Article written by FCX Performance, Inc
What is open area ratio (OAR)?
An open area ratio is the comparison of the “free area” in a strainer to the cross-sectional area of the pipeline it is installed in. Free area is the surface area of the strainer basket multiplied by the percent open area of the perforated plate or mesh the strainer basket is constructed of.
A perforated plate’s open area percentage varies depending on diameter of the perforations as well as their spacing. On the other hand, a mesh liner’s open area percentage varies depending on the size wire used, the weave of the wire and the resultant opening size.
How is an OAR calculated and called out?
Open area ratios are calculated by dividing the free area of the strainer basket to the cross-sectional area of the pipeline that it is installed in and are typically called out in ratio format, such as 10:1, 4:1, 1:1, etc. Additionally, for temporary cones and baskets (also known as Witch’s Hats and Pilgrim’s Hats) ratios are typically called out in a percentage format, such as 200% (2:1), 150% (1.5:1) and 100% (1:1).
What impact do OARs have on my strainer’s performance?
OARs have a direct relationship to the amount of clean pressure drop caused by the strainer as the process liquid or gas passes through it, otherwise known as differential pressure. As the strainer becomes clogged, the pressure drop increases and eventually blocks the flow. As long as the OAR is 1:1 or greater, the pressure drop can be minimized as much as possible. Increasing your OAR (beyond 1:1) can allow the strainer to maintain little-to-no pressure drop for an increased duration of time. For instance, a 4:1 OAR strainer can be 50% clogged but still have 2X more free area than the pipeline it is installed in. Overall, the higher the OAR, the longer you can run your strainer and the less time you spend cleaning it out.
How can I increase my OAR?
You can increase your OAR by upgrading your strainer as the options available for your desired OAR are dependent on the strainer selected. For example, if a Y-Strainer does not provide your desired OAR, you could consider using a basket strainer instead.
You can also increase the OAR of your cast strainer by installing a larger strainer than the pipeline size (Ex. A 4” size strainer can be installed in a 2” pipeline). However, standard design modifications are not available for cast strainers due to their standard and off-the-shelf nature.
If your desired OAR is not attainable in a cast product, you should consider using a fabricated strainer. Although manufacturers have standard designs for given OAR’s, fabricated strainers can be customized to increase the OAR due to their made-to-order nature.
In some cases, the most optimal solution is to use the internal support basket of a bag filter vessel as a strainer basket and customize it for the required perf or mesh size. This typically results in a substantial increase in OAR (10X or more) when compared to a standard basket strainer.
