D

FIGURE 3 Temperature-controlled system regulating condensate injection to feed pump shaft seals

Constant Drain Temperature-Controlled System The constant drain temperature-controlled seal system (see Figure 3) controls hot out-leakage by throttling the injection flow to maintain a preset seal drain temperature. This system has a temperature-sensing probe in each seal drain line. Each probe is connected to an indicating temperature controller, which provides an air signal to a pneumatic control valve in the condensate injection line for control of the seal injection flow rate. Electronic control systems are often used where thermocouples or RTD's sense drain temperatures. The signal output is then processed through an I/P controller that adjusts the position of the pneumatically operated throttle valve as required.

The cold condensate is injected into the stuffing box central portion and allowed to mix with hot water entering the seal from inside the pump. The drain temperature is maintained at a preset 140 to 150°F (60 to 66°C) to preclude flashing in the stuffing box or in the drains. Note that with this system some hot water enters the seal. Therefore, cold condensate does not enter the hot pump and does not adversely affect pump warming conditions, especially during extended idle periods. The required condensate injection pressure is at least equal to the internal stuffing box pressure plus interim frictional loss between the condensate supply source and the point of hot-water mixture. Note that this system may enable satisfactory operation even when the condensate supply pressure is nearly equal to boiler-feed pump suction pressure. In addition to providing a rapid response to variations in operating pump conditions, this type of control will always supply just enough injection water to maintain the recommended drainage temperature.

Intermediate Leakoff System The intermediate-leakoff shaft seal system (see Figure 4) has many variations but basically uses a bleedoff from a central portion of the stuffing box. This system may be used to reduce internal stuffing box pressure if high boiler-feed pump suction pressure exists. To create a positive leakoff flow, the intermediate bleedoff flow is piped back to a plant feedwater system low-pressure point, such as a plant condenser, heater, or booster pump suction where the pressure is less than the boiler-feed pump suction pressure. However, the back pressure of the leakoff destination must be above the bleedoff vapor pressure to suppress flashing in the leakoff lines. This back pressure may be the leakoff destination pressure or may be created by an orifice or a valve.

Cold condensate injection into the stuffing box is controlled by a pressure differential monitor maintaining a preset pressure above the bleedoff pressure (refer to Figure 4). A

FIGURE 3 Temperature-controlled system regulating condensate injection to feed pump shaft seals

FIGURE 4 Typical intermediate-leakoff shaft seal system. (From Power, September 1980, McGraw-Hill, New York, copyright 1980)

stuffing box drain temperature control can also be used. Note that the cold condensate injection pressure need not equal or overcome a high feed pump suction pressure. The condensate injection temperature must still be 85 to 110°F (30 to 45°C) to keep the drain temperature below the flashing condition. Condensate injection shaft seals without an intermediate bleedoff but that are subject to suction pressures in excess of about 250 lb/in2 (1725 kN/m2) must be extremely long for a proper pressure breakdown. Longer shaft seals require thicker pump case end covers, affecting pump cost, and a longer rotating element, which could adversely affect rotor dynamics. The intermediate-leakoff shaft seal is effective where there is high feed pump suction pressure imposed by boiler-feed booster pumps or in a closed feedwater system with no deaerating open heater, wherein a condensate pump discharge can be fully imposed on the feed pump at low plant loads.

In plant systems with feedwater heaters between a booster pump and a feed pump, a high pressure condensate from the cooler booster pump is injected into the seal to enable a cooler intermediate leakoff to help prevent flashing (refer to Figure 2).

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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