Pumps & Systems, February 2009
In the power industry, clean water is crucial for a variety of tasks, including extending the service life of turbine seals and protecting spray nozzles and heat transfer equipment. Raw water drawn from lakes, rivers and reservoirs must first be strained to create acceptable clean water for use. The power industry and other industries must continuously strain tens of thousands of gallons of water per minute to remove dirt and debris that can wreak havoc on critical process systems and equipment.
The raw water strainers used for straining are the first lines of defense for the entire plant's system. An inadequate strainer can lead to high maintenance and operating costs, periods of insufficient water supply, damage to process equipment and expensive downtime. Worse, the straining media of an overwhelmed water strainer can rupture or collapse, permitting debris to compromise critical plant operating components.
Unfortunately, such failures are not unusual, particularly when the strainer design does not allow for sufficient straining surface area. Single basket strainers sometimes become overwhelmed and clogged during periods of high volumes of debris in river water. Clogged strainers force excessive cleaning cycles (backwashing) and reduce water for process requirements.
"You never really know what you're going to experience with river water," says Sang Partington, a senior engineer with PPL Electric Utilities' Generation Technical Group. "It changes from season to season. During autumn and high water flow in the river, you may have a lot of debris such as tree branches, leaves and other solids in the water. Therefore, your water strainer has to be able to handle the solids and still maintain a continuous volume of water flow."
Optimizing Water Flow at PPL
About five years ago, Partington noticed that the old, basket-type water strainers at his Brunner Island plant required high maintenance and continuously shifted to backwash mode. "The old system was constantly backwashing," says Partington.
At PPL's electric power generation utilities, the priority is maintaining sufficient volume and pressure, although there is certainly concern about the debris and other solids in the rivers that feed water to the coal-fired plants.
PPL began to upgrade the raw water strainers at its 1,500-MW Brunner Island and Montour plants, which both feed off the Susquehanna River in central Pennsylvania. It is critical to ensure sufficient clean water to keep the plants on-line continuously.
According to Partington, the outflow of clean, filtered water through the strainers was also at lower-than-optimal volume when backwashing was taking place, so he began to look for a more advanced and efficient strainer technology. After reviewing several more advanced designs, Partington selected a multi-element strainer.
Multi-element, automatic self-cleaning strainers were developed as an alternative to basket strainers in the 1960s. By replacing the basket with multiple tubular elements, the design provides three to four times the straining surface area of a typical basket strainer. As a result, debris and solids-including heavy seasonal debris-are efficiently removed without downtime. The increased surface area of the multi-element design allows for fewer backwashes, equating to lower operational costs, less maintenance and greater overall efficiency.
Although initially designed for raw water applications, the multi-element strainer that PPL chose can actually remove solids as small as 25 microns. The strainer can serve as the first line of defense in water filtration, or can be installed at a point of use for critical plant operations requiring fine levels of separation.
Another significant feature of the multi-element design is in the engineering of the backwash mechanism. With basket strainers, the backwash mechanism directly contacts the straining media. This can be problematic, as large, suspended solids in raw water can become lodged between the straining media and the backwash arm. The result is straining media damage and/or rupture that can compromise plant operations.
The multi-element design uses a tube sheet to separate the straining media from the backwash mechanism. Without contact between the backwash mechanism and the media, large solids will not lodge between the media and backwash arm and damage the elements of the strainer.
Furthermore, the multi-element design provides three to four times the surface area of a basket strainer, which means less frequent backwashing, less water wasted, less power consumed and less maintenance required. These strainers could meet the necessary plant water requirements even while in backwash mode.
"The new units will not backwash unless the differential pressure of the strainer is high enough to activate backwashing automatically or by the timer, thereby saving us money on the power consumption," explains Partington. "We should also save significant money on maintenance too, but we don't know how much yet because the units are so new."
Partington also appreciated the fact that the strainer manufacturer was willing to customize the strainers to meet PPL's design specifications, and offer an exchange program whereby he could replace the strainer elements to a different micron size if he needed to do so.