Steel process lines and tubes are commonly cleaned using waterjet systems with pressures up to 40,000 psi. The risk of damaging these lines depends on a variety of operating parameters, such as jet pressure, jet angle, jet rotation, and rate of traverse.

These lines vary from small diameter heat exchanger tubes to larger pipes. The highest energy concentrations with the greatest risk typically occur in small-diameter tubes. Testing was performed to identify the operating parameters for which no damage would be caused to steel process lines.

Testing showed that it is important for operators to avoid letting stationary or rotating jets stand still inside tubes when operating at pressures above 10,000 psi.

Overlooked risk

Waterjet cleaning of steel tubes, piping and vessels is routinely conducted at pressures from 5,000 to 40,000 psi. The concerns of plant operators typically have been damaged by high-pressure waterjets and mechanical wear caused by the rubbing of the nozzle against the wall of the tube.

Another sometimes overlooked failure mode in piping and tubing is corrosion related to the plant process, in both carbon steel and stainless steel materials. There have been instances of blame placed on waterblast contractors for damage that was actually due to corrosion, because the waterjet cleaning removed material from the corrosion pits and cracks that had been keeping the tubes from leaking. A corrosion-damaged stainless steel tube is shown in Figure 2.

The pitting caused by corrosion has relatively sharp edges. A waterjet attacks steel through the process of cavitation erosion by water droplet impact, which looks much more smooth and rounded. The purpose of this testing was to determine operating parameters that will not result in waterjet damage to the tubes, and to illustrate what damage caused by waterjet action looks like.

Testing parameters

Tests were performed using a single fixed jet and multiple rotating jets. Figure 3 illustrates the typical test arrangement for multiple rotating jets. The standoff distances used would be typical of tube and small-pipe cleaning, but would be considered relatively close for pipe sizes larger than six inches unless provisions were made to place the jets closer to the pipe wall.

For tests performed at and below 20,000 psi, two types of nozzles were used: one made of drilled steel of poor quality, and the second high-quality stainless steel nozzles with flow straighteners. Tests performed at 36,000 psi used good-quality sapphire nozzles. The tube samples on which the tests were conducted consisted of new 304 stainless steel and 1018 DOM carbon steel, both 1.88 inches inside diameter.

The amount and type of damage was found to be quite similar in the stainless steel and the carbon steel, so most of the testing was done in the 1018 carbon steel to allow for better visual contrast. The damage was quantified and compared in terms of depth of material removal.

Stationary jets

The first test with stationary jets used a poor-quality drilled steel nozzle orifice, typical of small-tube cleaning nozzles without replaceable inserts. An orifice size of 0.042 inch was drilled at 90 degrees in the head and was tested with the jet perpendicular to the tube wall, at a standoff distance of 0.38 inch. Tests were run on both carbon steel and stainless steel 10, 30 and 60 seconds at pressures of 10,000, 15,000 and 20,000 psi. Figure 4 shows the damage created at 20,000 psi with a drilled jet in carbon steel.

There was very little difference in results between carbon steel and stainless steel. At 10,000 and 15,000 psi, there was no damage at the 10-second exposure, while slight damage occurred at 20,000 psi. The damage at 10,000 psi can only be felt as surface roughness. After exposure for 120 seconds, the damage did not increase.

The next series of stationary jet tests used a high-quality steel nozzle with a flow straightener and an orifice diameter of 0.038 inch, exiting a nozzle head at 90 degrees to the tube wall. Due to the nature of the cavitation mechanism by which the waterjet damages the tube wall, this more coherent jet did not produce any damage up to or including 20,000 psi after 60 seconds when the standoff distance was 0.38 inch.

Cavitation and damage did occur when this nozzle was tested with increasing standoff distance at 15,000 psi for 30 seconds at each point.

The final series of stationary jet tests, conducted at 36,000 psi, used a sapphire nozzle insert, diameter 0.024 inch, tested at 80 degrees to the tube surface. Results showed that allowing a nozzle operating at this pressure to stop rotating or traversing can result in significant damage to the tube wall. Figure 9 shows the test sample used and the resulting damage.

Rotating jets

Use of rotation as a means of keeping the jet moving over the surface can greatly reduce or eliminate damage to steel tubes and pipes. A stationary jet at 20,000 psi can cause more damage than rotating jets at 36,000 psi.

Damage by rotating jets will still be incurred if the tool is left rotating in the same place. The amount of time to cause damage is dependent on pressure. At 10,000 psi, a head rotating 500 rpm with three drilled steel jets of 0.033-inch-diameter at 85 degrees in the same path, at a standoff distance of 0.38 inch, did not cause any damage after four minutes in the same location, and did only slight damage after six minutes.

With the same conditions at 15,000 psi and 0.031-inch-diameter jets, no damage was caused after 60 seconds, and slight damage after 120 seconds. At 20,000 psi with 0.031-inch-diameter jets, slight damage was caused after 30 seconds. The damage produced at the latter condition is shown in Figure 12.

At 36,000 psi, a small amount of damage was caused after 10 seconds, and a fair amount was created after 60 seconds (Figure 14). When operating at 36,000 psi, even with rotation, the tool should be kept moving along the tube.

Jet angle

The effect of jet angle of impingement on the tube wall was studied at 36,000 psi with rotating jets to determine if a shallow angle resulted in reduced damage. A single 0.024-inch sapphire nozzle was used. At 10 degrees, no damage occurred after 60 seconds of exposure, but a small amount of damage did begin to occur at 20 degrees after 30 seconds, and after 10 seconds with a 30-degree angle.

Keep it moving

Waterjet damage to steel tubing and pipe is quite dependent on the operating pressure, rotation or other motion of the jet, and the amount of time the jets are left in the same place. Damage also depends on the standoff distance and somewhat on the quality of the jet.

There is a fairly high risk of damage at 36,000 psi; the jets must be kept continuously rotating and traversing along the tube or pipe. If the rotation or linear motion stops for as much as 10 seconds, damage may be caused. There is a decrease in risk when the pressure is lowered to 20,000 psi, but a stationary jet is still likely to cause damage at that pressure, and the operator should not allow a rotating tool to be left in the same place for 30 seconds or more.

The risk decreases considerably further at 15,000 psi. Rotating jets require more than a minute in the same location to begin to create damage, while a stationary jet could cause a small amount of damage after 30 seconds. Operating pressures at or below 10,000 psi pose a very slight risk of damage with any combination of conditions. n

Doug Wright is head of engineering, John Wolgamott is president, and Gerald Zink is vice president with StoneAge Inc. in Durango, Colo. They can be reached at 970/259-2869.