On the most basic level, a mechanical seal is a device that joins together two objects. An adhesive sealant holding pipes together in a plumbing system would therefore qualify as a mechanical seal. Sealants work by gluing together two objects. Gaskets and cartridge seals, found in power transmission products, operate through compression. Simple o-rings are one type of compressive seal. Mechanical seals may also be complex, with multiple parts milled to the millionth of an inch.
End face mechanical seals, often referred to as just “mechanical seals” in power transmission operations, hold together rotating pieces of equipment, as found in pumps, blowers, and compressors. This form of mechanical seal may be found where a rotating shaft comes together with the pump casing, for instance. It’s not easy to prevent fluid leakage here, as the rotation of the shaft naturally forces parts away from each other. Other systemic vibrations have the same effect, working elements apart. To counteract this tendency, it’s important for seals to be rigid enough for a tight fit, and yet flexible enough to allow for some movement.
Ultimately, mechanical seals are fluid sealing products. Their main purpose is to prevent the leakage of liquids being pumped through a system. Sealing out contaminants and containing pressure are secondary roles for mechanical seals. Depending on the liquid at hand, liquid leakage may be extremely dangerous. Leaking connections can also cause malfunctions and breakages, so operators have real motivation to prevent seal leakage as much as possible.
Every mechanical seal will eventually wear out, as its primary sealing surfaces erode. However, too often mechanical seals fail long before their specified lifetime ends. There are many reasons for early seal failure; by eliminating these elements, operators can cut costs and decrease system downtime.
Read on for tips on how to conduct preventative maintenance for mechanical seals.
Mechanical Seals: How to Extend their Lifespan through Preventative Maintenance
1. Proper installation. Improper installation is a common reason for seal failure. Correct installation depends on many factors, including shaft and plate alignment, fastener torque level, number and location of fasteners, port connections, and proper venting, among other factors. Always follow the manufacturer’s recommendations when installing seals.
2. Operate within pump specifications. If alignment, torque levels, and other factors are off, the seal will often be the element to absorb the repetitive force. This drastically reduces seal lifespan. Ensure that pumps are running according to the manufacturer’s specifications.
3. Proper seal selection also maximizes seal life. Each system will have its own unique needs. It is unwise to rush the seal selection process, which should include consideration of operating temperature, pressure, top shaft speed, and the supply source for any cooling/flushing systems. (Some mechanical seals include liquid flushing arrangements to minimize heat between sealing surfaces.) When hazardous materials are being pumped, plant engineers will select double seal systems. In these systems, it’s best to opt for alarm systems that are triggered when leakage rates exceed a specified safety level. Ongoing record taking of operation conditions, such as vibration and temperature, can help operators discover when something is throwing off system performance, to prevent seals from being damaged.
4. Strong seal support systems reduce friction and heat by pumping a liquid over interfacing seal faces. Generally, the manufacturer will provide instructions on whether a certain seal requires liquid flushing.
Consistent plant tours will help operators identify seal issues before they blow up into plant-halting problems. As mentioned above, it’s also smart to track certain variables, such as leakage rate and vibration levels. Most seals will allow for a minimum amount of acceptable leakage. If the rate of leakage climbs steadily, you can bet the seal is failing.
Posted under Machinery Maintenance on Thursday, October 16th, 2014