Case Study: Water Straining
Bulk raw water users, such as PPL Electric Utilities, protect process and downstream equipment by selecting multi-element water strainer technology
Whether used for cooling or processing, raw water drawn from lakes, rivers, and reservoirs must first be strained to remove dirt and debris that can harm critical process systems and equipment.
Raw water strainers that accomplish this task are the first line of defense for an entire plant's system. An inadequate or overwhelmed strainer can lead to high maintenance and operating costs, periods of insufficient water supply, damage to process equipment, and expensive downtime. In the power industry, for example, clean water is crucial for various tasks, including extending the service life of turbine seals and protecting spray nozzles and heat transfer equipment.
"You never really know what you're going to experience with river water," said 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."
Water strainers for mass raw water filtration have been around for decades. One of the more significant advances occurred in the 1960s when the first multi-element, automatic self-cleaning strainer design was developed. This design provides a durable and reliable alternative to the classic basket-type strainer that is limited by its surface area, which can quickly become clogged and force excessive cleaning cycles (backwashing) and reduce water for process requirements.
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 from seasonal peaks, 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.
About five years ago, Partington noticed that the old, basket-type water strainers at its Brunner Island plant required high maintenance. "The old system was constantly backwashing," he said.
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 that can be in the rivers that feed water to the coal-fired plants.
PPL began to upgrade the raw water strainers at its Brunner Island and Montour plants, both feeding off the Susquehanna River in central Pennsylvania. Both are large generating facilities with approximately 1,500-megawatt capacity, and it is critical to ensure sufficient clean water to keep the plants online continuously.
According to Partington, the outflow of clean, filtered water through the strainers also was 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 the multi-element strainer from R.P. Adams.
Although initially designed for raw water applications, the R. P. Adams multi-element strainer can remove solids as small as 25 microns. Another advantage is the device's multi-element design, which uses a tube sheet to separate the straining media from the backwash mechanism. This prevents the backwash mechanism from coming into contact with the media and damaging the elements caused by large solids becoming lodged between the media and the backwash arm.
Partington decided on this strainer because it could provide the necessary plant water requirements even 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," explained 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 company customized the strainers and offered an exchange program to replace elements to a different micron size if needed.
"The element exchange program allowed us to go for greater straining efficiency, which helped us optimize the raw water system," Partington said, adding, "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.