Pressure testing

As with any PTFE hose assembly, it is necessary to pressure test the finished product. Unlike rubber hose, the dimensional range for a properly attached, leak proof fitting is small. This requires a precise crimp or swage.

Pressure testing method

One of two methods is most often recommended for testing; hydrostatic or pneumatic. Each has its own advantages. Hydrostatic (water) testing is used most often because it does not compress and is therefore considered safer than pneumatic (air). Pneumatic testing is done with shop air or nitrogen under water.

Compatibility consideration (PTFE hose)

Three factors give PTFE its unique properties with regard to wide ranging fluid compatibility and resistance to corrosion:

  • A strong interatomic bond between the fluorine and carbon atoms, making it very difficult for other atoms to bond with it
  • The structure of the molecule itself. Carbon atoms form the backbone of the molecule and are completely shielded by the fluorine atoms surrounding them
  • The molecules form long polymer chains which result in a higher molecular weight than most polymers, giving stability to the material

Chemical inertness

For these reasons PTFE is chemically inert. This means that PTFE can be in continuous contact with a given substance with no detectable chemical reaction taking place. Few chemicals attack PTFE. The reactants are limited to: Violent oxidizers and reducing agents. For example, sodium, in its elemental state, removes fluorine atoms from the PTFE molecule.Other alkali metals such as potassium and lithium.Extreme oxidizers such as fluorine, chlorine trifluoride and related compounds can be handled by PTFE hoses only.

Chemical reactants

At the upper limits of service temperature and high concentration, some chemicals do react. Sodiuma and potassium hydroxide, boranes, metal hydrides, aluminum chloride, ammonia, certain amines and nitric acid are among those that should first be tested. Each application should be treated as unique. Always consult Unigasket if there is any question.

Compatibility considerations (fittings)

Because of the corrosion resisting limitations of metal, fitting choice becomes a critical element. Most often in chemical applications, the fitting material determines the ultimate life of the hose assembly unless the assembly is mishandled or operated outside of recommended limits.

Fitting corrosion

Regarding fitting material, it is often not a question of ‘Will it corrode?’ but ‘How much and how quickly will it corrode?’ It is therefore important to select the best available fitting material. Temperature, concentration, the exact nature of the fluid conveyed, flow rate, and occasionally even fitting design must be known.

Fitting testing

As with hose material, selected fittings should always be tested. Unigasket should always be contacted for recommendations and application advice. In all cases, it is necessary to know the specifics of temperature, pressure, concentration, and the exact nature of the chemical being handled.

Fundamentals of handling steam

Steam is frequently used as a process medium due to its unique thermal properties. Because of these properties, steam is also inherently dangerous. It is important to understand some of the facts about steam and how it is best handled with hose.
At atmospheric pressure (14.7 psi), water becomes steam, or boils at 212°F (100°C). When under increased pressure, boiling is delayed. At 200 psi (13 bar), the boiling point becomes 388°F (200°C). At 600 psi (40bar , the temperature approaches 500°F (260°C).
If there is water and steam in the system, the steam is said to be in a wet saturated state. As long as some liquid remains, the steam will stay at the boiling point. Any additional heat changes all the water into steam, at which point it becomes dry saturated steam. At this point, additional heat raises the temperature above the boiling point and the steam becomes superheated.

Steam hose failures

In an elastomer hose application, dry saturated or superheated steam at or below the temperature rating of the hose is acceptable, though the hose will heat age over time. Wet saturated steam poses a more severe problem for rubber and other elastomers because the inner core will absorb water. This will cause the tube to swell and deteriorate faster. In some cases the action of trapped water turning into steam can physically tear the inner core, creating a phenomenon called ‘popcorning’. Steam hose failures are usually caused by heat which either softens or hardens the hose material over time.

Best choice for steam

For this reason, Unigasket smooth bore PTFE hose is the best choice for steam. Unigasket hose can handle steam in any state up to its operating temperature of 260°C. Even steam and cold water cycles are easily handled with smooth bore PTFE hose.

Vacuum and hose performance

To ensure the selection of the right hose for a particular application, it is important to understand the potential effects of vacuum and its relationship to hose construction. Vacuum is also properly referred to as negative pressure which further helps to explain its effects on hose.

How vacuum is measured

Vacuum is measured as the height of a column of mercury that is a specified number of inches tall. Full vacuum in an industrial process, is considered to be 28” of Hg (mercury); theoretical full vacuum being 30” Hg. Expressed as negative pressure, theoretical full vacuum is the equivalent of one atmosphere (14.7 psi). In a hose application, full vacuum equates to the same 14.7 psi applied externally to the hose. This may seem to be a small amount, but since hose is designed primarily to hold internal pressure, this can be enough to cause collapse under certain conditions and with certain types of hose.

Factors that affect hose performance

The primary factors that affect hose performance under vacuum are hose size and construction, temperature and bend radius. Hose construction determines the ‘hoop’ strength or the relative ability of the hose to resist collapse. Generally, smaller ID’s, thicker walls, external reinforcement such as wire wrap or covers and bonding together the hose layers serve to increase hoop strength. As the temperature of a hose increases, the hoop strength typically decreases due to material softening. As a hose approaches its static bend radius limit, hoop strength can be adversely affected because the hose profile will go from round to oval. If combinations of these factors exist (i.e., large ID, thin wall, high temperature, extreme bending) hoop strength is reduced even further.

How to overcome vacuum collapse

PTFE hose is a relatively thin walled product and is therefore subject to vacuum collapse if not properly specified and protected. With single braided smooth bore hose the unbonded metal braid (the pressure handling element the hose) is of limited value in a vacuum application, especially as temperatures increase. The addition of internal or external springs or bonded covers is the best way to overcome potential vacuum collapse. We suggest than to use double braid smooth bore hose, our type FHMDC, or the corrugated our type CLWBSP reinforced with an external spiral.