Surface Condenser
- FIGURE 35 Typical hookup for submergence controlled condensate pump
CAPACITY, IN GPM
FIGURE 36 Characteristic of a condensate pump operating on a submergence-controlled system
CAPACITY, IN GPM
FIGURE 36 Characteristic of a condensate pump operating on a submergence-controlled system ate a shock wave of a high enough intensity to inflict physical damage on the pump parts. If, however, higher values of NPSH were required—as for instance with vertical-can condensate pumps because of their generally higher operating speed—operation in the break would result in a rapid deterioration of the impellers. It is for this reason that submergence control is not applicable to can-type condensate pumps.
The main advantage of submergence control was its simplicity and the fact that the power required for any operating condition was less than with any other system. Disadvantages occur when the pump is operating at very light loads, however, because the system head may require as little as one-half of the total head produced in the normal head-capacity curve. In this case, the first stage of a two-stage pump produces no head whatsoever and, if the axial balance was achieved by opposing the two impellers, a definite thrust is imposed on the thrust bearing, which must be selected with sufficient capacity to withstand this condition. In addition, no control is available to provide the minimum flow that may be required through the auxiliaries, such as the ejector condenser.
The condensate pump discharge can be throttled by a float control arranged to position a valve that increases the system-head curve as the level in the hot well is drawn down. This eliminates the cavitation in the condensate pump, but at the cost of a slight power increase. Furthermore, the float necessarily operates over a narrow range, and the mechanism tends to be somewhat sluggish in following rapid load changes, often resulting in capacity and pressure surges. This transient condition is often the root cause of failed thrust bearings and axial rotor shifting. Another critical piping arrangement feature is the discharge piping check valve. The check valve in every condensate pump must be below the condenser hotwell level to ensure prevention of air entrapment and start-up waterhammer. [Refer to Section 8.3.]
When condensate delivery is controlled through bypassing, the hot well float controls a valve in a bypass line connecting the pump discharge back to the hot well. At maximum condensate flow, the float is at its upper limit with the bypass closed and all the condensate is delivered to the system. As the condensate flow to the hot well decreases, the hot well level falls, carrying the float down and opening the bypass. Sluggish float action can create the same problems of system instability in bypass control as in throttling control, however, and the power consumption is excessive because the pump always operates at full capacity.
A combination of throttling and bypassing control eliminates the shortcoming of excessive power consumption. The minimum flow at which bypassing begins is selected to provide sufficient flow through the ejector condenser.
A modification of the bypassing control for minimum flow is illustrated in Figure 37, which shows a thermostatic control for condensate recirculation. With practically constant steam flow through the ejector, the rise in condensate temperature between the inlet and outlet of the ejector condenser is a close indication of condensate flow rate through the ejector condenser tubes. Therefore an automatic device to regulate the condensate flow rate can be controlled by this temperature differential. A small pipe is connected from the condensate outlet on the ejector condenser back into the main condenser shell. An automatic valve is installed in this line and is actuated and controlled by the temperature rise of the condensate. Whenever the temperature rises to a certain predetermined figure, indicating a low flow of condensate, the automatic valve begins to open, allowing some of the condensate to return to the condenser and then to the condensate pump, which supplies it to the ejector at the increased rate. When the temperature rise through the ejector condenser is less than the limiting amount, indicating that ample condensate is flowing through the ejector condenser, the automatic valve remains closed.
As condensate flow design demand increased, the vertical multistage pumps were installed as two half-capacity pumps. Flow variation is accomplished by operating one or two pumps and by utilizing the regulating discharge valve.
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