Condensate Pumps - What Do They Do?

Feb 7

The condensate it handles gives this kind of centrifugal pump its name. Under a technical vacuum, it is employed in condensers to remove water from condensed steam (near vapor pressure). In an open circuit, the condensate is pumped into a storage tank (like a feed water tank), whereas in a closed circuit, the condensate is pumped into the boiler feed pump through a low-pressure feed heater.

The maximum mass steam flow rate of the steam turbine is what establishes the condensate pump's capacity.

Structure of the human head:

Difference between feed water tank and condenser water levels is expressed as a geodetic head.
Static pressure heads vary between the feed water tank and the condenser.
Pipeline head losses due to components like installed valves (gate valves, swing check valves, etc.) and the pipeline itself (e. g. suction strainer, condensate preheater).

The condensate pump's design is determined by 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 induced by the condenser's position inside the building. If we take the geodetic head between the condenser's standard water level and the first stage's impeller level and deduct the flow losses in the intake line, we get the input head.

For best operation and to prevent cavitation damage, the system's available NPSH must be more than or equal to the NPSH required at the impeller of the first stage. Nothing but the truth.

The intake head on the system side may be raised by doing the following:

Using larger nominal pipe sizes to reduce flow losses in the intake line is one example.
A dry installation, which is arranged vertically, reduces the height of the first-stage impeller above the installation floor, hence increasing the geodetic head difference.
To increase the geodetic head difference, the vertical "can-type pump" has its suction stage positioned below the floor of the installation.
Input and outlet lines are situated above the installation floor.

Multiple strategies exist for enhancing the suction capabilities of a condensate pump:

Attachment of a suction impeller.
Putting in an Inductor.
They are putting in an impeller with two entrances.
Decreased rotational velocity.
Modifications made to the pump's piping.

Design variants include a condensate pump with intermediate extraction (re-entry). After the first or second pump stage, the whole flow is sent to the condensate cleaning system (condensate booster pump with single or dual entry suction stage).

A device consisting of a booster pump and a condensate main pump, the latter of which is situated above the floor, further boosts the pressure.

When flows exceed 150 l/s (540 m3/h), the presence of high cavitation loads necessitates careful consideration of cavitation intensity, velocity conditions, and the lengths of the cavitation bubbles' trail. Cavitation severity is measured in terms of the material loss rate LM.

When working with a constant flow rate, it is impossible to drastically alter the flow velocity at the leading edge of the impeller vane, thus the bubble trails must be as short as possible.

The shaft seal of a condensate pump has to be able to provide vacuum tightness even at low technical vacuum levels. The sealing element requires barrier fluid from the system-side barrier system to prevent air intrusion. In the case of gland packings, this is accomplished by inserting a lantern ring between the packing rings. Twin mechanical seals may be installed either inside or outside of a vessel. The barrier fluid is sent to the lantern ring, or the chamber between the inboard and outboard mechanical seals.

Squirrel cage three-phase motors are often used as condensate pump drives if they are compatible with a closed loop control system. The following set of controls is provided to allow the pump to adjust to different turbine loads and avoid dry running of the condensate pump.

There are many options for managing a condensate pump's operation:

Modifying the system's characteristic curve by closing a valve in the outlet line.
The characteristic curve of the system may be modified by redirecting flow excess to the condenser (bypass modification) (see Bypass).
Varying pump speed modifies the H/Q relationship (speed control).
The H/Q curve may be modified by allowing the flow rate to vary in response to the intake head. This kind of management is also known as self-regulation.

When some of the condensate evaporates before or during the first stage, the characteristic curve H(Q) shifts, causing the head H(Q) of this stage to drop by an amount proportional to the amount of steam blockage (Hcav). When the head breakdown curve (which is impacted by the degree of cavitation, Hcav(Q)) meets the system characteristic, Hsys(Q), that's the operational point (OP).

Due to the high cavitation loads, the initial stage of a condensate pump is particularly constrained by the need for self-regulation. This kind of regulation is hence not used on modern, larger pumps.