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DIFFERENTIAL HEAD DEVICES

Presented by Tony Barr at Fluor Canada, Calgary, Alberta on May 23, 2006

TECHNICAL OVERVIEW ENGINEERING INSTALLATION

LEVELMEASUREMENT

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INTRODUCTION: Level measurement is considered to be of primary importance in any continuous process. Without it common sense will tell us that vessels can overfill and cause catastrophic damage to equipment such as compressors and the environment by way of fire and explosion. Failing of alarm management systems can also lead to overfilling of vessels, so it is very important alarm systems are maintained periodically and set correctly. Emergency input to ESD systems is essential for closing of critical valves and equipment shutdown.Today’s seminar will give an appreciation of the workings of differential pressure type instruments used in level measurement. You are encouraged to use this information as a reference tool when selecting and designing level instrumentation.I have tried to explain the workings inside the instruments as well the application so that a fuller appreciation of fluid technology as well as other sciences is appreciated by the viewer.

TB

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Level measurement in vessels or tanks can

be determined in many ways with differing technologies.

Following list indicates the most popular ways of doing

this in today’s oil and gas industry:

1) Gauge glass. 2) Bubblers.3) Differential pressure 4) Displacers. 5) Magnetic.6) Radar.7) Ultrasonic.8) Capacitance.9) Nuclear.

Today we take at items 1 thru 5 of the above in a presentation on differential head measurement.

LEVEL TYPES

Rosemount Model 1151DP Transmitter

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Flow transmitters work on the difference of applied

Pressure across diaphragms and are generally known as

“Differential Pressure Cell Transmitters.” Some of the

operating principles are as follows:1) Strain Gage.2) Variable Capacitance.3) Semi-Conductor.4) Digital.Rosemount transmitters work on the Variable Capacitance Principle – The flow of a dielectric

fluid is used to move a plate (or diaphragm) relative to

the fixed capacitor plates. This movement will change thecapacitance of the fixed plates. The next two

slides will describe this principle.

DIFFERENTIAL PRESSURE TRANSMITTERS

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SensingDiaphragm

Rigid Insulation

FixedCapacitor Plates

Isolating Diaphragm

Fill Fluid

Lead Wires

The Rosemount DP Cell - The Variable Capacitance Principle – The flow of a dielectric fluid is used to move a plate (or diaphragm) relative to the fixed capacitor plates. This movement will change the capacitance of the fixed plates.

High PressureProcess Side

Low Pressure Process side

Copyright with permission Spartan Controls / Rosemount

Movable plate

Rosemount Model 1151 DP Cell

6Copyright with permission Spartan Controls / Rosemount

Measuring cell input.Variable Capacitance Principle.

Lead wires

Transmitter Electronics

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Atmospheric pressure(weight of air)

Liquid Head (H)

P

Pressure (P) x Area (a) = Force (F) on diaphragm. This determines amount diaphragm will move and displacement of internal fill fluid.

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High Level

Norm Level

Low Level

Visible RangeSimple sectional gauge glass level detector.Use on clear liquids.

Blind areas between sections

Shut-off valve

Bottom tap immersed in liquid zone

Top tap vented to atmosphere

SIMPLE SOLUTIONSVENTED TANK

LP

HPExtended DiaphragmArea ‘a’

Transmitter range = H (inches) x SG of fluid.Increasing pressure head ‘P’ on side of vessel.Low Pressure (LP) side of transmitter is vented to atmosphere.

4-20 ma output

DP TransmitterDirect mount.Internal Diaphragm area (a1)

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Simple gage glass or Magnetic design type - Principle of operation.

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SST Float Chamber Sealed Indicator Glass tubeIndicating beads Can flip showing Different colour for level present.Magnetic field Attracts follower(indicating beads)Magnets repeated360 deg inside of Float (float willRotate during Operation.Small space isPresent betweenFloat chamber andSealed glass tube.Float protectors are fitted at bottom and top (springs)

Float

Simple gage glass

Process in

Process out

Norm Level

Lower liquid

Upper vapour

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P atmos

Liquid Head (H)

High Level

Norm Level

Low Level

SIMPLE SOLUTIONS – Bubblers for atmospheric vessels. A small constant flow of air is bubbled from a dip pipe and the back pressure is measured.

Constant purge regulator can also be used to keep purge pipe from plugging and to eliminate errors due to back pressure effects on varying tank liquid heights.

Purge Meter w/reg + check valve set at 0 - 2.5 scfh. Purge orifice must be smaller than dip pipe tip.

Transmitter range = H (inches) x SG of fluid.

Platform or Grade Elevation

DP Transmitter

4-20 ma output to controller

Air Supply

Filter Regulator

Transmitter DiaphragmLP side vented to atmos.

Wat

er su

pply Constant

differential purge

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Process Max Level

Process out

Platform or Grade Elevation

Vessel Tan Zero

Calibrated range of transmitter is (Y.SG1 – Z.SG2) to (X.SG1 + Y.SG1 – Z.SG2)

Liquid Head ‘H’SG1

Vessel Pressure ‘P’

Top sensing tap is connected to low pressure side of DP transmitter (static Pressure of vessel must be connected to both sides of transmitter diaphragm).

HP

LP

DP Transmitter

Transmitter at bottom process connection

3-valve manifold

4-20 ma output to controller

P

PRESSURIZED VESSELWET LEG

Seal Pot

Wet Leg-SG2Min Level

X

Y

Isolation Valve

Z

Seal fluid to top of seal pot

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ProcessMax Level

Process out

Platform or Grade Elevation

Vessel Tan Zero

Calibrated range of transmitter is (Y.SG1 – Z.SG2) to (X.SG1 + Y.SG1 – Z.SG2)

Suppressed zero amount is(E.SG1 – E.SG2)

Liquid Head ‘H’SG1

Top sensing tap is connected to low pressure side of DP transmitter (static Pressure of vessel must be connected to both sides of transmitter diaphragm).

HP

LP

DP Transmitter

Transmitter installed below bottom process connection

3-valve manifold

4-20 ma output to controller

P

PRESSURISED VESSELWET LEG

Wet Leg-SG2

Min Level

X

Y

Isolation Valve

Z

Seal fluid to top of seal pot

E

Transmitter DiaphragmTube BundleRefer to slide 19

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Process in

Process out

Platform or Grade Elevation

Vessel Tan Zero

Calibrated range of transmitter is (Y.SG1 + (X-Y).SG3 – Z.SG2) to (X.SG1 + Y.SG1 – Z.SG2) Suppressed zero E amount is(E.SG1 – E.SG2)

Vessel Pressure ‘P’

Top sensing tap is connected to low pressure side of DP transmitter (static Pressure of vessel must be connected to both sides of transmitter diaphragm).Seal fluid will fill seal pot.

HP

LP

DP Transmitter

Transmitter installed below bottom process connection

3-valve manifold

4-20 ma output to controller

P

PRESSURIZED VESSELWET LEG - INTERFACE

Wet Leg-SG2

Min Interface

X

Y

Isolation Valve

Z

E

Transmitter Diaphragm

SG3 InterfaceSG1

Max Interface

Upper fluid must cover top vessel nozzle

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DIAPHRAGM SEALS

Seal systems prevent the process medium from contacting the transmitter diaphragm. They consist of the transmitter, a diaphragm seal, a fill fluid and either a direct mount or capillary style connection. Our next slide details this assembly.Process pressure is applied to the diaphragm and the measured pressure is transferred through the filled system and capillary tubing to the transmitter element. The transferred pressure displaces the sensing diaphragm in the pressure sensitive element of the transmitter. The displacement is proportional to the process pressure and is electronically converted to an analog or digital signal.Diaphragm seal systems should be considered when:- Process temp. is outside limits of norm. operating temp. of the transmitter.- Process is sufficiently corrosive enough to damage transmitter.- Process is viscous enough to plug the sensing lines.- Wet legs become too unstable and need frequent refilling. - Density or interface measurement is required.- Process will freeze inside sensing lines.

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SensingDiaphragm

Lead Wires

Measuring Cell internal diaphragm

Measuring Cell Internal fill fluid

Rigid Insulation

Process SideP P

Area of diaphragm = aP x a = Force F

Movement of dielectric fluid causes variable capacitance.

The Diaphragm Seal and Measuring Cell Principle of Operation

Fixed Capacitor Plates

HP SideLP Side

Capillaries filled with fluid eg Silicon.Process pressure deflects diaphragm and liquid is displaced. Sensing diaphragm is deflected.Physical characteristicsof material determinesize and amount of deflection of diaphragms.Under force F diaphragmIs deflected small amount.

Totally sealed unitNo vapour pocketspermitted.

o o

Fill fluidCharge port

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CAPILLARY FILL FLUIDS

Fill fluids are used for transferring head pressure by displacing fluid as described in slide 13.All fill fluids expand and contract with changes in temperature so it is important to install the capillaries away from direct sun rays. The amount of expansion depends on the type of fluid used. With ambient or process temperature changes, the fill fluid density can also change. This can have added effects to the fluid and will cause errors in the head reading. A larger volume of fill fluid increases the potential for volume expansion, so by minimizing the length and inside diameter of the capillary tube, the fill volume can be kept as low as possible to reduce temperature effects. This keeps the thermal expansion coefficient as low as possible and minimizes errors.The most common fill fluid used in transmitters is Dow Corning Silicone DC 200-10 good for -37 to 232 deg C. Viscosity is 10 cSt @ 24C; viscosity @ -37C is 65 cSt (centistokes).DC 510 fill fluid is good for -51C to 204C and has a viscosity >350 cSt @ -51C. Increases in viscosity at low temperatures will thicken the fluid and give less fluid movement resulting in slower reaction time. DC 550, 704 and 710 have viscosities of >1,000,000 @ -37C and are not recommended for use at sub zero temperatures.

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ProcessMax Level

Process out

Platform or Grade Elevation

Vessel Tan Zero

Liquid Head ‘H’SG1

Top sensing tap is connected to low pressure side of DP transmitter (static Pressure of vessel must be connected to both sides of transmitter diaphragms).Generally calibrated range of transmitter is: – Z.SG2 to Z.SG1 – Z.SG2 inches WC.Alternative range taken from Min to Max level is: X.SG1 – Z.SG2 to Y.SG1 – Z.SG2Note calibration formula for vacuum or atmospheric tank will vary.DP Transmitter

Transmitter Diaphragm

HP

LP

3-valve manifold

PRESSURIZED VESSEL - DIAPHRAGM SEALS Transmitter installed at any elevation relative to process connections.

Seal LegsSG2

Min Level

Isolation Valve

Z

6” Min.

Platform

X

Y

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Process

Max Interface

Process out

Platform or Grade Elevation

Vessel Tan Zero

Liquid Head ‘H’SG1

Top sensing tap always covered by upper fluid is connected to low pressure side of DP transmitter (static Pressure of vessel must be connected to both sides of transmitter diaphragms).

Generally calibrated range of transmitter is: – Z.SG2 to Z.SG1 – Z.SG2 inches WC.

Alternative range taken from Min to Max level is: X.SG1 – Z.SG2 to Y.SG1 – Z.SG2

Note calibration formula for vacuum or atmospheric tank will vary.DP Transmitter

Transmitter Diaphragm

HP

LP

3-valve manifold

PRESSURIZED VESSEL - DIAPHRAGM SEALS INTERFACE Transmitter installed at any elevation relative to process connections.

Seal LegsSG2

Min Interface

Isolation Valve

Z

6” Min.

Platform

X

Y

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Process in

Process out Platform or Grade Elevation

Vessel Tan Zero

Pressure ‘P’

4-20 ma output

PRESSURISED VESSEL DISPLACER Level Transmitter installed across process connections

Min Level

Isolation Valve

Z

Vertical force movement of displacer and rod converts to angular rotation of torque arm

Torque tube assembly

SG-1

Transmitter

Calibrated range of transmitter is: Z.SG of process fluid.

Level measurement using Displacers works on the ARCHIMEDES principle. When a body sinks in water it displaces its own volume of water….......orAn object in a fluid is buoyed up by a force equal to the weight of the displaced fluid

Displacer cage

P

0 100

Min 6”

Max LevelP

LIC001Controller

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The knife-edge bearing support minimizes friction and a limit stop on the torque arm is used to prevent over-stressing of the torque arm.Rotary motion of the torque tube moves a magnet attached to the lever assembly and changes the magnetic field that is sensed by the Hall-Effect sensor *. This is converted to an electronic signal which is processed to provide linearity, temperature and sensitivity corrections. From this a 4-20 ma signal is generated.* - see slide 19

0 100

Knife edges support rod

Fixed flange located Behind transmitter case

Rotary shaft Torque tube is fixed to back of flange and turns angularly as float rises. The rotary shaft inside will also turn angularly

Rod rises linearlyand pivots on knife edges

Rotary shaft

When the displacer rises or falls, the corresponding angular displacement of the torque rod is linearly proportional to the displacer movement and therefore to the liquid level.

Displacers work on Archimedes principle – The buoyancy force exerted on a body immersed in a liquid is equal to the weight of the liquid displaced.

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Hall Effect Sensors:

When a current carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. This principle is known as the Hall Effect.

o

oV = 0oI o

oVH = VoI

Hall Effect – Magnetic field present

To signal conditioningamplifier

The Hall Voltage is proportional to the vectorcross product of current “I” and the magnetic field.

Hall Element (semi-conductor material)with current passing. Output connections Are perpendicular to the direction of current.When no magnetic field is present there isno potential difference across the output.

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Process in

Process out Platform or Grade Elevation

Vessel Tan Zero

Calibrated range of transmitter for non-interface is: Z.SG of process fluid. For interface measurement calibrated range is: Z.SG-2 (lowest SG value) to Z.SG-1 (highest value)

Pressure ‘P’

Displacer Level Transmitter installed across process connections

4-20 ma output

PRESSURISED VESSELINTERFACE

Min Interface

Isolation Valve

Z

Vertical force movement of displacer and rod converts to angular rotation of torque arm

Torque tube assembly

SG-2

SG-1

Transmitter

Interface

Displacer cage

P

0 100

Min 6”

Max InterfaceP

LIC001Controller

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TUBING BUNDLES

Self Regulating Heating Cable

14 AWG Copper Bus WiresSemi conductive insulated heater.Nickel Plated Copper Braid.

Heat reflective tape

Polymer outer jacketProcess tubes1/8 to 1” sizeNon Hygroscopic Glass Fibre Insulation

As the heat is dissipated over the length of the tube assembly the heat output of the cable increases and when the heat increases the cable output decreases. Self regulating cable is protected from overheating and burning out.

Twin Tube Bundle Assembly used for flow transmitters.

HP

LP

Single Tube Bundle Assembly used for level transmitters

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Following references were made to complete this presentation:

1) Rosemount Instruction Manuals.2) Rosemount / Spartan Controls slides.3) Rosemount product data sheets.4) Siemens product information.5) Thermon product information.6) Sensors Magazine Online Jan 2003.7) Special thanks to Simon Lucchini and Carey Sloan for

their very helpful critique. 8) Special thanks to Kelvin Downs, Control System

Department Manager Fluor Canada for making this possible.

This seminar was prepared and presented by Tony Barr,Fluor Canada on May 23, 2006.

REFERENCES:

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THE END THANKS FOR

YOUR ATTENTION

THAT WAS LEVEL

ARCHIMEDES