Page 6 of 7
Similarly, the outlet has to be protected from the collection of excess fluids from desuperheating or condensation. The valve outlet and condenser feed pipe normally has greater quantities of fluid to be removed than the inlet and the flow energy is greater, thus compounding the situation. The elimination of excess fluid can be handled in several ways. The first is to make sure that only the correct amount of spray is injected into the discharge pipe. Thus, if all is evaporated effectively, there will be no excess fluid fall-out. The second is to make sure that the backpressure device is equipped with a drain connection near its termination point to continuously drain excess fluid build-up to the hotwell or some other low-pressure collection location. This is normally the case in that the backpressure device orifices point radially out and away from the condenser tube bundle, leaving a natural collection point at the end of the header.
Preheating is used not only to keep the valve warm and to prevent thermal shock when the system is commanded to operate, but also to keep the piping warm so that excess fluid condensation is minimized, reducing the cyclic thermal stresses which can lead to thermal fatigue. The quantity and method of preheating are dependent on the piping layout of the system, the design of the valve, and the mode of operation of the system, i.e., long term stand-by vs. daily start and stop. In many situations, little or nothing is done to assist in preheating beyond considerations for conduction through the pipe and valve walls. This will certainly have a long-term affect on the reliability and longevity of the equipment. In other situations, a small bypass line is installed between the upstream and downstream piping. A small needle valve is placed in the line and used to control the quantity of steam being bypassed around the main valve. Temperature readings are monitored until the desired thermal conditions are achieved. The system can be set up as automatic with temperature controls included or as a manual system requiring monitoring by the operators. Table 3 indicates recommended temperatures maintained through pre-heating of the valve dependent on temperature service.
It is also important that the slope of the discharge piping be pitched, negative, with drainage towards the condenser. This forces the entrapped fluid to move towards the internal condenser drain and discharges into the hotwell. If the fluid is not adequately removed from the system prior to bypass valve operation, the resultant momentum transfer could create a water hammer-like situation that could be seriously damaging to both the backpressure device and the condenser.
In some installations, use of isolation valves upstream of both the attemperator spraywater control valve and upstream of the bypass valves is utilized. Though an added expense, some owner/operators perceive that the use of a motor-operated block-valve upstream of the spraywater valve reduces the chance for a constant leak of spraywater through the bypass. The use of a motor-operated block valve upstream of the bypass valve is less common, but is sometimes used to eliminate steam leaks through bypass valves, which can cause significant MW losses that are "hidden" in many cases. A metal-to-metal seat valve is at best good for Class IV shutoff, which means a small amount of steam will leak by at all times. By definition, leakage allowed for this type of valve is 0.01 percent of rated valve Cv capacity at ANSI test conditions. Under actual service conditions, the leakage is more significant due to the high system pressure.