Usage — Who & Why?The chemical and coatings industries are prime users of pigging systems. Examples of companies that are using these systems include Sherwin-Williams, Sun Chemical and Bulk Molding Co. Pigging is used for displacing the bulk of the product during the initial phase of a clean-in-place process. It is used to capture valuable product between product changeovers, and it is sometimes used as a simple inexpensive batching system acting as a single or multiple positive displacement measurement system without all the moving parts of metering systems.
Motivation – The Bottom LineIf all your process lines were dedicated, then pigging would most likely be confined to cleaning or simple evacuation at the end of a production run. However, if your process lines are time shared, then pigging becomes a very important part of your operation. This is especially true where you are changing formulations, as pigging will reduce downtime between product runs. With the new environmental regulations being enacted, discharge wastes and usage of clean-in-place chemicals have become a major item in cost benefit studies as to pig or not to pig. There are situations where the value of the end product may not be great, yet the environmental costs for not pigging will drive the decision to pig. It would not be unusual for a company that is using a high-end hydrocarbon CIP chemical to have the used chemicals hauled to a recycling plant to be filtered, distilled and returned for reuse. This is a very costly proposition. Over time, the movement of hazardous chemicals brings all sorts of governing bodies into the scheme of things. In fact, a cost benefit study run in 2001 could very well have completely different results than that same study run in 1999.
Pig Types — What’s Out There?For a new pipeline network, it is best to start with a Power Brush Pig. This is a foam body urethane coated pig with nylon bristles for stainless pipe or flame-hardened steel bristles for carbon steel pipe. Driving this pig through the system prior to start-up will save much wear and tear on the process pigs to be used during normal manufacturing operations.
The Solid Cast Pig is the process operations displacement pig. Standard elastomers available are polyurethane (both standard and chemical resistant); nitrile; ethylene propylenediene methylene (EPDM) and DuPont’s Viton® (two rubber-like elastomers); neoprene; and other compounded materials. The choice of which elastomer to use for the pig material depends on the coating or ink’s chemical characteristics, as well as temperature.
Wiping Pigs are open cell urethane foam pigs of varying density often used as a follow-up swap when changing from a dark-colored product to a light-colored product, for example. Foam pigs are also pressed into service in piping networks, which were not originally designed for pigging.
Food Grade Pigs are usually confined to white polyurethane or white neoprene.
Generally, the Urethane Pig is the most popular for a number of reasons, including cost. Urethane lends itself to ease of changing the architecture, durometer and compounding with materials such as Teflon (durometer is a measurement term for the hardness of an elastomer). Furthermore, urethane pigs can be compounded to prevent the plasticizers from being leached out by certain products that might contain amines or other leaching chemicals. Urethane pigs are limited to about 200ºF continuous operating temperature. This limitation only applies to a bi-directional system where the pig remains in the system. In a unidirectional system, where the pig is removed after each run, this temperature limitation is not as stringent.
Temperature limitations for neoprene, nitrile, EPDM and Viton are 230ºF, 250ºF, 300ºF and 400ºF, respectively. These materials might be selected when urethane or chemical resistant urethane is not chemically compatible with the product being pigged. Again, exposure time comes into play. Does the pig remained continuously in the system in contact with the product or, as with a unidirectional system, is it removed after each pigging run? The correct approach on this issue is a simple soak test to look for swelling or color change, which usually indicates leaching out of the plasticizers. If the product is of a proprietary nature, a soak test will more than likely be required.
Existing Systems — Always Room for ImprovementAs operations change with time, and the system remains fixed, problems can unfold. A survey of the pigging system and its operation can quite often result in improved performance merely by addressing pig design, thus avoiding expensive piping modifications. Often an existing system will have pipe or tubing of one ID, hose of another ID, and couplings of still another ID. In this case, open cell foam pigs are usually pressed into service as a one-size-fits-all-type answer to the problem. However, the open cell foam pig soaks up product, looses its structural integrity and its life is limited. In this case, a closed cell urethane pig, which has an architecture with small diameter guide discs and sufficient diameter wiping discs, will greatly improve the overall pigging operation.
At times, a survey can uncover a situation where the plant air filter regulators used for launching the pigs have been undersized, forcing the operators to boost the pressure higher than required in an attempt to compensate for the lack of sufficient volume. Again, this is not a serious problem as filter regulators are relatively inexpensive. The correct pressure with sufficient volume of drive gas gives operations the best pig velocity control. This is most important on the return run for bi-directional systems, where the pig is in a gas to gas situation. Generally the operation should drive the pig at three to five feet per second during the fluid displacement run, and to extend pig life, no more than five to seven feet per second on the return run to home station.
Design — “Grass-Roots” SystemA basic product recovery pigging system does not require a high capital expenditure. Therefore, there is no need to “cut corners” with the design as any initial savings will be lost to higher operating labor costs in a short period of time.
With either a unidirectional or bi-directional system, there must be a launcher to launch the pig for the product displacement run and a receiver to catch the pig at the end of this run. A bi-directional system merely has two identical launcher/receivers at each end of the piping network. The “piping network” may be made up of carbon steel, stainless-steel tubing, hose or a polymer such as high-density polyethylene tubing. Launcher/ receivers are normally offered in carbon steel, stainless steel or, in some cases, fabricated out a polymer such as HDPE. Launchers and receivers are offered with blind flange closures or quick-opening closures. Each launcher/receiver is usually fitted with a pressure gauge, a product drain and a pressure/regulator flow control valve.
PiggingUsually, a pig detector is mounted in close proximity to both ends of the piping network. This takes the guesswork out of locating the pig. It further lends itself to interfacing with a PLC when the time comes to automate the process. A pig detector is either intrusive or non-intrusive. The latter type requires that a magnet be imbedded in the process pig to trip the switch.
All tees, Y-types fittings and such must have rail or bars welded across to guide the pig past these alternate passages. All inline valves must be of the full port variety. In more complex systems, pig switches are used to divert the pig from one path to another. Additional complexities would be the addition of flow transfer panels or a “scramble box” to allow maximum flexibility between raw material storage tanks and the product mixing or compounding areas.
Pressure transmitters and piggable inline mass flow meters can be added that provide full automation of the pigging system; CIP, product compounding, and batch sizing can all be computer controlled to formula.
Just as EPA regulations must be considered, OSHA should be kept firmly in mind when designing a system. Ten years ago, safety personnel would have thought nothing of catching a pig in an open basket affair. Not today! More often, safety engineers and industrial insurance people require a pig catcher that is a closed bullet-proof pressure vessel.
Finally, let us not forget Mr. Murphy and his laws. When designing a pigging system, always design for a worst case situation. On a gas driven system, which most are, make a provision with fittings to hydraulically drive a stuck pig all the way through the system. This takes the “fear” out of the equation.
For more information on pigging, contact John Nehrbass, P.E., Fluid Recovery Systems Inc., 429 Kerry Court, Palatine, IL 60067; phone 847/359.7552; fax 847/776.0311; e-mail email@example.com.