4:1 FoS origin

[planecrazy79]

I was talking to my lead recently and while we both agreed that most fluid power devices we've used cited a 4:1 factory of safety against burst (occasionally only 3:1, but generally speaking), neither of us could point to where exactly that came from. Is this buried in an ASME standard somewhere? Is it just convention? Does it come from the BPVC before they reduced it to 3.5:1?

Thanks in advance.

 

[LittleInch]

"Most fluid power devices" Eh?

Pumps?

Can you provide some links or data as it didn't make any sense to me.

ASME B31.3 sets allowable stress as UTS/3.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[akpipelineengr]

BPVC did historically have a nominal 4:1 safety factor.

But many other codes and standards have lower FOS. Pipeline codes go up to 80% of SMYS, or ~2/3 of SMTS.

 

[Andrew ONeill]

It's one of those historical things that's converged over time. At the heart of B31.3 / B31.3 there is currently a safety factor of 3 on "burst". By why 3? well it used to be much higher, because that's what it needed to be to stop people dying in the old days. As we got better at fabrication and quality control, and process control (stopping over pressure) the powers that have slowly lowered the fundamental safety factor in that code. The lowering is not fully scientific, it is subjective, based on stuff not blowing up and general consensus about quality. Also the powers that be look at what other countries are doing and try to align (if I can use the UK's code and it's cheaper, people will stop buying stuff from the USA), so there is also a natural global competition that results in most codes ended up being very similar.

Now in context of B31.3 / 31.1 this applies to power piping and refinery piping. But that same evolution would have happened over time for whatever codes / standards cover other hydraulic systems, wire ropes, structures, concrete etc.

Related, it's the same reason why for a lot of welds we only test 10% to be code compliant, why not 20% or 17% or 42.6%?, well after 100 years of history, the powers that be (i.e. B31.3 / B31.1 committees) have found out that only testing 10% of welds with some rules about individual welders dropping welds is the right balance to achieve the intent of the codes (not killing people by having pipes blow out), without spending too much money checking things.

Hope this helps, might be a little long winded.



Andrew O'Neill
Specialist Mechanical Engineer
Australia

 

[1503-44]

They are typically hydrotested afterwards as well.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."