Pressure Measurement in Petrochemical Industries

Volume 1 Pressure measurement in Petrochemical Industries

A.Gowthaman

ME,MBA

Mail : [email protected] [email protected]

Gowtham Books

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Volume 1 Pressure measurement in Petrochemical Industries

A.Gowthaman

ME,MBA

Mail : [email protected] [email protected]

Gowtham Books

2 of 90

Gowtham Books

3 of 90

INSTRUMENTATION BOOKS SERIES

Volume 1

Pressure and Flow measurement in Petrochemical Industries

Volume

Temperature and Level measurement in Petrochemical Industries

Volume 3

Analytical Instrumentation in Petrochemical Industries

Volume 4

Control Valves Part I

Volume 5

Control Valves Part II

Volume 6

Control Valves Design

Volume 7

Digital Controllers

Volume 8

Distributed Control Systems

Volume 9

Programmable Logic Controller

Volume 10

Supervisory Control and Data Acquisition System

Volume 11

Vibration Systems

Volume 12

Interview Questions

Volume 13

Instrumentation in Process Industry

Volume 14

Logic Distributed Control Systems

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PRESSURE MEASUREMENT INTRODUCTION

Measurement of pressure, vacuum and differential pressure is very important in Oil companies, chemical processing and manufacturing industries. In general, pressure gauges will exceed any other type of instruments. A wide range of pressures, from high vacuum to pressure of 40,000 kPa or even greater must be measured and controlled accurately and reliably in process industries. Because of its wide range, several pressure-measuring elements are required. In all these pressure measuring elements since the fluid being measured usually fills the measuring system, care should be taken in the correct installation, selection of the type of measuring element, pulsation in the fluid pressure, corrosive nature of the fluid etc. Pressure is defined as, “ The normal force per unit area exerted on a imaginary or real plane surface in a fluid or a gas”.

Pressure = Force /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 (14.7 psi).

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 is the sum of gauge pressure and atmospheric pressure. Absolute pressure = Gauge pressure + Atmospheric pressure ABSOLUTE 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. DIFFERENTIAL PRESSURE :

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. VACUUM :

GRAPHICAL REPRESENTATION OF COMMON PRESSURE TERMS Pressure Pascal Pascal ( Absolute Pressure )

( Pressure Difference )

Pascal ( Gauge Pressure )

Atmospheric pressure ( 101. 325 kPa A )

Zero Gauge pressure Pascal ( suction )

Pascal ( Absolute Pressure )

Absolute Zero Pressure ( Perfect Vacuum )

STANDARD ENGINEERING UNITS AND THEIR INTER-CONVERSIONS psi(lb/in2)

Kg/cm2

kPa

Bar

In. H2O

mm of 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

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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



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1

MANOMETERS

Monometers are generally used for Low Range Pressure Measurement applications. The types of Manometers are,

1. U Tube manometer. 2. Single limb manometer. 3. Inclined tube manometer.

1.1 U TUBE MANOMETER

This is used to measure low range of Pressure measurement in Inches of H 2O 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. A glass U-tube is partially filled with liquid, and both ends are initially open to the atmosphere. When a gage pressure P2 is to be measured, it is applied to the top of one of the columns and the top of the other column remains open. When the liquid in the tube is mercury, for example, the indicated pressure h is usually expressed in inches (or millimeters) of mercury.

P1= HIGH PRESSURE P2= LOW PRESSURE h = DIFFERENTIAL PRESSURE

To convert to pounds per square inch (or kilograms per square centimeter), P2 = dh where P2 = pressure, psig (kg/cm2) 3 3 d = density, lb/in  (kg/cm ) h = height, inches (cm) 3

For mercury, the density is 0.490 lb/in  at 60◦F (15.6◦C), and the conversion of inches of mercury to pounds per square inch becomes P2 = 0.490h The density of water at 60◦F (15.6◦C) is 0.0361 lb/in3, and if water is used in a manometer, the conversion of inches of water to pounds per square inch becomes P2 = 0.0361h The same principles apply when metric units are used. For example, the density of mercury at 15.6◦ C (60◦F) may also be expressed as 0.0136 kg/cm3, and the conversion of centimeters of mercury to kilograms per square centimeters P2 = 0.0136h For measuring differential pressure and for static balance, P2 − P1=dh

1.2 SINGLE LIMB MANOMETER

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. The ratio of diameters is important and should be as great as possible to reduce the errors resulting from the change in level in the large-diameter well. The pressure difference can be read directly on a single scale. For static balance, P2 – P1 =d ( 1 +



) h



where A1 = area of smaller-diameter leg A2 = area of well If the ratio of A1/A2 is small compared with unity, then the error in neglecting this term becomes negligible, and the static balance relation becomes P2 − P1 = dh

1.3 INCLINED TUBE MANOMETER

This is used to measure very low range of pressure measurement in Inches of H 2O 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.

This produces a longer scale so that h = L sin α

2

PRESSURE GAUGES

Pressure Gauges are generally used for Medium & High Range Pressure Measurement applications. 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. Capillary type Pressure Gauge

2.1

BOURDON TUBE PRESSURE GAUGE

There are mainly three types of bourdon tubes available namely C-TYPE BOURDON TUBE , SPIRAL BOURDON TUBE, 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 0200 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.

2.1.1

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.

2.1.2

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.

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2.2

GLYCERINE FILLED PRESSURE GAUGES

Liquid-filled pressure gauges provide a number of advantages: The liquid absorbs vibration and pressure spikes. The dampening action of the liquid enables the operator to take reading during conditions of rapid dynamic loading and vibration. The liquid lubricates all moving elements, dramatically reducing wear in the movement. Because most liquid-filled gauges are filled with non-aqueous liquid and hermetically sealed, they perform in corrosive environments and are immune to moisture penetration and icing.

2.3

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.

2.4

DIAPHRAGM TYPE GAUGE

Diaphragm type gauges are used for l ow 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. A view facing the bottom of the flange reveals the thin metal isolating diaphragm keeping process fluid from entering the gauge mechanism. Only inert fill fluid occupies the space between this diaphragm and the gauge’s bourdon tube. The following illustration shows how the fill fluid transfers process fluid pressure to the gauge’s bourdon tube element while isolating that bourdon tube from the process fluid (shown here inside a pipe).

2.5

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.

2.6

CAPILLAIRE TYPE PRESSURE GAUGE

This is a Diaphragm seal type pressure instrument in which liquid as a pressure transmitting media is filled between a diaphragm seal parts and bourdon tube as an element. Diaphragm and the lower flange as wetted parts can be selected according to applications, so these instruments are appropriate for the measurement of highly corrosive fluid, high viscosity fluid, fluid which contains solid materials or fluid to be easily solidified. Because the high-corrosion resistant diaphragm can be used at the pressure receiving portion, this pressure gauge can be used for the measurement of highly corrosive measuring fluid. For a pressure gauge in which a diaphragm is attached by welding, the surface of a diaphragm can be easily cleaned (by loosening the casing bolts.). A zero-adjusting pointer has been applied, so calibration required due to errors of temperature, elevation, etc. can be easily preformed. With the application of a welded diaphragm the application for leakage of filled liquid has been decreased. (diaphragms made of some materials are excluded.)

The only difference between this chemical-seal gauge and a remote-seal gauge is the small diameter capillary tubing used to connect the gauge to a remote diaphragm. An illustration showing the internals of a remote seal system appears here.

3

PRESSURE TRANSMITTERS

Transducer and transmitter are sometimes used interchangeably but there is a difference between these two.

Force

Output Motion ( Mechanical ) Resistance, Capacitance

Input

Inductance ( Electrical ) Block diagram of a transducer

Transducer is the part that is in physical contact with the process fluid and produces a response or a change in motion or force when the measured process value changes. The types of transducers are

Pneumatic transmitter Potentiometric Pressure Transducers Capacitive Pressure Transducers LVDT Pressure Transducers Piezoelectric Pressure Transducers Optical Pressure Transducers

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.

Transducer ( Pressure Element )

Transducer ( LVDT, flapper - nozzle etc. )

Converting pressure to movement

Converting movement or electrical quantity

or electrical quantity ( v, A, etc. )

to a scaled output signal

Pressure Scaled Output signal

Input

4 to 20 mA ( Electrical ) 20 to 100 kPa ( Pneumatic ) Block diagram of a transmitter

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3.1 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 b ecome 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.

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3.2 POTENTIOMETRIC PRESSURE TRANSDUCERS

The Potentiometric pressure sensor provides a simple method for obtaining an electronic output from a mechanical pressure gauge. The device consists of a precision potentiometer, whose wiper arm is mechanically linked to a Bourdon or bellows element. The movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit.

The mechanical nature of the linkages connecting the wiper arm to the Bourdon tube, bellows, or diaphragm element introduces unavoidable errors into this type of measurement. Temperature effects cause additional errors because of the differences in thermal expansion coefficients of the metallic components of the system. Errors also will develop due to mechanical wear of the components and of the contacts. Potentiometric transducers can be made extremely small and installed in very tight quarters, such as inside the housing of a 4.5-in. dial pressure gauge. They also provide a strong output that can be read without additional amplification. This permits them to be used in low power applications. They are also inexpensive. Potentiometric transducers can detect pressures between 5 and 10,000 psig (35 KPa to 70 MPa). Their accuracy is between 0.5% and 1% of full scale, not including drift and the effects of temperature.

3.3 CAPACITANCE PRESSURE TRANSDUCERS

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.

Capacitance pressure transducers were originally developed for use in low vacuum research. This capacitance change results from the movement of a diaphragm element. The diaphragm is usually metal or metal-coated quartz and is exposed to the process pressure on one side and to the reference pressure on the other. Depending on the type of pressure, the capacitive transducer can be either an absolute, gauge, or differential pressure transducer.

Stainless steel is the most common diaphragm material used, but for corrosive service, highnickel steel alloys, such as Inconel or Hastelloy, give better performance. Tantalum also is used for highly corrosive, high temperature applications. As a special case, silver diaphragms can be used to measure the pressure of chlorine, fluorine, and other halogens in their elemental state. In a capacitance-type pressure sensor, a high-frequency, high-voltage oscillator is used to charge the sensing electrode elements. In a two-plate capacitor sensor design, the movement of the diaphragm between the plates is detected as an indication of the changes in process pressure.

As shown in Figure the deflection of the diaphragm causes a change in capacitance that is detected by a bridge circuit. This circuit can be operated in either a balanced or unbalanced mode. In balanced mode, the output voltage is fed to a null detector and the capacitor arms are varied to maintain the bridge at null. Therefore, in the balanced mode, the null setting itself is a measure of process pressure. When operated in unbalanced mode, the process pressure measurement is related to the ratio between the output voltage and the excitation voltage.

Single-plate capacitor designs are also common. In this design, the plate is located on the back side of the diaphragm and the variable capacitance is a function of deflection of the diaphragm. Therefore, the detected capacitance is an indication of the process pressure. The capacitance is converted into either a direct current or a voltage signal that can b e read directly by panel meters or microprocessor-based input/output boards.

Capacitance pressure transducers are widespread in part because of their wide rangeability, from high vacuums in the micron range to 10,000 psig (70 MPa). Differential pressures as low as 0.01 inches of water can readily be measured. And, compared with strain gage transducers, they do not drift much. Better designs are available that are accurate to within 0.1% of reading or 0.01% of full scale. A typical temperature effect is 0.25% of full scale p er 1000¡ F.

Capacitance-type sensors are often used as secondary standards, especially in lowdifferential and low-absolute pressure applications. They also are quite responsive, because the distance the diaphragm must physically travel is only a few microns. Newer capacitance pressure transducers are more resistant to corrosion and are less sensitive to stray capacitance and vibration effects that used to cause "reading jitters" in older designs. See Flow measurement book for more details.

3.4 LVDT PRESSURE TRANSDUCERS

Figure illustrates the use of a linear variable differential transformer (LVDT) as the working element of a pressure transmitter. The LVDT operates on the inductance ratio principle. In this design, three coils are wired onto an insulating tube containing an iron core, which is positioned within the tube by the pressure sensor.

Alternating current is applied to the primary coil in the center, and if the core also is centered, equal voltages will be induced in the secondary coils (#1 and #2). Because the coils are wired in series, this condition will result in a zero output. As the process pressure changes and the core moves, the differential in the voltages induced in the secondary coils is proportional to the pressure causing the movement. This voltage is amplified and rectified by an AC-DC Converter. The output of this AC-DC converter is a current signal proportional to the input voltage and hence proportional to the applied pressure. The output is 4-20 mA in most of applications.

LVDT-type pressure transducers are available with 0.5% full scale accuracy and with ranges from 0-30 psig (0-210 kPa) to 0-10,000 psig (0-70 MPa). They can detect absolute, gauge, or differential pressures. Their main limitations are susceptibility to mechanical wear and sensitivity to vibration and magnetic interference.

Reluctance is the equivalent of resistance in a magnetic circuit. If a change in pressure changes the gaps in the magnetic flux paths of the two cores, the ratio of inductances L1/L2 will be related to the change in process pressure (Figure 3-10B). Reluctance-based pressure transducers have a very high output signal (on the order of 40 mV/volt of excitation), but must be excited by ac voltage. They are susceptible to stray magnetic fields and to temperature effects of about 2% per 1000¡ F. Because of their very high output signals, they are often used in applications where high resolution over a relatively small range is desired. They can cover pressure ranges from 1 in. water to 10,000 psig (250 Pa to 70 MPa). Typical accuracy is 0.5% full scale.

3.5 PIEZOELECTRIC PRESSURE TRANSDUCERS

When pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the force applied. The fundamental difference between these crystal sensors and static-force devices such as strain gages is that the electric signal generated by the crystal decays rapidly. This characteristic makes these sensors unsuitable for the measurement of static forces or p ressures but useful for dynamic measurements.

Piezoelectric devices can further be classified according to whether the crystal's electrostatic charge, its resistivity, or its resonant frequency electrostatic charge is measured. Depending on which phenomenon is used, the crystal sensor can be called electrostatic, piezoresistive, or resonant. When pressure is applied to a crystal, it is elastically deformed. This deformation results in a flow of electric charge (which lasts for a period of a few seconds). The resulting electric signal can be measured as an indication of the pressure which was applied to the crystal. These sensors cannot detect static pressures, but are used to measure rapidly changing pressures resulting from blasts, explosions, pressure pulsations (in motors, engines, compressors) or other sources of shock or vibration. Some of these rugged sensors can detect pressure events having "rise times" on the order of a millionth of a second.

3.6 OPTICAL PRESSURE TRANSDUCERS

Optical pressure transducers detect the effects of minute motions due to changes in process pressure and generate a corresponding electronic output signal. A light emitting diode (LED) is used as the light source, and a vane blocks some of the light as it is moved by the diaphragm. As the process pressure moves the vane between the source diode and the measuring diode, the amount of infrared light received changes.

The optical transducer must compensate for aging of the LED light source by means of a reference diode, which is never blocked by the vane. This reference diode also compensates the signal for build-up of dirt or other coating materials on the optical surfaces. The optical pressure transducer is immune to temperature effects, because the source, measurement and reference diodes are affected equally by changes in temperature. Moreover, because the amount of movement required to make the measurement is very small (under 0.5 mm), hysteresis and repeatability errors are nearly zero. Optical pressure transducers do not require much maintenance. They have excellent stability and are designed for long-duration measurements. They are available with ranges from 5 psig to 60,000 psig (35 kPa to 413 MPa) and with 0.1% full scale accuracy.

3.7 CALIBRATION OF PRESSURE MEASURING INSTRUMENTS

Pressure instrument calibration is the process of adjusting the instruments output signal to match a known range of pressures. All instruments tend to drift from their last setting. This is because springs stretch, electronic components undergo slight changes on the atomic level, and other working parts sag, bend, or lo se their elasticity. Basic calibration procedure includes zero, span, and linearity adjustments. Proper calibration provides the desired beginning and ending pressures, and produces an output signal that is proportional to the process pressure. Calibration of the instrument is carried out by applying to it an air or liquid pressure whose value is accurately known. The method used depends upon the range of the instrument.

Calibration of Low Pressure Gauges

For very low pressures, upto 3 inch of water gauge, an inclined water gauge may be used. For pressure upto 144 in. of water gauge or 12 in. of mercury gauge, a water manometer may be used, although at the upper part of this range a mercury manometer becomes more manageable. For higher pressures upto 72 in. mercury gauge, a mercury manometer may be used. For still higher pressures, a dead-weight piston tester or h ydraulic pumps must be used. Calibration of Pressure Transmitters

Typical tools required:

24 VDC power supply Multimeter digital Pneumatic hand pump (up to 600 psig) / Hydraulic hand pump (up to 10.000 psig) / Low pressure hand pump / High precision digital test gauge HART communicator Screwdriver toolkit.

Isolate the pressure transmitter and bleed off pressure as per the site Lockout/Tagout Procedure. Do not perform any adjustments until all "as found" calibration data has been recorded. Note any deviation from this procedure in the remarks section of th e calibration form. 1. Make connections between the pressure source, pressure standard, and th e pressure transmitter sensing line. 2. Connect the digital multimeter to the current output of the transmitter. 3. Turn on the transmitter and allow the internal components to reach normal operating temperature 4. Using a dead-weight tester or a portable regulated air supply, exercise the transmitter from zero to full scale and back to zero. 5. Adjust the pressure (vacuum) source to 0, 25, 50 , 75,and 100% of the calibration range and record the following at each data point a. Digital pressure standard reading b. Current output 6. Adjust the zero to set exactly 4mA output, indicated on a precision meter. 7. Supply 100% pressure to the instrument. 8. Adjust the span to get a reading of exactly 20 mA. 9. Repeat steps 6 to 8 till no adjustment is required. 10. Apply 50% pressure (half-scale input pressure). 11. Adjust the linearity to bring the output signal within the specified tolerance of 12 mA (halfscale output current).

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In most instruments, adjusting the span may throw off the zero adjustment or vice-versa. Therefore, it needs to repeat the entire calibration procedure several times to achieve a properly balanced calibration. Disconnect the test equipment from the switch under test. Complete the calibration form and affix the applicable calibration label. If all checks are within tolerance, restore the pressure switch to operational condition and notify appropriate personnel of work performed.

Calibration Setup with HART configurator.

Calibration Record Sample sheet

3.8 Rosemount 3051 Pressure Transmitter

The Rosemount 3051C Coplanar™ design is offered for Differential Pressure (DP), Gage Pressure (GP) and Absolute Pressure (AP) measurements. The Rosemount 3051C utilizes Emerson Process Management capacitance sensor technology for DP and GP measurements. Piezoresistive sensor technology is utilized in the Rosemount 3051T and 3051CA measurements. The major components of the Rosemount 3051 are the sensor module and the electronics housing. The sensor module contains the oil filled sensor system (isolating diaphragms, oil fill system, and sensor) and the sensor electronics. The sensor electronics are installed within the sensor module and include a temperature sensor (RTD), a memory module, and the capacitance to digital signal converter (C/D converter). The electrical signals from the sensor module are transmitted to the output electronics in the electronics housing. The electronics housing contains the output electronics board, the local zero and span buttons, and the terminal block. The basic block diagram of the Rosemount 3051CD is illustrated below.

For the Rosemount 3051C design pressure is applied to the isolating diaphragms, the oil deflects the center diaphragm, which then changes the capacitance. This capacitance signal is then changed to a digital signal in the C/D converter. The microprocessor then takes the signals from the RTD and C/D converter calculates the correct output of the transmitter. This signal is then sent to the D/A converter, which converts the signal back to an analog signal and superimposes the HART signal on the 4-20 mA output.

4-20 mA HART wiri ng

Rosemount 3051 Pressure Transmitter Exploded View

Process Connection

Transmitter selection guide

3.9 Yokogawa Pressure transmitters

Ordering Information

4

SPECIAL APPLICATION.

4.1

DIAPHRAGM PRESSURE TRANSMITTERS

4.2

CAPILLARY PRESSURE TRANSMITTERS

The following photograph shows a Rosemount model 1151 electronic pressure transmitter equipped with a remote sealing diaphragm. Here we may see the coiled metal (“armor”) sheath protecting the capillary tube from damage.

A close-up view of the sealing diaphragm shows its corrugated design, allowing the metal to easily flex and transfer pressure to the fill fluid within the capillary tubing.

Just like the isolating diaphragms of the pressure-sensing capsule, these remote diaphragms need only transfer process fluid pressure to the fill fluid and (ultimately) to the taut sensing element inside the instrument. Therefore, these diaphragms perform their function best if they are designed to easily flex. This allows the sensing diaphragm to provide the vast majority of the opposing force to the fluid pressure, as though it were the only spring element in the fluid system.

The connection point between the capillary tube and the transmitter’s sensor capsule is labeled with a warning never to disassemble, since doing so would allow air to enter the filled system (or fill fluid to escape from the system) and thereby ruin its accuracy.

In order for a remote seal system to work, the hydraulic “connection” between the sealing diaphragm and the pressure-sensing element must be completely gas-free so there will be a “solid” transfer of motion from one end to the other23. For this reason, the capillary system must remain perfectly sealed at all times! Breaching this seal, even for just a brief moment, will ruin the system. A protective measure visible in this photograph is the orange compound painted on the screw head and on the capillary tube connector. This is simply a visual indication that the factory seal has never been broken, since any motion of the screw or of the tube connector would crack the brittle orange compound and betray the breach. A potential problem with using remote diaphragms is the hydrostatic pressure generated by the fill fluid if the pressure instrument is located far away (vertically) from the process connection point. For example, a pressure gauge located far below the vessel it connects to will register a greater pressure than what is actually inside the vessel, because the vessel’s pressure adds to the hydrostatic pressure caused by the liquid in the tubing.

This pressure may be calculated by the formula ρgh or γh where ρ is the mass density of the fill liquid or γ is the weight density of the fill liquid. For example, a 12 foot capillary tube height filled with a fill liquid having a weight density of 58.3 lb/ft3 will generate an elevation pressure of almost 700 lb/ft2, or 4.86 PSI. If the pressure instrument is located below the process connection point, this 4.86 PSI offset must be incorporated into the instrument’s calibration range. If we desire this pressure instrument to accurately measure a process pressure range of 0 to 50 PSI, we would have to calibrate it for an actual range of 4.86 to 54.86 PSI. The reverse problem exists where the pressure instrument is located higher than the process connection: here the instrument will register a lower pressure than what is actually inside the vessel, offset by the amount predicted by the hydrostatic pressure formulae ρgh or γh. In all fairness, this problem is not limited to remote seal systems – even non-isolated systems where the tubing is filled with process liquid will exhibit this offset error. However, in filled-capillary systems a vertical offset is guaranteed to produce a pressure offset because fill fluids are always liquid, and liquids generate pressure in direct proportion to the vertical height of the liquid column (and to the density of that liquid). A similar problem unique to isolated-fill pressure instruments is measurement error caused by temperature extremes. Suppose the liquid-filled capillary tube of a remote seal pressure instrument comes too near a hot steam pipe, furnace, or some other source of high temperature. The expansion of the fill fluid may cause the isolation diaphragm to extend to the point where it begins to tense and add a pressure to the fill fluid above and beyond that of the process fluid. Cold temperatures may wreak havoc with filled capillary tubes as well, if the fill fluid congeals or even freezes such that it no longer flows as it should. Fill fluid expansion/contraction effects may be mitigated by keeping the volume of the fill fluid to a minimum, which is why capillary (smalldiameter) tubes are used to connect remote seals with instruments.

Another problem with remote seal pressure instruments is a time delay caused by the viscosity of the fill fluid as it moves through the small-diameter capillary tubes. This makes the pressure instrument slow to respond to changes in process fluid pressure. While minimal capillary tube diameter reduces the effects of temperature changes, it increases the effect of time delay. Proper mounting of the instrument and proper selection of the fill fluid24 will help to avoid such problems. All in all, the potential for trouble with remote- and chemical-seal pressure instruments is greatly offset by their benefits in the right applications. Some diaphragm-sealed pressure transmitters are equipped with close-coupled seals rather than remote seals to minimize hydrostatic, temperature, and tim e delay effects caused by fill fluid inside a capillary tube. A Rosemount extended-diaphragm pressure transmitter appears in the left-hand photograph, while a Yokogawa transmitter of the same basic design is shown installed in a working process in the right-hand photograph:

4.3

FILLED IMPULSE LINES

An alternate method for isolating a pressure-sensing instrument from direct contact with process fluid is to fill the impulse lines with a harmless fluid, which in turn directly contacts the process fluid. Filling impulse tubes with a static fluid works when gravity is able to keep the fill fluid in place, such as in this example of a pressure transmitter connected to a water pipe by a glycerin-filled impulse line:

A reason someone might do this is for freeze protection, since glycerin freezes at a lower temperature than water. If the impulse line were filled with process water, it might freeze solid in cold weather conditions (the water in the pipe cannot freeze so long as it is forced to flow). The greater density of glycerin keeps it placed in the impulse line, below the process water line. A fill valve is provided near the transmitter so a technician may re-fill the impulse line with glycerin (using a hand pump) if ever needed. As with a remote diaphragm, a filled impulse line will generate its own pressure proportional to the height difference between the point of process connection and the pressure-sensing element. If the height difference is substantial, the pressure offset resulting from this difference in elevation will require compensation by means of an intentional “zero shift” of the pressure instrument when it is calibrated. With no isolating diaphragm to separate process fluid from the fill fluid, it is critical that the fill fluid be compatible25 with the process fluid. Not only does this imply a total lack of chemical reactivity between the two fluids, but it also means the two fluids should not be readily miscible (capable of mixing in any proportion).

An important consideration in filled-line systems is how to refill the impulse line(s) without damaging the pressure instrument. Hand-operated pumps are commonly used to refill impulse lines, but such pumps are often capable of generating greater fluid pressures than the instrument can safely withstand. If we were to connect a glycerin pump to the filled system pictured previously, it would be advisable to shut the transmitter’s block valve to ensure we did not accidently over-pressure the transmitter. This is especially true if the impulse line happens to become plugged with debris, and substantial glycerin pressure from the hand pump is required to dislodge the plug:

In fact, the issue of impulse tube plugging is another reason to consider filled-line connections between pressure instruments and process lines or vessels. If ever a plug develops in the line, repumping the lines with fresh fill fluid is a practical way to clear the plug without disassembling any part of the system. For processes where impulse line plugging is a chronic problem, another solution exists called purging impulse lines, discussed in the next section.

4.4

PURGED IMPULSE LINES

Another method for isolating a pressure instrument from direct contact with process fluid, particularly when the impulse line is prone to plugging, is purging the line with a continuous flow of clean fluid. Consider this example, where pressure is measured at the bottom of a sedimentation vessel:

In this system, a continuous flow of clean water enters through a pressure-dropping “purge valve” and flows through the impulse line, keeping it clear of sediment while still allowing the pressure instrument to sense pressure at the bottom of the vessel. A check valve guards against reverse flow through the purge line, in case process fluid pressure ever exceeds purge supply pressure. The continuous water purge maintains clean impulse tubing, and ensures the pressure transmitter never contacts process fluid directly. A very important element of any purge system is a restriction between the purge supply and the connection with the process and pressure-sensing device. It is important that the pressure-sensing instrument senses the pressure of the process fluid and not the (higher) pressure of the purge fluid supply. In the example shown, the purge valve fulfills the role of this restriction, which is why it must be left in a partially-open condition, rather than fully-open. If this purge restriction is not restrictive enough, the purge fluid flow rate will be too great, resulting in a dynamic pressure drop developed across the length of the impulse line. This pressure drop will add to the pressure of the process fluid inside the vessel, creating a positive pressure measurement error at the instrument (i.e. the instrument will register more pressure than there actually is in the vessel). The purge restriction should be set to allow just enough purge fluid flow to guard against plugging, and no more.

Purged systems are very useful, but a few requirements are necessary in order to ensure accurate and reliable operation: • The purge fluid supply must be reliable: if the flow stops for any reason, the impulse line may plug. • The purge fluid supply pressure must exceed the process pressure at all times, or else process fluid will flow backward into the impulse line! • The purge fluid flow must be maintained at a low rate, to avoid pressure measurement errors. • The purge fluid should be introduced into the impulse line as close to the process connection as possible, to minimize errors due to the purge flow rate through long lengths of tubing. • The purge fluid must not adversely react with the process. • The purge fluid must not contaminate the process. • The purge fluid must be reasonable in cost, since it will be continuously consumed over time. A useful accessory to include in any purge system is a visual flow indicator such as a rotameter. Such an indicator is useful for manual adjustment of purge flow rate, and also as a troubleshooting aid, to indicate if anything happens to halt the purge flow. In the previous example, the purge fluid was clean water. Many options exist for purge fluids other than water, though. Gases such as air, nitrogen, or carbon dioxide are often used in purged systems, for both gas and liquid process applications.

4.5

Troubleshooting table for 4-20 mA output Symptom

Corrective actions

Verify power is applied to signal terminals Transmitter milliamp reading is zero

Check power wires for reversed polarity Verify terminal voltage is 10.5 to 42.4 Vdc Check for open diode across test terminal Verify the output is between 4 and 20 mA or saturation levels Verify terminal voltage is 10.5 to 42.4 Vdc

Transmitter Not Communicating with Field Communicator

Verify clean DC Power to transmitter (Max AC noise 0.2 volts peak to peak) Check loop resistance, 250 W minimum (PS voltage transmitter voltage/loop current) Have Field Communicator poll for all addresses Verify applied pressure

Transmitter milliamp reading is low or high

Verify 4 and 20 mA range points Verify output is not in alarm condition Verify if 4 – 20 mA output trim is required Check test equipment

Transmitter will not respond to changes in applied pressure

Check impulse piping or manifold for blockage Verify the transmitter is not in multidrop mode Verify applied pressure is between the 4 and 20 mA set points Verify output is not in alarm condition Verify transmitter is not in Loop Test mode Check test equipment (verify accuracy)

Digital Pressure Variable reading is low or high

Check impulse piping for blockage or low fill in wet leg Verify transmitter is calibrated properly Verify pressure calculations for application

Digital Pressure Variable reading is erratic

Check application for faulty equipment in pressure line Verify transmitter is not reacting directly to equipment turning on/off Verify damping is set properly for application Verify power source to transmitter has adequate voltage and current

Milliamp reading is erratic

Check for external electrical interference Verify transmitter is properly grounded Verify shield for twisted pair is only grounded at one end

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5

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. While connecting the power, check the name plate details of the pressure switch for the contact rating to prevent burn out of the contacts.

COM NO NC Electrical connections Mounting screw Snap acting micro switch Switch button Beam ( lever ) F2

Pivot Spring Piston

F1

Spring support Threaded base

Set point adj:

Sensing diaphragm ( rubber ) Protecting diaphragm ( teflon )

Applied pressure



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 4. Capillary Type Pressure Switches

5.1 STANDARD TYPE PRESSURE SWITCH

5.2 DIFFERENTIAL PRESSURE SWITCH

5.3 DIAPHRAGM TYPE PRESSURE SWITCH

5.4 CAPILLARY TYPE PRESSURE SWITCH

Pressure Switch Calibration Procedure Equipment/ materials needed

Regulated air or nitrogen pressure source Digital pressure gauge accurate to 0.1% of reading Multimeter capable of reading a change of state in resistance or voltage Pressure hoses and fittings capable of withstanding applied pressures

Isolate the pressure switch and bleed off pressure as per the site Lockout/Tagout Procedure. Do not perform any adjustments until all "as found" calibration data h as been recorded. Note any deviation from this procedure in the remarks section of th e calibration form. Make connections between the pressure source, pressure standard, and t he pressure switch sensing line. Connect the multimeter leads across the switch contact. For high pressure switch:

1. Increase pressure until the switch changes state, as indicated by a change in voltage or resistance reading. 2. Record the pressure switch trip value as indicated on the digital pressure gauge. 3. Slowly decrease the pressure until the switch changes state, as indicated by a change in voltage or resistance reading. 4. Record the pressure switch reset value as indicated on the digital pressure gauge. For low pressure switch:

1. Increase pressure until the switch resets, as indicated by a ch ange in voltage or resistance reading. 2. Record the pressure switch reset value as indicated on the digital pressure gauge 3. Slowly decrease the pressure until the switch changes state, as indicated by a change in voltage or resistance reading. 4. Record the pressure switch trip value as indicated on the digital pressure gauge.

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If the actual pressure switch trip (and if required, reset) values are greater than 1/2 the specified tolerance, adjust the switch until values are less than 1/2 the specified tolerance. If adjustments are made, repeat steps 1 to 4 as applicable and record final values.

Disconnect the test equipment from the switch under test. Complete the calibration form and affix the applicable calibration label. If all checks are within tolerance, restore the pressure switch to operational condition and notify appropriate personnel of work performed.

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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 within  1%.

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.

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6

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 pu sh 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.

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7

PRESSURE INSTRUMENT PROTECTION DEVICES

PRESSURE SNUBBER

This is used as an attachment at the bottom of the pressure gauge to protect the gauge sensor from damage, excessive wear, pulsation, oscillating deflections d eflections etc. Wherever sudden or repetitive changes of pressures are anticipated, anticip ated, pressure pressure snubber or otherwise called pulsation dampener is is used. Reciprocating pumps and Compressors are typical examples in our PDO 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.

GAUGE PROTECTION AGAINST HOT AND CORROSIVE FLUIDS

Basically there are four types of protection of gauges against hot, slurry and corrosive fluids. They are

1. Condensing Chamber Type 2. U- Tube Siphon Type 3. Pig-Tail Type 4. Diaphragm Seal Type

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 th e top and a blow-down valve below. The U-tube Siphon is generally made of a straight ½” or 3/8” pipe or tube itself to help condensation of hot fluids.

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.

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.

A= Condensing Chamber B= U-Tube syphon C= Pig Tail D= Diaphragm Seal

8

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

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8.1 DEAD WEIGHT TESTER

Pressure gauges are calibrated on patented hydraulic screw pumps, which transmit th e 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.

In dead weight tester, the weights we place on the weight stand are not actually weighs as indicated, instated they are calculated based on the piston diameter, hence it is called dead weigh tester.

A deadweight tester (DWT) is a calibration standard which uses a piston cylinder on which a load is placed to make an equilibrium with an applied pressure underneath the piston.

The formula to design a DWT is based basically is expressed as follows : p = F / A [Pa] where : p

: reference pressure [Pa]

F A

: force applied on piston [N] : effective area PCU [m2]

It consists of a very accurately machined, bored and finished piston which is inserted into a closefitting cylinder. The cross- sectional areas of both the piston and the cylinder are known. At the top of the piston is provided a platform on which the standard weight, of known accuracy, can be placed. An oil reservoir with a check valve at its bottom is also provided. The oil from the reservoir can be sucked by a displacement pump on its upward stroke.

For calibration purposes, first a known (calculated) weight is placed on the platform and the fluid pressure is applied on the other end of the piston until enough force is developed to lift the pistonweight combination and the piston floats freely within the cylinder between limit stops. The error in a dead-weight tester is less than 0.1%. In order to reduce the friction between the piston and the cylinder, the piston is generally rotated while a reading is being taken.

Operation of the tester is controlled by the two valves A and B on the top of the reservoir/high pressure cylinder block. When initially priming the system valves A and B are opened to fill the system with oil from the reservoir. Valve B is then closed with valve A left open and the screw pump operated to provide the lower test pressures. To provide the higher pressures valve A is closed to seal off the test circuit from the low pressure part of the screw pump and valve B is opened to allow the liquid in the low pressure part of the screw pump to return to the reservoir as the pump is operated. This ensures that the pump can be operated without h aving to put large forces on the screw pump handwheel. To release the test pressure the screw pump is wound out and valve A is opened.

Filling the base unit with liquid

Remove filler plug from reservoir by slackening screw and prising plug out. (This plug should be left out whilst in use). Open valves A and B. Wind screw pump handle fully clockwise. Fill reservoir with appropriate liquid. Use the oil supplied or an approved substitute for oil systems. Do not use other liquids. Castor based oils, Skydrol, solvents or similar liquids will attack the seals fitted in the standard tester. Wind screw pump handle fully anti-clockwise. Top up reservoir if necessary.

PROCEDURE

Fit instrument to be tested to gauge stand. Load the weight carrier/overhang with the weights equivalent to the desired pressure. Each weight is marked with two pressures, one for the low pressure range and one for the high pressure range. The equivalent weight of the carrier/overhang must also be taken into account. The piston cylinder unit has a basic 10 lb/in² start. Close valve B (valve A remaining open). Wind screw pump handle clockwise until the handle becomes stiff to operate. Close Valve A and open valve B Continue to wind screw pump handle clockwise. This will generate pressure up to approximately 2600 bar or 40,000 lb/in2. When the piston rises and the piston head skirt floats, this indicates it is at its nominal desired pressure. When the piston is floating within the blue band this indicates the pressure generated on the low pressure range (i.e. 1/8in² piston). When the piston is floating within the red band this indicates the pressure generated on the high pressure range (i.e. 1/160in² piston). The bands are easily visible from a seated position. To engage the high pressure range apply further pressure with the screw pump until the piston lifts higher and the piston head skirt floats within the red band. A slight leakage through the vent hole of the piston/cylinder unit is normal.

DURING CALIBRATION

When the tester is correctly set up and there are no leaks the piston should “float” for many minutes without it being necessary to touch the screw pump handwheel. On the initial setting up, however, there may be some air trapped in the base of the piston/cylinder unit. As this leaks past the piston the weights may fall slightly but it will only be for a matter of a few minutes until the air has escaped. If the piston continues to fall, check the connections for leaks. During calibration, the weights should be rotated by hand. It is desirable that the weights should only be rotated when approximately the correct pressure is obtained. Changing from the low pressure range to the high pressure range with the weights spinning does no harm. Weights should not be brought to rest by fully releasing the pressure and allowing the piston head to rotate against its stop under the full load of the weight pile. Stops come into action if the pressure is too high or too low and it is essential that the weights should be spinning freely whilst taking readings. At the lowest pressures the weights will not spin for more than a few seconds unless a very thin oil i s used, but providing the weight is rotated by hand before taking a reading and is obviously “floating” an accurate reading will be given.

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COMPLETION

After the test is finished wind screw pump handle anti-clockwise to lower pressure. Gently open valve A or B to release residual pressure Ensure that both valves A and B are fully open.

The system is now ready for another test and any residu al pressure is relieved.

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with dead weight testers. 1. Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves. 2. Pressure to be released slowly and carefully. 3. Weights to be handled properly. 4. Use Teflon tapes with pressure gauges for sealing against leaks.

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Troubleshooting. Fault

Equipment does not provide any output pressure.

Possible cause

Remedy

No liquid in tester.

Check that tester is filled with liquid. Fill the equipment with fluid as necessary.

Valve B is open. Component being tested has a large volume. Missing or damaged liquid seals shown by signs of unexplained liquid leaks.

Close valve B and try again. Pre-fill component with liquid before test.

Valve B handwheel disconnected from spindle. Valve B assembly or valve seat damaged.

If unable to locate a cause. Incorrect operating procedure being used.

Equipment provides pressure but pressure decays to zero

Equipment provides pressure but pressure decays when valves A and B are operated.

Missing or damaged liquid seals shown by signs of unexplained liquid leaks. Valve A or Valve B valve assembly or valve seat damaged. If unable to locate a cause. Incorrect operating procedure being used.

If unable to locate a cause. Insufficient liquid in tester.  Air in the system

Equipment provides pressure but pressure decays to lower value then remains steady.

Tester screw press becomes very stiff to operate when tester is being used in range below 140 bar 2 (2 000 lb/in ) Tester screw press becomes very stiff to operate when tester is being used in range above 140 bar 2 (2 000 lb/in )

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If unable to locate a cause. Internal damage Incorrect operating procedure being used.

Examine seals on equipment to ensure they are fitted correctly and are undamaged. Replace as necessary. Examine valve B. Tighten up nut securing handwheel to spindle as necessary. Examine condition of valve B and valve seat. Replace valve assembly or return tester to Manufacturer for overhaul as necessary. Return tester to Manufacturer for investigation. Ensure that correct operating procedure is being followed Examine seals on equipment to ensure they are fitted correctly and are undamaged. Replace as necessary. Examine condition of valves A and B and valve seat. Replace valve assembly or return tester to Manufacturer for overhaul as necessary. Return tester to Manufacturer for investigation. Ensure that correct operating procedure is being followed Return tester to Manufacturer for investigation. Check liquid level in reservoir. Fill reservoir with correct liquid as necessary. Prefill component under test with appropriate liquid. If necessary re-fill tester with appropriate liquid. Return tester to Manufacturer for investigation. Return tester to Manufacture for investigation. Ensure that correct operating procedure is being followed

If unable to locate a cause. Internal damage

Return tester to Manufacture for investigation. Return tester to Manufacture for investigation.

Incorrect operating procedure being used.

Ensure that correct operating procedure is being followed.

If unable to locate a cause.

Return tester to Manufacture for investigation.

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8.2 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.

QUICK TEST HYDRAULIC PUMP

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with Hydraulic Oil Pumps. 1. Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves. 2. Pressure to be released slowly and carefully. 3. Select the correct range of test gauge. 4. Use teflon tapes with pressure gauges for sealing against leaks.

8.3 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.

1. Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves. 2. Pressure to be released slowly and carefully. 3. Select the correct range of test gauge. 4. Use Teflon tapes with pressure gauges for sealing against leaks.

8.4 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.

QUICK TEST VACUUM PUMP

SAFETY PRECAUTIONS

The following safety precautions are recommended while working with Pneumatic Vacuum Pumps.

1. Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves. 2. Pressure to be released slowly and carefully. 3. Select the correct range of test gauge. 4. Use Teflon tapes with pressure gauges for sealing against leaks.

8.5 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.

1. Wear Personnel Protective Equipment (PPE) namely safety shoes, coveralls and gloves. 2. Pressure to be released slowly and carefully. 3. Use clean, dry air o f maximum 7 bars. 4. Use Teflon tapes with pressure gauges for sealing against leaks.

Rosemount 3051 HART menus

IMPULSE PIPING

The piping between the process and the transmitter must accurately transfer the pressure to obtain accurate measurements. There are five possible sources of error: pressure transfer, leaks, friction loss (particularly if purging is used), trapped gas in a liquid line, liquid in a gas line, and density variations. The best location for the transmitter in relation to the process pipe is dependent on the process. Before connecting the transmitter to the process, study the transmitter installation location, the process piping layout, and the characteristics of the process fluid (corrosiveness, toxicity, flammability, etc.), in order to make appropriate changes and additions to the connection configurations. If the impulse line is long, bracing or supports should be provided to prevent vibration. The impulse piping material used must be compatible with the process pressure, temperature, and other conditions. A variety of process pressure tap valves (main valves) are available according to the type of connection (flanged, screwed, welded), construction (globe, gate, or ball valve), temperature and pressure. Select the type of valve most appropriate for the application. Use the following guidelines to determine transmitter location and placement of impulse piping: Keep impulse piping as short as possible. For liquid service, slope the impulse piping at least 1 in./foot (8 cm/m) upward from the transmitter toward the process connection. For gas service, slope the impulse piping at least 1 in./foot (8 cm/m) downward from the transmitter toward the process connection. Avoid high points in liquid lines and low points in gas lines. Make sure both impulse legs are the same temperature. Use impulse piping large enough to avoid friction effects and b lockage. Vent all gas from liquid piping legs. When using a sealing fluid, fill both piping legs to the same level. When purging, make the purge connection close to the process taps and purge through equal lengths of the same size pipe. Avoid purging through the transmitter. Keep corrosive or hot (above 250 °F [121 °C]) process material out of direct contact with the sensor module and flanges. Prevent sediment deposits in the impulse piping. Maintain equal leg of head pressure on both legs of the impulse piping. Avoid conditions that might allow process fluid to freeze within the process flange.

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MOUNTING REQUIREMENTS

Impulse piping configurations depend on specific measurement conditions. Liquid flow measurement

Place taps to the side of the line to prevent sediment deposits on the transmitter’s process isolators. Mount the transmitter beside or below the taps so gases can vent into the process line. Mount drain/vent valve upward to allow gases to vent.

Gas flow measurement

Place taps in the top o r side of the line. Mount the transmitter beside or above the taps so liquid will d rain into the process line.

Steam flow measurement

Place taps to the side of the line. Mount the transmitter below the taps to ensure that the impulse piping will stay filled with condensate. In steam service above 250 °F (121 °C), fill impulse lines with water to prevent steam from contacting the transmitter directly and to ensure accurate measurement start-up.

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Communication Distance and other parameters to be considered.

Use stranded lead wires or cables which are the same as or better than 600 V grade PVC insulated wire (JIS C3307) or its equivalent. Use shielded wires in areas that are susceptible to electrical noise. In areas with higher or lower ambient temperatures, use appropriate wires or cables. In environment where oils, solvents, corrosive gases or liquids may be present, use wires or cables that are resistant to such substances. It is recommended that crimp-on solderless terminal lugs (for 4 mm screws) with insulating sleeves be used for leadwire ends. Up to 2 km (1.25 miles) when using CEV polyethylene-insulated PVC-sheathed cables. Communication distance varies depending on type of cable used. Load Capacitance 0.22 μF or less Load Inductance 3.3 mH or less Input Impedance of communicating device 10 kΩ or more at 2.4 kHz. HART Communication Distance Up to 1.5 km (1 mile) when using multiple twisted pair cables. Communication distance varies depending on type of cable used.

Use the following formula to determine cable length for specific applications: L= – 65 × 10 6 (R × C) (Cf + 10,000) C Where: L = length in meters or feet R = resistance in Ω (including barrier resistance)

C = cable capacitance in pF/m or pF/ft Cf = maximum shunt capacitance of receiving devices in pF/m or pF/ft

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PRESSURE UNIT CONVERSION To convert

Into

Multiply by

To convert

Into

Multiply by

atmosphere

bar

1.01295

in. Hg

atmosphere

3.342 x 10

atmosphere

dynes/cm

1.01295 x 10

in. Hg

bar

3.385 x 10

atmosphere

in. Hg

29.9213

in. Hg

dynes/cm

3.385 x 10

atmosphere

in. water

406.86

in. Hg

in. water

13.598

2

6

2

-2 -2

2

2

4

-2

atmosphere

kg/cm

1.03325

in. Hg

kg/cm

3.4532 x 10

atmosphere

mbar

1012.95

in. Hg

mbar

33.85

atmosphere

mtorr or micron Hg

7.6 x 10

in. Hg

mtorr or micron Hg

2.54 x 10

atmosphere

Pa or N/m

in. Hg

Pa or N/m  

2

5

5

1.01295 x 10 2

4

2

3385

2

atmosphere

PSI or lb/in

14.696

in. Hg

psi or lb/in  

0.4912

atmosphere

torr or mm Hg

760

in. Hg

torr or mm Hg

25.4

bar

atmosphere

0.9872

in. water

atmosphere

2.458 x 10

bar

dynes/cm

1 x 10

6

in. water

bar

2.489 x 10

bar

in. Hg

29.54

in. water

dynes/cm

bar

in. water

401.65

in. water

kg/cm

2.5396 x 10

bar

kg/cm

1.02

in. water

in. Hg

7.354 x 10

bar

mbar

1000

in. water

mbar

2.489

bar

mtorr or micron Hg

7.5028 x 10

in. water

mtorr or micron Hg

1.868 x 10

bar

Pa or N/m

bar bar

2

2

5

5

2

-2

-3

Pa or N/m  

psi or lb/in

2

14.503861

in. water

torr or mm Hg

750.2838

in. water

atmosphere

9.872 x 10  

-7

dynes/cm dynes/cm

2

in. Hg

2.954 x 10  

dynes/cm

2

in. water

4.0165 x 10  

2

kg/cm

-6

bar

1 x 10  

2

dynes/cm dynes/cm

2

dynes/cm

2

Pa or N/m

dynes/cm

2

dynes/cm

2

248.9

psi or lb/in

2

3.612 x 10

torr or mm Hg

1.868

2

atmosphere

0.9678

2

bar

0.9804

kg/cm

dynes/cm

9.804 x 10

2

in. Hg

28.958

2

in. water

393.76

2

mbar

9.804 x 10

mtorr or micron Hg

7.3554 x 10

kg/cm   kg/cm

-6

kg/cm

-3

mbar

1 x 10  

kg/cm

mtorr or micron Hg

0.75028

kg/cm

2

0.1

kg/cm

psi or lb/in

2

1.4508 x 10  

torr or mm Hg

7.5028 x 10  

2

2

-2

2

-4

1.0200 x 10  

2

2

kg/cm -5

3

2.489 x 10

-3

in. water

2

Gowtham Books

2

1 x 10

2

dynes/cm  

-3

2

dynes/cm

-3

2

2

Pa or N/m

2

-5

kg/cm

-4

kg/cm

2

5

5

2

9.804 x 10

4

psi or lb/in  

2

14.223

torr or mm Hg

7.3554 x 10

2

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