Perfect repair of large size gate valve

Perfect repair of large size gate valve
The waste valves replaced from the working site are corroded by various media, fluid pressure, and affected by different temperatures. The parts including the valve body are affected by different degrees and different locations and cause valve malfunction. The drop cannot even be used. However, the used old valves that have been ineffective at the “validation” are not worthless and have no remediation value. They cannot be repaired and used, as long as they use the principles of metal structure, dynamics, fluidics, corrosion protection, welding, and heat treatment. Knowledge, the provision of essential maintenance equipment and the establishment of an experienced professional team, can fully implement deep-level, high-grade, high-quality repair and refurbishment of used valves.

The quality problem and situation for gate valve

In Jun 2017, there were 4 pcs wedge gate valve PN25 dn500  WZ41H-PN2.5MPa in the water supply circulation system of Sichuan Jiuda Group vacuum salt making equipment. The nominal diameter DN500mm gate valve was replaced by a large amount of leakage and sent to our maintenance (only one for example) .

After the dismantling check, a large area of erosion corrosion occurred on the valve body, the gate plate and the sealing surface at the lower part of the valve inlet. The valve body has 120mm2 of corrosion, the depth is 20mm, the gate plate has 300mm2 corrosion, the depth is about 22mm, the sealing surface It is completely eroded.

The valve body and gate plate A216 (WCB) of the valve, this type of carbon steel harden tends to be harder, welding is easy to produce cold cracks in the weld, welding performance is poor.

The sealing surface of the valve body and the sealing surface of the stem are A182-F6a, which is a martensitic stainless steel. It is easy to produce hard and brittle martensite in the welding process, resulting in hardening and embrittlement of the weld and heat affected zone. , poor welding performance.

The valve is a large piece, the structure is rigid, easy to produce cracks, strict repair welding process.

carbon steel-ductile iron valve, stainlesss steel valve waste, repair and replace
carbon steel-ductile iron valve, stainlesss steel valve waste, repair and replace

Repair measures:

According to the working conditions of the valve, the contact medium and mechanical processing and other factors, the CHSo52 stainless steel electrode (Atlantic brand series electrode) with good comprehensive performance and resistance to chloride ion corrosion was selected to perform surfacing welding on the sealing surface of the valve.

Before repair welding, first deal with surface pits, sharp corners and other defects, and then polished smooth and smooth.

Because of the poor weldability of the valve body and the gate base, a transition layer must be added between the base and the surfacing layer to reduce or avoid the risk of cracks. The mechanical properties of the weld can also be satisfied.

First, use A302 stainless steel with good plasticity and toughness to make the transition, and then fill it with J507 low hydrogen electrode with good heat resistance, so that the strength of the weld is the same as that of the substrate. When the transition layer is surfacing, first weld the place where the weld is uneven, and then begin the surfacing from the bottom of the groove. The weld is gradually expanded outwards from the spiral shape, and the short arc arc welding is applied. The thickness of the weld bead is about 4 mm. We strictly control the specification during welding and avoid stress concentration. For overlay welding, the same surfacing method as the transition layer is used. The height of surfacing is the same as the surface

Although the damaged valve body and the sealing surface of the valve vane have been removed and no longer exists, surfacing is still performed in the original arc. Starting from the starting point of radius R, a short-arc arc breaking arc welding method is used for welding. The weld bead was widened by 6mm on the basis of the original sealing surface, and the highly surfacing layer was 3mm higher than the original sealing surface to facilitate machining.

The gate sealing surface is machined on a lathe with special tooling, and the valve body sealing surface is specially processed with the VRM’s patented maintenance equipment.

After the assembly of the valve, the strength of the valve body and the test pressure of the sealing surface, etc. are in full compliance with national standards and the customer is very satisfied.

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