The world is still fighting the COVID-19 epidemic and vaccine shortages are a reality in most of the countries. In this scenario, imagine if one could boost supply by as much as 20% with a simple design change? Impossible, you say? However, that is what some physicians accomplished by changing standard syringes to low dead space syringes. The latter leaves almost no liquid trapped between plunger and needle, which allows a 5-dose vial to provide up to 7 doses.
The take away from this example is that, although syringes may receive very little thought, it does not prevent them from having a huge impact on the outcome. In hydraulic systems, there is an often-overlooked item that may have a similar impact: the hydraulic conduit. It is therefore important for users to be aware of the significance of tubing and hoses for hydraulic systems, and explore some of the engineering considerations related to choosing one over the other.
By Davi Correia, Senior Mechanical Engineer
Importance of Hydraulic Conduits
Any book on hydraulic systems has topics covering pumps, cylinders, and valves. Many of them do not, however, provide guidance on the advantages and disadvantages of hose and tubing assemblies. As choosing one over the other has safety, cost, and performance consequences, it is important to be aware of their respective differences.
A common occurrence, even in well-designed systems, is the shortage of replacement parts when a component has failed or needs replacing. Maintenance technicians must constantly decide if it is safe to substitute hose for tubing, or tubing for hose. Despite their lack of glamour, proper selection of fluid-conveying conduits goes a long way in lowering the risks of sudden failure and ensuring ideal performance.
Hydraulic conduits serve to connect all the major components together within a hydraulic system. There are two types: metallic tubing or flexible hoses. Regardless of the type, they must perform three tasks:1
- Facilitate flow without inducing unnecessary turbulence and pressure drop,
- Avoid burst and collapse from inside and outside pressure, and
- Prevent leakage.
While metallic tubing is typically made of steel, it is possible to find other materials in niche applications, such as aluminum and copper in aviation. Carbon steel and stainless steel are, however, the primary options for the average user. Steel tubing is generally manufactured as cold drawn seamless tubes or welded tubes. Cold drawn tubes are preferred for hydraulic systems, as they provide a better external and internal finish and have higher tolerance control; the surface finish on these hoses minimizes friction losses. For low-volume systems, tubing can also handle the pressure and flow requirements with less bulk and weight.2 One of primary advantages of tubing is that it is considered easier to handle and can be reused without any sealing problems.
The ends of metallic tubing are normally flared to create an angled finish, which is required for fitting assembly (see Figure 1). This figure also shows other typical components of a metallic assembly. The flaring operation requires special tools and procedures in order to avoid defects and to not overstress the endings.
Benefits of Tube Bending
Steel tubing can be bent in order to conform to specific routing needs and facilitate installation; this helps reduce the number of necessary fittings. However, there is a caveat: pressure drop and heat generation have a greater impact in a bend than in a straight path, thus lowering system efficiency. So, any designer has to strike the right balance between bends and efficiency.
If no bends are used, several problems can occur. For example, cutting the exact lengths between any two anchor points is extremely difficult. Even if successful, the fitting may cause undesired stresses on the flaring portion of the tube, likely weakening it. Lack of bending can also prevent tubing from flexing, and limits its ability to accommodate vibration and thermal expansion and contractions. This can easily lead to fatigue failures. Figure 2 shows examples of correct and incorrect tube layout.3
Companies employ different tools and methods to bend tubes, choosing one over the other depending on material, size, and wall thickness. Regardless of the chosen method, it is vital that the minimum bend radius for that specific tubing is not exceeded. Bend must be smooth, without signs of flattening or buckling. It is also worth mentioning that tube cutting also requires adequate tools and procedures, in order to provide the required finish for the subsequent flaring operation.
The Hydraulic Hose
Hoses are the go-to solution when connected components are subjected to movement or when vibration is a problem. They are made from the combination of different materials in layers; and the number and type of these layers depends on the application; pressure, size, chemical compatibility, etc. Generally, there are 3 layers: inner tube, reinforcement, and outer protective layer.
The inner tube material, commonly rubber, comes in direct contact with the hydraulic fluid. Hence, it must be chemically compatible with the temperature range that the hose will be subject to. The reinforcement is responsible for ensuring the pressure capabilities of the hose. Material type, pattern arrangement, and number of layers are the factors that determine the pressure range of the hose. Steel wire is the most common material for reinforcement, arranged in spiral, helical, or braided patterns, according to the application. For example, a helically-reinforced pattern is normally selected for suction application, lest collapsing the hose during operation, see Figure 3. The outer layer, or cover, has the function of protecting inner tube and reinforcement from the environment where the hose will be used. As such, common features of the outer layer are resistance to: abrasion, weather, and specific chemicals.
Hose construction has been standardized by several bodies, such as SAE, ISO, BSI, DIN, and API. Of those, the most common in the United States is the SAE J5-17, also known as the R series. This standard covers dimensions, construction, application, and pressure rating for mobile and stationary equipment.
Routing of hydraulic hoses is subject to constraints, in a similar way as tubing, see Figure 4. Although more flexible than tube, hoses need some slackness to allow for assembly, vibration, and thermal effects. They also have a minimum bending radius, which should be provided by the manufacturer. Even with proper selection and installation, hoses typically have a significantly shorter life span than tubing. One of the main reasons that hoses are less durable is that elastomers tend to deteriorate when exposed to the most industrial environments. An aggravating notion is that this deterioration can seldom be monitored and used for remaining life predictions.
The short version of the design guidelines for hydraulic systems can be summarized by following the guideline to, “use tubing; unless there is excessive vibration or relative movement between parts.” As in many other things in life, reality can be a bit more nuanced. Although the choice between hose and tube for many applications is obvious, some applications require careful consideration of the advantages and disadvantages of the two conduits. The final arrangement of an application may even combine both, as is the case in many pieces of mobile construction equipment, see Figure 5.
Hydraulic hoses are normally more expensive than tubing and, despite their limitations, they are often employed in applications where one would expect to find tubing. For example, noise reduction. Depending on the characteristics of the system (pumps, chambers, reservoirs, etc.), the normal operating noise level can be amplified in a phenomenon known as resonance. If that is the case, hoses have a natural dampening effect due to their being more elastic. In maintenance, it is often tempting to substitute tubing for hose due to challenges in fit. Despite the higher cost of the latter, they are easier to fabricate (or procure) and installation may be sped up, especially when dealing with misaligned ports or compact equipment. Also, special tools for bending and flaring tubes in larger sizes may be difficult to find. On the other hand, hose selection has more items to check than tubing. For example, a fluid-compatible hose capable of dealing with the operational pressure, may be too rigid or too heavy for routing. This is especially true when dealing with helical-reinforced hoses. Table 1 provides a summary of benefits and limitations of hoses and tubing.4
It is important for users to be aware of the difference between tubing and hoses for hydraulic systems, and take into account some of the engineering considerations related to choosing one over the other. By choosing the component that is best suited for the application in question, the operator can mitigate the risk of premature failure and enhance the safety of the user.
- Noah D. Manring & Roger C. Fales, Hydraulic Control Systems, Second Edition, Wiley, 2020.
- Ravi Doddannavar & Andries Barnard, Practical Hydraulic Systems: Operation and Troubleshooting for Engineers and Technicians, 1st Edition, Wiley, 2005.
- Qin Zhang, Basics of Hydraulic Systems, 1st Edition, CRC Press, 2008.
- https://www.mfcp.com/our-blog/hose-v.-tube-assembly-in-hydraulicsystems (accessed April 2021).
ABOUT THE AUTHOR
Davi Correia is a Senior Mechanical Engineer who has worked at a major Brazil-based oil company for the last 15 years. Correia is part of multidisciplinary team that provides technical support for topside piping and equipment of production platforms. During this period, he began to work with materials and corrosion, and later moved to piping and accessories technology, where he has become one of the lead technical advisors on valve issues. Correia was part of the task force that revised the IOGP S-562 standard, and wrote the S-611 standard. Correia has a master’s and a doctor’s degree in welding by the Universidade Federal de Uberlandia.