How to Read a Safety Valve Nameplate: ASME Stamps, NB Marks & Model Numbers Explained

Why Your Safety Valve Nameplate Matters

Every ASME-certified safety relief valve ships with a nameplate stamped or etched into the body. That small metal tag is not decoration — it contains the exact specifications you need to verify compliance, reorder the correct valve, and ensure your system is protected. If you cannot read it, you are guessing, and guessing with pressure relief is not an option.

This guide walks you through every marking on a typical safety valve nameplate, explains what ASME code stamps and National Board (NB) marks mean, and shows you how to use nameplate data to find the right replacement valve.

The ASME Certification Mark and Designator

Starting with the 2011 edition of the ASME Boiler and Pressure Vessel Code, a single unified ASME certification mark replaced all previous code stamps. The mark is the same clover-leaf symbol for all ASME-certified products, but it is always accompanied by a designator — a letter code that identifies the specific product type.

For safety and relief valves, the key designators are:

• “V” Designator — The valve is certified under ASME Section I (Rules for Construction of Power Boilers). Section I covers fired pressure vessels, typically steam boilers used for power generation. A “V” stamped valve has been tested to reach full lift at no more than 3% overpressure above set pressure.
• “UV” Designator — The valve is certified under ASME Section VIII (Rules for Construction of Pressure Vessels). Section VIII covers unfired pressure vessels — air receivers, process tanks, heat exchangers, and virtually any pressurized equipment that is not a boiler. “UV” valves must reach full capacity at no more than 10% overpressure.
• “HV” Designator — Certified under ASME Section IV for heating boilers (low-pressure steam and hot water boilers).

Why this matters to you: If your pressure vessel is built to ASME Section VIII, you must install a valve with the “UV” designator. Installing a Section I (“V”) valve on a Section VIII vessel — or vice versa — is a code violation. The overpressure and blowdown characteristics are different between the two, and using the wrong type can result in failed inspections or unsafe operation.

The National Board (NB) Stamp

The NB stamp indicates that the valve’s relieving capacity has been independently certified by the National Board of Boiler and Pressure Vessel Inspectors (NBBPVI) in accordance with NB-18 testing procedures. This is separate from the ASME certification mark.

When you see the NB stamp, it means:

• The valve has been flow-tested at a National Board-accepted lab
• The rated capacity on the nameplate is a certified value, not just a manufacturer’s claim
• The valve model and capacity are registered with the National Board

Most jurisdictions in the United States and Canada require NB-certified capacity for code applications. If your jurisdiction requires it, always verify the NB stamp is present on any replacement valve.

Reading the Model Number

The model or series number is your most important identifier for reordering. For Aquatrol valves, the model number system works as follows:

• Series 740 — Bronze/brass body with stainless steel internals. The most versatile series, rated for air, gas, liquid, and steam service. ASME Section VIII (“UV”) certified. Pressures to 350 psig for air/gas/liquid, 250 psig for steam.
• Series 741 — Same body configuration as the 740 but with full stainless steel nozzle, ring, disc, and springs. Rated to 1,500 psig for air/gas/liquid and 300 psig for steam.
• Series 742 — Carbon steel body with stainless steel internals. An all-steel option for higher pressures. ASME Section VIII (“UV”) certified to 1,500 psig air/gas/liquid and 300 psig steam.
• Series 743 — 100% stainless steel construction. Same pressure ratings as the 742 but fully corrosion-resistant for chemical, food processing, and pharmaceutical applications.
• Series 560 — Brass steam safety valve. ASME Section I (“V”) and Section VIII (“UV”) dual-certified for steam and air/gas service to 250 psig.

The digits following the series number typically indicate the specific configuration — connection size, seat material, and orifice designation. For example, on Aquatrol’s ordering system, you will see orifice letter codes (D, E, F, G, H, J) that correspond to specific inlet/outlet size combinations.

Set Pressure

The set pressure (sometimes labeled “SET” or “SP”) is the inlet pressure at which the valve begins to open under service conditions. This value is stamped in psig (pounds per square inch gauge) or bar.

Key rules from ASME:

• ASME Section VIII: If a single pressure relief valve is used, the set pressure shall not exceed the Maximum Allowable Working Pressure (MAWP) of the vessel. When multiple valves are used, only one needs to be set at or below MAWP; others may be set up to 105% of MAWP.
• ASME Section I: The set pressure of at least one safety valve must not exceed the MAWP. Additional valves may be set up to 103% of MAWP.
• Operating pressure should be at least 10% below set pressure for Section VIII valves and at least 3% below set pressure for Section I valves to prevent simmering or premature lift.

Rated Capacity (CAP)

The rated capacity, often abbreviated “CAP” on the nameplate, represents the maximum flow rate the valve can relieve at its rated overpressure. This is expressed in different units depending on the service:

• Steam: Pounds per hour (lb/hr or PPH)
• Air/Gas: Standard cubic feet per minute (SCFM) or cubic feet per minute of air
• Liquid: Gallons per minute (GPM) at a specific overpressure

The rated capacity on an NB-certified valve is the flow rate measured during official National Board testing. Your valve’s capacity must meet or exceed the relieving requirements of your system. If your system can generate pressure faster than the valve can relieve it, you have a dangerous under-sizing problem.

Inlet and Outlet Size

Nameplate sizing is shown as inlet x outlet. For example, “1” x 1-1/4″” means a 1-inch inlet and 1-1/4-inch outlet. The outlet is always equal to or larger than the inlet to prevent back-pressure buildup during discharge.

Connection types are also noted — NPT (National Pipe Thread), BSPT (British Standard Pipe Thread), or flanged (ANSI flange ratings). Aquatrol 740 Series valves are available in NPT MxF (male inlet x female outlet), NPT FxF, BSPT, and flanged configurations with inlet sizes from 1/2″ to 3″.

Temperature Range

Some nameplates include the maximum (and minimum) operating temperature. This is critical for material compatibility:

• Standard brass/bronze (Series 740): -60°F to 406°F (-51°C to 208°C)
• PTFE seat option: Extends the low end to -320°F (-196°C) for cryogenic service
• Full stainless steel (Series 743): -320°F to 425°F (-196°C to 218°C)

If your application involves temperatures outside these ranges, the valve is not suitable regardless of its pressure rating.

Seat Material and Service Type

The nameplate or model code indicates the seat (disc) material, which determines what media the valve can handle:

• Metal seat (M): Required for steam service. Also suitable for air, gas, and liquid. Provides the tightest seal at high temperatures.
• Viton/FKM seat: Suitable for air, gas, and liquid service. Better sealing for lower-pressure applications. Not rated for steam.
• PTFE seat: Chemical resistance and cryogenic capability. Suitable for air, gas, and liquid. Not for steam above 450°F.

How to Use Nameplate Data to Find a Replacement

When you need to replace a safety valve, pull these five values from the existing nameplate:

1. ASME code section (Section I “V” or Section VIII “UV”) — must match
2. Set pressure — must match your system’s MAWP requirements
3. Capacity — replacement must meet or exceed the original
4. Inlet/outlet size and connection type — must physically fit your piping
5. Service type (steam, air/gas, liquid) — determines seat material needed

With these five data points, you can use our Cross-Reference Tool to find the exact Aquatrol equivalent, or contact us directly with your nameplate photo for a fast quote. We sell the full Aquatrol product line as an authorized distributor, with factory-direct pricing.

Frequently Asked Questions

What if my nameplate is corroded or unreadable?

If the nameplate is too damaged to read, you should not continue operating the valve without verification. Contact the original manufacturer with whatever information you can gather (physical dimensions, connection sizes, any visible markings) or contact our team for help identifying the valve. In many jurisdictions, an unreadable nameplate is grounds for requiring valve replacement.

Can I change the set pressure of my existing valve?

ASME-coded valves (V or UV stamped) cannot be field-adjusted to a different set pressure. The set pressure is established during manufacturing and certified during testing. Changing it requires the valve to be returned to an authorized facility, re-set, re-tested, and re-stamped. Non-code valves like the Aquatrol Series 69 are field-adjustable by design.

What is the difference between a safety valve, relief valve, and safety relief valve?

A safety valve opens rapidly (pops) and is used for compressible fluids like steam and gas. A relief valve opens proportionally to overpressure and is used for incompressible fluids like liquids. A safety relief valve can function as either, depending on the application — it pops on gas/steam and opens proportionally on liquid. The Aquatrol 740 Series is a safety relief valve, making it suitable for all three service types.

Does my valve need both ASME and NB certification?

ASME certification confirms the valve is designed and built to code. NB certification confirms the capacity has been independently flow-tested. Most U.S. and Canadian jurisdictions require both for code-stamped installations. Always check your local jurisdiction’s requirements.

Need help identifying a valve? Send us a nameplate photo and we will identify the valve and provide a direct replacement quote, typically within one business day. Or use our Valve Finder to search by specifications.