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Emergency shut-down for a flue gas double disc slide valve failure

PROBLEM: In early August, 1997, RPC contacted Remosa for an emergency intervention at the ConCón refinery, in Chile. A severe problem with regulating the differential pressure in the regenerator had been experienced for a few days, finally forcing a plant shut-down. In less than 24 hours, a Remosa field mechanical engineer was at work on the troubled flue gas double disc slide valve (cold wall design, 64" ID, originally by an US manufacturer). Upon disassembling the valve, it was found that the trim had collapsed, due to the failure of the 3/4" bolts fixing the trim itself to the seating ring. The problem was traced to an inadequate size and number of bolts, while the material they were made of - B8 - was the one commonly used for that type of job.

More trouble was discovered. The seating ring was in a funnel-like shape (3mm maximum distortion). The stems were distorted as well - extremely, beyond repair. The discs showed signs of wear on their seating faces and on the face exposed to flow. Stellite on the sliding surfaces was suffering from considerable wear. The guides were distorted too. Stellite on the sliding surfaces and on the sides was eroded and showed web-like patterns of cracks. The orifice plate was in a good conditions.

SOLUTION: A complete valve overhauling was performed as follows:

1. Stems were replaced with brand new types.
2. All stud bolts were replaced - upgrading them to 1"-8 UNC wherever possible.
3. New internal stuffing boxes were manufactured and all packing rings were replaced.
4. Discs reconditioning steps:
   • RESCO demolition
   • Hex steel removal
   • Installation and welding of Hex steel and corner tabs, according to UOP Specification 3.25.4
   • Sandblasting and welding slag removal
   • Application of RESCO AA-22S according to UOP Specification 3.25.4
   • Manual fitting on the sliding surfaces and sealing area of the disc
5. Guides reconditioning steps:
   • Layering with Stellite Gr 1 on the eroded surfaces
   • Restoration of linearity
   • Grinding of sliding surfaces
6. Orifice plate reconditioning steps:
   • Flattening of guide seating surfaces
   • Enlargement to 30mm of the seating ring fixing holes
7. Valve general re-assembling:
   • Trim installation
   • All cold clearances checked according with original drawings
   • Installation of stuffing box bonnet cover
   • Manual stroke test
   • Discs mechanical calibration
   • Lip seals welding

WHAT THE CUSTOMER HAD TO SAY: RPC was definitely satisfied with the work done. Remosa had recommended to acquire new spare parts, with recalculated thickness and dimensions to allow a general holes enlargement as for using 1" bolts to fix the trim to the seating ring. Later on, RPC actually issued an order to Remosa for manufacturing a complete set of spare internal parts.

Copyright © REMOSA spa 1998-2001 Last updated on June 29, 1999