Dates: Mar 23, 2026
There are some distinct differences between a revalidation of existing relief devices vs new relief system designs. Understanding the differences between the two, and the opportunities available when designing a new relief system, is critical. The design phase has more flexibility as compared to a revalidation, and it is important to recognize the available methods and guidance.
Existing relief systems are required to be revalidated periodically to ensure that adequate overpressure protection still exists, either after changes in the process itself or changes to relevant Recognized and Generally Accepted Good Engineering Practices (RAGAGEP). This process involves evaluating the as-built relief system in the field and looking for potential deficiencies with the existing installation.
A proper revalidation starts with a data collection phase. Field verified data should be used wherever possible, but keep in mind that the data will also include electronic (and potentially even paper) information. The existing relief systems are then evaluated for proper sizing, performance characteristics such as inlet and outlet pressure losses, and general RAGAGEP compliance. It is important to recognize that these evaluations are performed on relief devices of known manufacturers and models and connected to known piping layouts and equipment items. Following that, any identified deficiencies can be addressed.
Contrast this with a new design that does not yet exist in the field. The design phase is a great opportunity to minimize hazards through inherently safer design. This approach looks at limiting the pressures that can develop in the first place, designing the equipment to handle that maximum pressure, and minimizing the consequences if there is a relieving event or Loss of containment (LOC). Examples of inherently safer design would be the use of less hazardous materials, a reduction of inventory, or operating under less severe conditions.
When it comes to relief devices, API Standard 526 is a widely recognized industry standard that serves as a purchasing specification for flanged steel pressure relief valves used in the process industries. The standard dictates the nominal diameter, flange pressure rating, center-to-face dimensions, flow areas, body and spring materials, and service limits for these valves, ensuring they provide adequate protection against overpressure. API Standard 526 also indicates which body sizes are available for a range of standardized orifice sizes. This is very useful when it comes time to specify which pressure relief valve to order; after all, it is critical to specify a valve that is already available. However, it must be understood that API Standard 526 does come with some limitations. Just because a valve theoretically exists in the standard does not necessarily mean it will perform well. For example, some valves with a larger orifice area relative to a given body size are prone to high inlet pressure losses, which can lead to problems such as chattering. The standard also does not include several common relief devices, such as small relief valves used for thermal relief, rupture disks, or low pressure vents.
The piping that will be attached to a relief valve is important when evaluating the performance of a relief device, i.e., whether it can function without chatter, flutter, or cycling, which are signs of instability caused by system dynamics such as excessive inlet pressure losses or built-up backpressure. However, depending on the stage of design, there may not be any proposed piping to evaluate. Therefore, it is important to establish design criteria for piping evaluation if a proposed routing does not already exist. Theoretical maximum equivalent piping lengths can be found to keep inlet and outlet pressure losses within defined limits for a pressure relief valve; however, consideration should be given to whether the piping is intended to match the body size of the valve or if reducers and expanders will be used on the relief path, as well as other anticipated fittings, such as block valves for maintenance purposes. Additionally, some relief valve manufacturers state that their valves have modulating characteristics. Consider whether credit for modulation will be taken in the initial design phase and, if so, ensure that a specific valve with these characteristics is ordered later on.
Because process design involves several steps, it is important to remember that changes affecting the associated relief system design are inevitable. Process changes may be made due to any number of reasons. For example, results from pilot plant experiments being run in parallel to the process design may fuel changes to the process design. Designs might take months or even years, and market conditions can change greatly, necessitating changes during the design process. Additionally, more detailed designs, such as updated piping plans, the choice of a preferred vendor, or the discovery of additional design constraints, may call for an iterative approach to the relief system design.
Given the inherent differences between a system that already exists in the field and a system that is in the design phase, a new relief system design is not as prescriptive as revalidating an existing relief design. It does provide the unique opportunity to introduce inherently safer design at the outset. A new design also has some flexibility during the design phase, which means design standards and criteria need to be understood and established.
The ioMosaic team has extensive experience designing new relief systems, ranging from small equipment additions in existing units to the design of a completely new facility. We use both Process Safety Office® and Process Safety Enterprise® for all our pressure relief and flare systems (PRFS) design analyses. Combined, these technologies deliver a complete life-cycle management solution for emergency relief systems.
Have questions? Call us today at 1.844.ioMosaic or send us a note. We'd love to hear from you.