Strainer or No Strainer?
For years, members of the backflow prevention industry have debated the value of strainers at the inlet of an assembly’s installation. Arguments on both sides of the issue have merit and should be carefully considered before a strainer is installed.
A strainer is essentially a screen placed in the flow path. It is intended to catch sand and debris that may foul the integrity of a check valve. A fouled inlet check valve in a reduced pressure principle assembly, for example, will cause the relief valve to discharge intermittently with small sand and potentially several gallons per minute with lunch buckets and two-by-four sized debris.
If a public distribution system has a reputation for water main breaks or silty sand from shallow wells, a strainer will keep the backflow preventer operational. Some backflow preventer assembly manufacturers may encourage including a strainer in the contractor’s installation bid price, as the strainer’s cost is far less than a return service call to clean a valve’s seat after installation.
The key concern rarely discussed, however, is the maintenance of the strainer. Strainers will have ports to open and flush the debris free of the screen. This must be performed periodically. If this is not done, the flow path will eventually become plugged resulting in a pressure loss to the downstream use-or worse.
Many years ago, the director of a prominent evaluation laboratory expressed serious concern over the use of strainers with backflow preventers. The basis of his argument was the strainer’s lack of long-term maintenance.
As the strainer catches debris in the piping system, the cross-sectional flow path within the pipe is reduced. Because water is not a compressible fluid, a Venturi is established, causing higher velocity flow through the restriction. Keep in mind that a stainless steel strainer screen by its nature already creates a higher velocity flow through the mesh openings, debris blocking the screen openings compounds the effect and a still higher velocity is established.
We know that when sizing water supply piping, an important consideration is to avoid high velocity flows. High velocity water causes erosion corrosion. The upper velocity limit in a domestic system should not exceed 7.5 to 8 feet per second.
Flow velocities through a partially plugged strainer easily exceed that velocity. With time, even a stainless steel screen will disintegrate. Because the strainer has been retaining a mass of debris, the sudden release of that material when the strainer’s integrity is lost will likely cause an immediate failure of the closest check valve.
Perhaps this argument from the evaluation laboratory had an impact on backflow preventer designs for the last thirty years. One manufacturer of 1970s-era backflow preventers included a wye strainer as part of the assembly. In fact, the assembly’s number one test cock was located on the strainer. Conceivably, part of this design was to encourage periodic flushing when the assembly was field-tested.
It should also be considered that the manufacturer was aware that the check valve’s perpendicular orientation to the direction of flow might not allow adequate flushing of debris from the check valve seats, thus resulting in increased failures.
Backflow preventers of the ’80s and early ’90s has a distinctive design change: check valve components were designed at an angle. A few incorporated a characteristic Y-pattern, with one check 45 degrees off the axis and the second check 90 degrees from the inlet check. The justification at the time was improved flushing characteristics for the check valves. Water flow would lift the check off the seats, and the flow’s direction would in turn sweep any debris through the checks reducing the failure risk. Typically, strainers were not provided for these assemblies.
The industry continued to make demands for backflow preventer improvements. Pressure or head loss has always been a concern, mainly for the fire-sprinkler industry. Material costs have risen. Lay length is critical. In order to accommodate these requested improvements, check valve orientations have generally returned to being perpendicular to the direction of flow. However, the newer designs have improved check valve flushing characteristics resisting fouling from piping system debris.
Can the strainer be installed between the backflow preventer’s inlet shut-off valve and the inlet check valve? No, absolutely not. Neither the Foundation for Cross-Connection Control and Hydraulic Research nor the American Society of Sanitary Engineering will consider the assembly “approved.” In order to provide for the strainer’s servicing, an additional shut-off valve must be installed upstream of the device. One consideration should be a bypass with a control valve around the strainer, so it can be maintained without service interruption.
Please remember this: An essential task prior to the installation of any backflow preventer, especially on a service line, is to flush the pipeline with a high-velocity flow. Most backflow preventer testers will report that a high percentage of assembly failures occur with new installations. The reason for the failures: construction debris in the check seats.
Please consider all aspects of using a strainer. Hopefully these simple questions will help lead to thoughtful decisions. If you have any questions regarding this subject or any issue relating to backflow prevention, please feel free to contact me.
"This article was originally posted on ww.reevesjournal.com."