How to manage hydraulic hose lifetime more efficiently and effectively
30 March 2016
Equipment downtime is a project or site manager’s worst headache, especially when it is unplanned. When it is due to a hydraulic hose failure, there could be additional complications and costs. In this article, Alexandra Benz of Eaton Hydraulics gives advice on the maintenance and management of hydraulic hose, often crucial elements in mission-critical equipment.
Hydraulic hose has a finite life, even when it is not actually being used. As with car tyres, the flexible rubber of the hose can deteriorate over time; sometimes this could be in as little as four years. This could be due to ultra-violet light from the sun, thermal stress from weather extremes, or just from mechanical stress.
Anticipating when a hydraulic hose needs to be replaced in an active piece of equipment is complex as there are several life-reducing mechanisms, even where a preventative maintenance program is rigorously enforced. These are:
* Heat – Hydraulic fluid gets hot. This can be due to incorrectly set pressure regulators, failure of the oil cooling system, or extreme pressure drops. The latter may be caused when components are not sized properly, filters become clogged, and hoses are kinked or subjected to tight bends, all resulting in inducing heat in the system.
* Mechanical stress – Pulling, kinking, twisting, crushing, and by far the worst offender, abrasion, can all cause a hose’s life expectancy to be reduced significantly. Abrasion may be caused by contact with other hoses or surrounding surfaces and could be the result of a replacement hose of the incorrect length. Multiple axis bending could also twist and stress a hose’s wire reinforcement. In the field, a 5-degree twist may reduce service life by as much as 70%; a 7-degree twist may be significantly higher.
* Harsh environments – Equipment can be required to operate 24 hours a day in extreme hot and/or cold temperatures, as well as hazardous, highly abrasive conditions. In such an operating environment, corrosion of hose material and fittings are key factors for consideration. The use of machinery in highly abrasive environments, such as coal mines, for example, may require the substitution of more rugged, abrasion-resistant high pressure hose for optimum life.
* Duty Cycle – Frequent and extreme pressure changes, such as those encountered on lift trucks and cold metal forming equipment, can exacerbate hose fatigue. Again this may require the substitution of more rugged high pressure hose.
External wear and tear can be minimised, however. Adding abrasion protection to the hose is one method. This can be in the form of low friction nylon sleeves, and metal or plastic spring guards. Metal spring guards could also help reduce the stress on the hose caused by tight radii, twisting and vibration. Plastic spring guards can also help protect the hose from environmental factors, such as UV from sunlight, water, pollutants from fuel and corrosive engine exhaust gases. Another method is to use hose with abrasion resistant cover materials, particularly if the hose is installed in such a way that it comes into permanent contact with other hose. It should be noted however that this recommended method doesn’t prevent damage from all external factors such as sharp items.
The downsides of using hose sleeves and guards are that there are additional costs involved, both material cost and assembly cost, as they are usually installed during the equipment manufacturing process or when the hose is being replaced. The use of hose sleeves and guards also restricts the use of visual inspection, during daily, or periodic preventative maintenance.
Incorrect maintenance of equipment may cause additional problems. Often time pressures to get a machine back into operation may result in a wrongly sized or unsuitable replacement hose being fitted. The industry acronym ‘STAMPED’, which stands for Size, Temperature, Application, Material (to be conveyed), Pressure, Ends (or couplings) and Delivery, is often followed as this relates to the seven major areas of consideration for hose selection.
The use of adapters to allow a different sized hose from the manufacturer’s recommendation should also be avoided, as this could impact negatively on predicted service life and the equipment manufacturer’s warranty. An example could be when substituting with a smaller diameter part, which could result in premature failure due to the higher coupling pressure and resulting heat being generated.
The cost of a hose failure frequently adds up to more than paying for a service technician and the purchase of replacement parts, particularly in remote locations. Schedule delays at a construction site or production downtime in the mining and oil & gas industries could become extremely expensive. The rupture of a high-pressure hydraulic line may affect workplace health and safety as hydraulic fluid is considered an environmentally hazardous material.
Traditionally, industry has used two tried and tested methods to reduce the risk of an unanticipated hose failure:
* Visual inspection - A routine of visual inspections is recommended, not just to identify wear and tear, but also any damage that has occurred during operation. As this is a manual process, human error may always be possible. Indications of a potential failure are limited to wetness and leaks, cracked or worn rubber and abnormal stiffness in the hoses. To avoid serious injury, hoses should not be inspected by running a hand over them when pressurised or when hot. The use of hose guards and sleeving, however, can hide potential issues.
* Preventative maintenance – This is practised widely. The potential cost of hose failure in terms of lost production, environmental impacts and possible injury to operators and others, means that many companies are now choosing to replace their hose on a time-based schedule, while equipment is already out of service as part of a planned preventive maintenance schedule, to prevent critical downtime and potential additional expense.
Concerns for hose failure in the field and its expensive consequences have led to many companies adopting a time-based replacement program, which can result in hose being removed from service prematurely. The routine replacement of undamaged hydraulic hose, however, may waste both time and money. In-house laboratory tests indicate that the vast majority of replaced hoses still retain 50% or more of their useful life.
Technology has recently been developed that can help monitor and predict when a hydraulic hose is approaching the end of its life, reducing the risk of a field failure. The basis of the predictions is decades of real-life and test data accumulated by hose manufacturers under a wide variety of conditions. This data has then been used to detect the changes in the characteristics of a hydraulic hose prior to the occurrence of a failure.
For example, a research project carried out by power management company Eaton in collaboration with Purdue University sought to identify measurable, structural phenomena associated with hose deterioration over time, and develop the required technology to monitor them accurately.
This led to an intelligent hydraulic hose condition monitoring system that detects failure-related events within a hose and provides advance notification that the product is approaching the end of its useful life.
Regardless of the technology used, it is advisable to always regularly schedule thorough inspections to check for damage from external conditions. This, when combined with a proactive preventive maintenance program and using the correct replacement parts, can promote operator safety and reduce the risk of equipment downtime. For further peace of mind, real time monitoring of hose can help allay concerns about idled equipment and environmental clean-ups.