Why Hydraulic Systems Crash - Pressure - Part 2

Pressure relief valve – the most abused component in a hydraulic system:

When a production machine goes down supervisors expect maintenance mechanics to fix the machine, el pronto. Ironically, maintenance supervisors are acutely aware most mechanics do not have the training and experience to troubleshoot hydraulic systems, safely and effectively. It’s hardly a point one can argue. The hydraulic troubleshooting error rate (HTER) in the U.S. is a staggering 80%. That means 80% of the components removed from hydraulic systems by untrained mechanics, are in perfect working condition. 

You know how most company’s brag that safety is number one. That’s true if production folks have smiles on their faces. However, when those smiles turn upside down, most supervisors don’t hesitate to put untrained mechanics’ lives on the line to make the production folks happy. If you want to learn the truth about plant safety, talk to the “boots on the ground;” the untrained people whose lives are sacrificed on the altar of production.

Now to my point! Untrained mechanics usually cannot differentiate between pressure and flow. Most believe that pressure is synonymous with actuator speed. Accordingly, when they are tasked to troubleshoot a hydraulic system’s most common ailment; reduced actuator speed, they charge for the hydraulic system’s pressure relief valve. Since most maintenance mechanics don’t have hydraulic diagnostic instruments; besides they wouldn’t know what to do with them if they had them, they will, without hesitation, screw the pressure relief valves’ adjusting screw in as far as it will go. This is an extraordinarily hazardous practice, because it shifts the responsibility of “weakness” in a hydraulic system, to the next weakest link in the chain.

A pressure relief valve is purpose designed to be a hydraulic system's "weak link."

What untrained people don’t realize is that pressure and flow are completely unrelated when it comes to actuator speed. Pressure is only related to system safety; like a circuit breaker, and actuator force/torque output. What puts this practice at the top of the list of the most dangerous things a maintenance mechanic can do, is the fact that they are so ignorant, they don’t have the slightest fear that what their hand is touching, which only 1.5 feet (0.45 meters) away from their faces, could literally explode, and either severely injure, or kill them.

When the speed of an actuator decreases, reduced flow is always the culprit. It is physically impossible for reduced pressure to reduce actuator speed. There is only one way to find the problem, and that’s with a flow meter. The use of flowmeters is limited to testing only hydraulic pump and hydraulic motors. The only safe and effective way to test ALL other components in a hydraulic system i.e. pressure relief valves, directional control valves, check valves, cylinders, etc., is with the technique I invented – MicroLeak testing.

Case history – A mechanic installed a new hydraulic pump on a coal mine’s massive front-end loader. A step in the start-up procedure was to re-adjust the system’s main pressure relief to the recommended pressure – 3,200-PSI (221 bar). Shortly after the loader was put back into production, the repair shop received a call from the operator complaining that the loader failed to consistently lift the bucket, which was loaded with coal. A mechanic was dispatched to troubleshoot the problem. He decided to increase the bucket control system’s main pressure relief valve setting gradually, pausing each time he made the adjustment, to see if increasing the pressure would solve the problem. He was aware the pressure relief valve was set at 3,200-PSI (221 bar), when it left the repair shop. However, he did not use a pressure gauge while he was performing the field repair. Eventually, the operator reported that the loader had the power it needed to raise the load. The mechanic returned to the repair shop.

I could write many more case histories like this. Untrained mechanics routinely increase vital operating pressure to appease production. It’s a simple case of “forgive them for they know not what they do.” If safety “professionals,” and supervisors, were interested enough in hydraulic safety to attend just one hydraulic safety training program, they would see how untrained workers harvest unsafe situations throughout their respective plants, every day.

Sure, I’m cynical. I have been picking up the pieces for three decades, with no end in sight. I have been conducting hydraulic safety workshops in companies throughout the U.S.A. for over three decades. I can count on one hand how many safety “professionals” showed up for a free safety workshop that was held on the same property as their offices. Don’t tell me you’re a safety “professional;" show me you’re a safety professional.  

Here are the true facts about pressure, and pressure relief valves:

1. When design engineers determine a given hydraulic system’s maximum operating pressure, that parameter cannot, and must not, be changed by anyone. Any, and all, changes to a pressure relief valve’s setting must come, in writing, from the respective manufacturer in the form of a service bulletin.
2. Safety “professionals’ MUST move swiftly to ensure that ALL pressure settings are consistent with the respective system’s design parameters.
3. Safety “professionals” MUST, sooner than later, put hydraulic safety on their agendas. It will take literally ten minutes to help workers understand the critical role a pressure relief valve plays in a circuit. Suggestion: Simply hold a circuit breaker up in front of the group and ask them what it is. Then, ask them if they would let anyone take the 20-amp circuit breaker out of the panel at their homes, and replace it with a 30-amp circuit breaker. Then hold up a pressure relief valve, and tell them it’s one and the same component. I think they will get the message.
4. Make it a punishable offense that, post safety training, if anyone tampers with a pressure relief valve, the punishment is equal to that of tampering with a circuit breaker.
5. A pressure relief valve must be set in strict accordance with the respective manufacturer’s recommendations. All pressure relief valve adjustments must be performed by trained and knowledgable mechanics.
6. All the troubleshooting information about “testing” relief valves is erroneous (as usual). The experts opine that if a pressure relief valve can be set at the recommended pressure, the condition of the seat is perfect. Nothing could be further from the truth. The only way to safely and accurately test any type of pressure relief valve for cross-port leakage is with a MicroLeak test. Come to my workshop, and I will show you how MicroLeak testing can revolutionize the fluid power industry.
7. DO NOT follow the advice of anyone that tells you to disassemble a pressure relief and inspect it. First, are you qualified to disassemble, inspect, and assemble pressure relief valves? Second, what if you inadvertently assemble a pressure relief valve incorrectly? You will find out the hard; and possibility very painful, way when you re-install it, dead-head the actuator, and it blows up in your face!
8. Do not buy into the fact that pressure relief valve’s springs get “weak.” Do the valve springs in an engine get weak? I think the valve spring in an engine works a little harder than the spring in a pressure relief valve.
9. While we are talking about misguided troubleshooting recommendations, let's talk about cross-port relief valves. The experts opine that if you suspect one of the two pressure relief valves contained within a single body is leaking, switch the cartridges, and see if the problem switches sides. Look carefully at the illustration.

You can clearly see if one cartridge is leaking, oil will leak past the cartridge in both directions. Again, if you want "truth in troubleshooting," attend my workshops. The difference between myself an others, is I don't partake of the proverbial Kool-Aid.

10. The pressure in a hydraulic system decreases over time; it’s normal. The rate at which pressure declines is in direct relationship to the target cleanliness level you set; and consistently achieve, for your hydraulic oil. It is impossible a pressure relief valve to be the sole culprit when oil pressure declines. When pressure in a hydraulic system declines, every component in the system is “guilty until proven innocent.” Bear in mind, all the components in a hydraulic system “eat the same food.” If the oil is contaminated, all the components in the system will wear. Their wear rates may differ based on their respective duty cycles.

11. Watch out for the port reference numbers on pressure control valves. The manufacturers of cartridge valves didn’t think safety was important enough to warrant a standard port reference numbering/lettering system for all cartridge valves. For example, manufacturer “A” might choose the number “1” as the reference for the inlet port of their pressure relief valve. On the other hand, manufacturer “B,” might choose number “2” as the reference for the inlet port on their pressure relief valve. While cartridge valve manufacturers monkey around with numbers, the lives of assembly line workers and mechanics are at stake, because they are left to play the “guessing game” with when making critical connections to pressure relief valves. An error can cause an accident that can result in severe injury, death and/or substantial property damage.

12. Also note that you must never become complacent with the numbers. Take respectable cartridge manufacturer, Sun Hydraulics for example. The same body the company uses for their pressure relief valve, is also used for their pressure reducing valve. It’s easy to make an error, because the inlet port on the pressure relief valve is “1,” however, the inlet port for the pressure reducing valve is port “2.” I am by no means complaining about the discrepancy, I simply want to bring it to your attention.

13. Poorly drawn hydraulic schematics exacerbate the non-standardized port marking problem. Most hydraulic system designers don’t think port reference numbers are important enough to note on schematics. The electrical folks put us to shame. They care enough to use standardized colors to ensure that the correct wires are connected to the correct “ports.” Whenever possible, I urge engineers to note port markings on the schematics they compose.

I also recommend that they put an asterisk on the schematics to make people aware of which manufacturers numbers the drawing reflect. Regrettably, most of my recommendations fall on deaf ears.

Two decades ago I wrote a guide to help engineers draw meaningful schematics. Just as well my next meal isn't riding on the guide's sales!

14. Some pressure relief valve design engineers, in my opinion, overlooked a vital safety factor when designing a pressure relief valve's adjusting screw mechanism. They ignored vital stops that would have prevented adjusting screws from becoming lethal projectiles, when mechanics reset pressure relief valves, while the pump is running.

14. If a so-called hydraulic troubleshooting “expert” tells you that pressure relief valve tested for cross-port leakage by removing the drain line, and starting the pump, they are obviously trying to kill you. These are the same yahoos that will tell you to test a hydraulic pump or motor by removing the case drain line and sticking the open end in a bucket. Have you ever heard an electrical instructor tell a worker to use a “coat hanger, in instead of a volt meter?

By the way, if you ever follow a so-called expert’s advice i.e. use a bucket instead of a flow meter, and you receive second and third degree burns in the process, hire an attorney. I will be willing to serve as your consultant free of charge. One way or another we must stop machinery and equipment manufacturers from talking out of both sides of their mouths. They warn that discharging oil, under any circumstances, to atmosphere, can severely injure or kill you, and, in the next breath, they tell you to “test” components by discharging the oil to atmosphere.

Coming up - See why owners and operators of hydraulic systems are hemorrhaging cash. I will describe a situation that resulted in two-weeks of downtime, and four new valve assemblies. Even the two manufacturer's technicians that arrived on scene couldn't find the problem. Had they attended my workshop, it would have taken one mechanic, 1-hour to find the problem. As long as technical colleges continue to have machinery and equipment manufacturers poorly trained hydraulic "technicians" on their respective advisory committees, America 's critical shortage of bona fide hydraulic technicians will go on unabated.