ReManufacture+ EfW HP Bypass Stop Valve

The Challenge

An HP bypass system controls and maintains main line HP turbine steam pressure. The HP bypass stop valve allows isolation of the HP bypass system for online maintenance activities.

An EfW customer recently suffered a failure of their HP bypass stop valve in service and required support to inspect and recover the assembly for installation during an outage, on an urgent basis. They held no spares and lead time of a new component was a factor of months rather than weeks.

This valve incorporated a common main plug and pilot (pressure balancing) plug design. The pilot plug was retained by a threaded back nut which is held in an open position by a stack of belleville washers. The result is a high load on the back nut during operation.

Solving the Problem

On receipt to SpiralWeld™ Ltd it was clear that the valve had suffered a failure of the retaining back nut. The thread had been stripped from the nut and main plug body. Nut retention had been achieved by tack welds around the the nut circumference.

The main plug and back nut material was a hardenable martensitic stainless steel which is widely used for high temperature power generation applications. After welding this material, it is necessary to perform a post-weld heat treatment (PWHT) to stress relieve the weld at the heat affected zone. Without this PWHT the weld strength was compromised and with no threads to support the operating loads the valve failed after a short period in service.

Other observations included corrosion of the plug spindle and corrosion/erosion of the main plug control surface.

A Successful Outcome

We were able to offer the customer a full ReManufacture+ solution within the time frame of recovery of this valve. This included:

• Reverse engineering of full valve assembly, technical review and development of full quality plan inclusive of qualified weld procedures.
• ReManufacture+ of main plug. Machining out the damaged thread to sufficient depth, multi-pass weld with equivalent overlay and re-establishment of thread. ReManufacture+ of main control seat with our advanced cobalt hard facing.
• Manufacture of new back nut from an equivalent material.
• Manufacture+ of a new pilot plug spindle in equivalent material including carbo-nitride surface treatment to improve corrosion protection.
• ReManufacture+ of pilot plug seat with advanced cobalt hard facing.

All SpiralWeld™ processes are performed in accordance with our ISO 9001:2015 quality system and delivered to our customer within the required window to support immediate installation on site. In this support we met our commitment to better-than-new cycle time and delivering these components OEM performance standard or, in this case, better.