The Process Piping Best Practices Series: Material Selection

In addition to reviewing specifications, codes, and standards, engineers and design technicians must also apply practical knowledge and experience to develop process piping designs that are cost effective, reliable, serviceable, and easy to build. Codes are simply a minimum requirement, and compliance doesn’t guarantee optimal design.  Following codes will usually result in safe solutions – that is the point of the codes – but process piping design expertise is important to achieve good constructability, process operability and maintainability, and to avoid conditions that may be detrimental to a specific process. As new materials, devices, and processes are developed, so too should work methods and design principles. This three-part series will present some practical ideas for new piping installations that will aid in building a cost-effective and well-thought-out facility.

When deciding which pipe system is best for an application, the material is usually the main focus. A range of piping materials, including metallic piping (e.g., carbon steels, stainless steels, nickel alloys, titanium and zirconium), plastic piping (e.g., PVC, CPVC, PP, and PVDF), composite piping (e.g., FRP and advanced composites), and lined piping (e.g., plastic or glass-lined metal or plastic-lined FRP), are used to meet specific process piping application requirements. Industrial and chemical process plastic piping is typically Schedule 80 (SCH 80) with greater wall thickness and pressure/temperature operating range than SCH 40 piping (commonly used for domestic applications).

First, evaluate the chemical compatibility with the material choice, taking into consideration the expected fluid properties and system design life. The selection of piping system materials requires careful evaluation of all possible internal fluid scenarios (including range of typical operations, operating extremes, and start-up, shutdown, and upset conditions) and external exposure and environmental factors.  It is not uncommon to have widely varying conditions inside a process piping system. There may be large swings in pressure, flow, temperature, pH, chemical concentrations, or other factors that may affect the piping and other wetted components. Therefore, it is important to evaluate the potential minimum and maximum extremes and compare them with the pipe and component ratings. Reviewing these potential conditions may well avoid a costly accident in the future.

For example, the introduction of unexpected conditions as a chemical dosing pump is injecting a small amount of acid into a pipe flowing water at 40 psi could result in widely varying scenarios:

Scenario 1: An operator closes the valves on either side of the injection location while the pump is operating. The dosing pump is capable of delivering at up to 250 psi, so line pressure quickly jumps from the usual 40 psi to 250 psi, causing a joint to crack and leak. With minimal flow in the dosing pump, the crack can go unnoticed until the valves are opened again and water begins to flow causing a spraying leak. Practically speaking, this could be avoided by implementing a pressure relief valve; however, it is always best to design a system to be inherently safe, using mechanical devices for supplemental protection or for convenience.

Scenario 2: The water flow stops and the dosing pump continues to pump acid, dropping the pH in the area of the injector to levels low enough to damage the material of the pipe or elastomers. This is an instance where the design fluid properties after mixing would suggest the materials were suitable, but the unexpected extreme drop in pH caused the piping to degrade and fail.

In this example of chemical dosing, the heat of the reaction at the dosing point must also be taken into account. Introducing acid into PVC pipe full of water can yield a sharp temperature increase and, if the water is not flowing, the heat build-up could have important consequences.

Secondly, when evaluating material suitability for the maximum design temperature and pressure, it is critical to review both together rather than independently. Pressure ratings on piping are temperature-dependent, as the material strength (most importantly, tensile) drops with temperature increases, reducing the total pressure the material can withstand before failure. Often, pipe manufacturers will publish a table with a maximum working pressure for each size pipe at 70 to 80°F. For temperatures higher than this, a table is usually provided with derating factors which must always be used to check the pipe selection for an application. It is important to remember that the average conditions are not sufficient to verify correct material selection; maximums must be considered, even if they are rare. In some cases (e.g., for some plastics), minimum temperatures and durations are important, as some materials can embrittle at low end temperatures at or below freezing and become more susceptible to damage/failure.

A common material selection mistake is the use of PVC instead of CPVC for elevated temperatures. The pressure rating of most PVC and CPVC is the same at room temperature and PVC is less expensive, making it an attractive choice. For this reason, erroneous premature decisions to use PVC are sometimes made before evaluating the pressure rating of the PVC pipe at maximum expected temperature. PVC has a steeper derating curve, causing it to have lower pressure ratings at elevated temperatures than CPVC. For example, 2” SCH 80 PVC and CPVC pipe is rated at 400 psi at 73°F, but at 120°F, the derating factor is 0.4 for PVC, but only 0.65 for CPVC. This means that at 120°F, SCH 80 PVC is rated at 160 psi, while SCH 80 CPVC is rated at 260 psi.

The environment outside of the pipe is another design criterion that is often overlooked. In most cases, there are no adverse effects from the ambient conditions outside the pipe, but it must always be reviewed. A potentially serious environmental influence on plastic pipe systems is UV light. If a pipe is run outside with exposure to the sun, over time the UV light can degrade certain materials, particularly plastics or non-metal coatings. This can lead to discoloration, chalking, or even embrittlement. If it is impractical to provide protection from the sun, then the best option is to select a material that is UV stable.

Piping in industrial settings may also be regularly exposed to chemicals, such as fuming acids or oxidizers. This can lead to corrosion on the outside of piping systems, particularly metals, though many plastics can be susceptible to degradation from fuming chemicals as well.

Many systems will require pipe materials to change in certain sections depending on the environment they are to be used. For piping transitions between dissimilar metals, galvanic corrosion may result due to the electromotive force generated by the galvanic cell from the dissimilar metals. This can be minimized by using a nonconductive barrier such as a dielectric union.

By taking a more thorough look at the environment, and possible conditions a specific pipe system will be subject to, some unexpected and potentially expensive short-term or future issues can be avoided.