Thermal relief valves in hydraulic systems open at a lower pressure than the system relief valve

Thermal relief valves: why they open before the main safety valve

Let me ask you a quick, practical question. In a hydraulic system, what happens when the fluid heats up and expands? Temperature can push pressure up just as surely as a blocked pump or a clogged line. That’s why there are two safety helpers in many systems: the system relief valve and the thermal relief valve. They’re not rivals; they’re teammates, each guarding a different kind of pressure spike. So, at what pressure does the thermal relief valve open? The short, practical answer is: lower than the system relief valve. Now, let me walk you through why that matters and how it works in everyday machines.

What these valves actually do

First, a quick refresher, in plain language. The system relief valve is the grown-up in the room. If pressure climbs too high—because the pump’s pushing hard or a line is blocked—it vents excess fluid to keep the whole system from failing. Think of it as the last-resort brake pedal that saves the day when the system tries to push past its safe limit.

The thermal relief valve, on the other hand, is the temperature-aware guardian. When hydraulic fluid heats up (through mechanical work, ambient heat, or poor cooling), it expands. That expansion raises pressure even if the pump hasn’t changed its behavior. The thermal relief valve sits in a position to sense that extra pressure and vent it early, before the main relief valve needs to bite.

In practice, this means the thermal relief valve doesn’t wait for catastrophe. It acts sooner, specifically at a pressure that’s lower than the main relief valve’s set point. By catching the expansion early, it protects seals, hoses, and fittings from the higher stress that follows if the fluid keeps warming.

Why the thermal valve is set lower

Here’s the intuitive bit. If the thermal relief valve opened at the same pressure as the system relief valve, you’d likely end up venting because of heat only after the main relief was already hitting its limit. That would force the system to ride the edge of a dangerous condition longer than necessary and could let heat-induced problems propagate.

Setting the thermal relief valve a notch or two below the main relief valve creates a buffer. When the fluid expands with temperature rise, the thermal valve opens and bleeds off the extra pressure early. The system relief valve remains available to handle more severe, non-thermal overpressure scenarios—like a sudden surge from a misbehaving pump or a blockage that requires venting at a much higher threshold.

In numbers, the exact relationship isn’t one-size-fits-all. The thermal relief is designed to be lower than the system relief by a margin that engineers determine based on the fluid, the operating temperature range, and the specific machine. In other words, you’ll see a design rule of thumb rather than a single universal number, because different hydraulic fluids behave differently under heat, and different machines tolerate different heat loads.

A simple mental model helps

Think of a hot day on a bike ride. Your tire pressures rise a bit as the sun beams down. If you had a secondary valve in the tire that opened early whenever the air pressure crept up due to warming, you’d stay more stable on the ride. Now imagine a second valve that only opens when the pressure is dangerously high. You’d want both: the early one to keep the ride smooth and the second to handle dramatic bursts. That’s essentially what the thermal relief valve does in hydraulic systems: a proactive guard before the bigger safety valve steps in.

How designers decide where to set them

Engineers don’t pick numbers at random. They model heat generation, fluid properties, and the system’s duty cycle. They also consider the fluid’s coefficient of thermal expansion, viscosity changes with temperature, and how fast heat is exchanged with the surroundings. If a machine runs hot—say, a hydraulic press with continuous movement or a mobile crane working in the sun—the thermal relief might be set with a bit more headroom below the main relief to avoid nuisance venting when the oil temperature fluctuates during the day.

Manufacturers like Parker Hannifin, Bosch Rexroth, Eaton, and others provide guidelines and data for how to set these valves in different fluids and pressure ranges. It’s common to see a combination valve assembly or an integrated valve block where the thermal relief sits in close proximity to the main relief valve, ensuring the same oil temperature is reflected in both. The practical takeaway? When you’re reading a schematic or assembling a system, look for how these two pressures relate and verify that the thermal valve’s setpoint actually anticipates temperature-driven expansion.

What to watch for in the field

If you’re on a job site or in a workshop, these quick checks help ensure the two valves play nicely together:

• Check the pressure relationship. The thermal relief should be set under the main system relief. If you find the thermal valve and the main relief valve set to the same pressure, or the thermal valve is higher, that’s a red flag that needs adjusting.

• Observe operation during heat buildup. If you run the system and notice venting happening primarily as oil warms, that’s a sign the thermal relief is doing its job. If venting only occurs at the final relief threshold, something’s off.

• Inspect for nuisance venting. If you hear or see venting during normal operation, the thermal setpoint might be too close to the main relief, or there could be poor cooling causing excessive heat buildup.

• Confirm fluid and temperature compatibility. Fluids with large thermal expansion coefficients or systems that see big temperature swings require careful calibration. When in doubt, consult the fluid manufacturer’s data and the valve manufacturer’s guidelines.

• Routine maintenance matters. Leaks, stuck valves, or clogged passages can throw off the intended relationship between the two valves. Regular inspection and test cycles help keep both valves in sync.

A few practical analogies

• A thermostat and a main furnace limiter. The thermostat (thermal relief) kicks in early to prevent the indoor air from getting uncomfortably hot. The furnace limiter (system relief) is there to prevent a furnace disaster if the system truly overheats.

• A crowd at a doorway. The thermal relief is the crowd control at the entrance—letting people (fluid) out a little sooner as the temperature-driven pressure grows. The system relief is the backup who steps in if more pressure builds than the exit corridor can handle.

A note on safety and design philosophy

Hydraulic safety isn’t about chasing the perfect number; it’s about creating a safe, stable operating envelope. The thermal relief valve adds a layer of protection against temperature-driven excursions that could push the system toward seal wear, component fatigue, or unpredictable behavior. Keeping it lower than the main relief valve is a deliberate design choice that reduces the risk of thermal-related incidents while preserving the main safety valve’s capacity to handle severe, non-thermal overpressure events.

If you’re ever reading a hydraulic schematic and spot these two devices, it’s worth pausing to think about what heat does in the system. Temperature isn’t just a background condition; it’s an active force that can change pressures in subtle, sneaky ways. The thermal relief valve is the engineer’s answer to that reality: a guard that acts earlier, quietly, so the machine stays safer and more reliable.

Putting it all together

To recap in simple terms: the thermal relief valve in a hydraulic system is generally set to open at a lower pressure than the system relief valve. This arrangement provides a proactive response to thermal expansion, helping to maintain safe pressures and protect critical components. It’s a small difference in the valve settings, but it makes a meaningful difference in durability, reliability, and peace of mind on the shop floor or in the field.

If you’re curious to see this in action, look for hydraulic components from reputable brands and study the way their relief and thermal relief valves are paired in the same block or circuit. You’ll notice the same pattern across different machines: a little early protection, followed by a stronger safety valve for the big stuff. It’s not flashy, but it’s the kind of practical engineering that keeps systems running smoothly, day after day.

Final thought: temperature and pressure aren’t separate problems; they’re two sides of the same coin. By designing thermal relief valves to open at a lower pressure, engineers create a smarter, safer hydraulic system that respects both heat and force. And that makes life easier for everyone who relies on this technology—technicians, operators, and designers alike.