How Do Pigs Move Along Pipes?

Propellants Used in Industrial Process Pigging.

Many industries which process liquids, for example food, beverages, household, personal care products, pet food, pharmaceuticals and paint, use pigging systems to recover liquids and clean in the inside of pipes. The pipeline pigs (the projectiles that travel along the pipes to transfer residual product) are solid objects with no moving parts. They are unable to propel themselves, so instead are propelled along the pipes by a liquid or a gas.

Pressure Required to Pig a System

To move the pig, the liquid or gas propellant is pressurised behind it. Typically, the pressure is similar to that required to pump the liquid during normal processing. Some systems may need a slightly higher pressure to get the pig started, but once this has happened it is usually about the same as the pumping pressure.

In industrial process pigging systems, there are several factors which determine the precise pressure needed to propel the pig. These include diameter and length of pipe, number and radius of bends, viscosity of the product and so on.  If you have implemented a pigging system provided by HPS, our commissioning engineers will set the propellant pressure to provide optimum performance.

Speed of Pigspig moving through a pipe

The speed at which pigs travel is variable along the length of the pipe. Like the pressure required to move the pig, the actual speed at which it travels depends on a variety of factors. For example, pigs will tend to slow in around bends, and speed up towards the end of travel if there is less product to push. Pig motion tends to be less smooth if propelled by gases rather than liquids because of gas compressibility.

Even so, most people are surprised by the speed at which pigs travel because it is usually extremely fast (10 metres per second is not uncommon). Any solid projectile travelling at this speed poses potential safety risks, which is one of the reasons pigging systems should only be provided and commissioned by experienced pigging system designers such as HPS.

Types of Propellant

The type of propellant used largely depends on the application, and there are a number of considerations when deciding the best solution.

The most commonly used gases are compressed air, nitrogen and carbon dioxide. If using air, HPS systems always include specialist filters to sterilise it down to around 1 micron.carbon dioxide as a pig propellant

The most commonly used liquid is water, either mains or sterilised. In some system designs, the next product being processed is used to propel the pig. Occasionally liquids are used such as cleaning agents. In some applications specialist liquids are used, for example in some chocolate processing systems, which cannot use water, the pig is propelled by buttermilk.

Gases Used to Propel Pigs

Compressed air is a popular choice of pig propellant, simply because of its convenience (many factories and processing facilities already have compressed air systems in place). Compressed air is also relatively cheap, which keeps the overall price of a pigging system down.

Carbon dioxide is usually used with products which already contain it, such as fizzy drinks or beer, and when air could potentially damage the product.Nitrogen as a pig propellant

Nitrogen, being inert, is often used in pigging systems where contact with air could damage the product, such as with wine (which needs to avoid dissolved oxygen or DO). It is also used with products in hazardous areas which are easily combustible, such as solvents.

Note that, compared to compressed air, there are additional safety implications to take in to account when using carbon dioxide or nitrogen. Although the gases are non-toxic, the system location must have adequate ventilation to ensure that they do not displace oxygen. Unless there are detectors in place, people in the vicinity are unlikely to notice lack of oxygen early on, because it is not evident by odour, colour or taste. Low oxygen levels can cause faintness, confusion, dizziness and other symptoms – even death.

Why Liquids Are Used to Propel Pigs

Sometimes it is preferable to use a liquid (usually water) to propel the pig rather than a gas. This is often just because it is convenient, for example there’s already a clean water supply set up. However, the non-compressible properties of water and other liquids can provide an advantage over gases.

Typically, liquids are used to propel pigs because the pig initially needs a fairly high pressure to move it. Liquid may also be used because higher levels of control are required, or the system demands a smooth, steady pig motion.

Liquids are usually the preferred propellant when the product is viscous. This is because the pressure required to move the pig using gas or air can be too great to be easily or safely provided by a gas. Water is much less of a potential hazard at pressure of say 20 bar.

Liquids are generally more controllable than gases because they are pumped, rather than being provided from an air supply or compressor. Combined with their incompressibility, this enables smooth, highly controlled pig movement.

Using Both Liquids and Gases

Sometimes pigging systems can use both a liquid and a gas. For example, water may propel the pig to move the product to its destination, and when complete the pig is then propelled back to source by compressed air.

Professional Advice

Occasionally there can be no clear deciding factors on whether to use a gas or liquid to propel the pig. It may come down to convenience, cost or personal preference. However, it is important that you choose the most effective, suitable and reliable solution for your particular process or processes. If you work with HPS, our pigging system design team will use their extensive experience in a wide range of industries, processes and products, to recommend the most suitable propellant for you.

If you’re thinking of implementing a pigging system, we strongly recommend you get professional advice. HPS is the world’s leading industrial process pigging specialist. Our friendly, knowledgeable engineers will be more than happy to discuss any aspect of pigging with you, so please get in touch.

For more information, please email, or find your nearest HPS office or representative here.