Convoluted stainless steel flexible hose

[engsup]

Hi
I have a component on my turbine plant that has many high (80 bar) and low (1 bar) pressure oil hoses manufactured out of convoluted stainless steel flexible hose (304L and/or 316L). The wall thickness of the hose is about 0,4mm and it is contained within a braided stainless steel sleeve. The OEM recommends replacement every 6 years. Seems strange to recommend periodic replacement of stainless flexible hose. Rubber products Yes, but stainless steel? Can you suggest reasons why it should be replaced? Have you had any experience with failure in such hoses?

 

[mcguire]

There are reasons besides corrosion for limited life in stainless components. Braids abrade themselves, creep under stress, even fatigue. The OEM should have some knowledge of potential failure modes. If this is an ISO 9001 supplier, the potential failure modes should be spelled out by him and available for you to audit.

 

[Kenneth]

"If this is an ISO 9001 supplier, the potential failure modes should be spelled out by him and available for you to audit."

?

 

[mcguire]

Kenneth
Many manufacturers have Their quality control systems certified to the ISO standard number 9001. It is a stringent system that among other things requires a designer to document how his product could conceivably fail in service and what testing he has done to make sure it won't. The standard further stipulates that customers must be given access to these records. No more caveat emptor.

 

[rhc]

Stainless steel if continually worked hard becomes stiff & brittle. We have tried ss springs but they did'nt last as long & were unpredictable. We went back to conventional spring steel.

 

[JOM]

engsup,

I know of a failure of flexible fuel lines on a ship that resulted in a disatrous fire in 1998. The cause was fatigue from pressure pulses from the fuel pump.

Do you want more info?

John.

Cheers,
John.

 

[boo1]

Yes this is common for SST braded flex lines. Studies we did showed additional supports, and reducing the bend radius increased service life.

 

[JOM]

Here's a report on a flexible fuel hose failure. The hose needed periodic replacement. The full report is available.


28 June 1999
D07/99

RUPTURED FLEXIBLE FUEL HOSE DISABLES RESEARCH SHIP

A worn flexible fuel hose caused a fire aboard the Australian supply ship Aurora Australis en route to the Antarctic, according to a report released today.

more...

http://www.atsb.gov.au/marine/incident/incident_detail.cfm?ID=143

The other important incident was the Westralia fire in 1998. This is an Australian Navy supply ship. The engines had 56 short s/s flexible fuel lines installed. After less than 50 hours engine time, one of these failed due to fatigue and two others showed signs of failure. There was a fire and four sailors died.

There is a picture of the failed hose - I'll try to grab it.

In both incidents, the vibration from positive displacement fuel pumps was thought to be the cause of cyclic stresses.

Cheers,
John.

 

[TEV]

Although it looks like fatigue is the issue in this post, let me add another story from the annals of experience.

I was involved in analyzing an incident in which hose reinforced with stainless steel braid failed at the connector. It was learned that the connectors were mounted onto the hose by peeling the outer cover back to reveal the braiding, lubricating the end with common hand soap (!), and sliding the connection on, which was secured by swaging. Well, the soap was found to contain 2000 ppm chlorides, and the braiding was highly strain hardened, so guess what? Chloride stress corrosion failure! No surprise.

Moral of story - avoid mechanical engineers making decisions that affect material performance. No offense intended to mechanical engineers, as I would consult them for assistance in their area, but I would request that they reciprocate.

 

[JOM]

TEV,

Apart from the soap, is that the normal procedure for fitting the connectors?

Cheers,
John.

 

[TEV]

JOM:

It depends on the design of both the fitting and the hose, but for some of the high strength hose-fitting applications, you want the connector to be tied to the strengthening braiding of the hose. But here I go, straying into the mechanical engineer's realm! I forgot to mention that the joint was recovered with a polymeric shield after swaging the fitting on, but the chloride in the soap was captured in the area of the braiding.

 

[monkeydog]

TEV Said: "I forgot to mention that the joint was recovered with a polymeric shield after swaging the fitting on, but the chloride in the soap was captured in the area of the braiding."

And all the experts collectively Said: "AHHHHH"