A basket strainer catches debris on a perforated screen. As the mesh fills, it takes more pressure to push fluid through, so the difference between the two gauges climbs until it tells you to clean it.
It is a coarse filter that protects pumps and valves from debris. A perforated basket sits inside a housing; fluid must pass through the holes, which catch anything large enough to damage equipment downstream.
The whole story is the relationship between collected debris and the pressure difference. A cutaway lets you watch the screen fill while the two gauges diverge, the very signal an operator uses to know when to clean.
With a clean screen, fluid passes through the holes with little resistance. Both gauges read nearly the same pressure, so the difference is small.
Particles too big to pass the holes lodge on the upstream face of the basket. Slowly the open area begins to shrink.
As the mesh fills, the fluid must squeeze through fewer holes. To keep the same flow moving, the upstream pressure has to rise, a sign the strainer is loading up.
With higher pressure before the screen and lower pressure after it, the differential gauge climbs. A rising difference is the standard warning that the strainer is clogging.
When the difference reaches a set limit, the strainer is taken out and cleaned, or the drain valve at the bottom is opened to flush the debris out. Then the screen is reset and the difference drops back.
By the pressure difference across it. As the screen fills with debris, the pressure upstream rises and downstream falls, so the difference between the two gauges grows, and that difference is the clogging signal.
A strainer is a coarse device meant to catch large debris that would damage pumps and valves, not fine particles. A finer mesh would clog far faster and add more resistance.
It lets you flush the collected debris out of the housing, either for cleaning or for a blow-down while the strainer is isolated.
Because the clogged screen restricts the flow. Less fluid passes per second, so less pressure is left downstream, and the downstream gauge drops.
When the differential pressure reaches a set limit chosen by the operator. Cleaning before that wastes effort; leaving it too long starves downstream equipment of flow.