some expensive basic engineering lessons

Some Expensive Basic Engineering Lessons

by

Neville W. Sachs, P.E.

(c) 2018 Neville W. Sachs, P.E., PLLC Mechanical and Materials Engineering and Failure Analysis 1

My plan is a review of some engineering challenges

• The goal of this presentation is to have you think twice before working on an engineering project that you haven’t done before.

• We try to be problem solvers and we sometimes tend to jump in and give an answer that doesn’t quite fit the problem.


Questions to be answered

• Why is a liquid necessary for wet corrosion?

• What is galvanic corrosion?

• What is a good rule of thumb describing when steel welds require preheating?

• When steel is thermally hardened, what happens to its volume?

• What are three requirements for stress corrosion cracking?


The leak – my first view at 60,000+ gpm



Ruptured 24” plant

cooling water pipeRuptured 15 psi steam line

This is a leak in a plant cooling water system and it’s draining

a 375,000 gallon water tower tank. At the time of this photo the flow

is down from a peak of 60,000+ gpm. (How can you estimate the flow?)


Steam turbine driven pump

Somewhere in there

is a check valve

The turbine driven pump was being

manually shut down when the

pump began to rotate backwards.

Then the check valve slammed

shut!

Marks’ Handbook for Mechanical

Engineers has a formula for

calculating the forces, but it is

dependent on the “time to closure”

So, why is that leak an engineering problem?

Engineers have many responsibilities. One of them is to be certain their job is done correctly, and I was the engineer in the area of that pump.

The plant had created a group to improve the reliability and economics of the boiler house where those pumps were located.

As part of that group, we investigated whether the check valves had to be PM’d every year. It was an expensive and difficult job and Operations Management assured us that they never had had any problems with the valves.

Inspections and interviews after the failure found that the operators had adopted a practice of always bringing along a second person when they shut down the pump. That person’s job was to hit the check valve body with a sledge hammer to be sure it closed!

I relearned a very valuable engineering lesson,

like President Reagan said, “Trust but verify”


In this case of water hammer the cost was minimal. Fortunately

there was another valve in the line and two

alternating crews of four men each on a hand

wheel shutthe leak off before

disaster struck.


Cross-section view of a 24” gate valve

In the next example, they weren’t so lucky

This looks like a significant leak!!

Yes, when the water runs out the door of the

power plant, the leak is a significant!!

We should have this fixed in no time!!1. Except the water actually went over the roof and did internal damage.

2. Notice the rust line on the pipe that shows the previous ground level.

What happened


This automated hydroelectric generating station was shutting down one of the turbines when the pressure pulse ruptured the line (penstock).

The leak turbulence eventually ripped out a six foot long section of the 72” diameter penstock and attempted to drain the holding pond.

The damage was that the station was off line for about a year and the generators had to be essentially totally rebuilt. As we understand it, the loss was well in excess of $10,000,000.

Inspection of the other three 100-year-old penstocks showed that they all had significant thinning on the bottom where they were exposed to damp soil.

The engineering lesson –Where there is moisture there

will be corrosion!

• In 1957 Melvin Romanoff published what is generally recognized as the best long-term detailed study of underground corrosion. Since then there have been similar analyses and there is lots of data available.

• The data essentially says that, if the soil is wet, there will be corrosion. The rapidity of the corrosion will vary with soil conditions, but we engineers have to recognize that

old wet pipes are always weak pipes. (c) 2018 Neville W. Sachs, P.E., PLLC Mechanical and Materials Engineering and Failure Analysis 13


Water hammer calculation

• The formula effectively says F = ma

• But, in the case of that slammed check valve, when

a = - ꝏ the force is also close to infinite

• One place we’ve frequently seen water hammer is with manually operated ¼ turn valves.

• The other place we’ve frequently seen water hammer-like forces is with poorly trapped steam lines when the phase change of flashing steam can drive a slug of water at incredible speed. (We’ve seen it literally explode a vented condensate sump!)

Analysis of the removed section showed long term

corrosion along the lower center of the pipe.

Going back to that hydro plant failure …

That was uniform corrosion. This was another corrosion problem - but in this

example it was galvanic

In uniform corrosion the anode and

cathode sites uniformly swap

positions. In galvanic corrosion they

are always the same.

In this galvanic example the bottom

cone was 0.43” thick Monel 400 and

for many years the corrosion rate

averaged about 0.004”/yr.

The material loss was concentrated

at the top of the cone and they

decided to replace the top portion

with Inconel.


Product Evaporator

Monel Cone

Galvanic Corrosion

The remaining Monel measured 0.38” thick when

they replaced the upper section with Inconel.

Ten months later the cone sprang a leak here. (The

bottom of the cone was expected to last at least another 40 years and it was the top that had always failed.)


Product Evaporator

Monel 400 Cone

Inconel 625 Replacement Section

Nine months and 3/8” gone!


Inconel 625

Monel

400Black material

is the external

support steel

Product Evaporator

Monel 400 Cone

Inconel 625 Replacement Section

Inconel 625 is

the cathode

while the Monel

400 is the anode

CONE

SECTION

Scale

Galvanic Corrosion

– Where the anode is always attacked.

This “Galvanic Series” is the

result of using sea water at 200C.

Other conditions may change the

way the metals interact

Active (corrodes)

Magnesium - 1.75

Zinc - 1.10

Aluminum-Zinc Alloy - 1.05

Aluminum (pure) - 0.8

Mild Steel (clean) - 0.5 to 0.8

Mild Steel (rusted) - 0.2 to 0.5

Gray Cast Iron - 0.5

Copper, Brass - 0.2

High Silicon Cast Iron - 0.2

Steel Mill Scale - 0.2

Graphite + 0.4

Platinum + 0.4

Noble (gets protected)

Metal Voltage

Galvanic Corrosion

– Where the anode is always attacked.

When you replace

pipe sections in

kind, the new piece

is ALWAYS the

anode.

Active (corrodes)

Magnesium - 1.75

Zinc - 1.10

Aluminum-Zinc Alloy - 1.05

Aluminum (pure) - 0.8

Mild Steel (clean) - 0.5 to 0.8

Mild Steel (rusted) - 0.2 to 0.5

Gray Cast Iron - 0.5

Copper, Brass - 0.2

High Silicon Cast Iron - 0.2

Steel Mill Scale - 0.2

Graphite + 0.4

Platinum + 0.4

Noble (gets protected)

Metal Voltage

What do you think about the chance of galvanic corrosion on an aluminum subway car ?

If it was held together with aluminum rivets that were anodic to the sheet structural aluminum,

what do you think happened?

They had to replace EVERY RIVET!


If there is a conductive liquid, there will be corrosion!

Important points:

• Different materials have different galvanic potentials.

• Those potentials can change with the solution and the temperature.

• Be very careful of the cathode anode ratio. (If there is a large cathode and a small anode, all of the corrosion current will be concentrated on the small area.)

• More conductive solutions are more efficient in conducting corrosion currents.


Speaking of expensive leaks

This plant had a gigantic press with eight 30” OD, 3000 psi, ≈ 150” stroke hydraulic cylinders. Each of these huge

cylinders had four 3¾” diameter, 193” long tie rods. Below is a plan view of the press with the cylinders and tie rods


Cylinder and Cylinder Tie Rod Locations

1

A

B C

D

2

A

B C

D

3

A

B C

D

4

A

B C

D

5

A

B C

D

6

A

B C

D

7

A

B C

D

8

A

B C

D

Cylinders

Tie rods

The plant had a fatigue failure of one of the tie rods on Cylinder #1 and we were

told the repair cost was $1,000,000.

In order to do the repair, the press had to be partially disassembled and the floor of the pit below it modified.

Their engineering staff hired an “experienced maintenance company” to do the repairs and that involved some massive

work. To make moving the cylinders easier, the maintenance company welded lifting eyes to the sides of the cylinders.


A side view of one of the cylinders with a lifting eye


Side view

of cylinder

wall

3¾” diam

tie rods

Lifting eye

with fluffy

debris

on top

The bad part is the leak


This is a leakage

pattern where

hydraulic fluid is

running down

the cylinder

sidewall

Lifting eye

with debris

on top

A wet fluorescent magnetic particle test of the lifting eye weld

that shows an extensive crack.


The “experienced maintenance

company” didn’t do any material testing

and didn’t use a formal welding procedure

when they added the lifting eyes.

Two eyes/cylinder,eight cylinders,

16 sets of cracks and 16 leaks!


Two eyes/cylinder,eight cylinders,

16 sets of cracks and 16 leaks because of a

lack of welding knowledge!

What would it cost to replace twelve 30”OD hydraulic cylinders?

They closed and sold the plant – because

their engineering people and their contractor didn’t understand basic

welding procedures.

Welding preheat requirements

ASM and AWS handbooks (and others) have detailed instructions about how you can

determine needed preheat temperatures. (I use a Lincoln Welding Preheat Calculator and there are numerous apps.)

To avoid cracking, you have to specify low hydrogen welding techniques and if the metal is thicker than ½” (12mm) and has

more than 0.27% carbon, you have to consider preheating.


Some shaft welding preheat guidelines*


Shaft

Material

Preheat Temperatures for LH Welds

1” diam 2” diam 3” diam 4” diam

AISI 1025 700F 1000F 1500F 2000F

AISI 1030 1000F 1500F 2000F 2500F

AISI 1035 1000F 2000F 2500F 3000F

AISI 1040 2000F 3000F 3000F 3500F

AISI 1045 3000F 3500F 3750F 4000F

AISI 4140 4000F 4500F 4500F 5000F

AISI 4340 5000F 5500F 5500F 5500F

LH = low hydrogen processes

* From Practical Maintenance Page #1 - Repair Welding of Motor and Machine Shafts

Engineered Chain Fractures and a similar welding lesson


• The photo shows a section of engineered chain from a bucket elevator out of a cement plant. (Photo courtesy of Reynold Jeffrey Plc)

• The plant had repeated problems with the side links coming off the chain.

• So they welded “tie pieces”between the side links.

A broken link and “tie piece” – and a repeated welding metallurgy lesson


The link is 5/8” thick and SAE 1045 steel, i.e., 0.45% carbon

A classic brittle fracture

The chevrons show the cracking started at the toe of the weld.

Welding heated the metal and the molten weld nugget solidified at about 27000F, and then, because it hadn’t been preheated, the mass of the part was enough to quench and harden the area around the weld.

When you harden steel you change the molecular structure and increase the volume by up to about 3%, creating an internal stress.

Then the added operating and misalignment stresses cracked the links.


Why is carbon content important?

• It largely governs the heat treatability of the metal and harder metals are almost always stronger. (This is one reason why hardness testing is important.)

• Below about 0.27% carbon, some thermal hardening can take place but ideal microstructures can’t be formed.

• The carbon allows higher hardness to be reached, while other alloying elements, such as chrome, nickel, and molybdenum, allow the hardness to penetrate deeper into a part.

© 2018 by Neville W. Sachs, P.E., PLLC Materials and Mechanical Engineering and Failure Analysis 34

Calculating the carbon equivalent (CE) is the best approach to prevent cracks

• CE = %C + (%Mn + %S)/5 + (%Cr + %Mn + %V)/6 + (%Cu + %Ni)/15

• If the carbon equivalent is more than 0.40 then preheating may be needed.

• If the equivalent is above 0.60 then preheating is generally necessary.

• There are lots of minor variations on this formula

• Some theoretical examples:

• SAE 1030 0.28 to 0.34 C + 0.60 to 0.90 Mn CE could be between 0.40 and 0.52

• SAE 1045 0.43 to 0.50 C + 0.60 to 0.90 Mn CE could be between 0.55 and 0.68

• But that assumes there are no trace alloys in the scrap!

© 2016 by Neville W. Sachs, P.E., PLLC Materials and Mechanical Engineering and Failure Analysis 35

Effect of alloying elements on steel hardening


0.25 0.75

Jominy specimens distance (in) from quenched end

10

Ro

ckw

ell

C h

ard

ne

ss

20

30

40

50

60

0.50 1.00 1.25 1.50 1.75 2.00

Jominy specimen distance (mm) from quenched end 10 20 30 40 50

Ap

pro

xim

ate

ten

sile

str

eng

thfr

om

AS

TM

A 3

70

(p

si x

10

00

)

110

138

156

182

215

255

301

123

0

1020 1040- added carbon

4042 – added 0.25% moly

4340– added 0.25% moly,1% chrome, 1.8% Nickel

4140– added 0.2% moly,1% chrome

In reality, the Rockwell C scale does not exist below HRC 20.

A little more on metallurgy and heat treatment

• The paper mill management was under economic pressure from corporate and trying to make every ton possible.

• Predictive monitoring had said one of the dryer bearings was failing badly. But production was important and they had been pumping grease into it, trying to keep it running for another week until the scheduled shutdown day.


What happened

Overgreasing allowed grease to collect here


Pillow block roller bearing

60" diam, 180" long shell

Head bolted to shell

Drive gear

Support beam

… And then the fire started

The fire was impressive!

They took about 10 minutes to get there, but the plant fire department rapidly hosed it down and put it out.





Drive gear

Support beam

… And the fun began 24 hours later

The decision was to pull the remains of the bearing off the shaft and replace it, and about 12 hours later they restarted the machine.

… when the shaft broke – and the dryer dropped!


Horseshoe-shaped hardened area from being quenched by the fire hose

Details on why it happened

The bearing was failing and grinding itself apart, and the resultant heat caught the grease on fire. The fire was burning for several minutes before they could get a hose on it and the shaft was red hot. Then the blast of water from fire hose quenched and hardened the shaft. That horseshoe-shaped section was about HRC50 while the remainder was HRC22.





Drive gear

Support beam

(In this example, the lesson is that the management should have listened to their Reliability Engineer who had argued that the dryer head should be changed.)

It doesn’t have to be heat treated to have thermal stress

Process vessel demolition


304 stainless steel reactor with carbon steel structural supports


Thermal expansion• Carbon steel – 6.3 x 10-6 in/in/F0

• Type 304 stainless steel – 9.6 x 10-6 in/in/F0

• The vessel runs at a constant temperature of about 1400F but the external steel holddownring and roof members fluctuate with the ambient

– and the sun!


The major stress was the residual weld stress but the thermal expansion stresses added to it

These 304 stainless vessels were downwind from a series of large cooling towers whose mist is high in chlorides.

The result was a series of stress corrosion cracks, eventually almost 1000, that continually grew deeper as time passed. (But interestingly, the length of the cracks didn’t grow past the immediate stress field from the welds.)

As engineers, we should realize that residual weld stresses are yield strength stresses and are enough to cause significant SCC when chlorides are present.


Notice the black product leakage(actually that is

oxidized product)


And the multiple stress corrosion cracks along

a weld

In this case the cost was astronomical

1. Two forty foot diameter and a sixty foot diameter reactor were replaced.

2. The plant went through several slowdowns.

3. Comment – The decision to replace the vessels was made because the projected number and size of the leaks was growing at a rate that would make them have to formally list the reactors as leakage sites, and the product was recognized as being spontaneously exothermic.


Stainless steel is also prone to pitting corrosion in stagnant locations

A primary metals producer installed a 304 stainless steel fire water line over an operating area where there was molten metal. They then filled the water line with “lake water”.

The lake water contained some solids and some minute organisms, including the type of anaerobic bacteria that can eat through steel and stainless steel pipes.

The microbes secreted acids that pitted through the pipe wall. The leaks then oozed down a vertical run, eventually evaporating, but as the water evaporated, the chloride concentration increased.


Stress corrosion cracking in a section of fire water line wall


When they ran a test of the main fire pump the line ruptured

and this photo shows an incredible network of SCC cracks.

Later NDT of the line found several other SCC cracked areas.

Guidelines for understanding SCC in austenitic stainless steel

1. SCC needs a combination of stress, a material that can be affected by the environment, and that environment.

2. As temperatures increase, the likelihood of SCC increases, but stagnant conditions greatly increase the probability.

3. Most SCC occurs in the range of 1000 to 3000F.

4. Usually, if the temperature is well below 1000F, 1000 ppm of chlorides is needed for cracking. But at temperatures above 1000F, 100 ppm is often enough to do damage. (One text says above 500C, 50 ppm can cause SCC in Type 304.)

5. Any time there are evaporating solutions the chance is extremely high.

6. Every know metal has an environment where SCC can occur. For example, steel with nitrates, copper with ammonia, etc.


Back to the first challenge

“Trust, but verify”

This failure happened because the engineering staff responsible for quality control checking

didn’t do their job.


This is the stack before it fell and

absolutely crushed the electrician’s

truck


The driver was killed 30 years to the day that his

dad was killed in an industrial

accident.

The crushed stack after it was lifted off the truck


The collection of bolts found around the base

of the stack. They failed from fatigue, i.e., they

weren’t properly tightened.

Why were the stack bolts – and many of the structural bolts –

not properly tightened?

Why did the electrician die?

- because the engineering staff responsible for quality control checking didn’t do their job.


Questions to be answered

• Why is a liquid necessary for wet corrosion?

• What is galvanic corrosion?

• What is a good rule of thumb describing when steel welds require preheating?

• When steel is thermally hardened, what happens to its volume?

• What are three requirements for stress corrosion cracking?


Answers

• Why is a liquid necessary for wet corrosion? Electrons flow through the metal from the cathode to the anode and the liquid enables the circuit to be completed.

• What is galvanic corrosion? Where the anode and cathode don’t change positions and the one, the anode, is always attacked.

• What is a good rule of thumb describing when steel welds require preheating? When the steel is thicker than ½” and greater than 0.27% carbon.


Why is a liquid necessary for wet corrosion?

What is galvanic corrosion?

What is a good rule of thumb describing when weld preheating is needed?

Answers

• When steel is thermally hardened, what happens to its volume? It increases by about 3%.

• What are three requirements for stress corrosion cracking? Needed are a material that is sensitive to an environment, the environment, and sufficient stress.


Why is a liquid necessary for wet corrosion?

What is galvanic corrosion?

What is a good rule of thumb describing when steel welds require preheating?

When steel is thermally hardened, what happens to its volume?

What are three requirements for stress corrosion cracking?

Thank you for listening

Any questions or comments?

If you think of questions later, please don’t hesitate to call or email me.

Phone - 315-436-1257

Email – [email protected]


some expensive basic engineering lessons

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