Why Steam Hoses Use EPDM Rubber (and Where EPDM Must Never Be Used)

Why steam hose is made of EPDM, and where EPDM must never go

The rubber that makes a steam hose last on saturated steam and hot condensate is the same rubber that hose and seal makers rate not recommended in mineral oil. Both facts decide where an EPDM steam hose belongs.

Flexihose rubber steam hose: heat-resistant EPDM tube · high-tensile brass-coated steel wire braid · EPDM cover resistant to heat, ageing and ozone · −40 °C to +195 °C one braid · −40 °C to +232 °C two braid

Every steam hose we make at Bhiwadi has an EPDM tube and an EPDM cover, and two questions about that rubber matter before you order. A buyer may want to know why the specification insists on EPDM; a maintenance engineer may want to know whether the same hose can take a line of oil. The answer to both is in what EPDM is. This guide covers what EPDM is in plain words, why it handles steam when natural rubber and nitrile do not, where EPDM fails, and why the wire braid under it is brass-coated.

What EPDM is, in plain words

EPDM is a synthetic rubber made from three building blocks: ethylene, propylene and a small share of a diene. The diene is there so the rubber can be vulcanised, and its leftover reactive part sits on a side chain rather than in the main chain. Once vulcanised, EPDM is a thermosetting synthetic elastomer: a rubber, not a plastic (NRCA technical glossary, “Ethylene propylene diene terpolymer”).

The letters are often written out as “ethylene propylene diene monomer”. Strictly, the M is not “monomer”. It is the M class in ASTM D1418, the standard practice that names rubbers by the chemistry of their main chain, so EPDM reads “ethylene propylene diene M-class rubber” (same NRCA source). Parker’s O-Ring Handbook sets out what the class means: the M group is rubbers with saturated carbon molecules in the main chain, while isoprene rubber, nitrile (NBR) and styrene butadiene sit in the R group, with an unsaturated carbon chain (Parker O-Ring Handbook ORD 5700, Table 2-1).

Why EPDM handles steam when natural rubber and nitrile don’t

A steam hose tube spends its life in saturated steam and the hot condensate that forms from it, and its cover spends its life in plant air, sun and ozone. Parker’s handbook rates the common rubbers on exactly those properties. Set EPDM against natural rubber and nitrile and the gap is in the two columns that matter most to a steam hose: water and steam, and ozone.

1. Steam and hot water: the compound sets the ceiling

Parker gives general EPDM a heat limit of 150 °C, rising to a maximum of 204 °C in water or steam, and lists special EPDM compounds for hot water and steam up to 260 °C (Parker ORD 5700, section 2.2.4). That last line is why steam hose is a compounding job, not just a choice of rubber family. Gates makes the same point in its own hose tables, which list a general EPDM compound and a separate EPDM compound “special for steam hose”, rated excellent for high temperature (Gates Chemical Resistance Tables). Our steam hose uses a heat-resistant EPDM tube rated −40 °C to +195 °C in the one-braid grade, which exceeds IS 10655 Type 2, and to +232 °C in the two-braid grade, which exceeds IS 10655 Type 3.

2. Ozone, weather and ageing: the cover’s job

The cover never sees steam, but it sits for years in plant air, sunlight and ozone. Parker rates EPDM excellent for ozone and weather and lists ozone, ageing and weather resistance among its strengths; natural rubber and nitrile are rated poor for ozone. That is why our cover is EPDM too, specified to resist heat, ageing and ozone, so it keeps protecting the braid after the tube has done years of work.

3. Why nitrile, the oil-resistant rubber, is wrong for steam

Nitrile is the rubber most often chosen for oil, and Parker rates it excellent there. On steam it falls short: Parker gives general nitrile a heat limit of 100 °C, with shorter life at 121 °C, water resistance only in special compounds up to 100 °C, and states plainly that nitrile is not resistant to weathering and ozone (Parker ORD 5700, section 2.2.1). Saturated steam at 7 bar g is already about 170 °C (Spirax Sarco), well past that limit.

4. Why natural rubber is not used for the tube

Natural rubber is strong and resilient, and Parker rates its tensile strength and abrasion resistance excellent. Against a steam hose’s duties it rates fair for heat, fair to good for water and steam, poor for ozone and fair for weather (Parker ORD 5700, Table 2-2). Gates likewise rates natural rubber and SBR compounds good, not excellent, for high temperature, and fair for weather and ozone. It is a good rubber for other hoses; on steam it is rated below EPDM on each of those properties. If you are still choosing between rubber, stainless steel and PTFE for a steam point, our guide to the types of steam hose compares the four families.

Where EPDM fails: mineral oil, hot oil and thermal fluid

EPDM’s weak point is the mirror image of nitrile’s strength. Parker lists mineral oil products, meaning oils, greases and fuels, under “not compatible with” for EPDM, and rates its oil resistance poor (Parker ORD 5700, section 2.2.4 and Table 2-2). Gates’ chemical resistance tables say the same row by row.

• Mineral and petroleum oils. The ratings above are at room temperature. A steam hose that is used for an oil line, or even flushed with oil, is being asked to do the one job both manufacturers say EPDM should not do.
• Hot oil and thermal fluid. We do not rate our steam hose for hot oil or thermal fluid at all, whatever the fluid’s chemistry. The Gates tables rate fluids up to +38 °C, so they say nothing in EPDM’s favour about a heat-transfer circuit running far hotter. Treat those lines as outside a rubber steam hose’s scope.
• Oil on the outside. The cover is EPDM as well. Hose that lies in oil spills or under a dripping gearbox is exposing EPDM to the same fluids from the outside, so route steam hose away from oil and replace any length whose cover has gone soft or swollen.

Gates’ own selection notes keep these duties apart: steam goes in hose from its steam hose line, and petroleum products in hose from its petroleum transfer line. We work the same way. If the line carries oil, choose a hose built for that fluid; our industrial hoses page lists the families we make, and we will tell you plainly if none of them fits. Our rubber steam hose is also not food or pharma grade, and not a replacement for fixed steam piping.

Why the wire under the EPDM is brass-coated

EPDM carries the steam and protects the hose, but it does not hold the pressure on its own. That is the braid’s job. We braid our steam hose with high-tensile steel wire rather than textile, because textile loses tension under repeated heat cycling. The wire is brass-coated so that it bonds to the rubber around it, and that bond is what stops the braid separating from the tube and cover as the hose heats and cools.

The chemistry behind a brass bond is documented in tyre research, because tyres use brass-plated steel cord for the same reason. A study at Japan’s Photon Factory by researchers from Tokyo Institute of Technology and Yokohama Rubber describes it plainly: with brass-plated steel cord, adhesion comes from chemical bonds between the rubber molecules and the sulphur atoms of copper sulphide that forms at the rubber-brass interface during vulcanisation. The same study notes that overgrowth of that copper sulphide layer is one proposed reason adhesion falls after long service (Ozawa et al., Photon Factory Activity Report 2010).

For a steam hose that matters because the wall is never still. Every time the line comes up to temperature the wall expands, and every time it cools it contracts. A braid that is bonded to the EPDM on both sides moves with it. A braid that has let go of the rubber can shift, and once it shifts the hose has lost the support its pressure rating depends on. That is one of the reasons we tell customers to retire a hose with a bulge or soft spot at once.