Most of us are aware that prolonged exposure to the sun—especially in hot weather—can wreak havoc on rubber, polymers, and other synthetics, rendering them weak and brittle. But prolonged exposure to the sun can also cause problems inside a hydraulic system.
Hydraulic component failures from pressure overload have been reported in agricultural equipment, which often is left exposed to the sun for long periods when it is not being used. These pressure overloads can be above and beyond system relief valve settings, and they occur while equipment is shut down, rather than when it is operating. These seemingly improbable events result from thermal radiation absorptivity from prolonged exposure to the sun.
Many designers believe that system relief valves will prevent such heat-related high pressures from developing or that expansion of hydraulic hoses will absorb any pressure increases. They will, but many system designers do not specify relief valves in the implement because they rely on relief valves in the tractor’s hydraulic system.
Obviously, the problem is more complex. Through laboratory stress analysis of a failed component, the pressure that caused the failure can be computed. Working backward from that number, an analysis of the situation can be made.
A Typical Scenario
Let’s begin with this scenario: Assume the failed component came from an agricultural implement. The implement had been disconnected from the tractor on a fall day. It was a heavy implement that could trap 2,500 to 3,000 psi in its hydraulic system. Further assume that the ambient temperature was between 20° and 30°F on that fall day. If we know what the failure pressure is, and subtract the trapped pressure, what remains is the amount of pressure that would have to be generated by external sources to cause failure.
For this thermal-expansion problem, the bulk modulus of the fluid is a key factor. The bulk modulus for typical hydraulic oils ranges from 240,000 to 250,000 psi and varies with temperature. For this example, we’ll use an average value to get a useful indication of the possibility of thermally generated pressure increases.
ß = ∆P /(∆V/V1)
where ß is bulk modulus,
P is pressure,
V is volume, and
V1 is the original volume.
∆V = V2 – V1 , or
V2 = V1 (r1 /r2 ),
where r1 and r2 are mass density values that can be found graphically, plotted against temperature, or calculated in a mathematical model.
Expansion of Hose
Hose expansion can be found by using similar combined bulk modulus data. The combined bulk modulus for oil and high-pressure hose is approximately 47,500 psi. To find the actual expansion of the hose, you must know the bulk modulus for the hose itself. This can be done by remembering that the compression of the system is the sum of the compression of its different parts. In other words, the expansion of the hose, plus the compression of the fluid, equals the total compression. By also remembering that compressibility is the reciprocal of bulk modulus, we can determine a bulk modulus for the hose itself. Knowing the bulk modulus of the hose allows us to compute the expansion of the hose.
From the failure pressure, the corresponding change in volume can be calculated using the bulk modulus equation. Adding this change in volume to the volumetric expansion of the hoses is the total volumetric expansion of the oil in the system.
From the initial volume and volume change, the temperature associated with the failure can be found using the mass density expression. Because the system in an unattended implement is completely closed (remember, the relief valve is on the tractor), the temperature increase can occur at any time before reuse of the equipment—even several months after it has been shut down.
Can failure temperature occur from solar radiation? Heat gain from solar radiation can run as high as 300 Btu/hr-ft2, depending on the location, time of year, and other factors. Although the implement would radiate some heat back to the atmosphere, much of the time there would be a heat gain in the system, increasing its temperature.
QG = QA – QL, or
QG = Î (AP) (t) Cos Q,
where Î is emissivity (in this case, for cylinders painted black and black hoses, Î = 0.96),
AP is the projected area of the hydraulic cylinders and hoses
Q is position on the earth’s surface in degrees latitude +15° (this factor takes into account the sun’s location in the sky. For this example, use 40° north latitude), and
t is time, in hours
QA = ∆T åMCP,
where ∆T is temperature change,
M is mass, and
CP is the specific heat for the various materials.
QL = h AP ∆T,
where h is the film coefficient for prevailing conditions—in this case, use 1.65 for still air (Ref: P.E. Reference Manual, by Michael R. Lindeburg, P.E.).
It’s About Time
Each factor in these equations is known except for the amount of time. Solving for t, we find that if the machine sits in the hot sun six to seven hours, it can gain sufficient heat to reach failure pressure.
Other factors that will impact the highest pressures attained are the actual tractor-system pressure, the amount of steel plumbing, the stiffness of hoses, and implement weight (which can elevate the trapped pressure). Also, the operator can take some positive actions, such as parking the implement in the shade and equalizing the pressure after disconnection. (Ironically, leaking quick-disconnects, normally a negative factor, will prevent pressure buildup.)
Overall, thermal relief protection (such as a relief valve) should be provided for mobile implements, even if they will be unused for only a few hours. Such protection will allow them to retain their designed-in safety margin and protect them against damage and downtime.
This information was submitted by Jim Walker, former senior vice president of engineering at Prince Manufacturing Corp., North Sioux City, S.D.