Many industrial plants utilize Hydraulic Power Units (HPUs), where pumps pressurize hydraulic fluid whose energy is then used to perform work. HPUs are found in many applications and require hoses and tubing to connect the various system components (pumps, valves, cylinders, etc.). While rigid hydraulic tubing may be used to connect stationary components, hydraulic hoses are also common, as they provide flexibility and versatility while safely conveying high-pressure fluids. In most hydraulic hose applications, rubber or thermoplastic hoses with wire or fabric reinforcement are used, as they are designed to handle normal hydraulic system operating conditions. However, there are instances where corrugated metallic hoses should be considered instead, in order to improve system safety and reliability.
By Frank Caprio, Corporate Trainer and Major Market Specialist, Hose Master
Temperature Extremes
Metal hydraulic hoses are common when temperature extremes preclude the use of rubber or thermoplastic hoses. Depending on the materials of construction, a rubber hydraulic hose is limited to a temperature range of roughly -40˚F up to around +250˚F; certain materials can extend these limits somewhat. When a rubber hose is exposed to temperatures above these limits, the hose tube and cover eventually harden from the heat and the hose becomes stiff, which in turn cause cracks to form internally or externally. At ultra-cold temperatures, rubber hoses may also experience shortened service life, as the low temperatures can cause the hose to freeze and become brittle. Alternately, corrugated metal hoses intended for hydraulic applications have a much higher temperature rating (in excess of 1,000˚F depending on the alloy), and can handle sub-freezing/cryogenic conditions without it negatively affecting their performance. In applications where either the media or the surrounding environment may reach these extreme temperatures, a metal hose should be considered.
Avoiding Fire Hazards
Recently, inspectors have increased their oversight of Hydraulic Power Units as a potential safety hazard. This is because after the hydraulic fluid runs its course through a hydraulic system, it is returned to a reservoir through a low-pressure return line. These return lines are often rubber or plastic drain hoses, as there are no extreme pressure or temperature requirements. However, if the hose is connected to the reservoir at a point below the fluid level inside the reservoir, it is a potential fire hazard; if a fire were to occur inside the hydraulics room, non-metallic hoses can melt which would allow the contents of the reservoir (which is often flammable) to empty into the fire. Replacing the return line with a metal hose or expansion joint will effectively mitigate this risk.
Non-metallic hoses located under hydraulic reservoirs should be converted to metal hoses to improve safety. A fire could cause rubber or plastic hoses to melt, allowing the stored hydraulic fluid to drain into the fire.
Application Highlight: Steelmaking
Here are some examples where metal hydraulic hoses can be found in a typical integrated steel mill.
Coke, one of the raw materials used in ironmaking, is produced by heating a special grade of coal in a coke oven. Hydraulic hoses are used to open and close the oven and damper doors, and to move the coal/hot coke in and out of the ovens. The coke is then loaded into a transfer car that also has hydraulics to control the braking and dumping mechanisms. Hydraulic hoses in coke oven batteries on these ‘quench cars’ are exposed to high heat as well as falling coke embers which would quickly destroy a rubber hose, so metal hoses are common. Some hoses may even be fitted with external armor or insulation to protect the hose from external damage while also keeping the hydraulic fluid within an acceptable temperature range.
Next, the blast furnace combines the coke with iron ore and limestone to produce molten iron. The blast furnace and related equipment (tap hole drills, bottle cars, etc.) also use metal hoses in their hydraulics systems. These hoses must be able to resist the high-heat environment during ironmaking, and when transferring the molten iron from the blast furnace to the furnaces that convert the iron into steel.
Steel is produced from molten iron in a Basic Oxygen Furnace (BOF), which may also use some scrap steel or Direct Reduced Iron (DRI) as a feedstock. Alternately, an Electric
Arc Furnace (EAF) predominantly uses recycled steel or DRI which is then melted down. Whichever furnace is used, the resulting steel must then be refined and cast into the appropriate shape (slabs, billets, beams, rebar, etc.). Each furnace style contains various hydraulically-controlled equipment, and the intense heat radiating from the furnace often requires the use of metal hoses.
Once the steel has been made, hydraulic systems are used on degassing systems, reheat furnaces, casting equipment, hot strip mills, and pickling/galvanizing operations, all of which emit radiant heat that can reduce a hose assembly’s service life. While certain applications (drive rolls, coilers, etc.) may permit the use of rubber hoses with external protective layers, the insulation used on these hoses may degrade or fall off during normal operations, losing its effectiveness. Each application should be reviewed by someone fluent in all hose types in order to implement the best solution.
Once the steel has been made, hydraulic systems are used on degassing systems, reheat furnaces, casting equipment, hot strip mills, and pickling/galvanizing operations, all of which emit radiant heat that can reduce a hose assembly’s service life. While certain applications (drive rolls, coilers, etc.) may permit the use of rubber hoses with external protective layers, the insulation used on these hoses may degrade or fall off during normal operations, losing its effectiveness. Each application should be reviewed by someone fluent in all hose types in order to implement the best solution.
While metal hydraulic hoses accommodate high-temperature media and environments, external insulation may be required to prevent the media from overheating or to protect nearby workers from burns.
Other Safety Considerations
When hoses are used at elevated temperatures, their rated working pressure must be decreased in order to account for softening of the hose material as the temperature rises. For a rubber hydraulic hose, derating factors must be applied (typically at temperatures exceeding 212˚F) in order to determine the reduced hose pressure rating at that temperature. Metal hose also has temperature derating factors, but they generally do not apply until temperatures exceed 300˚F, and the derating factors are much less severe than those for rubber and thermoplastic hoses. Each hose manufacturer should be able to provide the appropriate temperature derating factors for their products.
Regarding low temperature service, corrugated metal hose is preferred, as it does not get stiff or brittle as the operating temperature decreases. Many industry standards allow Type 316 stainless steel to be used in cryogenic applications down to -325˚F without any reduction in performance.
Fitting retention is another critical factor to consider when selecting the best hose for an application. Over time, rubber or thermoplastic hoses with crimped end fittings may become vulnerable to sudden fitting separation. This is because the hose material that was compressed under the crimped or swaged fitting connection can slowly migrate out from under the ferrule or clamp over time. Using mismatched hose and fittings or improper attachment methods may also result in hose failure. While there are special devices designed to minimize the effects of sudden coupling separation, using a metal hose eliminates the need for extra safety features as the fittings are permanently welded to the hose ends by specially trained and certified welders.
Chemical compatibility is also an important factor in proper hose material selection. While rubber and thermoplastic hydraulic hoses are designed to safely convey hydraulic fluids, the compatibility of the hose material to be media being conveyed should be verified. This is because some hydraulic fluids are oil-based while others are water or ester-based, requiring different elastomers to be used for each medium. Since most all hydraulic fluids are considered non-corrosive, stainless steel metal hoses can handle hydraulic fluids without concerns about chemical resistance. Another corrosion concern is galvanic corrosion, which may result from the presence of dissimilar metals in the hydraulic system. Metal hoses made with all stainless-steel components provide added resistance to galvanic corrosion, assuming proper sealing materials are used. Additionally, certain environments (e.g. saltwater docks, etc.) may cause chloride-induced corrosion in stainless steels, so hoses should be monitored and taken out of service if any signs of corrosion are present.
Lastly, if a metal hose does fail in a hydraulic application, it is generally not a catastrophic failure. Metal hoses tend to develop small cracks or pinholes that leak fluid at a slower rate than non-metallic hoses, where failures may be violent and pose a safety hazard from both the uncontrolled release of energy as well as the escaping fluid.
In conclusion, there are many applications where metal hydraulic hoses can offer benefits over hoses made from other materials. However, the proper components must be selected and the assembly should be fabricated by skilled personnel. Improper materials or joining techniques can compromise the integrity and ultimate service life of a corrugated metal hose, and special fabrication techniques have evolved that minimize any potential media entrapment areas where corrosion can initiate. Make sure to work with a supplier who can help identify potentially dangerous installations and make recommendations as to proper assembly construction, fabrication, testing, installation, routing, storage, and inspection.
ABOUT THE AUTHOR
Frank Caprio, Corporate Trainer and Major Market Specialist at Hose Master, LLC, has more
than 35 years of experience in hydraulic, industrial, metal hose, and expansion joint products and applications, and is the ‘Dean’ of Hose Master’s training program, Hose Master University.
He is nationally recognized as a leading authority in metal hose, has authored various articles for industry publications, and has become a sought-after source for industry facts and trends.