Oh well, the safety dance
Ah yes, the safety dance
Oh well, the safety dance
Oh well, the safety dance
Oh yes, the safety dance

("The Safety Dance," sung by Men Without Hats, 1982)

 

YOU DON'T KNOW WHAT YOU DON'T KNOW
ABOUT THE CONDITION OF THE METALLURGY

Many millions of automobile drivers operate a vehicle without realizing how their personal safety depends upon the design of the car's components and how well those components can perform in expected and non-expected conditions.

Likewise, Process Operators have no influence regarding the design of the Piping Network they are responsible for, yet their personal safety and well-being depend upon how well the components of the Piping Network are matched to wide variances in process operations that the hardware is exposed to.

The metallurgy awareness list for Process Operators working in any chemical processing plant includes:

• Understanding the importance of the heat treatment process during the manufacturing of metals because the treatment steps apply to Post-Weld Heat Treatment (PWHT) performed in the field.
• Awareness of processing areas in which Stress Corrosion Cracking (SCC) could occur.
• Knowing the location of "Pipe Breaks" (aka piping interfaces) because this is where tensile stress occurs with thermal expansion and contraction.

 

THE HEAT TREATMENT OF ALLOYED METALS

Why Metals are Heat Treated during Manufacturing

In PTOA Segments #246, PTOA Readers and Students learned how Carbon can be alloyed into base Iron metal to make Carbon Steel. In PTOA Segment #247 PTOA Readers and Students learned how additional alloys can be integrated to improve the metal's hardness, tensile strength, oxidation and corrosion resistance.

Guess what?

Incorporating the alloys into the base metal alone will not achieve the improvement in metal performance, nor is the addition of alloys into the metal structure the only way to change the composition of the metal.

The heat treatment process that a metal is exposed to while being manufactured will change the metal's physical, mechanical, and even chemical properties. The steps of the heat treatment process are targeted to yield improvements in the metal's ductility, hardness, temperature resistance, formability, and machineability.

For example, the Low Alloy Carbon Steels featured in PTOA Segment #247 retain ductility, durability and "weldability," if they undergo a careful heat treatment process during manufacturing.

Otherwise stated, the heat treatment process makes the product metal more robust and durable.Unfortunately, Steel which does not undergo a successful heat treatment process has a long history of failing, causing catastrophic loss of life.

 

The Heat Treatment Process Steps: Heating, Soaking, Cooling

The purpose of the heat treatment process is to achieve greater metal hardness and tensile strength.

As the nearby graphic illustrates, the heat treatment process involves heating the alloyed metal at a specific rate.

For example, a Carbon Steel alloy may be heated gradually and uniformly to 1200 °F.

Once the metal has been uniformly heated to the target temperature, the metal is held at that temperature for a specific period of time (aka "Soaking").

For example, the Carbon Steel alloy might be "soaked" at 1200 °F for 10 hours.

The entire piece of metal must be held at the target Soaking temperature until the desired changes in structure have occurred.

Afterward, the metal undergoes a crucially controlled Cooling step which allows the selection of desired metal properties.

For example, the Carbon Steel could be cooled gradually and intentionally to 850°F over a period of 8 hours.

The Cooling step could be altered to fabricate a metal that changes composition, stays the same throughout the process, or reverts to the original form if it has changed.

Eventually the metal will cool to the ambient temperature.

Note how the metal shown in the nearby graphic was originally made from 50% Austenite. By the end of the heat treatment process most of the Austenite has changed into Bainite.

Without heat treatment, the Low Alloy Carbon Steels featured in PTOA Segment #247 will lose their shock resistance upon exposure to PV Temperatures below -20°F.

Even after going through a heat treatment process, Low Alloy Carbon Steels need to be fortified with a Nickel alloy to withstand PV Temperatures in the -50°F to -150°F range.

 

Post Weld Heat Treatment (PWHT) aka Stress Relieving

Although Process Operators inherit the process made from engineered metals, s/he or they may from time to time witness a post-weld heat treatment (PWHT) process since this method of heat treatment occurs in the field after piping and/or pipe fittings have been replaced.

PWHT, or stress relieving as the process is often identified, repairs damage that occurs during the welding that joins two pieces of metal hardware together. A successfully completed PWHT reduces and then redistributes the stresses within the microstructure of the metal that were created by the action of welding.

The steps of PWHT mimic those of the manufacturing heat treatment process described above. The metal surrounding a heat-affected-zone must be uniformly heated to a specified target temperature. The target temperature must be held for a specific period of time. Then the metal must be cooled at a sufficiently slow rate that prevents the stresses from redeveloping.

Can we talk about the real world situation?

Process Operators will grow irritably weary of waiting for the PWHT process to be completed in the field because the process unit will necessarily have to be down and not making money while the PWHT takes place after line repairs. Everyone will be anxious to get the plant back online.

At times like these the Process Operator must remember that their personal welfare and safety literally depend upon a successful PWHT process being concluded, no matter how much time the process requires. The root cause of several notorious industrial fatalities has been determined to be due to incomplete, poorly executed PWHT.

 

Annealing Metals for Softness

Annealing is a different type of heat treatment process that takes place during the manufacturing process, not in the field as with PWHT.

The big difference between the annealing process and the heat treatment process described above is the purpose of the heat treatment; annealing softens the metal and increases its flexibility by refining the metal's microstructure. The nearby graphic attempts to illustrate how the soft annealing process of sheet metal (on the left) makes the metal more deformable and easily machinable (on the right).

To reiterate ...

An annealed metal is softer and more ductile than it was before the treatment. That's a totally different outcome than making the metal harder and with more tensile strength.

Annealing is associated with Low Carbon Steel described in PTOA Segment #247 because Low Carbon Steel is brittle.

During the manufacturing process of annealed Low Carbon Steel, the metal will be brought to a temperature that is just below its recrystallization temperature and then allowed to slowly and completely cool before being removed from the furnace.

The end result is Low Carbon Steel that is less brittle because the internal stresses and strains present in the metal's structure have been relieved.

 

BEWARE OF STRESS CORROSION CRACKING (SCC) IN STEEL!

The Stress Corrosion Cracking (SCC) of steel has been the root cause of too many deaths and casualties in the chemical processing industries.

Which brilliant PTOA Readers and Students remember that the corrosion process  ... like the rusting of a nail ... is a chemical reaction? SCC is also a chemical reaction.

The textbooks state that SCC is triggered when these three conditions co-exist:

• applied or residual stress, for example high PV Pressure.
• continually high PV Temperature.
• a water-based corrosive media which has chlorides or hydrogen sulfide present.

 

By the time SCC is detectable to the naked eye, a catastrophic failure has happened.

Process Operators should be aware of process services which can be incubators for Stress Corrosion Cracking (SCC).

Note that one contributing component to SCC is hydrogen sulfide (H₂S gas). Hydrogen Sulfide gas and liquid sulfur components called mercaptans are naturally found in crude oil. Several steps of the crude oil refining process are dedicated to the removal of sulfur.

A Unit Operation in a fuel refinery with the following names are a hint to the potential for sulfur-induced SCC:

• 

  By the time an SCC crack is visible to the naked eye, a catastrophic failure has occurred.

  Crude Distillation Tower Unit

• Unit Operations with a Reactor called a "Hydrotreater."
• Unit Operations with a Tower called a "Stripper."
• Amine Unit and Sulfur Unit.
• LPG Unit (Butane and Propane Separator Towers).
• Fuel Gas and Flare Utilites.

 

Removal of Sulfur: Hydrotreating and SCC

A popular process technology that is used for removing liquid sulfur (mercaptans) from hydrocarbon products is a catalytic reaction that converts the liquid sulfur into gaseous H₂S. The reaction takes place under high hydrogen partial pressure at elevated PV Temperatures.

The Combine Feed Exchanger (CFE) that transfers the heat from the Reactor Effluent into the Reactor Feed must be made from metal alloys that are susceptible to a potentially lethal combination of Hydrogen-induced embrittlement and sulfur-induced SCC.

Hydrogen-induced embrittlement AND sulfur-induced SCC

The nearby graphic attempts to illustrate the mechanism for hydrogen-induced embrittlement and H₂S-induced SCC.

H₂S gas interacts with the Iron in the steel and the corrosion breaks the H₂S into little black particles called Iron Sulfide (FeS). The particles of FeS form a layer on the Steel.

The presence of FeS particles anywhere in a processing plant is the tell-tale sign that metal corrosion is occurring somewhere. 

Once the particles of sulfur dissociate from the H₂S and interact with the Iron in the metal to form FeS particles, the elemental Hydrogen atoms (shown as "e" in the schematic) are free to float around.  The layer of FeS particles coating the metal surface help the elemental Hydrogen migrate into the underlying surface of the metal.

The accumulation of Hydrogen generates tremendous pressure within the grains of the metal. Hydrogen embrittlement starts, and when this situation is combined with further increases in static Pressure or cyclic stresses, the metal will fail due to corrosion fatigue ... aka SCC.

The SCC will start as a pit or a crevice. The zone around the tip of the crevice becomes less strong and more impressionable ... like plastic ... which allows the stress crack to develop.

 

The Amine Unit's Potential for Amine-induced SCC and Sulfur-induced SCC. 

The Amine Unit in a fuel refinery is a red flag zone for SCC potential.

The purpose of the Amine Unit is to circulate a specialty chemical called Amine which absorbs the accumulated H₂S gas (aka, Acid Gas) that has been removed from various sources. For example, the H₂S gas converted from mercaptans in the Naphtha Hydrotreater feed discussed above is a contributing stream to the Amine Unit's Acid Gas feed stock.

Amine that is loaded with absorbed H₂S is called "Rich Amine." The H₂S gas is then stripped from the Rich Amine in a Stripper Column. The "Lean Amine" that that exits the Stripper Tower is recirculated to absorb more H₂S gas.

"Lean Amine" solvent can cause Amine-induced SCC in Carbon Steel and Low Alloy Carbon Steel. The "Rich Amine" environment has potential to cause H₂S-induced SCC.  The take-home message is to be alert to SCC in the Amine Unit.

 

Naphtha Reformers and Chloride-induced SCC

Note that Chloride is another contributing factor to SCC.

Chlorides are not found in crude oil naturally but are rather added during the extraction process and/or as HCl (Hydrochloric Acid).

For example, a Naphtha Reformer Rector converts the hydrotreated naphtha into a high-octane gasoline blend stock. HCl is injected into hydrotreated naphtha to promote the performance of the Naphtha Reformer Reactor's catalyst.

Chloride-induced SCC can occur in austenitic stainless steel (the Carbon Steel that has a total amount of Nickle and Chromium over 20%).  When the stew of tensile stress (in the presence of oxygen), chloride ions, and high PV Temperatures exists, Chromium Carbide deposits start to settle in the metal grain boundaries of the austenitic stainless steel and ... voila! ... a pathway to Chloride-induced SCC is established.

 

"PIPE BREAKS" aka PIPING INTERFACES:
WHERE A DIFFERENCE IN THERMAL EXPANSION RATE IS EXPERIENCED

"Pipe Breaks" and the Thermal Expansion of Metals.

There is not one sole metal that can suit all processing possibilities within a complex, integrated processing plant.

"Pipe Breaks" are the interface between two types of metal within the Piping Network of a processing facility.

Process Operators should be aware of their location because metals expand and contract at differing rates with corresponding increases and decreases in the PV Temperature.

How many times so far in this PTOA Segment have PTOA Readers and Students been warned about the link of weakening metal tensile strength and corrosion?  The tensile strength in pipes is impacted by frequent changes in the PV Temperature and the rate of strain and fatigue these changes cause.

For example:

In PTOA Segment #247, PTOA Readers and Students learned the many benefits of austenitic stainless steel compared to carbon steel.

The benefits of using austenitic stainless steel must be accompanied by an awareness that stainless steel will expand at a significantly faster rate than Carbon Steel.

 

Decoding "Pipe Breaks" on the P&ID

A Piping and Instrumentation Diagram (P&ID) will note where the interface between two types of metals exists.  The exact location of the "pipe break" or "pipe interface" will be indicated on the Piping Isometric Drawings which are typically located in the Mechanical Engineering division of the Maintenance Department.

In the nearby P&ID snippet, a "pipe break" or "pipe interface" is indicated on the downstream side of Temperature Valve 10045 (TV 10045).

The "upstream" side of the valve indicates a metal described as K37 was used to fabricate the 6-inch TV.

The piping on the "downstream" side of the valve is described as fabricated from 6-in type S37 metal.

Furthermore, the P&ID indicates that the weld at the flange that joins the valve to the pipe must be stress relieved (otherwise stated, be PWHTreated).

A "pipe break" is also noted between the inlet and outlet sides of Pressure Safety Valve 10606 (PSV 10606). No stress relieving was deemed to be necessary at this junction, presumably because types C60A and C10 are sufficiently similar in composition to not require stress relieving.

 

TAKE HOME MESSAGES: The personal safety and well-being of Process Operators depends upon the condition of The Piping Network. Knowledge is power! Three areas of concern related to the metallurgy of The Piping Network are:

• The importance of accurately completing all steps of Post Weld Heat Treatment (aka Stress Relieving) of welded joints. 
• Areas in the processing facility where Stress Corrosion Cracking in its various forms can begin. 
• The location of Pipe Breaks (aka Piping Interfaces) since these are also the locations of accumulating tensile stress and strain due to the differing rates of thermal expansion of metals.

 

Iron Sulfide (FeS) particles are the tell-tale sign that metal is corroding somewhere in the process. 

©2023 PTOA Segment 0248

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

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