Within the pump, the mechanical seal is the component that acts as a barrier between the rotary components and the stationary elements.
The seal needs to be able to prevent leakage at three different places.
- In the space between the faces of the seal (both rotational and fixed).
- Between the rotating element and the shaft or sleeve of the pump.
- Between the stationary element and the housing for the seal chamber of the pump.
All seals share these fundamental parts and functions in common with one another. The form, style, and design might be different from one manufacturer to the next depending on their purpose, but despite this, the fundamental theory behind its function and purpose has not changed at all.
Through the use of the spring, the set screw that is responsible for transmitting the torque from the shaft is connected to the rotary face. In addition to this, it ensures the positive and accurate placement of all of the rotary components.
Because of the wear on the faces, the spring is allowed to lengthen, which keeps the rotating face in contact with the stationary face. Within the parameters of the operational tolerances of the bearings, the O-ring on the shaft should be able to move freely in the axial direction. This type of play is known as axial play.
The pressure exerted by the liquid inside the seal chamber not only keeps the faces of the seal together but also creates a thin coating that acts as a lubricant between the surfaces of the faces. This lubricant is the media that was pumped out. The only components of the seal that move relative to one another are called faces, and they were chosen for their low frictional properties. If the equipment is not properly aligned or if the bearings have a loose tolerance, then other parts could be moving relative to one another.
There are different types of mechanical seals used onboard vessels and those mainly are:
- Single, unbalanced, inside mounted mechanical seal
- Single, balanced, inside mounted mechanical seal
- Single, balanced, external mounted mechanical seal
The balanced type tend to be more and more present due their numerous advantages (less heat generated, balanced seals can seal vacuum, high pressure, less energy consumed, less wear, higher speed shafts etc.). This balance is not a dynamic balance; rather, it is a relationship between the forces that tend to open the faces in a mechanical seal and the forces that tend to close the seal faces. In other words, this balance is a relationship between the two types of forces.
Cartridge mechanical seals are constructed in such a way that the rotary and elements, the springs and secondary seals, the gland, the sleeve stationary, and all of the pieces that accompany it are all included into a single, unified unit. It installs as a single unit as opposed to the many separate parts previously required.
Same like any other machineries and mechanical elements, the mechanical seals fail at some point, sometimes prematurely and there are numerous reasons why seals fail prematurely.
The source of the problem might be the pumping system, the operation of the pump, the repairer, the warehouse, or even before the seal was even delivered to the vessel. The appearance of liquid on the floor is typically the earliest warning sign of a problem.
The failure could have originated on the seal assembly line, although the manufacturers perform a static pressure and vacuum test on their final product.
If the mechanical seal fails immediately, or within moments of the pump start-up, one should investigate the events before start-up. This problem most likely occurred during the installation process, or possibly during the handling or manufacturing of the seal. If the seal breaks after only a few days, the problem could be caused by an erroneous specification of a component such as an O-ring seal. However, if the mechanical seal fails after three weeks of service, or after two or more months of use, then it need to reevaluate the operation of the system as well as its design.
The pump must be operated at, or close to it’s best efficiency point (BEP) on the pump curve.
The pump will vibrate if it is operated away from its BEP on the curve, which might be either to the left or to the right. This causes the bearings and seal faces to become damaged, which ultimately leads to premature failure. In addition, operating the pump to the left of the BEP on the curve causes more heat to be added to the fluid, which has the potential to destroy the O-rings that are contained within the seal. In extreme circumstances, the fluid may evaporate, leaving the seal without any cooling or lubrication as it continues to run dry. This causes the seal to get damaged. In addition to the vibrations that will occur if the pump is operating to the right of the BEP on its curve, the bad pump may enter cavitation, which will almost likely result in the seal being destroyed.
Roughly half of all of the pumps that were in the shop at the moment had to be removed from operation because they were leaking, couldn’t maintain pressure, or wouldn’t pump. This is most likely due to an O-ring that is leaking.
The majority of mechanical seals have a rubber component known as an O-ring. The O-ring is responsible for regulating the mechanical seal’s performance in terms of temperature, pressure, and chemical resistance.
The O-ring is what differentiates a mechanical seal used in a pump for a fuel system from an O-ring used in a pump for a water system. It is not the ceramic face of the seal or the stainless steel face of the seal.
Depending on the nature of the chemical attack, the O-ring may swell, harden, dry and crack, soften, or even disintegrate if the pumped liquid is not chemically compatible with the O-ring.
Even though an O-ring will become more flexible as the temperature rises, exposing it to extreme heat will cause it to become more rigid, therefore should be compatible with the high temperature environment.
O-rings, particularly Buna-N (a compound made of nitrile), should be stored in a location that is not near fluorescent lighting or electric motors. These things produce ozone in the atmosphere. These elastomers deteriorate in quality as a whole when exposed to ozone.
The pumped liquid may contain suspended particulates, crystals, and silt, all of which have the potential to settle into the seal springs and impede their movement. Because they are jammed, the springs are unable to bend and keep the seal faces together while the shaft moves within the axial tolerance of the bearing.
The shaft O-ring will need to bend in order to maintain face contact if the mechanical seal is cocked or misaligned onto the shaft, or if the seal chamber face is not perpendicular to the shaft. Because of the bending, the shaft seal will rub against the shaft underneath the seal, which can cause the shaft to erode or eat away at a groove. Either the pumped fluid may leak under the O-ring, or the O-ring could dangle into the groove and pull the seal faces as the shaft moved within the bearing tolerance. Both of these scenarios are possible. This is commonly thought to be a problem with the seal, although it is actually an alignment issue.
Seal faces that are loaded with an excessive amount of spring tension might generate an excessive amount of heat. This will cause damage to the O-ring, and the seal faces may potentially shatter as a result of the temperature expansion. If the spring tension is not sufficient, the faces may leak after a short while when the softer face wears against the harder face and the spring tension entirely relaxes. This occurs when the softer face rubs against the harder face. When it comes to a lengthy seal life, the installation dimension is quite important.
Pump and motor alignment is yet, another important matter that must take into consideration when investigate the mechanical seal failure. A misaligned shaft will exert a higher load on the pump bearings which will overheat due overloading. This overheat will affect the mechanical seal O-ring which will harden in time, thus leading to its failure.
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Source and Bibliography:
- Know and Understand Centrifugal Pumps – Larry Bachus & Angel Custodio
- Youtube video training credit – Engineering Dots ; Mechanical Seal Engineering Pty Ltd; Paul Brimhall;