Pushing a piston like object through a pipe to clean the pipe walls is known as ‘pigging’. This technique is widely used in the hydrocarbon recovery and processing industries and is beginning to be adopted in others such as food and pharmaceutical production. Pigging can do more than just cleaning; it can be used to enhance product recovery from a pipeline and can be strategically used to separate different products in the same duct.

Conventional pigs can only be used in relatively simple geometries. Specifically, pigs can be ‘pushed’ through uniform pipes with constant diameter, but find it difficult to negotiate bends or minor changes in cross sectional area/shape. Clever mechanical design and use of compliant materials to act as piston rings, has extended the use of conventional pigs to ducts with minor cross sectional area change (typically no more than 20%), however they cannot be used in complex topologies or when the geometry changes rapidly with position. This means that pigs cannot be used in relatively simple situations such as branching ductwork, pipes with valves, pipes containing intruding instrumentation or heat exchangers.

When topology constraints makes conventional pigging impossible it may still be possible to achieve some of the benefits of pigging by ‘flushing’ the system with a suitable fluid. As the flushing shear rates at the containment walls are modest compared to those achieved by a solid pig scrapping the surfaces, flushing tends to be less effective than pigging in cleaning duties. What is required is some smart material that acts like a fluid squeezing through complex geometries but behaves as though it were a solid scrapping the surface structures clean. Further, this smart material should be such that it never gets stuck in the geometry it cleans.

The Department has developed and patented the use of crushed ice in water to achieve many of the desirable characteristics of the smart material described above. The ‘ice-pig’ can cope with very complex topologies, can clean, product separate and product recover. Further, is does not get stuck, and even if it did, one would just have to wait a while for it to melt into water, which can then be easily drained from the ductwork.

Over the past 5 years the technology has been extended and applied to a variety of industries and application. For example it has been used in the food manufacturing industry to displace valuable food material from process equipment. This has many benefits including product recovery (actually recovering product and selling this rather than throwing it away), reduced effluent treatment (with less product being thrown away there is less demand for land fill and less demand on the water companies to clean up effluent), and reduced need for harsh cleaning chemicals. Other areas where the technology is being used includes the paints and coating industries (to clean out their pipes), the potable water supply industry (to remove loose fine sands from their drinking water supply pipes), even the nuclear industry is investigating the use of the methodology to remove difficult materials from ducts whilst keeping the effluent volumes to a minimum.

Figure 1 shows the back of the ice pig (ice is white) when it is followed by water (water has been dyed red). Both the ice pig and the water have been through a piston pump, into a 1 inch diameter duct, through two 90 degree bends, one 180 degree bed and then expanded out into the 3 inch plastic pipe which is viewed in the picture.

Figure 2 shows the demanding geometry of the system. The flow is from right to left. One can see the red water behind the with ice pig.

The three pictures in Figure 3 show a sequence during a simulated pigging process through a static mixer.

The flow is from left to right in the one inch tube at the bottom of the picture, the tube then goes through a 180 degree bend so that the flow is then from right to left.The tube then enters the 3 inch section containing the static mixer.

Whatever material is seen in the one inch tube, will, within 5 to 6 seconds find itself in the 3 inch tube. Hence on the first picture where we see the 1 inch tube full of ice, this ice is seen to have almost completely filled the three inch tube in the second picture. At this point in time, the 1 inch tube has red water flowing in it and this is seen to be displacing the ice in the three inch tube in the third picture (still remarkably sharp).

Static mixers

Although remarkably sharp interfaces are maintained between the ice and the water, the ice pig is still susceptible to the mal-distribution which occurs in static mixers.