PMP discover cause of failure of lever plug valves in Thirlmere Aqueduct

The downstream non-return valves of the Thirlmere Aqueduct syphon pipes were Victorian plug valves, 54" Ø cast iron, opened by the water flow upthrust and balanced by lever arm counterweight assemblies. From previous reports, United Utilities were aware that some valve plugs were detaching from their vertical control rods and instructed Daniel Contractors to investigate.

DCL appointed PMP to develop a safe method of removing the detached plug valve, to investigate the root cause of its failure and to provide a detailed report.

PMP designed, fabricated and tested a bespoke lifting device to enable the detached valve plug to be safely lifted from the bellmouth and removed from the well. In May 2008, the 500kg detached cast iron plug was safely removed from Whaves Brook SW and, together with its lever arm counterbalance mechanism, was taken to our Haslingden workshop.

Once the control mechanism was disassembled and examined the cause of the failure was readily apparent - inappropriate material choice for submerged control rod: mild steel tube with no internal protective coating and no means of future inspection or maintenance.

PMP produced a detailed report on the condition of each component, reported the findings to Daniel Contractors and to
United Utilities designers Montgomery Watson Harza.

From PMP's wealth of experience in providing bespoke engineering solutions for the water industry, PMP proposed a new design solution backed by metallurgical analysis and design calculations. The proposed replacement control rod would use new components manufactured from 316 stainless steel and phosphor bronze to provide a corrosion and maintenance free solution.

During August and September, PMP fabricated 8 new lever plug valve assemblies which were replaced during the TA 2008 Outage. This was followed by a further 12 valve assemblies in the 2009 Outage and 34 in the final Outage of 2010.