Understanding the non return valve symbol is critical for anyone working with piping diagrams, hydraulic systems, or industrial schematics. Also called a check valve symbol, this simple graphic communicates a critical function: ensuring fluid flows in only one direction. But with multiple standards (ISO, ASME, DIN) and variations for different valve types (spring-loaded, sanitary, stainless steel), misinterpreting the symbol can lead to system failures, contamination, or costly rework. In this guide, we’ll break down the most common check valve symbols, explain their nuances, and show you how to apply them—especially in specialized fields like sanitary or high-pressure applications.
What is a Non Return Valve Symbol?
At its core, a non return valve symbol is a visual shorthand for a valve that prevents backflow. It appears on P&IDs (Piping and Instrumentation Diagrams), hydraulic schematics, and plumbing blueprints. The symbol’s design varies slightly by standard, but all versions share two key elements:
- A flow direction arrow: Indicates the allowed flow path (never reversed!).
- A check mechanism icon: Represents the valve’s internal component (e.g., a swing disc, ball, or spring).
Standard Non Return Valve Symbols (ISO vs. ASME vs. DIN)
Engineers rely on global standards to ensure consistency. Here’s how the check valve symbol appears across the most common systems:
1. ISO Standard (Global)
The ISO 1219-1 symbol is the most widely used. It features:
- A straight line (representing the pipe) with an angled arrow (flow direction).
- A curved “flap” or “disc” perpendicular to the arrow, indicating the one-way seal.
Example: In a water treatment plant’s P&ID, this symbol tells operators where to install a stainless steel non return valve to prevent contaminated water from backflowing into clean reservoirs.
2. ASME Standard (U.S.)
ASME Y14.35M uses a similar design but with a more angular “V” shape for the check mechanism. The arrow remains, but the flap is drawn with sharp edges—common in U.S. industrial diagrams.
Pro Tip: When specifying an ASME check valve symbol for high-pressure systems, pair it with a note for “316L stainless steel” (ideal for corrosion resistance).
3. DIN Standard (Europe)
DIN 2403 symbols are more detailed, often including a small circle or dot to represent the valve seat. This is common in German-engineered systems, such as pharmaceutical processing lines using sanitary check valves.
Key Variations: Symbols for Specialized Check Valves
Not all check valves are the same—and their symbols reflect that. Here’s how to identify specialized types:
Spring-Loaded Check Valve Symbol
Adds a small “spring” coil to the basic symbol, indicating the valve closes via spring pressure (not just gravity). Critical for vertical installations or low-flow systems (e.g., medical gas lines).
Application: A sanitary spring-loaded check valve (symbol with coil) in a brewery prevents beer from siphoning back into fermentation tanks during cleaning.
Swing Check Valve Symbol
Features a hinged flap (curved line) instead of a flat disc. Used in large-diameter pipes (e.g., HVAC systems). The symbol’s “swing” shape warns engineers to avoid installing it in tight spaces where the flap can’t fully open.
Sanitary Check Valve Symbol
There’s no official symbol for sanitary check valves (material isn’t shown in symbols), but best practice adds a “3-A” or “FDA” label next to the standard check valve icon. This signals the valve meets sanitary standards (e.g., 316L stainless steel, smooth bore) for food/pharma use.
How to Use Non Return Valve Symbols in Real-World Design
Step 1: Match Symbol to Valve Type
- For a stainless steel non return valve in a chemical plant: Use the ISO symbol with a “316L” annotation.
- For a sanitary check valve in a dairy: Add “3-A Certified” next to the ASME symbol.
Step 2: Avoid Common Mistakes
- Reversing the arrow: A fatal error! Always double-check flow direction matches the system’s needs (e.g., from pump outlet to tank, never the reverse).
- Overcomplicating symbols: Stick to one standard per diagram (e.g., ISO or ASME, not both) to avoid confusion.
Step 3: Pair Symbols with Material Notes
Symbols don’t show material, but annotations do. For example:
“[Check valve symbol] – Sanitary 316L SS, Tri-Clamp, 3-A Certified” ensures contractors install the right valve for a pharmaceutical line.
Why Symbols Matter: A Case Study
A food processing plant once misread a non return valve symbol as a globe valve, installing a brass valve instead of a sanitary stainless steel check valve. During an audit, inspectors flagged the brass as non-compliant (risk of lead leaching), halting production for 48 hours. Correct symbol interpretation would have prevented this $50,000 mistake.
FAQs: Non Return Valve Symbols
Q: Is there a difference between “non return valve symbol” and “check valve symbol”?
A: No—they’re interchangeable. “Non return valve” is common in Europe, while “check valve” is preferred in the U.S.
Q: Can I use the same symbol for a plastic and stainless steel check valve?
A: Yes—the symbol indicates function, not material. Use annotations (e.g., “316L SS” or “PVC”) to specify.
Q: How do I show a failed check valve on a diagram?
A: Draw a diagonal line through the symbol and label it “FAILED – REPLACE.”
Final Thoughts: Master Symbols, Avoid Disaster
The non return valve symbol is small, but its impact is huge. Whether you’re designing a sanitary brewery system or an industrial hydraulic line, correctly interpreting and using these symbols ensures safety, compliance, and efficiency. Remember: the symbol is just the start—pair it with clear annotations for material (like stainless steel) and certifications (like 3-A) to get the full picture.