One of the best features of braided packing is its flexibility and ease of use. In today's world of maximizing plant efficiency with limited maintenance resources, some important steps to follow when installing packing are often overlooked. One of these is cutting braided packing.
As Ben Franklin stated, "An ounce of prevention is worth a pound of a cure." This is true about packing, since installation is the key to long packing life. Cutting packing rings incorrectly can result in a low mean time between failure (MTBF) for valves and pumps and is easily preventable with some basic training. The best way to cut packing accurately is to focus on a few simple steps:
- Use a mandrel to cut rings.
- Use a sharp knife.
- Use a forceful cutting motion.
Sometimes, bad habits that are developed when cutting packing can start at a plant and severely limit packing performance. Training and education will help prevent bad maintenance practices and provide an understanding of the importance of the task. Unacceptable practices for cutting packing rings can be handed down from technician to technician because it is hard to see the effect that they have on the sealing failure of the equipment.
One of the more common practices is using the packing rings that were removed when unpacking a valve or a pump as a length guide for cutting the new rings. The problem is the rings that were removed could have been incorrectly sized the last time so the error will just be repeated. Also, the rings that are unpacked might have been chemically attacked and may have shrunk or become deformed while in service.
The most accurate way to cut packing rings is to use a mandrel that is the exact size of the shaft or stem. By taking the mandrel and placing it in a vice and wrapping the packing around it, a better-sized packing ring is cut because the ring is in the same tension and stress as it would be in the equipment.
The rings should either be cut on the mandrel at a 45-degree angle for a skive cut or a 5-degree, almost-straight cut for a butt cut. Check the installation instructions of the packing manufacturer for the correct cutting type required by the application.
After each ring is cut, it should be wrapped around the mandrel as a double- check to ensure that the ends meet each other. This is similar to a "go/no-go" gauge. Getting good at cutting rings with a mandrel does take some practice with the packing/pulling and wrapping technique. Focusing on the end result of the ring fitting on the mandrel with no gap will quickly provide a feedback loop to the perfect practice. Each style of packing will pull around the mandrel slightly differently.
The second component to an accurately cut packing ring is the tools that are used. Another major poor practice is using a pocket knife to cut packing rings. A number of major issues can arise from using this type of cutting tool.
The first problem is that most knives are not very sharp, and a dull knife will make the cutter use a back-and-forth cutting motion instead of a clean slice with the packing. This back-and-forth motion causes the packing to open up (bloom), making installation and sealing more difficult. A sharp knife makes a clean cut that keeps the strands together in the original shape. Cutting packing will dull any type of knife quickly, so make sure there is easy access to knife sharpeners.
The correct way to cut packing is to have the knife at an angle to the packing (handle higher) and not parallel to it. This method results in better leverage on the packing, resulting in a better cut. Many tutorials are available that highlight this method for cutting in the kitchen. Correctly cutting a piece of packing is very similar to cutting in a professional kitchen.
A couple of other simple ways to create a good environment that will result in a better cut ring are to order (or get a machine shop to create) accurate mandrels for all stem and shaft sizes first. The other is to have some good knives and access to a knife sharpener in the cut ring area.
For more information, visit www. chesterton.com, call (978) 469-3993 or email Ron.Frisard@Chesterton.com.