1. Preliminary Preparation: Design and Material Confirmation
Before steel butterfly valve fabricating manufacturing begins, engineers will complete a detailed design based on project requirements, including:
• Nominal Diameter (DN)
• Pressure Rating (PN/Class) This is particularly important because welded valve bodies, compared to integral castings, place greater demands on the pressure-bearing capacity of the joints.
• Working Medium
• Working Temperature
• Corrosive Environment
• Drive Method (Manual, Worm Gear, Electric, Pneumatic, Hydraulic)
Then, the steel plate material is determined, for example:
• Carbon Steel (A36, S275, Q235, Q355)
• ASTM A516 Gr.70
• ASTM A572
• Stainless Steel 304 / 316
• Duplex Stainless Steel 2205
• Super Duplex 2507
The first three are commonly used carbon steel plates and are also the most commonly used welding materials.
All steel plates should have material certification (Mill Test Certificate, MTC) and incoming material acceptance.
2. Raw Material Inspection
All steel plates undergo various inspections upon entering the workshop:
* Dimensional Inspection
* Thickness
* Length
* Width
* Flatness
* Material Confirmation
Large projects typically use:
* PMI (Positive Material Identification)
* Spectroscopic Analysis
To ensure correct material grade.
* Visual Inspection
Check for:
* Cracks
* Delamination
* Inclusions
* Rust
* Scratches
Ultrasonic testing (UT) is performed if necessary to check for internal defects in the steel plate.
3. CNC Plate Cutting
Cut according to CAD drawings:
* Valve body plate
* Flange plate
* Reinforcing ribs
* Support plate
* Bearing housing
After cutting:
* Remove slag
* Grind cut edges
* Bevel weld joints
4. Rolling the Cylindrical Body
Use a plate rolling machine to roll the steel plate into a cylinder.
Requirements for control:
• Roundness
• Concentricity
• Ovality Large butterfly valves typically allow deviations of only a few millimeters.
Tack welding is performed after rolling.
5. Longitudinal Seam Welding
The longitudinal seam of the cylinder needs to be welded to form a single unit. Due to the typically thick plates, multi-layer, multi-pass welding is required.
Welding process control:
• Heat input
• Interpass temperature
• Welding deformation
6. Circumferential Welding
This welding method is required when welding the following locations:
• Flange
• Reinforcing ring
• Support plate
• Connection structure
Large butterfly valves especially require attention to the following issues:
• Weld consistency
• Welding efficiency
• Weld quality
7. Welding Reinforcement Ribs
To improve resistance to deformation, the following are welded:
• Circumferential reinforcing ribs
• Radial reinforcing plates
• Support ribs Butterfly valves above DN2000 almost always have a reinforcing structure.
The location of the reinforcing ribs is usually determined based on finite element analysis (FEA) to avoid stress concentration.
8. Stress Relief Heat Treatment (Optional)
PWHT (Post Weld Heat Treatment)
Functions:
• Eliminates residual welding stress
• Reduces deformation risk
• Improves weld toughness
9. Weld Inspection (Important)
Weld inspection is the most important quality control step in welding valve bodies.
Common inspections include:
* VT (Visual Testing)
Inspects:
• Weld appearance
• Weld leg size
• Undercut
• Weld bead
* PT (Dye Penetrant Testing)
Inspects:
• Surface cracks
• Porosity
Applicable to stainless steel.
* MT (Magnetic Particle Testing)
Inspects:
• Carbon steel surface cracks
* UT (Ultrasonic Testing)
Inspects:
• Internal cracks
• Incomplete penetration
• Lack of fusion
• Slag inclusions
Most large butterfly valves require 100% UT.
* RT (Radiographic Testing)
Some projects require:
X-ray inspection of weld quality.
10. Machining
The welded valve body enters a large CNC machining center, similar to the cast valve body.
Machining includes:
• Flange sealing surface
• Valve shaft bore
• Bearing seat bore
• Positioning surface
• Valve seat mounting groove
• Sealing surface
Key control points:
• Coaxiality
• Perpendicularity
• Flatness
• Roundness
Because these dimensions directly affect the sealing performance of the butterfly valve.
11. Surface Preparation
Typically, the following is performed:
Sandblasting
Achieving:
Sa2.5
Removing:
• Oxide scale
• Weld slag
• Rust
Increasing coating adhesion.
12. Coating
Select according to the operating environment:
General operating conditions:
• Epoxy primer
• Epoxy topcoat
Marine environment:
• Zinc Rich Primer
• Epoxy Intermediate Coat
• Polyurethane Top Coat
Drinking water: Epoxy coating meeting drinking water certification requirements.
13. Assembly
Installation:
• Valve plate
• Valve shaft
• Bearing
• Valve seat
• O-ring
• Packing
• Drive unit
Adjustment:
• Opening/closing angle
• Sealing pressure
• Operating torque
14. Pressure Testing
Performed according to standards such as API 598, EN 12266, or ISO 5208:
• Shell strength test
• Sealing test
• Opening/closing torque test
For large-diameter butterfly valves, the smoothness of fully open and fully closed operation is also verified.
15. Final Inspection and Documentation
Completed before shipment:
• Dimensional re-inspection
• Coating thickness inspection (DFT)
• Label verification
• Visual inspection
• Packaging inspection
And compile quality documents, including:
• Material Test Certificate (MTC)
• Welding Procedure Specification (WPS)
• Procedure Qualification Record (PQR)
• Welder Qualification Record (WQ/WPQR)
• NDT Reports
• Hydrostatic Test Report
• Coating Inspection Report
• Dimensional Inspection Report
• Certificate of Conformity (COC)