Weld Overlay Repair of Annular Sealing Flange Sealing Surfaces on High Pressure Jet Pumps in Urea Plants

Literature Overview

This 2020 technical paper published in Large-Scale Fertilizer Technology documents a practical engineering solution for restoring the sealing integrity of annular flange faces on high-pressure jet pumps (ejectors) in urea production facilities. The work was authored by Li Fangquan from CNOOC Chemical Co., Ltd., representing frontline field maintenance engineering experience in the fertilizer industry. The technical challenge addressed is the degradation of sealing surfaces on critical high-pressure equipment where replacement of entire components is either impractical, prohibitively expensive, or unavailable due to long lead times.

Technical Background and Problem Statement

Urea production plants operate under severe conditions with high-pressure ammonia, carbon dioxide, and their reaction products. The high-pressure jet pumps (ejectors) used for vapor compression and circulation operate at pressures typically ranging from 15 to 25 MPa with temperatures of 180-220 °C. The annular sealing flanges on these pumps are subjected to:

Cyclic thermal loading from start-up and shutdown operations
Chemical erosion from ammonia and urea solution
Mechanical wear from flange bolt tightening and loosening cycles
Galling and scoring from relative movement during assembly

When the sealing surface is damaged beyond the tolerance limits specified in ASME B16.5 or equivalent standards (typically surface roughness exceeding Ra 3.2 μm or concentricity deviation beyond 0.05 mm), leakage occurs at the flange joint, creating safety hazards and production losses.

Repair Methodology and Process Parameters

The repair methodology employs multi-pass GTAW (gas tungsten arc welding / TIG) overlay to rebuild the sealing surface geometry and restore surface finish requirements. The process design follows a systematic approach:

Process Parameter	Specification	Rationale
Welding process	GTAW (TIG)	Low dilution, precise control, clean weld
Filler material	ER308L or ER316L	Compatible with 304/316L base, low carbon to prevent sensitization
Shielding gas	Argon (99.99%)	Inert atmosphere, prevents oxidation
Wire diameter	1.6 mm	Balance between deposition rate and control
Current	120-180 A	Adequate penetration without excessive heat input
Travel speed	30-50 mm/min	Controls bead width and profile
Heat input	0.5-1.0 kJ/mm	Minimizes distortion and HAZ effects
Preheat temperature	50-100 °C	Reduces cracking risk, removes moisture
Interpass temperature	Below 150 °C	Prevents sensitization in austenitic stainless steel

The repair sequence involves: (1) thorough cleaning and surface preparation of the damaged area by grinding back to sound metal with a 30° chamfer groove, (2) application of a root pass to establish full fusion with the base material, (3) filling passes to rebuild the required thickness (typically 2-4 mm total overlay), (4) final dress-up pass to achieve the required surface profile, and (5) post-repair machining to achieve the specified flatness and surface roughness.

Quality Assurance and Inspection Protocol

Given the criticality of the repaired component operating at high pressure and temperature, a comprehensive inspection protocol is essential:

Visual inspection (VT): 100% examination of all weld passes for surface defects, undercut, and porosity
Penetrant testing (PT): Applied to the finished machined surface to detect surface-breaking cracks and porosity
Magnetic particle testing (MT): If the base material is magnetic (carbon steel backing), applied to detect subsurface defects
Hardness testing: Overlay layer hardness should be within 150-250 HV, ensuring adequate machinability and avoiding work hardening during machining
Macrographic examination: Cross-sectional examination of a coupon to verify complete fusion and absence of internal defects
Hydrostatic test: Final pressure test at 1.5 times design pressure to confirm sealing integrity

A critical quality consideration is the prevention of intergranular corrosion in the heat-affected zone. The use of low-carbon (L-grade) filler materials and strict interpass temperature control are essential to prevent chromium carbide precipitation at grain boundaries.

Engineering Practice Reflections

This case study exemplifies the practical engineering approach to component restoration that is frequently required in process industries where unplanned shutdowns are extremely costly. Several lessons emerge from this type of repair work:

Root cause analysis is essential: Before proceeding with overlay repair, the root cause of sealing surface degradation must be identified. If the cause is improper bolt torque sequencing or gasket material incompatibility, repair without addressing the root cause will result in repeat failure.
Welding procedure qualification: The repair welding procedure must be qualified in accordance with NB/T 47014 or ASME IX, with qualification coupons tested for mechanical properties and, where applicable, corrosion resistance.
Post-repair monitoring: After reassembly and return to service, the repaired flange should be included in enhanced monitoring schedules with periodic leak detection checks during the first operating cycle.
Documentation: Complete documentation of the repair including welder identification, consumable lot numbers, procedure parameters, and inspection results must be maintained for traceability and regulatory compliance.

The paper's practical value lies in demonstrating that weld overlay repair can be a technically sound and economically advantageous alternative to component replacement, provided that appropriate procedures, qualifications, and quality controls are implemented.

Summary and Conclusions

This technical documentation provides valuable field-level insight into the practical repair of high-pressure sealing surfaces through weld overlay techniques. The GTAW-based repair approach, when properly executed with appropriate filler selection, process control, and inspection protocols, offers a reliable means of restoring service life to critical components without the cost and delay of full replacement. For maintenance engineers in the urea and ammonia production sectors, this work validates the feasibility of in-situ repair strategies while emphasizing the non-negotiable requirements for procedure qualification, welder certification, and comprehensive post-repair inspection to ensure long-term reliability under high-pressure operating conditions.