Hydac VD Differential Pressure Clogging Indicator, up to 420 bar

The indicator that tells you when a high-pressure filter element is full

The Hydac VD is a differential pressure clogging indicator for inline filters in lines up to 420 bar. It reacts to the pressure difference across the element, which rises as the element fills with dirt, and it indicates visually, electrically or both when the element needs changing. Without an indicator, changes have to follow a time or running-hour schedule, which risks either throwing away good elements or running a blocked one.

The VD screws into the G1/2 cavity of the filter head. A lower-pressure VM version covers up to 210 bar, and the VD is the choice for the high-pressure inline filters such as the DF, DFDK, HFM and MDF families.

Technical specifications

How to order a VD indicator

The order examples in the brochure follow the pattern VD 5 B.1 for a visual indicator with a 5 bar setting and VD 5 D.0 /-L24 for a visual and electrical version with a 24 V lamp. The number after VD is the pressure setting in bar, the letter code is the type of indication, and the supplementary details give lamp voltage (L24, L48, L110, L220), LED, or connector variants for vehicle-style plugs.

The same brochure covers vacuum, return line and differential pressure indicators for all Hydac filter families, with a quick selection table by filter type. If you do not know which indicator your filter uses, send the filter nameplate and we will check the model code against the brochure.

Installation and cautions

Hydac filter housings must be earthed. When an electrical indicator is used, switch the power supply off before removing the connector. Observe the maximum torque of 100 Nm when fitting a VD. For electrical versions on low-current inputs, such as PLC inputs, the brochure points to the SO135 option.

Seals are NBR as standard with Viton available. Please provide the filter type, operating pressure, setting and the electrical requirements with your enquiry.

How the visual and electrical versions compare in the brochure:

Why differential pressure monitoring matters on high-pressure filters

In a hydraulic system the pressure filter protects sensitive components such as servo and proportional valves, pumps and actuators. As the element collects particles, the pressure drop across it rises. If the element is not replaced in time, the filter bypass valve (where fitted) opens and unfiltered oil passes to the system; on filters without a bypass, the element itself can be overloaded. The VD indicator gives a clear signal before either of these conditions occurs, so that the element is changed based on its actual condition rather than on a fixed interval.

Choosing the pressure setting

The VD is offered with 2, 5 and 8 bar settings. The setting is normally chosen to suit the filter and its bypass valve, so that the indicator responds before the bypass opens. When replacing an existing indicator, take the setting from the old unit’s type code or the filter nameplate, and do not change it without checking the filter documentation. Note also that cold oil at start-up has a higher viscosity and causes a higher pressure drop; an indicator may show a warning during a cold start that disappears once the oil reaches operating temperature.

Visual or visual-plus-electrical?

• VD B (visual): suitable where operators walk past the unit regularly and can see the red/green band, for example on mobile machines or small power units.
• VD D (visual and electrical): preferred where the filter is out of sight or the plant uses a PLC or DCS; the switch signal can raise an alarm or schedule maintenance automatically.
• Versions with switching at set points, manual reset or electronic evaluation are covered in the same brochure for more demanding monitoring.

Use in Oman and GCC applications

High-pressure hydraulic filters with VD indicators appear on presses, injection moulding machines, test benches, steel and aluminium processing, cranes, and hydraulic power units serving oil and gas wellhead control or valve actuation. High ambient temperatures in the region raise oil temperatures and accelerate oil ageing, while dust ingress through breathers and seals increases the particle load on filters. Both make reliable clogging indication more valuable. For electrical versions installed outdoors, make sure the connector and gasket are fitted correctly so that the IP65 rating is achieved, and protect cables from direct sun.

Maintenance tips

1. Check the indication with the system at normal operating temperature.
2. When the indicator responds, depressurise the filter and replace the element with the correct type and filtration rating.
3. Inspect the indicator O-rings when removing it, and use the seal material that matches the fluid (NBR or Viton).
4. After element change, confirm that the visual indicator has reset and the electrical signal has cleared.

What to include in your RFQ

Send the full type code of the indicator (for example in the VD 5 D.0 /-L24 format), the filter type and size, operating pressure, required pressure setting, seal material, switching function and voltage, and any special approvals such as ATEX.

Documents and quotation

The Hydac clogging indicator brochure (E 7.050.15/11.16, 46 pages) is attached; it covers the VD together with other indicator types. Related: filter housings and the Hydac RFM return line filter.