In many large or complex hydraulic circuits, especially in steel mills, one hydraulic power unit (HPU) generally drives multiple functions. In many cases, this HPU may feed ten or more manifolds that, in turn, route fluid to ten or more actuators. Each line to each function may contain ten or more fittings. So it would not be unreasonable to assume that well over a thousand potential leak points exist in a large or complex hydraulic system. Furthermore, many of these leaks will be difficult to reach because of the physical position of the equipment and the way the piping was installed through a foundation. Even more difficult to find would be internal leaks in the 100 or so valves and cylinders.
By measuring the temperature of various pipes, tubing, or components, the existence of internal leakage can be detected. This is because an increase in surface temperature often is directly related to power loss. The following discussion describes a proposed method of determining internal and external leakage rates by measuring the surface temperature of components.
In steel mill applications, valve stands may be mounted 100 ft or more from the cylinders. In selecting the proper diameter of pipe, hose, or tubing for a projected flow rate, a velocity of 10 ft/sec is normally used. This low velocity keeps pressure drop reasonable for piping runs, including pressure drop from interconnecting fittings.
In the following examples, assume a 6-in. bore hydraulic cylinder holds a load in place with a system pressure of 2250 psi. In the normal condition, pressure is applied to the cap end of the cylinder to hold the load. No flow should occur.
Depending on the piping ID, length of the lines, and the time between cylinder strokes, the cylinder cap- and rod-end piping should reach a temperature of near ambient. In many cases, the run of the tubing from the directional control valve to the cylinder contains a greater volume of fluid than the volume used to fully stroke the cylinder. In this case, if no internal leakage occurs, the temperature of the fluid in the cylinder can increase only if the valve shifts to extend or retract the piston rod.
If, however, the cylinder extends 12 in. in 2 sec, the cap-end flow rate required would be 44 gpm. If the cylinder has a 4-in. rod, the flow required for the same return speed would be 24 gpm. If a schedule 80 1¼-in. pipe is used, the maximum velocity would be about 11 ft/sec.
It is common practice to plumb both cylinder lines with the same size of pipe or tubing. Doing so would produce a rod-end flow velocity of 6.12 ft/sec. The actual volumetric exchange of fluid would be 339 in.3 on the cylinder cap end and 188 in.3 on the rod end. Because the 1¼-in. piping has an internal bore of 1.278 in., every foot of piping contains 15.39 in.3 of fluid. This means that increasing the temperature of the fluid in the cylinder would require a pipe length of less than 22 ft on the cap end and 12 ft on the rod end.
Heat generation and propagation
The main method of introducing heat to components and piping is to use power loss in reference to flow rate as a heat transfer device. If a 50-ft piping distance is used for the preceding example, and the valve is shifted once every minute, then the temperature gradient relative to flow may appear as shown in Figure 1.
At a certain distance from the directional valve, the pipe or tube is heated by stroking the cylinder then cooled during the reverse cylinder stroke. The amount of heat transferred through both tubes is directly related to ambient temperature, tube diameter, and the internal temperature of the fluid (at the specific distance from the directional valve). The amount of heat loss through a non-insulated tube can be approximated from the following:
Lheat= A × K × ∆T,
where Lheat = heat loss — btu/(hr • ft)
A = pipe or tube surface area — ft2/ft,
∆T = temperature difference between the fluid and ambient — °F
K= 2.15 btu/(hr•ft2•°F) for ∆T to 100 °F
K= 2.66 btu/(hr•ft2•°F) for ∆T from 100° to 200 °F
For sake of discussion, assume rating tables list heat loss for 1 ft of 1¼-in. schedule 80 pipe with an internal fluid temperature of 140° F and ambient temperature of 60° F as 74.8 btu/(hr•ft). If the circuit just mentioned maintained a fixed state, such as holding a load for a prolonged time, then the temperature gradient relative to zero flow may appear as shown in Figure 2.
This example demonstrates that a transition temperature point may be established on the tubing leading to and from a cylinder that defines the natural action of the cylinder in reference to cycle times and a zero flow, zero leakage condition. With normal ambient temperature changes, this point should not vary more than 2 ft. Any variance beyond this tolerance would indicate leakage.