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PUMPS, VALVES & ACTUATORS FULL STEAM AHEAD FOR INNOVATION


Mark Wheat, Celeros Flow Technology, outlines how innovative valve design has improved the conditioning of superheated steam used in power generation and other essential industries


paper mills. The basic principles, using a boiler, steam turbine and generator, have remained largely unchanged over the years, however, the pressures and temperatures involved have increased considerably. This article explains how Celeros FT brand


M


Copes-Vulcan developed the critical steam conditioning valve technology that meets the rigours of modern steam energy generation. The origins of steam generators can be


traced back to as early as 1884. Today, a steam power plant essentially uses a boiler to generate steam at high pressure and high temperature. A steam turbine converts the heat energy of steam into mechanical energy, and a generator then converts the mechanical energy into electric power. Water boils at 100ºC under normal


atmospheric pressure [0.101 MPa]. As pressure increases, the boiling temperature of water also increases. When the pressure is increased to 22.12 MPa, and at a temperature of 374ºC, water is directly converted into steam. This is called the critical point. Pressure above this critical point, with a temperature equal to or more than 593ºC, is called ultra-supercritical pressure. Handling superheated steam at these


pressures and temperatures places enormous strain on flow control equipment. Inadequately specified 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. 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 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 high-pressure 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


14 OCTOBER 2023 | PROCESS & CONTROL


any industrial activities rely on steam for their processes, including power generation plants, refineries, and


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 high- pressure steam unbalancing the trim. If 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, high-


pressure 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 minimise 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. Innovation #2: steam atomization This has 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


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