A patented lip design and the patented combination of PTFE sealing lip and sliding bearing in the lip seal element provide the new dry running seal “SeccoLip” from EagleBurgmann with particularly high flexibility. These technical features help the lip seal compensate directly and safely radial deflections of the shafts in agitators, mixers and reactors.
The sliding bearing tracks the complete lip seal element to the shaft movements. Since the lip and bearing are in one element, the sealing gap between the rotating shaft and the sealing lip remains virtually constant and the seal remains tight over the long term. Compensating elements such as O-ring, expansion washer or metal bellows are not required for reliable operation.
The modular seal was specifically designed for the operating conditions in the chemical, pharmaceutical, food industry as well as in water and wastewater technology. One or more sealing elements are combined in different possible arrangements to comply with the requirements.
Connections for a supply system are available. Due to the design features, a rolling bearing is not necessary but is optionally available.
The cartridge design makes the SeccoLip easy to install and safe to use. It is particularly suitable for a sliding velocity of up to 2 m/s (6 ft/s) and a pressure range of -1 to 6 barg.
How this application fits as a versatile solution.
Stem packing is a familiar product. The most common type is braided compression packing. Braided packing is used in a wide range of applications. Depending on the service, construction materials can be as diverse as plants or animal derivatives, mineral fibers or synthetic plastics and even metal. The process of cutting rings from rope packing, inserting them into a stuffing box and torquing them to the right density is common, but it is not always the best choice.
Another widely used manufacturing method is die-molding. It is the process of wrapping a material around a mandrel, placing it in a die and preforming it to make a seal. Using these and other manufacturing technologies, packing is found to work in applications as different as aerospace, heavy trucking and power generation. A review of some unusual applications demonstrates the versatility of compression packing as a sealing solution.
The Origin of Packing
Compression packing is an ancient technology dating back more than 5,000 years. Boats and ships used a rudder as a steering mechanism. The rudder shaft penetrates the hull of the vessel below the water line, so water can leak into the bilge. Ancient sailors, using the top technology of the day, would take pieces of clothing, sail cloth and rope, cover it with animal fat or wax and stuff it into the gap around the shaft. Eventually, a box was secured around the shaft and a gland, which could be tightened to compress the packing material, was created to improve sealing and longevity. The terms compression packing, stuffing box and gland come from these early sailors.
Over time, many improvements in packing construction and materials were made. Packing today can be made of flax, Kevlar, polytetrafluoroethylene (PTFE), graphite or metal. It typically has a square cross-section and is sold in precut rings or in large coils, as shown in Image 1. Synthetic aramid fibers are abrasionresistant and can handle higher temperatures and shaft speeds. PTFE has excellent lubricity and chemical resistance. Graphite coupled with mica or an aramid fiber can stave off the heat generated by a rotating shaft and provide long life in challenging applications.
Die-formed compression packings are excellent in terms of sealing performance and reliability and offer a wide range of long-term, low-emission and low maintenance products. See Image 2.
Not only are die formed rings easier and quicker to install, but the pre-compression increases the density of each ring and reduces the gland loads necessary to seat and compress multiple rings in the stuffing box. The result is lower friction on the shaft or the spindle, with improved sealing performance and a longer life.
Factor in STAMPS
As mentioned in an article previously published by the Fluid Sealing Association, (Sealing Sense, Pumps & Systems, March 2005), there are several key factors to consider when choosing the right packing. They include:
size or stuffing box bore
temperature inside the stuffing box application: whether it’s a pump, valve, mixer, refiner, process, characteristics such as pH level and chemical compatibility
motion: rotary, helical or reciprocal
pressure inside the stuffing box
surface speed expressed in feet per minute or meters per second
Over-tightening, excessive speed and improper installation can cause a system to falter.
In many respects, troubleshooting and failure analysis of compression packing materials is similar to the investigation of a crime scene. A good investigator knows how to gather clues from many different sources and put them together to understand what has happened. A good troubleshooter uses the same information gathering method, familiarizing themselves with the sealing materials, the process equipment and the systems where they are used.
Start by Interviewing Witnesses
The troubleshooter should seek information from the people who work with the equipment on a regular basis. Seal installers, maintenance personnel, operators, process engineers and others can all shed light on potential causes of failure. Some key questions should be:
How is failure defined? Some examples include excessive leakage, overheating, high rate of flush water consumption, excessive friction load and blowout.
Is this application the source of chronic seal failures, or was this an unexpected event?
Were there any changes to the seal material, the equipment or the overall process that preceded the failure?
Were there any system upsets or cleaning cycles that preceded the failure?
Can you describe the installation procedure?
Gather Information About the Victim
Knowing the limitations of the sealing product is a key step. The acronym “STAMPS” will help remember the key elements to ensure the right packing is selected for the application.
S: Size. Is the correct packing cross-section being used? Are the rings cut or formed to the correct length?
T: Temperature. Check the system temperature against the packing manufacturer’s established temperature ratings for the product.
A: Application. Some packings are made specifically for rotary equipment while others are intended for valves or static seals. Check to make sure the packing is suitable for the equipment where it is being used.
M: Media. This refers to the fluid being sealed. Check with the manufacturer or with compatibility charts to be sure the seal material is compatible with the media. If the media is slurry, abrasion-resistant materials may need to be specified. If the media is toxic, explosive or required to be contained within certain maximum allowable leakage requirements, then a packing must also be selected on the basis of its ability to seal at low leakage levels.
P: Pressure. Check the system pressure against the packing manufacturer’s established pressure ratings for the product.
S: Speed. Check the equipment speed against the packing manufacturer’s established surface speed ratings for the product. Surface speed is expressed in feet per minute or meters per second and not revolutions per minute.
Investigate the Crime Scene
When possible, observe the equipment while it is running. Can you see, hear, feel, smell or use a sensor to make observations? Smoke, vibration, grinding noises, the scent of burning fibers and system pressure fluctuations are only a few of the clues that can be noticed or measured while the equipment is up and running.
Examine the condition of the equipment. Most packings are robust seals that can handle less than perfect equipment condition, but there are limits to the amount of degradation they can withstand.
Valve stems and pump shafts or sleeves should be checked for scratches, corrosion pitting and general surface roughness. Rough surfaces can damage the sealing surface and result in excessive leakage and quick wear of the seal.
Excessive clearances at the top or bottom of the stuffing box can lead to extrusion of the seal material and intrusion of large solid particle into the seal area (see image 1).
In severe cases, excessive clearance may result in a seal blowout.
Most packings are not meant to function as both a seal and a bearing. In rotating equipment, poor bearing condition may result in shaft runout that “wallows out” the inside diameter of the seal. Misalignment may result in shaft/stuffing box offset that causes one side of the packing set to be heavily compressed while the other side is compressed much more lightly. A similar side loading of a packing set can occur in large horizontally oriented valves where the packing is forced to bear the weight of the stem.
Check to make sure all the parts are in place. During the breakdown, repair and reassembly of equipment it is possible to misplace parts. Equipment might be put back into service without seat rings, bushings, lantern rings, O-rings and other parts that are essential to proper equipment operation.
Look at the seal and the equipment as a part of a big picture.
Consider how this piece of equipment is affected by other equipment and control devices in the system. For example, is there a downstream valve that creates pressure spikes in an upstream pump seal when the valve closes and the pump is still operating?
Gear motors, pumps and stirring units keep process material in constant motion in the process industry’s production facilities. A large number of shaft seals are used at drive shafts to keep liquids securely within the equipment. But leaks may be more likely to occur if the pressure acting on the seals becomes too great. Freudenberg Sealing Technologies has developed a new rotary seal, the Gerromatic, which has a wave-shaped sealing lip. This increases the maximum amount of pressure that can be applied. The sinusoidal contact path also reduces friction and provides self-cleaning, which extends operating life.
In the process industry, including the food and beverage sector, shaft seals used in equipment mostly have a rotation-symmetrical seal lip, which abuts the rotating shaft with a groove-like contact pattern. During wet-running, this can cause the medium to be displaced at the contact surface. The seal then runs in a more or less dry condition, leading to increased friction and higher temperatures. The increased friction increases wear and reduces the efficiency of the equipment. The accompanying rise in temperature is not desirable, especially when the process media are temperature-sensitive. If the seal lip is also exposed to high temperatures at high rotational speeds – for example, due to a process material that applies pressure to the seal lip in a vessel with a stirring unit below it – the lip can fold down on the low-pressure side, which would result in immediate leakage and the seal’s failure.
Polytetrafluoroethylene (PTFE) is commonly known as a coating for pans under the DuPont trade name Teflon™. It is also superbly suited as a sealant and is superior to many materials in specific ways. For example, it can be used at low and high temperatures and in combination with gasoline, solvents, water and other polar media such as lyes, standard lubricants and brake fluid. PTFE’s chemical resistance is nearly universal.
In 1938, while working for DuPont, American chemist Roy Plunkett was looking for a substitute for the fluorohydrocarbon Freon, which his employer was only allowed to sell to General Motors’ Frigidaire division for patent-related reasons. For his research, he had obtained a supply of tetrafluoroethylene (TFE), which was used as refrigerator coolant. He stored it in small pressurized gas cylinders at low temperatures. When he was ready to use the gas after a fairly long storage period, none was left in the container. But its weight was unchanged. After it was opened, there were white crumbs inside and the inner walls of the container were covered with a thin layer. Plunkett quickly realized that the TFE gas had been polymerized into a plastic. This new plastic, PTFE, proved to be completely resistant to chemical exposure. Not even aqua regia¹ could harm it in any way. But its production was so costly that practical uses seemed inconceivable.
There are many factors to be considered when choosing the right type of hose for your application. There are many different types of hose available on the market. They include metal, rubber, composite, PTFE and fabric. The decision of which hose type to buy depends on the application for which the hose is being used.
Metal hose is ideal for absorbing vibration, misalignment correction, thermal expansion or contraction of piping systems, and protecting equipment from excess motion. Typically, metal hose is used when no other (non-metallic) constructed hose will work. In other words, metal hose is used as a last resort.
Here are some of the factors that should alert you it’s time to use metal hose:
In our latest video, Gallagher Fluid Seals Engineering Manager Craig Beil discusses the various PTFE fillers used in making seal materials. Fillers include carbon, glass, stainless steel and molybdenum disulfide. Watch the video to learn more about what PTFE fillers can do, and be sure to watch the other videos in our series.
Spring energized PTFE seals use one of three different types of springs to energize the jacket. When choosing a spring design, consider two elements: load value and deflection range.
The spring’s load affects its ability to seal, friction, and wear rate. As the load increases, the lips seal tighter, while friction and wear increase proportionately.
The deflection range affects the seal’s ability to compensate for variations in gland tolerances and for normal seal wear. Each spring size has a specific deflection range. The available deflection increases as the seal and spring cross-section grows. This increase could be a deciding factor in choosing a cross-section. Use springs with a wide deflection range when sealing surfaces are not concentric. Continue reading What Springs Do Spring Energized PTFE Seals Use?→