π www . valve-world . net Valve World February 2024
Valve design valves could lead to inefficiencies through leakages and unplanned outages . What the 21st century required was a valve that could deliver operational reliability and withstand the extreme operating conditions within modern power plants – particularly during plant start-up , shutdown and turbine trips .
21st century solution
The DSCV-SA ( Direct Steam Converting Valve – Steam Atomization ), addresses the issues encountered by older , base load designed bypass valves when employed on modern high frequency , rapid ramp rate plants . Key to the DSCV-SA ’ s performance are a number of unique technical innovations developed following extensive consultation with power generation customers .
Innovation # 1 : high-pressure balance Unlike conventional turbine by-pass valves , the DSCV-SA is designed to use highpressure balance rather than low-pressure balance . This eliminates risk of wear , damage or breakage relating to piston rings and balancing systems , which are a major problem with traditional valves . When an open command signal is received , the DSCV-SA actuator retracts , and the pilot plug is the first to open . This allows P1 steam to flood through the large pilot plug port to the underside of the main plug , which in turn balances it and reduces the actuation thrusts required . In traditional low-pressure or P2 balancing designs , auxiliary balancing seals such as piston rings and close tolerance sealing surfaces are needed to prevent the highpressure steam unbalancing the trim . If
Bonnet split ring
Pressure seal spacer
Pressure seal ring
Trim split ring
Trim retaining cap screws
Trim retainer ring these seals or surfaces become worn or damaged , it can unbalance the trim and stem loads can fluctuate dramatically , causing the valve to oscillate violently or not open on command . When the DSCV-SA pilot plug is open , highpressure inlet steam floods the underside of the main plug and the steam atomizing unit ( see Innovation # 2 below ) operates in preparation of the incoming cooling water from the water control valve . The pilot plug shoulder engages with the underside of the tandem cap of the main plug , which then starts to lift , and the main seat opens . As the main plug opens , steam first enters the valve via a heavy-duty distribution spacer . The steam passes through the spacer by means of numerous holes evenly positioned around the circumference . This distribution spacer is specifically designed to negate any upstream pipework-induced flow disturbance being communicated to the main plug . This is important because long radius bends or isolation valves can be fitted directly to the valve inlet to minimize installation space . The main plug is fully guided by the cage and spacer to ensure complete plug stability through full travel . After the inlet steam has passed through the distribution spacer , it then travels through the main seat area to the underside of the main plug via large feed ports . With the main plug lifted , the pressure reducing ports of the cage now open to allow the steam to be pressure reduced in a controlled manner . As the main plug opens further , more pressure reducing ports are exposed and the steam flow rate increases .
Distribution spacer
Key features of the DSCV-SA design , which facilitate high turndown in steam atomization applications .
The Copes Vulcan DSCV-SA turbine bypass valve has evolved to be at the forefront of today ’ s thermodynamic engineering in steam conditioning .
Innovation # 2 : steam atomization Steam atomization is a technology that has significant benefits over mechanically spraying the cooling water via nozzles . Traditional mechanical spray nozzles - even spring-loaded types - are limited in their turndown . This is because the water atomization and spray pattern degrade as the water flow rate and available pressure differential reduces . As the water demand reduces , the spray water control valve closes and the spray valve trim absorbs the water pressure differential , which leaves little pressure differential for the spray nozzles . This lack of pressure differential does not allow atomization of the spray water , which results in the water pouring into the steam rather than producing a fine atomizing mist . Mechanical spray nozzles rely on the surrounding steam velocity to provide adequate mixing . When the steam load reduces , so too does the steam velocity and the ability of the mechanical spray nozzles . This effect manifests itself with poor downstream steam temperature control and water ‘ drop-out ’, which can be very damaging as cold water can track along the bottom of the inside wall of the downstream pipe whilst un-cooled superheated steam travels along the top and sides . This produces high thermal shocks which can lead to steam header fracture . With steam atomization however , cooling water is pre-heated ; significantly accelerating the evaporation and desuperheating process . Equally important is the finely atomized incoming cooling water . Very fine atomization produces extremely small water droplet sizes with a vastly
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