Open area ratio decides how often it blocks
A strainer is specified by two independent numbers and buyers usually give only one. The first is the aperture — the mesh count or perforation diameter that sets what gets caught. The second is the open area ratio: the total free area of the screen divided by the pipe bore. For a Y-strainer in continuous service you generally want a ratio around 3 to 4, and considerably more on start-up duty where the debris load is heavy. Halve the aperture without increasing the screen area and clean-condition pressure drop rises sharply and the element blinds off in a fraction of the time. If you only quote a mesh number, you have specified the filtration and left the service interval to chance.
Collapse differential is a separate rating from the body rating
The A105 body may be rated to Class 2500, but the screen inside it is a thin perforated cylinder, typically SS304 or SS316, and it has its own collapse pressure that is orders of magnitude lower. The dangerous case is not normal operation — it is a blinded element. As the screen loads with debris the differential across it climbs until the cylinder buckles, and once it collapses the entire debris load passes straight through to the equipment the strainer was protecting. Specify the collapse differential you need, and fit a differential pressure gauge or at least tapping points across the strainer on any duty where blinding is plausible.
Temporary start-up screens versus the permanent strainer
Most of the debris a system will ever see — weld slag, grinding dust, gasket offcuts, the occasional tool — arrives during commissioning, not during operation. Sizing the permanent Y-strainer for that one-off load means oversizing it for the following twenty years. Standard practice is a temporary conical or witch-hat screen dropped into a flanged joint for the flushing and start-up period, then removed once the system is clean, with the permanent strainer sized for the ongoing duty. Plan the blow-off connection at the same time: it wants to be large enough to actually clear the pocket under line pressure, and valved so it can be operated without shutting the line down.
What to include on the enquiry
- Aperture — mesh count or perforation diameter — and the required open area ratio
- Maximum allowable clean pressure drop at design flow
- Collapse differential for the screen, quoted separately from the body rating
- Screen material, normally SS304 or SS316 regardless of the carbon steel body
- Blow-off connection size, and whether it must be operable on line
- Whether a magnetic insert or a temporary start-up screen is needed
Material behaviour and standard references on this page are general engineering guidance for grade selection. Confirm the final specification against your line list and the applicable design code for the project.
