Steam, Air-Cooled Condensers Play Critical Role in Combined-Cycle, CHP Power Plants

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Researched by Industrial Info Resources (Sugar Land, Texas)--Surface condensers, which are water-cooled shell and tube heat exchangers, are typically installed on the exhaust steam outlet of a power plant's steam turbine, then used to convert steam into liquid at atmospheric pressures or below. Air-cooled condensers are a similar technology, typically used in areas where water is in short supply. But air-cooled condensers are significantly more expensive and cannot reach lower exhaust pressures than water-cooled condensers.

Condenser maintenance, a critical aspect of power plant maintenance outages, keeps the generating units operating at peak performance and efficiency.

Most thermal power plants have a condenser if steam is used in a power plant Rankine cycle. They are common in combined-cycle power plants, as well as combined heat and power plants (CHP), because they can improve efficiencies and convert steam to a liquid form.

The condenser consists of a shell, or outermost body, and is fabricated from steel plates and manufactured to induce the desired flow path of the condensate steam as it flows through the condenser and arrives in the bottom area, or "hotwell." The hotwell is a holding area for the condensate liquid before it is pumped back to the deaerator and the boiler feedwater pump, starting the cycle over again.

A vacuum system removes any non-condensible gases in the condenser. Along with the condensing of steam back to condensate, this helps to maintain the condenser below atmospheric pressure, which in turn keeps the steam flowing through the steam turbine and through the condenser. Typical condensing pressures are 0.75 inches to about 10 inches of mercury absolute (HgA) in an air-cooled condenser.

The tubes in a condenser are made of stainless steel--or copper alloys in older power plants--and attached to the cooling water boxes. However, they are separated to avoid contamination of the steam and condensate and cooling water systems.

Problems that occur in condensers that can be addressed during maintenance outages include air leakage problems, which can be found in turbine gland seals, bonnet flanges, shaft seals, valve packing, and vacuum pump seals. These problems become evident when the condenser's performance becomes an issue.

Other problems include the tube side fouling from silt, sediment, biofilm, scaling or invasive species. These species include zebra mussels, which can be introduced from the millions of gallons of water circulating through the condenser when it is not monitored or properly addressed during operations and maintenance activities.

To maintain the condenser for optimum performance, chemical or mechanical cleaning is performed during outages or as a measure to prevent condenser tube failures. This is vital for plant reliability, as even a single condenser tube leak can take a unit off line and can easily exceed several million dollars in repairs.

Another option for a plant owner is a complete condenser retubing, which can extend a unit's lifespan and is usually part of a major overhaul and capital project. This project is very labor-intensive and requires cutting out the old condenser tubes and installing the new tubing into the tubesheet during a set timeframe; it is always a priority in the overhaul schedule.

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