Selection Criteria For Tubes and Hoses

Figure 1: Hose

Choosing the appropriate tubes and hoses is essential for maintaining the safety and efficiency of fluid handling systems. These parts are crucial for the transportation of liquids and gases in various applications, including industrial machinery, medical devices, and household appliances. It is important to understand the selection criteria, material composition, and pressure and temperature ratings of a hose to make well-informed choices.

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Tubes And Hoses

Tube, hose, and pipe definitions

Hoses, tubes, and pipes can sometimes be used interchangeably. However, they do have differing characteristics:

Tube: A tube is a hollow cylinder made of plastic or metal that transports liquids or gasses.
Hose: Although it can be rigid,a hose is often a flexible tube made of plastic, rubber, or metal used to transport liquids or gasses. Hoses are typically made of numerous materials.
Pipe: A pipe is a rigid tube made of plastic or metal used to transport liquids or gasses. It can be flexible, but that is not often the case.

Selection criteria

When selecting a tube or hose, the three main criteria are length, diameter, and material. Incorrectly specifying a hose or tube could lead to a burst or failure, causing leakage and system downtime.

Length

The length of a hose or tube should be kept to a minimum to avoid unnecessary pressure drop. However, it must be long enough to avoid tension and have enough slack to compensate for length changes due to temperature, pressure, vibrations, or component movements. The inner diameter of the hose or tube can be increased to decrease pressure drop.

Diameter

The inside diameter (ID) of a hose or tube affects its flow capacity (Q) and flow speed (v). Increasing the inner diameter will decrease the pressure drop over the length of the hose or tube. However, the flow speed will be reduced. There should be a balance between optimizing energy transfer and lowering the cost. Undersized hoses can lead to a large performance loss, and oversized hoses can lead to installation problems due to lack of space or increased costs.

Using flow capacity (Q in l/min) and flow speed (v in m/sec), use the following equation to find an application's appropriate inner diameter (mm):

Note: Equation should only be used for general fluids, not gasses or viscous fluids.

For common applications, the average recommended pressure line flow speeds are:

Compress air/pneumatic lines: 15 - 25 m/s
Household water lines: 1 - 3 m/s
Hydraulic oil lines: 3 - 6 m/s
Hydraulic suction lines: 0.6 -1.3 m/s
Hydraulic return lines: 1.7 - 4.5 m/s

There are three ways to indicate a hose or tube diameter:

Inside diameter
Outside diameter (OD)
Both inside and outside diameters

For example, a 3x2 hose means an OD of 3 mm and an ID of 2 mm. If only the outside diameter is specified, look at the hose's specification sheet to understand the inner diameter to ensure it is suitable for the application. The outside diameter is important for connection purposes, such as for specifying a fitting. If necessary, the hose or tube's wall thickness can be specified using the following formula:

Material

Tube and hose material selection is crucial to ensure it doesn’t fail during operation. A hose or tube's material needs the following properties:

Resistant to the atmospheric temperature
Resistant to the media's temperature and pressure
Chemically compatible with the media
Flexible or rigid enough to meet the application's requirements
To learn more, read our hose and tube material guide

Pressure and temperature

Using hoses and tubes within their designated pressure and temperature ranges is essential. Manufacturer tests determine these specifications and are typically presented as a maximum safe pressure at a specified temperature. For hoses carrying pressurized fluids, it is crucial not to exceed the maximum operating pressure, also known as the static pressure limit. Additionally, any spikes in pressure should remain below the hose's burst pressure rating to prevent failure.

The temperature rating of a hose or tube must also be considered. It should operate at a temperature lower than that of the fluid it is transporting and the ambient environment. Temperature directly impacts pressure; as the temperature rises, the pressure inside the hose increases as well (according to Charles' Law). Consequently, a hose rated for full operating pressure at 20°C (68°F) may have a reduced pressure rating at 60°C (140°F). The choice of material for the hose or tube will ultimately determine its pressure and temperature operating limits.

Reinforced hoses and tubes

Most hoses and tubes are designed as single, standalone units. However, it is sometimes beneficial to bind multiple hoses and tubes together for organizational and safety reasons. This practice prevents tangling and allows a neater arrangement of the lines. Encasing these combined hoses and tubes in a protective jacket, commonly made of PVC, offers additional protection. This outer layer shields them from external forces, adverse weather conditions, and exposure to oils and other potentially damaging substances.

Furthermore, hoses and tubes can be made of multiple materials to enhance their functionality and durability. Typically, this involves an inner hose made from one material that is then coated with another material on the outside. The choice of materials is strategic; the inner layer is selected for its compatibility with the fluid being conveyed, while the outer material is chosen for its ability to withstand the surrounding environment.

There is a wide array of material combinations available, each providing distinct benefits tailored to specific requirements. To further augment the hose's capabilities, manufacturers may incorporate inlays or braiding, a process commonly referred to as reinforcement. Reinforcement significantly enhances the hose's performance characteristics. For instance, a hose made from a material that typically handles low pressure, such as silicone, can be reinforced with a layer of tightly woven textile. When this is then coated with an additional layer of silicone, the result is a hose that retains its compatibility with the media while gaining the strength to withstand high-pressure applications.