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I was once asked by a technician that when the hydraulic system heats up and temperature goes past a certain acceptable working fluid temperature limit, what should be the first part or component of the hydraulic system to be checked.
The relief valve is the first component of the hydraulic system to be checked when the temperature gets past the maximum acceptable working point. &#;Really, how do you explain this?&#; He further quizzed.
A relief valve essentially serves as a pressure limiter in the system and so, if the system&#;s pressure goes past a certain threshold, the relief valve kicks in to protect the system from self-destruction by rerouting the working fluid back to the oil reservoir. The way positive displacement pumps operate is that they will keep on pumping out or creating flow (remember a positive displacement pump does not create pressure but flow) unless there is a deliberate system is place to regulate it. If the system is devoid of a pressure limiter (Relief valve, Load-sensing system, etc) the weakest component or part in the system will fail thanks to over-pressure. In short, positive displacement pumps are not intelligent enough to know that if they continue pumping and oozing out flow into the system unmonitored and against a dead end, the system pressure may rise to a level where failure would be the ultimate product and so, to fend off any failures due to over pressure, pressure limiters are inherently designed into the hydraulic system.
As alluded to, a relief valve&#;s prime function is to ensure that the system&#;s pressure does not go past a certain point, commonly referred to as maximum working pressure, by diverting excess working fluid back to the tank. The relief valve is normally set at about 10% above the system working pressure. As I was about to delve further into explaining the working principles governing relief valves, he interrupted me and asked, &#;sir, you have not explained why the valve heats up, where does the heat come from?&#; I said well, bid you time sooner than later you will be enlightened. Without much ado, I went straight into explaining where and how the heat comes about as follows:
Whenever a valve opens due to high pressure encountered in the system, it is accompanied by a pressure drop and this pressure drop is what manifests as heat resulting in the system&#;s temperature rise as the heat is dissipated and carried along partly by the valve body itself and partly by the working fluid. So, when you touch valve body it feels hot. Pressure is a form of energy which can neither be created nor destroyed but can be converted from one form to the other. In this case, this excess pressure, a form of energy, is what appears as heat.
The introduction of the term &#;Pressure Drop&#; seems to have pushed the technician into further confusion. He then asked me if I could paraphrase and make it simpler for him to understand. I asked him to look at it from this perceptive; when the valve first cracks open, the opening is so small that as the fluid particles (molecules) attempt to pass through it to cycle back to the oil reservoir, chaos ensues; the particles struggle, pull each other from side to side, toss each other about and rub against each other and against the walls of the valve in a bid to go past the minute valve opening. Consequently, part of the energy the particles had when they arrived at the relief valve entrance gets transformed into another form due to this chaos. Then after I dropped this explanation, I asked him to tell me where he thinks the heat comes from. Without hesitation he exclaimed, &#;eureka! The heat comes from the frictional forces resulting from the particles rubbing against each other and against the walls of the valve as they attempt to pass through the valve&#;
Before I could conclude another technician popped up and asked, &#;Sir, are we able to tell what might have induced shaft failure by merely analyzing the broken surfaces, what is this animal called fatigue and its associated phenomenal terminologies; striations, bench marks, river marks?&#; This is the next matter we shall address in a few weeks&#; time.
Pressure relief valves are used to prevent over pressurization in the hydraulic circuit. They are located close to the pumps and reservoir tank or close to the equipment to safeguard from over pressurization.
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This is an illustration of a cartridge style "pilot" operated pressure relief valve. (There are also "poppet" operated relief valves, which have no pilot function.) Pilot operated relief valves, vs. poppet style relief valves, can be sized to handle more flow than a poppet style valve and have better response and flow characteristics. This type of valve has a spool that is controlled by a variable force spring. Utilization of spring tension is the most common means of controlling pressure from pumps, pressure relief valves, and pressure reducing valves. The spring&#;s force constant along with where the spring is set determine at what pressure the valve will dump to the port 2.
Turning the pilot spring screw clockwise increases the force exerted on the pilot check ball. This will increase the pressure at which the valve opens up. As pressure in port 1 increases, oil flows up the main spool orifice and lifts the pilot check ball. When the check ball opens, a small amount of oil is exhausted to port 2. This creates a pressure drop across the main spool orifice. This pressure drop lifts the main spool and opens the main path from port 1 to port 2. Pressure in port 1 will remain at the valve pressure setting. Dumping continues as long as there is flow past the pilot check ball.
Stage 1: Pressure is below valve setting
In stage 1, the pressure is below the valve setting. There is small leakage around the main spool to port 2.
Stage 2: Pressure in line 1 increases
In stage 2, as pressure in line 1 increases above the valve setting, the pilot check valve opens and some oil flows to port 2. This flow creates a pressure drop across main spool orifice.
Stage 3: Pressure drop opens main spool
In stage 3, the pressure drop across the main spool orifice opens the main spool, dumping main port 1 to main port 2. When port 1 pressure decreases, the pilot ball check valve will reseat itself, eliminating flow through the pilot check. With no flow, the main spool spring will reseat the main spool and stop flow to port 2.
Pressure relief valves are adjusted to stay closed under normal operating conditions. They are not an efficient way to set system pressures as heat and noise are generated when the valve is open (recall that temperature rises when no useful work is being done). If a relief valve and downstream oil lines are hot, the valve is open and dumping oil. The relief valves are the first things to check if your oil is running hot. Verify system pressures are on target as well as the setting of the valve.
For more information on maintaining your hydraulic system, contact your Valmet representative.
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