GC Site Maintenance and Monitoring

8/17/2019 GC Site Maintenance and Monitoring

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New Ideas on Gas Chromatograph SiteMaintenance and Monitoring

By Anwar Sutan

[email protected]

www.i-Vigilant.co.uk



Objectives

• Review current Gas Chromatograph maintenance regimes

– Current practices subect to !i""erent interpretations – #mplementation re$uires skille! technicians

– GC is vulnerable to unsatis"actory per"ormance

– &he use o" automate! system to gather important parameters

– &he use o" log-log chart

– &he use o" live uncertainty value as per"ormance benchmark





Repeatability Test

• &olerance is normally base! on AS&' ()*+, stan!ar!

• ormally !one between / , runs



Repeatability Test

• , runs o" repeatability test on methane

AS&' ()*+, tolerance



Repeatability Test

• +0 hours o" repeatability test 1'ethane (ata2

Calibration gas concentration3 4+.**+

AS&' ()*+, tolerance3 4+.4*+ / 4,.0*+

'inimum measure!3 4+.*45,

'a6imum measure!3 4,.0



Repeatability Test

• +0 hours o" repeatability test 17ropane (ata2

Calibration gas concentration3 ).408

AS&' ()*+, tolerance3 ).588 / ).458

'inimum measure!3 ).40)9

'a6imum measure!3 ).40,)



Repeatability Test

• #s repeatability test really worth !oing:

• GC is capable to per"orm much better than normally use!limits

• 'any people think that i" GC pass repeatability test; it means

– #n "act GC can provi!e a repeate!ly wrong results

• 'ost o" the times; when repeatability test "ails; test was

per"orme! on un-normali<e! basis



Control Charts

• #t is plot o" response "actors an! retention times o" GC

• #t only re"lects the per"ormance o" GC against calibration gas• ormally a contractual re$uirement

• &here are variation on how people !o this

– 8 st! !eviation o" "irst , !ata is set as control limit – 8 st! !eviation o" all !ata is set as control limit

• =hat woul! logical>practical limit be:

– #" GC per"orms well "rom beginning..

– #" GC per"orms poorly "rom beginning..

• =hat woul! pass>"ail criteria be:



Control Charts Simulation

• &o prove a point; consi!er the "ollowing theoretical values

• charts; both o" methane R? have 8 st! !eviation

Current practice say

both GC works

e$ually goo!




• ?irst control chart has synchronous components change

• Secon! control chart has ran!om component changes

Comparing the two

charts; it is visible

that the top chart is

better than bottom

chart






%ooking at the secon!

chart in!ivi!ually;

everything looks






Better 7er"ormance

=orse 7er"ormance





Control Charts

• ealthy GC Control Chart

• e6ane an! eptane per"orm slightly worse than the rest o"

the components



Control Charts

• Actual GC uncertainty



Control Charts

• ?ew points3

– Currently use! control chart looks at components in!ivi!ually an!not as a whole

– &here is no stan!ar!i<e! limit "or the control chart monitoring

• 8 st! !eviation o" the "irst , !ata or all !ata is totally !epen!ent on how

goo! > ba! the GC per"ormance is• &his shoul! not be use! as control limit

– 7er"ormance o" in!ivi!ual component tren! in control chart cannot

be use! to !etermine the actual per"ormance o" the GC

– uncertainty value can be !erive! "rom control chart an! may be abetter representation o" the actual GC per"ormance



Calibration Gas Correctness

• Scenario )3

– Calibration gas has been e6pose! to the winter col! below themi6tureDs !ew point temperature

– Some heavy en!s have migrate! to the bottom o" the bottle

– 'i6ture is non-homo enous an more

– &echnician believes the value written in the certi"icate

– &his can lea! to $uite a signi"icant measurement error



Calibration Gas Correctness

• Scenario 3

– (uring shut !own; a technician put new calibration gas; an! "orgot toput the new component !ata table

– e also "orgot to put a note that new calibration gas has been

installe!

– ?ew months later; ne6t technician came in; wante! to start up the GC – e !i! a "orce calibration on the GC

– &his lea!s to a signi"icant measurement error



Composition Comparison

• #n many places un-normalise! composition between two GC is

compare!



Composition Comparison

• =hen normali<e!; in "act all is goo!



Not Enough Confidence in GC

• E6ample3

– GC result shows that the gas is o""-spec – &echnician is usually calle! to check what is wrong with the GC



Current Regime Summary

• Repeatability test most likely pass

• ?ailure normally !ue to wrong proce!ure• Control chart in!ivi!ual limits cannot be use! to !etermine the

per"ormance o" the GC

• urt er ata process ng rom contro c art ea s to uncerta nty

value that can be use! to !etermine GC per"ormance

• Calibration gas correctness check is o"ten misse!

• =rong composition is compare! lea! to "alse !oubts o" GC

• %ittle con"i!ence in GC measurements



So How can We Detect Such Issue

andImprove Our Confidence?



Automate! System

• A!vantages

– &akes away re$uirement to manual entry !ata to a sprea!sheet

• A lot o" hours o" technician time is save!

• Removes operator error

• Better interpretation o" the !ata

– Alarm "lag in case one o" the parameters "aile! – istorical !ata in the "inger tips



Log-log Chart

• othing new about log-log chart; but3

– &he current practice is to look all gases together in the same chart

– Check is per"orme! to see i" the chart looks linear

– &his !oes not necessarily mean that GC is not healthy



Log-log Chart

• %og chart to be mo!i"ie! as per the GC gas path

• =hen put together result "ails 1R F 0.**42• =hen separate! base! on the gas path in the column; all

results pass



Log-log Chart

• =hat can be !etermine! "rom log-log chart:

– #" the GC valve timings are correct

– #" the calibration gas composition is as per the certi"icate

– &his will provi!e with con"i!ence that GC "unctions correctly on that

moment

– nce we have the con"i!ence on the GC; how can we monitor that itmaintains its repro!ucibility:



Live CV uncertainty value

• &his is !erive! "rom repro!ucibility !ata > control chart

• &he !i""erence is3 all !ata are looke! at together as a whole• ncertainty value can be !erive!

• istorical !ata o" uncertainty can be tren!e!

• Check both long term uncertainty value an! short termuncertainty value



Live uncertainty value



Live uncertainty value with Limit



Last Calibration Date

• n automate! system; no up!ate in calibration !ata means3

– Calibration in the GC has "aile!

– &here is a communication "ailure in the system

– Both issue can be easily checke!



Daily Activities

• Check that the status o" the GC is healthy



How About the Control Chart?

• Control chart is integral part o" the uncertainty calculation

• =e can now interpret it better



Analysis Parameters

• #n case o" "ailures; knowing the healthy value o" the analysisparameters will help troubleshooting

– GC ven temperature

– Carrier gas pressure

– Valve actuating pressure

– Carrier gas "low rate 1through each gas path2

– Bri!ge balance

– Sample vent "low

– Sample pressure

• =hen GC per"ormance is poor; one o" these parameters willhave change!

• &his helps pin-pointing where the issue is





Conclusions

• (oes the current GC maintenance regime provi!e "ull

con"i!ence o" GC per"ormance:

• =hen implemente! the new i!eas will provi!e the "ollowing

bene"its3

–

– Early "ailure !etection base! on uncertainty values

– igher con"i!ence o" the GC

– %ess maintenance time

– %ess unnecessary errors



Thank You

Documents

GC Site Maintenance and Monitoring