Clean, contaminant-free oil within hydraulic systems is well known to be vital in prolonging the service life of components. However, what’s less commonly understood are the factors that determine the appropriate level of cleanliness for a given system.
Within a hydraulic system, contamination breeds contamination. Initial damage, including corroded surfaces or eroded seals, sets off a chain reaction of wear – gaps grow larger, leaks increase in size and metal-to-metal contact increases, further contaminating oil and leading to operating inefficiencies and control inaccuracies.
While contamination can occur as a result of wear to internal parts or through water or air ingress, a common cause is insufficient filtration or cleanliness of lubricants, meaning the contamination is being introduced to a previously acceptable system. As today’s systems are smaller and more powerful, so the demands of cleanliness become much higher and the size of particle which can disrupt and damage hydraulics becomes imperceptible to the human eye.
On average, the human limit of vision is a particle of 40µ – put into perspective, a grain of salt is typically 100µ while a hair is around 70µ. Typical component clearances, however, range from 25µ down to a microscopic 0.5µ.
Most new oil is supplied at a filtered level of 20µ, which for most hydraulic systems will be insufficient as any oversized particles can cause blockages and negatively affect pressure in the system.
ISO 4406:1999 is the standard by which the level of acceptable contamination by solid particles can be set. Also known as the ISO cleanliness code, it helps to set the maximum level of particles present per millilitre of oil, at three different sizes – 4µ, 6µ and 14µ. This maximum level then correlates to a range number, giving each component a three-number code (for example 16/14/11 for a servo control valve) – and once this code is known, sampling or live monitoring devices can be set to this level to alert engineers of any deviation from the allowed limits. Corrective action can then be taken in advance of catastrophic failure and costly unplanned downtime. But what factors need to be taken into account when determining the ISO code?
According to Terry Davis, national technical manager at Brammer, there are seven steps that help establish targets and, in turn, optimise the longevity of hydraulic systems.
These steps to achieving the correct levels of hydraulic cleanliness were originally introduced by the manufacturers of filters used within hydraulic systems. They provide a means of selecting the correct filter media and depth, and they can also be used to determine the filtering requirements of oil before it enters a hydraulic system. They are:
1 Establish component sensitivity: “The ISO code of a hydraulic system should always be based on the most sensitive component – that is, the one with the smallest clearance levels. Often a central reservoir will supply a number of systems and, in this case, either the whole system must be maintained at the cleanliness required of the most sensitive component, or an internal filter must be put in place to protect that component by cleaning fluid before it is reached. Piston pumps are among the most sensitive, while gear pumps and manual valves can be considered the least sensitive.”
2 Gauge operating pressure and duty cycles: “Normal operating pressure should be taken into account alongside its severity of change. A light duty cycle would see a system operating at its rated pressure or lower for continuous periods with minimal fluctuation – typically at 150bar or below. Other systems operate with medium pressure changes up to the rated pressure, while heavy or severe duty cycles would experience frequent changes from zero to full pressure and would operate at 300bar-plus with frequent, high-magnitude changes in pressure. The pressure rating will determine the material and strength of filter used, which in turn contributes to oil cleanliness.”
3 Look at the life expectancy of the equipment: “For equipment which is expected to last for more than 20,000 hours of operation, it is prudent to select a greater level of cleanliness than is perhaps required to derive the true value from the machinery.
“If equipment is only predicted to last around 1,000 hours or less (around 125 days of operation at eight hours a day), component failure is less costly and the minimum level of cleanliness is more acceptable.”
4 Evaluate the cost of component replacement: “Similar to the above, the most expensive assets should benefit from higher levels of protection. Large piston pumps or high-speed, low-torque motors are among the more costly components to replace, meaning failure due to oil contamination cannot be countenanced. However, as line-mounted valves or gear pumps come at a much lower cost, again the minimum cleanliness level can be set.”
5 Calculate the cost of downtime: “Realistically, the operational economic liability of downtime must come into play, too, for similar reasons to the above. Where production is 24/7, any downtime of equipment can be catastrophic and equally some non-production equipment may be critical especially in temperature-controlled environments. Conversely, equipment which would not heavily impact on production if taken out of action would have a much lower liability due to downtime being less costly. Seasonal production schedules, particularly in the FMCG sector, may also need to be accounted for as downtime is likely to be more costly in peak periods.”
6 Assess safety aspects: “Components critical to safety must use a more strict cleanliness rating than standard equipment to safeguard employees, contractors and site visitors. Similarly, any equipment which could endanger individuals in the event of a failure must employ a lower rating than would be required of equipment with no discernible safety risk.”
7 Look at the machine environment: “Operating conditions can also impact upon the cleanliness rating required. Cleanrooms, labs and high-care manufacturing facilities are, by their nature, less likely to pose a contamination risk to hydraulic fluids and are therefore allocated a low risk rating.
“Average risk is assigned to most general manufacturing facilities, while mills, food manufacturing facilities or anywhere likely to experience dust particles is classified as a poor hydraulic environment.
“Hostile environments, where the risk of contamination is likely to be very high, should also have their ISO code set at one or more values lower for each size (4µ, 6µ and 14µ). Other environmental factors that would lead to a lower ISO code include high temperatures or humidity, frequent cold starts or very high levels of vibration among other extreme operating conditions.”
Once an ISO cleanliness code has been established, ongoing monitoring must be put in place to ensure the rating is successfully maintained. This will allow the site to reap the benefits of minimised downtime, as well as a reduction in the costs associated with component repair, fluid replacement and disposal.
Today, real-time monitoring devices are available which, when set to the correct ISO rating, will display a green, amber or red light to alert engineers of dangerously high levels of particle contamination – ideal for production, or safety-critical equipment. Lab-based sampling should also be completed and documented regularly in the absence of real-time monitoring, and may be sufficient for higher ratings and less critical equipment.
Safety in the compressed air industry
It could be dangerous if you do not maintain your compressor regularly using a Written Scheme of Examination. Compressed air is often regarded as the fourth utility, yet, despite being vital in many components of manufacturing and industrial processes, its safe use can sometimes be overlooked.
Just like other items of industrial equipment, a compressor needs to be installed and maintained correctly to ensure it operates safely and efficiently. If a system isn’t properly and regularly maintained, it could end up posing a danger – and, in extreme cases, could even catch fire or explode.
“There are continuing concerns across the industry that, despite the risks, there are currently no formal accreditation schemes for designing, installing and maintaining compressed air systems. End-users could well be receiving poor advice and safety could be compromised,” Chris Dee, executive director of the British Compressed Air Society, explains. “Every compressed air system, virtually without exception, should have a Written Scheme of Examination in place, which the system should be regularly inspected in accordance with.”
Written Schemes of Examination are legal requirements under the Pressure Systems Safety Regulations 2000. The document contains a wide range of information, including the parts of the system that need to be examined, the nature of the examination required, the preparatory work needed and the maximum interval allowed between examinations.
“The Written Scheme of Examination has been in place for over 14 years and carries a potential fine if you are caught without one,” Dee continues. “However, the worry is that many businesses running compressed air systems either ignore this or are simply not aware of it, as there is no thorough policing.
“The British Compressed Air Society offers courses for those involved in the installation and maintenance of compressed air systems. However, these are voluntary, and so there is a real need to introduce recognised training programmes and an official accreditation scheme for service engineers working on compressed air systems, as well as for designers and installers that can work in support of the Written Scheme of Examination.”
Hydraulic hose awareness
More often than not, hydraulic hoses have a critical role to play in the efficient running of many types of plant. However, because of the range of hose available, it is important to bear in mind a number of key points to ensure that equipment is safe for operators and maintenance personnel, while also incurring minimal downtime.
“Hydraulic hose should always be clearly marked with information concerning the type/material, flow rating, pressure rating and size,” Chris Buxton, director and CEO of the British Fluid Power Association (BFPA), says. “Different makes or types of hose on OEM equipment may have different pressure ratings. If the flow and pressure for each hose is correct for the specific requirements asked of it, then all should be well, but it is highly advisable never to operate plant without having confirmation that the specifications of hose used are fit for each specific task in question.
“It is also inadvisable to mix different suppliers’ fittings for use in the same hydraulic hose application. Fittings have thread sizes that are often measured using a number of different formats, such as metric, BSP and NPT. Therefore if size and thread-form information is not written on the fitting, you should consult specialist personnel who are capable of determining the correct thread-form before a suitable fitting is installed.
“Always tighten a hydraulic fitting to the torsion level specified by the manufacturer or supplier. Too much torque can result in damage to the fitting’s thread, making it difficult to unscrew and potentially reducing its efficiency and lifespan. It is always inadvisable to mix fittings with different material specifications. Fittings made of different materials on the same hose application may cause corrosion to occur more easily, potentially resulting in leaks.
“Sourcing the right grade of hose and type of fitting can also help to avoid injury. A high-pressure hydraulic oil injection injury may not happen very often. However, when it does, the consequences can be very serious.”
The BFPA has established a number of training programmes aimed at raising the awareness levels of people who work with hydraulic hose at all levels. The BFPA has introduced the following courses: ‘Foundation Course in Working Safely with Hydraulic Hose and Connectors’; ‘Hose Assembly Skills Training Programme’; and ‘Hose Integrity, Inspection and Management’.
A number of reputable hose equipment and service providers are licensed to carry out these courses and they are available throughout the UK and Ireland.