How do I determine the correct inside diameter and working pressure of a hose?

The working pressure is the maximum pressure at which a hose may be used in practice; the burst pressure is the pressure at which the hose fails. The ratio between the two is the safety factor, which depends on the standard the hose is manufactured to.

Some examples from the range:

• NARCORFLEX® OILROUNDER: 5 bar working pressure, 20 bar burst pressure (factor 4)
• FLOWINFLEX® OILOHM: 10 bar working pressure, 30 bar burst pressure (factor 3)
• FLOWINFLEX® OILREXTOR and FLOWINFLAT® PROBUNKER: 20 bar working pressure, 60 bar burst pressure (factor 3)
• FLOWINTUBE® VAPORTOR-18-D (steam, EN ISO 6134): 18 bar working pressure, 180 bar burst pressure (factor 10)

For hoses manufactured to an EN or ISO standard, the ratio is fixed by the standard; some EN standards for chemical hoses, for example, prescribe 16 bar working pressure with 64 bar burst pressure. For non-standardised hoses, the ratio is agreed with the manufacturer.

Important: the stated working pressure applies at 20 °C with water as the medium. At a higher temperature or with a different medium, the permissible pressure may differ. In that case, have the suitability of the hose checked in advance, for example for a product at 50 °C pumped at 10 bar. SCHIEMA can also test and certify hose assemblies.

As a rule of thumb, plastic hoses have a maximum temperature of 60 °C. Depending on the rubber compound and construction, rubber hoses can handle considerably higher temperatures. An EPDM steam hose such as the FLOWINTUBE® VAPORTOR-18-D, for example, is suitable for saturated steam at up to 210 °C continuous.

Take into account not only the temperature of the medium, but also the ambient temperature and direct sunlight. Experience has shown that with plastic hoses exposed to extreme heat and full sun for long periods, such as in the Middle East, the inner and outer layers could separate due to overheating. Such effects are difficult to predict in advance. At high ambient temperatures, rubber is therefore the better choice.

That depends on the type of reinforcement. You need a helix as soon as the hose must be suitable for suction or may be exposed to vacuum: without a helix, the hose collapses. For discharge only, a helix is not necessary. Textile or steel plies determine the pressure resistance of the hose; without plies, a hose would expand under pressure.

This is how a rubber hose is built up: unvulcanised rubber is first wrapped around a mandrel to form the inner lining. This is followed by the plies, then the helix, and then another layer of unvulcanised rubber for the outer cover. The complete mandrel, often with a hose length of 40 metres, then goes into a vulcanising autoclave, where heat bonds all layers into a single unit.