LFL Monitoring - FPA · PDF file– NFPA 325 Guide to Fire Hazard Properties ......

1January 18, 2006

LFL Monitoring

MEGTEC Systems

2January 18, 2006

Flammability

• For each volatile flammable substance there is a concentration in air (usually expressed as % of volume, known as Lower Flammable Limit – LFL). Below LFL concentration is too lean to support combustion.– ASTM E681 Test Method for Flammable Limits– NFPA 325 Guide to Fire Hazard Properties…

3January 18, 2006

LFL Monitoring

• LFL – Lower LFL increases safety of processes– Higher LFL reduces energy usage

• Lower process operating costs• Lower emission control operating costs

4January 18, 2006

LFL & Safety Margins

Safety authorities required a 4:1 margin of safety below the LFL.

–NFPA 86 requires enough dilution air to always maintain less than 25% of LFL (for systems w/o monitors)

5January 18, 2006

Safety Margins

• Drying systems or oxidizers are allowed to operate with a 2:1 safety margin (50% LFL) when a continuous flammability monitor is used. (NFPA 86, 9.2.6.1 and 9.2.8)

– Must be real-time– Must provide fast-response– Must be connected to trigger corrective action

at predetermined alarm points

6January 18, 2006

Safety Margins• Processes without monitors typically run at

10% - 12% LFL to avoid reaching 25% in case of accidental upset.

• Adding a monitor allows much higher concentrations to be run.

• Result is cost savings in operation of process (as long as higher LEL does not impact product retained VOC)

7January 18, 2006

Impact of Temperature

• LFL published values are typically calculated at room temperature.– When mixture is heated, it’s flammability

increases and the concentration to achieve 100% LFL is less.

– Therefore a safe level at 75 degrees may become dangerous at elevated temperatures.

– NFPA 86 9.2.5.2• LEL°F = LEL77°F[1-0.000436(T°F-77°F)]

8January 18, 2006

LFL Detectors

• NFPA 86 Annex E lists 4 types– Catalytic– Flame Ionization– Flame Temperature– Infrared

9January 18, 2006

Catalytic

Detectors

10January 18, 2006

Catalytic

• Low cost• Small size• VOC response specific• Poisons and Masking• Slower response time 15-20 sec• Typical for area monitoring not process• Sample passes over catalyst. Temp rise

from oxidation converted to LFL.

11January 18, 2006

Flame Type

Detectors

12January 18, 2006

Flame Type LFL

• Higher initial cost• Requires fuel source for operation• Requires regular calibration

– Auto calibration often available• Senses wide range of hydrocarbons• Proven technology• Response times down to 1 second• High accuracy

13January 18, 2006

Flame Type

• Flame ionization (FID) device good for low LFL ranges.– Typically used for area sources (like catalytic)

• Sample conditioning needed to keep sensing chamber clean.

Detector

Flame Source

Sample Flow

14January 18, 2006

Flame Type

• Pass metered sample across a flame. Oxidation of VOC produces measured signal.

• Flame temperature device good for wide LFL ranges

Thermocouple

Sample Flow

Flame Source

15January 18, 2006

Flame Type LFL

16January 18, 2006

FTA Layout

17January 18, 2006

Infrared

Detectors

18January 18, 2006

Infrared LFL Monitors

• Lower initial cost• Does not require fuel source• Does not require O2 for operation• VOC specific responses• Calibration needs to be done at least once per

year.• 2-5 second response time• Sample conditioning needed to keep sensing

chamber clean.

19January 18, 2006

Infrared LFL Monitors

• IR LFL monitors are sensitive to what VOCs are to be monitored and at what levels.– This requires client inputs into what will be

used in the process to insure the proper monitors are supplied.

– Not only are types of VOCs required, but ratios of each are also required to minimize nuisance alarms.

20January 18, 2006

Infrared LFL Monitors

• If mixtures are used in the process, IR LFL monitors need to be calibrated for worse case VOC and could limit maximum VOC load for process

• One manufacturer has develop their product to use 4 different parameters to change (automatically) according product code

• This may be risky as the inputs are manual and may be missed by operator

21January 18, 2006

IR Layouts

22January 18, 2006

IR Layout

23January 18, 2006

Response Factors

24January 18, 2006

Group AGroup A for IR LFL

0 0.5 1 1.5 2 2.5

Me t hanol

Et ha ne

Ace t ic Ac id

n-He xa ne

P ent a ne

Group A

But a ne

D-Et hyl Et he r

n-P ropyl Ac e t a t e

R e l a t i v e S e n s i t i v i t y

25January 18, 2006

Group BGroup B

0 0.2 0.4 0.6 0.8 1 1.2 1.4

P e nt a ne

n-But yl Ace t a t e

Me t hyl Iso But yl Ke t one

Et hyl Ace t a t e

Et hoxy P ropa nol

Group B

Cyc lohe xa ne

Iso-P ropyl Ace t a t e

But a none

R e l a t i v e S e n s i t i v i t y

26January 18, 2006

Group CGroup C

0.67 0.68 0.69 0.7 0.71 0.72 0.73 0.74 0.75 0.76

P -Xyle ne

Group C

D-Me t hyl Forma mide

P rope ne

R e l a t i v e S e n s i t i v i t y

27January 18, 2006

Group DGroup D

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Me t ha ne

O-Xyle ne

Tolue ne

Group D

Ac e t one

Et he ne

Be nz e ne

R e l a t i v e S e n s i t i v i t y

28January 18, 2006

Typical Example for IR LFL

Client VOC Mixture:– Ethyl Alcohol 39.06%– N-Propyl acetate 14.76%– N-Propyl Alcohol 31.50%– Isopropyl Alcohol 5.53%– N-Heptane 0.29%– Heptane 0.43%

29January 18, 2006

IR LFL Manufacturer Recommended System

• Relatively tight bundle of responses• Calibration "Band B" with 50%

Propane/bal Nitrogen cal check mixture• Presuming that the gases in your mixture

have a relatively constant concentration ratio between them

• 50% LEL propane cal check mixture will read about 75%LEL with our analyzer

30January 18, 2006

Installation Locations

Duct mount– Protect sensor from

• Condensation• Dirt• Heat• Vibration

– Faster response time– Limited access for

maintenance

Remote Mount– Add sample transport

time to safety calculations

– Easy access– May need heated

sample line

31January 18, 2006

Flame or IR?

• Traditional Flame Type LFL are widely used in flexible packaging in North America

• IR LFL systems popular in Europe.– Alcan, Amcor, and Printpack use these in Europe.

• W&H and PCMC now use IR as standard detector.

• IR LFL less costly• Jury is out on which system is best for your

application.

32January 18, 2006

Questions/Thank You

Source material provided by:

Jeff Sampson, Control Equipment CorporationChet Anderson, Honeywell Zellweger Analytics, Inc.