Mechanical Seals/packing/valves Flashcards
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?
- A set of primary sealing surfaces
- A mechanism for holding the seal faces together to form a seal
- A set of secondary seals (commonly known as shaft packing or insert mounts)
- 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?
.0001”-.0002”
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
Chevron seals
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
What is a gland jacket?
A cavity for holding, and circulating a heating or cooling fluid near the seal
Gland jackets come equipped with pipe connections to allow for internal or externally supplied fluids for:
Flushing Quenching Lubricating Buffering Barrier fluid ( in this case it would need vent and drain connections)
What is bypass flushing?
It uses the pumping medium from the discharge to remove heat, or to maintain positive pressure in the seal chamber
Describe flushed gland seals
They allow circulation of fluid around the sealing faces to provide lubrication, cooling, heating cleansing
What are quench glands?
They are fitted with drains and vents, to introduce and drain quench fluids. They are introduced at the low pressure side of the double seals. The quenching accomplishes adding, or removing heat, cleanses buildup due to coking
What is the purpose of a barrier and buffer system in double seal applications
Barrier system put barrier fluid at higher pressure than pumped medium, so if it leaks, it leaks into the pump
Buffer system uses a lower pressure, and looks for a pressure increase to detect a leak
What are ways of contamination control?
A filter on the flush line, or a cyclonic separator
What is the maximum pressure on an unbalanced seal?
The seal chamber pressure can not exceed 300 PSI
At what speeds should stationary seals be used?
When shaft speeds exceed 4500 ft/min. Distortion would take place with a rotating seal
What are the two configurations for mounting seals?
Inside mounted, or outside mounted
What are advantages of inside mounted seals?
Seal chamber pressure helps the spring in applying a closing force, clean themselves with centrifugal force, can come balanced for higher pressures. These are the most common arrangements
When are outside mounted seals used?
- stuffing box is too small
- low pressure (15-75 psi) unless specially designed balanced, then up to 400 psi
Where are double seals required?
Non lubricated products
Anything hazardous, carcinogenic, toxic
What are the three arrangements on double seals?
Face to face
Back to back
Tandem
How is fluid pressure in a back to back arrangement?
It must use a barrier fluid. It should be approx 10% higher than fluid pressure
What is fluid arrangement on face to face seal arrangements?
They can use either a barrier fluid, or a buffer fluid
What is fluid method on tandem seals?
They must use buffer fluid
What is the main benefit of a cartridge seal?
The main benefit is that the setting of the seal on the shaft for proper loading is eliminated
How much is one light band
11.6 millionths of an inch
What are some common installation errors?
Assembly error, secondary seal damage, seal face damage, incorrect positioning, misalignment
How do you determine if the seal is repairable?
If there is heat cracking or is the seal face has where in excess of 1/32 it should be replaced
Should seal faces be lubricated?
No
Where is the Seal reference point to set the seal which required length?
Seal housing face or stuffing box face
What does a wide track pattern or a narrow track pattern show you on a wear pattern on a seal face
A wide track pattern is an indication of pump misalignment, and a narrow track pattern shows signs of over pressurizing
What is the difference between compression packing on the shaft and for a valve?
Compression packing on the shaft must control leakage. 40-60 drops in a minute is acceptable. Zero leakage is allowed in a valve. The gland follower is tight
Where is most of the wear on a shaft?
Roughly 70% of the wear is on the outer most rings closest to the gland. This is because the mechanical pressure applied by the gland follower is greatest at this point
Which way are lips always oriented?
The lips always face towards pressure
What type of packing is used for high-pressure? Low pressure?
High pressure- braid over braid works well for pressure, but interlocking braid packing has highest
Low pressure - twisted packing
Where must the lantern ring be located
Directly below the seal water connection. It’s purpose is lubricating, sealing, flushing, and cooling
When is lubrication not required
When the packing is a self lubricating material such as PTFE
What material cannot be used for acids or strong oxidizers
Carbon yarn
What material is found extensively in the food industry?
PTFE fibre
What are some reasons packing may fail prematurely?
The wrong type of packing, improper installation, wrong size, or it was exposed to excessive heat
What should never be done to packing?
It should never be struck with a hammer
How should joints be installed in success of packing rings
Joints should be staggered at least 90° apart
Why is bronze used in valves?
It is used environments that need to be non-sparking
What should all valves have marked on them?
The service ratings, and the CSA marking
What is a WOG rating and what is the other name for it?
It is also the CWP rating. W OG stands for water oil and gas, and CWP stands for cold working pressure
What does WSP stand for?
Working steam pressure. Sometimes it is simply stamped with a number and a S example 125 S
What are clamped joints also called? What are these joints good for
They are also called Victaulic joint. They are very good if the pipes it to be quickly disconnected and reconnected
What a gate valves never used for?
Throttling
What is a plug valve
Similar to a ball valve, but it is a straight cylinder down. They are a quarter turn valve
What is important to remember with butterfly valves?
Accurate alignment is very important to prevent binding of the pivoting disk which swings into the pipe when open. It is a very narrow valve
What are needle valves good for?
They are used for precise fluid flow control are very accurate for throttling
Describe a globe valve
They cause the flow of the fluid passing through change direction twice. They work well for throttling or regulating flow.
Describe an angle valve?
They are similar to a standard globe valve but the change 90° direction.
Describe the diaphragm valve
Similar to a diaphragm pump the fluids that go through are completely isolated and there is no leakage which makes it good for corrosive and Toxic fluids
What is it called when a pressure relief valve first starts to open?
Cracking pressure. The difference between cracking pressure in the pressure required to fully open the valve is called pressure override
How is a ruptured disc reset?
It must be rebuilt because the disc will be ruptured.
What is a transducer?
It measures process variables and provides a signal to a command system for decision-making ( PLC )
What is a stepping motor
Rather than rotating all the time, it makes small turns that are steps
What cannot be used with valves that are in oxygen service?
Oil or grease
