By Tom Horner and Neil Hoehle, Inpro/Seal
Packing is likely the oldest means ever devised to seal a rotating shaft. It’s still widely used on pumps, agitators, mixers, conveyors and other equipment. Mechanical seals are also used extensively since the mid-20th century.
More recent technologies, i.e., air- and gas-purged seals, are proving, for many applications, to be a better, more cost-effective solution than packing for rotating equipment.
An air- or gas-purge “clearance” seal means no contacting parts are critical to sealing. The seal is maintained because the sealing medium is at a higher pressure than that of the sealed unit or equipment.
Problems with packing
Packing works because the pressure asserted by the gland, in effect, prevents the product from passing. Product and shaft friction eventually wear the packing down. The gland must be periodically tightened to maintain the pressure seal.
Packing is notorious for damaging shafts. It also demands significant maintenance. Moreover, because it creates heat via friction, to stay cool packing must be flushed. Flushing can be with water but, in most cases, the product being sealed — such as light gas oil in refineries — does the job. So, flushing costs can vary from minimal to substantial.
Packing does have benefits. It’s relatively inexpensive, comes in many different materials and is a proven solution. It works particularly well in services where the product is neither dangerous nor valuable, such as vertical pumps in cooling towers.
In the end though, packing leaks and it does so sooner if not properly attended to. It leaks if not tightened properly and fails prematurely if over-tightened. It only can be tightened so much before it can’t be tightened any further and is replaced.
Mechanical seals are another proven rotating-equipment option found in process industries. They became popular after World War II and design evolution has improved their efficacy. Each includes a drive-mechanism, primary-seal, secondary-seal and actuator component. The primary seal is key, having two seal faces — each made of a different material — one rotating, the other stationary.
Mechanical seals work great on pumps, compressors, reactors and critical, precision equipment. But, due to the flat, small clearance between the two seal faces, they can be sensitive to system upsets and the installed environment. The two faces mustn’t touch but must stay exceedingly close. The seal fails if the faces rub or are pulled apart.
More recent alternatives
Two relatively recent alternatives to packing or mechanical seals are air- and gas-purged seals.
Their clearance seal means no contacting parts are critical to sealing, and thus exhibit minimal wear. They maintain the sealing medium at a higher pressure than that of the equipment being sealed.
As a sealing solution, air seals are “permanent” and don’t need periodic tightening or complex secondary systems. All that’s needed is that pressure inside the seal is greater than the pressure it’s sealing against.
Moreover, seal pressure is adjustable — for example, if there’s a change in the system. The seals are maintenance-free to the extent there are no contacting faces or parts — and might very well outlast the equipment being sealed. Some air-seals allow for shaft run-outs and misalignment, making them very adaptable to equipment circumstances.
Air seals aren’t the answer for all rotating equipment, though. They have their shortcomings. The unit must be vented to allow air or gas to escape and not over-pressure the piece of equipment being sealed. In addition, product being sealed must be compatible with the seal medium. These constraints make it very difficult to use these seals on pumps because the air or gas will cause cavitation.
Air- or gas-purged seals rely heavily on the tight clearance between the seal and the shaft and, as a result, can fail or become inefficient if that clearance opens up.
Because pressure inside the seal must exceed the product pressure, available air or gas pressure at a plant may limit their use.
For all these reasons, air-purge seals are found effective in low-pressure, low-speed applications that have misalignment, high vibration and shaft run-outs. They typically don’t do well in high-speed and high-pressure applications.
There’s no one-size-fits-all sealing solution for rotating equipment. Mechanical seals are needed in high-speed, high-pressure and highly critical applications. However, the precision they require makes them unsuitable for a large number of services.
Where mechanical seals aren’t required or suitable, packing is usually found. Air/gas seals are a great alternative to packing for the majority of such services. Not only can they seal the same applications but also to so with less maintenance, higher mean time between repairs and without damaging equipment.
Tom Horner recently retired from Inpro/Seal. Neil Hoehle is Inpro/Seal’s global director of technology. E-mail him at firstname.lastname@example.org.
Headquartered in Rock Island, Ill., Inpro/Seal is a world leader in design and manufacture of permanent bearing protection and complete shaft seals for rotating equipment. Since inventing the original Bearing Isolator in 1977 Inpro/Seal has maintained a leadership position in the niche markets it serves.
Inpro/Seal is part of Waukesha Bearings Corp., a world leader in hydrodynamic bearings and magnetic bearing systems, and an operating company of Dover Corp.