Choosing the Right Pipeline Pig for Process, Hygienic and Sanitary Applications
The pig (the projectile that travels through a pipeline to transfer and recover product), is arguably the most important part of a pigging system.
There are a wide range of pig designs available for industrial processes and hygienic or sanitary applications. They come in different shapes, sizes, materials and designs. For example, some pigs have fins, some are made of assembled parts, some are spherical, cup shaped, bone shaped, cylindrical or bullet shaped.
This article provides information, guidance and advice to help you choose the right type of pipeline pig for your liquid transfer operation or application.
Categories of Pig
One of the key factors when choosing a pig is to ensure it will effectively work with the product being processed. There are generally three categories of pipeline pigs for processing applications. These are:
Used in processing food, beverages, chocolate, confectionery and so on, these are pigs made from FDA approved materials
Mainly used in processes in the manufacture of personal care and household products which contain traces of silicone such as shampoos.
For processing chemicals, solvents, paints and similar products.
General Requirements for Choosing a Pig
Like any system component, pigs should have a long working life and include and a minimum usage guarantee. They should be available in a range of sizes to suit different pipeline specifications. To enable full automation, pigs should be fully detectable and ideally be provided with a purpose-designed pig detection system.
To reduce contamination risks, pigs should not contain any solid magnets. For use with beverages and food, pigs need to be manufactured from food grade material. If using clean in place (CIP), pigs should be compatible with your processes and allow steam cleaning to a reasonable temperature without degradation.
While being flexible (so that they can travel around bends and still efficiently recover product), ideally the pig will be of one-piece, full contact design rather than assembly; that is, not include caps, fins, or assembled components that could catch, break or fall off.
Above all, they should be effective. They should maintain complete contact with the inside of the pipe at all times and recover upwards of 99% of product.
The Importance of Two Way Operation
To enable efficient operation and closed-loop automation, it is essential that pigs should be able to travel in both directions (i.e. they are bi-directional) without having to be adjusted or turned around. Having to physically turn a pig around causes disruption and delays during processing. Single or uni-directional pigs are also hinder pigging system automation.
For automation and PLC control, there needs to be some form of reliable pig detection at key points within a system. Usually, a pig contains magnetic material which is detectable using pig detectors or pig detection systems. When a pig passes a pig detector, it creates a signal for the control system to act on. This could be, for example, starting a CIP sequence, venting the line, returning the pig, reverting to normal processing and so on.
Magnetisation is the preferred method of pig detection because it is precise, reliable and enables non-intrusive detection.
Avoid Pigs with Solid Magnets
For detection, HPS pigs have a flexible, silicone-based magnetic core. This is moulded to the outer of the pig so it in effect becomes part of it. However, many other designs of pig contain solid magnets. We do not recommend using pigs which contain solid magnets for a number of reasons.
Solid magnets can break free from the pig, shatter and contaminate the product. This renders the entire batch unusable. Some types of pig, often the finned type, contain solid magnetic strips along the spine of the pig. These strips pose an additional risk as they can snap, and in some cases the whole pig will break in to two or more pieces.
The flexible silicone-based magnetic core used in HPS pigs will not shatter, fragment or break free from the rest of the pig and so avoids the risk of contaminating the processed product. The flexibility of the magnetic core also means HPS pigs can travel around bends down to 1.5 D bends while still efficiently recovering product.
Problems with Finned Pigs
Over the years, HPS engineers have experimented with finned pig designs, both new and existing. However, we do not recommend using finned pigs. While initial trial runs may show that finned pigs recover similar levels of product to full contact designs, they present a number of problems once they are deployed in working systems.
One of the main issues with finned pigs is product bypass. Each fin will let a small amount of product past. While initially this has no effect on overall product recovery, the product builds up in the gap between the first and second fin. Then, the second fin will let this built up product past it in to the next gap, and so on. Eventually the product bypass becomes significant. Even if the pigging process is stopped before this is allowed to happen, the shape of finned pigs makes them difficult to clean. Removing the product between the fins takes time and effort to perform effectively, making them far less efficient and effective than streamlined, full contact pig designs.
A further problem with fin shaped pigs is that the spine of the pigs is usually of small diameter, making the pig weak and needing to be replaced regularly. If they are left to break, this can cause serious contamination issues, especially if they contain solid magnets as described earlier.
Many people are surprised by the speed at which pigs travel through pipes: it is extremely quick! This is a great advantage because the entire process is fast, often completed in a few seconds causing minimal delays to processing and production. While the actual speed of pig travel depends on a variety of factors, 10 metres per second is not uncommon. Therefore, it is important that the pig design, materials and shape, can cope with high speed without damage, excessive wear or deterioration.
Alternative Pigging Technologies
Recently there has been interest in using other types of pigging technology in process and hygienic applications. An interesting development is ice pigging.
Put simply, Ice Pigging is the process in which an ice slurry is pumped into a pipe and forced along inside in order to remove sediment and other unwanted deposits.
While ice is suited to some larger-scale utility applications such as those used by water and sewage companies, at present we do not believe ice pigging is a viable alternative for the majority of hygienic and process pigging applications. This is because ice pigging is much less effective at product recovery than projectile pigging, while at the same time being more expensive to set up and run.
Ice pigging product recovery levels are low, leaving upwards of 10% of product in the pipe, compared with less than 1% with HPS pigging solutions. Product seeps in to the ice pig so there’s significant bypass, plus ice pigs melt in the line, which dilutes the processed product with water. The ice or water remaining in pipe has to be removed after pigging, which adds time and resources to the overall process.
The ice pigging process is relatively slow and uses a lot of power, requiring a large machine to constantly freeze the ice, making operational costs expensive. Ice and slush is also difficult to detect with precision, so reliable automation and PLC control is not yet possible.
What’s the Best Type of Pig?
There are pros and cons to nearly every design of pig. However, HPS has been in the pigging business for over 21 years and helped bring the benefits of pigging and product recovery systems to a wide range of businesses throughout the world.
The HPS Research and Development Department has assessed most pig designs for food, beverage, homecare, paint, personal care, pharma, chemical and many other applications. We consistently find that the full-contact, fully moulded, flexible pigs with magnetised silicon core outperform all others. That’s why it is the primary type of pig we supply.
Need Help with Choosing Pigs or Pigging System Design?
From spare pigs to comprehensive pigging system design and implementation projects, HPS pipeline pigging products, customer service and specialist knowledge are second to none. HPS Pigs are extremely robust, have a long working life and recover over 99.5% of product from a full pipeline. We have thousands of systems installed worldwide and unmatched pigging expertise in our best practice solutions.
To find out more about pigging and get your project started, then please get in touch.