Multi-Element Strainers Handle Raw Water Debris
April 24, 2009
Bulk raw-water users such as PPL Electric Utilities in Allentown, Pa., protect process and downstream equipment by selecting multi-element water strainer technology.
Whether used for cooling or the process itself, the raw water drawn from lakes, rivers and reservoirs first must be strained. In many industries, this means continuously straining tens of thousands of gallons of water per minute to remove dirt and debris that can wreak havoc on critical process systems and equipment.
In essence, the raw-water strainers that accomplish this task are the first lines of defense for a plant's entire system. Choosing an inadequate strainer can lead to high maintenance and operating costs, periods of insufficient water supply, damage to process equipment and downtime. Worse, an overwhelmed water strainer can rupture or collapse, permitting debris to compromise critical plant operating components. In the power industry, for example, clean water is crucial for many tasks such as extending the service life of turbine seals and protecting spray nozzles and heat transfer equipment.
Unfortunately, failures are not unusual, particularly when the strainer design does not allow for sufficient straining surface area. In applications using raw water from rivers, for instance, single-basket strainers sometimes become overwhelmed and clogged. This situation usually occurs during periods when there are high volumes of water debris due to seasonal conditions such as a large amount rainfall, which carries dirt, leaves and other loose particles into the water supply.
"You never really know what you're going to experience with river water," says Sang Partington, a senior engineer with PPL's 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."
Different TechnologiesWater strainers for mass raw-water filtration have been around for decades, and today manufacturers offer many designs, including those that operate automatically. One of the more significant advances in strainer design occurred in the 1960s when the first multi-element, automatic self-cleaning strainer design was developed.
The design was particularly significant because it provided a durable and reliable alternative to the classic basket-type strainer. Although it sometimes carries a lower price tag, the basket-type design also is limited by its strainer surface area, which can quickly become clogged and force excessive cleaning cycles (backwashing) and reduced water for process requirements.
On alternative to the basket-type strainer is a design with multiple tubular elements. This design provides three to four times the straining surface area of a typical basket strainer, according to the manufacturer. As a result, debris and solids, including those from seasonal peaks, are removed without downtime. The increased surface area of the multi-element design allows for fewer backwashes, equating to lower operating costs, less maintenance and greater overall efficiency.
Case in Point: PPLAbout five years ago, PPL's basket-type water strainers at the Brunner Island plant required high maintenance and continuously shifted to backwash mode. According to Partington, the outflow of clean, filtered water through the strainers was also at lower-than-optimal volume when backwashing took place, so he began to look for another strainer technology.
At PPL, the priority is maintaining sufficient volume and pressure, although there also is concern about the debris and other solids that can be in the rivers that feed water to the coal-fired plants. To satisfy this priority, PPL decided to upgrade the raw water strainers at its Brunner Island and Montour plants, both feeding off the Susquehanna River in central Pennsylvania. The large generating facilities each have approximately 1,500 MW capacity, and both require sufficient amounts of clean water to keep the plants on line continuously. After reviewing designs, Partington selected a multi-element strainer from R.P. Adams, Tonawanda, N.Y.
Although designed for raw-water applications, the R.P. Adams multi-element strainer can remove solids as small as 25 microns, so it can be used as the first line of defense in water filtration. It also can be installed at a point of use for critical plant operations requiring fine levels of separation.
The multi-element design uses a tubesheet to separate the straining media from the backwash mechanism, which prevents element damage from large solids becoming lodged between the media and the backwash arm. In addition, the multi-element design provides three to four times the surface area of a basket strainer, which results in less frequent backwashing. The strainers also satisfy the 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," Partington says. "We should also save significant money on maintenance too, but we don't know how much yet because the units are so new."
R.P Adams set up an exchange program through which Partington can replace the strainer elements with a different micron size if necessary.
"The element exchange program allowed us to go for greater straining efficiency, which helped us optimize the raw water system," Partington says. "We elected to exchange the original elements for a smaller micron size. It has worked very well, so we're going to stay with that size for that particular installation. But as we continue to upgrade our water strainers at various locations, we can do the same thing – in effect, fine-tune the solid removal and water flow as the situation warrants."
To date, PPL has upgraded to eight R. P. Adams strainers at the Brunner Island plant and has installed the first unit at its Montour site.