Proper hydraulic safety begins with knowledge, maintenance
Everyone who has direct, or indirect, contact with fluid power systems should take the proper steps to prevent injury to people and damage to equipment and property from fluid leakage.
It is impossible to overemphasize the importance of implementing and adhering to safe practices when working with pressurized fluids and components in hydraulic and pneumatic systems. A fluid system failure can result in equipment damage, production losses, personal injury, or even death.
Everyone who has direct, or indirect, contact with fluid power systems should take the proper steps to prevent injury to people and damage to equipment and property from fluid leakage and component failures in pressurized fluid systems.
There are three major areas for maintaining safe conditions and efficient operations with regard to fluid systems:
• Implementing a hose and component selection process for safe and efficient fluid-systems design and use.
• Following proper hose assembly and routing guidelines to help prevent premature failures and promote hose safety.
• Avoiding injection injuries and understanding the importance of timely treatment should an injury occur.
Hose and component selection
Implementing a hose and fluid connector selection process enhances system performance and reliability, as well as facilitated safe and efficient fluid systems design and use. This process helps ensure that the components will meet the application's requirements. Selection factors to consider for hose and fluid connectors include:
• Electrical conductivity of components. Some applications require that hose be nonconductive to prevent electrical current flow. Other applications require that the hose, the fitting, and the interface be sufficiently conductive to drain off static electricity.
• The right hose size is necessary to ensure proper flow and velocity to meet the application's hose-rate requirements. Very high or low ambient and system temperatures can have severe adverse affects on the hose cover and reinforcement materials, resulting in reduced service life.
• The application in which the hose and components will be used impacts the selection process. Equipment, environment, mechanical loads, hose routing, fitting sealing methods, proximity to heat sources (such as exhaust systems) and the potential for hose abrasion are examples of factors to be considered.
• Any specified hose must be compatible with oils, chemicals and other media conveyed in the application, as well as with the operating environment. The inner tube of the hose, and its cover, hose fittings and O-rings must be compatible.
• Pressure must be identified both in terms of working pressure and surge pressures and spikes. The published maximum recommended working pressure of the hose assembly must be equal to or greater than the maximum system pressure. Surge pressures or peak transient pressures in the system must also be below the published maximum working pressure for the hose. It is a common assumption that if a hose has a maximum working pressure of 3,000 psi and a design safety factor of 4:1, the hose can be used in applications of 4,000 psi because it is within the safety factor. Making these assumptions can be dead wrong - literally. Published hose-burst pressure ratings are for manufacturing test purposes and do not indicate that the product can be used in applications with pressures above the published maximum recommended working pressure.
Operating conditions such duty cycle and impulse must be considered and evaluated.
Duty cycle identifies how frequently different functions occur at specified times in the application's power circuits and impulse identifies extreme, dynamic pressure fluctuations. Accurately understanding and measuring duty cycle and impulse are critical when specifying fluid connector components.
A high impulse, high-duty cycle application may require the use of a spiral-wire reinforced hose instead of a braid reinforcement hose, even though both meet all other application criteria. Short, rapid pressure impulses accelerate hose deterioration and reduce service life. In applications with similar conditions, a spiral-wire hose should be considered.
Hose assemblies and fittings can, for many reasons, fail without warning. Improper selection, installation, and use of hose, tubing, fittings or related accessories are common reasons for failure - which can cause equipment damage, personal injury or death. Aging, wear, and lack of scheduled maintenance have the potential to cause failure as well.
Hose systems and equipment should be designed in a fail-safe mode, so that failure of the hose or any of the fittings and components will not endanger people or damage property and equipment. Because there are so many operating conditions and unique requirements for each hydraulic hose application, hose specification is ultimately the user's responsibility.
Assembly and installation
Because installation and assembly procedures vary for different hose styles, sizes, materials and manufacturers, users should follow some basic and general guidelines, listed below, that are often overlooked during assembly and installation. Before beginning the assembly (or hose fabrication) process, it is essential to inspect the components to make sure they are correct for the application. The parts should be clean and examined for visible damage or defects such as obstructions, blisters, kinks, cracks or cuts.
During installation, follow the stated bend radius specifications from the hose manufacturer. It is imperative that if a hose is kinked during installation, it must be discarded. Care must be taken not to twist hose during installation. Hydraulic hose is designed to bend and move - but not twist. Twisting hose will greatly reduce its operational life. When installation is complete, all air entrapment must be eliminated from the system before the system is pressurized to maximum system pressure for system status check. Personnel must stay out of potential hazardous areas while testing and using.
Hose and fitting maintenance
Proper selection and installation are critical to extend the hose and component service life. However, without an ongoing maintenance program, hose life may be significantly reduced. Hoses can and do fail, in many cases without warning. Inspection and replacement of hose and components is usually determined by the severity of the application, risk potential from a possible hose failure, and a history of hose performance in similar applications.
A typical preventative maintenance program includes visual inspection of hoses, fittings and assemblies; periodic functional tests; and scheduled component replacement intervals. Upon a visual inspection, hoses showing any of the following conditions require immediate shut down and replacement of the hose assembly:
• Fitting slippage on the hose
• Damaged, cracked, cut or abraded cover (any reinforcement exposed)
• Hard, stiff, heat cracked, or charred hose
• Cracked, damaged, or badly corroded fittings
• Leaks at fitting or in hose
• Kinked, crushed, flattened or twisted hose
• Blistered, soft, degraded, or loose cover
When working with hydraulic hose, fittings and assemblies - whether during installation, preventive maintenance, or scheduled component replacement, exercise extreme caution. Fluids under high pressure can be dangerous and potentially lethal. Make sure that system pressure is relieved so that hoses and components may be examined safely.
When hoses fail, tiny holes in the hose, commonly known as pinholes, can eject small, dangerously powerful, yet hard to see streams of hydraulic fluid. Often, the stream or spray of hydraulic fluid is not visible to the user. Under no circumstance should the user attempt to locate the leak by "feeling" with their hands, arms or any other part of the body.
It only takes 100 psi to penetrate the surface of the skin and fluid escaping through a pinhole leak in a hydraulic system can be in excess of 600 feet per second. Penetration can occur in distances of up to four inches between the fluid source and the skin.
Consider the safety of the tools used for the job as well. A common tool found in maintenance garages, manufacturing facilities and home workshops is the grease gun. Hand grease guns can generate pressures in excess of 7,000 psi and can inflict life-threatening injuries to operators. The grease gun's flex-hose is subjected to constant abuse, aging and wear from crushing, kinking, and twisting of the hose. Regular and proactive replacement of the hoses on grease gun(s) is a simple step people can take in preventing injection injuries.
Injection injuries come from many sources. Hydraulic fluid, grease, paint and paint thinners can cause injection injuries with devastating outcomes. Very often these injuries seem quite minimal to unsuspecting examiners. Even though most patients report only a minor stinging sensation or no pain at all, high-pressure fluids that penetrate the skin can cause severe tissue damage and possibly loss of limb. Even seemingly minor hydraulic fluid injection injuries must be treated immediately. Unfortunately, many patients delay seeking treatment until symptoms develop.
Obtaining proper treatment from a medical professional familiar with treating injection injuries is critical. Injection injuries are often misdiagnosed by health care providers in emergency rooms, plant first aid stations, and triage centers. Because an injection injury appears benign on the surface of the skin, most injured parties and responders often brush them off as minor. However, the real damage is happening under the surface of the skin. An injection injury is a surgical emergency and should be treated with the highest priority.
Prompt medical intervention is essential to saving the injured party's limb and even life. Within an hour or two after an injection injury, swelling may begin. Conservative therapy such as warm soaks or antibiotics following the injury - rather than prompt surgical consultation - can be catastrophic and eventually lead to tissue loss. After four to six hours, intense throbbing pain that is unresponsive to pain medication and only manageable by nerve blocks may set in. Failure to pursue proper care may lead to amputation of affected parts. In fact, amputation is most likely required if the contaminated tissue is not surgically removed within 10 hours of the injection.
Unfortunately, many people who work with hydraulic systems - including maintenance personnel and machine operators - never receive the proper hydraulic safety training. It is of paramount importance to understand safe procedures for working on an operating hydraulic system equipment to prevent injuries. And it is equally important to understand what to do if an injury occurs. Injured patients need to receive immediate treatment from a doctor experienced in the treatment of injection injuries.
Gary Kleiner is technical development manager for Parker Hannifin Corporation
Taking a few simple steps before equipment installation or maintenance work begins goes a long way toward preventing costly and painful injection injuries:
• Be sure that all pressure is bled/released from the hydraulic system. Inspect all gauges for zero pressure and review the hydraulic schematic for pressure traps, such as accumulators, and check valves.
• Never loosen or tighten a hydraulic connection when the system is under pressure. The connection could fail catastrophically and cause and injection injury and/or damage to property.
• Any hydraulic lines and components that are exposed and routed near the equipment operator should be shielded to project the operator from potential danger associated with hydraulic injuries.
• Avoid routing hose assemblies in areas where the ambient heat is excessive. Heat may cause the hose to fail and potentially ignite an oil fire, injure people, or damage equipment. Specify the correct temperature rating of the outer hose cover material and use fire sleeve when necessary.