Ultrasound for Reliable Hydraulic Systems

Ultrasound testing is a powerful solution for assessing the condition of our assets. The technology hears defects in the ultrasonic range and transfers these inaudible faults to low frequency sounds we can hear. Ultrasound signals from failing assets are measured, trended, and analyzed so we can execute an action plan that minimizes the impact.  

Reliable plants need FIT assets.  

Most defects that threaten uptime produce Friction, Impacting, and Turbulence (FIT). These conditions exist silently in the ultrasound region. Find one – or any combination – of these and you now have data to monitor the asset condition at an advanced level.  

Why is Ultrasound Useful? 

Ultrasound Owns the Apex of the P-F Curve. Newly formed defects present themselves ultrasonically first, and only become audible once they’ve advanced closer to the moment of failure. Ultrasound testing is considered one of the earliest indicators that an asset will soon be in need of attention.  

Ultrasound instruments are particularly effective in high noise situations. Inspections can be carried out any time of day, regardless of background noise.  

Ultrasound is introverted. It prefers to remain isolated to its source. This unique characteristic makes ultrasound extremely effective for distinguishing specific anomalies occurring in close proximity to each other; like finding that one needle, in a haystack full of needles.  


Ultrasound’s versatility is unlike any other condition monitoring technology. SDT identified eight application pillars and within each pillar there are virtually hundreds of ways to increase safety, eliminate unplanned downtime, and reduce energy waste. Ultrasound is the first line of defense for industries seeking higher profits and a sustainable future.  

Ultrasound for Reliable Hydraulic Systems 

What is the asset and what function does it perform? 

Hydraulic systems represent another way industry converts electricity to mechanical energy. A pump, normally powered by an electric motor, moves hydraulic fluid through a system. The system is typically made up of a reservoir to hold the hydraulic fluid, high pressure lines to carry the fluid, control valves that ensure the fluid is sent where needed, relief valves for safety, and cylinders to produce mechanical energy.  

Some reliability minds seem obsessed with machines that rotate, while failure modes of non-rotating asset components seldom receive the routine surveillance they need. The pump and motor receive attention while the balance of the system is run-to-failure. Ultrasound testing could change this lackadaisical approach; especially considering its versatility, ease of use, and applicability to most hydraulic defects.  

What are the most common failure modes? 

Ultrasound is particularly useful for detecting early stages of defects and work wonderfully in environments characterized as “high-noise”. Symptoms that manifest themselves through friction, impacting, and turbulence (FIT) are quickly revealed with high-quality ultrasound technology in the hands of a competent inspector.  

Failure modes in hydraulics systems include external leakage, internal leakage, by-passing, and blockages. They are detected with ultrasound because they produce Friction, Impacts, and Turbulence. 

  • Friction 
    • Bearings in the motor and pump 
    • Rubbing of the head against the inside of the barrel 
    • Rubbing of the rod against the gland seal 
  • Impacting 
    • Cavitation on the vacuum (inlet) side of the pump 
    • Cavitation on the discharge (pressure) side of the pump 
    • Bearing and coupling failure 
  • Turbulence 
    • Passing fluid past the head seal or wiper 
    • Passing fluid past the valve body of an isolator valve 
    • Bursting of small bubbles on any internal leak. 

External leaks can often be found by visual inspection. Some external leaks, like a ruptured hose or fitting, are easy to spot. Simply follow the trail of spilled oil. Others are not so obvious. These slow seeping leaks weaken system function gradually until failure. It is uncommon for them to produce any usable ultrasound signal for inspectors to detect. The best one can hope for is to trace the pooling of oil back to its source.  

Internal leakages are surprisingly more detectable with ultrasound than external. That’s actually opposite thinking when compared to compressed air and vacuum leaks. An internal leak in a hydraulic cylinder slowly passes by the seal on the piston. By carefully positioning a magnetic mounted ultrasound sensor near the piston head, one can hear bubbles or even streams of oil (depends on severity of leak) pushing past. The best position for the contact probe is downstream from the fluid flow.  

Since these assets are run-to-failure, it becomes difficult to predict just when they may fail. Three symptoms that reveal the presence of an internal leak are  

  1. Lack of power 
  2. Slow response 
  3. Stalling when under load 

These conditions may represent a seal leak, but they could be revealing much worse. Stroke and pressure tests are intrusive and disrupt the process. Alternatively, listen with an ultrasound instrument equipped with a highly sensitive contact probe tuned at 38kHz. The instrument should also have high clarity headphones with separately adjustable volume amplification.  

Testing a hydraulic ram for an internal leak requires patience. Place the contact probe at several positions along the barrel length. If the leak is minor and the symptoms reported are moderate, expect to hear bubbles popping when the probe is placed adjacent to the leak. A rushing sound means the oil is pouring past the cylinder head. This is a more severe issue. The cause might be contaminated oil that damaged the seals. Worse yet, a bent rod or barrel may result in the head being cocked inside the barrel.  

Testing Hydraulics on a Timber Grapple Koch Cellulose   

A timber grapple is a large, hydraulically operated hand that can pick up large quantities of logs from a collection pile and deliver them to a processing area. In the case of Koch Cellulose that processing area will produce pulp for the next stage of the paper process.   

Koch Cellulose uses grapples to transport timber from the yard to the mill. Any malfunctions pose threats to both safety and production.   

Purpose of Ultrasonic Test:  Determine location of fluid bypass or internal leakage of hydraulic components. Reduce equipment downtime by limiting disassembly and repair to only defective components.   

Theory: Properly holding hydraulic cylinder stop valves will produce no sound as heard by detection equipment. Defective valves will produce a continual “hissing noise” as the fluid leaks past the valve head or rod.   

Test Procedure: Prior to beginning the ultrasound inspection ensure all safety procedures including equipment lockouts are in place and the area is free from anyone not directly associated with the test.   

  1. Position grapple bucket in the working platform so that all four stop valves are within safe reach for the ultrasound inspector. The inspector should wear fall arrest safety harnesses as necessary, as well as any other prescribed personal protective equipment.  
  2. Using the electric hydraulic pump pressurizing system, instruct the operator to close the grapple.  
  3. Ensure the ultrasonic detector is equipped with a magnetic mounted resonant contact probe (RS2T). The inspection requires headphones as well as digital RMS value in dBµV.  
  4. With the RS2T contact sensor positioned on the stop valve body, instruct the operator to pressurize the system once again by continuously forcing the grapple into the closed position. Observe the amplitude change and dBµV RMS fluctuations on the display.  
  5. While the operator charges the system with hydraulic pressure inspectors must watch for a change in RMS of around 20 dBµV. This indicates a properly seating, non-leaking stop valve. There should also be a clear difference in sound through the headphones. A continual hissing sound in combination with a constant amplitude value means the stop valve is passing.  
  6. Continue the test for each hydraulic cylinder on the grapple. Most are equipped with four cylinders.  

Test Validation: One of the four cylinders was found to have no fluctuation in amplitude value or signal. It was declared defective but before it could be scheduled for repair the ultrasound test required validation.   

  1. All four stop valves were manually tested by removing the return oil lines and pressurizing the hydraulic cylinder and stop valves. With the system operating at approximately eighteen hundred PSI, a substantial amount of oil was observed leaking from the suspect valve. No oil leakage was observed from the other three valves.
  2. Final validation was confirmed by disassembling the suspect stop valve and performing a visual inspection. This revealed a clipped O-ring around the valve body. The stop valve was replaced with a rebuilt valve from stores but it too leaked. It was found to also have a clipped O-ring which led to a re-evaluation of the installation procedure. It was discovered that the mechanic performing the installation was mishandling the stop valve. During assembly he was clipping the O-ring without realizing it.   

Grapples can also be found in other forest products operations such as saw mills. The timber grapple is attached to an overhead crane which moves laterally on a track. Four hydraulic rams work in concert with long steel fingers to empower the grapple with its prehensile ability.  

Should there be any compromise in the seals of the hydraulic rams during timber transfer the result is dropped timber. Consequently the grapple must be shut down to both remove the spilled wood and isolate the cause of failure. Production stoppage can reach several hours if the problem cannot be found quickly. It is here that ultrasound wins time for production.  


Valves play a critical role in hydraulic systems. Simple or complex, their job is to direct the flow of fluid to where it’s needed. There are many opportunities for ultrasound to detect problems in hydraulic valves. 

A two-way valve controls fluid flow through two paths. One path allows mechanical movement to extend the cylinder and the other contracts. When that valve is either passing or blocked the system loses functional ability.  

 An isolation valve stops the flow of hydraulic fluid in a given direction. Isolators are usually used for maintenance on the system or for safety reasons. They may be used to temporarily stop the system while ancillary equipment is connected. A functional isolator should have no downstream flow. Place a contact sensor above, on, and below the direction of flow. Listen for turbulence either in the form of a steady stream or bursting bubbles on the downstream side. The differences may be slight so comparing dBµV values at all three positions should prove enlightening. 

Excessive vacuum conditions on the inlet side of the hydraulic pump can cause the formation of vapor bubbles in the hydraulic fluid. As the oil moves from the inlet to the discharge (pressure) side the vapor bubbles collapse or cavitate. The condition will deteriorate pump components over time but its avoidable. Consider training data collectors to listen for the bursting sound of cavitation. They already make frequent visits to the pump to check bearings.  

What Happens if we do Nothing? 

Many opportunities exist to advance the reliability of hydraulic systems with ultrasound. Just because they don’t rotate does not mean they cannot contribute to unscheduled downtime. Reliability teams need to elevate the importance of hydraulic systems in their condition monitoring regimens. Today’s technology is simple and effective. Run-to-failure on hydraulic systems powering critical processes is inexcusable.  

It’s really a matter of understanding what the failure modes are and how an ultrasound inspector can be trained to spot them.  

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