DESIGN OF CONDENSER ONCE-THRU CIRCULATING WATER PIPING OUTLET SYPHON SEAL-WELL & OUTFLOW STRUCTURES
All once-through circulating water systems are normally designed with an outfall structure to return the discharge of circulating water flow back to the reservoir. Most of such system piping arrangements can incorporate siphon recovery to reduce the system pumping head requirement. A siphon piping arrangement works on the principle that no pumping head is lost in a pumping system between two points with the same elevation, irrespective of elevation differences that may occur between these points. When pumping from one point to another at the same elevation, the only losses are those caused by friction and piping appurtenances (valves, elbows, sudden divergence and convergences, exit kinetic losses etc.) A pipe that first rises in the direction of flow and then descends is called a siphon, whereas a pipe that first falls and then rises is called c alled an “inverted siphon”. On rising, the pressure head is transformed into elevation head and the reverse takes place on falling. Most once-through circulating water systems in power plants can use a siphon to some extent because the condenser elevation is usually well above the standing water level of the water reservoir (ocean, sea, lake, artificial pond etc). Although a natural siphon effect shall in any case occur, irrespective of whether a sealwell is incorporated ahead of the final outflow structure or not, engineering prudence dictates that a “seal well” is incorporated in to the return piping arrangement, ahead of the final outflow structure, for the following two reasons: 1) THE SYPHON EFFECT NEEDS TO BE LIMITED : The siphon principle works only if the circulating water piping flows full and is free of vapor and air . These requirements impose a limiting height for an effective siphon. The absolute pressure in a siphon is a minimum at the highest point in the system (which is normally the top of the outlet condenser water box). To prevent vaporization of the liquid at the highest point, the ABSOLUTE pressure must exceed the vapor pressure of the water. Thus, the siphon will, in most cases, have to broken broken in the circulating water discharge piping ahead of the outfall structure by installation of a “seal “seal well” well ”. The seal well exposes the circulating water flow to atmospheric pressure and the seal well SWL elevation is so selected to maintain the back pressure needed to prevent flashing (sudden evaporation) evaporation) of the circulating water in the outlet condenser water box, that can result due to excessive lowering (by siphon effect) of the static pressure at that point to below the saturation pressure corresponding to the pressure temperature of the water water at that point. If the SWL of the sea-well is too low or alternatively if no seal-well is constructed, the absolute pressure at the critical point may indeed fall below the saturation pressure.
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2) THE SYPHON EFFECT NEEDS TO BE CONTROLLED TO MAKE SURE THAT IT IS MAINTAINED:
Even if the critical system elevations and flow conditions are such that, the absolute pressure at the condenser outlet waterbox remains above the saturation point, thus ostensibly obviating the need for the incorporation of an interim elevation seal-well, it must still be made sure that the siphon is not broken during operation due to ingress of air, resulting in uncontrolled exposure to atmosphere. Incorporation of no seal-well shall mean that the total length of the return piping structure is exposed to risk of air ingress through several appurtenances. Moreover, during wavy sea conditions, SWL will naturally fluctuate and this may lead to approach to the critical saturation pressure at condenser outlet waterbox. Therefore it would still be best to incorporate an interim elevation seal-well, with a well defined and controllable SWL, in turn translating to the maintenance of a well defined, fixed minimum absolute pressure in the highest siphon point.
Notwithstanding the above, it would be theoretically possible to “oversize” the sea water circulation pumps by not taking into account any siphon effect in calculating pump total dynamic head requirements. However, it should be noted that, this over sizing will, under normal conditions where the natural siphon is still maintained, translate into higher than required flow rate and naturally higher flow velocities. First and foremost this means excessive power consumption. On the other hand; all condensers may not be conducive to excessive tube velocities, such as those with brass and CuNi tubes. Excessive velocities shall result in copper transportation phenomenon and eventual tube failure. Therefore, the excessive flow may need to be throttled down via control valves, again translating into power wastage. One would also naturally circle back to the issue of flashing risk at the minimum absolute pressure point in the system, hence forming a vicious circle. In conclusion, all once-thru condenser circulation system must be equipped with siphon control seal-wells, and the circulation pumps must be so selected to provide optimum total dynamic lift during normal operation. The pumps should also be capable of developing the siphon autonomously, by providing sufficient additional lift during the condenser filling operation, at reduced flow rate. Considering such diverse operational requirements, varying winter and summer condenser flow rate requirements and the need to provide at least 75% of full flow requirement per condenser during the outage of a single pump etc, adjustable angle of attack propeller type pumps should be selected. These pumps operate at fixed low speed, with large propeller diameters, with minimum NPSH requirements and have an essentially flat efficiency curve across a wide range of operating conditions, leading to significant kwhour savings in operation. Such features can simply be not achieved with fixed angle variable frequency drive type pumps, hence should be avoided.
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Good engineering practice dictates that, the minimum absolute pressure at the highest point in the siphon (outlet water box top) must be maintained above 6 feet water column, i.e. at least 1.8 meter (absolute, not gauge, hence a vacuum of around 8.2 meters). This would mean a flashing temperature of around 58 Deg C, hence a safe figure, even under condenser warm water recirculation operation for biological growth control, where the objective is to achieve temperatures of slightly above 43 degrees only. In the original Ambarlı Fuel-Oil plant units 4 & 5 design, the system is designed to maintain a minimum absolute pressure (not gauge) of around 3 meters, at the worst condition of -2 meter SWL at pump pit. This translates in to min 5 meters absolute pressure at a SWL of 0 meter. S.Erkan, EPP JV, 21.7.2010
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