Automatic Plasma Cladding Tracking Control System for Three-Eccentric Butterfly Valve Disc

Literature Overview

This 2006 study by Li Heqi, Gao Dongfeng, Li Chunxu, Chen Kexuan, and Jiao Lei from the School of Materials Science and Engineering at Lanzhou University of Technology investigates the development of an automatic plasma transferred arc (PTA) cladding tracking control system specifically designed for three-eccentric butterfly valve discs. Published in the Journal of Lanzhou University of Technology, this research addresses a practical manufacturing challenge in the production of high-performance butterfly valves used in pipeline control applications.

Core Technical Content

Three-Eccentric Butterfly Valve Design

A three-eccentric butterfly valve features three offsets between the disc center and the shaft axis:

First eccentricity: The shaft axis is offset from the disc center (single eccentricity)
Second eccentricity: The shaft axis is offset from the disc centerline (double eccentricity)
Third eccentricity: The shaft axis is tilted at an angle relative to the disc centerline (triple eccentricity)

This geometry eliminates metal-to-metal contact between the disc and seat during operation, enabling the valve to function as a shutoff valve under high-pressure and high-temperature conditions. The disc surface requires a hard, wear-resistant, and corrosion-resistant cladding layer to ensure long-term sealing performance.

PTA Cladding Process for Valve Discs

Plasma transferred arc cladding is the preferred process for butterfly valve disc cladding due to:

High energy density: Enables deep penetration with narrow heat-affected zone
Precise deposition control: Suitable for thin, uniform layers required on curved surfaces
Low dilution: Typically 5–15% base metal dilution, preserving cladding composition
High deposition rate: 1.0–3.0 kg/h with appropriate powder feed rates
Good layer quality: Dense, pore-free deposits with fine microstructure

Tracking Control System Architecture

The automatic tracking control system consists of:

Component	Function	Specification
Position sensor	Detects disc geometry	Laser triangulation or capacitive
Tracking sensor	Maintains torch-to-workpiece distance	Arc voltage or optical
Motion controller	Drives multi-axis robot or gantry	3–5 axis CNC
Powder feeder	Delivers cladding powder	Vibratory or screw, 0.5–3.0 kg/h
Plasma power source	Provides arc energy	50–300 kW, DC
Gas supply system	Shielding and carrier gas	Ar or Ar+H₂
Control software	Coordinates all functions	Real-time feedback control

Process Parameters for Valve Disc Cladding

Parameter	Value	Notes
Plasma arc current	100–250 A	Depends on layer thickness
Arc voltage	15–25 V
Torch travel speed	100–300 mm/min	Slower for thicker layers
Powder feed rate	0.8–2.5 kg/h
Powder particle size	-325 mesh (45 μm)	Spherical preferred
Shielding gas flow	15–25 L/min	Ar or Ar+5% H₂
Torch-to-workpiece distance	5–10 mm	Maintained by tracking
Layer thickness	0.5–2.0 mm per pass
Number of layers	2–5	For total thickness of 1.5–8 mm

Control System Design Challenges

Geometric Complexity of Three-Eccentric Discs

The three-eccentric geometry presents unique challenges for automated cladding:

Variable curvature: The disc surface has complex, continuously varying curvature that requires real-time trajectory adjustment
Limited access: The eccentric geometry restricts torch access angles, requiring careful path planning
Seam alignment: The disc typically has a weld seam or forging seam that must be avoided or specifically addressed
Concentricity: The cladding layer must maintain concentricity with the valve seat to ensure uniform sealing

Tracking Sensor Selection

Sensor Type	Advantage	Disadvantage	Application
Arc voltage	Simple, low cost	Limited to conductive surfaces	Basic tracking
Laser triangulation	High accuracy, non-contact	Expensive, sensitive to surface	Precision tracking
Capacitive	High resolution	Requires conductive workpiece	Close-range tracking
Optical (CCD)	Real-time imaging	Complex processing	Full-surface monitoring

The study recommends a hybrid approach combining arc voltage sensing for distance maintenance with laser triangulation for geometric tracking, providing both robustness and accuracy.

Quality Control and Inspection

In-Process Monitoring

Arc stability monitoring: Real-time arc voltage and current monitoring to detect anomalies
Powder feed rate control: Closed-loop control to maintain consistent deposition rate
Temperature monitoring: Infrared thermography to detect overheating or insufficient fusion
Layer thickness measurement: Post-weld ultrasonic or eddy current measurement

Post-Weld Inspection Requirements

Inspection Method	Purpose	Standard
Visual examination (VT)	Surface defects	JB/T 4730
Magnetic particle testing (MT)	Surface and near-surface cracks	JB/T 4730
Ultrasonic testing (UT)	Subsurface defects, layer thickness	JB/T 4730
Hardness testing	Layer hardness uniformity	ISO 6507
Metallographic examination	Microstructure, porosity	ASTM E3
Bond strength testing	Layer-to-base adhesion	ASTM A263

Typical Acceptance Criteria

Parameter	Acceptance Criterion
Surface porosity	≤1% area, individual pore ≤1 mm
Cracking	No cracks in cladding layer or interface
Hardness	Uniform within ±50 HV of target
Layer thickness	Within ±0.5 mm of nominal
Bond strength	≥30 MPa (or no failure at interface)
Surface roughness	Ra ≤6.3 μm (or as specified)

Engineering Practice Implications

The development of an automatic PTA cladding tracking system for three-eccentric butterfly valve discs represents a significant advancement in valve manufacturing technology. Key implications include:

Productivity improvement: Automated cladding reduces cycle time by 40–60% compared to manual PTA
Quality consistency: Automated systems produce more uniform layers with reduced operator variability
Scalability: The system can be adapted for different valve sizes with minimal reprogramming
Cost reduction: Reduced labor costs and scrap rates improve overall economics

Application to Other Curved Components

The tracking control technology developed for butterfly valve discs can be extended to:

Ball valve balls and seats
Gate valve gate surfaces
Check valve disc surfaces
Turbine blade airfoils
Pump impeller surfaces
Heat exchanger tube sheets

Study Insights and Reflections

This research exemplifies the integration of advanced welding technology with automated control systems to solve specific manufacturing challenges. The three-eccentric butterfly valve disc presents a particularly demanding cladding application due to its complex geometry and critical functional requirements.

The key insight from this work is that successful automated cladding requires not only appropriate welding parameters but also sophisticated sensor integration and real-time control algorithms. The tracking system must compensate for geometric variations, maintain consistent torch-to-workpiece distance, and adapt to surface conditions in real time.

From a materials perspective, the PTA process is well-suited for butterfly valve disc cladding because it produces dense, pore-free layers with minimal dilution. The cladding material selection depends on the service environment—stainless steel (316L, 321) for corrosion resistance, nickel-based alloys (Inconel 625, Hastelloy C276) for severe corrosion, or hardfacing alloys for erosion resistance.

The research also highlights the importance of process qualification and standardization. For critical valve applications, the cladding process must be qualified per NB/T 47014 or ASME IX, with documented procedures for each valve size and material combination. The tracking control system parameters must be included in the qualified welding procedure specification to ensure reproducibility.

This work demonstrates that the combination of advanced materials science, welding technology, and automation engineering can significantly improve the manufacturing of high-performance valve components, enabling more reliable and longer-lasting products for critical pipeline applications.