ASME recommends a 4 X 1 factor for appliances such as valves, tubes, fittings, etc. The reason, it appears, is that these devices are either small in size, or large but not weight sensitive, are subject to abuse and other factors, or are made of materials which are difficult to test or verify long term health. However, there are many high pressure containers such as SCUBA tanks which are manufactured to 2.5 X 1 minimum. There are practical reasons for this such as weight, ease of manufacturing and cost. However, this latter category are made of materials for which excellent data are available for strength, durability and service life. Hydrostatic tests and inspections are mandated by DOT of approved containers made to known specifications to monitor their integrity over time.
Some materials for which cycle life (fatigue) are difficult to quantify include some stainless steels and some aluminum alloys. Hydraulic accumulators are often made of these materials. Aircraft aluminum, a typical material for accumulators, is very strong in tension and gives fantastic numbers when tested to destruction. However, this material does not hold up well to repeated pressure cycles due to its low ductility. This is one reason why a 4 X 1 factor is used, to minimize flexing when pressurized and to ensure a long life. Manufacturers probably apply this safety factor to yield which is about 12,000 psi for an ASME spec'ed aluminum vessel rated at 3000 psi.