C05- Four-Gas Monitors for Upstream Oil & Gas Conference/C05 Four-Gas Monitors... · A worker was in process of refurbishing a tank ... was performed prior to hot work ... Hydrogen

C05- Four-Gas Monitors for Upstream Oil & Gas: Understanding the Proper Use and Limitations of Monitors

JOHN SNAWDER, NIOSH

EVGENY ANDRONOV, WHITING PETROLEUM CORPORATION

45TH ANNUAL NORTH DAKOTA SAFETY COUNCIL CONFERENCE FEBRUARY 21-24, 2018 1

DisclaimerThe findings and conclusions in this presentation have not been formally disseminated by NIOSH and should not be construed to represent any agency determination or policy.


Course ObjectivesAt the end of this course you will be able to:Understand the atmospheric hazards associated upstream oil and gas activitiesExplain the functionality of the sensors and list key indicators of proper operationIdentify the limitations of the meterSet up the instrument for different applicationsDiscuss real-world monitoring situations


Incident 1

20 year old male flow tester found unresponsive on a well pad site face down in the upper hatch of a crude oil storage tank The victim was gauging the tank There was no H2S exposure(may or may not have been an H2S monitor)


Incident 2

A truck driver pumping and hauling crude oil from a tank battery was found slumped over and non responsiveHe appeared to have been measuring the volume of liquid from the top of the tank batteryHis H2S monitor did not alarm


Incident 3

39 year old truck driver was transferring crude oil from a tank batteryA pumper showed up and found the victim slumped over the railing at the top of the tank battery He was wearing an H2S monitor, no alarm indicated.


Incident 4

A 59 year old oil tanker driver died while collecting crude oil samples from an open thief hatchThe employee was wearing a 4 gas monitor which showed an oxygen deficient atmosphere and the presence of hydrocarbons exceeding 100% of the LEL at the time of his death


Incident 5

A well tender foreman hand dug a 4' deep hole to repair a 2 inch gathering line. The foreman was found with his head in the hole at approximately 11:00am, after the company was notified that he was not answering his phone. The employee was not wearing a 4 gas monitor and post-mortem analysis of the workers blood indicated the presence of hydrocarbons at the time of his death.


Incident 6

A worker was in process of refurbishing a tank to be used for crude oil storage. The tank was presumed to be empty. The worker was on top of the tank, possibly using an acetylene torch to cut a hole in the tank lid. The tank erupted/exploded, throwing the lid and worker off the tank. No monitoring for flammable atmospheres was performed prior to hot work


Incident 7

Employee was unloading salt wastewater from the oil fields. After unloading the truck, the employee went to use the bathroom located next to the hooking valves for unloading. The bathroom caught fire and the employee was killed from after-burns to 90 percent of his body

No monitoring for flammable atmospheres was performed


Incident 8

A trucker was dispatched to a well pad to load a tanker truck with oil. Worker was found lying on the ground between truck and loading box. Victim was unresponsive and could not be revived. Biomarkers of H2S were found in his blood at autopsy

Worker had H2S monitor in the truck but was not wearing it on location


Incident 9

An employee was dispatched to an oil and gas wellsite to pick up produced water. At the worksite, he set his truck in position, connected the hose, and engaged the vacuum pump. A fire then occurred, causing second and third degree burns to both of his arms and part of his torso, as well as minor burns to his neck, face, and ear.

No monitoring for flammable atmospheres was performed


Incident 10

Eight water heaters housed in a shipping container were being used on a hydraulic fracturing job. High demand for water would trip a breaker and workers would have to enter the shipping container and restart the system. Worker #1 and Worker #2 were taking 30-minute turns monitoring the water heaters from inside the shipping container. Worker #1 was found unresponsive in the shipping container by other crew members. The crew removed worker #1 from the shipping container, started chest compressions, and called for help. Worker #1 died in the ambulance. Worker #1 was fatally exposed to carbon monoxide (CO) levels of 5,273 parts per million for approximately 25 minutes while inside the shipping container monitoring the heaters. Adequate CO detection equipment was not used at the site.


What’s wrong here?Each case illustrates a limitation of the gas monitor or worker training

◦Limitations present risks that must be managed

◦There must be awareness of the limitations of the equipment as much as there must be awareness of the hazards


Monitor LimitationsMust have the right detector for the application◦H2S detector is not intended to detect combustible gases or lack of oxygen

Cannot accurately detect combustible gases in a low oxygen environment◦most combustible gas detectors require 10-15% oxygen


Monitor LimitationsMust know the detector actually works◦ monitor must be bump tested and calibrated

Cannot detect combustible gases above the UEL◦ UEL condition doesn’t mean there is no immediate fire/explosion

hazard◦ monitor may initially alarm and quickly go out of alarm leading to false

sense of security


How do we manage the limitations of the equipment?

There must be awareness of the limitations of the equipment as much as there must be awareness of the hazards

Must develop a process that adequately evaluates and indicates the hazard while protecting the worker and accounting for the equipmentTraining, Training, Training


Three Basic Atmospheric Hazards

•Oxygen (deficiency and enrichment)

•Flammable gases and vapors

•Toxic contaminants


Hazardous Atmospheres*Atmosphere that has the potential to expose entrants to the risk of death, incapacitation, impaired ability to self-rescue (e.g. escape unaided from a permit required confined space), injury, or acute illness from one or more of the following causes: atmospheric oxygen concentrations below 19.5% and above 23.5%

flammable gas, vapor, or mist in excess of 10% LEL

atmospheric concentration of any substance for which a dose or OEL is published in applicable government regulations, safety data sheets (SDS), standards, or other published or internal documents and could result in responder exposure in excess of its dose or PEL;

any other IDLH atmospheric condition


Source: API RP 98, Personal Protective Equipment Selection for Oil Spill Responders, First Edition, August 2013. Global Standards

http://www.api.org/

http://www.iadclexicon.org/personal-protective-equipment/

IDLH: Immediately Dangerous to Life and Health “The IDLH is considered a maximum concentration above which only a

highly reliable breathing apparatus providing maximum worker protection was permitted”

“Atmosphere that limits the ability of a worker to escape without loss of life or irreversible health affects”

Critical symptoms that could retard escape are: Blindness Unconsciousness Impaired judgment


What is IDLH?

Primary Atmospheric Hazards in Oil and Gas Exploration and ProductionOxygen ◦Deficiency = too little oxygen to support life◦Enriched = increased fire hazardFlammable gases and vaporsHydrogen SulfideCarbon Monoxide


Oxygen


There is no antidote for too little oxygen!

Flammable Gases and VaporsFlash Point◦ The minimum temperature of a liquid at which a spark or flame can cause an instantaneous flash in the

vapor the liquid forms with air◦ As flash points drop, fire hazard increases

Flammability Limits◦ Lower Explosive Limit (LEL) – the lowest concentration of a gas or a vapor where an ignition source can

produce a flash of fire◦ Upper Explosive Limit (UEL) - the highest concentration of a gas or a vapor where an ignition source

can produce a flash of fire


Hydrogen SulfideHydrogen Sulfide or sour gas (H2S) is a flammable, colorless gas that is toxic at extremely low concentrations.It smells like "rotten eggs" at low concentrations and causes you to quickly lose your sense of smellIt is heavier than air, and may accumulate in low-lying areasMany production basins where H2S is found have been identified, but pockets of the gas can occur anywhereAll oil and gas sites should be classified according to areas of potential and/or actual exposure to H2S, The four hazard levels are:◦ No Hazard Condition◦ API Condition I - Low Hazard◦ API Condition II - Medium Hazard◦ API Condition III - High Hazard


Carbon Monoxide

CO is a colorless, odorless, toxic gas which interferes with the oxygen-carrying capacity of blood

CO is non-irritating and can overcome persons without warning

Effects of CO poisoning◦ Severe carbon monoxide poisoning causes neurological damage, illness, coma and death

Symptoms of CO poisoning◦ Headaches, dizziness and drowsiness◦ Nausea, vomiting, tightness across the chest


Carbon monoxide (CO) is a toxic gas resulting from the incomplete burning of fuels containing carbon; sources of CO at oil and gas sites may be heater treaters, engines of all kinds and flares/combustors

Quick Chemistry ReviewThere are a few basic principles that can help you in better understanding atmospheric hazards, how they behave and how to use the 4 gas monitor to detect them


Gas versus Vapor

A gas is a substance that has a single defined thermodynamic state at room temperature.

A vapor is the gaseous phase of a substance that is normally a liquid (or solid) at room temperature.

There are two “Numbers” that help us make this determination◦ Vapor Pressure◦ Boiling Point

Take away message Heavy sticky vapors are harder to detect than light gases


Vapor pressureTells us how readily a liquid (or solid) wants to evaporate into to a vapor stateLow vapor pressure chemicals don’t want to make vapors

High vapor pressure chemicals want to become gases

A chemical with a vapor pressure over 1 ATM, 760 mm/Hg or Torr, 14.7 PSIA or 1,013 mb is a GASVapor pressures of over 40 mm/Hg are more likely to move around and are considered to be an INHALATION HAZARD

Water has a vapor pressure of about 20 mm/Hg


Temperature at which a liquid transitions to a gas. Can tell you how readily a liquid wants to move to a vapor state. Low boiling point chemicals◦ Want to become vapors ◦ Have relatively higher vapor pressures◦ Are relatively easier to measure with a vapor monitor◦ Ex Gasoline

High boiling point chemicals◦ Don’t want to become vapors◦ Have relatively lower vapor pressures◦ Are harder to measure with a vapor monitor◦ Ex Diesel

Boiling Point


VPs & BPs for some common hydrocarbonsName Formula Vapor Pressure

(@20oC) mm HgBoiling

Point (oC)Boiling

Point (oF)Water H2O 17.54 100 212Ethane C2H6 >760 -67 -89

Acetone (CH3)2CO 200 13 56

Isopropanol C3H8O 40 27 81Propane C3H8 >760 -43 -45Methane CH4 >760 -107 -161Butane C4H10 >760 -18 -0.5Pentane C5H12 465 2 36Hexane C6H14 260 20 68Heptane C7H16 46 37 98Octane C8H18 5 52 126

Gasoline C5-10 15 10-93 50-200Decane C10H22 2 79 174Diesel C11–25 0.4 160-371 320-700

Dodecane C12H26 0.3 102 216Hexadecane C16H34 ~0.01 114 237Docosane C22H46 <0.001 164 327

Triacontane C30H64 <0.0001 232 450

# of Carbonsincrease.

Vapor Pressure decreases.

Boiling Pointincreases

ArgonAr

(and other gases)

1.1%

OxygenO2

20.9%

Composition of Air, Molecular Weight

NitrogenN2

78%

Gas % in Air MW Relative MWN2 78% (0.78) 28 21.84O2 20.9% (0.209) 32 6.69Ar 1% (0.01) 39 0.39

OxygenO2

20.9%

ArgonAr

(and other gases)

1.1%


Molecular Weight of AirFormula: % in air x MW = relative MW

Add the relative MWs to get the MW of air:MW of air = 21.84 + 6.69 + .39 = 28.92 ≈ 29

Gas % in Air MW Relative MWN2 78% (0.78) 28 21.84O2 20.9% (0.209) 32 6.69Ar 1% (0.01) 39 0.39


Molecular Weights - QuestionThe molecular weight of air is 29 g/mol. Using this information, where should you sample for each gas?

Chlorine (Cl2)

MW = 70

Methane(CH4 )

MW = 16

Propane(C3H6)

MW = 44


Vapor Density

The relative weight of a gas or vapor compared to air. Air has a vapor density of 1.

VD less than 1 means the gas will rise

VD greater than 1 means the gas will sink


Percentage and ppm

Example:The flammable range of propane is 2.1% - 9.5%. What is that in ppm?

2.1% = 21,000 ppm(because 0.021 x 1 million = 21,000 ppm)

Therefore a 10% LEL = 2,100 ppm

Key equivalency: 1% = 10,000 ppm


Measuring in Parts Per Million (ppm)ppm %

1,000,000 100500,000 50250,000 25100,000 10

10,000 15,000 0.5

500 0.05100 0.01

10 0.0011 0.0001

• 1 ppm is the same as 1 inch in 16 miles• 1 ppm is the same as 1 oz in 10,000 gallons

ppm = mg/m3 x 24.5molecular weight


Oxygen Sensor Learning Objectives

What is air?Oxygen levels◦Deficiency◦EnrichmentOxygen sensorsO2 as a broadband toxic sensors


Oxygen SensorsThere is no antidote for lack of Oxygen

Understanding Oxygen Sensors used in portable handheld detectors

MIDDLETOWN OH — Investigators are trying to unravel the mystery of how a routine check of a sanitary sewer line turned deadly Friday morning, May 7, when a city worker was killed and three firefighters were hospitalized after being overcome.

A 31-year-old maintenance worker, was found dead at the bottom of a manhole. He had opened the manhole around 8 a.m. to do some routine sewer line work when he apparently was overcome, police said.


Composition of “Fresh Air”

◦78 % Nitrogen◦20.9 % Oxygen◦1.1 % All other gases◦Water vapor◦CO2

◦Argon◦Other trace gases

Nitrogen (N2)

Oxygen (O2)

All Other Gases

Constiuents of Air



Gas Detection – Key Terms

Partial Pressure of Oxygen - The pressure exerted by oxygen gas in a mixture of gases.

Oxygen Deficient Atmosphere - An atmosphere containing less than 19.5% oxygen by volume.

Oxygen Enriched Atmosphere - An atmosphere containing more than 23.5% oxygen by volume.

Partial Pressure of Oxygen (pO2)


Key points:• % of O2 does not change with altitude• pO2 of the atmosphere:

with increasing altitude with low pressure events with increasing water vapor

Oxygen Deficiency Air is oxygen deficient whenever its concentration is less than 19.5%


Causes of Oxygen Deficiency

Causes of O2deficiency include:◦Displacement◦Microbial action◦Oxidation◦Combustion◦Absorption


Health Effects of Oxygen Deficiency


% Oxygen Physiological Effect19.5 – 16 No visible effect.

16 – 12 Increased breathing rate. Accelerated heartbeat. Impaired attention, thinking and coordination.

14 – 10 Faulty judgment and poor muscular coordination. Muscular exertion causing rapid fatigue. Intermittent respiration.

10 – 6 Nausea and vomiting. Inability to perform vigorous movement, or loss of the ability to move. Unconsciousness, followed by death.

Below 6 Difficulty breathing. Convulsive movements. Death in minutes.


Oxygen EnrichmentOxygen enrichment is dangerous because it:

Proportionally increases the rate of many chemical reactions.Can cause ordinary combustible materials to become flammable or explosive.

Photo courtesy NASA


Oxygen Enrichment

Most conservative approach is to use 22% as take action point.

23.5 % is oxygen enriched (based on the OSHA code 29 CFR 1910.146)

Most Sensors in Portable Monitors are Fuel Cell Oxygen SensorsHow they work:Air sample enters through diffusion capillary or membrane.Sensor generates an electrical current proportional to the O2 concentration.Oxygen entry limited by either a capillary pore or membrane.


Oxygen Sensors

Fuel Cell Sensor Capillary Pore

Capillary pore◦Most commonly used in confined space monitors.◦About the size of a human hair.◦Gives a reading in % volume.


Electron flow

49

Fuel Cell - ChemistryOxygen reduced to hydroxyl ions at cathode:

O2 + 2H2O + 4e- → 4OH-

Hydroxyl ions oxidize lead (anode):2Pb + 4OH- → 2PbO + 2H2O + 4e-

Overall cell reaction:2Pb + O2 → 2PbO


Capillary PoreBenefits

True % by volume sensorNot influenced by changes in pressure due to:

◦Barometric pressure◦Pressurized buildings◦Altitude

51

Fuel Cell Sensor MembraneMembrane◦Directly measures the partial pressure of oxygen.

◦Not as common in handheld devices



Membrane Fuel Cell Oxygen SensorsIn a membrane fuel cell oxygen sensor there is a very thin, plastic membrane over the top of the sensor which is a solid barrier in which the oxygen molecules must dissolve in order to reach the sensing electrode◦ Entry into the cell is driven by the differential of oxygen partial

pressure across an oxygen permeable membrane◦ This results in a partial pressure reading which is usually

corrected to %Oxygen◦ Minor changes in pressure and barometric pressure can change

the sensor readings unless it is corrected for pressure

O2N2

N2

N2N2

N2N2

N2N2

O2

O2

O2 dissolves through the

membrane and gets into the

sensor

O2O2

O2


Pressure Effects◦As partial pressure is important for diving applications this sensor is commonly used in diving where partial pressure and matrix affects of dilutant gases are of concern.

◦At the top of Everest (29,029 ft), the standard barometric pressure is 34kPa (253 mmHg), this means that there is 33% of the oxygen available at sea level and a meter using the membrane oxygen sensor would read only 6.9% if it had been calibrated at sea level


Capillary Most Common

Reads % Volume

+Reading doesn’t change with pressure−Affected by molecular weight of matrix gases

−Acid gases like CO2 can get in the sensor and can affect performance

RAE, ISC, MSA, Draeger

MembraneLess Common

Read % Volume corrected from partial pressure

+Unaffected by changes in matrix gases+Membrane is selective to O2 and prevents neutralization from acid gases like CO2

−Reading may change with pressure

RKI, Draeger (0-100% sensors only)

Sensor Type Comparison


Fuel Cell FailuresFailure modes which lead to lower current output where reading ultimately goes to zero:◦ All available surface area of Pb anode converted to PbO2◦ Electrolyte poisoned by exposure to contaminants ◦ Electrolyte leakage ◦ Desiccation◦ Blockage of capillary pore◦ Frozen sensor ◦ Electrolyte neutralized by exposure to high levels CO2


+ Proven technology+ Fail safe (they typically fail

to zero)+ Required for properly

measuring combustibility

− Limited and finite life span−Contain toxic heavy metal

(lead)− Leaking electrolyte can ruin

meter−Only can detect gross levels of

toxic industrial chemicals (TICs)

Fuel Cell Oxygen SensorsAdvantages Disadvantages


Oxygen SensingOxygen sensors can act as “broad-band” toxic sensors.

Air is 20.9% or 209,000

ppm oxygen

(O2)

Air is 78%or 780,000

ppm nitrogen

(N2)

Oxygen sensors as a “Broad-Band” Toxic Sensor

20 / 80 = 1000 / X1000 x 80 = 20X

80,000/20 = X

X = 4000 PPM


Composition of Air % Percent PPMNitrogen N2 78% (80) 780,000Oxygen O2 20.9% (20) 209,000

Decreasing O2

20.9 20.8% 1000 ppm

Air is 20% O2 so that means that the other 80% of N2 must also be displaced.

Every 0.1% Oxygen drop is 5000 ppm of “something else”

DO THE MATH

4000 ppm


“Dead-band”◦ Is placed on gas monitors around 20.9%

oxygen, forcing the meter to read “20.9”◦ It can also reduce the monitor’s effectiveness

as a broad-band toxic sensor

While oxygen is only a gross broad-band sensor sometimes it is all you have.

Oxygen Sensing

20.920.8 21.121.020.7

While dead-band can reduce the perceived “jumpiness” of oxygen sensors but it can reduce their effectiveness as a broad-band toxic sensorIf the oxygen sensor jumps from 20.9 to 20.7 you won’t notice 5000 ppm of “something else” you might only see the first 10,000 ppm of itWhile oxygen is only a gross broad band sensor sometimes is all you’ve gotIf oxygen drops AT ALL you have a LOT OF SOMETHING else in the air, so much so that you should expect response from most electrochemical sensors if only as a reading from cross-sensitivity


Oxygen Sensing

Combustible Gas Sensors

•Wheatstone bridge catalytic bead•Thermal Conductivity-Sensit•Photoionization Detector (PID)

How do they work & what are the limitationsHow to choose the right meter


Flammable vs. Combustible

Flammable liquida liquid that has a flash point below 100°F (37.7°C).

Combustible liquida liquid that has a flash point above 100°F (37.7°C).


Flash Point

The minimum temperature of a liquid at which a spark or flame can cause an instantaneous flash in the vapor the liquid forms with air.As flash points drop, fire hazard increases.


Flammable Limits

Between the LEL & UELthe mixture is said to be explosive or flammable.

LELUEL

Why are LEL and UEL important?

Below the LEL, a mixture is too lean to ignite.

Above the UEL, a mixture is too rich to ignite.

Lower Explosive Limit (LEL) – the lowest concentration of a gas or a vapor where an ignition source can produce a flash of fire

Upper Explosive Limit (UEL) - the highest concentration of a gas or a vapor where an ignition source can produce a flash of fire


Common Flammability Ranges

• Note that LELs and UELs can vary between reference sources

• Meter accuracy can drastically affect your LEL readings

• Therefore, always be VERY CONSERVATIVE when making LEL decisions

* NFPA 325 “Guide to Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids, 1994 edition

Flammable Limits

Compound (vP)* Lower Explosive Limit (LEL)

Upper Explosive Limit (UEL)

Methane (40 atm) 5.0% 50,000 ppm 15% 150,000 ppm

Ethane (20 atm) 3.0% 30.000 ppm 12% 120,000 ppm

Propane (8.4 atm) 2.1% 21,000 ppm 9.5% 95,000 ppm

n-Butane (2.8 atm) 1.6% 16.000 ppm 8.4% 84,000 ppm

i-Butane (3.1 atm) 1.8% 18,000 ppm 9.6% 96,000 ppm

n-Pentane (420 mmHg) 1.5% 15,000 ppm 7.8% 78,000 ppm

Flammable Limits Compound (vP) Lower Explosive Limit (LEL) Upper Explosive Limit (UEL)

n-Hexane (134 mm HG)

1.1% 11,000 ppm 7.5% 75,000 ppm

Benzene (75 mmHg)

1.2% 12,000 ppm 7.8% 78,000 ppm

Heptane (40 mm Hg)

1.05 % 10,500 ppm 6.7 % 67,000 ppm

Ethyl Benzene (7 mm Hg)

0.8% 8000 ppm 6.7% 67,000 ppm

Toluene (21 mm Hg)

1.1% 11,000 ppm 7.1% 71,000 ppm

Xylenes (7 mm Hg)

0.9% 9000 ppm 6.7% 67,000 ppm

Crude Oil (50-720 mm Hg)

0.4%-1.5% 4000 -15,000 ppm 7.5-15% 75,000-150,000 ppm

Total Hydrocarbons* (2-20 mm Hg)

0.8 – 1.1 % 8000-11,000 ppm 5.4-7.0% 54,000-70,000 ppm

Combustible Gas/Vapor Instruments typically read in “% LEL” not “%Volume”

0%(0% Methane)

LEL(eg. 5% Methane)

Gas Concentration

Flammability Range

100% Volume(100% Methane)

Measuring Flammability

LEL Meter

100%0%

UEL(eg. 15% Methane)

69

Catalytic Bead LEL Sensor is like an electric stove

One element has a catalyst and one doesn’t

Both elements are turned on low

The element with the catalyst “burns” gas at a lower level and heats up

As this is a combustion process a minimum of 12-16% oxygen is required

The hotter element has more resistance and the Wheatstone Bridge measures the difference in resistance between the two elements

This is a primary measurement because if something burns it will burn on this sensor

Photo courtesy of Whirlpool


Wheatstone bridge catalytic bead LEL Sensor Shortcomings

Two mechanisms affect the performance of Wheatstone bridge LEL sensors and reduce their effectiveness when applied to all but methane:

Gases burn with different heat outputs at their LEL“Heavier” hydrocarbon vapors have difficulty diffusing into the LEL sensor and reduce its output


Methane (CH4)Heavier Hydrocarbons

Heavier Hydrocarbons Rejected by the Flame Arrestor

Catalytic LEL Sensor Cut-Away

Active beadCompensatingbead

Flame arrestor

LEL Sensors were designed to measure Methane

Catalytic LEL Sensor Response

Gas/Vapor LEL (% vol) Sensitivity (%)* Ignition Temp. F0(C0)**Methane 5 100 999 (537)Hydrogen 4 91 932 (500)Propane 2 63 842 (450)Gasoline 1.4 48 536 (280)Acetone 2.2 45 869 (465)Benzene 1.2 45 928 (498)n-Pentane 1.5 45 500 (260)MEK 1.8 38 759 (404)Toluene 1.2 38 896 (480)Diesel 0.8 30 NA

LEL sensor sensitivity varies with the gas/vapor* Relative sensitivities are for example only, please consult your detector manufacturer for sensitivities specific to your product** NFPA 325 “Guide to Fire Hazard Properties of Flammable Liquids, Gases and Volatile Solids, 1994 edition



Example Response to C1-C5 (Model LEL-2)

Making LEL DecisionsIt is impossible to make a decision with an LEL meter unless you know the scale in which you are measuring

Measurement scale is usually the calibration gas

Correction factors allow you to change scale without changing calibration gas

Correction Factor (CF) is also a measure of the sensitivity of the LEL sensor to a gas

Know your LEL meter measurement scale


Catalytic LEL Sensor Poisons

Carbonization: sensor in combustible gas too high for too longAcute Poisons act very quickly, these include compounds containing:◦ Silicone (firefighting foams, waxes, glues)◦ Tetra Ethyl Lead (old gasoline)◦ Phosphates and phosphorous◦ High concentrations of combustible gas◦ Armor All

Chronic Poisons/Inhibitors◦ Sulfur compounds (H2S, CS2)◦ Halogenated Hydrocarbons

(Freons, trichloroethylene, methylene chloride)◦ Styrene

CHRONIC

Sensor Lifetime Sensor Lifetime

ACUTE

Sens

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utpu

t

Sens

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t45TH ANNUAL NORTH DAKOTA SAFETY COUNCIL CONFERENCE FEBRUARY 21-24, 2018 76

Catalytic Bead LEL Sensors

+Proven technology+Direct measurement of flammability

−Can be poisoned−Cannot measure above 100% of LEL−Needs at least 12-16% oxygen for measurements−Difficulty measuring diesel, jet fuel, kerosene and similar vapors−Not sensitive enough for toxicity measurements

AdvantagesDisadvantages


NIOSH FOG Report

Occupational Exposure Limits

Compound (vP)* REL/PEL IDLH* LEL

Methane (40 atm) Asphyxiant 5000 ppm (10% LEL) 5.0% 50,000 ppm

Ethane (20 atm) 1200 ppm/Asphyxiant 3000 ppm (10% LEL) 3.0% 30,000 ppm

Propane (8.4 atm) 1000/1000 ppm 2100 ppm (10% LEL) 2.1% 21,000 ppm

n-Butane (2.8 atm) 800/- ppm 1600 ppm (10% LEL) 1.6% 16.000 ppm

i-Butane (3.1 atm) 800/- ppm 1800 ppm (10% LEL) 1.8% 18,000 ppm

*vP = vapor pressure, 1 atm = 760 mm Hg = 14.7 psi

Health EffectsAsphyxiation:◦ Support life◦ Self-rescue may be impaired

Narcosis (i.e. depression of the central nervous system; anesthesia), ◦ Self-rescue may be impaired◦ A few breaths can rapidly induce unconsciousness◦ Narcotic potency generally increases with molecular weight◦ The effects of individual components in a mixed exposure can be regarded as additive

Cardiac arrest:◦ Abnormal heart rhythm and cardiac arrest, particularly where exposure is accompanied by stress

and exercise◦ Certain pre-existing cardiac conditions may lead to increased sensitivity from the exposure

Upstream Oil and Gas industry needs a simple and reliable method for evaluating the VOC exposure real time in the field

Making sense of LEL (pellistor) Sensor Response

Sensor calibrated to Pentane will produce a reading of 10% LEL when it’s exposed to 10% LEL

Pentane LEL is 1.4% or 14,000ppm

10% LEL is 0.14 % of Pentane or 1,400ppm

Therefore, if we are exposed to Pentane and the reading is 10% LEL, that means we are exposed to appx. 1,400ppm of Pentane

Making sense of LEL (pellistor) Sensor Response

Sensor was calibrated to Pentane; however, the exposure is to Methane

Methane LEL is 5% or 50,000ppm

Correction factor is 0.5

The instrument will show twice the real % LEL

In a nutshell, if we are exposed to 1000ppm (2% LEL) of Methane, the instrument will read 4% LEL

If sensor is calibrated by pentane

1,000ppm of Methane (2% LEL) will read 4% LEL

1,000ppm of Ethane (3.3% LEL) will read 4.7% LEL

1,000ppm of Propane (4.7%LEL) will read 6% LEL

1,000ppm of Butane (5.5% LEL) will read 6.7 % LEL

1,000ppm of Pentane (7% LEL) will read 7.1% LEL

Mean 17.98 25.76 25.87 3.77 11.73 2.51 3.81 1.15 1.83 0.27 0.06 0.22 0.08 2.38 2.57

VOC Mixture Composition

12.66 27.20 29.28 3.94 12.15 2.82 4.24 1.26 2.27 0.32 0.01 0.13 0.03 1.49 2.193

19.89 23.50 20.36 2.53 7.37 2.98 5.34 4.09 7.24 0.57 0.00 0.04 0.00 5.16 0.925

18.34 22.64 29.26 4.08 12.02 2.08 2.59 0.50 0.81 0.06 0.01 0.14 0.03 1.63 5.812

12.37 29.81 34.34 4.28 11.22 2.22 2.95 0.07 0.99 0.09 0.01 0.03 0.03 0.89 0.716

27.34 23.62 23.10 3.22 11.86 1.96 3.32 0.55 0.36 0.04 0.38 0.04 0.02 0.89 3.294

17.16 26.34 28.74 4.36 12.77 2.09 2.84 0.40 0.30 0.03 0.00 0.05 0.02 0.74 4.136

28.35 23.47 20.85 2.71 8.99 2.12 3.59 1.67 2.36 0.49 0.07 0.54 0.08 3.51 1.227

17.42 25.38 29.08 3.77 10.98 2.32 3.17 0.84 1.72 0.25 0.06 0.19 0.17 2.16 2.486

26.46 23.78 22.13 3.00 9.21 2.45 3.67 1.31 2.42 0.35 0.02 0.20 0.04 1.76 3.2

23.72 26.65 24.74 2.97 9.30 2.02 3.05 1.05 1.80 0.38 0.05 0.41 0.08 2.91 0.858

16.66 24.22 28.72 4.25 14.89 2.34 3.11 0.55 0.38 0.04 0.00 0.04 0.01 1.53 3.251

11.22 24.40 25.88 4.75 13.07 3.07 4.69 1.75 3.20 0.58 0.11 0.56 0.19 3.86 2.68

17.31 26.44 26.05 4.12 13.76 2.98 4.05 0.81 1.37 0.17 0.02 0.17 0.02 1.21 1.537

9.37 20.67 23.30 4.65 18.37 4.10 8.35 2.13 1.32 0.14 0.02 0.27 0.08 4.08 3.144

16.81 28.56 25.36 3.86 11.71 2.54 3.90 1.15 1.80 0.33 0.06 0.25 0.10 2.27 1.318

13.41 28.98 27.57 4.18 12.13 2.27 2.94 0.61 0.96 0.25 0.11 0.28 0.21 2.65 3.454

25.33 24.21 23.01 2.84 9.91 2.45 3.61 0.98 1.93 0.30 0.05 0.16 0.12 2.01 3.094

17.76 30.46 24.00 3.54 11.04 2.52 3.63 1.17 1.53 0.36 0.02 0.17 0.04 2.61 1.16

16.82 32.40 25.83 3.61 10.09 1.89 2.49 0.46 0.72 0.13 0.06 0.12 0.07 1.03 4.291

Stressor Concentrations MOLE % (VOLUME %)

Methane Ethane Propaneiso-

Butane Butaneiso-

Pentane Pentane n-Hexaneother

Hexanes BenzeneEthylbenz

ene Toluenem, p, o-Xylene

Other VOC

H2S; N2; H2S

LEL Reading is a function of:

Concentration of the VOC mixture in the air

VOC mixture composition

Correction factor for each mixture constituents

LEL of each constituency

ppm

Making Sense of LEL Reading

Methane(%)

Ethane(%)

Propane(%)

Butanes(%)

Pentanes(%)

Hexanes(%)

Benzene(%)

Ethylbenzene(%)

Toluene(%)

m,p,o- Xylene(%)

Other VOC(%)

17.98 25.76 25.87 15.50 5.51 2.980 0.27 0.06 0.22 0.08 2.38

0.5 0.7 0.8 0.8 1.0 1.4 1.0 1.3 1.3 1.3 1.2

50000 30000 21000 18000 14000 12000 13000 10000 12000 11000 18000

0.719 1.227 1.540 1.037 0.394 0.175 0.021 0.005 0.015 0.006 0.110

Total: 5.247 % LEL

1000 ppm of VOC

0.00

5.00

10.00

15.00

20.00

25.00

30.00

Methane Ethane Propane Butanes Pentanes Hexanes Benzene

% LEL (pellistor) response to various gas compositions

0.00

5.00

10.00

15.00

20.00

25.00

30.00


0.00

10.00

20.00

30.00

40.00


1000ppm- 5.25 % LEL2000ppm – 10.5 % LEL

420 ppm of Propane and Butanes

1000ppm- 6.11 % LEL1600ppm – 10 % LEL

960ppm of Propane and Butanes

1000ppm- 4.71 % LEL2,120ppm – 10 % LEL

420ppm of Propane and Butanes

Experiment

Calculated LEL Sensor Response VOC Concentration (PPM) LEL (%) CF (by pentane) LEL% (in the air) LEL% (reading)

Methane 300 50,000 0.50 0.60 1.20

Ethane 555 30,000 0.70 1.85 2.64

Propane 545 21,000 0.80 2.60 3.24

Butane 360 18,000 0.83 2.00 2.41

Pentane 145 14,000 1.00 1.04 1.04

n-Hexane 70 12,000 1.40 0.58 0.42

Benzene 10 13,000 1.00 0.08 0.08

Toluene 9 12,000 1.26 0.08 0.06

p-Xylene 6 11,000 1.30 0.05 0.04

Total 2000 8.9 11.1

Experiment Results Readings were within 20% of the predicted value:◦ Instruments calibrated to Pentane

◦ 10% LEL on the screen (11% LEL calculated)

◦ Calibrated to Methane as twice LEL% of Pentane:◦ 12% LEL on the screen (11% LEL calculated)

◦ Calibrated to Propane as Pentane ◦ 10% LEL on the screen (11% LEL calculated)

What is the ideal calibration gas?

Proposed Study (Benzene)

The goal is to develop a methodology for modeling the VOC exposure hazard/risk for a certain basin and task using personal gas monitors (benzene exposure risk assessment)

Need other producers to participate

Study DesignResearch team provides Industry Partners with whole-air samplers and instructions on how to use them;

Industry Partners collect whole-air sample when the plume is released (during the initial vessel opening) and return collected samples to NIOSH;

Research team characterizes the “expected gas composition” based on the formation, location in the infrastructure (treater vs. separator vs. battery) and other conditions;

Research team develops a methodology to use personal gas monitors to control a potentially elevated exposure to VOCs.

Equipment Supplied

◦ Whole-air sampling tools

◦ Sample log template

◦ Boxes for shipment

Thermal Conductivity sensors to measure combustible gases

The Thermal Conductivity (TC) sensor operates on the principle of the cooling effect caused by the gas as it passes over a heated coil.Flammable gases tend to conduct heat better (“air conditioning”) than nitrogen which is a great insulator.

As the coil cools, the resistance decreases in proportion to the thermal conductivity of the gas.LEL: the coil acts as the reference bead and the catalytically active bead is connected to the bridge.

V2

V1

Fixed Resistor

VOUT

Fixed Resistor

Active LEL Bead; Disconnected for

Vol% Mode

Deactivated LEL Reference Bead; Used for TC/Vol%


Thermal Conductivity (TC) Response Factors*

•Gases has a unique TC

•Gases have unique relative responses. •Almost any gas can be measured if its TC higher or lower than that of the matrix gas.

•The gas does not need to be combustible

•No oxygen is required for its operation


Thermal Conductivity Sensors for Combustible Gases

+Great for high range measurements up to 100% by volume+Do not require oxygen

−Secondary measurement (uses cooling affect)−Other cooling gases in matrix can cause “false”alarms−Less sensitive at low levels (0-10% of LEL)

AdvantagesDisadvantages


Non-Dispersive InfraRed (NDIR) Sensors for Combustible Gases

NDIR sensors use the absorption of infrared light to make gas measurements.Many molecules can absorb infrared light, causing them to bend, stretch or twist.The amount of IR light absorbed is proportional to the concentration.

NDIR Sensors for Combustible Gases

The energy of the photons is not enough to cause ionization, and thus the detection principle is very different from that of a photoionization detector (PID).

Ultimately, the energy is converted to kinetic energy, causing the molecules to speed up and thus heat the gas.

NDIR sensorsLight passes through the gas sample and is absorbed in proportion to the amount of C-H bonds present

The filter in front of the detector removes all the light except that at 3.3-3.5 µm, corresponding to C-H bonds

Reference detector provides a real-time signal to compensate the variation of light intensity due to ambient or sensor changes

Concentration = Detector B – Detector A

CH4CH4CH4

CH4CH4 CH4CH4

Measurement side

Reference side

3.3-3.5 µm filters

IR detector A

IR detector B

Nonlinear Molecules

Symmetric Asymmetric BendStretch Stretch

“Bond Stretching” and “Bending” Vibration

Chemical bonds absorb infrared radiation

For infrared energy to be absorbed (that is, for vibrational energy to be transferred to the molecule), the frequency must match the frequency of the mode of vibration

Thus, specific molecules absorb infrared radiation at precise frequencies

NDIR LEL sensors will miss some flammable gases

Flammable gases and vapors that lack the C-H bond will not be seen by the NDIR LEL sensorsSome examples of flammable gases/vapors that NDIR LEL sensors miss◦Hydrogen◦Carbon monoxide◦Ammonia

NDIR SensorsAdvantages

Can measure to 100% by volume

Do not require oxygen

Resist poisons

Disadvantages

Secondary measurement (measures IR absorption of the C-H bond)

Misses some common combustible gases**

More expensive

High power consumption

** Consult manufacture for limitations and correction factors.

NDIR LEL Sensor vs Catalytic LEL

Methane ppm

Ethane ppm

Propane ppm

i-Butane ppm

n-Butaneppm

i-Pentaneppm

n-Pentaneppm

267677 114194 103114 27200 50902 22793 11529

Topics: Electrochemical Sensors

How Electrochemical (EC) toxic gas sensors work.

Understand sensor specifications and how they might impact decision-making.

Understanding sensor cross-sensitivities.

Electrochemical Toxic Gas SensorsGas diffusing into sensor reacts at surface of the sensing electrode.Sensing electrode made to catalyze a specific reaction.“EC” sensors are often called “3-wire” sensors as they have a sensing, reference and counter electrodes.Use of selective external filters further limits cross-sensitivity for new sensors.Unlike “fuel-cell” oxygen sensors EC sensors are not a “one-way trip”.Similar to dry cell battery in construction.

Electrochemical Toxic Gas Sensors

EC Sensors - Regenerative Process

• Unlike “fuel cell” oxygen sensors which have a one-way trip from lead to lead oxide, electrochemical toxic gas sensors are more of a circular process

• Chemical comes in, reacts, generates electrical current, uses up water and then current from the battery is returned to the sensor to regenerate water in the presence of oxygen

• Really a regenerative or circular process as long as you stay within the operating parameters (specs) of the sensor

Stay within the operating parameters and you stay in balance

Another way to look at EC sensors is that they are like a “see-saw”.

Under normal operation the amount of toxic gas in can be balanced by the electricity returned

Gas sample in CO e- Current from battery

Exceed the operating parameters and youruin the balance (and the cell)

However, if too much toxic gas (or sometimes interferent) is added the sensor MAY not be able to balance back out

Gas sample in CO

e- Current from battery

This may exceed the “maximum over-range” of the sensor or “Sensor IDLH”

What do sensor specs mean?

Pumped units should be faster responding unless they have extra tubing in placeSome sensors take significantly longer to respond

Sensors don’t respond instantly

The common O2, LEL, CO, H2S and PID all respond in less than 30 seconds after the gas gets to the sensors

Diffusion units may take longer to respond

0

20

40

60

80

100

120

140

160

180

200

Common Sensor T90 Time in Seconds

→ Response Time: time for sensor to reach its final stable reading. Typically called T90,or time to 90% of response and usually expressed in seconds.

What do sensor specs mean?Response time increases on pumped monitors when extension tubing is used

For most monitors drawing 250-500 cc/min through 1/8” tubing add at least 1 second of lag time for every 10’ of tubing

Response time will increase for larger bore tubing

Always check your pump for strong flow through the tubing because older pumps may not be up to the task

Check with manufacturer on maximum tubing to be used, only under unusual situations should more than 25’ of tubing be used

Always use sample tubing that will not absorb the chemicals that may be present

◦ Teflon is always best

What do sensor specs mean?

Cross-Sensitivity: every sensor has a cross-sensitivity. It can see gases other than the specified gas or vapor that are not filtered out and can react with the electrolyte. These can also be called “interferences”◦The gas can either decrease the signal (negative cross-sensitivity) or increase the signal (positive cross-sensitivity)

◦The actual values may vary between batches because the cross sensitivity is not typically controlled during the manufacturing process

What do sensor specs mean?For safety concerns, a negative cross-sensitivity may present more risk than a positive one, as it will diminish the response to the target gas and so prevent an alarm.

CO sensor cross-sensitivity*

* Cross-sensitivity chart for example only, consult your manufacturer for specific cross-sensitivities

Note: High levels of some chemicals including alcohols, ketones, and amines give a negative response.

Used sensors show increasing response to VOCs

Gas Concentration Response#

H2S 24 ppm 0 ppm

SO2 5 ppm 0 ppm

Cl2 10 ppm 0-1 ppm

NO 25 ppm 0 ppm

NO2 5 ppm 0 ppm

NH3 50 ppm 0 ppm

PH3 5 ppm 0-1 ppm

H2 100 ppm 40 ppm

Ethylene 100 ppm 16 ppm

Acetylene 250 ppm 250 ppm

Ethanol 200 ppm 1 ppm

Ethylene Oxide 125 ppm >40 ppm

Propane 100 ppm 0 ppm

Isobutylene 100 ppm 0 ppm


Hexane 500 ppm 0 ppm

Toluene 400 ppm 0 ppm

Nitrogen 100% 0-4 ppm

H2S sensor cross-sensitivity*Note: High levels of some chemicals including alcohols, ketones, and amines give a negative response.

*Estimated from similar sensors.

Gas Conc. Response

CO 300 ppm <1.5 ppm

SO2 5 ppm about 1 ppm

NO 35 ppm <0.7 ppm

NO2 5 ppm about -1 ppm

H2 100 ppm 0 ppm

HCN 10 ppm 0 ppm

NH3 50 ppm 0 ppm

PH3 5 ppm about 4 ppm

CS2 100 ppm 0 ppm

Methyl sulfide 100 ppm 9 ppm

Ethyl sulfide 100 ppm 10 ppm*

Methyl mercaptan

5 ppm about 2 ppm

Ethylene 100 ppm < 0.2 ppm


Toluene 10000 ppm

0 ppm*

Turpentine 3000 ppm about 70 ppm*

Understanding EC Sensor Cross-sensitivities

“When you hear stampeding hooves think horses not zebras”But sometimes when you can’t find the horses it is time to start looking for zebrasThere is a saying in detection “one man’s noise is another man’s sensor” and sometimes cross-sensitivities can be used to our benefit

+Easily measures “heavier”chemical and fuel vapors

+Continuous readings+Proven technology +Reasonably specific

−Easily measures heavier chemical & fuel vapors−Exotic sensors can be expensive to purchase and to calibrate−Exotic sensors typically have 1 year life

Advantages Disadvantages

Electrochemical Toxic Sensors


Module Seven

Instrument Calibration

119

Learning ObjectivesDescribe the difference between instrument calibration, functional (bump) check, and calibration check.Explain the importance of verifying equipment function.Define the frequency for completing calibration and functional checks.


What is Calibration?A full calibration is the adjustment of the sensor’s response to match the desired value compared to a known traceable concentration of test gas.


What is a Functional (Bump) Test?

A qualitative function check where a challenge gas is passed over the sensor(s) at a concentration and exposure time sufficient to activate all alarm indicators to present at least their lower alarm settings.


What is a Calibration Check?

A quantitative test using a known traceable concentration of test gas to demonstrate that the sensor(s) and alarms respond to the gas within manufacturer’s acceptable limits.


Important Differences

Functional (Bump) test only verifies gas is getting to sensors and the alarm works. Calibration check provides verification of sensor performance in addition to alarm function.Full calibration includes adjustment to the sensor’s response.


Calibration Frequency

A functional test or calibration check is required before each day’s use.• If instrument fails the functional test or calibration check, then a full calibration is required before use

A full calibration should be conducted at regular intervals according to:• Instrument manufacturer’s instructions• Internal company policy• Regulatory agency


Why is Calibration Important?

1 out of every 2,500 gas detectors not tested before use will fail to respond and alarm properly to a dangerous concentration of gas.

Bump test failure rate for instruments tested daily was ~0.3%

A functional test or calibration check should be completed immediately if there is any doubt, at any time, about the sensor’s performance


Fresh Air Set-Up

“Zero” instrument in fresh air prior to:• Calibration, calibration check, or functional test • AND before each day’s use

Use “clean” ambient air because:• It contains humidity representative of the local area • Synthetic air may affect sensor performance due to lack of water vapor

Compare readings in fresh air indicate the correct values ◦ 20.9% for oxygen◦ 0 % LEL for combustible gas◦ 0 ppm for toxic contaminants ◦ 0 ppm for total VOCs (if using a PID)

If they don’t, re-zero again in a known clean air environment.


Regulation References Calibration Requirements

Per OSHA 29CFR 1910.146 “Before an employee enters a Confined Space, the internal atmosphere shall be tested, with a calibrated direct-reading instrument, for”:

OxygenContent

FlammableGases and

Vapor

Potential Toxic Air

Contaminants


Importance of Validation

Improper or irregular calibration and maintenance of instruments can lead to serious incidents, such as asphyxiation, explosions, etc.Only way to determine if an instrument is working properly is to expose it to a known concentration of gas.


Do you know how the person before you used the instrument?What chemical substances or environmental conditions have interacted with the sensors?Can you see the causes of sensor response changes?

Importance of Validation


Oxy % 20.9

% LEL 00.0

CO ppm 0.0

H2S ppm0.0

1) Wear the monitor, preferably in the breathing zone.

2) Turn the monitor on in clean, fresh air away from vehicles and other engines, heaters and hydrocarbon sources.

3) Ensure the monitor is calibrated and perform a function or bump test each time the monitor is used

4) Ensure the monitor is equiped with appropriate sensors for the known hazard. For example, an H2S sensor will only alarm when there is too much H2S

5) Ensure workers are aware of potential hazards, understand instrument readings and proper response to alarms. The 4-gas monitor should continuously monitor for oxygen, combustible gas and vapor, and alarm when toxic gases might be present

Oxy % 20.9

% LEL 00.0

CO ppm 0.0

H2S ppm0.0

Oxy-The oxygen sensor measures the percent of oxygen in the air. Normal air contains between 20.8 and 21 percent oxygen, OSHA defines an oxygen deficient atmosphere as less than 19.5 percent oxygen, and oxygen enriched atmosphere contains more than 22 percent oxygen. Alarm setting < 19.5 % or >22 %

% LEL-Lower Explosive Limit/Lower Flammability Limit sensor is used to measure combustible gas. The value reported is the amount of any combustible gas or vapor present relative to the lower explosive concentration of the calibration gas. The LEL for methane is 5% or 50,0000 ppm. Most meters are set to alarm at 10% of the LEL of the calibration gas (typically methane) and alarms will lock when the LEL is exceeded. Many gases and vapors may have LEL values lower or higher than methane so monitors may need to be calibrated to those gases.

CO ppm- The CO sensor is used to measure the amount of carbon monoxide in air. CO is a toxic gas produced by incomplete combustion. CO is a colorless, odorless gas that interferes with the ability of blood to deliver oxygen to body tissues. CO exposure limits are 50 ppm for a full shift and NIOSH has a ceiling of 200 ppm. Alarm settings > 50 ppm

H2S-H2S sensor measures hydrogen sulfide. Hydrogen sulfide is a toxic gas that has a characteristic rotten egg odor. H2S is very poisonous, flammable, explosive, and corrosive gas. Because high concentrations of H2S destroy your ability to smell the gas, trusting your ability to monitor H2S by its odor is not safe. Health effects vary with how long, and at what level of exposure. At low concentrations H2S can irritate the eyes, nose, throat, or respiratory system; moderate concentrations lead to more severe eye and respiratory effects, headache, dizziness, nausea, coughing, vomiting and difficulty breathing and high concentrations can lead to shock, convulsions, unable to breathe, coma, death; effects can be extremely rapid (within a few breaths). OSHA PEL 20 ppm Ceiling, Alarm 5-20 ppm

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