piping in the process industry

Metallurgical selection for piping in the process industry

Many industrial accepted standards for pressure design address all aspects of piping design for chemical plants and refineries; none discuss all of the considerations for metallurgical selection. These code standards do list materials that are acceptable for use in chemical plants and refineries.This paper discusses some but not all of the resources that a designer can reference in properly selecting materials for an application.
 
1. Introduction: background leading to metallurgical selection

Engineering, procurement, and 
construction companies provide 
professional services in the fields of 
engineering, procurement, construction 
and maintenance as well as project 
management services on a global basis. 
This is an overview of how those 
companies manage technical requirements. 
Engineering, procurement, and construction 
companies are also ISO 9000 certified 
companies. With that status the company 
must maintain its certification by having 
approved company practices, work 
instruction, and checklists. Engineering has 
hundreds of practices and are applied to 
all projects.

The application of the practices 
is based on the scope of work/scope of 
services. Each engineering discipline lead is 
responsible for completing their discipline’s work instructions. Each instruction is completed by the lead engineer plus input from commodity engineer. The commodity engineer is the individual responsible for the technical requirements of the item being procured. In the case of piping, it is the piping commodity engineer. This instruction is the road map to providing a world class facility for any client. Part of the instruction addresses pressure design. Requirements for pressure design are usually to a recognized industry standard.

One of these recognized industry standard 
is ASME (American Society of Mechanical 
Engineers). Pressure design in ASME falls 
under the responsibility of BPTCS (Board 
on Pressure Technology, Codes and 
Standards). All ASME pressure standards 
and codes fall under the umbrella of 
BPTCS. Examples are ASME Section VIII 
rules for pressure equipment, B16 
standards for piping components, and 
B31 standards for piping design. 
For work instructions covering piping, B31 
standards are referenced. In the realm of 
chemical plant and refineries, ASME B31.3 
the Process Piping Code is specified.

2. The importance of metallurgical selection

Metallurgical selection is crucial for equipment engineers, and piping commodity engineers. The metallurgical selection will dictate the material, corrosion allowance, valve requirements, and any special metallurgical fabrication requirements. Taking the information, the piping engineer will build piping material line classes that are in turn associated with the mechanical piping & instruments diagrams. Piping material line classes are the heart of piping. They describe piping components of certain metallurgy, nominal pipe size, and wall thickness.

Piping components are defined as mechanical elements suitable for joining or assembly into pressure-tight fluid-containing piping systems. Components include pipe, tubing, fittings, flanges, gaskets, bolting, valves, and devices such as expansion joints, flexible joints, pressure hoses, traps, strainers, inline portions of instruments, and separators. Once the line classes are developed electronically, that electronic file is loaded into the PDS (Plant Design System), a three dimensional modeling program. Once the designer models his/her piping system and the system is reviewed and approved the designer can issue the isometric drawing for fabrication. The electronic data is extracted to the material management system for procuring and tracking material. Those piping line classes contain full purchase descriptions for the piping components being procured. You should now see why piping material line classes are the heart of piping on projects.

As indicated earlier, piping material line classes are developed from the metallurgical selection data. The data is extracted early in the project development from the Process Flow Diagrams (PFDs). PFDs are the basic design of the unit being designed for a client diagrammatically. These diagrams are then converted into Metallurgical Selection Diagrams (MSDs). Other information is required for the metallurgist to make his/her selection. One way to present process data is through Metallurgical Selection Templates (MST). The MST in combination with the PFD builds the road map for piping engineers.

3. Tools and data for evaluating process streams

Engineering, procurement, and construction companies have their own methods of acquiring the needed data to initiate selection of materials. NACE SP0407 – Format, Content, and Guideline for Developing a Materials Selection Diagram[2] can offer a novice design engineer some idea of where to start. Other references can be found, but the one used most by this author is David Hansen’s Materials Selection for Hydrocarbon and Chemical Plants[3]. Another good reference is API RP 571, Damage Mechanisms Affecting Fixed Equipment in the refining industry[4]. Basic information for facility design is communicated through what is known as a Basic Engineering Design Data (BEDD). The BEDD will list typical seasonal conditions, estimated design life of the facility, any guarantees for output, etc. Typically process units are designed for twenty years of service. In determining the selection, the metallurgist must look at economics of the materials selected, and difficulty of servicing at a period of time if a fitness for service evaluation is determined most economical.

4. Evaluation of a process stream

Metallurgical templates have been developed for the following stream. Commodity: hydrocarbon, hydrogen, and hydrogen sulfide

Phase:
Mixed stream MDMT: -20° F (-29 °C) @ DP
Operating temperature: 728° F (385°C)
Operating pressure: 2000 psig (138 barg)
Design temperature: 850° F (455°C)
Design pressure: 2200 psig (152 barg)
Possible free water, normal operating: No
Possible free water, upset: No
Wet Sour Service: No
H2S in vapor phase: 1.14 mol %
Sulfur in liquid phase: 4.2 wt %
Partial pressure of hydrogen: 1940 psia (134 bar)
Partial pressure of hydrogen temperature: 728° F (385°C)

First evaluate the hydrogen component in the stream. Using API-941 Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants[5], Fig. 1, and following hydrogen partial pressure to 1940 psia at 728° F (385 °C), Fig. 1. API Recommended Practice 941, fig. 1. as a minimum 2.25 chrome - 1 moly steel would be specified. The selection is not finished. Also present in the stream is hydrogen sulfide. The Couper-Gorman Curves[6] must
be referenced. The minimum low alloy material that can be evaluated is 5 percent chrome.

Entering the 5 percent chrome curve, Fig. 2 at 728° F (385 °C) the rate of corrosion is about 45 mils per year. Total corrosion allowance of 0.9 inch (23 mm) over 20 years. Industry allowed corrosion allowance has a maximum of 0.25 inch (12 mm) overall. 0.9 inch (23 mm) corrosion allowance is more than the industry standard. Evaluation other low alloys greater than 5 percent chrome results with about the same corrosion allowance. Using the 18/8 curve, Fig. 3, corrosion is estimated at 1.0 mils per year for a total corrosion allowance of 0.02 inches (0.5 mm) over 20 years. Typical Industry minimum corrosion allowance for stainless steels is 0.03 inches (0.8 mm). Material selected: 316 SS with a corrosion allowance of 0.03 inches (0.8 mm). Is this now correct? No. Operating cases have not been addressed.

5. Conclusion

Material selection for the process industry can be costly if the wrong material is specified for a given application. Premature failure, cost involved in replacing after installation, and lost production with other considerations not evaluated lead to additional cost. Industry standards and codes provide guidance in pressure design, but not until a material is specified. Engineering, procurement, and construction companies have work instructions and Checklists for material selection and make that mandatory for all projects. The metallurgist sets the selection through an analytical process, and the design engineer executes the details for providing safe, reliable plants to your client. Practices, work instructions, and checklists should exist for engineering, procurement, and construction companies/firms.


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About the Author:

Roy A. Grichuk, Director of Piping, Fluor Enterprises, USA. Chairman of the Stainless Steel World Conference 2017

Click here to go to the Event page: Stainless Steel World Conference & Exhibition 2017

References

[1] ASME B31.3, Process Piping Code; American Society of Mechanical Engineers; Latest Edition.
[2] NACE Standard Practice (SP) 0407; Format, Content, and Guidelines for Developing a Materials Selection Diagram; NACE International; Latest Edition.
[3] David Hansen, Robert Puyear; Materials Selection for Hydrocarbon and Chemical Plants; Marcel Dekker, Inc.; Copyright 1996.
[4] API Recommended Practice 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry; American Petroleum Institute; First Edition.
[5] API-941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants; American Petroleum Institute; Latest Edition.
[6] A.S. Couper, J.W. Gorman; Computer Correlations to Estimate High Temperature H2S Corrosion in Refinery Streams; Materials Protection and Performance; Volume 10, Number 1, pages 31-37; Copyright 1971.

 

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