Advantages are that they are inexpensive and commercially available in a wide range of sizes and configurations.
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The primary disadvantage of this seal type is that it is prone to secondary seal hang-up and fretting of the shaft or sleeve, especially when the seal is exposed to solids. A pusher seal type should not be selected if the secondary seal is likely to hang-up. Can small deposits of solids form ahead of the secondary sealing member?
The non-pusher or bellows seal does not have a secondary seal that must move along the shaft or sleeve to maintain seal face contact. In a non-pusher seal the secondary seal is in a static state at all times, even when the pump is in operation. A secondary sealing member is not required to make up the travel as the rotary and stationary seal faces wear. Primary seal face wear is typically accommodated by welded metal or elastomeric bellows which move to assist in the compression of the rotary to stationary seal faces.
The advantages of this seal type are the ability to handle high and low temperature applications (metal bellows), and that it does not require a rotating secondary seal, which means it is not prone to secondary seal hang-up or shaft/sleeve fretting. Elastomeric bellows seals are commonly used for water applications.
The disadvantages are that thin bellows cross sections must be upgraded for use in corrosive environments, plus the higher cost of metal bellows seals.
Cartridge seals have the mechanical seal pre-mounted on a sleeve (including the gland). They fit directly over the shaft or shaft sleeve, and are available in single, double, and tandem configurations. Best of class pump users give strong consideration to the use of cartridge seals.
The advantages are that this seal configuration eliminates the requirement for seal setting measurements at installation. Cartridge seals lower maintenance costs and reduce seal setting errors.
The primary disadvantage is the higher cost, plus in some cases they will not fit into existing stuffing box/seal housings.
Mechanical Seals Arrangements
Single seals do not always meet the shaft sealing requirements of today&#;s pumps, due to the small amount of required leakage when handling toxic or hazardous liquids; suspended abrasives or corrosives in the pumpage getting between the seal faces and causing premature wear; and/or the potential for dry operation of the seal faces. To address these situations, the seal industry has developed configurations which incorporate two sets of sealing faces, with a clean barrier fluid injected between these two sets of seal faces. The decision to choose between a double or single seal comes down to the initial cost to purchase the seal vs. the cost of operation, maintenance and downtime caused by the seal, plus the environmental and user plant emission standards for leakage from the seal.
This article will explain what a mechanical seal is and the key design features that make them work.
A mechanical seal is simply a method of containing fluid within a vessel (typically pumps, mixers, etc.) where a rotating shaft passes through a stationary housing or occasionally, where the housing rotates around the shaft.
When sealing a centrifugal pump, the challenge is to allow a rotating shaft to enter the &#;wet&#; area of the pump, without allowing large volumes of pressurized fluid to escape.
To address this challenge there needs to be a seal between the shaft and the pump housing that can contain the pressure of the process being pumped and withstand the friction caused by the shaft rotating.
Before examining how mechanical seals function it is important to understand other methods of forming this seal. One such method still widely used is Gland Packing.
Gland packing is a braided, rope like material that is packed around the shaft - physically stuffing the gap between the shaft and the pump housing.
Gland packing is still commonly used in many applications, however increasingly users are adopting mechanical seals for the following reasons;
Mechanical seals are designed to overcome these drawbacks
A basic mechanical seal contains three sealing points.
The stationary part of the seal is fitted to the pump housing with a static seal &#;this may be sealed with an o-ring or gasket clamped between the stationary part and the pump housing.
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(Highlighted in red below, left the stationary part and right the rotary portion)
The rotary portion of the seal is sealed onto the shaft usually with an O ring. This sealing point can also be regarded as static as this part of the seal rotates with the shaft.
The mechanical seal itself is the interface between the static and rotary portions of the seal.
One part of the seal, either to static or rotary portion, is always resiliently mounted and spring loaded to accommodate any small shaft deflections, shaft movement due to bearing tolerances and out-of-perpendicular alignment due to manufacturing tolerances.
While two of the sealing points in a seal design are simple static seals, the seal between the rotating and stationary members needs a little more consideration. This primary seal is the basis of all seal design and is essential to its effectiveness.
The primary seal is essentially a spring loaded vertical bearing - consisting of two extremely flat faces, one fixed, one rotating, running against each other. The seal faces are pushed together using a combination of hydraulic force from the sealed fluid and spring force from the seal design. In this way a seal is formed to prevent process leaking between the rotating (shaft) and stationary areas of the pump.
The surfaces of the seal faces are super-lapped to a high degree of flatness; typically 2-3 Helium light-bands (0.&#; / 0.mm).
If the seal faces rotated against each other without some form of lubrication they would wear and quickly fail due to face friction and heat generation. For this reason some form of lubrication is required between the rotary and stationary seal face; this is known as the fluid film
In most mechanical seals the faces are kept lubricated by maintaining a thin film of fluid between the seal faces. This film can either come from the process fluid being pumped or from an external source.
The need for a fluid film between the faces presents a design challenge &#; allowing sufficient lubricant to flow between the seal faces without the seal leaking an unacceptable amount of process fluid, or allowing contaminants in between the faces that could damage the seal itself.
This is achieved by maintaining a precise gap between the faces that is large enough to allow in a small amounts of clean lubricating liquid but small enough to prevent contaminants from entering the gap between the seal faces.
The gap between the faces on a typical seal is as little as 1 micron &#; 75 times narrower than a human hair. Because the gap is so tiny, particles that would otherwise damage the seal faces are unable to enter, and the amount of liquid that leaks through this space is so small that it appears as vapor &#; around ½ a teaspoon a day on a typical application.
This micro-gap is maintained using springs and hydraulic force to push the seal faces together, while the pressure of the liquid between the faces (the fluid film) acts to push them apart.
Without the pressure pushing them apart the two seal faces would be in full contact, this is known as dry running and would lead to rapid seal failure.
Without the process pressure (and the force of the springs) pushing the faces together the seal faces would separate too far, and allow fluid to leak out.
Mechanical seal engineering focuses on increasing the longevity of the primary seal faces by ensuring a high quality of lubricating fluid, and by selecting appropriate seal face materials for the process being pumped.
When we talk about leakage we are referring to visible leakage of the seal. This is because as detailed above, a very thin fluid film holds the two seal faces apart from each other. By maintaining a micro-gap a leak path is created making it impossible for a mechanical seal to be totally leak free. What we can say, however, is that unlike gland packing, the amount of leakage on a mechanical seal should be so low as to be visually undetectable.
A recent case study indicated - Upgrading from packing reduces water usage and operating costs.
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