In the next example, an internal leak occurs in a cylinder that is used to hold a load in place with a system pressure of 2250 psi. Figure 2 represents a cylinder in the hold condition — under pressure with zero leakage. If, however, fluid in the cylinder leaks past the piston seals, fluid flow will cause heat to migrate toward the leak point, Figure 3. This heat is generated by high-pressure fluid flowing through an orifice (the leakage path through the piston seals) to the low-pressure side of the piston without performing any work. Heat detected more than 2 ft from the heat point established in Figure 2 indicates that fluid flow is occurring. Internal cylinder leakage serves as the vehicle for this flow.

Any internal leakage of pressurized fluid generates heat because fluid energy that is not transmitted as mechanical power will be transformed into heat. Internal leaks transmit the lost power directly into the downstream fluid flow, which increases the temperature of the downstream fluid. The result is that the temperature exiting the cylinder exceeds that of the fluid entering it.

With external leaks, the heat transfers lost power directly to external environment. But some heat does transfer to the tubing leading to the leak, Figure 4. Depending on the amount of the leakage flow, the tubing’s outer surface temperature will increase closer to the leak point. However, at some point past the leak, approximately 1 ft in Figure 4, temperature returns to ambient because flow is (or should be) static.

Again, fluid flow is the main vehicle that transmits heat to piping surfaces and housings. Furthermore, a pressure drop across a leak path or orifice will increase the downstream fluid temperature by converting unused power into heat. From these two facts, we can approximate the actual fluid flow based on the temperature difference across the pressure drop.

### Quantifying leakage

Ideally, temperature measured across a cylinder should be static relative to flow. But in actuality, a temperature differential exists. In Figure 4, for example, assume temperature at the pressure port is measured at 141 °F, and 151° F at the tank port. This clearly shows a temperature rise of 10° F.

Using ∆T in° F and the pressure differential (∆P) in psi, the flow through the orifice (leak path) can be approximated. From the familiar equation for hydraulic horsepower, at 2250 psi, a flow of 0.7623 gpm would produce 1 hp, which is 2546 btu/hr. From standard conversions, we can calculate that 1 gpm of water is equivalent to 8.345 lb/min, or 500.7 lb/hr. Furthermore, if we define the specific gravity of hydraulic oil to be 0.90, and specific heat as 0.45, we can calculate heat generation:

btu/hr = m × c × ∆T.

Plugging in known values and solving for ∆T:

T = 15.9° F.

Using 15.9° F as a slope, any change in ∆T can be related directly to a change in flow based on a set pressure drop. Therefore, to approximate the fluid volume of an internal leak at 2250 psi:

1. Define the flow required to generate 1 hp across an orifice at a specific pressure drop.

2. Find the temperature differential in °F at 2546 btu/hr (1 hp) input using the flow value in gpm defined in Step 1.

3. Measure the temperature difference across the inlet and outlet ports of the cylinder. Use the actual measured ∆T in °F to alter the input power based on 1 hp being equal to 15.9° F at 2250 psi.

4. Recalculate the new flow in reference to the new horsepower value.

The heat transfer discussed so far is a constant value relating to flow across two points. This does not account for the heat transfer through system piping and components. It also does not account for the heat transferred through reservoir walls or heat exchangers. It basically defines two set points of temperature in close proximity to an internal or external leak. By setting two temperature points across a cylinder or across a line, a constant inlet-to-discharge temperature difference can be found. Using that difference, you can calculate internal or external leakage.

### Heat detection identifies leakage

To actively trace heat through a system requires knowledge of how the valves in a circuit function and how many times they function per unit time. The use of an infrared heat gun for troubleshooting is mainly centered on finding internal or external leakage in components or, in some cases, improperly set or faulty pressure control valves.

The detection of heat, regarding internal leakage, can reveal seal wear before it becomes excessive. This allows replacing seals during normal scheduled maintenance, rather than waiting for leakage to become so severe that it slows down production enough to justify interrupting production to allow replacement. Heat from external leaks — even if they are relatively inaccessible — can reveal the location and severity of problems, such as loose or misaligned fittings, before they result in excessive fluid loss.

Identifying internal leakage in a 2-position valve is labor intensive using conventional troubleshooting techniques. Because the valve directs the cylinder to the end of its stroke and holds it there with a constant flow of pressurized fluid, heat transfer in the cylinder lines can quickly reveal internal or external leaks and allow for scheduled maintenance repairs.

In a 3-position valve, heat reveals leakage only when the cylinder is moving. Cylinder drift with the valve in neutral defines the occurrence of a leak in the cylinder or the control valve. Some amount of leakage should be expected with spool valves because this leakage provides lubrication to prevent stick-slip operation (stiction) of the spool.  Often, a pilot-operated check valve is used in combination with a 3-position valve to hold a load more positively. The pilot-operated check valve changes the center condition of the valve to P blocked with A and B ports routed to tank. Any leakage from the P port goes directly to tank, which causes a heat rise in the directional control valve and the tank line.

The number of times a function cycles per unit of time period determines the distance heat is transferred to the piping from the directional control valve to the cylinder. System functions may be defined as continuously operating, intermittently operating, and static in reference the cycling of one product through an operation.