The quality of the cooling water intake and the amount of debris in that water affects the operation and performance of the condenser and therefore the thermal performance of the typical steam plant.
Material such as slime, calcium carbonate, calcium sulfate, magnesium dioxide, silt, petroleum products, corrosion products, and the like adhere to the inside of the heat exchanger and condenser tubes. The accumulation and growth of such deposits reduce the overall heat transfer coefficient and will have an adverse effect on the operation of process equipment, plant availability, production, and maintenance cost.
Traditionally, plant maintenance teams isolate and open each heat exchanger or condenser unit periodically in order to clean the tubes manually using high-pressure water jets or mechanical scrapers. This process is labor-intensive, costly, and may require process shutdown. Furthermore, when the equipment returns back to operation, the fouling process begins anew.
There is an alternative. Two excellent online cleaning systems can keep your condensers clean and your unit heat rate down: a debris filter and an automatic tube-cleaning system (Figure 4). Both automate the process of continuously keeping heat exchange surfaces clean.
|4. Keep it clean. Shown in blue is a self-flushing debris filter on the inlet of a typical steam plant condenser. The debris is discharged in a bypass pipe connected to the condenser discharge. Shown in red is a typical online automatic tube cleaning system that is an effective method for preventing the formation of, and for removing, any soft or hard scale inside the condenser tubes while the heat exchanger remains online and at full load. Courtesy: Ovivo USA, LLC|
Self-Flushing Debris Filter
The purpose of the self-flushing debris filter is to eliminate clogging in condenser tubes, thus improving unit heat rate.
Intake screens are often ineffective in protecting heat exchanger tubes from debris. Many plants designed with through-flow traveling screens located upstream of the condenser still experience heavy debris carryover or are victims of macro fouling via mollusk or crustacean growth within the intake tunnel, which causes many clogged condenser tubes. The normal practice is to clean the tubes manually, but this requires an outage or a period at reduced load, depending on the condenser design.
A debris filter or automatic pipe strainers provide the best filtration, ranging from 50 microns to 10 mm. These strainers can be sized to remove the carryover and fine debris that pass through intake screens to prevent buildup inside the condenser or exchanger tubes.
Figure 5 illustrates a typical installation for such in-line debris filters prior to a heat exchanger or condenser. The debris filter replaces a section of the cooling water piping. Water and debris enter into the filter housing, and filtered water exits the screen mesh. Debris larger than the mesh size is trapped over the screen area. Based on the desired setpoint for the increased pressure drop above the clean screen, or on a preset time interval, the debris-cleaning rotor travels over the screen mesh to lift and remove debris from the screen surface. The debris-cleaning cycle lasts only a few minutes. The debris is then discharged into the sewer or into the cooling water discharge from the plant using a small quantity of water.
|5. Stop the big pieces. Typical debris filters are installed prior to a heat exchanger or condenser. Courtesy: Ovivo USA, LLC|
Basket or Balls?
Automatic tube cleaning systems (ATCS) are a mechanical method of cleaning heat exchanger tubes while the equipment remains in operation and in full production. There are two types of ATCS: the brush and basket type and the recirculating ball type.
Brush and Basket. The brush and basket system consists of a set of baskets (cages) that are permanently installed at the two ends of each condenser (Figure 6). A single brush is inserted inside each basket. For large condensers, the existing flow reversal/backwash system can be used to periodically and automatically reverse the flow through the heat exchanger and shuttle the brushes through the tubes. The flow returns to its normal direction within a few minutes. In the case of the power plant condenser application, the flow reversal is performed at night and only through one-half of the condenser. The plant must reduce load for this type of condenser cleaning. For small heat exchangers such as auxiliary coolers, a single in-line flow diverter valve is used to accomplish the periodic backwashing without requiring any reduction in plant load.
|6. Brush and basket. Cleaning of the condenser occurs when the flow is reversed by a series of butterfly valves installed in the crossover piping. At this plant, baskets (with purple brushes inside) are being added to each condenser tube. The force of the water pushes the brushes through the tubes to keep the inside wall of the tubes clean. Courtesy: Ovivo USA, LLC|
Balls. The other, more common, system used to remove micro fouling and scaling from the heat exchanger and condenser tubes employs recirculating balls (Figure 7). This system consists of uniquely designed, slightly oversized elastomer balls that are periodically or continuously injected upstream into the heat exchanger or condenser cooling water inlet.
|7. Recirculating balls. Elastomer balls injected upstream of the condenser will remove micro fouling from condenser tubes. The external ball collector is used to replace the balls once a month as they wear down. Courtesy: Ovivo USA, LLC|
The balls are passed through the tubes by the water flow. A strainer/ball collector is installed at the water piping exiting the condenser. A nonclogging pump removes the balls from the strainer and reinjects them back into the water supply line after they have passed through external collectors.
—Contributed by Kaveh Someah ([email protected]), Ovivo USA, LLC, formerly Eimco Water Technologies