What ASME Section VIII Covers
ASME Section VIII, Division 1 of the Boiler and Pressure Vessel Code (BPVC) establishes the rules for the design, fabrication, inspection, testing, and certification of unfired pressure vessels operating at internal or external pressures exceeding 15 psig. This covers the vast majority of pressurized industrial equipment that is not a boiler — including air receivers, heat exchangers, process reactors, separators, accumulators, storage tanks, and compressed gas vessels.
If your facility operates any pressure vessel built to ASME Section VIII, the code requires that it be protected by one or more pressure relief devices. This guide covers the specific requirements for those devices as defined in ASME Section VIII, Division 1, paragraphs UG-125 through UG-137.
General Requirements for Pressure Relief (UG-125)
UG-125 establishes the fundamental requirement: all pressure vessels within the scope of Section VIII, Division 1 shall be provided with overpressure protection in accordance with the code requirements. There are no exceptions for vessel size, pressure level, or service type — if it is a Section VIII vessel, it needs overpressure protection.
The relief devices must prevent the pressure from exceeding specific limits based on the scenario:
Standard Operating Conditions
Under normal operating conditions, the pressure relief valve(s) must prevent the pressure from rising more than 10% or 3 psi above the MAWP (Maximum Allowable Working Pressure), whichever is greater. This is the most common scenario and applies to the majority of installations.
Multiple Valve Installations
When multiple pressure relief valves are installed on a single vessel and are set per UG-134(a), the combined capacity must prevent the pressure from rising more than 16% or 4 psi above the MAWP, whichever is greater.
Fire Exposure (External Heat)
When the vessel may be exposed to fire or other unexpected external heat sources, supplemental pressure relief must prevent the pressure from rising more than 21% above the MAWP. This is the most generous allowance because fire scenarios are considered emergency conditions.
Safety and Relief Valve Requirements (UG-126)
UG-126 defines the specific requirements for reclosing pressure relief valves — the spring-loaded safety valves, relief valves, and safety relief valves that open to relieve excess pressure and then automatically reclose when pressure returns to a safe level.
Valve Type
UG-126(a) requires that pressure relief valves be of the direct spring-loaded type. This means the valve is held closed by a spring, and it opens when the inlet pressure overcomes the spring force. Pilot-operated safety relief valves (POSRVs) are also permitted under UG-126(b), but only if the main valve will open fully even if the pilot mechanism fails.
Set Pressure Tolerances (UG-126(c))
The set pressure of the valve — the pressure at which it begins to open — must fall within specific tolerances:
| Set Pressure Range | Allowable Tolerance |
|---|---|
| Up to and including 70 psig | +/- 2 psi |
| Over 70 psig | +/- 3% of set pressure |
This means a valve set at 100 psig must actually open between 97 and 103 psig. These tolerances are verified during factory testing and certified on the valve’s nameplate.
ASME Certification Requirement
All pressure relief valves used on Section VIII vessels must bear the ASME “UV” certification mark. This mark confirms that the valve was designed, manufactured, and tested in accordance with ASME Section VIII requirements by a manufacturer holding a valid ASME Certificate of Authorization. Valves without the UV mark cannot be used on Section VIII code vessels.
Nonreclosing Pressure Relief Devices (UG-127)
UG-127 covers nonreclosing devices — primarily rupture discs (bursting discs) and breaking pin devices. These are one-time-use devices that must be replaced after activation.
Key requirements for rupture discs:
- May be used as the sole relieving device on a vessel, or in combination with a reclosing pressure relief valve
- When used alone, the burst pressure at the coincident disc temperature must not exceed the MAWP at the operating temperature
- When installed in combination with a pressure relief valve (upstream of the valve), the space between the rupture disc and the valve must be equipped with a pressure gauge, try cock, or free vent to detect disc leakage
Set Pressure Requirements (UG-134)
UG-134 establishes the rules for how the set pressure of pressure relief valves relates to the vessel’s MAWP:
Single Valve Installation
If a single pressure relief valve is used, its set pressure shall not exceed the MAWP of the vessel. The valve must be set at or below MAWP — never above it.
Multiple Valve Installation
When multiple valves are used to meet the required relieving capacity:
- At least one valve must be set at or below the MAWP
- Additional valves may be set up to 105% of MAWP
- The combined capacity of all valves must meet the total required relieving capacity
Fire Case Supplemental Valves
Supplemental valves installed specifically for fire exposure protection may be set up to 110% of MAWP.
Capacity Requirements
The total relieving capacity of all pressure relief devices on a vessel must be sufficient to prevent the pressure from exceeding the accumulation limits defined in UG-125. The required capacity is determined by:
- The maximum expected heat input or pressure generation rate — for process vessels, this is based on the worst-case operating scenario
- Fire exposure calculations — for vessels subject to fire, API 521 provides methods for calculating the required relieving rate based on wetted surface area
- Blocked outlet scenarios — the valve must be able to handle the full flow rate if downstream equipment is blocked
The rated capacity of the installed valve(s) must meet or exceed the calculated required capacity. This capacity must be certified by the National Board (NB) through independent flow testing — not just calculated by the manufacturer.
Operating Pressure vs. Set Pressure
A critical design consideration is the margin between your normal operating pressure and the valve’s set pressure. ASME recommends that the operating pressure be at least 10% below the set pressure for Section VIII valves.
Why this matters:
- If operating pressure is too close to set pressure, the valve may “simmer” — leak slightly due to the spring being nearly overcome
- Simmering causes seat damage, reduces valve life, and wastes product
- Frequent cycling (opening and reseating near set pressure) accelerates wear and reduces the valve’s ability to seal tightly
Example: If your valve is set at 150 psig, your normal operating pressure should not exceed 135 psig (150 x 0.90 = 135).
Overpressure and Accumulation Summary Table
| Scenario | Max Accumulation Above MAWP | Set Pressure Limit |
|---|---|---|
| Single valve, standard conditions | 10% or 3 psi (whichever is greater) | At or below MAWP |
| Multiple valves, standard conditions | 16% or 4 psi (whichever is greater) | One at MAWP, others up to 105% MAWP |
| Fire exposure (supplemental valve) | 21% above MAWP | Up to 110% MAWP |
Inspection and Testing Requirements
While ASME Section VIII defines the design and installation requirements, ongoing inspection and testing of installed pressure relief valves falls under jurisdictional regulations and standards like the National Board Inspection Code (NBIC) and API 510 (Pressure Vessel Inspection Code).
General best practices include:
- Visual inspection during each scheduled vessel inspection — check for corrosion, leaking, paint over the outlet (which blocks discharge), and missing or damaged nameplates
- Bench testing at regular intervals (typically every 3 to 5 years, depending on service and jurisdiction) — the valve is removed, tested for set pressure accuracy and seat tightness, and recertified or replaced
- In-service testing where available — some installations allow testing the valve at operating pressure using an assisted lift device
- Documentation — maintain records of all inspections, tests, and replacements for each valve, including the NB number, set pressure, test date, and test results
Choosing the Right Valve for Your Section VIII Application
When selecting a pressure relief valve for a Section VIII vessel, consider:
- Service type: Air/gas, liquid, steam, or combination. This determines whether you need a safety valve (gas/steam), relief valve (liquid), or safety relief valve (both). The Aquatrol 740 Series is a safety relief valve suitable for all three service types.
- Pressure range: Standard brass/bronze construction (Series 740) handles up to 350 psig for air/gas/liquid. For higher pressures up to 1,500 psig, the stainless steel Series 741, 742, or 743 are required.
- Temperature: Standard materials handle up to 406°F. For cryogenic service down to -320°F, PTFE seat options are available. For temperatures up to 425°F, full stainless steel (Series 743) is recommended.
- Back-pressure: If your system has significant built-up or superimposed back-pressure at the valve outlet, a balanced bellows valve may be required to maintain accurate set pressure.
Frequently Asked Questions
What is the difference between MAWP and design pressure?
Design pressure is the pressure used for the design calculations of the vessel. MAWP is the maximum pressure permitted at the top of the vessel in its operating position for a designated temperature — it is determined after the vessel is designed and built, and may be equal to or greater than the design pressure. The pressure relief valve set pressure is based on MAWP, not design pressure.
Can I use a Section I valve on a Section VIII vessel?
No. Section I (“V” stamp) and Section VIII (“UV” stamp) valves have different performance requirements. Section I valves must reach full lift at 3% overpressure, while Section VIII valves are designed for 10% overpressure. Using the wrong type is a code violation and may result in unsafe operation. For more on this, see our guide on ASME Section I valves.
How often must Section VIII pressure relief valves be tested?
ASME Section VIII does not specify a testing interval for installed valves. Testing frequency is governed by your jurisdiction’s regulations, your company’s inspection program, and industry standards like API 510. Common practice is to bench-test valves every 3 to 5 years, with more frequent testing for severe service applications.
What happens if my valve’s capacity is not National Board certified?
For code installations, the valve’s rated capacity must be certified by the National Board through independent flow testing (NB-18 certification). A valve without NB-certified capacity cannot be used on an ASME code vessel in most U.S. and Canadian jurisdictions. All Aquatrol ASME-stamped valves carry NB-certified capacity, and we ship them as an authorized Aquatrol distributor with factory-direct pricing.
Need help selecting the right valve for your Section VIII application? Use our Valve Finder to search by pressure, size, and service type, or browse our full selection of ASME Section VIII certified valves. For complex applications, contact our engineering team for sizing assistance.