A reversible-flow filter can use four check valves as a routing bridge. For either external flow direction, one inlet check and the diagonally paired outlet check open so fluid crosses the same filter element from its dirty side to its clean side.
Reversible Filter Interactive Calculator
Follow the check-valve routes as external flow reverses. The calculator below explores an illustrative pressure-drop equation; its defaults are examples, not filter specifications.
Equation Used
- K is element pressure drop divided by flow squared at the 32 cSt reference, if that relationship fits the filter data.
- Viscosity is scaled linearly from 32 cSt for this example; actual filter corrections may differ.
- Entered check-valve ΔP is an aggregate value, not calculated from valve geometry.
- Valve dynamics, bypass flow, filtration efficiency, contamination loading and strength are excluded.
The companion diagram shows the same two routing states. Green circles identify the open checks; the blue line marks the active flow path.
How the check-valve bridge works
With external flow from port A to port B, checks A and B open while checks C and D close. When external flow reverses, checks C and D open while A and B close. Both paths take fluid into the dirty side of the filter element and out through its clean side.
The four checks act as one-way gates. The animation alternates between these two ideal states; it does not simulate how pressure moves the valve parts. Green circles mark open checks, and the blue line traces the active route.
Why keep element flow in one direction?
A hydraulic line can carry flow in either direction, while its filter element is intended to work in one direction. A routing bridge lets either external port supply the dirty side without reversing flow through the element. The patent references show examples of this arrangement and an alternative that uses a sliding valve.
This function differs from backwashing: a backwash deliberately reverses flow through the filtering material. The bridge illustrated here keeps dirty-to-clean flow through the material in both external directions.
Illustrative pressure-drop equation
The calculator uses ΔPelement = (μ/32 cSt)KQ² and ΔPtotal = ΔPchecks + ΔPelement. Q is flow in gpm, μ is kinematic viscosity in cSt, and K is entered in psi/gpm².
At 32 cSt, K is element pressure drop divided by Q². The example assumes pressure drop varies with flow squared and scales linearly with viscosity. These assumptions must be checked against the actual filter curves; they are not general laws for all filter elements.
Margin = max(0, setting − total). Setting ratio = 100 × total/setting. These compare two numbers; they do not calculate whether a bypass valve opens. The pressure difference across a real bypass may span only the element rather than the complete filter assembly.
Worked example
With 10 gpm, 32 cSt and K = 0.06 psi/gpm², the equation gives 6 psi element drop. Adding a combined check-valve loss of 6 psi gives 12 psi total. Compared with an entered 75 psi setting, the margin is 63 psi and the setting ratio is 16%.
These are example inputs, not a specification for a particular filter. Within this equation, doubling flow multiplies element loss by four, while the entered check-valve loss stays unchanged.
Using the results
Use the diagram to understand routing and the equation to explore its stated assumptions. The default coefficient, viscosity correction and check-valve loss are not a measured filter performance curve. A real check-valve loss also changes with operating conditions, whereas this calculator simply adds the value you enter.
For a particular installation, consult the element pressure-drop curve, the recommended viscosity correction, the check-valve flow data and the pressure points used for its bypass setting. Filtration efficiency, contaminant capacity, collapse strength and service intervals are outside this calculation.
The animation changes routes on a viewing cycle, not in response to calculated pressure. Flow rate adjusts playback speed; no valve dynamics or bypass path is animated.
Reversible-filter questions
Does the element reverse filtration direction?
No. The check-valve bridge reroutes external flow so fluid always enters the dirty side of the element and leaves its clean side. This is different from backwashing, which reverses flow through the element itself.
What does the coefficient K represent?
In this illustrative equation, K is element pressure drop divided by flow squared at the reference viscosity of 32 cSt. A real filter may not follow that flow-squared relationship or the assumed linear viscosity scaling. Check its pressure-drop curves before applying either assumption.
Does a setting ratio above 100% predict bypass flow?
No. It means only that the calculated total exceeds the entered setting. The calculator does not model a bypass valve, its flow, or its response. Real operation depends on the actual pressure difference across that valve and its manufacturer data.
Why does the margin stop at zero?
The margin output is limited to zero when calculated total pressure drop reaches or exceeds the entered setting. A zero margin therefore covers both equality and exceedance; the setting ratio distinguishes them.
Does changing pressure drop change which checks open?
No. The animation alternates between two ideal routing states. Flow affects playback speed, while the other inputs affect the equation results. Check-valve motion and cracking pressure are not simulated.
Primary references
- EP3148670A1, Filter device — reversal device keeps flow through the element in one predefined direction.
- RU2075328C1, Filter for reversible flow — four-check routing between external ports, dirty annulus and clean element cavity.
- US4657040A, Bypass reverse flow valve — distinguishes reverse-flow checking from differential-pressure bypass operation.
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