Tubing and Hosing Selection Criteria
Figure 1: Hose
Tubes and hoses are used to transport a media. They are used for industrial purposes as well as personal uses like gardening. Even for simple applications like gardening, you still need to select the appropriate one (length, diameter, etc.) to make sure it meets your needs. For industrial applications it can get more complicated. This article aims to give you all the available selection criteria to make an educated decision on selecting a hose or tube for your application.
Tube, Hose, and Pipe Definitions
A hose, tube, and pipe can almost be used interchangeably, but they do have characteristics that differ.
- Tube: A hollow cylinder typically made out of plastic or metal that is used to transport liquids or gases.
- Hose: A flexible tube that is typically made out of plastic or metal that is used to transport liquids or gases. However, it can also be rigid. A hose is also typically made out of numerous materials.
- Pipe: Typically, a rigid tube that is made out of plastic or metal that is used to transport liquids or gases. However, it can also be flexible.
As you can tell, all three definitions are very similar. Here at Tameson, we typically refer to “rigid tubes as pipes and “flexible tubes as hoses. The terms hoses and tubes are also used interchangeably.
Invention of the hose
Jan van der Heyden, a Dutch Baroque-era painter, who lived in the late 1700s in Amsterdam, became famous not only for his townscape paintings. Besides being a canvas artist, a glass painter and a writer, he also was the superintendent of the Amsterdam fire brigade. Together with his son, also called Jan, he made substantial improvements to fire pumps in Amsterdam and in 1673 they invented the first fire hose. By sowing long pieces of leather like bootlegs together, water could be transported closer to the fire. Although these first hoses could not take much pressure, it became possible to spray directly on the fire. Later Jan van der Heyden and his son also invented the suction hose by applying wire to keep it rigid.
When selecting a tube or hose, the three main criteria are length, diameter, and material. Knowing your application criteria and media allows you to properly select these three components of your tube or hose. Incorrectly specifying a hose or tube could lead to a burst or failure causing leakage and system downtime.
It is important to keep the length of any hose or tube to a minimum to avoid unnecessary pressure drop. However, it needs to be long enough to avoid tension and enough slack to compensate for length changes due to temperature, pressure, vibrations, or component movements. Your hose/tube length should be specified at the maximum length of all movements.
You also need to take into consideration the pressure drop through the length of the hose/tube. The longer the hose/tube, the larger the pressure drop will be. To decrease pressure drop, the inner diameter of the hose or tube can be increased.
When sizing a hose or tube, the inside diameter (ID) needs to specified as this will affect the flow capacity (Q) and flow speed (v). It is often a balance between optimizing the transfer of energy and lowering cost. However, undersized hoses can lead to a large performance loss and an oversized hose can lead to installation problems due to lack of space or increased costs.
From knowing your application requirements, you should already know your flow capacity (Q in l/min) and flow speed (v in m/sec) as you can then determine the appropriate inner diameter (ID in mm). Use the equation below to find your appropriate inner diameter.
Note: Equation should only be used for general fluids, not gases or viscous fluids.
There are three ways to indicate a hose or tube diameter. By just the inside diameter, just the outside diameter (OD), or both diameters. For example, a 3x2 hose means an OD of 3 mm and an ID of 2 mm. If they only specify the outside diameter, you need to look at the specification sheet to understand the inner diameter value to ensure it works for your application. The outside diameter is important for connection purposes, like for specifying a fitting. If needed, you can then calculate the wall thickness:
As previously mentioned, increasing the inner diameter will decrease the pressure drop over the length of the hose/tube. However, then the flow speed is reduced. You need to be aware if the flow is laminar or turbulent and at what speeds it will switch from laminar to turbulent. With a laminar flow, the pressure increase is linear with the increase in flow speed. However, when it increases too much the flow becomes turbulent and the pressure loss increases exponentially. 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
A hose or tubes material needs to withstand the environment temperature, media temperature, pressure, and be resistant to the media. It also needs to be flexible, or rigid enough, for installation as it may need to bend around obstacles and/or be durable enough from external objects. For a more in-depth look at materials, read our Hose and Tubes Material Guide article.
The material that your hose/tube is made out of is crucial to ensure it doesn’t fail during operation. The material of a hose/tube can affect the following:
- Operating pressure
- Burst pressure for pressure lines
- Collapse pressure for vacuum lines
- Operating temperature
- Outside and inside diameter and tolerance
- Kink resistance
- Abrasion resistance
- Chemical resistance
- Working cycle
Types and Reinforcement
The majority of hoses and tubes are ‘single’, meaning that it is just one hose. However, hoses and tubes can be glued together from a tidiness and safety perspective, to ensure that they don’t become entangled. Having multiple hoses and tubes glued together also allows you to encase them into a jacket, for example, made of PVC. This jacket gives them additional protection from external impacts, weather, oils, etc.
Hoses and tubes can also be made of more than one material. In general this is done by embedding an inner hose with an outer material. You can have a different material on the outside of your hose/tube than on the inside to ensure it is durable enough for the environment, but still resistant to the media characteristics. There are many different available combinations, each with their own advantage.
In addition to this, inlays or braiding can be used, which is often called reinforcement of the hose. Reinforcing a hose allows you to expand its properties. For example, a low-pressure enduring material like silicon can be embedded in a layer of tightly wrapped textile and encapsulated in an outer silicon layer, to make it suitable for high-pressure applications.
Pressure and Temperature
Ensuring that your hose or tube is operating within its pressure and temperature ratings is critical. Manufacturers do thorough testing to derive these numbers and they are often listed as a certain pressure at a given temperature. For pressure lines, it is important to operate below the maximum operating pressure (can also be referred to as static pressure limit), but to also consider any pressure peaks to ensure they are below the burst pressure rating. The temperature rating needs to be below your media’s temperature as well as the environment. One thing to consider is that temperature can affect your pressure. As temperature increases, the pressure increases as well (Charles’ Law). Therefore, at elevated temperature the pressure rating of the tube may be reduced. For example, if a hose is rated at 100% of operating pressure at 20°C it may be rated at 50% at 60°C. Your material selection will determine the operating pressure and temperature.
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