Mechanical Seals, Compression Packing and Valves Flashcards
Why are mechanical seals not to be used in fires safety?
when a mechanical seal fails it fails completely
What are the 4 basic types of components mechanical seals are made up of?
a set of primary sealing faces (rings), a mechanism for holding the seal faces together (ex: springs), a set of secondary seals, metal components
Where is the primary sealing point on a mechanical seal?
between the stationary face and the rotating face
What are the factors that govern the amount of friction between the sealing faces in a mechanical seal?
seal ring material, smoothness and flatness of the seal faces, the amount of closing force
What feature does a unidirectional gas seal have that keeps the mating faces from coming into contact while running? What is the typical gap produced in these seals?
One of the faces has gas grooves only a few microns deep machined into it. The typical gap would be 0.0001” to 0.0002”
In an emergency, what can the seal faces in a mechanical seal be hand-lapped and polished to?
three light bands
What are the different types of secondary seals that can be used on a mechanical seal?
O-rings, PTFE wedges, V-rings, U-cups
What secondary seal can only take pressure in one direction, and is not tolerant of any misalignment and axial movement.
PTFE wedges
What secondary seals are normally used in sets of 3 or more?
V-rings (chevron seals)
What is used with a U-cup seal to keep it loaded?
an energizer/spreader ring
What type of seal has a secondary seal that is pushed back and forth axially to maintain constant contact at the seal faces?
pusher seal
What is the major disadvantage of a multiple spring closing mechanism?
tendency to clog at the spring pockets
What type of seal has a static secondary seal, and is inherently hydraulically balanced?
non-pusher (bellows) type seal
What are the 3 materials commonly used for the bellows in non-pushers seals?
rubber (elastomer), PTFE, welded metal
What type of pumped medium would a friction drive not be used with?
any lubricating medium (ex: oil)
What system has a fluid that is introduced at a higher pressure than that of the pumped medium?
barrier system
Where would quenching fluids be introduced?
to the low-pressure side of the double seals
What is normally recommended when steam is used for quenching?
throttle bushing
What type of contamination control is used when there is a constant introduction of particles into the system?
cyclone seperator
What is the maximum seal chamber pressure for an unbalanced seal?
300 psi
What seals would be used when shaft speeds exceed 4500 ft/min?
stationary seals
What seal setup is recommended when the stuffing box is too small, and/or the product is very corrosive?
Outside mounted single seal
What pressures can an unbalanced single outside mechanical seal handle? A balanced?
15-75 psi, 400 psi
What would a double mechanical seal arrangement be used for?
non-lubricated products, carcinogenic products, toxic products, hazardous fluids
What are the two types of double mechanical seal arrangements?
face-to-face, back-to-back
At what pressure is barrier fluid recommended to be at in relation the fluid pressure on the inner seal?
approx. 10% higher (min. 25 psi difference)
When is a face-to-face arrangement used, and when does it act the same as a tandem seal?
When the stuffing box is too small to accommodate a back-to-back seal, acts the same as a tandem seal when the fluid pressure in the seal chamber is lower (buffer fluid) than that of the product as opposed to higher (barrier fluid)
What are tandem seals also known as?
environmental seals
What is one light band equal to?
11.6 millionths of an inch
What tools would you use to check a seal face for flatness?
optical flat, monochromatic light
What are some important points to remember when handling mechanical seals?
do not lube faces, keep hands clean when handling seals
What would a wide track pattern on a seal face indicate? What about a narrow track pattern?
pump misalignment, overpressurizing
What are the two types of closing mechanisms found on pusher seal?
single springs, multiple springs
What are 4 reasons for providing flushing fluid?
lubrication, cooling, heating, cleansing
How many light bands would seals be lapped to from the manufacturer?
1-3
How many drops per minute must leak from compression packing?
40-60
What type of fluid would a tandem seal use?
buffer fluid
What type of non-pusher seal (bellows) would be good for high pressures?
the welded metallic type
What is the purpose of compression packing?
to control leakage along moving parts
In what application would compression packing be set to allow no leakage?
on a valve stem, also acts as a bearing in this case
What are 3 types of motion compression packing may have to seal against?
rotary, reciprocating, helical
What are the 3 types of automatic packing? What material are they normally made of?
cup packing, u-cup packing, v-cup packing, normally some kind of elastomeric
Why is it important not to over-tighten jam packing?
The increase in friction between moving parts will act as a brake.
Which way is automatic packing orientated in relation to the pressure its sealing against?
the lips face the pressure
What jam packing type is the best for high pressures?
interlocking braid
What packing component is installed in the stuffing box to aid in lubricating, sealing, flushing, and cooling?
a lantern ring
Where does a lantern ring need to be located when used for cooling and sealing?
under the seal water connection
When should carbon yarn not be used as packing material?
in the presence of strong oxidizers
What type of packing material is used extensively in the food industry, but is limited in temperature and shaft speed?
PTFE fibre
What are some causes of premature packing failure?
wrong type or size of packing used, improper installation, exposed to excessive heat
Where does most of the wear on a pump shaft occur within the stuffing box?
On the outermost part of the shaft in contact with the packing.
What are 3 metals commonly used in metallic compression packing?
Babbitt, aluminium, annealed soft copper
What does the “S” preceded by a number indicate on a valve?
the steam rating of the valve
What does WOG indicate on a valve?
water, oil, and gas rating
What are gate valves not used for and why?
Throttling, because wire drawing (erosion) can occur on the gate and the gate seats.
Why is an accurate alignment important during installation of a butterfly valve?
to prevent binding of the pivoting disc
What valve is used for precise fluid flow control?
needle valve
What does a globe valve work well for?
throttling or regulating flow
What valve operates the same as a standard globe valve but has reduce turbulence, flow restriction and pressure drop? When is it used?
angle valve, used when a 90 deg change is needed in a pipeline
What type of valve would be used for throttling when handling corrosive and toxic fluids?
diaphragm valve
What is the function of a pressure relief valve?
to prevent equipment damage
What is the difference between the cracking pressure and pressure required to fully open a pressure relief valve called?
pressure override
Name the different categories of valve actuators:
manual, power, pneumatic, hydraulic, electric, variable position
When would you not apply oil or grease to a valve?
when the valve is in oxygen service
What can be installed and adjusted to greatly reduce the mechanical stresses on a valve?
pipe hangers
What are the hazards you may encounter when working on valves?
high pressures, high temperatures, reactive agents, flammable liquids or gases, poisonous substances
What should be done to the splits on new packing when installing?
they should be staggered
What is the purpose of a balance valve?
smaller actuators may be used to operate the valve because less force is required
