Remaining Life Assessment of Refinery

Remaining Life Assessment of Refinery Furnace Tubes Using Omega Simulations

Jerry WilksCITGO PetroleumLemont Refinery

IR Scan of a Coker Furnace

at Lemont Refinery

Summary

The Importance of Refinery Furnace Reliability inProcessing Opportunity Crudes

Causes of Poor Furnace Reliability

Creep Damage and Creep as a Process

Coking Furnace Operations and Why Coker Furnaces Are Difficult to Simulate

Long-Term Changes in Process Conditions

Results and Conclusions

The Importance of Furnace ReliabilityThe production of gasoline, diesel, coke, hydrogen, and other refinery products requires heating hydrocarbons to as high as 1400˚F (760˚C) during processing, and furnace reliability is an important aspect of refinery operations.

As a result of the process temperatures, the metals used for furnace tubes are exposed to temperatures up to 1750˚F (950˚C). Furnace tube degradation occurs primarily due to corrosion, metallurgical changes, and creep. Furnace tube failures occur primarily due to creep.

The highest process temperatures occur in catalytic cracking, 1350°F (718°C) in the catalyst regenerator and 1400°F (760°C) in methane-steam reforming. The coking process temperature is ~950°F (510°C) and 9Cr-1Mo tubes in a Coker furnace typically are heated to a maximum of 1250°F (705°C) when coked. Stainless tubes can be heated to a maximum of 1500°F (815°C).

Processing Opportunity Crudes

The opportunity crudes available in the Midwest come primarily from Canada.

Many Canadian opportunity crudes are defined as “synthetic” crudes – mined instead of pumped & pre-processed.

The mined raw crude is processed to remove rock and produce usable products - crudes that refineries can process like natural crudes. This processing removes lighter distillates in Canada making the available synthetic crudes heavy.

These synthetic crudes also contain sulfur and they are referred to as sour crudes. In general heavy, sour crudes are cheaper and are therefore considered “opportunity crudes.”

Heavy, sour crudes result in significant “resid” production when they are refined. Resid is processed by US refineries into coke – some can be cracked to produce more products, but there will still be more coke produced – making Coker reliability important.

Why Cokers Are Important to Processing Opportunity Crudes

Coker Furnace Tube Reliability

Coker units operate 4-5 years between shut downs. Unscheduled Coker outages due to furnace tube failures are expensive, and the entire refinery can be impacted because crude rate may have to be cut or crude blends changed when the Coker is down or cut back. Therefore, being able to predictfurnace tube life is essential to maintaining refinery furnace reliability and refinery profitability.

Creep is the #1 cause of furnace tube failures. Creep is the time-dependent deformation occurring when metals are subjected to stress (internal pressure) at elevated temperatures.

Predicting Coker furnace tube life using simulations is difficult because the process conditions change continuously due to the formation of coke in the tubes: tube metal temperature, pressure distribution, and flow characteristics change with time.

What a Tube Failure Looks Like

Bulge

Rupture at the BulgeTube Background

Unit: Aromatics Hydrotreater

Alloy: 9Cr-1Mo

Cause ofFailure: Flame

Impingement

Creep is the long-term deformation of metals that typically occurs at elevated temperatures.

Creep deformation is rapid for a short time period when equipment is first put in service (primary creep). Then the deformation rate becomes constant and relatively low for a long time period (secondary creep). Toward the end of life the deformation again becomes rapid (tertiary creep) . Refinery furnaces typically operate in the secondary creep region.

Time

Stra

in (m

/m)

*

I II III

Characteristics of the Creep Phenomenon

Characteristics of the Creep Phenomenon

Creep is a combination of three mechanisms

• Diffusion• Crystal boundary sliding or grain boundary sliding • Dislocation motion – dislocations are linear defects in metal

crystals that cause deformation when they move through the crystal.

Some Similarities between Creep and Chemical Processes:

• There is an activation energy for creep.• Increasing temperature increases the rate creep occurs.• Pressure or stress also effects the creep rate.• Creep can be simulated and the simulation used to predict

what will occur in the future – the remaining life of a furnace tube.

Omega Creep AnalysisBackground: • Technology developed by the Oil Industry in a joint industry

project initiated in 1986 – “Project Omega”.

• Materials Properties Council did the research funded by API (Refining) – Martin Prager was the project leader.

• Omega analysis involves a metal database and applying Omega is geometry specific – the geometry of the component at high temperature has to be included in simulation software.

Omega Definition: Ω is a creep damage coefficient that is related to the strain and the strain rate: Ω = ∂lnε/∂ε. Omega defines the rate at which strain rate accelerates as a result ofcreep strain.

See “Development of the MPC Omega Method for Life Assessment in the Creep Range,” 1994, Martin Prager, ASME for a complete explanation of Omega technology.

.

ε = ln(1-ε0 Ωt)/Ω

ε = true strain (today)

ε0 = initial strain rate

Ω = a material propertythat is a function oftemperature andstress.

t = time elapsed

If you know the strain that occurs at rupture and the initial strain rate for new steel, you can use this equation to determine remaining life.

A Section of the Omega Database

.

.

Temperature – it has a major influence on the creep rate and the remaining life of equipment.

Stress or Pressure – The forces acting on the metal are also important to the rate at which the metal will creep. If the internal pressure is low a furnace tube can operate at higher temperatures before experiencing significant creep damage.

Corrosion or Wall Damage – since corrosion causes the equipment to lose thickness it influences the stress and therefore influences creep. Other mechanisms that reduce equipment wall thickness also play a role – an example is provided later.

Past Service – unlike most types of refinery process simulations, what occurred in the past effects the creep phenomenon today and in the future.

What’s Important in a Creep Simulation?

Omega Software

MPC software & Equity Engineering software both contain the Omega database and equations to do Omega calculations

MPC software is somewhat easier to use. Initially it was only available to companies that participated in the Omega project.

Equity Engineering software is more sophisticated, and it does other types of engineering calculations.

A Simple Example – New Boiler Superheater Tubes

1000

10000

100000

1000000

950 975 1000 1025 1050 1075 1100 1125 1150Temperature -˚F

0

43 2 1

5

Corrosion Rates – mils per year

Omega Remaining Life vs. Temperature for New Boiler Superheater TubesIn a Boiler Temperatures and Pressures Remain Relatively Constant

Rem

aini

ng L

ife -

Hou

rs

Tube Alloy: 2¼Cr-1MoSize: 5.563”OD - 0.25”Wall

141mm OD - 6.35mm WallTemperature: 915˚F - 491˚CPressure: 775 psig – 5343 kPa

Maximum Operating Temperature with

No Corrosion1042˚F - 561˚C

Maximum Operating Temperature

With 5 mpy (0.13mm/year) Corrosion

1019˚F - 548˚C

Temperature -˚C525 550 575 600

1.14 Years

11.4 Years

22.8 Years

The Effects of Past Service

Temperature -˚C525 550 575 600

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1000000

950 975 1000 1025 1050 1075 1100 1125 1150

20

1050

Rem

aini

ng L

ife -

Hou

rs

22.8 Years

Temperature -˚F

1.14 Years

11.4 Years

Tube Alloy: 2¼Cr-1MoSize: 5.563”OD - 0.25”Wall

141mm OD - 6.35mm WallTemperature: 1025˚F - 551CPressure: 775 psig – 5343 kPa


No Corrosion1042˚F - 561˚C


No CorrosionAfter 20 Years at

979˚F - 526˚C

Omega Remaining Life vs. Temperature for New Boiler Superheater TubesTemperature Increased to 1025˚F - 551C

Failure occurs at 20.92 years

if the temperature isn’t lowered.

What This Example of Omega Analysis Shows

The results of Omega analysis can be displayed with a curve thatmakes it possible to quickly evaluate what will happen in the future - a remaining life vs. temperature curve. Pressure vs. remaining life curves can also be generated.

The future maximum operating temperatures depend upon the past operating conditions – this ties into unit productivity & $$.

The effect of corrosion on remaining life – the corrosion effect is not linear with time. Longer design life means larger corrosion effect.

How the design life chosen impacts the maximum operating temperature: longer design life – lower max. operating temperatures.

Past service effects remaining life by gradually lowering the maximum operating temperature. The relationship between the effect of past service and time of past service is not linear – its an exponential relationship with damage occurring more rapidly as time passes.

400

500

600

700

800

900

1000

1100

1200

Tem

pera

ture

-°F

Pres

sure

-ps

ig

Decoking Outages

Average Temperature and Pressure Trends in 13B 2 Furnace During 2005Best Correlation Between Temperature and Pressure Excluding Shutdowns & Startups: 0.9042

400

350

150

200

300

400

350

250

150

Time

Decoking Outages

-Best Correlation Between Temperature and Pressure Excluding Shutdowns & Startups: 0.9042

Average TITemperature

InletPressure

The Coking Process VariablesTe

mpe

ratu

re -

°C

250

400

300

550

450

350

600

500

2750

2000

1750

1500

1250

2250

2500

Pres

sure

-kP

a

Decoking occurs when one of the tube TI’s reads 1250˚F - 705˚C

Improved Temperature and Pressure CorrelationWhen Instrumentation Errors and Ramp-Up Effects Are Removed

Y = 2.1832X + 511.85 R2 = 0.957

800

900

1000

1100

1200

1300

150 200 250 300 350

2400220020001800160014001200

Tem

pera

ture

-˚C

650

600

550

500

450

700

Tem

pera

ture

-˚F

Pressure (psig)

Pressure (kPa)Temperature & Pressure Correlation for a Single Cycle

Y = 2.1832X + 511.85 R2 = 0.957

800

900

1000

1100

1200

1300

150 200 250 300 350

2400220020001800160014001200

Tem

pera

ture

-˚C

650

600

550

500

450

700

Tem

pera

ture

-˚F

Pressure (psig)

Pressure (kPa)

Y = 2.1832X + 511.85 R2 = 0.957

800

900

1000

1100

1200

1300

150 200 250 300 350

2400220020001800160014001200

Tem

pera

ture

-˚C

650

600

550

500

450

700

Tem

pera

ture

-˚F

Pressure (psig)

Pressure (kPa)Temperature & Pressure Correlation for a Single Cycle

This correlation is good enough to use temperature

to calculate pressure.

Important Aspects of the Coker Furnaces

Note that the temperature and pressure follow similar trends, but these variables don’t correlate very well – this is a process modeling problem – it can be dealt with in the simulation by entering data frequently – daily averages were used for this analysis.

Decoking outages occur when any of the temperature indicators on the tubes reaches 1250˚F (705˚C). Only 12 tubes in a firebox have TI’s – 12 out of 50 tubes. Periodic infrared (IR) scans are used to help monitor tube metal temperatures and check for hot tubes and hot spots. Periodic IR scans ensure that the tubes we monitor are representative of furnace conditions.

Coke is forming in the tubes and coke formation effects both the temperature and pressure distributions. Local hot spots due to coke also occur

Throughput is maintained at a nearly constant level so inlet pressure increases as flow drops off due to coking. Outlet pressure also varies with coke build-up downstream of the furnaces. The changing pressure drop in the furnace due to coking makes it necessary to simulate individual tubes.

Past service impacts the remaining life or the maximum operating temperature of the furnace tubes. There is a tradeoff between tube life and maximum operating temperature.

900

950

1000

1050

1100

1150

1200

1250

J-91 J-92 J-93 J-94 J-95 J-96 J-97 J-98 J-99 J-00 J-01 J-02 J-03 J-04 J-05 J-06

Time - Years

Tem

pera

ture

-˚F

13B-2 Coker Furnace - Average Tube Metal Temperatures – Coils 3 & 4Average of Readings from 12 TI’s

Change in OperationsHigher Temperatures

Tem

pera

ture

-˚C

650

600

550

500

Pigging Replaces Steam–Air Decoking

Major Process Changes

Important Aspects of Major Process Changes

In mid-1995 the maximum tube wall temperature reached each cycle increased by ~50°F (27.8°C). That magnitude of change in maximum temperatures has a major effect on creep life.

In March of 1996 the decoking process changed from steam-air decoking to mechanical pigging. In the end this turned out the be the most important major process change.

Due to the more rapid decoking, the overall furnace cycle time was shortened. In a given year the tubes would reach maximums more often.

Also increased processing of opportunity crudes began in 1997 – more on this later.

Before Major Process ChangesA Nearly Normal Distribution

900-

925

925-

950

950-

975

975-

1000

1000

-102

5

1025

-105

0

1100

-112

5

1075

-110

0

1050

-107

5

1175

-120

0

1200

-122

5

1225

-125

0

1250

-127

5

1125

-115

0

1150

-117

5

1275

-130

0

Temperature Ranges (˚F)

Day

s in

Eac

h Te

mpe

ratu

re R

ange

482-496 496-510 510-524 524-537 579-593537-551 649-662566-579551-566 621-635 635-649 662-677 677-691593-607 607-621 691-704Temperature Ranges (˚C)

250

200

150

100

50

0

13B-2 Daily Maximum Tube Temperature Distribution 5/1/91-5/1/95

Average 1131.6˚F610.9˚C

900-

925

925-

950

950-

975

975-

1000

1000

-102

5

1025

-105

0

1100

-112

5

1075

-110

0

1050

-107

5

1175

-120

0

1200

-122

5

1225

-125

0

1250

-127

5

1125

-115

0

1150

-117

5

1275

-130

0

900-

925

925-

950

950-

975

975-

1000

1000

-102

5

1025

-105

0

1100

-112

5

1075

-110

0

1050

-107

5

1175

-120

0

1200

-122

5

1225

-125

0

1250

-127

5

1125

-115

0

1150

-117

5

1275

-130

0


Day

s in

Eac

h Te

mpe

ratu

re R

ange

482-496 496-510 510-524 524-537 579-593537-551 649-662566-579551-566 621-635 635-649 662-677 677-691593-607 607-621 691-704Temperature Ranges (˚C)

250

200

150

100

50

0

250

200

150

100

50

0


Average 1131.6˚F610.9˚C

900-

925

925-

950

950-

975

975-

1000

1000

-102

5

1025

-105

0

1100

-112

5

1075

-110

0

1050

-107

5

1175

-120

0

1200

-122

5

1225

-125

0

1250

-127

5

1125

-115

0

1150

-117

5

1275

-130

0


0

100

200

300

400

500

600

Day

s in

Eac

h Te

mpe

ratu

re R

ange


482-496 496-510 510-524 524-537 579-593537-551 649-662566-579551-566 621-635 635-649 662-677 677-691593-607 607-621 691-704

Average 1133.5˚F611.9˚C

Temperature Ranges (˚C)90

0-92

5

925-

950

950-

975

975-

1000

1000

-102

5

1025

-105

0

1100

-112

5

1075

-110

0

1050

-107

5

1175

-120

0

1200

-122

5

1225

-125

0

1250

-127

5

1125

-115

0

1150

-117

5

1275

-130

0

900-

925

925-

950

950-

975

975-

1000

1000

-102

5

1025

-105

0

1100

-112

5

1075

-110

0

1050

-107

5

1175

-120

0

1200

-122

5

1225

-125

0

1250

-127

5

1125

-115

0

1150

-117

5

1275

-130

0


0

100

200

300

400

500

600

0

100

200

300

400

500

600

Day

s in

Eac

h Te

mpe

ratu

re R

ange


482-496 496-510 510-524 524-537 579-593537-551 649-662566-579551-566 621-635 635-649 662-677 677-691593-607 607-621 691-704482-496 496-510 510-524 524-537 579-593537-551 649-662566-579551-566 621-635 635-649 662-677 677-691593-607 607-621 691-704

Average 1133.5˚F611.9˚C

Temperature Ranges (˚C)

After Major Process ChangesDistribution Skewed to Higher Temperatures

Before Major Process ChangesMaximum & Average Temperatures

900

1000

1100

1200

1300

5/1/91 5/1/92 5/1/93 5/1/94 5/1/95

650

600

550

500

700

Tem

pera

ture

-˚C

Tem

pera

ture

-˚F

Time - Years

Average & Maximum Temperatures Tend to Be Significantly Different

After Major Process ChangesMaximum vs. Average Temperatures

900

1000

1100

1200

1300

5/1/95 5/1/96 5/1/97 5/1/98 5/1/99

Time - Years

Tem

pera

ture

-˚F

Maximum & Average Tube Metal Temperatures from 5/1/95 to 5/1/99Readings from 12 TI’s on Coils 3 & 4

Tem

pera

ture

-˚C

Maximum Temperatures

Average Temperatures

650

600

550

500

700

900

1000

1100

1200

1300

5/1/95 5/1/96 5/1/97 5/1/98 5/1/99

Time - Years

Tem

pera

ture

-˚F

Maximum & Average Tube Metal Temperatures from 5/1/95 to 5/1/99Readings from 12 TI’s on Coils 3 & 4

Tem

pera

ture

-˚C

Maximum Temperatures

Average Temperatures

650

600

550

500

700

650

600

550

500

700

Tem

pera

ture

-˚C

Tem

pera

ture

-˚F

Time - Years

Average & Maximum Temperatures Were Closer After the Process Changes

Pressures Trends

150

200

250

300

350

400

O-96 O-97 O-98 O-99 O-00 O-01 O-02 O-03 O-04 O-05

History of Average Inlet Pressures from 10/27/96 to 5/1/06

Pres

sure

(psi

g)

Time - Years

2600

2400

2200

2000

1800

1600

1400

1200

Pres

sure

(kPa

)

150

200

250

300

350

400

O-96 O-97 O-98 O-99 O-00 O-01 O-02 O-03 O-04 O-05

History of Average Inlet Pressures from 10/27/96 to 5/1/06

Pres

sure

(psi

g)

Time - Years

2600

2400

2200

2000

1800

1600

1400

1200

2600

2400

2200

2000

1800

1600

1400

1200

Pres

sure

(kPa

)

Gradual Downward Pressure TrendSince the Process Changes

Beneficial but Why?

Pressure Increased with 95-97 Process Changes

The pressure distribution was a normal bell curve.

Petro-Chem Pressure Simulation of the Furnace with Coked Tubes

50

100

150

200

250

300

Y = 324.07-0.004X3 + 0.0636X2 -1.469XR2 = 0.9995

The red curve shows the result of polynomialregression of the Petro-Chem simulation data.

C-1

C-3

C-5

C-7

C-9

C-1

1

C-1

3

C-1

5

R-1

5

R-1

7

R-1

9

R-2

1

R-2

3

R-2

5

R-2

7

R-2

9

R-3

1

R-3

3

R-3

5

R-3

7

R-3

9

C-2

C-4

C-6

C-8

C-1

0

C-1

2

C-1

4

C-1

6

R-1

6

R-1

8

R-2

0

R-2

2

R-2

4

R-2

6

R-2

8

R-3

0

R-3

2

R-3

4

R-3

6

R-3

8

R-4

0

Tube Inlet Pressures vs. Tube NumberTu

be In

let P

ress

ures

ps

ig

Tube Identification (C= Convection R= Radiant)

The simulation results for pressureare plotted in blue. The data points

are the tube inlet pressures.

2000

1500

1000

500

Tube

Inle

t Pre

ssur

es

kPa

50

100

150

200

250

300

Y = 324.07-0.004X3 + 0.0636X2 -1.469XR2 = 0.9995

The red curve shows the result of polynomialregression of the Petro-Chem simulation data.

C-1

C-3

C-5

C-7

C-9

C-1

1

C-1

3

C-1

5

R-1

5

R-1

7

R-1

9

R-2

1

R-2

3

R-2

5

R-2

7

R-2

9

R-3

1

R-3

3

R-3

5

R-3

7

R-3

9

C-2

C-4

C-6

C-8

C-1

0

C-1

2

C-1

4

C-1

6

R-1

6

R-1

8

R-2

0

R-2

2

R-2

4

R-2

6

R-2

8

R-3

0

R-3

2

R-3

4

R-3

6

R-3

8

R-4

0

Tube Inlet Pressures vs. Tube NumberTu

be In

let P

ress

ures

ps

ig

Tube Identification (C= Convection R= Radiant)

The simulation results for pressureare plotted in blue. The data points

are the tube inlet pressures.

2000

1500

1000

500

Tube

Inle

t Pre

ssur

es

kPa

The pressures used in the simulation for individual tubes were determined with ratios between the pressure drop across the entire

furnace and the pressure drop in this simulation.

Note: Tubes C-15 & C-16 are actually roof tubes.

The Corrosion Process Change Coker Resid Sulfur Content Began Increasing in 1997 Increasing Sulfidic Corrosion Potential

Sulfur increased due to increased processing of opportunity crudes.

Analysis of Tube Inspection Data

Radiant Section Wall Loss Summary

1968-2006 1994-2006 1997-2006 2002-20063.42 mpy

0.0867 mm/year11.80 mpy

0.299 mm/year12.83 mpy


0.366 mm/year

1968-2002 1994-2002 1997-20022.50 mpy



0.345 mm/year

1968-1997 1994-19970.55 mpy


0.193 mm/year

1968-1994-0.29 mpy

-0.0074 mm/year

Tube wall loss rate began increasing between the 1994 and 1997 inspections.

Negative corrosion due to changes in thickness measurement technology. Readings from inspections conducted in the 1980s indicated no corrosion.

The Pigging Process Change

Contrary to what the companies doing pigging tell you, pigging operations sometimes do damage furnace tubes.

Pigging impacts corrosion because protective corrosion product layers are removed during pigging.

Pigging mechanically damages the tubes.

Damage is not uniform so it is difficult to measure.

What’s a Pig?

Tungsten CarbideTips to Cut the Coke

also cut the steel

Hard Rubber

The pig fits snug ina furnace tube and is

forced to move through the tube with water

pressure

What Happens at a Turn?

The Pig Spikes Dig in and

Shave off Metal

A return bend failed on the inside sweep

during the spring of 2006

Wall loss increased significantly when pigging started &

before sulfur content in resid began

increasing

The most severe damage occurred on the inside

sweep of the return bends where the pig was forced into the return bend wall.

Grooves Cut in Return Bends

Failure occurred on the inside sweep of a return bend.

Other Evidence of Damage

9Cr-1Mo Steel Shavings Found in the Coke Debris after Pigging

So How Is This Process Simulated?Use as much available data as possible to accurately represent the process information: 15 years of temperature data and 10 years of pressure data.

Sample process data at high frequency – daily maximums and averages used in this analysis.

Divide the total 37 years of furnace tube life into sections at major process changes. Interview people who worked on the unit years ago to try to fill in time periods where electronic data is not available.

Model each time period separately and link the time periods with creep damage.

Include pigging damage with corrosion damage.

Use the Petro-Chem pressure drop simulation to develop a relationship between the pressure in the tubes and the pressure drop across the entire furnace. Tie the tube pressures to the TI outputs to determine the remaining life of individual tubes.

Omega Analyses Results for the Most Damaged Tube

Omega Simulation Results for Tube C-16

1000

10000

100000

1000000

10000000

100000000

1000000000

900 1000 1100 1200 1300 1400 1500Temperature ˚F

5

25

1015

3

Rem

aini

ng L

ife -

Hou

rs

Rem

aini

ng L

ife -

Year

s

Life Consumed by 2006 = 25.6%



Temperature ˚C650600550500 700 750 800

x x x

Omega Simulation Results for Tube C-16

1000

10000

100000

1000000

10000000

100000000

1000000000

900 1000 1100 1200 1300 1400 1500Temperature ˚F

5

25

1015

3

Rem

aini

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ife -

Hou

rs

Rem

aini

ng L

ife -

Year

s




Temperature ˚C650600550500 700 750 800

xx xx xx

1217˚F (658˚C) is needed to continue current operations for 3 years till the next turnaround opportunity. Typically 25˚F is subtracted to

provide a safety factor. Therefore, the 1230˚F maximum isn’t high enough and it was recommended the tubes be replaced.

1230˚F (665˚C)

ConclusionsThe tubes in this coker furnace needed to be replaced but they didn’t need to be replaced right away. It was recommended that they be replaced within 18 months or at the half life of tube C-16. The furnace is being retubed in October 2006 – over a year earlier.

High temperature sulfidic corrosion did not appear to play a major role in tube wall damage. From 1968 to 1994 there was no significant wall loss. Wall loss began in 1996 when decoking with pigs began, and this was before the sulfur content of the resid increased. Consequently, the wall loss cannot be attributed to high temperature sulfidic corrosion because the increase in sulfur would not have resulted in the significant change in wall loss that occurred.

Furnace tube simulations need to include past major process changes by dividing the Omega analysis into time periods starting at those major process changes. Changes to the decoking process should be considered significant process changes.

Process variations can be accommodated in Omega simulations by taking into account all the factors effecting creep of the tubes: long term and short term temperature variations, long term and short term pressure variations, changes in corrosion, & changes that increase corrosion and other changes that increase wall loss. Use data collected over short time periods, i.e. – averages of data collected during 24 hour time periods. Averages over longer periods could miss significant process variations.

Remaining Life Assessment of Refinery

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