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What Is Accumulation? | Process Engineering Glossary
What Is Accumulation?
Accumulation is the pressure rise above a vessel’s maximum allowable working pressure that is permitted to occur while a relief device is discharging. It is expressed as a percentage of the vessel’s rated pressure, and pressure vessel codes specify different allowable accumulation limits depending on the scenario, non-fire overpressure events are held to a tighter margin than fire exposure cases, and installations with multiple relief devices are generally allowed a larger margin than a single device.
Accumulation exists because a relief valve needs some overpressure above its set point to reach full rated flow. The vessel is therefore designed with enough margin above operating pressure to absorb both the valve’s own overpressure requirement and any additional accumulation the applicable code allows.
Applications of Accumulation
Vessel Design Margin
The allowable accumulation percentage directly sets how much margin a vessel needs between its normal operating pressure and its mechanical design pressure. Vessels intended for services with multiple credible relief scenarios are typically designed toward the more generous accumulation allowance so a single relief device can be sized economically.
Fire Case vs Non-Fire Case Scenarios
External fire exposure generates vapor at a much higher rate than most process upsets, and codes recognize this by allowing greater accumulation during a fire case than during a non-fire overpressure event. Distinguishing which scenario governs a given vessel’s relief requirement is one of the first steps in a relief system design.
Multiple Valve Installations
When a vessel is protected by more than one relief device, the codes generally permit a higher combined accumulation than when a single device provides all the protection, on the reasoning that the combined installed capacity carries its own inherent margin. This affects how set pressures are staggered across the installed devices.
Benefits of Knowing Accumulation
Clarifies vessel margin requirements. Knowing which accumulation limit applies tells the engineer exactly how much headroom the vessel needs above operating pressure before relief sizing calculations even begin.
Distinguishes credible scenarios. Separating fire case from non-fire case accumulation limits forces an explicit determination of which overpressure scenarios actually govern the vessel’s protection.
Supports economical multi-valve designs. Understanding the more generous accumulation allowance for multiple devices can make a staged, multi-valve protection scheme more cost-effective than a single oversized device.
Limitations to Consider
Code-specific values. Allowable accumulation percentages vary by the applicable code, the type of vessel, and the specific scenario, so the correct figure has to be confirmed against the governing code for that installation rather than assumed from a different project.
Doesn’t account for downstream effects. Accumulation describes pressure rise inside the vessel, not conditions in the downstream vent or flare system, which need their own separate back pressure evaluation.
Static design assumption. Accumulation limits assume the vessel’s mechanical rating and relief device performance stay as designed; corrosion, fouling, or unaccounted-for modifications can erode the margin the original calculation assumed.
Accumulation FAQ
How does accumulation relate to set pressure and overpressure?
Set pressure is when the valve begins to open, and overpressure is the additional pressure rise above set pressure the valve needs to reach full flow. Accumulation is the overall pressure rise the vessel itself experiences above its rated pressure during the relief event, which is closely related to but not identical with the valve’s own overpressure requirement.
Why do fire case and non-fire case accumulation limits differ?
A fire case generates vapor at a substantially higher rate than most non-fire upsets, such as a blocked outlet scenario, so codes allow more accumulation during fire exposure in recognition of the more severe but shorter-duration nature of the event. This distinction directly affects the required capacity in a relief valve sizing calculation.
How does accumulation affect storage tank and vessel design?
Vessels, storage tanks, and atmospheric vessels are all designed with enough margin above operating pressure to absorb the allowable accumulation, which becomes a direct input to pressure vessel design. This margin matters as much for conventional single-phase relief as it does for reactive systems protected under two-phase relief sizing.
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