Filter bypass valve: the spring-loaded valve in a hydraulic or lube filter head that opens at a set pressure difference
Filter bypass valves · cracking pressure against element collapse and burst ratings, clogging-indicator trip points, cold-start opening, non-bypass options
Inside a hydraulic or lubrication filter head, a small spring-loaded poppet sits in parallel with the filter element. While the element is clean it stays shut. When dirt loading, a cold start or a flow surge raises the pressure difference across the element to the valve's cracking setting, it opens and lets part of the flow go round the element, unfiltered, so that the element does not collapse or burst. Specifying one means choosing the cracking pressure against the element's collapse or burst rating, setting the clogging indicator to warn before the valve opens, and deciding whether the circuit can accept unfiltered oil at all.
Differential-pressure bypass valves built into hydraulic and lubrication oil filter heads and housings, with their cracking settings, matching clogging indicators and non-bypass alternatives. Choosing a hydraulic filter by ISO 4406 target and Beta ratio is covered on the Hydraulic filter page; pump-inlet strainers and their bypass on the In-tank hydraulic suction strainer page; screw-in relief cartridges that limit system pressure on the Hydraulic cartridge pressure control valve page. Refrigeration hot-gas bypass valves and differential bypass valves for heating circuits are different products outside this page.
CNFX editorial research · Sources checked October 12, 2026 · 4 sources
Cracking pressure against element collapse and burst ratings
The valve's cracking pressure has to sit well below the pressure difference the element can survive. HYDAC lists its FMND duplex pressure filters with bypass codes B3.5 and B7 and publishes the element collapse rating beside them; Schroeder lists the cracking setting of its BRT in-tank return filter beside the element burst rating.
Editorial calculation: the HYDAC B7 setting leaves the element at 7 / 20 = 35% of its collapse rating when the valve cracks, and the B3.5 setting 17.5%; Schroeder's BRT cracks at 2.5 / 6 ≈ 42% of the element burst rating. A valve that cracks but cannot pass enough flow lets the pressure difference keep rising above the cracking point, so ask for the full-flow pressure difference as well as the cracking value. [1][2][4]
HYDAC states that its bypass valve seals positively in normal operation and opens fast during cold starts and flow surges, and offers a non-bypass version as an option. [2] Schroeder's BRT runs inside-out flow and lists Buna N or Viton seals and an operating range of −30 to 120 °C. [4]
Give the filter supplier the element's collapse or burst rating, the coldest start-up viscosity, the peak flow (including flow surges from cylinders), and whether the circuit downstream can accept any unfiltered oil.
02
Clogging indicator trip points 1 bar below the bypass setting
HYDAC's stated default, with exceptions listed below, is to set its clogging indicators to trip about 1 bar (14–15 psid) below the bypass valve's cracking pressure, so that the operator is warned before dirty oil starts to go round the element. Its examples: a pressure filter bypass that begins to crack at 6 bar (87 psid) with the indicator at 5 bar (72 psid), and a return filter bypass at 3 bar (43 psid) with the indicator at 2 bar (29 psid). [3]
HYDAC puts the time between indication and bypass at 5 to 15% of element life: an element that lasts 100 days gives 5 to 15 days to change it. It notes exceptions: lubrication filters that cannot take a high pressure drop may have indicators set below 15 psid, filters on a pump's suction side are limited to about 3 psid, and some non-bypass HYDAC filters use indicators set at 8 bar to make full use of element life. [3]
Cold oil raises the pressure difference without any dirt, so an indicator can trip falsely at start-up; HYDAC offers a thermal lockout that stops an electrical indicator from tripping until the fluid reaches a set temperature. [3] Editorial calculation: applying HYDAC's 1 bar rule to the FMND B7 bypass gives an indicator trip at about 6 bar, and to Schroeder's 2.5 bar BRT bypass about 1.5 bar. Confirm the actual indicator setting with each maker. [3][2][4]
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When to order a non-bypass filter instead
Without a bypass valve, nothing protects the element once it loads. HYDAC explains that in a non-bypass filter whose indicator warning is ignored, the pressure difference builds until system performance degrades, the element fails or the system relief valve opens; in critical systems the indicator signal can be used to shut the machine down. [3]
The choice therefore depends on what sits downstream. Where any unfiltered oil is unacceptable, order a non-bypass housing with a high-collapse element and wire the indicator to an alarm or shutdown; where the element must never be the cause of a stoppage, order the bypass setting and keep the indicator margin above. HYDAC lists the non-bypass option on its FMND series. [3][2]
State in the order: bypass or non-bypass, cracking setting in bar or psid, element collapse or burst rating, indicator type (visual, electrical, thermal lockout) and trip setting, and seal material for the fluid.
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Filter bypass data to send
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What cracking pressure do filter bypass valves use?
HYDAC's FMND filters come with 3.5 or 7 bar settings (codes B3.5 and B7); Schroeder's BRT return filter cracks at 2.5 bar (36 psi). [1][2][4]
Where should the clogging indicator be set?
HYDAC's default is about 1 bar (14–15 psid) below the bypass cracking pressure, with exceptions for lube and suction filters; for example 5 bar with a 6 bar bypass and 2 bar with a 3 bar bypass. [3]
How much warning does the indicator give?
HYDAC gives 5 to 15% of element life between indication and bypass, for example 5 to 15 days on a 100-day element. [3]
How far below the element rating should the bypass open?
On HYDAC's FMND B7 filter the 7 bar setting is 35% of the 20 bar collapse rating; Schroeder's 2.5 bar BRT setting is about 42% of its 6 bar burst rating (editorial calculations). [2][4]
Sources and what still needs a direct check
Sources checked 2026-10-12. HYDAC and Schroeder Industries are filter makers; each publishes the settings of its own filters and indicators.
