Shannon Creek Dam and pump station is the core feature of the Regional Water Supply scheme, a joint venture of the Coffs Harbour City Council and the Clarence Valley Council. With an inundation area is 37 square Km, the reservoir provides 30,000 mega litres of supplementary water supply to The Clarence Valley and Coffs Harbour, located on the Mid North Coast of NSW, Australia. This award-winning project will secure water supply to an estimated 220,000 people in the region over the next few decades. The dam crest can be raised a further 10 meters to increase storage a further 45,000 megalitres.
The Nymboida River supplies the Shannon Creek Dam with its water and has an inlet weir at the river takeoff that is approximately 128 m above the pipe work in the valve pit. The dam delivery head is 33 m making the normal head loss across the valve 95 m. This can increase to a maximum of 105 m when the Nymboida River is at its maximum level as a result of heavy rainfall.
Clarence Valley Council (CVC) who manage the scheme, have an operating license that allows for up to 54 mega litres/day which amounts to 624 l/sec flow into the dam. This can increase to 94 mega litres/day (1087 l/sec) in certain emergency situations. The dam inlet control valves have to be able to operate between closed and maximum flow rate, depending on current rainfall in the catchment area, and levels in the Nymboida River.
Since commissioning and start up, CVC has experienced ongoing cavitation problems and seal failures of the main inlet control valve. The original installation included an inlet valve - DN750, PN35 double flanged butterfly valve, along with a DN250, PN35 low flow bypass butterfly valve. The problem was the DN250 bypass valve would cause severe noise and vibration when operated, which lead to the DN750 valve doing the majority of the work. It was operating at approximately 8 to 9% open to achieve the normal required flow rate of 250 l/sec into the dam. This is really not an ideal situation for a butterfly valve, as the butterfly valve’s original design is for fully open or fully closed.
After the second valve seal failure on the main valve that resulted in severe leakage when closed, CVC contacted TOTAL Flow Control to provide a solution that would deliver to CVC long term operating reliability and control. It was determined that with the significant head loss across these valves and the variable flow range required, the installed DN750 butterfly valve was oversized, and was an inappropriate valve type for this application, resulting in severe cavitation damage to the valve seats, leaf and seal. The consequences of cavitation are numerous and included loud noise, extreme vibrations, choked flow, erosion of the control valve, its components, and connected pipework resulting in the failure of the main inlet valve.
During discussions regarding these problem valves, CVC provided TOTAL Flow Control and Singer Valve with specific flow ranges, inlet pressure ranges and outlet pressure requirements enabling Singer to offer and recommend an engineered solution to their specific application.
CUSTOM ANTI-CAV CONTROL VALVE SOLUTION
Singer’s Anti-Cavitation technology contains two heavy stainless steel sliding cages that maximize the full flow capacity. The first cage directs and contains the cavitation recovery, allowing it to dissipate harmlessly, while the second cage allows further control to a level as low as atmospheric pressure downstream. The cages are individually engineered by entering the data into proprietary software which calculates the size and placement of the orifices on both inlet and outlet cages. The valve bodies are specifically designed to fit a larger cage allowing higher Cv values (increased flow) while also allowing for reasonable space between the anti-cavitation trim and the body wall. This separation allows for consistent uniform entry around the cage area ensuring the vapour bubbles collapse symmetrically towards the center of the anti-cavitation cage. Extreme pressure drops, even to atmosphere and up to 21Bar head loss across the valve will still not allow cavitation to occur.
The DN750 main inlet valve and DN250 bypass valve were replaced with a single DN600 electronic dual solenoid control valve, with custom engineered - Anti Cavitation Trim. CVC already had Singer valves in other parts of the distribution network, so were familiar with their reliability and use of quality materials that give them a long life. Part of this assurance is warranty that covers the operation of the AC Trim for the life of the valve when used within the specified operating parameters.
TOTAL Flow Control was contracted to supply and install the new valve, including removal of the existing valve assemblies, and providing various fittings and labour to complete the installation and on site commissioning of the valve. In removing the existing valves, the cavitation damage had badly damaged the resilient seat and there were obvious signs of erosion on the body, disc, and the concrete liner of adjoining pipework, as well as on the downstream dismantling joint and fittings. The replacement of the inlet control butterfly valve with the Singer control valve may have prevented a catastrophic failure of the original butterfly valve in the event of fatigue failure of the butterfly spindle. There was evidence of fatigue failures of some of the seal retaining screws.
The actual installation of the new valve was completed over a two-day period with local contractors, AJ Pipelines.
Andrew Parker from TOTAL Flow Control worked with CVC Engineers and Operators, to successfully commission and start up the new Singer control valve. The valve was tested over a variable flow range capacity that was within allowable licensed flows on the day. It was able to control flow and deal with the head loss pressure easily with minimum noise and no vibration from 0 to 400 l/sec on the day of commissioning.
The operators are now able to locally and remotely control the inlet flow of water into the dam with no further concerns for valve failure or leakage from cavitation. As a bonus, the project came in at just over 1/3 of the $300,000 budget thanks to the clever solution offered by the Contractor to use the existing valve pit, with no need for a rebuild.