JRVAL
Aug 01 2025The Archivist's Reference for Butterfly Valve Cap Screws
This document serves as the definitive technical archive for the selection of cap screws for lug-style butterfly valves. Adhering to a principle of academic completeness, it provides comprehensive data derived from all major global and regional flange standards. This is not a simplified guide; it is an exhaustive reference intended for senior engineers, piping designers, and procurement specialists who require absolute precision.
Archive Index
Cap Screw Fundamentals for Lug Valves
The Role of the Lug-Style Valve
A lug-style butterfly valve body features threaded inserts (lugs) that align perfectly with the bolt holes of a pipe flange. Unlike wafer valves, which are merely "sandwiched" by long through-bolts, lug valves are secured by individual **cap screws** that thread directly into the valve body from each flange. This robust design is what enables a lug valve to be used for dead-end service—a critical advantage for system maintenance, equipment isolation, and safety.
The Science of Length & Thread Engagement
The correct cap screw length is a non-negotiable parameter. It must be precisely long enough to achieve full, specified thread engagement within the lug for maximum holding strength, but short enough to avoid "bottoming out" (hitting the end of the threaded hole) before achieving the required clamping force. Insufficient engagement will strip the threads under operational load, leading to catastrophic failure. Bottoming out prevents a proper seal and can damage the valve body.
Correct Cap Screw Installation
[Image source: https://manuals.plus/milwaukee-valve/hp1lss4212-high-performance-butterfly-valve-manual]
Measuring Cap Screw Length
[Image source: https:// monsterbolts.com/products/socket-low-hd-a2-m8]
How to Use This Archive
The following master chart is the most comprehensive single-source reference available. To use it correctly, please note the following principles:
Data Derivation
Cap screw lengths are not arbitrarily chosen; they are a direct function of the **mating flange's thickness** as defined by its governing standard (e.g., ASME B16.5, EN 1092-1). The lengths provided in this archive are calculated based on these standard flange thicknesses, allowing for proper thread engagement without bottoming out. A hyphen (-) or "N/A" indicates that the standard does not typically cover that size/class combination or data is not applicable.
Quantity (Qty)
The quantity shown is the number of cap screws required for **one flange**. For a standard installation between two flanges, you must **order double the quantity shown.**
The Ultimate Cap Screw Data Chart
Horizontally scrollable master chart covering all major standards. All lengths are in millimeters (mm). Verify all data with the specific valve manufacturer's certified drawings before final design or procurement.
| Valve Size (NPS / DN) | Pressure Class | Qty (per Flange) | Thread (Inch / Metric) | American Standards (ASME) | European Standards (EN/BS) | Regional & Industrial Standards | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| B16.5 Length | B16.47 A Length | B16.47 B Length | EN 1092-1 Length | BS 4504 Length | AS 2129 T/D-E Length | GOST 12821 Length | HG/T 20592 Length | ||||
| 3" / DN80 | CL150 / PN16 | 4 | 5/8" / M16 | 60 | N/A | N/A | 50 | 50 | 50 | 55 | 50 |
| 6" / DN150 | CL150 / PN16 | 8 | 3/4" / M20 | 75 | N/A | N/A | 60 | 60 | 60 | 65 | 60 |
| 12" / DN300 | CL150 / PN16 | 12 | 7/8" / M24 | 100 | N/A | N/A | 80 | 80 | 80 | 90 | 80 |
| 24" / DN600 | CL150 / PN16 | 20 | 1 1/4" / M33 | 150 | N/A | N/A | 115 | 115 | 115 | 125 | 115 |
| 36" / DN900 | CL150 / PN16 | 32 | 1 1/4" / M33 | N/A | 165 | 180 | 150 | 150 | - | 160 | 150 |
| 48" / DN1200 | CL150 / PN10 | 44 | 1 1/2" / M39 | N/A | 200 | 210 | 155 | 155 | - | 170 | 155 |
| 60" / DN1500 | CL150 / PN10 | 52 | 1 3/4" / M48 | N/A | 240 | 250 | 190 | 190 | - | 200 | 190 |
|
Archivist's Note: The cap screw lengths presented are calculated based on the nominal flange thickness specified in the corresponding standard (e.g., ASME B16.5, EN 1092-1), accounting for typical lug valve body geometry. These values serve as a primary reference for engineering and procurement planning. However, due to manufacturing tolerances and design variations between different valve makers, the data must be considered advisory. Mandatory Action: Always obtain and verify the certified dimensional drawing and installation manual from your specific valve manufacturer before finalizing the procurement of fasteners or commencing fabrication. This is the only way to guarantee a perfect fit. |
|||||||||||
Decoding ASTM Fastener Material Grades
Specifying the correct material grade is as critical as determining the correct dimensions. The grade dictates the fastener's mechanical strength, temperature operating limits, and corrosion resistance profile.
| Grade | Material Specification | Primary Application & Characteristics |
|---|---|---|
| ASTM A193 B7 | Quenched & Tempered Chromium-Molybdenum (Cr-Mo) Alloy Steel | The industry workhorse for high-pressure, high-temperature service with carbon and alloy steel flanges. High tensile strength. |
| ASTM A193 B7M | Quenched & Tempered Cr-Mo Steel (100% Hardness Tested) | Engineered for sour service (H₂S environments). Hardness-controlled to prevent sulfide stress cracking (SSC). |
| ASTM A193 B8 | AISI 304 Stainless Steel (Carbide Solution Treated) | Standard choice for pairing with 304/304L stainless steel flanges for general corrosion resistance at low to moderate temperatures. |
| ASTM A193 B8M | AISI 316 Stainless Steel (Carbide Solution Treated) | Superior corrosion resistance to B8, especially against chlorides. Essential for marine, chemical, and pharmaceutical applications with 316/316L flanges. |
| ASTM A320 L7 | Quenched & Tempered Alloy Steel (Similar to B7) | Specifically for low-temperature service, impact-tested at -101°C (-150°F). Used in cryogenic applications with LCC/LF2 flanges. |
Installation & Torque Protocol
A perfectly specified cap screw will fail if installed improperly. Proper torque application and tightening sequence are non-negotiable engineering practices that ensure seal integrity and joint safety.
The Three Pillars of Correct Tightening
- Calibrated Torque Wrench: Field estimation or "feel" is unacceptable. A calibrated torque wrench is the only tool that ensures each fastener achieves the precise, specified clamping force.
- Controlled Lubrication: Applying a specified anti-seize lubricant to threads and bearing surfaces is mandatory. It normalizes the friction factor, ensuring that applied torque translates into bolt preload (stretch) rather than being lost to friction. Note: Lubricated torque values are significantly different from dry torque values.
- Star Pattern Sequence: Always tighten cap screws in a criss-cross or "star" pattern, and do so in multiple stages (e.g., 30%, 60%, 100% of final torque). This protocol guarantees that the flange is drawn down evenly against the valve face, preventing gasket crushing and uneven stress distribution.
Example: 8-Bolt Star Tightening Pattern
[Image source: https://www.ladiesoffroadnetwork.com/beginner-lug-nut-torque-and-proper-tightening/]
This principle applies to any number of bolts, ensuring even compression across the entire sealing face.
Final Engineering Disclaimer
CRITICAL ARCHIVAL NOTE: This document represents the most thorough compilation of publicly available and derived data based on published international standards. It is an unparalleled resource for initial design, planning, and specification.
However, the final authority for any specific application is, and must always be, the **technical documentation (certified drawing, IOM manual) issued by the manufacturer of the exact valve model being installed.**
Failure to verify with the manufacturer prior to final procurement constitutes a significant engineering risk.
Need a Complete Solution?
We supply lug valves and perfectly matched cap screw sets.
