Friday, December 28, 2012

Waterjetting 4c - hoses and tubing

One of the first decisions one makes in connecting a pump to a nozzle is to select the size of the high-pressure pipeline that will take the water from the pump to the cutting nozzle. This choice has become a little more involved as ultra-high pressure hose has come on the market since this can be used at pressures that once could only be served with high-pressure tubing. However, at higher pressures the flexibility of hoses becomes reduced, both because of that pressure, and also because of the layers of protection that are built into the hose structure.

Much of the original plumbing, in the earlier days of the technology, used 3/16th inch inner diameter, 9/16th inch outer diameter, steel tubing. One reason for this was that, at this diameter, the tubing could be quite easily bent and curved into spiral shapes. And that, in turn, made it possible to provide some flexibility into an assembly that would otherwise have been quite rigid.


Figure 1. Early cutting nozzle with spiral coils in the high-pressure waterjet feed line to the nozzle.

When cutting nozzles were first introduced into industry, they were fixed in place, because of the rigid connection to the pump. The target material had, therefore, to be fed underneath the nozzle, since it was easier to move that than to add flexibility to the water supply line.


Figure 2. Early slitting operation (courtesy of KMT)

However, because feed stock can vary in geometry some flexibility in the positioning of the cutting nozzle above the cutting table would allow the jet to do more than cut straight lines. A way had to be found to allow the nozzle to move, and this led into the development of a series of spiral turns that high-pressure tubing can be turned through, as it brings the water to the nozzle. (See Figure 1). That, in turn, allowed a slight nozzle movement. By adding this flexibility to the nozzle, a very significant marriage could then take place between robotics and waterjet cutting.

The force required to hold a nozzle in a fixed location becomes quite small as the flow rate reduces and the pressure increases. (at 40,0000 psi and a flow rate of 1 gpm the thrust is about 10 lb). The first assembly robots that came into use were quite weak, and as their arms extended the amount of thrust they could hold without wobbling was small, but critically more than 10 lb. And this gave an initial impetus to adding jet cutting heads to industrial robots of both the pedestal and gantry type, to allow rapid cutting of shapes on a target material, such as a car carpet, where the ports for the various pedals and sticks need to be removed.

But this marriage between the robot and the jet required that the jet support pipeline be flexible, so that it could allow the nozzle to be moved over the target, and positioned to cut, for example, the holes for retaining bolts, without damaging the intervening material.

The pipe had to be able to turn and to extend and retract, within a reasonable range, so that it could carry out the needed tasks. Bending the pipe into a series of loops produced that flexibility.

A single full circular bend in the pipe will acquire sufficient flexibility that the end of the pipe (and thus the nozzle) can be moved over an arc of about 9 degrees.


Figure 3. Coils on a pedestal-mounted robot allowing 3-dimensional positioning of the cutting nozzle.

A large number of coils were required, since the tubing has only a very limited amount of flexibility in every turn. For example, if one wanted to stretch the connection by lowering the nozzle, then the several coils would act in the same way that the steel in a spring would as it extended. The movement can perhaps be illustrated with the following representation of a set of spirals, with metric dimensions.


Figure 4. Schematic of a series of coils, arranged to allow the nozzle to feed laterally.

Each spiral will also allow a slight angular adjustment also, and these add up, as more spirals are added to the passage.


Figure 5. Angular movement allowed per spiral. This should not exceed 9 degrees per turn.

While, in many modern assemblies this may seem to be a quaint way of solving the problem, back when these systems were first put together it was very had to find high-pressure swivels that would operate at pressure for any length of time. In those days we had one source that provided a swivel that would run for many hours provided that all the external forces could be removed from the swivel irself. But the moment an out-of-alignment force hit the swivel it was ruined. In another application we had tested every swivel we could find, that would fit down a six-inch diameter hole, and had found one that would run for ten minutes. To finish our field demonstration, where we had to drill out 50-ft horizontally from a vertical access well, we had to continuously pour water onto the joint to keep it cool, and the manufacturer stood by with a pocket full of bearing washers that we had to replace every time one started to gall.

But that was over thirty years ago. Now the connections from the pump to the nozzle can flow through ultra-high-pressure hose with a flexibility that we could barely imagine. And ultra-high pressure swivels will run for well over a hundred hours each without showing any loss in performance. It was, however, a gradual transition from one to the other.


Figure 6. Ultra-high-pressure feed to a nozzle, using coils and swivels

There are a couple of additional cautions that should be born in mind, when laying these lines out. While hose is more flexible, it is liable to pulsing, and moving slightly on a bearing surface, under pump cycling. In most places this is not a problem, but if the hose is confined and bent, then it may cause the hose to rub against a nearby surface. Over time this can generate heat, and can even wear through the various hose layers.


Figure 7. Worn hose and the scuff mark where it was rubbing on a plate.

There are other issues with hose, smaller high-pressure lines can kink, when used in cleaning operations and this is a seriously BAD thing to happen, and I will discuss that in a future article. Similarly one must consider the weight of hose, particularly in hand-held operations, where it is important to address hose handling as part of the procedure, but again this will be discussed later.

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