A condensate pump is a centrifugal pump that gets its name from the fluid it deals with. It's used in condensers to extract condensed steam as water (condensate) under a technical vacuum (near vapor pressure). Condensate pumps carry the condensate into a tank (for example, a feed water tank) in an open circuit; in a closed circuit, and pumps the condensate directly into the boiler feed pump through a low-pressure feed heater.
The capacity of the condensate pump is determined by the steam turbine's maximum mass steam flow rate.
The head's composition:
- Water levels in the feed water tank and the condenser differ by a geodetic head
- In the feed water tank and the condenser, the static pressure heads differ
- Head losses in the pipeline, including installed valves (such as gate valves and swing check valves) and system components (e. g. suction strainer, condensate preheater)
The vapor pressure of the water on the suction side (about 56.2 mbar for pure water at 35 °C) and the low intake head caused by the condenser's location within the structure determine the condensate pump's design. The input head is computed by subtracting the flow losses in the inlet line from the geodetic head between the condenser's standard water level and the first stage's impeller level.
The available NPSH of the system must be more than or equal to the necessary NPSH at the impeller of the first stage to ensure optimal operating behavior and avoid cavitation damage. This is true across the board.
Increase the system-side inlet head by taking the following steps:
- Minimizing intake line flow losses, for example, by using bigger nominal pipe diameters
- Vertical arrangement, such as a dry installation that lowers the first-stage impeller's height above the installation floor and increases the geodetic head differential
- Vertical configuration as a "can-type pump," with the suction stage lowered into an input "can" positioned below the installation floor to maximize the geodetic head differential
- Above the installation floor, the input and discharge lines are located
Suction qualities of a condensate pump may be improved in a number of ways:
- Suction impeller installation
- Inducer installation
- A dual-entry impeller is being installed
- Rotational speed reduction
- Improvements to the pump's flow channels
A condensate pump with intermediate extraction is an example of a design variation (re-entry). The whole flow is transferred to the condensate cleaning system after the first or second stage of the pump (condensate booster pump with single or dual entry suction stage).
The condensate main pump, which is located above the floor and above the booster pump with which it forms a unit, increases the pressure even further.
High cavitation loads necessitate special consideration of cavitation intensity, velocity conditions, and the lengths of the cavitation bubbles' trail when flow rates surpass 150 l/s (540 m3/h). The material loss rate LM is used to determine the severity of cavitation.
Because the flow velocity at the impeller vane leading edge cannot be changed significantly at a fixed flow rate, the lengths of the bubble trails must be as short as feasible.
When a condensate pump is stopped, the shaft seal must offer sealing against a low technical vacuum. In order to avoid air infiltration, the sealing element must be supplied barrier fluid from the system-side barrier system. A lantern ring is fitted between the packing rings in the case of gland packings to accomplish this. Inboard and outboard twin mechanical seals are available. The lantern ring or the chamber between the inboard and outboard mechanical seals receives the barrier fluid.
If they are acceptable for operation with a closed loop control system, three-phase motors with squirrel cage are commonly utilized as condensate pump drives. To adapt the pump to varying turbine loads and prevent the condensate pump from running dry, the following control options are offered.
Controlling a condensate pump may be accomplished in a variety of ways:
- Throttling with a control valve in the discharge line to adjust the system characteristic curve
- Returning surplus flow to the condenser (bypass adjustment) helps alter the system's characteristic curve (see Bypass)
- Pump speed may be changed to adjust the H/Q curve (speed control)
- By allowing the flow rate to adapt to the inlet head, the H/Q curve may be adjusted. Self-regulation is another term for this form of control based on impending cavitation
The self-regulation of condensate pumps takes use of a shift in the characteristic curve H(Q) when part of the condensate evaporates upstream of or in the first stage, lowering the head H(Q) of this stage by a specific amount depending on the degree of steam blockage (Hcav). The operating point is determined by the junction of the head breakdown curve (affected by the extent of cavitation Hcav(Q) and the system characteristic Hsys(Q) (OP).
Self-regulation of condensate pumps imposes difficult constraints, especially on the first stage of the pump, due to the large cavitation loads. As a result, on today's bigger pumps, this sort of control is no longer employed.