The primary distinction between Wafer, Semi-Lug, and Full-Lug butterfly valves lies in their method of installation and their ability to handle dead-end service, not the total number of bolt holes.

2. Origin of Bolt Hole Quantity (4, 8, 16 Holes Explained)

A frequent misconception is that the valve's type, such as 'Semi-Lug', determines its number of bolt holes—often incorrectly associating it with having four holes. To be clear: the quantity of bolt holes (whether 4, 8, or 16) is not defined by the valve's body style. Instead, it is strictly governed by the applicable flange standard, which specifies the bolt pattern based on pipe size (NPS/DN) and pressure class.

The table below shows an example using the ASME B16.5 Class 150 standard. Any valve type matching this standard will have the same number of hole positions.

A Critical Distinction: "Full-Lug" in Appearance vs. Function

After reviewing the bolt charts, a sharp-eyed engineer might notice a discrepancy: the charts demand a high number of bolts for large valves (e.g., 20 for an NPS 24 valve), yet many large-diameter "full-lug" valves in the market have far fewer lugs (e.g., 16).

This observation is correct, and it highlights a critical difference between market terminology and strict engineering standards. The term "Full-Lug" is used in two distinct ways, especially as valve sizes increase.

4. Visual Guide to Bolt Tightening Sequence

Proper bolt tightening is critical for achieving a leak-free seal and preventing damage to the valve or flanges. The universal best practice is the star pattern (or criss-cross pattern), which ensures gasket load is applied evenly.

The Multi-Pass Process:

• Step 1: Hand Tighten. Snug all bolts by hand in the star sequence to bring flanges into uniform contact with the valve.
• Step 2: First Pass (20-30% Torque). Using a calibrated torque wrench, tighten each bolt in the star sequence to 20-30% of the final required torque.
• Step 3: Second Pass (50-70% Torque). Repeat the star sequence, bringing each bolt to 50-70% of the final torque.
• Step 4: Final Pass (100% Torque). Repeat the star sequence a third time to bring all bolts to their final torque value.
• Step 5: Rotational Pass (Final Check). Perform one final pass, tightening the bolts sequentially (e.g., clockwise) to the final torque to ensure no bolt has relaxed.

5. Authoritative References & Further Reading

• API 609 - Butterfly Valves: Double Flanged, Lug- and Wafer-Type

  The definitive industry standard specifying design, including the mandatory capability of lug-type valves to handle dead-end service.

  View Standard at IHS Markit
• ASME B16.5 - Pipe Flanges and Flanged Fittings

  This standard governs flange dimensions, including the number and pattern of bolt holes for different sizes and pressure classes.

  View Standard at ASME.org

6. Full Lug Butterfly Valve Bolt Charts

Data below represents the required Cap Screws for Full-Lug style valves (Functional Definition). The quantity is double the flange bolt hole count, as each flange is bolted to the valve body independently.