg /cm2 kPa Bar In. H2O mm Hg Atmosphere
1 0.07031 6.895 0.06895 27.70 51.71 0.06804
14.223 1 98.05 0.9805 394.0 735.5 0.9678
0.1450 0.0102 1 0.01 4.016 7.502 0.00987
14.50 1.020 100 1 401.6 750.2 0.987
0.03610 0.002456 0.2490 0.002490 1 1.867 0.002456
0.0193 0.001360 0.1333 0.001333 0.5357 1 0.001316
14.70 1.0333 101.3 1.013 407.2 760 1
PRESSURE MEASURING DEVICES
SELECTION CRITERIA
Pressure Measuring Devices Selection criteria depends upon the following factors
Range of the pressure to be measured
Application
Corrosive nature of the fluid
Hot and slurry nature of fluids
Based on these factors following devices are used for pressure measurement applications.
MANOMETERS
PRESSURE GAUGES
PRESURE TRANSDUCERS
PRESURE TRANSMITTERS
PRESSURE SWITCHES
PRESURE REGULATORS
MANOMETERS
Monometers are generally used for Low Range Pressure Measurement applications. If small, near atmospheric pressures are to be detected, the manometers are the appropriate choice although manometers are widely used in laboratories and calibration workshops. The types of Manometers are,
1. U - Tube manometer.
2. Single Limb Manometer.
3. Inclined Tube Manometer.
1. U- TUBE MANOMETER
` P1= HIGH PRESSURE
P2= LOW PRESSURE
h = DIFFERENTIAL PRESSURE
This is used to measure low range of Pressure measurement in Inches of H2O Column and normally used in the workshop facilities for calibration. This consists of a U- tube closed at one end or open at both ends. A manometric liquid, such as mercury, glycerine or water is filled to half of the tube.
The liquid is generally coloured by ink or some dye. One of the ends of the U- tube is connected to the pressure tapping and the other is open to the atmosphere. The height difference of the liquid will give the pressure or vacuum applied. A scale fitted with the limbs is calibrated in centimeters or inches.
2. SINGLE LIMB MANOMETER
P1= HIGH PRESSURE
P2= LOW PRESSURE
h = DIFFERENTIAL PRESSURE
This is used to measure low range of Pressure measurement in Inches of H2O Column and normally used in the workshop facilities for calibration. The single limb well-type manometer does not differ much from the U- tube manometer. However in place of one leg of the manometer, a well is installed which has sufficient capacity to cause the level to remain practically constant regardless of the height of the liquid column. This arrangement permits the use of only one glass column and makes it possible to read the pressure directly on the graduated scale without making any zero adjustment of the scale as is necessary with the U- tube.
3. INCLINED TUBE MANOMETER
This is used to measure very low range of Pressure measurement in Inches of H2O Column and normally used in the workshop facilities for calibration. This is identical to Single limb manometer but is used to measure very small range of pressure or vacuum. This is achieved by magnifying the level
difference. To increase the sensitivity, a less dense liquid may be used. Ranges of measurement using this type of manometer are usually few millimeters of water column.
P1= High Pressure P2= Low Pressure h = Differential Pressure
PRESSURE GAUGES
Pressure Gauges are generally used for Medium & High Range Pressure Measurement applications. In fact, it will be found that in any industrial plant, the quantity of pressure gauges will far out-number the other types of instruments. When local pressure indicators are required in the ranges between 10” H2O column up to 100,000 Mpa, the conventional bourdon, bellows, diaphragm or capsule sensors can be used.The types of Pressure gauges are,
1. Bourdon tube Pressure Gauge 2. Glycerine filled Pressure gauges 3. Bellows Type Pressure Gauge 4. Diaphragms Type Pressure Gauge 5. Capsules Type Pressure Gauge 6. Capillairy type Pressure Gauge
1. BOURDON TUBE PRESSURE GAUGE
There are mainly three types of bourdon tubes available namely
A. C-Type Bourdon Tube B. Spiral Bourdon Tube C. Helical Bourdon Tube
We are normally using stainless steel bourdon tubes although other material of bourdon tubes are used for specific applications in other industries because of their ruggedness, long life and corrosive nature of crude oil. Stainless steel bourdon tubes are used to measure pressures from 0-200 Kg/cm2 to 0-1500 Kg/cm2. Brass bourdon tubes are used to measure pressures from 0-1 Kg/cm2 to 0-75 Kg/cm2.
Bourdon Tube pressure gauges are available with different dial sizes and different connections. We are using normally 4” and 6” dial sizes with ¼” or ½” NPT (M) bottom and back connections. Most of the pressure gauges are fitted with one main isolation valve either ball or gate valve and a block and bleed valve to safely isolate the pressure gauge from service for calibration or replacement.
A. C- TYPE BOURDON TUBE
The bourdon in a pressure gauge is a C- shaped flattened or oval tube,
bent into an arc of about 250 degrees. One end of the tube is fixed onto a fitting
where the pressure to be measured is admitted and the other end is
sealed/brazed. When the applied pressure is increased, the two sides of the
tube are forced apart as a result of increase of the radius of curvature of the
tube.The movement or lift of the closed end of the bourdon tube resulting from the internal pressure change is converted into rotary motion by means of a sector and pinion arrangement. A pointer attached to the extension of the pinion moves on a calibrated dial to read the pressure in the desired units.
The material of the tube should be
Hard enough to withstand the pressure
Stable enough to retain its calibration indefinitely.
Immune to corrosion from the fluid.
Easy to fabricate.
The most common material used for construction of the bourdon tubes are Phosphor bronze, Beryllium Copper, Alloy Steel, Carbon Steel, Stainless Steel and Brass to name a few.
B. SPIRAL AND HELICAL BOURDON TUBES
Spiral bourdon tube is used to measure lower pressure from 0 to 14 bars while helical bourdon tube is used for higher pressure from 0 to 5600 bars. The main advantage of this type over conventional C- type bourdon tube is that it eliminates springs, sector and pinion arrangements thereby increasing the life span of the instrument. This is achieved by increasing the number of turns in the spiral or helical type bourdon tubes. In this way an enlarged movement of the free end of the tube is obtained. The movement of the free end of the tube is transmitted to the pen or pointer through a flexible metal connecting strip, which joins the free end of the tube with the pointer shaft.
2. GLYCERINE FILLED PRESSURE GAUGES
3. BELLOWS TYPE GAUGE
Bellows assembly is often compared to a spring. Available ranges on this type are from 0-5 in.Hg to 0- 3 bar. The material of construction of bellows is 80% Copper- 20% Zinc, Brass, Phosphor Bronze, Beryllium Copper or Stainless Steel.
In actual construction, a thin- walled tube is taken and formed mainly by special hydraulic presses onto a corrugated shape. One end of the bellows is completely sealed and the other end soldered/brazed to a fixture with an opening to apply either pressure or vacuum.
4. DIAPHRAGM TYPE GAUGE
Diaphragm type gauges are used for low- pressure measurement, between 25mm water column and 0.3 bars. Diaphragm seals are used along with C- type bourdon gauges to protect bourdon tubes against corrosive/clogging fluids.
In construction, it consists of a hardened and tempered stainless steel corrugated diaphragm of about 65mm diameter held between the two flanges. Pressure is applied to the underside in the chamber shown, and movement of the center of the diaphragm is transmitted through the ball-and-socket joint and high magnification link to the pointer as in the bourdon gauge.
5. CAPSULE TYPE
GAUGE
This is the most precision type of pressure indicator. This instrument is available in ranges from 0-30 kPa to 0-10 bar. This has a sensitivity of 0.01% of full range and an accuracy of 0.1% of full scale. This instrument may be used to measure both differential pressure and gauge pressure.
In case of differential pressure measurement, the higher pressure is applied to the inside of the capsule and the lower pressure is applied to the pressure-tight case.
In case of gauge pressure measurement, the measured pressure is applied to the capsule and the case is open to atmosphere.
In both cases the meter or pointer movement is similar to diaphragm type gauge which was discussed earlier.
6. DRAFT PRESSURE GAUGE
PRESSURE GAUGE PROTECTION DEVICES
A pressure gauge is one that always requires special attention because the measuring element of the instrument is usually exposed to the fluid whose pressure is to be measured. In the case of pressure measuring elements, the fluid being measured usually fills the measuring system and it is likely to cause the trouble if the installation is not carefully made. Furthermore, the instrument may be subjected to violent pulsations in fluid pressure which can completely destroy the accuracy of the instrument or cut short the life of the instrument.
Basically there are five types of protection of gauges against hot, slurry & corrosive fluids and to protect the gauge sensor from damage, excessive wear, pulsation, oscillating deflections etc. They are,
1. Pressure Snubber
2. Condensing Chamber Type 3. U- Tube Siphon Type 4. Pig-Tail Type 5. Diaphragm Seal Type
1. PRESSURE SNUBBER
This is used as an attachment at the bottom of the pressure gauge Wherever sudden or repetitive changes of pressures are anticipated, pressure snubber or otherwise called pulsation dampener is used. Reciprocating pumps and Compressors are typical examples in our applications where Snubber is used.
Two types are available, one with a fixed throttle and the other with adjustable needle valve throttling. Basically both these types reduce the pulsation thereby eliminating direct impact of the process medium on the measuring element.
2. CONDENSING CHAMBER
The Condensing Chamber type is made out of a 2” pipe welded onto ½” nipples tapped to take the isolation valve of gauges at the top and a blow-down valve below.
3. U- TUBE SIPHON
The U-tube Siphon is generally made of a straight ½” or 3/8” pipe or tube itself to help condensation of hot fluids.
4. PIG-TAIL
The Pigtail type is also generally made of a straight ½” or 3/8” pipe or tube. Both these U-tube Siphon and Pigtail types trap a certain quantity of the condensed fluid always, say water in steam applications.
5. DIAPHRAGM SEAL
The diaphragm seal type is used in such places where impulse line clogging can occur by hot, slurry and corrosive fluids. The interconnecting piping and the space above the diaphragm is normally filled with the silicon oil.
6. GAUGE SAVERS
Gauge Savers also known as over pressure protectors are applicable where pressures exceed the maximum pressure rating of the pressure gauge.
PRESSURE TRANSMITTERS
Transmitter is a device that responds to a measured process value and produces an output that becomes the input to a receiver or a controller in a control room.
Block diagram of a transmitter Pressure
Input Scaled Output signal
4 to 20 mA ( Electrical ) 20 to 100 kPa ( Pneumatic ) Converting pressure to movement
or electrical quantity ( v, A, etc. )
Transducer ( Pressure Element ) Transducer ( LVDT, flapper - nozzle etc. ) Converting movement or electrical quantity
to a scaled output signal
Pressure Transmitters are generally used for Medium & High Range Pressure Measurement applications.
The types of Pressure Transmitters are,
1. Pneumatic Flopper-Nozzle Pressure Transmitters 2. Electronic Pressure Transmitters
3. Diaphragms Type Pressure Transmitters 4. Capillary Type Pressure Transmitters
1. PNEUMATIC FLAPPER - NOZZLE PRESSURE TRANSMITTER
An increase in the measured pressure will move the force bar. The flapper is attached to the force bar by means of a flexure strip (A spring). This movement of the force bar will make the flapper move towards the nozzle. The nozzle backpressure will subsequently increase and this increased nozzle backpressure will be amplified by the relay to produce the output signal. The output signal is also applied to the feedback bellows. As the pressure increases in this bellows, the bellows will apply a force on the bottom end of the range bar. This force makes the range bar to move in the opposite direction to that caused by the force bar. The range bar is also attached to the flexure strip and the movement of the range bar will cause the flapper to move away from the nozzle. During the stable condition of the transmitter, that is, when the process pressure is not changing, the two forces are balanced. Any change in the measured pressure will upset this balance.
The sequence of events that will follow such an upset is as follows.
A change in the measured pressure will cause the forces to become unequal.
This will change the flapper-nozzle relationship.
The nozzle backpressure will change.
The changed nozzle backpressure will be amplified by the relay and will be given as the output and also to the feedback bellow.
The output pressure will now create a new feedback force to counteract the force created by the force bar.
At the balanced condition the flapper-nozzle relationship is such that the output will neither increase nor decrease. This specific position of the flapper with respect to the nozzle is called as the throttle position.
The feedback force is said to be negative because this force is opposite to or opposes the force that is produced by the diaphragm capsule and the force bar. The amount of the pressure required in the feedback bellows to generate sufficient force to counteract the force produced by the feedback bellow would depend upon the following.
The effective area of the bellow (Usually a Constant)
The distance between the bellow and the range wheel (Movable Fulcrum)
It can be seen from the drawing on the next page that as the distance is increased, the mechanical advantage of the range bar will increase and a lesser pressure is required to balance a given force. Conversely, when the distance is decreased, a larger pressure is needed to balance a given force. Changing the mechanical advantage of the feedback mechanism is a convenient means of changing the gain or the span of the transmitter.
A bias spring (reference adjustment) is provided to preload the feedback mechanism to obtain a desired output when the pressure measured is zero. This is the zero adjustment provided by the manufacturer of the device.
2. ELECTRONIC PRESSURE TRANSMITTERS
An electronic-type transmitter is shown in the figure above. This particular type utilizes a two- wire capacitance technique.
Process pressure is transmitted through isolating diaphragms and silicone oil fill fluid to a sensing diaphragm in the center of the cell. The sensing diaphragm is a stretched spring element that deflects in response to differential pressure across it. The displacement of the sensing diaphragm is proportional to the differential pressure. The position of the sensing diaphragm is detected by capacitor plates on both sides of the sensing diaphragm. The differential capacitance between the sensing diaphragm and the capacitor plates is converted electronically to a 4-20 mA dc signal.
3. DIAPHRAGM PRESSURE TRANSMITTERS
4. CAPILLARY PRESSURE TRANSMITTERS
PRESSURE SWITCHES
A pressure switch is an instrument that automatically senses a change in the measured pressure and opens or closes an electrical switching element when a predetermined pressure is reached. In other words, a pressure switch is a digital instrument as compared to other pressure gauges discussed so far, which are analog instruments. Pressure switches have pressure-sensing elements that make a small movement when the measured pressure varies. Most common sensing elements are
Diaphragms
Bourdon tubes
Bellows
The pressure measuring elements (sensors) produce the necessary movement to actuate the electrical switching element. These switches are normally snap acting single pole double throw (SPDT) types. Since they are SPDT switches, they have one normally open (NO) and one normally closed (NC) and one common (C) terminal to which electric power can be connected. The switch is said to change over when the common pole changes from NC to NO. By doing this either the electric power can be connected or disconnected instantly on actuation of the switch.
Applied pressure Set point adj:
Spring support
Spring Snap acting
micro switch
Sensing diaphragm ( rubber ) Protecting diaphragm ( teflon ) Pivot
Beam ( lever )
Piston
Switch button Electrical connections
NO NC COM
Mounting screw
F2 F1
Threaded base
Pressure multiplied by the area of the diaphragm = F1
Spring tension (varied by turning the set point adj.) = F2 When F1 > F2,
The beam actuates the switch button and switch contacts changes over. NO contact becomes closed and NC contact becomes open.
The types of Pressure Switches are,
1. Standard Type Pressure Switches 2. Differential Pressure Switches 3. Diaphragm Type Pressure Switches
1. STANDARD TYPE PRESSURE SWITCH
2. DIFFERENTIAL PRESSURE SWITCH
3. DIAPHRAGM TYPE PRESSURE SWITCH
GLOSSARY OF TERMINOLOGIES RELATED TO PRESSURE SWITCHES
SET POINT
The point or the value at which the switch is actuated is called as the actuation point or the set point. This point (value) is expressed in terms of an appropriate pressure unit (e.g. kPa, bar, psi etc.).
SWITCH DIFFERENTIAL
Due to practical reasons and constructional limitations the switching mechanism will not actuate and re - actuate at the same pressure value. Normally there will be a difference between these two values and this difference is called as the switch differential or dead band.
ACCURACY
Accuracy is defined as the ability of a pressure switch to repetitively operate at its set point. For example if a pressure switch is set to actuate at 100 kPa, repeatedly actuates from 99 kPa to 101 kPa then it is considered to be accurate within1%.
ADJUSTABLE RANGE
It is defined as the pressure range within which the actuation point of a pressure switch can be set.
For example, the adjustable range of a switch is given as 0.5 kPa (g) to 300 kPa (g) then this switch can be set to actuate at any pressure value between 0.5 to 300 kPa (g).
TOLERANCE
Tolerance is the variation that may happen at the re - actuation point for pressure switches. For example, three switches with the same specification have a set point of 100 kPa (g). They all will actuate at the same pressure value of 100 kPa (g). However one pressure switch may re - actuate at 94 kPa (g), another at 95 kPa (g) and the third one at 96 kPa (g).
PROOF PRESSURE
Proof pressure is the highest pressure (including transients) to which a pressure switch may be subjected without damage.
CONTACT RATING
Contact rating is defined as the capacity of the contacts designed to pass the current at the given voltage without burning out the contacts.
AIR PRESSURE REGULATOR
Instrument air enters through the inlet port and then to the drip well. Any dirt or moisture carried along with air will be collected at the bottom of the drip well and can be drained through the drain cock (valve).
A supply cum exhaust valve supported by the inlet valve spring controls the air pressure. The inlet valve spring and the supply cum exhaust valve are housed in spring housing. The valve sub-assembly comprising of parts are separated by a filter element. Practically clean air is available at the supply valve port.
The upper section of the filter regulator comprises of an adjusting screw, a lock nut, spring case, range spring, spring button and a diaphragm sub-assembly. The orifice (exhaust valve seat) at the center of the diaphragm is in contact with the exhaust valve plug.
When the adjusting screw is turned clockwise it compresses the range spring, which applies a definite amount of force on the diaphragm. This closes the exhaust port and pushes down the supply valve to open the supply port admitting the filtered air to pass through the passage in the filter body and then to the outlet port. The air pressure in the outlet port is also communicated to the underside of the diaphragm through the aspirator hole to produce the necessary balancing force to counteract the force generated by the range spring.
When the downward and the upward forces on the diaphragm are equal, the exhaust valve is closed and the supply valve is open to supply the set pressure through the outlet port to the downstream equipment. The outlet pressure is also tapped to a pressure gauge, which is mounted on the regulator to indicate the set pressure.
In case of a decrease in air pressure in the outlet port, the force acting on the under side of the diaphragm will reduce and the spring force will push the supply valve to open the supply port to admit more air to meet the new requirement and the increased pressure will restore the equilibrium condition of the diaphragm assembly.
In case of an increase in the output pressure, the force acting on the under side of the diaphragm will overcome the force generated by the spring. This unbalance in forces will move the diaphragm upwards to make the exhaust valve seat to lift off from the plug to allow the excess air to bleed to the atmosphere through the bleed hole (B) in the spring housing. This process will continue automatically till the forces acting on the diaphragm are in equilibrium. The lock nut on the adjusting screw prevents it from turning due to vibration and not to cause any changes in the set point.
CALIBRATION DEVICES
There are many calibration devices available within PDO or any other industry depending on the range of the pressure measuring device and the location to be used. Calibration devices are used mainly to simulate the pressure required.
CALIBRATION ADJUSTMENTS
Generally instruments are provided with a system of linkages, screws, springs etc. in order to do three basic adjustments namely Zero, Span and Linearity. Hence calibration is required to check and adjust, if necessary, all these adjustments to maintain the reliability of the reading.
Zero adjustment shifts the entire scale up or down by the same amount.
Span adjustment progressively increases or decreases readings over the range, without changing the Zero.
Linearity adjustment speeds up or slows down the calibration at either end of the scale to eliminate intermediate errors.
COMMON CALIBRATION DEVICES
These are the following commonly used devices for calibration of pressure instruments.
1. Dead Weight Tester 2. Hydraulic Oil Pump 3. Pneumatic Hand Pump 4. Pneumatic Vacuum Pump 5. Pneumatic Calibrators
1. DEAD WEIGHT TESTER
USING TESTED GAUGE USING WEIGHT
Pressure gauges are calibrated on patented hydraulic screw pumps, which transmit the oil pressure upon an accurately manufactured piston cylinder arrangement. The oil pressure is increased or decreased by the screw pump and the piston top is loaded with known weights, which can be conveniently added or subtracted. The weights themselves are manufactured in accordance to the exact cross sectional area of the piston. The oil pressure is also fed to a suitable pressure gauge mounting on the tester itself.
With the gauge on the tester, the application of the hydraulic pressure is adjusted, continuously rotating the stack of weights so as to maintain dynamic balance. Thus the pressure gauge can be checked for zero, full-scale and other intermediate points. Necessary zero and range adjustments are done to make the readings uniform on the scale. Alternately a hydraulic screw pump with standard gauge also can be used for calibration.
SAFETY PRECAUTIONS
The following safety precautions are recommended while working with dead weight testers.
Wear Personnel Protective Equipment namely safety shoes, coveralls and gloves.
Pressure to be released slowly and carefully.
Weights to be handled properly.
Use Teflon tapes with pressure gauges for sealing against leaks.
2. HYDRAULIC OIL PUMP
The hydraulic Quick Test Hydraulic Pump is a portable source of hydraulic pressure (up to 14000 kPa) for field calibration. Internal parts are brass, aluminium and stainless steel, compatible with a variety of hydraulic fluids including petroleum-based oils and water.
A transparent fluid reservoir permits a quick visual check of fluid level. The bleed-off valve allows a slow bleed-off of pressure and fluid back to the reservoir.
Included with the pump is a rugged Test Gauge. Gauges are mounted in the same swivel fitting and are easily removable. Gauges available include 0-1000, 1500, 2000 psi (0.5% full-scale accuracy). Also available: 0-70 and 0-140 bar. The pressure probe is connected to the hose with a standard flare connection facilitating replacement with a variety of fittings to meet the requirements.
SAFETY PRECAUTIONS
The following safety precautions are recommended while working with Hydraulic Oil Pumps.
Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves.
Pressure to be released slowly and carefully.
Select the correct range of test gauge.
Use Teflon tapes with pressure gauges for sealing against leaks.
3. PNEUMATIC AIR PUMP OR QUICK TEST AIR PUMP
The Quick Test Air Pump is a portable, hand operated source of air pressure (up to 200 psi) for the use in field calibration of pressure instruments.
Included with the pump is a rugged test Gauge. Gauges are mounted in a quick-change swivel fitting which is easily removable from the pump assembly. This allows you to exchange gauges and match the gauge range with calibration range. Gauges in the following ranges are available: 0-5, 15, 30, 60, 100, 160 and 200 psi (0.5% full-scale accuracy). Also available: 0-1, 2, 4, 7 and 14 bar.
The pressure probe is connected to the hose with a standard flare connection facilitating replacement with a variety of fittings to meet the requirements. A bleed-off valve permits a slow bleed-off of pressure to atmosphere for downside calibration.
SAFETY PRECAUTIONS
The following safety precautions are recommended while working with Pneumatic Air Pumps.
Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves.
Pressure to be released slowly and carefully.
Select the correct range of test gauge.
Use Teflon tapes with pressure gauges for sealing against leaks.
4. PNEUMATIC VACUUM PUMP OR QUICK TEST VACUUM PUMP
The Quick Test Vacuum Pump is a portable lightweight pump, which will generate approximately 23” Hg vacuum. It includes 0.5% full scale accuracy, 0-30” Hg test gauge which makes it a convenient method to field calibrate vacuum instruments.
It is complete with hose, bleed-off and pressure probe. Probe is identical to that discussed with Air and Hydraulic pumps and fits all Quick Test Fittings.
SAFETY PRECAUTIONS
The following safety precautions are recommended while working with Pneumatic Vacuum Pumps.
Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves.
Pressure to be released slowly and carefully.
Select the correct range of test gauge.
Use Teflon tapes with pressure gauges for sealing against leaks.
5. PNEUMATIC CALIBRATOR
This calibrator is a highly accurate, portable instrument, designed primarily for the field checking of pneumatic instruments using non-corrosive gases, mainly compressed air. It is available in several ranges, from as low as 0 to 300 mbar, up to 0 to 7 bars. Basically the Calibrator is a shock-mounted, precision, dial manometer in a robust, suitcase type carrying case. The main component of this Calibrator is the dial manometer. Its pressure-measuring element is a precision C type Capsule, specially formed and heat-treated to minimize any change from aging.
When connected to a source of air pressure, the Calibrator can apply and measure accurately two different pressures using two different regulators through ports P1 and P2. Port P3 is used to measure any pressure within the range, for example, output of a pressure transmitter. The Calibrator can also be used to measure the difference between two pneumatic signals where neither signal exceeds the Calibrator range.
SAFETY PRECAUTIONS
The following safety precautions are recommended while working with Pneumatic Calibrator.
Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves.
Pressure to be released slowly and carefully.
Use clean, dry air of maximum 7 bars.
Use Teflon tapes with pressure gauges for sealing against leaks.
MEASUREMENT - FLOW
TYPE OF FLOW METERS:
Various types of meters are used to measure flow in process industries. The
most important ones are classified as under.
1. Differential Pressure Flow meters
2. Variable Area Flow meters - Rota Meter 3. Force Balance Principle - Target Flow Meter 4. Positive Displacement Meter
5. Turbine Flow Meter
6. Coriolis Principle - Mass Flow Meter 7. Ultrasonic Flow Meter
1. DIFFERENTIAL PRESSURE TYPES:
They are commonly called the DP cell type and are the most widely used flow measuring instruments.
Bernoulli's Principle:-
Bernoulli's Principle states that for an ideal fluid (low speed air is a good
approximation), with no work being performed on the fluid, an increase in velocity occurs simultaneously with decrease in pressure or a change in the fluid's
gravitational potential energy.
This principle is a simplification of Bernoulli's equation, which states that the sum
of all forms of energy in a fluid flowing along an enclosed path (a streamline) is
the same at any two points in that path. It is named after the Dutch/Swiss
mathematician/scientist Daniel Bernoulli. In fluid flow with no viscosity, and therefore, one in which a pressure difference is the only accelerating force, the principle is equivalent to Newton's laws of motion.
Principle of pressure drop:
When a fluid flows through a closed pipeline, whether it is by gravity or by force (by a pump or compressor), there is a gradual pressure drop due to friction. The pressure drop depends on the diameter, length and the rate of flow. If the diameter of the pipe is restricted or reduced, the pressure drop increases because of the increased velocity. (Energy can neither be created can be converted to another). For a standard restriction in the flow-path, the pressure drop varies proportional to the flow.
If the pressure drop can be measured the flow can be calculated. This is the principle of flow measurement by the method of differential pressure.
Flow elements – different types:
There are different types of flow elements, which depend on the principle of
pressure, drop for measuring flow of fluids.
A. ORIFICE PLATE B. VENTURI TUBE C. PITOT TUBE
A.ORIFICE PLATE:
The orifice meter is by far most common type used in flow measurement. The primary measuring element in this case is the orifice plate secured in the line by a suitable holder.
An orifice plate is a flat circular piece of metal (usually stainless steel) that has a precision metering hole (orifice) bored into it. The diameter of this orifice is carefully calculated using a complicated formula. Its purpose is to create a restriction in the line and cause a drop in pressure of the fluid passing through it.
An increase in flow will cause a corresponding increase in pressure drop.
Therefore, the amount of flow passing through the orifice is related to the difference in pressure measured across the orifice. The rate of flow is proportional to the square root of the differential pressure across the orifice.
There are several specific features about an orifice plate that should be noted.
Information on the tab welded to the plate:
There is a tab welded to the top edge of the plate. It has pertinent informations stamped on the upstream side, such as instrument identification tag number and orifice bore diameter. Sometimes the line size and identification number appear also. This information always appears on the upstream side, so that it can be determined if the plate is installed properly after it has been secured in the line.
This is important because special care is used in machining the upstream edge of the orifice. Also, in some sizes the downstream side of the orifice is usually beveled. Should the plate be installed back-wards a large flow error would result.
Weep hole in the orifice plate:
Usually an orifice plate will have a small hole drilled adjacent to the metering hole. This is called a “weep” hole. Its purpose is to pass materials through, that rate apt to be stopped by the orifice plate in the line. A concentric orifice plate has the metering hole in the Centre so that a dam is formed. Without the “weep’
hole, any liquids in a gas line or any vapors in a liquid line can be trapped on the upstream side of the plate. Metering accuracy is reduced when the orifice plate traps any material. Whereas “weep” holes can take care of unwanted liquids or
LINE SIZE
DIRECTION INDICATION TAG NO
BORE DIA
SS 316
LIQUID SERVICE GAS SERVICE
vapors, it is necessary to pull the plate and clean out the line when trash or debris is trapped.
Different shapes of the orifices:
Eccentric or segmental orifice plates are used where excessive amounts of detrimental materials are in the line. The metering hole is offset to be tangent with the bottom of the line to take advantage of the larger opening to permit slurry, debris, etc., to pass on through.
These types of plates are used only when necessary because they are not as accurate as concentric orifice plates. The normal flow pattern of fluid in a line is for the maximum velocity to be in the centre of flow and decrease to zero out to the wall of the pipe. The metering hole design formula is based on the hole being in the middle of the line No adequate correction data are available for ecentric or segmental orifice plates.
Install Orifice plates with tabs “UP”
Because of “weep” holes and ecentric type orifice plates it is imperative that the plate be installed with the metal tab extending straight up.
The tab hole:
A hole usually appears also in the metal tab. This is for the purpose of storing the plate while waiting to be installed. Protruding nails on the walls are used to hang the plates. Since the metering hole is manufactured so precisely, rubbing against a metal object can damage the required sharp edge of the orifice. Therefore, a special hole is provided so that the metering hole will not be used to hang on the nail.
Orifice flanges:
The most common type of orifice flanges are those with the pressure measuring taps drilled into the flange and extended all the way into the pipe. This type of flange permits measuring the pressure in the line both upstream and downstream of the orifice plate.
Meter run:
The orifice plate and flanges and the piping immediately upstream and
downstream are called the meter run.
Although a meter run may be located in either a horizontal or a vertical line,
installation in a horizontal line is preferred. For liquid metering, location of an
orifice plate in a vertical line with down ward flow is not advisable since under
some conditions the liquid can fall free and may not fill the line. However, when
gases contain condensable constituents, downflow installation may be desirable
in that it allows any condensed liquid to be blown through the orifices.
INSTALLATION:
The Transmitter installations for the lquid and gas ervices are shown above.
B. VENTURI TUBE:
The Venturi effect is an example of Bernoulli's principle, in the case of fluid flow through a tube or pipe with a constriction in it. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is supplied by a drop in pressure or a pressure gradient force. The effect is named after Giovanni Battista Venturi, (1746–1822), an Italian physicist.
LIQUID SERVICE GAS SERVICE
Another method of measuring flow using the same D.P.principle is a Venturi tube. The pressure drop across the reduced section can be correlated with flow rate. A Venturi is generally used when the system pressure is low and further pressure drop is undesirable. These are also used in heavier fluids, which might plug up an orifice plate.
C. PITOT TUBE:
A Pitot tube is a pressure measuring instrument used to measure fluid flow
velocity. The Pitot tube was invented by Italian-born French engineer Henri Pitot
in the early 1700s, and was modified to its modern form in the mid 1800s by
French scientist Henry Darcy.
The orifice plate and the Venturi tube were based on the principle of pressure drop. A Pitot tube converts velocity directly into pressure to measure flow. This type is primarily used to measure flows in large diameter pipes (20” and above)
2. ROTAMETERS:
Rota meters are classified as variable orifice meters.
Principe:
It works on the principle of varying the orifice diameter for the same pressure drop.
The common name for this type of instrument is rotameter. In its simplest from it
consists of s “float’ and a calibrated glass tube of varying inside diameter. The
tube has its smallest diameter at the bottom and increases continuously, having the largest diameter at the top. When the “float” is inserted into the tube there is very little clearance at the bottom and more clearance at the top.
With the float in the tube and the assembly mounted vertically the flow enters the bottom of the tube. The float is lifted off its bottom stop and carried up the tube.
The higher it goes the more free passage exists between the float and the tube wall, until the differential pressure above and below the float is just sufficient to support the weight of the float.
The rate of flow is then read off the calibrated markings on the glass tube.
Rota meter are suitable for metering small rates of flow, below the practical range of the orifice meter or for special applications such as very waxy oils, which must be kept moving, to stay in the fluid state. The rotameter may be made considerably more complicated for other applications but its basis of operation remains the same.
3. THE FORCE BALANCE PRINCIPLE - TARGET FLOW METER:-
This type of instrument operates by measuring the force exerted by the moving
fluid, on a disc held centrally in the line by a lever. The lever is connected
through a flexible membrane to a sensor. Movement of the lever actuates
sensor assembly generating 4 to 20 ma signal, which is fed back to the lever until
balance occurs.
This type of instrument is comparatively inexpensive, but since part of the instrument itself is in the fluid stream, valves for isolation and bypass are required in order to service it. Further, the instrument itself is in close proximity to the line, which may be kept cool.
4. POSITIVE DISPLACEMENT TYPE:
When positive displacement meters are used, the entire flow must pass through
the meter, filling a calibrated chamber on its path through. The number of times
the chambers is filled is counted and displayed as a total volume having passed
through the meter. The cost of this type meter goes up rapidly with the size of the
pipeline and meters for high-pressure service are very expensive.
5. TURBINE FLOW METER:
The turbine meter has a limited range of uses in refineries as it is either very simple in construction or subject to inaccuracies or it is rather sophisticated in design and construction and expensive. The turbine meter employs a multi- vaned rotor mounted in an in-line housing. The total flow passes through the meter and causes the rotor to spin at a speed, which is proportional to the velocity of the flow. In its simplest form the rotor motion is transmitted to an external gear train, which operates a simple counting mechanism. The counter indicates the total flow that has passed through the meter. In its more complex form the rotations are electronically measured and converted to a flow reading.
6. MASS FLOW METER:-
DRIVE COILS MASS BAR TORSION ROD
PICK-UP COILS TUBE LOOPS
ELECTRICAL
JUNCTION BOX FLANGE
CONNECTIONS
CORIOLIS FORCE:-
A person standing on a rotating turntable was only to lean slightly inward to counteract centrifugal force (left). A person moving from the centre of the rotation towards the edge of the turntable (Right), encounter steadily increasing rotation speed and inertia comes in to play has to overcome a force known as coriolis force. This coriolis force acts to deflect the person from the shortest route from the turntable.
The coriolis force is therefore proportional to the moving Mass (m), Angular velocity (w) and Radial Velocity (Vr) in the rotating system. The rotary motion described above that generates the coriolis force is replaced in the flow meter by exciting the measuring tube to oscillate at its resonance frequency.
At zero flow, when the fluid is at a standstill, there is no linear movement hence
no coriolis force occurs. Once the mass is flowing, the movement induced by
oscillation in the measuring tube superimposes itself on the linear movement of
the flowing fluid. The coriolis force causes the measuring tube to twist.
CORIOLIS FORCE IN MASS FLOW METER:-
Fluid mass has a linear velocity as it flows through the sensor tube.
Vibration of the tube, as its natural frequency about an axis generates an angular velocity. These vibrational tube forces, proportional to fluid flow, cause the fluid to Accelerate on the inlet side and Decelerate on the outlet side. The fluid exerts an opposing force at its own whish resists the proportional tube forces, causing the tube to twist. A et of pick-up coils is mounted on either side of the tube.
Electronic unit measures this very small twist force induced by the flowing process fluid on the vibrating sensor tube. The fluid force is proportional to the mass flow rate.
5. ULTROSONIC FLOW METER:
END VIEW OF FLOW TUBE SHOWING TWIST TWIST ANGLE
TWIST ANGLE
VIBRATING FLOW TUBE
FLUID FORCES REACTING TOVIBRATING FLOW TUBE
FLOW METER SELECTION
METER TYPE
CONCENTRIC ORIFICE
PLATE (SQUARE
EDGED)
SEGMENTAL
WEDGE VENTURI
TUBE FLOW
NOZZLE
TARGET METERFLOW
VARIABLE AREAFLOW METER
MAGNETIC METERFLOW
TURBINE METERFLOW
CORIOLIS TYPE MASS
METERFLOW
CLEAN LIQUID DIRTY LIQUID SLURRY VISCOUS LIQUIDS CORROSIVE / EROSIVE LIQUID CLEAN GAS DIRTY GAS STEAM FULL BORE SIZE AVAILABILITY
> 1 in.
>25 mm
> ½ in
>15 mm
> 2 in.
>50 mm
> 2 in.
>50 mm
> 1/2 in
>15 mm
< 3 in.
<75 mm
> 1/16 in
< 1 mm
> 1/4 in
>6 mm
< 6 in
<150 mm TYPE OF
MEASUREMENT
Square Root Volumetric
Square Root Volumetric
Square Root Volumetric
Square Root Volumetric
Square Root Volumetric
Linear Volumetric
Linear Volumetric
Linear Volumetric
Linear Mass Volumetric ACCURACY 0.5% to 3% 0.5% to 5% 0.5% to 1.5% 1% to 2% 0.5% To 5% 0.5% To 5% 0.2% To 2% 10:1 To 50:1 0.15% To 2%
TYPICAL RANGE
ABILITY 3:1 to 5:1 3:1 to 5:1 3:1 to 5:1 3:1 to 5:1 3:1 To 20:1 10:1 30:1 To 100:1 10:1 To 50:1 40:1 To 100:1 REYNOLDS NO.
OTHER LIMITATION
>10,000 >500 >100,000 >75,000 >1,000 Fluids under
3 Cp None Fluids under
10 CST NONE
SENSITIVITY TO INSTALLATION EFFECTS
High LOW LOW MED HIGH NONE LOW HIGH NONE
STRAIGHT PIPING REQUIREMENT
100 to 400 Up
20 to 60 Down 50 to 100 p
20 to 50 Down Upstream Runs Shorter than Orifice by Factor of 2-9 limes
Same as for
Orifice Plate Same as for
Orifice Plate None 5 To 100 Up
3D Down Similar to
Orifice Plate NONE
TYPICAL ANCILLARY EQUIPMENT
Drain. Vent, Blow-off and Shut-down Valves
with Remote
Seal Version Same as Orifice, Air Purge and ventcleaners on dirty liquid
Same as Orifice Plate
-- --
Jock Valves May Be Required to Isolate Meter for Servicing
Strainers, Filters. Air Eliminators.
Steam Traps May Be required
Special Supports May Berequired for Meter
OTHER
CONSIDERATIONS
Pressure and/or Temperature Compensation May be Required
Same as
Orifice plate Same as
Orifice plate Same as
Orifice plate Viscosity Affects Performance Below Critical Rd.
--
AC Designs May Give Better Performance On Some Slurries
Viscosity Can Affect Performance
Entrained Air May Cause Problems
PREFERRED METER ORIENTATION
Pressure Tap Orientation Depends on Pipe Orientation and Fluid Being Metered
No limitations onRemote Seal Elements
Same as
Orifice Plate Same as
Orifice Plate Zero May NeedAdjustment in Vertical Installations
Can Only Be Installed In Vertical Pipe with Flow Up
Electrodes Must Be In Horizontal Plane. Flow Should Be Up ard In Vertical
Some Designs Must Be Oriented as Calibrated
Specific Orientations Vary With Meter Designs
INITIAL COST L To H H M To H M To H L To M L To M M To H L To H H
INSTALLATION
COST M To H L M M L L L To M M To H L To M
MAINTENANCE
COST M To H L L L M To H L L To M M To H L
OPERATING
COST M To H L To M L M M M L M L To H
PERFORMANCE STABILITY
Performance Affected by edge and Tap
Wear GOOD GOOD GOOD
Performance Affected by Wear of
Target GOOD GOOD
Performance Affected by Wear of Bearings and Other parameters
GOOD
STANDARDS OR RECOMMENDED PRACTICES
AGA3, ANSI/API 2530ANSI/ASME MFC 3M. ISO 5167 ASME Fluid Meters
...
AGA3, ANSIIAPI2530 ANSIIASME MFC 3M. ISO 5167 ASME Fluid Meters
AGA3, ANSIIAPI2530 ANSIIASME MFC 3M. ISO 5167 ASME Fluid Meters
... ... ISO 6817
AGA 7, API 2534, ISO 2715 ASME Fluid Meters, API Manual for Petroleum Measurement Standards
ANSI/AS ME MFC 11M, California Weights and Standards Bureau
TEMPERATURE INSTRUMENTS:
There are two type of temperature measuring instrument in general process use:
1. Non – electrical thermometers 2. Electrical thermometers
1. NON – ELECTRICAL THERMOMETERS:
The most important classes of non electrical thermometers are the expansion type:
A. Liquid expansion B. Gas expansion C. Vapor pressure
D. BI - Metallic (solid expansion)
A. LIQUID EXPANSION THERMOMETERS:
These thermometers are made in a variety of forms from the common glass tube thermometer to the more elaborate remote recorder controller. The glass tube thermometer is common enough to require no explanation. The more elaborate type of liquid expansion thermometer is made up of a bourdon tube connected to a metal bulb by small bore metal up of a bore metal tubing called ‘Capillary tubing’, the whole being filled with a suitable liquid. A completely sealed system is formed. The size of the bulb depends on the range of the instrument. To avoid troubles with vapor pressure effects, the systems are filled and sealed at a very high pressure.
When the bulb is heated the liquid expands, this expansion is transmitted through the capillary and in to the bourdon tube. The increase in volume causes the bourdon tube to deflect in a similar manner to the pressure gauge. This deflection is amplified by linkage and it causes a pointer to sweep over a calibrated scale indicating the temperature at the bulb is so large that temperature change of the tube will give only negligible effect on the indicated temperature.
B. GAS EXPANSION & VAPOR PRESSURE THERMOMETER:
The operation of these types is very similar to that described above, Except that gas or vapor is used instead of liquid.
Care of expansion type Thermometers:
The capillary tubing used in these instruments has a bore approximately 0.01”
and it may or may not armored. It must never be bent at a short radius or cut.
These instruments are susceptible to permanent damage by “Over – ranging”
and this must be avoided.
C. BI-METALLIC THERMOMETERS:
Bi-metallic thermometers are widely used for local temperature indication. A bi- metal strip consists of two metals welded has an expansion co-efficient much greater that the other. With one end of the helix fixed and the other left free, the latter can be made to give a rotary movement to a spindle when heat is applied.
Attaching a pointer to the spindle and adding a scale may make thermometers.
2. ELECTRICAL THERMOMETERS
These are two main types of electrical measuring instruments in general use and two types of temperature sensitive elements used with these instruments. The more common is the thermocouple used with a potentiometer type instrument the second one is a resistance bulb used with a Wheat-stone bridge type instrument.
A. THERMOCOUPLES:
PRINCIPLE:
A thermocouple is made up of two dissimilar metal wires jointed at one end (known as hot junction) and connected to a potentiometer at the other end (Cold junction). If heat is applied to the hot junction a mill volt is generated which is proportional to the difference in temperature between the hot and the cold junction may be determined.
TYPES OF THERMOCOUPLES FOR DIFFERENT TEMPERATURE RANGES:-
The three most common types of thermocouples are iron-constantan, Chromel- Alumel, and Copper-Constantan.
An Iron-Constantan thermocouple is one made by joining a pure iron wire and a constantan (an alloy of copper and nickel) wire. It is generally used in temperature measurement in the minus 0 to 200
oC range. Iron-constantan is the most efficiency thermocouple commonly used in that its emf or voltage output is higher (10.779 millivolts at 200
oC).
A Copper-Constantan thermocouple is one made by joining a pure Copper wire and a Constantan wire. It is generally used to measure temperatures that are below O
oC and up to 200
oC. As a comparison it has a 9.288 millivolt output at 200
oC through it is seldom used at this high a temperature.
A Chromel-Alumel thermocouple is one made by joining a Chromel (an alloy of
Nickel, Manganese, Aluminium, and Silicon) wire. It is generally used to measure
temperatures that are over 200
oC up to about 1000
oC. As a comparison it has a 8.138 millivolt output at 200
oC through it is seldom used at this low a temperature.
Each of the three types of thermocouples covered here has unique features that dictate their use. Though Ion-Constantan is the preferred thermocouple because of its high efficiency pure iron wire is easily damaged by extreme heat or cold. It oxidizes and deteriorates rapidly at high temperatures (above 650
oC) and becomes extremely brittle at low temperatures approaching-15
oC. therefore, a Chromel-Alumel thermocouple is used at high temperatures because it holds up well up to 1000
oC, and a Copper-Constantan thermocouple is used at low temperatures because it stands up well down to – 150
oC.
For temperatures above 1000
oC, special thermocouples of Platinum-Platinum Rhodium are used.
TYPE LEG METALLIC COMPOSITION
MELTING POINT USABLE PRACTICAL TEMPERATURE RANGE
° C
B + PLATINUM - 30% RHODIUM
1825 200 TO 1680 ° C
- PLATINUM - 6% RHODIUM
C * + (TUNGSTEN - 5% RHENIUM)
2480 0 TO 2300 ° C
- (TUNGSTEN - 26% RHENIUM)
E + CHROMEL
1220 -200 TO -450° C
- CONSTANTANT
J + IRON
1220 0 TO 400° C
- CONSTANTANT
K + CHROMEL
1400 -200 TO 1000° C
- ALUMEL
N + NICROSIL
1340 0 TO 1000° C
- NISIL
R + PLATINUM - 13% RHODIUM
1770 200 TO 1500° C
- PURE PLATINUM
S + PLATINUM - 10% RHODIUM
1770 200 TO 1500° C
- PURE PLATINUM
T + COPPER
1080 -270 TO 200° C
- CONSTANTAN,
