Fire and Gas Detector

Fire and Gas (F&G) System

· Manual fire call points

· Fire detectors

· Flammable gas detectors

· Toxic gas detectors

· Local Fire alarm panels

· F&G PLC

· Alarms

· Loud speaker – For unit

· Optical Beacon

· Motor siren

· Strong sound

· For fire only

· Local Bell – Use in building

· Has a mimic panel to know where the fire originated

· Only for buildings, for plants, we rely on DCS.

· Purpose

· For fire brigade

· Because buildings are common alarm we need to know where fire is

· Types

· Addressable

· More diagnostics

· Chain loop topology. If 1 detector fails, the other detectors can till work

· Conventional

· One loop per room

· If 1 detector fails, all other detectors won’t work

· A method to ensure robustness against failure

· Do IPF study for fire and gas – to be implemented

· Normally use 1oo2D

· Originally 1oo2, If 1 fault become 1oo1. Unlike safeguarding if 1 fault, fail safe

· For automatic, 2oo2

Fire Detection System

· Flaming – From hydrocarbons

· Smoldering – Fire burning without flame

· Flash

· Smoke first

· Flame Second

· Heat last

· Flame

· Smoke

· Heat

· Human visual

Type

Advantage

Disadvantage

When to use

Flame

Fast detection

Smoke

Early warning

Closed areas, for early warning

Heat

Hazard specific, less nuisance alarms, reliable

Cannot function test, damage replace, Slow response

When rapid fire spread is unlikely,

Person

· Manual Call Points

· In process area Installed at side of roads every 100 meters

· In offsite area installed at side of roads every 200 meters

Type

Principle

Advantage

Disadvantage

When to use

Infrared

Detect IR from flame flickering

Open fires

Cannot detect smouldering fires, Cannot detect H2 based fire,

Solar radiation interference

Preferred technology for hydrocarbon

Ultraviolet

Extremely Fast detection, open fires

Heavy smoke will foul the lens.

Welding, flare, black body interference.

UV-IR

Expensive

The best

IR multiple Freq

Visual CCTV

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· Smoke detectors provide the earliest warning for fires.

· From critical buildings, a concept called VESDA(Very early smoke detection apparatus) is utilized. In our plant, the system is called USSD (Ultra sensitive smoke detection). The concepts of USSD is

· Aspiration units are installed throughout the building. Installed normally on ceilings

· The smokes are sucked and brought to a central unit

· The central unit hosts an optical type smoke detector

Type

Principle

Advantage

Disadvantage

When to Use

Optical / Scattered light type

Smoke deflects beam of light

Good for smouldering fires,

Preferred choice, most common

Ionization

Smoke particles reduce current created from ion flow of radioactive materials

Sensitive to small smoke particles

Must be applied together with heat detectors

Only when ionization type cannot be used. Use in printing workshops, highly flammable places

· Most commonly used inside plants since it’s hazard specific

Type

Principle

Advantages

Disadvantages

When to use

Fusible

Heat will melt a tube containing pressure

Cheapest

False alarms, easily

Continuous-line

Heat will cause insulators to melt and 2 conductors come in contact

Bimetallic

Heat causes metal to bend

· Best is use of F&G mapping software

· No standards, just based on principle (owner) study

·

· Installed at most likely place for leaks

· Mechanical seals

· High pressure Flanges

· If gas tend to go up, the detector should be place on top and vice versa

Gas Detection System

· Analyzer house

· Pit

· HVAC air intake for buildings in plant

Type

Principle

Advantages

Disadvantage

When to use

Electro Catalytic

Combustible gas oxidizes and produces heat

Robust, detects any combustible gasses, Cheaper

Catalyst can be poisoned,

Single point detection, fail-dangerous

Use when hydrogen detection is needed

Electro Chemical

Electrolysis of gas creates voltage

PPM Level, Good repeatability,

Cannot be used at high temperature and low humidity,

Must change electrolytes, Not fail safe, Gas specific (H2S, SO2,CO2,NO2, CL2)

Old technology, typically avoided, used mainly for toxic gas (see below)

Point type Infrared Gas Detector

Long service life, background gas does not effect service life, has diagnostic, fail safe

Cannot detect hydrogen

Preferred choice for HC plants.

LOS Infrared Gas detectors

Larger coverage

Expensive, high prone to nuisance alarms

Unit of measure is PPM Meter

· 2 categories

· Category 1 – Immediate health effect gasses (H2S, CO, HCL)

· H2S < 10 PPM, more than that is dangerous. 1000 PPM = dead

· HCL < 5 PPM

· Category 2 – Long Term (Vinyl Chloride, benzene, toluene)

·

Type

Principle

Advantages

Disadvantage

When to use

Electro Catalytic H2S gas detector

Combustible gas oxidizes and produces heat

Robust

For H2S detection

Electro Chemical

Electrolysis of gas creates voltage

PPM Level, stability, immune to environment change

Must change electrolytes, Not fail safe, Gas specific (H2S, SO2,CO2,NO2, CL2)

Preferred choice

Semiconductor

Analyser Instruments

Foundation Support

  • Large Instrument analysers when mounted on the ground are bolted to a concrete block. The concrete block is usually casted outside the plant. The earth is then excavated and the concrete block is inserted into the excavated hole. Analyzer is then bolted on to this concrete The concrete block is usually designed by a civil engineer

Analyzer Sampling Systems

  • Purpose of analyser sampling system is
    • To obtain representative sample
    • To condition the sample
    • To accomplish stream switching
    • To ensure safety
    • To allow adequate sample time
  • The 5 main elements of sampling system
    • Sample take off
    • Sample pre conditioning system
    • Sample transport line
    • Sample conditioning system
    • Sample return/dispose
  • Sample Take off
    • Sample take off probe is usually angle cut to avoid solid particles from entering the sampling system.
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    • Very thick schedule (80 and above) to avoid it from getting corrode
    • Is usually flanged typeclip_image003clip_image004clip_image005
  • Rules for using sample take-off probe or sample tap
    • For relatively clean process streams of 2" and below pipe, a simple sample tap may be adequate, however sample take-of probe is preferred.
    • A sample probe is preferred because solid particles tend to move at the rear of the pipe
    • For both types, take sample from the TOP of the process line for Vapours
    • For both types, take sample at the side of the process line for liquids.
    • Never take sample from the bottom of the process line since there may be sediments (such as piping sediments)
    • General guideline is to use sample take-off probe to ensure representative sample
  • Sample preconditioning
    • Purpose is to for the process sample to suit the analyser requirements (such as pressure, temperature and flow rate)
    • Main components are pressure regulator, pressure gauge, cooler, sample vaporizer
    • Sample vaporizer is to provide heat to maintain the gas sample at a gas phase after pressure is reduced
    • Sample cooler is to provide cooling. Conventionally, we use a tube-in-tube method
  • Sample Transport Line
    • Type of transport line
      • Single transport line
        • For short transport line (<30m)
      • Fast loop system
        • Long transport time
        • Lag time is undesirable
      • Sample needs to be in single phase along the transport line
        • Use electrical heater to ensure sample is heated
        • Gas sample need to be maintained above it’s dew point
        • Liquid sample needs to be maintained below it’s boiling point
      • The sample is usually returned back to process. In case if pressure is inadequate, the sample may be returned back to the following:-
        • For hydrocarbons, it can be returned either to:-
          • DOC(Oil contaminated drain) or Oily water sewer . The sample will then flow to a waste water treatment plant. Some DOC design goes straight to slop tank this however will run a risk causing slop to be over contaminate
          • SOP(Slope drain). The sample will flow straight to slop pit and pumped back to slop tank and the reprocessed back. This is different to DOC since it does not go through a waste water plant. Normally SOP’s are seen in tankage area where WWTP is too far away for the waste to be pumped.
        • For clean water, it can be returned back to DAC(Accidentally oil contaminated drain) or oily water sewer. The sample will then go straight to final pond/bio pond

Gas Chromatograph Analysers

  • GC is used to analyse product concentration. A constant volume sample is injected into the GC column. The injection is done by a sample injection valve
  • GC must be temperature controlled. Temperature control is important for
    • Column separation - A 1 degree C temperature change will have a 3% effect in retention time
    • Gas sample injection – Temperature is inversely proportionate with volume injection
  • GC’s can be used for both liquid and gas sample
  • For liquid sample,
    • The sample boiling point must be adequately low to ensure that the GC oven can vaporize the sample. If the sample boiling point is too high, a vaporizer may need to be use
    • The sample pressure must also be adequately high (~>2kgcm2) to ensure. However, if the pressure is too high , a pressure reducer need to be installed
  • For liquid, there are 2 types of sampling valves.. These valves range from 0.1 to 3ul injection
    • Liquid sampling valve
      • is used when the boiling point is high.
      • The LSV incorporates a vaporizer to vaporize the sample
    • Rotary sampling valve
  • TCD reading – Small temperature deviations in TCD will cause significant zero/span shifts
  • Having a lower temperature will prolong the lifetime of columns and valves
  • Carrier Gas System
    • The important purpose of carrier gas system is to provide a stable transport and detection medium for the components of the sample
    • Nitrogen, Hydrogen, helium are used as carrier gas as they do not have interactions with the solutes. Carrier gas selection also depends on the type of detector used
    • The carrier gas must fit the following criteria
      • Water or oxygen in carrier gas will destroy most column materials (dewpoint must be less then -60 degC)
      • 99.99% purity
      • Organic components must be less than 5ppm
    • Some GCs have a dehumidifier at their carrier gas system where a desiccant such as molecular sieves is use
  • Carrier Gas Maintenance
    • There should be 2 cylinder manifolds, one cylinder pressure set higher so that when the higher pressure cylinder finishes, the lower one will take over.
    • There are 3 types of GC columns
  • GC Ovens
    • The most important function of the GC oven is to ensure constant temperature selectable between 5 degC up to 350 degC.
    • Oven temperature is important as the higher the temperature, the faster components move in the GC oven. It is therefore practical to put it lower for better separation but slower cycle time. To high temperature in the other hand provides faster analysis but will reduce column life time due to column bleeding. It is best to put temperature as low as possible
    • Aside from stability, temperature should also be evenly distributed. The unevenness of temperature distribution should not be more than 0.8 degC if a same performance with lab GC is expected
    • Oven designs include
      • Using a heater element.
      • Heated-are blow-in method where air is heated and the blown in 4 sides pf the oven
      • Air bath circulation method where air is heated also but is circulated with a fan. This method ensures good stability and evenness
  • Gas Chromatograph Detectors
    • The most common detectors are FPD and FID
  • Slope Processing
    • User will usually have to configure how the GC detects the slope. Slope detection is the prime method used for GC to recognize peaks. There are a few things that needs to be configured in slope detection which are
    • The Gate time
      • Gates are the point on which the GC will turn on a slope detection algorithm.
      • There are 2 types of gates
        • ON gate – This is the gate when the GC will start to detecting an increasing slope
        • OFF Gate – This is the gate when the GC will start to detect when it should start find a decreasing slope
      • User need to first configure the time when both Gates start
    • Detection Slope
      • Once a chromatograph signal have passed an ON gate time, the GC will start the slope detection algorithm.
      • Before that, user will have to configure the ‘Detection Slope’ a parameter with a mV/s unit. Typically this value is set at 0.005mV/s. Increasing this value higher may cause the slope detection too late but decreasing the value may cause the slope to detect noises. The time when the slope is detected is called the Peak-ON time
      • As the chromatograph further traverses, it will soon find the OFF Gate. This off gate will start another slope detection algorithm however, this time it will start to determine the PEAK-OFF time. Different GCs have different method of determining a gate off time. The usual method is to find a 0mV/s slope. Some GC
    • The gate cutting method
      • Gate cutting is to determine where to cut the baseline to determine the borders of integration
      • There are 2 types of gate cutting methods:-
        • Slope Gate – The GC will draw a straight line from the start gate and end gate
        • Time Gate
      • Detection slope
    • Detection level
    • The integration method
    • The 2 most common gate methods are
      • Time gat
      • Slope gate
  • The calibration of GC is done by using the following formula
    • Concentration x Std Area = Measured Area x Reading Range x Calibration Factor
    • Based on this formula, calibration is possible by altering 2 value namely “Std Area” or “Calibration factor”. However, in PPTSB/AMSB GC application, we only change “Std Area” as our standard practice.

Gas Chromatograph Columns

  • When changing a GC column, one must allow 1 night of flushing. One must slightly increase the oven temperature
  • Types of Column
    • Packed
      • Short length (1.5 to 10m)
      • ID 2-4mm
      • Have solid supports which is inert such as diatomaceous earth
      • Diatamaceous earth is a naturlly found compound containing 86% silica, 5% sodium, 3% magnesium and 2% iron.
      • This solid support is coated with a liquid or solid satationary phase
      • The selection of this stationary phase coating depends on the type of analysis to be done
      • Slow and inefficient
      • Have higher capacity, though new technologies have allowed capillary column to have large capacity to
    • Capillary Column
      • Long length (25-60m)
      • ID 0.25mm
      • Mostly made of fused-silica with polyimide outer-coating
      • Best for speed
      • However only small samples
      • Type of capillary column
        • Wall coated open tubular (WCOT)
          • Has tube walls coated with stationary phase
          • Most commonly used
          • Does not separate very light component well
          • Some also call it FSOT (Fused silica open tubular)
        • Support coated open tubular (SCOT),
          • Has solid support on the tube walls which is coated with the stationary phase
          • Solid support used is also normally diatomaceous earth
          • Can handle larger samples than WCOT, so it is the best choice for very light components
          • Also, for specific type of stationary, SCOT must be used.
          • Most common stationary phase which must use SCOT is
            • Molecular Sieve
            • Divinylbenzene(DVB) - Used for C1 to C3 isomers analysis
            • Alumina Al2O3 - Separation of isomers of C1 to C10
        • PLOT (Porous Layer Open Tubular)
          • Has porous layers on the walls of the tube which can be coated with stationary phase
          • The pores itself can sometime be used as stationary phase
          • Usually used for compounds that are gas at room temperature as it is able to create sharper peaks
          • Common trade name for stationary phase columns. This table is taken from aegilent product brochur
  • Most supports use diatomaceous earth. However, some advance supports have trade names such as
    • Celite
    • Chromosorb W, Chromosorb P, Chromosorb G, Chromosorb S
    • Some chromosorbs have a AW notation, which means acid wash.
    • Agilent Phase Composition Polarity Approximate Temperature Range (C)  (Isothermal/Programmed)* Phases With Similar Selectivity
      General Applications
      HP-1ms, DB-1ms, HP-1, DB-1 Amines, hydrocarbons, pesticides, PCBs, phenols, sulfur compounds, flavors and fragrances 100% Dimethylpolysiloxane Non-polar From -60 to 325/350 BP-1, SPB-1, CP-Sil 5, Rtx-1, OV-1, SE-30, 007-1, ZB-1
      HP-5ms, DB-5, HP-5 Semivolatiles, alkaloids,drugs, FAMEs, halogenated compounds, pesticides, herbicides 5% Phenyl 95% dimethylpolysiloxane Non-polar From -60 to 325/350 SPB-5, XTI-5, Mtx-5, CP-Sil 8CB, SE-54, Rtx-5, BPX-5, MDN-5, Rtx-5ms, BP-5, ZB-5
      DB-5ms Semivolatiles, alkaloids, drugs, FAMEs, halogenated compounds, pesticides, herbicides 5% Phenyl 95% dimethyl arylene siloxane From -60 to 325/350 SPB-5, XTI-5, Mtx-5, CP-Sil 8CB, SE-54, Rtx-5, BPX-5, MDN-5, Rtx-5ms
      DB-1301 Aroclors, alcohols, pesticides, VOCs 6% Cyanopropyl-phenyl 94% dimethyl polysiloxane Mid-polar From -20 to 280/300 Rtx-1301, Mtx-1301, CP-1301
      DB-35, HP-35 CLP-pesticides, aroclors, pharmaceuticals, drugs of abuse 35% Phenyl 65% dimethyl polysiloxane Mid-polar From 40 to 300/320 Rtx-35, SPB-35, AT-35, Sup-Herb, MDN-35, BPX-35
      DB-35ms CLP-pesticides, aroclors, pharmaceuticals, drugs of abuse 35% Phenyl
      65% dimethyl arylene siloxane
      From 50 to 340/360 Rtx-35, SPB-35, AT-35, Sup-Herb, MDN-35, BPX-35
      DB-1701, DB-1701P Pesticides, herbicides, TMS sugars, aroclors 14% Cyanopropyl-phenyl 86% dimethyl polysiloxane Mid-polar From -20 to 280/300 SPB-1701, CP-Sil 19 CB, Rtx-1701, CB-1701, OV-1701, 007-1701, BPX-10
      HP-50+, DB-17 Drugs, glycols, pesticides, steroids 50% Phenyl
      50% dimethylpolysiloxane
      Mid-polar From 40 to 280/300 Rtx-50, CP-Sil 19 CB, BPX-50, SP-2250
      DB-17ms Drugs, glycols, pesticides, steroids 50% Phenyl
      50% dimethyl arylene siloxane
      From 40 to 320/340
      HP-88 FAMES Approximately 88% Cyanopropyl arylene siloxane High From 0 to 260
      DB-200 Residual solvents, pesticides, herbicides 35% Trifluoropropyl
      65% dimethyl polysiloxane
      Polar From 30 to 300/320 Rtx-200
      DB-210 50% Trifluoropropyl
      50% dimethyl polysiloxane
      From 45 to 240/260
      DB-225ms, DB-225 FAMEs, alditol acetates, neutral sterols 50% Cyanopropyl-phenyl
      50% dimethyl polysiloxane
      Polar From 40 to 220/240 SP-2330, CP-Sil 43 CB, OV-225, Rtx-225, BP-225, 007-225
      HP-INNOWax Alcohols, free organic acids, solvents, essential oils, flavors and fragrances Polyethylene glycol Polar From 40 to 260/270 BP-20, 007-CW, CP-WAX 52 CB, Stabilwax, Supelcowax-10
      DB-WAX Solvents, glycols, alcohols Polyethylene glycol Polar From 20 to 250/260 Rt-Wax
      CAM Amines, basic compounds Polyethylene glycol-base modified Polar From 60 to 220/240 Carbowax Amine, Stabilwax-DB, CP-51 WAX
      HP-FFAP, DB-FFAP Organic acids, alcohols, aldehydes, ketones, acrylates Polyethylene glycol-acid modified Polar From 40 to 250 OV-351, SP-1000, Stabilwax-DA, 007-FFAP, Nukol
      DB-23 FAMEs (requiring cis/trans resolution) 50% Cyanopropyl
      50% dimethyl polysiloxane
      Polar From 40 to 250/260 Rtx-2330, 007-23, SP-2330/2340/2380/2560
      CycloSil-B Chiral compounds (general purpose) 30%-heptakis (2,3-di-O-methyl-6-O-t-butyl dimethylsilyl)-B-cyclodextrin  in DB-1701 Mid-polar From 35 to 260/280 LIPODEX C, Rt-BDEXm, B-DEX 110, B-DEX 120
      HP-Chiral b Columns Chiral compounds (using a Nitrogen selective detector, NPD) beta-Cyclodextrin in phenyl-based stationary phase Mid-polar From 30 to 240/250 LIPODEX C, Rt-BDEXm, B-DEX 110, B-DEX 122
      PLOT Column Applications
      HP-PLOT Molesieve Permanent and noble gases. Argon and oxygen separation at 35�C 5� molecular sieve zeolite From -60 to 300 Rt-Molesieve 13X, Molesieve 5�
      HP-PLOT Al2O3 �KCl� C1-C6 hydrocarbons in natural gas, refinery gas, fuel gas, synthetic gas, dienes Aluminum Oxide "KCl" deactivated Least polar From -60 to 200 AluminaPlot, Rt-Alumina,CP-Al2O3/KCl Plot
      HP-PLOT Al2O3 �S� C1-C6 hydrocarbons in natural gas, refinery gas, fuel gas, synthetic gas, dienes Aluminum Oxide "Sodium Sulfate" deactivated Mid-polar From -60 to 200
      GS-Alumina C1-C6 hydrocarbons in natural gas, refinery gas, fuel gas, synthetic gas, dienes Aluminum Oxide with proprietary deactivation Most polar From -60 to 200
      HP-PLOT Q Hydrocarbons including isomers, CO2, methane, air/CO, water, polar solvents, sulfur compounds Polystyrene-divinylbenzene From -60 to 270/290 PoraPlot Q/S, Rt-Q, Supel-Q PLOT
      HP-PLOT U C1 to C7 hydrocarbons, CO2, methane, air/CO, water, oxygenates, amines, solvents, alcohols, ketones, aldehydes Divinylbenzene/ethylene glycol dimethacrylate From -60 to 190 PoraPlot U
      GS-GasPro C1 to C12 hydrocarbons, CO2, trace-level sulfurs, hydride gases, inorganic gases, halocarbons, SF6,  oxygen/nitrogen separation at �80�C Proprietary, bonded silica-based From -80 to 260/300 CP-SilicaPLOT
      GS-CarbonPLOT C1 to C5 hydrocarbons, CO2, air/CO, trace acetylene in ethylene, methane Bonded monolithic carbon layer From 0 to 360 CP-CarboBond
      Specialty Phases - Environmental Applications
      DB-624 6% Cyanopropyl-phenyl
      94% dimethyl polysiloxane
      Mid-polar From -20 to 260C Rtx-624, AT-624, SPB-624, CP-624, PE-624,007-624
      DB-VRX Volatile Organic Compounds using MSD, ELCD/PID Proprietary phase Non-polar From -10 to 260C Rtx-VRX
      DB-35ms CLP Pesticides, Chlorinated Herbicides, PCBs, 508.1 Pesticides 35% Phenyl
      65% dimethyl arylene siloxane
      Mid-polar From 50 to 340/360C MDN-35, BPX-35
      DB-XLB (Confirmation Column) Proprietary phase Non-polar From 30 to 340/360C No equivalent
      HP-5ms Semivolatiles by EPA Method 8270 5% Phenyl
      95% dimethylpolysiloxane
      Non-polar From -60 to 325/350C SPB-5, XTI-5, Mtx-5, CP-Sil 8CB, SE-54, Rtx-5, BPX-5, MDN-5, Rtx-5ms
      DB-XLB PCB Congener Analysis (209 Congeners) CLP Pesticides, Chlorinated Herbicides, PCBs, 508.1 Pesticides Proprietary phase Non-polar From 30 to 340/360C
      DB-TPH Leaking Underground Fuel Tank (LUFT) testing Proprietary phase Non-polar From -10 to 290C
      DB-MTBE MTBE in Soil and Water Proprietary phase Non-polar From 35 to 260/280
      Other Specialty Phases
      HP-Fast GC Residual Solvent Column Residual Solvents 6% Cyanopropyl-phenyl
      94% dimethyl polysiloxane
      Mid-polar From -20 to 260C
      DB-ALC1 Blood Alcohol Testing Proprietary phase Mid-polar From 20 to 260/280C
      DB-ALC2 Proprietary phase Mid-polar From 20 to 260/280C
      HP-Blood Alcohol Column Proprietary phase From -60 to 270/290
      *Temperature limits vary with column dimensions. Always confirm temperature limits prior to column use.

Flame Scanners

Flame Scanners

Temperature Instruments

Thermowells

· Thermowells are usually inserted in a vessel, pipeline and is used to protect the thermo element from process conditions

· Thermowell are inserted either by

· Flange joint – Most applications used flange joints

· The thermowell is clamped between a piping nozzle flange and the temperature transmitter’s cover flange. There will be a gasket between the piping nozzle flange and the thermowell

· It is important that the thermowell and the piping nozzle flange have a serrated finish to help the spiral wound gasket in between it arrest leaks. Serrated finish are small groves between 3.2 to 6.3 micro meter. There will be around 45 to 55 groves per inch.

· There was an incident in PPTSB where the thermowell was covered with satellite material to prevent erosion. The satellite cover came all the way to the connection joint. There should be a serrated finished here instead of the satellite coating. The thermowell was sent to a machining workshop so the satellite material is removed and groves are installed instead

· Thermowell cover flanges are typicall 1 ½” in size. The pressure rating will usually be a pressure rating higher than the piping spec. For example if the pipe rating is 300#, the thermowell cover flange (and gasket) will be 600#.

· Threaded joint – This is not used in high pressure applications. Also, thereded joints require teflon tape. This tape usually melts at high degrees.

· Threaded joint with seal weld – This is used since welding alone is not strong enough to hold the well. A thread is used to ensure stronger well contact

· Before installing a new thermowell, the following values should be calculated:-

· Natural frequency (fn) of vibration when installed in its service

· Vortex shedding frequency (fp) for full range of process flow rates

· For safe applications fn > 2fp

· The insertion length (from flange top to end of thermowell) of a thermowell according to PTS standard:-

· 230 mm for DN80 and DN80 and DN 100

· 255 mm for DN 150

· 305, 355, 405, 455 mm for equipment

· For small pipes (<3”) , the thermowell can be inserted in the elbow of the pipe

· The standard set is to take into account the mechanical strength of the thermowell due to flow and vibrations

· The thermowell should be immersed as much as possible to allow proper conduction of heat to the element i.e. to ensure that the measured temperature is exactly the same as the actual temperature.

· The sensor must touch the end of the thermowell and is inserted into a ½” NPTF thread.

· Thermowell should not be used in high fluid velocity and high vibration

· General Rule of Thumb : The length of the thermowell must be slightly more than half the pipe length.. This is to ensure that the thermowell will always be touching the process liquid. Why not just make it long enough? The length cannot be too long to avoid vibrations

Temperature Elements

· Temperature elements are put inside a thermowell

· The layer structure inside a temperature element

· The outer well layer – Ceramic type for high temperature or just plain SS

· Magnesium oxide powder (MgO) layer - ~0.5 mm

· Secondary sheath – Made of SS to protect the temperature element wires. Usually 6 mm

· The element tip must touch the thermowell. This is often ensured by having a spring

· Insertion of an element is typically screwed on a cover flange.

Thermocouple

· The thermocouple use in our plant is type K, chromel-alumel. Type K is the most common used because it has he highest range

· Thermocouple are usually used for high temperature measurements

· If a thermocouple with no transmitter’s wire disconnects, the reading would go maximum high, this is because thermocouple measures voltage. An open circuit would of course give the highest voltage.

· The 2 Thermocouple wires are colored as

· Red

· Red is positive and is Chromel

· Yellow

· Yellow is negative and is Alumel

· Tip types

· Grounded tip – good heat transfer

· Ungrounded tip – No noise (EMI)

· Exposed – Highest Sensitivity

· If you do not know the color coding of the Thermocouple, there are 2 methods to identify which one is positive or negative :-

· Short the wires to create a junctions and use a voltmeter to know which on is +Ve

· Use a magnet to detect chromel, since chromel is ferrous and response to magnets. Chromel is positive and alumel in negative

RTD (Resistance Temperature Detector)

· RTD’s are used because of its extreme accuracy and sensitivity up to 0.001 degree C

· 3 Wire RTD’s are used in process plants to reduce the connection wire error. The connection wires used are usually of copper type.

· When verifying the direct resistance of an RTD, one needs to compensate the wire errors. This can be done by directly measuring the resistance of the 2 common connections. The Sensor resistance is therefore the resistance of the 2 terminals minus the resistance of the 2 common terminal wires divide by 2.

· 3 wires RTD are used to compensate the resistance from the transmission wire. Since there are 2 transmission wires, we assume the wires are of same resistance. Having a 4 wire RTD will compensate each wires differently

· Most Transmitters compensate the wire error from the sensor to the transmitter. For example, in the Machine Monitoring System (MMS), the RTD is a kilometer length from its’ transmitter to the system. This would of course bring some resistance. To prevent this, RTD transmitters compensate the value of this resistance. Therefore, to measure the direct resistance of the RTD, one has to measure the sensor directly at the nearest terminal to the RTD.

· The most common RTD material used is platinum and nickel. Platinum is much more accurate and has less span shift (linearity).

· Copper RTD are sometimes used to eliminate thermocouple effect on the copper wire extensions.

· The error of RTD for old type is +0.15% and for SMART type is +0.05% of full span

· Causes of RTD error : trapped moisture, vibration, contact with sheath.

· RTD hardly drifts. Only 0.1% span shift in 6 months and most o the time longer.

· RTDs must not be used in vibrating services.

· RTD should not be used on high temperature measurements. This is because at high temperature, the RTD has a self heating. In PTS it is stated that RTDs must not be used above 650 degrees C. However, based on experience, even service at 400 degrees C would reduce the life time of the RTD and later cause problems.

· 4 Wire RTD’s are never used for high temperatures

Level Instruments

Level Instruments

· Zero Suppression – is when the transmitter is below the tapping point

· Zero Elevation – is when the transmitter is above the tapping point

· The issue with zero suppression or elevation is only during calibration. Zero is not 0 mmH20 but is calculated on the height of the elevation or suprression

· Types

Type Advantage Disadvantage When to use Special needs
DP Dry leg Cheap Cannot measure interface, Cannot measure corrosive or plugging liquid Clean liquid
Sealed Type DP/ Remote seal/ capillary tube Avoid plugging, avoid leaks due to corrosion Expensive Corrosive liquid
DP wet leg Cheaper than sealed, long length Cannot measure interface Process tends to condense, Slanted tube, need to fill up tube
Unguided wave radar level (radio detection and range) Cheaper for long distance Can have false returns Large tank
Guided wave radar False returns avoided Expensive for long tanks, cannot work if have emulsion layer Small to medium sized vessels
Float Type Disturbed by surface ripples
Capacitance Error with temperature Avoid, old technology
Ultrasonic Error with temp change, sensitive to foaming Avoid, old technology
radioactive For extreme high temperatures, pressure, corrosion troublesome Only when nothing else works Source need to be replaced after half life elapsed
displacers Can measure interface, severe applications Affected by density
bubbler Cheap, for corrosive/slurry type

 

 

 

 

 

DP Level Instrument

· Suppressed zero is when a DP level transmitter is installed below the tapping point. The issue with suppressed zero is when setting the millibar signal at 4mA and 20mA. For suppressed zero, the 4mA is set at the length below the tapping times with gravity and density

· Elevated zero, is vice versa, when the DP level transmitter is installed above the tapping point

· Whether one elevates zero or suppresses zero, the range is always the same. The range will always be between the top and bottom tapping points

Wet Leg Level Instrument

· Wet leg uses the principle of DP, where the low side of the leg is filled with product and tapped from the top of the measured vessel

· The wet leg measured DP is 0 at full and negative at empty vessel

· The wet leg manifold does not have any equalizing valve to eliminate the effect of valve parsing. A parsing equalizing valve would cause the low side filled product to drop down, reducing the low side pressure. If this occurs, we would expect a higher DP (high minus low) and would of course result in a higher level measurement

· To verify 100% level i.e. 0 dp, vent both high side and low side manifolds

· To verify 0% level i.e. lowest dp (negative), vent high side manifold first. Then, the lowside must me vented at the top tapping point. By this way you measure atmospheric pressure at high side and wet leg pressure at low side. This low dp should be equal to the LRV

· Some transmitters allow the setting of 0% level at 0 dp and 100% level at made-positive lowest dp. This is done be adding an offset or zero trimming the output with a +|lowest dp|, setting 100% level at +lowest dp and 0% level at –lowest dp + lowest dp = 0. Although this gives an error to the actual dp, one should know that this is just a transmitter setting.

· Wet Leg level transmitter relies on Specific Gravity (SG).

· Wet Leg is preferred to remote seal type due to its lesser cost.

Radar Type Level Instrument

· High accuracy

· Depends on dielectric constant

· Used for replacement of the displacer type

· Instrument has no moving parts

· Disadvantage

· Requires tuning

· Two types

· Open path

· Cannot be used in side chambers as the signal might reflect

· Guided wave

· Most common use

Displacer Type Level Instrument

· Displacer Type transmitter uses the principle of buoyancy to obtain level. The measured weight of the Displacer will be converted to the signal. The weight is measured using a hall effect sensor

· For side mounted chamber Displacer, the 0-reference point is at the center of the bottom side flange. The 100% point is at the center of the top flange.

· However, the ability to measure accurately would be from the start of the bottom flange to the top of the top flange.

· An easier way to obtain the point of measure would be to hang the displacer together with its top mounted flange outside of the chamber. The point where the measured liquid starts to hit the displacer would be the start of measure. This of course requires compensation in interface measurements(liquid-liquid), where the low SG should be compensated on the measured freefall weight.

· 2 flanges are required for a displacer. The bottom side flange is used to indicate the starting point of measurement. The top side flange displacer is required to equalize the pressure between the measured vessel and the chamber. Trapped pressure would impede the level increment of the product.