By Steve Gutai
Backwash valves are the unsung heroes of many a fine hydraulic system. Seldom considered other than when in use, these handy devices simplify filter maintenance, significantly extend filter cycles and even serve to stretch the service lives of a filter’s internal components. Ensuring that level of reliable performance, says hydraulics expert Steve Gutai, is a matter of understanding the role these valves play and selecting the right one for the given application.
Backwash valves are simple in concept: They reverse the flow of water through a filter and, in so doing, dislodge dirt and debris that has built up on the surface of and in the filter medium.
This procedure has a couple of key benefits in both sand and diatomaceous-earth filters: First, it improves filter performance by breaking up and flushing out the near-solid cakes of dirt and oil particles that build up in the media over time. Second, it prolongs filter cycles and extends the time between major (and messy) cleanings. Third, because they minimize those invasive cleanings, backwashing helps to extend the service lives of a filter’s internal components.
For all that, I keep seeing systems in the field in which backwash valves are not properly specified or no provisions have been made for routine backwashing – that is, systems in which there’s no backwash valve to divert the flow and initiate the cleaning process. And these devices aren’t particularly complex or expensive, which leads me to believe there’s a lack of information about the value of these valves, a gap we’ll begin to fill here.
REVERSING THE FLOW
Some watershape systems are set up with cartridge filters that don’t require backwashing, but a good portion of filtration systems – especially those for swimming pools, spas, and waterfeatures – use either sand or diatomaceous earth (D.E.) to rid the water of particulates.
Setting up these systems for proper backwashing isn’t the most complicated procedure or the sexiest topic going, but it really does cut to the heart of proper filtration and water quality. In fact, I’d say the presence of backwashing capability is crucial to the long-term enjoyment and sustainability of any watershape system.
As is the case with most simple-seeming topics, there is, of course, more to backwashing and backwash valves than meets the eye.
Consider what happens inside a filter when the system is in backwash mode. In a D.E. filter, the water flows backwards through the filter grids, thereby blowing the caked filter medium and all the debris imbedded in it from the polyester-fiber/nylon-mesh grids. The water containing all the dirt particles, oil, debris, and caked diatomaceous flows to waste via a P-trap, a drain system or a reclamation/backwash tank.
In a sand filter, the flow is similarly reversed: Instead of the water flowing downward through the sand bed, it comes up from the bottom of the filter and effectively churns the top few inches of the sand bed up into solution. Dirt is removed when the water flows up and out of the filter body through the diffuser at the top of the tank. The water is disposed of via the options described just above for a D.E filter.
Again, that’s elegant and simple, but there are a couple of key points that bear consideration in greater detail.
For starters, backwashing lengthens filter cycles, that is, the time between serious cleanings. As we all should know, emptying and recharging D.E. filters is a laborious and dirty job that involves opening the tank, removing and cleaning the grids – then reassembling and recharging the system with D.E. Removing sand from a sand filter is even worse, a nasty task that requires not only disassembling the filter but also scooping out the muck and sand by hand in most cases.
In addition, backwashing not only enables the media in a D.E. filter to last longer, it also lengthens the service life of the grids and internal components simply by reducing handling and allowing the filter to operate at lower pressures. For sand filters, backwashing also helps prevent the solidification of the surface of the sand bed and can thereby lengthen the span between major cleanings by several years.
The ultimate benefit in both cases is proper filtration and proper flow throughout the system, which can have a huge impact on a range of other issues such as the proper functioning of heaters and sanitization systems as well as the overall performance of spa jets, fountains, waterfalls or interactive waterfeatures.
There are several types of backwash valves on the market these days, and each has its own set of characteristics, advantages, disadvantages and proper applications. For all of their differences, however, all backwash valves have the same five-port setup: filter inlet and outlet; pump discharge to valve inlet; valve outlet port to return circulation; and port to waste.
As you’ll see in the summary of options below, the way those five connections are accommodated differs widely from valve to valve.
[ ] Slide or push-pull valves: These are the least expensive and most often used backwash valves on the market. Sometimes referred to as piston-style valves, they are essentially two-position devices that run in either filtration mode or backwash mode.
Slide valves feature a simple cylinder – a shaft with two plungers that move through the internal part of the valve body. The plumbing ports in and out of the valve are positioned so that sliding the shaft will redirect the flow of the water.
Slide backwash valves are typically made of PVC or ABS plastic material. Most are glued to a set of unions that are ultimately mounted onto the side of the filter tank, which makes them easy to service and replace as needed. They cannot be automated.
These valves are in common use in the Sunbelt (other than Florida, where cartridge filters dominate), but they are seldom used in regions where watershapes must be winterized. In these places, multi-port backwash valves are generally used because of the additional benefits they can provide.
[ ] Multi-port valves: These valves offer a range of functions and configurations that go well beyond backwashing, offering a versatility that is highly desirable for many applications. In addition, multi-port valves are self-draining, which makes them suited for use in cold climates.
Typically made of ABS plastic and fitted with threaded socket connections that make installation, removal, repair and replacement convenient, multi-port valves are also commonly referred to as dial valves because they look like sundials on the inside, with compartments divided into six or seven wedge-shaped slices. Water is sent in various, distinct directions through these compartments by a rotating diverter assembly that moves into different positions over a spider gasket.
Multi-port valves also have a re-circulation mode, which is basically a bypass that allows for water circulation without any flow through the filter. In addition, some have a winterization mode that allows the valve to drain itself.
These valves come in two basic types: The first is a top-mounted style most often seen on smaller sand filters (30 inches or less). The second is a side-mounted style used on larger systems (30 inches and up). The key difference between the two is the way the filter’s internal plumbing interfaces with the backwash valve connections.
With a top-mounted valve the filter diffuser is part of the valve and the water enters the valve and is distributed from the valve itself. By contrast, with a side-mounted multi-port valve, the valve sends water to a diffuser that is integrated into the sand filter’s tank. Despite these physical differences, water flows to the sand bed in exactly the same way with both valve types.
For the most part, side-mounted multi-port valves provide a second option to slide valves because the former are more versatile and have the benefit of being side mounted. Moreover, multi-port valves – especially of the side-mounted variety – are easily automated using specialized valve-actuator mechanisms.
[ ] Full-flow valves: These valves have completely different design than multi-port and slide valves. They have two diverter gates that are attached to one common shaft. When the shaft is rotated, both diverter gates are rotated. They direct water through the same sets of ports as do other backwash valves, but with much less pressure drop as the water flows through the valve.
When water flows through a slide or multi-port valve, it must make at least two 90-degree turns or travel through different chambers, resulting in additional friction loss within the system. By contrast, full-flow valves divert the water flow while adding virtually no extra resistance – something they accomplish because the water flows straight through the valve and there are no bends or turns to contend with in the filtration mode of operation.
In addition to backwash valves, there are a couple of other components that can be crucial to the backwashing process.
The first is a simple sight glass – nothing more than a small window that allows the operator to look at the water flowing within the filter tank to see when it’s clear and backwashing can be stopped. These are found on many multi-port and tandem filter piping kits.
Reclamation or separation tanks offer another clear-water assist. They look like small filters, but they have a polyester-cloth sack on the inside that captures debris from the backwash effluent and sends the water back into the circulation system rather than to waste. These systems are used most often in conjunction with D.E. filters in drought-prone areas where water conservation is a key issue.
[ ] Rotary valves: Valves in this fourth category are used only with D.E. filters and, unlike the valves discussed above, are mounted on the bottoms of the tanks and can be used for the purpose of backwashing.
These two-position valves are integrated into the body of the filter tank (usually in older stainless steel models) and are operated by a handle that moves an internal rotor assembly. They cannot be automated.
These valves have been in reliable service for many, many years and are still in use in some areas, particularly southern California. Their biggest drawback is the fact that you have to take apart the entire filter to access the valve for service and repairs. With a big tank, that means crawling into the filter itself to reach the valve.
[ ] Tandem-filter piping kits: These are basically series of pipes and butterfly valves that form manifold structures used to connect large sand filters in series. These arrays are typically found on large commercial projects that require special backwash valve systems designed for the specific types and numbers of filters.
These systems enable operators to take individual filters off line for backwashing or service while the others are still running, thus preventing facility down time. There are a variety of permutations and possibilities with these complex manifold systems that are specific to each project. Many of these systems are automated.
DOWN TO CHOICES
In evaluating what sort of backwash valve system should be used for a specific application, a watershaper needs to weigh all sorts of details, including filter type, pressure drop, whether the system will be automated or not, physical access to the valve and whether one or more filters will be used.
From a systems standpoint, however, the largest design consideration has to do with pressure drop. You need to know what degree of pressure drop you will experience with each possible backwash valve and come up with an overall system that works with the greatest efficiency and optimal performance.
There are a few helpful rules of thumb. You know, for example, that most multi-port backwash valves experience the highest level of pressure drop, so you won’t spend much time considering them for a pool with an in-floor cleaning system. In that sort of application, slide valves or full-flow valves are the best choices.
The other big question has to do with which operational modes are needed for the given project. In the northeast, where pools are winterized and all sorts of leaves and debris can accumulate in the water, valves that have bypass positions and enable water to be directed to waste without flowing through the filter are a big plus. In warm weather areas, you generally won’t need winterizing or bypass modes.
Cost can be a factor as well. Simple slide valves are far less expensive than multi-port or full-flow valves – perfect for straightforward backyard pools or spas, but generally less advisable for complicated projects where multi-port or full-flow valves may be the answer.
The final consideration is serviceability. If you look at all of these valves, the slide valve is by far the easiest to service, with removal of just one nut allowing removal of the plunger and most repairs involving little more than replacement of a few O-rings. Multi-port valves are equally accessible, but they are much harder to work on because there are usually eight to ten bolts to remove on the top plate of the valve, and the spider gaskets can be tricky to reinstall because of their complex web shapes. Full-flow valves are easy to access and repair, so much so that some come with lifetime warrantees, while rotary valves, as mentioned above, can be a service challenge.
In the short run, the type of backwash valve you choose may not be the most critical or thought-provoking of all the decisions you’ll make about a hydraulic system. But if you make the effort, do some homework and settle on just the right system, it’ll be one decision that won’t require any second-guessing down the line.