|main forms of steam attack on rubber hoses||cause|
|peeling (popcorning), separation of inner layer||the condensate penetration through inner layer|
|corrosion of steel reinforcement||the condensate and gas penetration through inner layer and reinforcement|
|thermoplastic effect||effect hot water at too high a temperature|
|overheating, oxidation and cracking of inner layer||superheated steam, dry steam|
Popcorning and separation of the inner layer – condensate penetration
The effect known as popcorning occurs when condensate (water) penetrates through the inner wall of the hose. This effect is typical for operation at a low temperature with a high proportion of condensate fraction in steam (wet steam) or when the steam cools down and the hose is not drained. Basically, the occurrence of the popcorning effect is mainly determined by how the hose is used. The steam must be drained from the hose after use to avoid the accumulation of condensate. The condensate penetrates the elastomeric material, filling its micropores. When the steam flows again, the condensate trapped in the pores is heated and changes into steam. The conversion into steam causes rapid expansion (many times more steam is produced from one volume of condensate), and an “explosive” ejection of the condensate from the material, tearing the inner layer and causing damage that looks similar to the surface of popcorn or cauliflower. A similar mechanism occurs in the case of separation (delamination) of the inner layer, which can eventually result in the blockage of the flow through the hose.
Cool steam condensate should never be left in the rubber hose, it should be drained after use (“DRAIN AFTER USE” – a recommendation often given by steam hose manufacturers).
Corrosion of steel reinforcement – condensate and gases penetration
Corrosion of steel wire reinforcement can be caused by the exposure of the braid through cracks in the outer layer and by the penetration of condensate through the micropores of the inner layer. In addition, the gases from air dissolved in the water (oxygen and carbon dioxide), present in the steam, also enter the hose material and are strong corrosive agents and corrosion can progress very quickly at high temperatures. Therefore, the reinforcement wires should be protected by, for example, brass coating. Moreover, proper chemical treatment of boiler water from which the steam is generated and its deaeration (removal of air) are very important.
Thermoplastic effect – hot water vs. saturated steam
Thermoplastic effect is the plasticization and loss of mechanical properties of the hose wall material due to the warming caused by the flow of heat from steam or hot water into the hose wall. Rubber steam hoses are designed to handle saturated steam. Saturated steam is a specific medium in which both gas (steam) and liquid (condensate) can coexist at the same temperature and pressure. When saturated steam flows through a hose or pipe, it gives up its heat to the cooler inner surface of the hose wall and condenses as droplets or a thin layer of condensate that separates the hot steam from the hose wall. In addition, air is usually dissolved in the water used to generate steam, and during heating, this air is released and forms a very thin layer between the steam and the condensate. These two layers (air and condensate), with low thermal conductivity, insulate the hot steam from the inner surface of the rubber hose wall (fig. A).
In the case of hot water, these insulating layers are absent. Therefore, the temperature of the rubber on the inner surface of the hose wall and throughout the cross-section of the wall and on the outer surface of the hose, can be much higher, even when the hose is operating with hot water at a lower temperature than for saturated steam (fig. B). The result is a large decrease in the pressure resistance of the hose, as well as rapid, significant, and permanent deformation of the rubber under the clamps of the hose fittings.
Rubber hoses for steam can be used for hot water with a maximum temperature of about 90 ÷ 100ºC, unless otherwise stated by the manufacturer.
Overheating, oxidation and cracking of the inner layer – dry steam, superheated steam
Oxidation is ageing of hose material caused by oxygen in steam. This process is a very rapid at high temperatures, causing the rubber of the inner layer to harden and crack. It is evident under dry steam and superheated steam conditions, as there is no thin layer of liquid condensate directly on the hose wall lowering its temperature (fig. C), thus the rubber of the inner layer is exposed directly to dry, gaseous, hot steam, which makes it cook, harden, and crack. Some types of hose are approved for temporary use under superheated steam conditions (up to 18 bar, up to 232ºC), but longer use under these conditions will drastically shorten the life of the hose. Superheated steam or dry steam destroy the hose quickly, even at temperatures below the maximum working temperature of the hose.
That is why::
- Do not use standard saturated steam rubber hoses for superheated steam or dry steam. This may be permitted only temporarily, if hose specification allows for this option.
- Avoid spontaneous conversion from saturated steam to superheated steam in the system through a sudden change, e.g. increasing cross-section (diameter) of the system or opening a valve, which result in a pressure drop.
Metal hoses or PTFE hoses can be used for superheated steam – within their performance parameters.