The Inspectors Most Important Calculations

A Study Guide
for
Pressure Equipment Inspectors

The Inspectors Calcs

Corrosion Rates
Equipment Remaining Life
Inspection Intervals
Next Inspection Date

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December 2014

The Inspectors Most Important Calculations

How are these calculations related?

An important role for Authorized Inspectors is setting appropriate inspection intervals. Intervals are often
based on the Remaining Life of the equipment. The Remaining Life of the equipment is most often
determined using the Corrosion Rate of the equipment. Thus to set the interval, the inspector must know
how to determine the Corrosion Rate and Remaining Life.

Why are these calculations important?

During the API certification exam, you will perform many calculations. e.g. retirement thickness, hydrotest
pressure, etc. At the job-site, engineers usually perform most calculations. But the calcs that every
inspector must successfully perform are the corrosion rate, remaining life and interval calculations. These
calcs belong to the inspector.
Sure, most inspection records programs perform these calculations. But dont be a wimp inspector who
just relies on the computer! Remember, autopilot computers can fly and land an airplane, but we still want
a live pilot in the seat! Lets look at the following situations. If you were this inspector, imagine how silly
you could look!
Situation 1: During the turnaround the plant spends $250,000 to clean, isolate and open a large vessel.
You were the one that had last inspected the vessel and had set the internal interval at 3 years. During the
current inspection, the vessel is found to be in better shape than expected. Another inspector reviews the
old thickness data and determines that you blew the calculations! The internal inspection could have been
set at 10 years. The Process Manager is extremely upset about wasting the money required to prepare this
vessel for this unnecessary inspection.
The result: Youre embarrassed!

Situation 2: You are assigned as the bundle inspector during a chemical plant turnaround. You are
organized, have copies of all the old inspection data, and are basically having a great time inspecting
remote from all the normal turnaround hub-bub. But one of the last bundles inspected has unexpected
deep corrosion and is made of an exotic alloy. The word quickly gets out and suddenly the key Operations
& Maintenance Leads show up at the bundle cleaning area. Their first question is not, Hows the
weather?, or, Hows the bundle cleaners doing?, but ... Will the bundle make the next operational
run??? You sheepishly look at the past records and tell them you will not know until after you have
plugged the data into the computer. They walk away disgusted, muttering something about your lack of
ability.
The result: Youre embarrassed!

Situation 3: A vessel fails and results in a tremendous fire. Two contract craftsmen are injured and will
have permanent disabilities. During the post-fire investigation, it is determined that you made a mistake
when calculating the corrosion rate. The actual corrosion rate was 0.020 ipy, but you missed a decimal
and used 0.002 ipy! The primary cause of the fire was your mistake.
The result: Youre embarrassed! You sorrow for those hurt, youre depressed, you may be fired, you may
have your API certs pulled, and youll probably have to admit your mistakes in a lawsuit. All because of a
silly little decimal point!

These Calculations are Important!!

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The Inspectors Most Important Calculations

Whats a Subscript?
Before proceeding further, lets understand subscripts! In math, subscripts are used to differentiate
between values that use a similar symbol. For example, t often represents thickness. However, many
thicknesses are important to us, e.g. nominal thickness, minimum thickness, etc. Subscripts clearly
communicate which thickness you are using in a calculation. Always use subscripts in Corrosion Rate
calculations. YES THAT MEANS YOU!

tmin

a subscript

Table 1 lists the most common thicknesses and the common math symbol.

Table 1:
Types of
Thicknesses

Thickness

Math Symbol

Nominal thickness

tnom or tnominal

Initial thickness

tini or tinitial or tbase

Previous thickness

tpre or tprevious

Last thickness

tlast or tcurrent or tactual

Retirement thickness

tretirement or tmin or tminimum

Notes
Based on std pipe or plate thickness
At a TML - First thickness reading.
Also called base or first reading.
At a TML - The previous(prior)
thickness reading
At a TML - The most recent thickness
reading. Also called current or actual
thickness
Minimum allowed thickness. Also
called minimum thickness

What is a Speed?
A Speed is how fast something moves. Speed always has units of a distance per time. Speeds are
expressed different ways depending on what units are most convenient. For example, traveling in a car,
speed is measured in miles-per-hour (mph). It would be ridiculous to say were traveling 3,801,600 inchesper-hour, but thats exactly the same as 60 mph. Typical speed units are illustrated in Table 2.

Table 2:
Speed Units

Speed Types
Automobiles
Bullets
Light
Corrosion

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US Speed Units
miles per hour
feet per second
miles per second
inches per year, or
mils per year

US Symbols
mph
fps
mps
ipy or mpy

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A Corrosion Rate is a Speed!

A Corrosion Rate is a Speed! The Corrosion Rate is the speed of Corrosion BUG eating through the
metal! The units commonly used to measure the speed of the corrosion bug are inches-per-year or milsper-year. (A Corrosion Rate in metric measurements would be mm/yr)

Current Speed vs. Average Speed

If were on a trip and want to estimate our arrival time we need to know our average speed. Our current
speed is not that useful. At one moment, we may be driving at 70 mph, and then a few minutes later we get
stuck in traffic and are only going 15 mph. Or, we may be on a long trip and will have to stop for gas and
meals. This reduces our average speed and lengthens the duration of the trip.
Likewise, when we calculate the Remaining Life of equipment, we need to know the Average Speed of the
Corrosion Bug, not an instantaneous speed. Some days the bug may be eating faster, and other days
slower. The arrival time to the retirement thickness will be based on the average speed of the Bug.

How is Average Speed Calculated?

Current speeds are measured by various devices. For example, the speedometer measures the current speed
of a car, and a radar gun measures the current speed of a baseball. But these devises do not measure the
average speed of an object. An average speed must always be calculated. The formula for average speed is
always:

Average Speed = Distance Traveled

Travel Time

Illustration: An Average Car Speed

Youre on a trip and have traveled 340 miles in 6 hrs. Your high speed was 73 mph, but you stopped once
for lunch and were slowed by some construction work. What is your average speed?

Speed =

340 miles
6 hours

= 56.7 mph

Key! All speeds always use the term per, as in miles-per-hour, or inches-per-year. In mathematical
terms, what is the meaning of per? PER means DIVIDED BY. Remember this and you will
always be able to write the formula for a corrosion rate or any other speed. So miles-per-hour is just
miles-divided by-hours.

Determining a Speed
Automobile Speed (mph):
Corrosion Speed (ipy):

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Miles Traveled DIVIDED BY Hours Traveled

Inches Traveled DIVIDED BY Years Traveled

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The Inspectors Most Important Calculations

Illustration: Taking a Vacation

Determining the Corrosion Rate and Remaining Life is a major role for an authorized inspector. The best
way to learn these calculations is by Taking a Vacation. Lets roll! Alamo here we come!
We are heading west from Louisiana and want to see the Alamo in San Antonio. At 1:00 p.m. we cross the
Texas/Louisiana border, highway mile marker 880. At 3:00 p.m. we pass downtown Houston, mile marker
750. If we maintain the same average speed, when will we reach San Antonio, mile marker 555? Lets
calculate!

Travel direction
El Paso
0

Fort Stockton

San Antonio

220

555

Mile Markers

750 @ 3:00 p.m.

Step 1: Calculate Average Speed

Speed =

First we must calculate our

Average Speed

Speed =

Step 2: Calculate Remaining Trip Time

880 @ 1:00 p.m.

Distance Traveled
Time Traveled
880 - 750
=
3:00 1:00

Time Remaining =

Next, we must determine the

Remaining Life of our trip

Louisiana
Border

Houston

Time Remaining =

130 miles
2 hours

= 65 mph

Distance To Go
Speed
750 555
195 miles
= 3 hrs
=
65
65 mph

Step 3: Determine Arrival Time

Arrival Time = Current Time + Remaining Time
Now, lets determine our
Arrival Time

Arrival Time = 3:00 p.m. + 3 hours = 6:00 p.m.

The Key!!! Understand these simple steps! When calculating the

equipments Next Inspection Date, we use the same basic steps!

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The Inspectors Most Important Calculations

How do I calculate the Corrosion Rate?

The introduction is complete. Time to direct our attention to real problems. Lets first learn how to
calculate a Corrosion Rate. Later well learn how to calculate Remaining Life and the Next Inspection
Date. Remember a Corrosion Rate is a Speed! Its how fast the Corrosion BUG is eating through the
metal! And speeds are always distance traveled divided by the time traveled. In corrosion, the
DISTANCE TRAVELED is the METAL LOSS. Here is the formula.

Formula #1:
Corrosion Rate

Corrosion Rate =

metal loss
time period

tprevious tlast
time period

Illustration: Calculate the Corrosion Rate

Here is the thickness data for a specific TML. Calculate the Corrosion Rate.

Thickness Data
TML #

Jan. 2014

Jan. 2004

01

0.450

0.500

CR =

tprevious tlast
time

CR =

0.500 0.450
2014 2004

0.050
10

= 0.005 ipy

Always add units to your answer!

Dont say the corrosion rate is 0.005.
Is that 0.005 mm/yr or 0.005 ft/second?

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The Inspectors Most Important Calculations

Time to Practice: Calculate the Corrosion Rate in the following exercises. An Answer
Key is provided in the website as another pdf file to download.

Exercise 1: Calculate the Corrosion Rate

Thickness Data
TML #

Jan. 2015

Jan. 1995

08

0.260

0.400

CR =

tprevious tlast
time

CR =

ipy

Exercise 2: Another Corrosion Rate Calculation

Thickness Data
TML #

Nov. 2014

Nov. 1999

C-5

0.675

0.750

Remember! Always write the formula!

CR =

CR =

ipy

Always write the formula! Failure to write the formula

is the Number 1 cause of calculation mistakes!

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The Inspectors Most Important Calculations

Illustration: Calculate a Corrosion Rate using Mils

You can also work corrosion rate problems in mils. A mil is equal to a thousandth of an inch (1 mil =
0.001). I like to work these problems in mils since there are fewer decimals and it is much easier to
say mils per year vs. thousandth of an inch per year!
Here is the thickness data for a specific TML. Calculate the Corrosion Rate.

Thickness Data
TML #

March 2015

March 2008

TH-3

0.280

0.350

Convert thickness readings to mils; 280 mils & 350 mils

CR =

tprevious tlast
time

CR =

350 280
2015 2008

70
7

= 10 mpy
Since we used mils, the units on
the answer are in mils-per-year

Exercise 3: Calculate the Corrosion Rate with Mils

Calculate this Corrosion Rate using mils.

Thickness Data
TML #

Nov. 2014

Nov. 1992

32

0.410

0.480

Remember! Always write the formula!

CR =

CR =

mpy

What is this Corrosion Rate in ipy? __________ ipy

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The Inspectors Most Important Calculations

Calculating a Corrosion Rate when readings were taken on different Months

In our previous examples, the thicknesses readings were taken exactly on the same month. Therefore, the
length of time between the thickness readings was in whole years. In real life, the readings are often
taken at different months. This makes the calculation a bit trickier. Lets illustrate!
The Key to this Problem! We have to convert the Month into a fraction of a Year. For example, suppose a
reading was taken in May 2003. May is the 5 th month. Since there are 12 months in a year, May is 5/12ths
of a year. (and 5/12 = 0.42). Therefore, May 2003 can be represent as 2003.42.

Exercise 4: Convert Months to Years

Complete this Table. Row #1 is completed as an example.

Date

Month #

Fractional
Year

Decimal
Year

Total Years

April 1998

4th

4/12

0.33

1998.33

Feb. 1995

June 2011

Nov. 2007

Illustration: Calculate a Corrosion Rate with Odd Months

Heres the data. Lets do it!

Thickness Data
TML #

March 2015

Oct 2012

N-2

0.275

0.288

Step 1 - Convert the months to years.

March 2015 = 2015.25 (March is 3rd month, so 3/12 of a year, which = 0.25 yrs)
Oct 2012 = 2012.83 (Oct is 10th month, so 10/12 of a year, which = 0.83 yrs)

Step 2 - Calc Corrosion Rate

CR =
CR =

tprevious tlast
time
0.288 0.275
2015.25 2012.83 =

0.013
2.42

= 0.0054 ipy

Note! The dates have been

converted to numbers!!!

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The Inspectors Most Important Calculations

Exercise 5: Calculate a Corrosion Rate with Odd Months

Calculate this Corrosion Rate. (either in ipy or mpy)

Thickness Data
TML #

Sept 2014

April 2010

HD-2

0.521

0.565

Step 1 - Convert the months to years.

September 2014 =
April 2010 =

Step 2 - Calc Corrosion Rate

Remember! Always write the formula!

CR =

CR =

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ipy

December 2014

The Inspectors Most Important Calculations

How do I calculate the Remaining Life?

The Remaining Life at a specific TML is the length of time remaining until this point corrodes to the
retirement thickness. The Remaining Life is the life from date of the last thickness reading. It is not the
remaining time from todays date (unless the last reading was taken today). The Remaining Life is
basically how long until the Corrosion Bug eats through the Remaining Corrosion Allowance.
Retirement thickness is also called minimum thickness (tmin). The retirement thickness (minimum
thickness) is calculated using appropriate formulas from the construction code.
Here is the formula for Remaining Life:

Formula #2:
Remaining Life

Remaining Life =

Remaining Corrosion Allowance (RCA)

Corrosion Rate (CR)

tlast tmin
CR

Note! The Remaining Life for an Equipment Item is based

on the TML with the lowest Remaining Life!

Illustration: Calculate Remaining Life

Here is the thickness data for TML #6. The corrosion rate at this TML is 0.005 ipy. Calculate the
Remaining Life.

Thickness Data
TML #

tminimum

Jan. 2015

Jan. 2005

0.270

0.300

0.350

Life =

tlast tmin
CR

Life =

0.300 0.270
0.005

No need to calculate the Corrosion Rate.

It was given in the problem.

0.030
0.005

= 6 yrs

The Equipment Life at this TML is 6 years from

the last inspection date (January 2015).

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The Inspectors Most Important Calculations

Exercise 6: Calculate the Remaining Life

Calculate this Remaining Life for the following TML.

TML Data
TML #

Corrosion Rate

tminimum

March 2015

D-7

0.007 ipy

0.150

0.198

Remember! Always write the formula!

Life =

Life =

yrs

Exercise 7: Another Remaining Life

Calculate this Remaining Life for the following TML.

TML Data
TML #

Corrosion Rate

tminimum

March 2015

D-8

0.003 ipy

0.150

0.211

Remember! Always write the formula!

Life =

Life =

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12

yrs

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The Inspectors Most Important Calculations

How do I determine the Inspection Interval?

Inspection Intervals are determined based on the applicable requirements from the Inspection Code.
Table-3 summarizes the current rules for setting the Inspection Intervals.
Note! When establishing the interval for most inspection types, the Remaining Life must be calculated
first. Without knowing the Remaining Life, the Interval cannot be set!
Table 3:
Interval Rules

Inspection Code
API 510 - Vessels

API 570 - Piping

API 653 - Tanks

Inspection Interval Rule

Inspection Type
Internal

a) Normally, lesser of: 10 yrs or Life, or

b) 2 yrs - if Life is from 2-4 yrs, or
c) Full Life - if Life is < 2 yrs

External

Lesser of: 5 yrs or the Inspection Interval

Thickness Readings

Lesser of: Life or maximum interval specified in

API 570 Table 6-1. (Max interval is either 5 or 10
yrs based on Piping Class)

External

No relationship to Life. (Seems a bit weird, but

thats the way the Code is written)

External

Lesser of: Life of Shell or 5 yrs

Internal

Lesser of: Full Life of Bottom or 20 yrs

UT of Shell

Lesser of: Life of Shell or 15 yrs

How do I determine the Next Inspection Date?

How do you determine when to change the oil in your car? The interval for an oil change is usually
3000 miles. So when is the next oil change due? Its the Last Oil Change plus the Interval.
For inspection schedules, the Next Inspection Date works exactly the same. When is an equipment
inspection due? Its just the Last Inspection Date plus the Interval. Here is the formula.

Formula #3:
Next Inspect Date

Next Inspection Date = Last Inspection Date + Interval

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The Inspectors Most Important Calculations

Illustration: Calculate the Next Inspection Date

Here is the data for a vessel. Determine the Interval and Next Inspection Date for the Internal Inspection.

Vessel Data
Vessel

Remaining Life

Last Inspection

V-100

12 yrs

January 2015

Step 1 - Calculate the Interval

Interval = Lesser of: life or 10 yrs (Per API 510 Code)
= Lesser of: 12 x = 6 yr, or 10 yrs
= 6 yrs
Step 2 - Calculate the Next Inspection Date (NID)
NID

= Last Inspection Date + Interval

= January 2015 + 6 yrs
= January 2021

Exercise 8: Calculate the Next Inspection Date

Here is the data for a vessel. Determine the Interval and Next Inspection Date for the Internal Inspection.

Vessel Data
Vessel ID

Remaining Life

Last Inspection

V-200

14 yrs

March 2015

Step 1 - Calculate the Internal Interval

Interval = Lesser of: __________________________________
= Lesser of: __________________________________
Remember!
Always write the formula!

=
Step 2 - Calculate the Next Inspection Date (NID)
NID

=
=
=

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December 2014

The Inspectors Most Important Calculations

Illustration: Calculate the Next Inspection Date with Odd Months

Not all inspection intervals are in nice round years. Suppose a vessel has a Remaining Life of 9.5 yrs.
Then the Internal Inspection Interval is 4.75 yrs. Determining this Next Inspection Date is a bit more
difficult. Lets show you how its done. Here is the data for this vessel. Determine the Next Inspection
Date for the Internal Inspection.

Vessel Data
Vessel

Remaining Life

Last Inspection

V-300

9.5 yrs

October 2014

Step 1 - Calculate the Interval

Interval = Lesser of: life or 10 yrs (Per API 510 Code)
= Lesser of: 9.5 x = 4.75 yr, or 10 yrs
= 4.75 yrs
Step 2 - Convert the Last Inspection Date to a Number)
October 2014 = 2014-10/12 = 2014.83
Step 3 - Calculate Next Inspection Date (NID)
NID

= Late Inspection Date + Interval

= 2014.83 + 4.75 yrs
= 2019.58

Step 4 - Convent the Partial Year to Months

Partial Year is 0.58 yr (there are 12 months in a year)
# of Months = 0.58 yr x 12 months/yr = 6.96 months = 7 months
The 7th month is July
Step 5 - The Final Answer
July 2019

Looking ahead to the

next Inspection.

To convert year & months

Partial Yr to Months: # of Months = Partial Year x 12
Months to Partial Yr: Partial Year = # of Months / 12

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The Inspectors Most Important Calculations

Exercise 9: Calculate the Next Inspection Date

Determine the Next Inspection Date for the Internal Inspection for each of the vessels in this Table. The
first row is completed as an example.

Last Insp
Date

Last Insp
Date
(number)

Remaining
Life
(yrs)

Inspection
Interval
(yrs)

Next Insp
Date
(number)

Convert
Partial Yr
to Months

Next Insp
Date

V-10

April 2014

2014.33

18.7

9.35

2023.68

8.16

Aug 2023

V-20

Feb. 2015

V-30

Sept 2014

5.5

V-40

Nov. 2011

20

V-50

May 2014

11.2

Short-Term vs. Long-Term Corrosion Rates

Most Codes suggest you calculate the Long-Term and Short-Term corrosion rates. The Short-Term
corrosion rate is based on metal lost between the last two reading thickness at a TML. The Long-Term
corrosion rate is based on metal lost between the first and last thickness readings at a TML.
Advantages: Since process conditions often change over the life of the equipment, the Short-Term
corrosion rate is usually a better indicator of the current conditions. But this corrosion rate is subject to
greater inaccuracy when there are inaccurate thickness readings. The Long-Term corrosion rate is less
affected by inaccurate thickness readings.
The Corrosion Rate used in Remaining Life calculations is usually the greater of the Long-Term and ShortTerm rates.

Formula #4:
Short-Term Rate

Short-Term Rate =

metal loss
time period

tprevious tlast
time period

Formula #5:
Long-Term Rate

Long-Term Rate =

metal loss
time period

tfirst tlast
time period

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The Inspectors Most Important Calculations

Illustration: Short & Long-Term Corrosion Rates

For this TML, we have multiple thickness readings taken at different dates. Calculate the Short-Term and
Long-Term Corrosion Rates and the Remaining Life for the TML.

Thickness Data
tminimum

Jan 2015

Jan 2010

Jan 2007

Jan 2005

0.380

0.440

0.465

0.480

0.500

Step 1 - Calculate Short-Term Corrosion Rate

ST Rate =

tprevious tlast
time period

465 440
5

= 5 mpy

Step 2 - Calculate Long-Term Corrosion Rate

LT Rate =

tfirst tlast
time period

500 440
10

= 6 mpy

Step 3 - Pick the Controlling Corrosion Rate (the highest)

Highest between: 5 & 6 mpy = 6 mpy
Step 4 - Calculate Remaining Life
Life

tlast tmin
=
Corrosion Rate

440 380
6

= 10 yrs

This problem is worked in mils.

If you work it in inches, the
answer is the same!

Which Corrosion Rate should you use?

Long-Term or Short-Term?
Select the larger rate!

What do we do with the Reading on January 2007?

For this TML, absolutely nothing!
Its not needed for either the short or long-term calculations.

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The Inspectors Most Important Calculations

Exercise 10: Short & Long-Term Corrosion Rates

For this TML, we have multiple thickness readings taken at different dates. Calculate the Short-Term and
Long-Term Corrosion Rates and the Remaining Life for the TML. .

Thickness Data
tminimum

Jun 2015

Jun 2009

Jun 2006

Jun 2002

0.640

0.718

0.742

0.745

0.750

Step 1 - Calculate Short-Term Corrosion Rate

ST Rate =

Step 2 - Calculate Long-Term Corrosion Rate

LT Rate =

Step 3 - Pick the Controlling Corrosion Rate (the highest)

Step 4 - Calculate Remaining Life

Life

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December 2014

The Inspectors Most Important Calculations

Illustration: The Monster Calc

This calculation will be more detailed than anything you will have on the API exam. But this is real life!
For this vessel, calculate the Next Inspection Date for the Internal Inspection.

Thickness Data
tminimum

May 2015

Oct 2010

May 2007

Jan 2003

0.395

0.454

0.477

0.492

0.500

Step 1 - Convert Dates to Numbers

May 2015
Oct 2010
Jan 2003

= 2015-5/12 = 2015.42
= 2010-10/12 = 2010.83
= 2003-1/12 = 2003.08

Step 2 - Calculate Short-Term Corrosion Rate

ST Rate =

tprevious tlast
time period

477 454
=
2015.42 2010.83

23
4.59

= 5 mpy

Step 3 - Calculate Long-Term Corrosion Rate

LT Rate =

tfirst tlast
time period

500 454
2015.42 2003.08

46
= 3.7 mpy
12.34

Step 4 - Pick the Controlling Corrosion Rate (the highest)

Highest between: 5 & 3.7 mpy = 5 mpy
Step 5 - Calculate Remaining Life
Life =

tlast tmin
=
Corrosion Rate

454 395
5

59
5

= 11.8 yrs

Step 6 - Calculate Inspection Interval

Per API 510: Lesser of: Life (0.5 x 11.8 = 5.9 yr) or 10 yrs = 5.9 yrs
Step 7 - Calculate the Next Inspection Date (NID)
NID

= Last Insp Date + Interval

NID

= 2015.42 + 5.9 = 2021.32

Step 8 - Convert Partial Year to Month

# of Months = 0.32 x 12 = 3.8 months, about 4th month (April)
Step 9 - Final Answer:

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April 2021

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Exercise 11: The Monster Calc

Heres a Monster Calc for you to do. The steps are listed as an aid. For this vessel, calculate the Next
Inspection Date for the Internal Inspection.

Thickness Data
tminimum

Feb 2015

May 2009

Jun 2004

Nov 1997

0.212

0.288

0.296

0.305

0.322

Step 1 - Convert Dates to Numbers

Step 2 - Calculate Short-Term Corrosion Rate

Step 3 - Calculate Long-Term Corrosion Rate

Step 4 - Pick the Controlling Corrosion Rate (the highest)

Step 5 - Calculate Remaining Life

Step 6 - Calculate Inspection Interval

Step 7 - Calculate the Next Inspection Date (NID)

Step 8 - Convert Partial Year to Month

Step 9 - Final Answer:

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December 2014