Wednesday, 13 July 2016

Braided Flexible Hose

Leading up to the March 6th 2016 deadline for the 50th anniversary of the closure of the Somerset and Dorset Railway, I was faced with many outstanding jobs to get Sentinel 7109 in steam for the event. I knew I had eleven or so pipe runs to fabricate for carrying superheated or saturated steam as well as water and oil, all at 275psi boiler pressure.

While I could have kitted myself out with copper pipe tooling, it was going to make the job too lengthy to meet the deadline. I’d been recommended stainless steel braided hose as a possibility and after some internet searching found Guyson’s Kaptech hosing. (
Stainless Steel Braided Hose
After some discussion with Guyson’s technical department, I found that they could make to order in a fairly short time the hoses I wanted with the right lengths and connections to suit the existing fittings on 7109. In addition, each pipe run had to suit the pressure and temperature ratings required.

All hoses would have to withstand 275psi. Superheated steam hose would have to withstand the pressure at up to 380DegC, the max superheated temperature measured by Sentinel in their test department. Saturated steam would be up to 230DegC. Cylinder oil from the mechanical lubricator and water from the feed pump would be at relatively cool temperatures.

I chose all the steam hose to be 3/8” Nominal Bore (NB) annularly corrugated 321 stainless steel lined; the water feed hose to be 1” NB PTFE lined and the cylinder oil feed to be ¼” NB PTFE lined. All would need only the single outer braiding layer.

The maximum pressure and other ratings for the annularly corrugated hose are in the following table (from Guyson):
Stainless Steel Hose Ratings
Thus my 3/8” NB hose with single braiding would take 100 bar (~1500psi) at room temperature.

The pressure derating table for temperature is below.
Pressure derating for temperature
At 380DegC, the pressure rating would still be around 74% of 100bar, i.e. ~1100psi. This is a very high specification indeed with plenty in hand for a Sentinel. The PTFE lined hose was not so highly specified for temperature but not a weakling by any means.
PTFE Lining rating
The ¼” NB cylinder oil hose would thus handle 240 bar (~3600psi) and the 1” water feed hose 55 bar (~825psi).

In order to purchase the various hoses, I prepared a spreadsheet table with the following information (two examples shown).
Information to specify a hose
I sent this to Guyson who responded with their own internal manufacturing version for me to check. After a few iterations, we settled on a final spec for manufacture and I placed the order. It all arrived in a few weeks.

The following photos show how the hose was used.
3/8" hose as part of the blower supply
These short sections of hose were later connected to a ‘T’ piece leading back to the blower valve. Despite ordering a 6” length, I found that the flexible part was much shorter than 6” due to the end fittings. Thus the curve radius had to be tighter than I wanted and the final appearance not ideal.

This was not the only time I found the fitting length getting in the way of a tight bend.
3/8” hose from the feed pump regulating valve
The connection to the feed pump valve required a hex nipple to fit a female hose end fitting. These fittings are a hydraulic type but similar in many ways to the original for copper pipe. Hydraulic fittings generally use a cone seat rather than flat face with copper washer to seal so I had to use a lathe to convert the nipple cone into a flat face.
3/8” elbow fitting at feed pump
Elbow fittings can make life simpler but need to be thought out carefully before ordering. Again I found I needed a parallel thread hex nipple and copper washer to fit the pump. Being superheated steam, I was not happy with a tapered thread fitting here.
3/8” in and out feeds for steam brake valve
The pressure capability of the flexible hose is provided by the outer braiding with an annularly corrugated inside tube providing the flexibility. As such, the failure of flexible hose is down to abrasion of the braiding. Where the hose had to pass through a hole and could chafe the sides, I wrapped silicone fire sleeve (silicone coated glass fibre) around as protection. While the hose can move about, it is now well protected from chafing. For hose such as the steam brake supply, pressure is applied and released. The hose hardly moves with the changes in pressure.
3/8” whistle and pressure gauge feeds, the latter incorporates the loop
For the boiler pressure gauge feed, I was able to wrap the hose into a loop to keep the steam out of the gauge.

The whistle has been supplied by flexible hose too; however, this may be the cause of the whistle seeming to be choked with condensation unless used frequently. Despite using various sizes of orifice to regulate the steam flow rate to the whistle, it always has to ‘clear its throat’ before coming on tune. I suspect that condensation accumulates in the inner tube corrugations with the hose not being lagged so further investigation is required. Suggestions?
1” PTFE water feed hose from feed pump to single check valve
With the pulsing of the Worthington Simpson pump, the 1” PTFE hose moves about quite a bit. I’d be better off with an anti-shock bulb at the pump end to smooth out the pulses.
¼” PTFE hose, 11 feet long section in the cylinder oil feed
The cylinder oil feed takes a tortuous route around the pipe work underneath and then circumvents the boiler. There was an eleven feet gap in the original pipe to the regulator assembly. The ¼” PTFE hose filled this gap very elegantly. I used ‘nutserts’ to attach the hose to the boiler cladding.

In summary, I would say that flexible hose is an elegant alternative to copper tubing; however, there are various factors to be borne in mind.

1. It lacks the authentic appeal of copper piping.
2. It can be ordered ready to fit (provided you get your specification right – not trivial and prone to mistakes).
3. The hose is flexible but cannot twist and can thus be awkward to fit at times.
4. The length of the fittings may get in the way. Consider an elbow fitting if a right angle exit is needed.
5. For short hose lengths, take great care about the minimum bend radius specification.
6. For joins in the flexible hose such as a ‘T’ joint, the standard hydraulic cone fitting can be retained. They work very well between themselves but won’t mate with the original copper washer fittings.
7. Any type and size of fitting can be ordered for any diameter of hose. This could give some pretty bizarre assemblies but technically it is possible.
8. Lengths have to be thought out carefully. It is tempting to think that a bit extra will make life easier. In fact the extra can get in the way and it’s ‘return to factory’ if it needs shortening.
9. Chafing must be prevented. An annual inspection of the braiding should be performed – no different to any other pressure tubing.
10. If vibration is likely, make sure the hose is secured in place as with any other tubing.
11. Stainless steel fittings are the norm. There is little saving to be had with none stainless types.
12. Copper washers are needed for flat face parallel thread fittings (same as with copper tubing fittings). I also used Steamseal to make sure.
13. The PTFE inner lining can kink if twisted.

The entire set of flexible hoses was in the region of £1000. Would I recommend this approach? Certainly!

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