Seven steps to hydraulic cleanliness

01 December, 2015

Within a hydraulic system, contamination breeds contamination. Initial damage, including corroded surfaces or eroded seals, sets off a chain reaction of wear meaning 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 as follows:

• Gear pump – 0.5-5µ

• Vane cell pump – 0.5-5µ

• Piston pump – 0.5-1µ

• Control valve – 5-25µ

• Servo valve – 5-8µ

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?

The ‘seven steps’ to achieving the correct levels of hydraulic cleanliness were originally introduced by the manufacturers of filters used within hydraulic systems as a means of selecting the correct filter media and depth, and can also be used to determine the filtering requirements of oil before it enters a hydraulic system.

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 150 bar 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 300+ bar 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 1000 hours or less (around 125 days of operation at 8 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 start 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 manufacturer 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.

Brammer’s specialist technical team of hydraulic and pneumatic experts is able to quickly determine ISO ratings for hydraulic components and systems, undertake root-cause analysis in the event of contamination and perform the necessary corrective actions to ensure assets as well as production schedules are protected.

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