Clogging indicators are warning devices that signal visually or electrically that a filter element is filled with contaminants and should be changed or cleaned. These indicators activate when the flow of fluid causes a pressure drop (∆P) across the filter element that exceeds a predetermined setpoint. In filters that incorporate bypass valves, contaminated fluid will flow past the element if the operator does not respond to the indicator’s signal within a reasonable time. In non-bypass filters, if the indicator warning is not heeded, the pressure across the filter will build up to the point where system performance is degraded, the element fails, or the system relief valve opens.

The indicator is set to trip well before the element becomes fully clogged — usually at about 15 psid (1 bar) lower than the bypass pressure. This gives the operator enough time to take corrective action. The indicator warning may be a visual signal at the filter site (pop-up button, light, etc.) or some form of signal at a remote location (an indicator light, audible alarm, etc.). In the most critical applications, the signal from the indicator may be used to shut down the system, which would call for immediate service from personnel.

Some users install filters without indicators, preferring instead to change or clean elements according to hours of operation. There is some risk in adopting this approach. It may be difficult to establish a reliable schedule for installing new elements because the rate of dirt ingression is not known, and, in fact, may vary with time and from machine to machine.

Clogging indicators have two primary benefits. First, they eliminate the need to guess when the element will clog. Second, they avoid the unnecessary cost of replacing elements too soon.

Indicator settings

For most applications, we set our indicators to trip 15 psid (1 bar) below the bypass valve cracking pressure. For non-bypass filters, we set them 15 psid below the element’s replacement pressure. A Hydac pressure filter bypass valve typically begins to crack at 87 psid (6 bar), so we set the indicator to trip at 72 psid (5 bar). Our return filters normally begin to bypass at 43 psid (3 bar), so we set the indicator to trip at 29 psid (2 bar). Consequently, the operator has time to change or clean the element
before the bypass valve opens, which allows contaminated fluid to flow to components downstream of the filter.

Typically, the time from indication to bypass is 5% to 15% of the life of the element. For instance, if the element has a normal service life of 100 days, a grace period of five to 15 days occurs before the filter begins bypassing. Nevertheless, the element should be changed as soon as the indicator trips.

Non-standard indicator settings are often used for specific reasons. For instance, filters in lubrication systems may not be allowed to have a high pressure drop. Therefore, the indicator may be set to trip at less than 15 psid. When the filter is installed on the suction side of a hydraulic pump, it is normal to limit the ∆P across the filter to only 3 psid and set the indicator at a correspondingly low amount. This prevents starving the pump, which could cause cavitation.

On the other hand, some non-bypass filters are set at higher pressures. For example, Hydac’s DFDK duplex and DFZ series of sandwich filters, use indicators set at 116 psid (8 bar) in order to maximize dirt retention and service life of the elements.

In most cases, our pressure and return line filters bypass at higher pressures than other commonly used filters, meaning that indicator settings also are higher than usual. This has the advantage of extending element service life.

Types of indicators

Filter assemblies may be ordered with or without indicators. When ordered with an indicator, Hydac’s assembly model code includes a letter symbol for the indicator, such as B, C, or D.  Type B or BM visual indicators are suitable when only a local warning is required. To signal a remote warning device, control panel, or PLC, an electric switch should be specified. Various kinds of switches are available to provide a range of electrical configurations, contact ratings, and connections. Our Type D indicator incorporates a switch and built-in light for both local and remote warning signals.