Nailing hydraulic logic element leakage

One of our readers wrote to me recently with the following question:

 "In one of our applications we are using NG 40 cartridge valves (sleeve, poppet and logic cover). With the valve closed and the inlet port pressurized to 315 bar, we are seeing a leakage from the outlet port in the order of half a liter per minute. Is this level of leakage acceptable?"

The first thing to consider is whether the logic element has been configured for leakless operation. If the direction of flow is from A to B this is referred to as base flow. If flow is from B to A this is know as annulus flow (see figure 1). A logic element can be configured for flow in either or both directions.

logic element flow paths

Figure 1. Logic element base and annulus flow configurations (Industrial Hydraulic Control).

To establish whether a logic element is configured for zero leakage, it is necessary to consider the direction of pressure drop across the poppet when it is closed. Consider a logic element configured for check valve function in both base and annulus flow directions. When configured as a check valve for base flow (A to B) see figure 2, the direction of pressure drop across the poppet when it is closed is from B to A. In this configuration the logic element is leakless.

logic check - base flow

Figure 2. Logic element; check valve function; base flow (Industrial Hydraulic Control).

When configured as a check valve for annulus flow (B to A) see figure 3, the direction of pressure drop across the poppet when it is closed is from A to B. In this configuration the clearance between the poppet and its sleeve results in leakage from A to B. The magnitude of this leakage may increase over time as a result of wear between the poppet and sleeve.

logic check - annulus flow

Figure 3. Logic element; check valve function; annulus flow (Industrial Hydraulic Control).

Assuming our reader's logic elements have been configured for leakless operation, other possible explanations for the leakage include:

  • damage to the poppet and/or its seat
  • degradation or damage to the elastomeric seal at the base of the sleeve
  • incorrect machining tolerance in the logic housing

In this, and all other troubleshooting situations, the first place to look for guidance is the machine's circuit diagram and your reference library. From there on, it is a logically process of elimination.

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