The Aquatrol 700-Series Stainless Steel Family at a Glance
The Aquatrol 741, 742, and 743 are the stainless steel safety relief valves in the Aquatrol catalog — the right choice when bronze body construction is unsuitable due to corrosive media, higher temperatures, or sanitary requirements. All three share the same core stainless steel design platform but are differentiated by service type. Choosing the wrong one means specifying a valve that is not certified for your application, which is both a code violation and a safety hazard.
This guide explains the practical differences between the 741, 742, and 743 — what each one is engineered for, how to decide which one fits your application, and where to verify the spec details that matter for your specific build. As an authorized Aquatrol distributor, we sell the full 700-series stainless line at factory-direct pricing.
Aquatrol 741 — Stainless Steel for Air and Gas Service
The 741 is the air, gas, and other compressible-fluid stainless steel model. It is ASME Section VIII (“UV”) certified for unfired pressure vessels and National Board (NB) registered. Use the 741 when your application requires:
- Stainless steel body and trim for corrosive gas service (chlorine compounds, hydrogen sulfide, certain process gases) where bronze would corrode
- Higher operating temperatures than bronze can handle
- Cleanable construction for industries with sanitary or contamination-sensitive requirements
- The same set pressure/capacity range as a 740 but in stainless construction
Standard sizes range from 1/2″ through several inch-class connections in NPT, with PTFE or metal seat options depending on the application. Confirm specific sizes and pressure ratings against the Aquatrol 741 spec sheet for your build.
Aquatrol 742 — Stainless Steel for Steam Service
The 742 is the steam-service stainless steel model. Steam imposes design requirements that differ from compressible gas service: thermal cycling, condensation, higher latent energy on relief, and (depending on the host vessel) ASME Section I or Section VIII certification considerations. The 742 is designed and certified for steam relief on ASME Section VIII unfired vessels.
Use the 742 when:
- Your relieving service is steam (process steam, sterilization, district heat, jacketed reactors)
- Your host vessel is ASME Section VIII (unfired) — if the host is an ASME Section I power boiler, you need a Section I valve, not the 742
- You need stainless construction for sanitary, pharmaceutical, or food-grade steam applications
Note: Do not substitute a 742 for a Section I boiler valve, and do not use a 741 (air/gas) on a steam system. The certification stamp on the nameplate determines what installations the valve is legal for.
Aquatrol 743 — Stainless Steel for Liquid Service
The 743 is the liquid-service stainless steel model, available in both ASME-coded and non-code variants depending on whether the host vessel and application require formal ASME certification. Liquid relief differs from gas/steam relief mechanically: liquid valves open proportionally to overpressure rather than “popping” rapidly, and sizing uses different formulas because liquid is incompressible.
Use the 743 when:
- Your relieving fluid is a liquid (water, hydraulic fluid, chemical process liquid)
- You need stainless construction for corrosive, high-purity, or sanitary liquid service
- You need either an ASME-coded or non-code variant depending on jurisdictional and code requirements
Verify whether your application requires ASME certification before specifying coded vs. non-code. Most pressure vessels in industrial settings require coded; some non-code applications (open systems, certain hydraulics, some non-pressure-vessel applications) can use the non-code variant.
Side-by-Side Quick Reference
| Model | Service | Body / Trim | Certification | Typical Use |
|---|---|---|---|---|
| 741 | Air / Gas / Compressible fluids | Stainless Steel | ASME UV (Section VIII), NB-registered | Corrosive gas, high-temp gas, sanitary gas service |
| 742 | Steam | Stainless Steel | ASME UV (Section VIII), NB-registered | Process steam, sterilization, jacketed steam vessels |
| 743 | Liquid (incompressible) | Stainless Steel | ASME-coded or Non-code variant | Corrosive liquid, hydraulic, chemical process liquid, sanitary liquid |
Decision Tree: Which One Do You Need?
Three questions get you to the right model:
- What is the relieving fluid? Air or gas → 741. Steam → 742. Liquid → 743.
- Is the host vessel ASME-coded? Yes → specify ASME-stamped valve (UV stamp). No (non-code application) → the 743 has a non-code variant; the 741 and 742 are ASME-only.
- Do you need stainless specifically? If your application can use bronze, the Aquatrol 740 Series is typically lower cost and equally code-compliant. Use the 700-stainless line when stainless is required for the service.
How to Order
To specify any of the 700-series stainless models, you need:
- Inlet and outlet size (NPT, BSPT, or flanged)
- Set pressure (psig)
- Required relieving capacity (SCFM for gas, lb/hr for steam, GPM for liquid)
- Service fluid identification (specific media, not just “gas”)
- Operating and relief temperature
For sizing assistance, see our 740 Series Buyer’s Guide sizing section (the methodology is the same for the 700-stainless line). For replacement of an existing stainless steel safety valve, use our Cross-Reference Tool with the data on your existing valve’s nameplate.
Request a quote with your application details and we will spec the right 741, 742, or 743 model for your build, or call us at (262) 320-7101 for sizing help.
Frequently Asked Questions
Can I use a 741 (gas) on a steam system?
No. Steam service requires the 742, which is certified for steam. The certification stamp on the nameplate determines code-legal installations — a 741 stamped for gas service is not code-legal on a steam vessel regardless of physical similarity.
What is the difference between the 743 coded and non-code variants?
The coded 743 carries the ASME UV stamp and NB registration, required for installation on ASME code vessels. The non-code 743 is mechanically similar but lacks the formal certifications, suitable only for non-coded applications (some hydraulic systems, open vessels, equipment where ASME compliance is not required by jurisdiction or specification).
Are the 741, 742, and 743 interchangeable physically?
The trim and disc assemblies are engineered for the specific service. Even when external dimensions are similar, the internal components are tuned for compressible (gas), high-energy (steam), or incompressible (liquid) flow. Substituting models across services voids the certification and creates a real risk of inadequate relief performance.
What is the lead time on stainless steel models?
Stock items at Safety Control Valves typically ship next business day. Custom configurations (specific set pressures, alternate connections, special trim) are built to order; lead time depends on Aquatrol’s production schedule and the specific configuration.