Unloading Valve Overheating

7 min read

A design engineer at a mining equipment maker called me about a prototype machine. The oil around the unloading valve was overheating so fast that within five minutes, you couldn’t touch it. Now and then, the fan motor stopped as well.

The valve wasn’t faulty, but there was a problem with where the dump oil went. On a standard unloading valve, the tank port also drains the spring chamber. Send that oil to a working load, like a fan drive, and the load’s pressure adds straight onto the valve setting. If you want to use the dump flow for a second circuit, you need a priority unloading valve with a separate drain port.

For how pressure control valves work in general, start with [LINK: topic 10].

How does an unloading valve keep accumulators charged?

This customer has bought unloading valve systems from us since the 1990s. On their machines, one valve looks after three accumulators. The pump charges them through a check valve. Once they reach the setting, the valve dumps the pump to the tank at very low pressure, about 1 to 3.5 bar. The check valve keeps the accumulators charged. (More on check valves in [LINK: topic 29].)

When any accumulator loses pressure, the sensing line picks it up. The valve brings the pump back on load, charges the accumulators again, and dumps the pump to the tank again. The pump spends most of its life running at almost no pressure, so very little heat goes into the oil. If you want to see how many ways accumulator circuits get used, Power & Motion has a good walk-through of common accumulator circuits.

A valve that worked for 36 years

In the 1990s, the only unloading valve we had that could handle 250 lpm was our priority unloading valve, the PUL 250 [LINK: product page – PUL 250 priority unloading valve]. Their circuit never needed the priority function. It was simply the only valve big enough, so that is what they got.

It has worked on their machines for about 36 years. In all that time, we have not had a single complaint.

What changed on the new circuit

On the new machine, the job was different. The engineer wanted to charge a brake circuit, and once the brakes were charged, run a fan drive with the oil.

He took our UL C65, a standard 150 lpm unloading valve, from the catalogue on our website. To him, it was the same kind of valve they had always used. Charge the brakes, unload, and the unloaded oil is free to go wherever you need it. So he piped the tank port to the fan drive.

That is a reasonable assumption. On paper, an unloading valve is an unloading valve. The difference between a standard valve and a priority valve sits inside the body, where the spring chamber drains. Nothing in a part number or a flow rating tells you that. And the valve on their older machines had a feature nobody there knew it had.

What causes the unloading valve to overheat when the tank port feeds a fan?

In a standard unloading valve, the spring chamber drains to the tank port. That is fine when the tank port really goes to the tank. The spring chamber sees a bar or two, and the setting stays where we set it.

Put a fan drive on that port, and the spring chamber sees the fan’s working pressure. That pressure pushes along with the spring. Now the setting is the spring plus whatever the fan needs. The more the fan loads up, the higher the setting goes. I’d call it an infinite loop, with the valve and the fan fighting each other.

Exactly what the valve does after that, we haven’t measured on this machine. We don’t know if there is a separate relief valve in the fan line. What we do know is that the setting no longer means what it says, and the unloading valve was overheating within minutes. For other valve-related causes of hot oil, see [LINK: topic 31].

Priority unloading valve: why the drain port matters

A priority unloading valve has four ports. One is the sensing line, which tells the valve whether the system needs recharging. One is the pump inlet. What a standard valve does through a single tank port, the priority valve splits into two. One port feeds the secondary circuit, in this case, the fan drive. The other is a separate drain.

The drain connects the spring chamber straight to the tank. It doesn’t matter what pressure the fan drive is running at. The spring chamber never sees it, so the setting stays where it was set. The brakes get charged first, and the fan gets the oil after that.

The same idea, a spring chamber picking up pressure it shouldn’t, shows up in counterbalance valves too. See [LINK: topic 22].

Mining adds its own rules

Here, the customer hit two more problems. Mining sites don’t allow aluminium valve bodies, and we have no 250 lpm priority unloading valve in stock with a non-aluminium body. Our smaller unloading valves also carry an integrated check valve. The PUL 250 doesn’t.

So for now, the PUL 250 goes on with an external check valve and some extra piping. If the quantities grow, we will make them a custom housing with the check valve built in.

[RESULT: what happened once the PUL 250 was fitted. Oil temperature, fan behaviour, any numbers.]

What I would tell a young engineer

Always check with the valve maker, even if you think you know.

This engineer wasn’t careless. He had a valve that had worked on his company’s machines for 36 years, and a catalogue for what looked like the same valve. What he couldn’t see was which port drains the spring chamber. A two-minute call would have told him, and saved a prototype from unloading valve overheating.

When you write the spec, say where the dump oil goes, not just the flow and pressure. “Unloaded oil goes to a fan drive” changes the valve we would pick. More on writing a spec, a supplier can quote right the first time in [LINK: topic 40].

FAQ

Why is my unloading valve overheating?
First, check where the dump oil goes. If the tank port feeds another circuit, that circuit’s pressure may be acting on the spring chamber and raising the setting. A valve that unloads properly to the tank should run cool, since the pump sits at a few bar most of the time.

Can I use the unloaded oil from an unloading valve to run another circuit?
Yes, but not through a standard valve’s tank port. Use a priority unloading valve, which has a secondary port and a separate drain for the spring chamber.

What is the difference between an unloading valve and a priority unloading valve?
A standard unloading valve has one tank port that also drains the spring chamber. A priority unloading valve splits this into a secondary port for another circuit and a separate drain to the tank, so secondary pressure never changes the setting.

Can a priority unloading valve do the job of a standard one?
It did on our customers’ machines for 36 years. Ask your valve maker how to pipe the secondary and drain ports for your circuit.

Using unloaded oil for a second circuit?

If your circuit charges accumulators and you want the dump flow to do some work, send us the circuit and tell us where every port goes through our technical enquiry form. We will tell you which valve we would fit, and why.

Raj Dabholkar is the Technical Director of Tucson Hydrocontrols, a hydraulic cartridge valve maker in Umbergaon, Gujarat, founded in 1987. A jet propulsion engineer by degree and a hydraulic design engineer by trade, he has spent over 20 years designing valves and solving field problems with OEMs in construction, defense, earthmoving, power generation, agricultural, and industrial equipment.