Atlas Copco Rental is the number provider of 100% oil-free air (standard and high pressure) to the pipeline construction and service industry.
Pigging, a technique whereby a cylindrical plug is blown through a pipe, is often used to clean pipelines and other conduits; the plug seal fits in tightly against the interior wall of the pipe and thus cleans it. A plug of this type is called a pig because of its appearance.
However, the comparison in appearance has to be carefully sought in many cases because various types of pigs now exist, each with their own applications. The differences lie in the material type used, the type of seal and the erosion that is caused as a result. In some cases it is more like a solid sponge, while in other cases it may be a metal structure to which a seal is attached. A pig of this type can weigh several hundred kilograms.
The most common application is to clean pipes, with the pig scraping the edges to remove debris, material sediment and corrosion. Another application is water removal, where, for example, water left behind in a pipe after a hydraulic pressure test is removed. Pigging can also be used in some cases to equip the interior wall of a pipe with coating. A train is formed with two pigs with the product used for treatment located between the two. A train of this type can be allowed to move very slowly through a pipe to allow the product to work on the walls and the train can be allowed to stop at regular intervals, equal to the length of the train, to increase the impact time.
A final application is primarily found in the petroleum sector where pigs are used in pipelines to keep various crudes (petroleum types) separated from each other when these are being transported through the same pipe. Compressed air is used to drive the pigs. Incidentally, pigging is one of the classic applications in the compressor hire market.
CORRECT PRESSURE AND THROUGHPUT
The need for compressed air, both in terms of pressure and throughput depends firstly on the type of pig that is selected for a given application. Each pig is characterized by a Differential Pressure (DP), which complies with the power needed to get the item moving. This power depends on the pig’s weight but also the friction with the pipe wall. In many applications a light pig with less friction is used first to be certain that it does not become stuck when underway. If this succeeds, a changeover can be made in the next run to a pig with greater friction, which will also clean more effectively.
In any case, the pig must fit in well against the pipe walls; otherwise the air will be blown past it and the pig will remain behind in the pipe. In the case of sea pipelines, the DP has to be increased by the pressure difference that matches the depth of the pipeline. For trains with several pigs and liquid in between, the extra pressure required to bring the entire system back up to the surface can increase dramatically.
In addition to the pressure difference, the speed applied also depends on the pig type. If the speed is too high the seal against the wall can burn due to the friction, which means the scraping effect is lost and in the worst scenario the pig can get stuck if the compressed air can escape past it. A typical speed for pigging is one or a couple of m/sec. This speed determines the compressed air throughput that the compressor must supply.
CLEANING IN 3 STEPS
A pipe is usually cleaned in 3 stages. In the first phase, the pig is not used; compressed air alone is used to blow the pipe empty. To do this the pipe is closed at one end and placed under maximum pressure. When the valve is opened quickly afterwards, rapid expansion occurs, thus removing much of the debris and water present from the pipe. This is quite a dangerous operation in itself given the high speeds that can arise when the material is leaving the pipe. A large number of big installations are equipped with a “silencer”, a tower to which the pipe outlet can be attached so that the expansion can occur inside it.
The second step is the actual pigging, whereby a difference can be made between long and short pipes. In short pipes it is usual to work with one pig and then to examine the quantity of dirt and water it has removed. This process is subsequently repeated until it is clear that hardly any dirt is being removed. The big challenge here is to ensure that the pig does not get stuck, because then there is frequently no option other than to cut the pipe open. As was said, this is why a pig with a smaller Dp is first used. Moreover it must be ensured that some reserve capacity is available in the compressors. After all, by increasing the pressure the pig can be moved on in some cases, although this remains limited to the pressure that the pig can withstand itself.
Otherwise the risk that the seal will fail arises again. In this process, not only the pig’s Dp has to be taken into account; the fact that the pig will push quantities of water ahead of it during the first runs is also an influencing factor. If the pig does nonetheless get stuck, the question of where it is located comes into play. Use is occasionally made of intelligent pigs which have electronics on board and can even indicate their position. In other cases an idea of the location can nonetheless be determined using resources.
The pipe can be put under pressure in advance and then allowed to expand again, with the time required for this being measured. If the same is done with the pig stuck inside somewhere, the same exercise will take a fraction of the time, in proportion to the pig’s location. In long pipes it is more common to use a train with several pigs because the entire process takes longer and there is a wish to limit the number of runs. The separate pigs can be kept at a distance from each other with the help of compressed air or with materials such as glycol which also absorb some water while underway.
DRYING IN ACCORDANCE WITH CUSTOMER SPECIFICATIONS
Depending on the application another third step follows this pigging, in which the pipe is dried further. If the pigging has been carried out properly, the largest quantity of water will certainly have been removed, in the sense that no more puddles will be left in the pipe.
However, quite a lot of water can still be found overall in recesses at flanges and drains. For applications where this is unacceptable, dried air is blown through the pipe after pigging to take the leftover water away. The dried air can be obtained through partial expansion of compressed air, which means the relative humidity falls or, if this is inadequate, with the help of adsorption dryers that are placed after the compressor.
The length of time this is proceeded with depends on the customer’s requirements and can be verified by measuring the compressed air’s relative humidity when it leaves the pipe again. If possible, drying is encouraged by heating the pipe.
Some insulated pipes are equipped for this and contain heating tracks with electrical resistances or steam-based heaters. If these are not available, the compressed air can be heated before transmitting it through the pipe. The throughput applied during this drying can be selected at will. With a higher throughput drying will occur faster as a rule, but more air will also have to be heated.
In the case of pipes intended for transporting hydrocarbons nitrogen can be used in this last phase instead of dried air. Nitrogen has the advantage that it is absolutely dry and the pipe is also rendered inert immediately (which means that no oxygen is left in the pipe).