Flashcards in Mechanical Seals/packing/valves Deck (81)
What is the main difference between compression packing, and mechanical seals?
Packing must leak so it can lubricate, and/or seal the shafts.
What is the rule of thumb for leakage in compression packing?
A rate of 40-60 drops per minute leakage
Where are mechanical seals never permitted?
On pumps used for fire safety. This is because when a mechanical seal fails, it fails completely.
What are the four basic types of components that make up mechanical seals?
1. A set of primary sealing surfaces
2. A mechanism for holding the seal faces together to form a seal
3. A set of secondary seals (commonly known as shaft packing or insert mounts)
4. Metal components- eg-Set screws, springs, drive lugs, collars, pins, retainers, snap rings.
Where is the primary sealing point?
Between the stationary face, and the rotating face
Where is the secondary sealing point?
At the outside or around the stationary face, by means of packing, such as gaskets, O rings, and V rings
Where are two additional sealing points for mechanical seals?
The contact point between the gland and the pump (static seal- gaskets, O rings), and if a shaft sleeve is used the point between the shaft and the sleeve.
How closely lapped are the two primary sealing faces?
They are lapped to 1-3 light bands
What are the factors that govern the amount of friction between the sealing faces?
Seal ring material, smoothness and flatness of the seal faces, the amount of closing force.
Describe gas lubricated seals.
The mating surfaces do not come in contact with each other while running due to the spiral grooves
How big is the gap between surfaces on a gas lubricated seal when it is running?
How closely lapped are gas seals?
1-2 light bands, and emergencies can be hand lapped to 3 light bands
What are some common materials for seals?
Bronze, ceramics, tungsten carbide, silicon carbide
What are some things used as a secondary seal?
O rings, PTFE wedges, V rings, U cups
Where can O rings be used?
They can be used in both static, and dynamic applications as a secondary seal. The material they are made of must make sure it is compatible with the pumping medium, and pressures
Describe PTFE wedges
They are used on pusher seals, and the wedge seal can only take pressure in One Direction. It cannot be used in applications which the seal is subjected to vacuum situations. It is not tolerant of any misalignment and axial movement
Describe V rings
They are usually made of PTFE. They are an automatic type of packing in which the pressure in the seal chamber aids in the ceiling. They can only handle pressure in one direction. They are usually used in sets of three or more
What is another name for V rings
What way are V rings installed?
The open part of the V must face the pressure
What is the big difference between the V ring and the U cup?
The U cup uses a single U per seal
What is a pusher seal?
It is called this because it secondary seal is pushed back and forth axially. This movement maintains constant contact at the seal faces to compensate for seal face wear and misalignment
What are some advantages of pusher seals?
They are cheap, and available in a wide range of sizes to accommodate a large variety of equipment
What are some disadvantages of pusher seals
The secondary packing is susceptible to hang up by the pumping medium. Movement of the secondary seal allows dirt to embed in the packing which causes the packing to act as an abrasive fretting the shaft or sleeve
What is the benefit of a multiple spring over a single spring closing mechanism?
Multiple spring requires less operating strength, and the face loading is more evenly distributed. The major disadvantage of the multiple spring is the tendency to clog at the spring pockets.
Describe non-pusher seals
They are a bellow style, usually made of rubber, PTFE, or welded metal. The shaft seal is static, therefore no seal hang up, or shaft fretting. These types of seals are hydraulically balanced.
Where would PTFE bellow seals be used?
In conditions where metal will not handle the corrosive environment.
What are the benefits of bellows seals?
Can be used in high temperature and pressure applications (metal bellows), do not require a secondary dynamic seal, hydraulically balanced
What are the two means of providing drive for the rotating ring?
Mechanically (pins or tabs to engage seal ring, and locking collar held to shaft by set screws
Friction drives (relies on the vulcanization of the rubber to the shaft)
Where are friction drives normally used?
Pumps whose medium is non lubricating, such as water