 | Aviation Fueling Hose Assemblies, Permanent vs Reusable couplings (published in GSE today September 1998) |
By Bill Lesser, Director Marketing & Sales, Hewitt USA
with thanks to Trevor A. Sanderson, Fuelling Equipment Specialist
Shell Aviation Ltd.
Until now, customers in the United States have purchased aviation fueling hoses with permanent style couplings. These couplings must be assembled by an approved coupler using special equipment and they are designed not be recoupled in the field.
For at least 20 years, the rest of world has accepted use of both the permanent type and the specific type of reusable coupling to be discussed here.
These reusables have the unique capability of being safely recoupled in the field. This can facilitate replacement of damaged couplings and/or the cutting back of the hose assembly to remove kinks (very common near the reel or fueling nozzle) and damaged or worn areas.
Current acceptable reusables are made to specifications which meet or are equivalent to the German VG.85.328 standard. . Others types of reusables are not acceptable - such as Bandit clamps (2 steel bands) on ribbed hose tails, and the Scovill 3R / United Metal style with a compressible /spring coil like ferrule.
Hewitt USA has supplied aviation hose assemblies with reusable couplings to our export customers for more than five years. Hewitt USA performed API 1529 coupling security tests on reusables as far back as 1993 and re-qualified reusables for use on Hewitt 4113/4053 hose at the end of 1996. We have assembled and shipped hundreds of hose assemblies with reusable couplings.
Since the current API 1529 4th edition1, the new proposed API 1529 5th edition, and the new EN 1361 standard, regulations do accept reusable couplings as a suitable coupling
for aviation fueling hoses provided that the appropriate security burst and cycling tests are performed. We believe it is necessary, therefore, to provide information on the reusable couplings for US customers.
Reusable couplings come in two varieties: Four Bolt Clamp and Pin Clamp
Four Bolt Clamps are the most commonly used type.. Pin types are also used but are only offered by three other hose non US manufacturers.
Four Bolt Aluminum Clamp Reusable (see photo) - Four bolt reusables use two forged
/ hot stamped aluminum half clamps joined together by four bolts and nuts. European
specifications do not allow cast material to be used. The two clamp halves are placed
over the hose and fitting.
There are two methods used for initial assembly of the clamps to the hose. Using a vise,
the two halves are closed together until two bolts (diagonally opposite) can be inserted
and started. These bolts are tightened until the other two bolts can be inserted and
started.
The second method is to use two long bolts to start the tightening process until the
appropriate short bolts can be inserted and started on the opposite bolt holes. The long
bolts are removed and the other two short bolts are inserted and started.
After either of the above starting methods, the bolts are tightened alternately (similar to tightening a gasket cover) until the acceptable clamp gap and/or torque is reached.
Hewitt uses a combination of both criteria as we believe that only using the clamp gap can give a false reading. This will be discussed later.
Four bolt clamps are more forgiving than the pin type as they will work over a slightly wider range of hose OD per hose ID size. Also, multiple OD sized clamps are available for the same ID hose size. This allows for better matching of clamp size to hose OD.
Clamps must be properly matched to hose OD. If the clamp, after tightening, completely closes together leaving no gap between the halves, than a smaller clamp should be selected. On the other hand, a clamp that is too small for the hose may leave a gap (over 3 mm) that is unacceptable or if tightened too much may cause overcompression of the hose and/or fitting. In this case, the clamp gap (1 to 3 mm) does not necessarily mean the fitting is assembled properly.
We can not emphasize enough that it is very important that the correct size aluminum clamp be used to match the hose OD . There are different sizes of four bolt clamps available for the same ID size fitting and one clamp that is correct for one manufacturer's hose may not be correct for another. One must contact the manufacturer to confirm acceptability.
Pin Type Aluminum Clamp Reusable (see photo) - Pin type reusables use two aluminum half clamps joined on one side with a permanent pin acting as a hinge. The clamp is placed over the hose and fitting. Using a vise, the two halves are brought together until a removable pin can be inserted into the pin hole to permanently lock the clamp in place on the hose. This pin can be removed later if the user wants to salvage the fitting and recouple it.
Pin clamps have the advantage of easy assembly with only a vise and hammer, but are suitable only for hoses that fit within the rated hose OD range of the coupling and should not be used on hoses outside these OD ranges.
We cite the above because the allowable tolerances for hose OD, as noted in API 1529, are such that acceptable aviation fueling hose can be produced that falls outside the recommended OD ranges for available pin type reusable couplings. Hewitt USA does not use pin type clamps on our aviation fueling hose for this very reason.
Matching the clamp to the hose OD is not something that is new to aviation hose assembly and is not just a reusable problem. Permanent couplings (internally expanded type) require the proper matching of ferrule to hose OD. Ferrules come in several sizes per hose ID.
Since all assembly is done in the factory, the proper matching is done by experienced and qualified assemblers. Improperly matched ferrules to hose do not leave the factory due to tight controls, experience, and the required API 1529 proof test.
Important tests:
This brings up the most important point of assembling reusable couplings in the field. The hose assembly must be retested to API 1529 hose assembly proof test requirements before returning the hose to service.
Finally, whether the reusable couplings are assembled by the hose manufacturer, qualified distributor, or are assembled in the field, it should be a common practice to check and retighten the four bolt clamps one or two days after assembly. The reason for this is that the rubber under the clamp may continue to flow into the clamp shape after completion of assembly to proper specifications and, therefore, cause one or more bolts to become slightly loose. Users, upon receipt of a new hose, should also perform this routine procedure.
How good are reusable couplings?
Properly assembled, they are excellent. Hewitt has not experienced any problems with the sealing and/or the holding power of reusables. Outside the USA, most fueling operations are done by the International Oil Companies such as Esso Air, Shell Aviation, Air BP, Texaco, Caltex, etc. and, almost exclusively, they specify reusable couplings of the type discussed. Indeed, Shell Aviation Limited, who operates at over 650 airports outside the USA, has told Hewitt that they do not favor permanent couplings for safety reasons.
Benefits to the customer?
The ability to reuse the coupling and do field assembly and repair is an extremely strong benefit both as a cost and a time saver. If a hose becomes damaged, the inconvenience and difficulty of replacing a permanent coupling, or perhaps the cost of having to buy a new hose earlier than anticipated, may lead to a "fault tolerance syndrome" and give rise to an increased hazard.
Furthermore, it is clear that reusables serve a useful purpose in remote locations which do not have access to assembly equipment.
Weaknesses??
Initially, the reusable fitting is more expensive than permanent couplings, but over an expected life of 5 to 10 years, this extra cost becomes a marginal expense and may well save the customer from having to buy a new hose.
It may be difficult to control the proper assembly and test in the field. It should be noted that according to the experience of Shell Aviation Ltd, where couplings have to be replaced, the clamps themselves are clearly marked with the range of hose outer diameters they are suitable for, and it is a simple matter to compare those on the new coupling with those on the coupling being removed. Written work practices and training, together with the simplicity of assembly, have not led to any incident of significance over many years even though the typical test pressure after assembly is 300 psi (Shell procedure) as opposed to 600 psi cited in API.1529 (Grade 2 ref. )
It remains to be seen whether or not reusable style couplings become an acceptable method of attachment by US customers. Assembly control and potential liabilities should be assessed. But, most certainly, we must take a look at these specific and well proven types of couplings to see if they are a viable and/or necessary product for the US market.
1 - It should be noted that API 1529, 4th Edition wording, while allowing reusables, does state in appendix C under recommended recoupling practice that recoupling must be done by the original manufacturer or qualified distributor . The new proposed API 1529 5th Edition fully allows reusables both for factory and field assembly provided a written procedure is in place. (refer to paragraph 7.1 and Appendix C)