Hydraulic Filter Clogging Indicator Tripped? What to Do Next

A tripped hydraulic filter clogging indicator means the pressure drop across the element has reached the indicator's setting, which on most filters is 1 bar below the point where the bypass valve opens. Change the element now. The one exception is a cold start: thick oil can trip a clean filter, so let the system reach working temperature and check again before you order anything. If it is still tripped when the oil is warm, the element is full.

Below: what the indicator measures, where the trip point sits relative to the bypass valve and the element's collapse rating, a step by step for the next ten minutes, and what it means when a new element trips again within days.

What a clogging indicator measures

As an element loads with dirt, the oil has to push harder to get through it. The pressure upstream of the element rises while the pressure downstream stays where it was. The difference between the two is the pressure drop, written ΔP. The clogging indicator is a small spring loaded piston, usually magnetically coupled to a pop-up pin or a switch, that moves when ΔP exceeds its setting.

There are two families, and it matters which one your filter has:

• Differential pressure indicators read upstream minus downstream directly, whatever the system pressure is. They are used on most pressure filters and in-line return filters.
• Static pressure indicators read only the pressure upstream of the element and assume the downstream side is at atmospheric pressure. HYDAC recommends them only on filters that discharge straight into a vented tank with minimal back pressure. If a cooler, valve or long return line sits after the filter, a static indicator adds that restriction to its reading and trips early.

Where the trip point sits: indicator, bypass, collapse

Three pressure figures stack on top of each other in every filter housing that has a bypass valve. HYDAC's published practice sets the indicator 1 bar below the bypass cracking pressure. On their return filters the bypass opens at 3 bar and the indicator trips at 2 bar (29 psi). On their pressure filters the bypass opens at 6 bar and the indicator trips at 5 bar.

• Indicator trip: the warning. The element is close to full but every drop of oil still passes through the media.
• Bypass valve opens: some or all of the flow now goes around the element, unfiltered, to the components downstream. The filter has stopped protecting the system, and the dirt it already caught can be washed loose.
• Collapse pressure: the ΔP the element itself can stand before the pleats or the centre tube give way. A bypass valve should open well before this is reached.

Our own catalogue shows how these settings cluster. Of the 913 hydraulic elements we list, 56 carry a published bypass setting: 34 at 3 bar, 16 at 2 bar and 2 at 1.75 bar. Of the 341 that carry a collapse rating, 114 are rated 10 bar, 108 are rated 20 bar and 78 are rated 210 bar. The 10 and 20 bar elements are built for housings with a bypass valve. The 210 bar elements are high collapse designs for housings without one, where nothing relieves the element if it plugs.

How long do you have between the warning and the bypass? HYDAC puts it at 5 to 15% of the element's life. An element that normally lasts 100 days gives you 5 to 15 days of grace after the indicator trips. ΔP rises slowly for most of an element's life and then climbs steeply as the last open pores fill, which is why the margin is short. HYDAC's own advice is to change the element as soon as the indicator trips, and that is the right call: the grace period is a buffer for getting the part, not a second service interval.

First check: is the oil cold?

Pressure drop depends on viscosity as much as on dirt. On a cold morning, mobile and outdoor machines start with oil that is far thicker than at working temperature, and a clean element can show enough ΔP to trip the indicator. HYDAC lists this as the main source of false indications.

• Auto reset indicators (most electrical switches and visual types such as HYDAC's type B) drop back on their own once ΔP falls. If the warning clears as the oil warms, the element is not full yet. Note the date, because it will be soon.
• Manual reset indicators (HYDAC type BM) latch once tripped and stay up after shutdown until someone pushes the button down. Reset it with the machine at working temperature and see whether it comes back.
• Thermal lockout is an option on many electrical indicators: a bimetal switch in series that blocks the signal until the fluid is warm. HYDAC offers one that holds the signal off below 38 °C (100 °F), and cold start suppression up to 30 °C on its electronic indicators. If your machine throws false alarms every winter morning, this is the fix, not a coarser element.

What to do when the indicator trips

• 1. Read it warm. Run the system to normal temperature at normal flow. A trip at idle on cold oil is not a reading.
• 2. Confirm it is still tripped. Reset a manual reset indicator and watch it. If it returns, the element is loaded.
• 3. Get the element part number before you open the housing. Read it from the housing label, the old element's end cap or your service records, and check you have the replacement in hand. A housing left open waiting for a part lets dirt in.
• 4. Change the element. Isolate and depressurise the circuit, drain the bowl, fit the new element with a new O-ring lubricated with clean system oil, and close the housing to the torque given in its manual.
• 5. Run and recheck. With a clean element at working temperature the indicator must stay down. If it trips straight away, go to the section below.
• 6. Record the date and hours. The interval between changes is the most useful number you have for spotting a problem early.

While the old element is out, look at it. Note how long it lasted against the last two changes. Bright metallic particles in the pleats point to wear inside the system rather than dirt getting in from outside, and that is worth an oil sample before the next change. For general change intervals across compressors, engines and hydraulics, see when to change an industrial oil filter.

The new element trips again within days

A fresh element should run for roughly the same time as the one before it. When it does not, one of these is usually the cause:

• A finer grade than the housing was sized for. Moving from a 10 µm to a 3 µm element raises the clean pressure drop and shortens life at the same flow. Our catalogue shows how wide the spread is: of the elements we list with an inorganic microfibre rating, 100 are 5 µm, 92 are 7 µm, 180 are 12 µm and 148 are 25 µm. Match the grade on the original part number, or read our guide to hydraulic filter beta ratios before changing it on purpose.
• A shorter element. Many housings take several lengths of the same diameter. A shorter element has less media area and fills faster. Check the height against the old one.
• A cleanup after a repair. After a pump, cylinder or hose failure the filter is catching the debris from that event. Two or three short intervals in a row are normal until the system is clean again.
• Dirt getting in. A failed tank breather, worn cylinder rod wipers, or top-up oil poured from an open container all add load the filter has to catch.
• A static indicator reading a downstream restriction. If the indicator reads only upstream pressure and something after the filter has changed, such as a blocked cooler, it can trip with a clean element.
• The indicator itself. A switch can stick or be wired normally open where the controller expects normally closed. If a known clean element shows a trip on warm oil, test the indicator before you change another element.

Finding the right replacement element

The indicator tells you when; the part number tells you what. Hydraulic element codes carry the size, grade and seal material in a few characters, and the same element is sold under several brands. Take a common HYDAC return element, 0660 R 010 BN4HC. Our aftermarket equivalent of Hydac 0660R010BN4HC is 114.5 mm in diameter and 333 mm tall, with 12 µm inorganic microfibre media, a 3 bar bypass setting and a 20 bar collapse rating (see current price). The same housing also takes a 10 µm paper element, our Epe 10660P100006P equivalent, which is rated to collapse at 10 bar rather than 20. The longer 0850 size is our Hydac 0850R010BN4HC equivalent at 413 mm.

Across the range we cover 160 HYDAC return element numbers (the R and RN series) with 57 elements, and 327 other HYDAC numbers with 108 more. Browse the full hydraulic filter range, search any brand's number on our compatibility page, or send the number from the old element through request a quote and we will match it.

Frequently asked questions

Can I keep running after the clogging indicator trips?
For a short time. On a typical HYDAC filter the gap between the warning and the bypass opening is 5 to 15% of the element's life. Once the bypass opens, oil reaches your pumps and valves unfiltered, so change the element as soon as you can.

Why does the indicator trip in the morning and clear later?
Cold oil is thicker and raises the pressure drop across a clean element. An auto reset indicator clears once the oil warms. If it happens daily, fit an indicator with thermal lockout, which ignores readings below a set fluid temperature.

Can I clean and refit a hydraulic filter element?
Microfibre and paper elements are single use; washing them damages the media. Wire mesh elements are designed to be cleaned and reused, and they are usually found in suction and coarse return duties.

Does changing to a coarser element stop the indicator tripping?
It makes the element last longer, but it lets more dirt through to every component downstream. Fix the cause (cold starts, ingress, wrong length) and keep the grade the system was designed for.

Should I replace the indicator when I replace the element?
No. The indicator is part of the housing and lasts many element changes. Replace it only if it fails a test with a clean element on warm oil.