Overlay Welding of Flow Orifice Plates in Heavy Oil Catalytic Cracking Units

Literature Overview

The paper by Pei Boyuan, published in Welding (1991, No. 6, pp. 25-26), addresses the overlay welding requirements for flow orifice plates used in heavy oil catalytic cracking units at Changzhou Energy Equipment General Factory. The technical challenge is the protection of orifice plate surfaces exposed to aggressive process media at elevated temperatures (up to 500 °C), where conventional materials cannot provide adequate wear and corrosion resistance. This is a specialized application of overlay welding to flow measurement and control components in petrochemical processing.

Technical Background and Operating Conditions

Flow orifice plates in catalytic cracking units serve as differential pressure elements for flow measurement. In heavy oil catalytic cracking service, the orifice plates are subjected to:

High temperature: Operating temperatures up to 500 °C for critical velocity nozzles.
Abrasive erosion: High-velocity slurry flow containing catalyst particles causes severe erosion at the orifice edge.
Chemical corrosion: Sour gas components (H2S, CO2) and acidic species attack the base metal.
Thermal cycling: Startup and shutdown cycles create thermal fatigue conditions.
Cavitation: Pressure drop across the orifice can cause cavitation damage.

Service Conditions and Material Requirements

Parameter	Specification
Operating temperature	400–500 °C
Flow velocity	30–60 m/s at orifice
Media	Heavy oil + catalyst particles + H2S
Base material	15CrMo or 12Cr1MoV
Required overlay hardness	40–50 HRC
Required overlay thickness	1.0–2.0 mm
Critical dimension	Orifice bore diameter tolerance ±0.05 mm
Service life requirement	> 2 years before replacement

Overlay Welding Process Selection

The selection of overlay welding process for orifice plates is constrained by:

Dimensional accuracy: The orifice bore must maintain precise diameter after overlay welding.
Surface smoothness: The overlay surface must be machinable to tight tolerances.
Low heat input: Excessive heat input can distort the thin plate geometry.
Minimal dilution: The overlay layer must maintain its corrosion resistance.

The chosen process was GTAW (Tungsten Inert Gas Welding) for the following reasons:

Precise heat input control.
Excellent bead geometry control.
Low dilution with proper technique.
Suitable for thin sections.
Minimal distortion.

GTAW Overlay Welding Parameters

Parameter	Value
Shielding gas	Argon (99.99%)
Wire material	ER309L or ER316L stainless steel
Wire diameter	1.0–1.6 mm
Current	80–120 A (DC)
Travel speed	150–300 mm/min
Stick-out	5–8 mm
Gas flow rate	8–12 L/min
Preheat	100–150 °C
Interpass temperature	≤ 150 °C
Post-weld treatment	None (to preserve dimensional accuracy)

Critical Technical Challenges

The overlay welding of flow orifice plates presents several unique challenges:

1. Bore diameter control: The overlay must be applied to the outer surface and edge of the orifice plate without affecting the bore diameter. This requires precise torch positioning and possibly a backing plate to protect the bore.

2. Edge overlay: The orifice edge (the critical flow-measuring surface) requires overlay protection but must maintain a sharp, smooth edge. This is typically achieved by:

Applying overlay to the outer face first.
Carefully overlaying the edge with a fine wire and low current.
Precision machining of the edge after overlay.

3. Distortion control: Thin orifice plates (typically 6–12 mm thick) are susceptible to warping. Control measures include:

Low heat input (minimum current and maximum travel speed compatible with fusion).
Symmetric welding pattern to balance thermal distortion.
Fixture clamping to restrain movement during welding.
Backing plate with thermal mass to absorb heat.

4. Dimensional verification: After overlay welding and machining, the orifice bore diameter must be verified using precision bore gauges or optical measurement. The tolerance is typically ±0.05 mm for the bore diameter and ±0.02 mm for concentricity.

Quality Verification Protocol

Given the critical nature of flow measurement components, the quality verification is extensive:

Visual inspection: All overlay welds examined for surface continuity, undercut, and porosity.
Dye penetrant testing (PT): Applied to all overlay surfaces to detect surface cracks and pores.
Ultrasonic testing (UT): Used to verify overlay thickness and detect subsurface defects.
Hardness testing: Minimum 3 points per overlay area to verify hardness uniformity.
Dimensional verification: Precision measurement of bore diameter, thickness, and flatness.
Corrosion testing: Immersion test in simulated process fluid at 500 °C for 100 hours to verify overlay integrity.
Flow coefficient verification: The orifice plate must meet the specified discharge coefficient (C) within ±0.5% of the theoretical value.

Engineering Practice and Service Performance

The overlay welded orifice plates produced at Changzhou Energy Equipment General Factory demonstrated:

Service life extension: From 6 months (unprotected) to 24+ months (overlay protected).
Dimensional stability: Bore diameter change during service remained within ±0.02 mm.
Surface integrity: No cracking, spalling, or delamination observed after extended service.
Measurement accuracy: Flow measurement deviation remained within ±0.5% throughout service life.

The successful application of overlay welding to flow orifice plates demonstrates the versatility of this technique for protecting critical functional surfaces in petrochemical equipment.

Study Insights and Reflections

This paper, though brief, addresses a highly specialized and technically demanding application of overlay welding. The key insight is that overlay welding is not limited to thick-section pressure vessels or large structural components — it can be successfully applied to thin, dimensionally critical components when the process is carefully controlled. The emphasis on low heat input, precise torch control, and post-weld dimensional verification highlights the importance of adapting welding parameters to the specific geometry and functional requirements of the component. For engineers working on flow measurement instrumentation in petrochemical service, this paper provides practical guidance on achieving the required surface protection without compromising the critical dimensional accuracy of orifice plates. The 500 °C service temperature requirement also underscores the need for overlay materials with excellent high-temperature oxidation resistance, which austenitic stainless steels (309L, 316L) provide through their stable chromium oxide scale.