The Natural Refrigeration

Creating Your Pressure Vessel Replacement Framework

When it’s time to replace a pressure vessel in an industrial refrigeration system, the stakes can be high and the questions are many. To help operators, engineers, and manufacturers navigate this complex process, IIAR recently hosted a member webinar titled Considerations for Pressure Vessel Replacement. The presentation laid out a question-and-answer framework designed to guide decision-making and ensure long-term mechanical integrity for your refrigeration system.

Creating a framework of operating conditions that describe a system before making a repair or replacement decision can help guide decision makers upfront, said David Crement, Director, Quality, ASME Code & Design for vessel manufacturer RVS, Refrigeration Vessels & Systems Corp.

“Part of this is taking good engineering practices and saying – when was the last time we did a thickness check on this vessel and are the original conditions really what we’re operating with,” said Crement.

“We find with older vessels, someone may have made a decision with one set of conditions in mind such as higher temperatures, but really, they’re operating in a different set of conditions like lower temperatures in a vacuum. We want to make sure we look at all the conditions accurately to make sure if you get audited by an agency, you have a pressure vessel that really fits your operating conditions.”

Creating a framework of operating conditions and taking that to the decision process with a manufacturer is vital because a manufacturer may not know a facility’s specific operating conditions,” said  Tony Lundell, IIAR’s Senior Director of Standards and Safety. “These questions will help prepare you to communicate effectively with your manufacturer about what you need. These questions are good for anyone to consider, but they’re especially useful for someone new to the industry who is responsible for pressure vessels.  They remind everyone what we should be asking.”

Whether you’re planning a replacement now or preparing for future upgrades, these questions yield essential insights every operator can use to build a framework for making vessel replacement decisions:

1. How long will this replacement pressure vessel need to operate safely once installed?

This question sets the foundation for replacement decisions. If the system is expected to run continuously, without planned shutdowns or decommissioning, the pressure vessel should be designated as designed for long-term operation. However, if the pressure vessel is intended for a use period less than 10 years, a similar replacement at the current minimum design pressure may be sufficient and the vessel should be designated as designed for short-term use.

2. What is the minimum design pressure and the lowest operating temperature of the system that the replacement pressure vessel will be operating under once installed?

This question is essential for defining the operating envelope of your replacement pressure vessel. The answers help ensure the vessel is designed to withstand the full range of pressures and temperatures it will encounter—both now and in the future.

Start by assessing the minimum design pressure and the lowest operating temperature based on your system type and installation location. These values vary depending on whether your system is water-cooled, evaporative cooled, air-cooled, or operating on the high pressure side or low-pressure side of the refrigeration system.

The pressure vessel must also be stamped with a Minimum Design Metal Temperature (MDMT) that reflects the lowest expected operating temperature. If you plan to reuse the vessel on another system in the future, consider designing for a higher or lower minimum pressure now. This forward-thinking approach can reduce future costs and expand the pressure vessel’s versatility across multiple installations.

3. Will this new pressure vessel be considered for future use on a different system that has a higher minimum design pressure and/or a lower operating temperature?

If there’s a chance your replacement pressure vessel may be reused on a different system in the future, it’s worth designing for that possibility now. This question helps operators think beyond the immediate installation and consider long-term flexibility.

A pressure vessel designed for a higher minimum pressure can still be used in a lower-pressure system today, protected by appropriate relief devices. But when that pressure vessel becomes available for reuse, it will already be rated for more demanding conditions—saving time, cost, and complexity down the road.

Ultimately, while higher ratings may increase upfront costs, they also enhance mechanical integrity and future-proof the pressure vessel for broader operational use. If reuse is part of your long-term plan, this is a question worth answering.

4. Will this new pressure vessel be carbon steel and/or subject to an environment that could result in external corrosion?

Start by evaluating the installation environment. Is it mild, mildly harsh, or harsh? This determination will guide whether additional wall thickness or protective coatings are needed beyond the standard corrosion allowance included in ASME B&PVC, Section VIII, Division 1 design formulas.

If the pressure vessel is made of carbon steel and exposed to moisture, chemicals, or other corrosive elements, a suitable increase in material thickness is recommended. This helps ensure the pressure vessel maintains its mechanical integrity throughout its service life. Even if the pressure vessel meets minimum thickness requirements, added protection may be necessary depending on the conditions.

Most importantly, manufacturers must be informed of the environmental conditions the pressure vessel will face. This ensures the design accounts for corrosion risks from the onset. If there’s any doubt about the severity of the environment, lean and focus on the side of caution—thicker material is a small price to pay for long-term reliability.

5. Should you install a horizontal or vertical replacement pressure vessel?

Orientation matters—and in most cases, the replacement pressure vessel will match the original layout. But whether horizontal or vertical, the choice should be informed by space constraints, system function, and separation performance.

Vertical pressure vessels are often preferred when floor space is limited, but they require sufficient overhead clearance. Their design is fixed in terms of cross-sectional area, so the vertical distance between the liquid level and the suction outlet becomes critical. If the pressure vessel is used to separate refrigerant liquid from vapor, this spacing must be carefully calculated to avoid liquid carryover and ensure buffer capacity.

Horizontal pressure vessels, on the other hand, offer more flexibility. Designers can adjust the high-level cutout or install internal baffle plates to create a dual-flow approach, effectively altering the vessel’s capacity and separation dynamics. This can be especially useful in systems with variable flow rates or where fine-tuning is needed to maintain performance.

By evaluating these factors early, operators can ensure the replacement  pressure vessel is not only a physical fit—but a functional one.

6. Should the heads of the pressure vessel be hot-formed or stress relieved after cold-forming?

This question is all about managing risk—specifically, the risk of Stress Corrosion Cracking (SCC). SCC is the result of stress (from forming, welding, or external forces) interacting with a corrosive environment and oxygen. While it’s a relatively low risk in closed-circuit refrigeration systems, it’s still a critical factor in vessel design.

If the internal surfaces of the replacement pressure vessel will be exposed to refrigerant, the heads must either be hot-formed or stress relieved after cold-forming. This treatment helps prevent SCC from developing over time. If the pressure vessel will primarily contain oil, the designer should evaluate whether stress relief is necessary based on the specific application.

Post-weld heat treatment (PWHT) is a common method for relieving stress, but it’s not always feasible—especially in pressure vessels with internal gaskets, such as specialty chillers. In those cases, hot-forming the heads during fabrication is the preferred approach. If PWHT is used, specifying a purge gas during the process can help minimize scale buildup, which could otherwise impair system performance.

Ultimately, the owner should work closely with the designer to confirm whether SCC mitigation is required. While SCC is less common in closed systems—especially those with water content up to 0.2%, which can scavenge oxygen—it’s still a risk worth addressing. Getting this detail right at the design stage can prevent costly nuisances and/or failures later.

Click here to find IIAR’s Considerations for Pressure Vessel Replacement webinar, which answers several other questions needed to build out your design framework and built a working “win-win” relationship with the pressure vessel manufacturer.

Led by Tony Lundell, IIAR’s Senior Director of Standards and Safety, the webinar features detailed images of pressure vessels to illustrate common issues and practical solutions—bringing clarity to a process that often involves regulatory nuance, design complexity, and operational risk.

Each question covered by IIAR’s webinar is paired with technical guidance rooted in IIAR’s recognized standards, including the newly revised IIAR 2 Standard, which supports safe design and compliance across the refrigeration industry.