Distance pieces in hose assemblies for oxygen service

[Prometheus21]

Hello everyone,

Rubber and plastic (flexible) hoses in oxygen system often incorporate what is a called a distance piece, and is described in ISO 14113. In short it is there to lessen the effect of adiabatic compression. Now the standard describes that the distance piece should have a volume of 3280 mm^3 (including the fitting end) for a hose with a nominal internal diameter of 6,4 mm, and with a length of 910 mm; this is for systems pressures up to 200 bar (20 MPa). (Taken from the standard, rf. CGA E-9-1987 based on ASTM STP 1319).

Now this volume should be increased and decreased in direct proportion to the hose length and the system pressure.

Now does anyone know if the volume should increase in direct proportion to a hose with a larger internal hose diameter? e.g. 10 mm ? I have yet to find any standards describing anything other than 6,4 mm hoses, while plenty of filling plants use 10 mm in their operations due to capacity demands.

For now we have used the simple assumption that the distance piece should be increased in direct proportion to the hoses' inner diameter size increase - and the preliminary adiabatic shock testing based on this assumption has been successful. Still, anyone with any input on the topic? Whether it is opinions, experience or standard references; its all appreciated.

 

[LittleInch]

I love the way something in nominal units gets converted to a different one.

This looks like a 1/4" pipe 3 ft long with a volume max of 0.2 cubic inches.

Why this volume I have no idea and hence not sure if it really makes any difference if its bigger, smaller or in proportion to size. The

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

 

[georgeverghese]

See page 9 of this document - there is also a reference to an ASTM paper - ref 3 - for calculating this

3. A.J.Santay, I.D.Becker and B.L.Werley.
“Design Strategies for polymer lined flex-hose Distance/ Volume pieces”
Paper presented at ASTM Symposium on “Flammability and sensitivity of Materials in Oxygen
Enriched Atmospheres” 1997 ASTM STP 1319

https://asiaiga.org/uploaded_docs/D...ions in hp gas systems_ reformated Jan 12.pdf

 

[Prometheus21]

LittleInch - Its always fun when the customer insists on 6 mm and 10 mm sizes, and you explain that when it comes to flexible hoses its 1/4'' and 3/8''. The real fun begins when the customers demand 6.35 mm hose, calls it a 6 mm hose and refuse to accept the term 1/4''; demanding all drawings, specifications and documents to refer to it as 6 mm.

georgeverghese - thank you, and correct; its the paper ISO 14113 is partly based upon. I finally got around to buying the "Design Strategies for polymer lined flex-hose Distance/ Volume pieces"; and while it does not describe a larger inner diameter directly, it does offer design guides of the distance piece as a function of total system volume, temperature and pressure.

It's interesting to consider the context in which this document was written; while the cylinder-filling pressure for oxygen is still 24 MPa in many filling plants, we do have some filling plants going up to 35-39 MPa these days in Europe. I assume the US has transitioned to 24 MPa now, or is it still 18.3 MPa?

Another question for the Americans; since the document describes the hose material. Do you guys mostly use PTFE or PFA in your flexible hoses these days? PFA has been the standard the last few decades in northern Europe, but nowadays it is getting slowly phased out in favour of PTFE due to shortage, PFAS restriction and other local and regional legislatives being implemented.