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4th Year Millwright > Mechanical Seals, Compression Packing and Valves > Flashcards

Mechanical Seals, Compression Packing and Valves Flashcards

1
Q

Why are mechanical seals not to be used in fires safety?

A

when a mechanical seal fails it fails completely

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2
Q

What are the 4 basic types of components mechanical seals are made up of?

A

a set of primary sealing faces (rings), a mechanism for holding the seal faces together (ex: springs), a set of secondary seals, metal components

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3
Q

Where is the primary sealing point on a mechanical seal?

A

between the stationary face and the rotating face

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4
Q

What are the factors that govern the amount of friction between the sealing faces in a mechanical seal?

A

seal ring material, smoothness and flatness of the seal faces, the amount of closing force

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5
Q

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?

A

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”

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6
Q

In an emergency, what can the seal faces in a mechanical seal be hand-lapped and polished to?

A

three light bands

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7
Q

What are the different types of secondary seals that can be used on a mechanical seal?

A

O-rings, PTFE wedges, V-rings, U-cups

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8
Q

What secondary seal can only take pressure in one direction, and is not tolerant of any misalignment and axial movement.

A

PTFE wedges

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9
Q

What secondary seals are normally used in sets of 3 or more?

A

V-rings (chevron seals)

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10
Q

What is used with a U-cup seal to keep it loaded?

A

an energizer/spreader ring

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11
Q

What type of seal has a secondary seal that is pushed back and forth axially to maintain constant contact at the seal faces?

A

pusher seal

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12
Q

What is the major disadvantage of a multiple spring closing mechanism?

A

tendency to clog at the spring pockets

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13
Q

What type of seal has a static secondary seal, and is inherently hydraulically balanced?

A

non-pusher (bellows) type seal

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14
Q

What are the 3 materials commonly used for the bellows in non-pushers seals?

A

rubber (elastomer), PTFE, welded metal

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15
Q

What type of pumped medium would a friction drive not be used with?

A

any lubricating medium (ex: oil)

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16
Q

What system has a fluid that is introduced at a higher pressure than that of the pumped medium?

A

barrier system

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17
Q

Where would quenching fluids be introduced?

A

to the low-pressure side of the double seals

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18
Q

What is normally recommended when steam is used for quenching?

A

throttle bushing

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19
Q

What type of contamination control is used when there is a constant introduction of particles into the system?

A

cyclone seperator

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20
Q

What is the maximum seal chamber pressure for an unbalanced seal?

A

300 psi

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21
Q

What seals would be used when shaft speeds exceed 4500 ft/min?

A

stationary seals

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22
Q

What seal setup is recommended when the stuffing box is too small, and/or the product is very corrosive?

A

Outside mounted single seal

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23
Q

What pressures can an unbalanced single outside mechanical seal handle? A balanced?

A

15-75 psi, 400 psi

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24
Q

What would a double mechanical seal arrangement be used for?

A

non-lubricated products, carcinogenic products, toxic products, hazardous fluids

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25
Q

What are the two types of double mechanical seal arrangements?

A

face-to-face, back-to-back

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26
Q

At what pressure is barrier fluid recommended to be at in relation the fluid pressure on the inner seal?

A

approx. 10% higher (min. 25 psi difference)

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27
Q

When is a face-to-face arrangement used, and when does it act the same as a tandem seal?

A

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)

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28
Q

What are tandem seals also known as?

A

environmental seals

29
Q

What is one light band equal to?

A

11.6 millionths of an inch

30
Q

What tools would you use to check a seal face for flatness?

A

optical flat, monochromatic light

31
Q

What are some important points to remember when handling mechanical seals?

A

do not lube faces, keep hands clean when handling seals

32
Q

What would a wide track pattern on a seal face indicate? What about a narrow track pattern?

A

pump misalignment, overpressurizing

33
Q

What are the two types of closing mechanisms found on pusher seal?

A

single springs, multiple springs

34
Q

What are 4 reasons for providing flushing fluid?

A

lubrication, cooling, heating, cleansing

35
Q

How many light bands would seals be lapped to from the manufacturer?

A

1-3

36
Q

How many drops per minute must leak from compression packing?

A

40-60

37
Q

What type of fluid would a tandem seal use?

A

buffer fluid

38
Q

What type of non-pusher seal (bellows) would be good for high pressures?

A

the welded metallic type

39
Q

What is the purpose of compression packing?

A

to control leakage along moving parts

40
Q

In what application would compression packing be set to allow no leakage?

A

on a valve stem, also acts as a bearing in this case

41
Q

What are 3 types of motion compression packing may have to seal against?

A

rotary, reciprocating, helical

42
Q

What are the 3 types of automatic packing? What material are they normally made of?

A

cup packing, u-cup packing, v-cup packing, normally some kind of elastomeric

43
Q

Why is it important not to over-tighten jam packing?

A

The increase in friction between moving parts will act as a brake.

44
Q

Which way is automatic packing orientated in relation to the pressure its sealing against?

A

the lips face the pressure

45
Q

What jam packing type is the best for high pressures?

A

interlocking braid

46
Q

What packing component is installed in the stuffing box to aid in lubricating, sealing, flushing, and cooling?

A

a lantern ring

47
Q

Where does a lantern ring need to be located when used for cooling and sealing?

A

under the seal water connection

48
Q

When should carbon yarn not be used as packing material?

A

in the presence of strong oxidizers

49
Q

What type of packing material is used extensively in the food industry, but is limited in temperature and shaft speed?

A

PTFE fibre

50
Q

What are some causes of premature packing failure?

A

wrong type or size of packing used, improper installation, exposed to excessive heat

51
Q

Where does most of the wear on a pump shaft occur within the stuffing box?

A

On the outermost part of the shaft in contact with the packing.

52
Q

What are 3 metals commonly used in metallic compression packing?

A

Babbitt, aluminium, annealed soft copper

53
Q

What does the “S” preceded by a number indicate on a valve?

A

the steam rating of the valve

54
Q

What does WOG indicate on a valve?

A

water, oil, and gas rating

55
Q

What are gate valves not used for and why?

A

Throttling, because wire drawing (erosion) can occur on the gate and the gate seats.

56
Q

Why is an accurate alignment important during installation of a butterfly valve?

A

to prevent binding of the pivoting disc

57
Q

What valve is used for precise fluid flow control?

A

needle valve

58
Q

What does a globe valve work well for?

A

throttling or regulating flow

59
Q

What valve operates the same as a standard globe valve but has reduce turbulence, flow restriction and pressure drop? When is it used?

A

angle valve, used when a 90 deg change is needed in a pipeline

60
Q

What type of valve would be used for throttling when handling corrosive and toxic fluids?

A

diaphragm valve

61
Q

What is the function of a pressure relief valve?

A

to prevent equipment damage

62
Q

What is the difference between the cracking pressure and pressure required to fully open a pressure relief valve called?

A

pressure override

63
Q

Name the different categories of valve actuators:

A

manual, power, pneumatic, hydraulic, electric, variable position

64
Q

When would you not apply oil or grease to a valve?

A

when the valve is in oxygen service

65
Q

What can be installed and adjusted to greatly reduce the mechanical stresses on a valve?

A

pipe hangers

66
Q

What are the hazards you may encounter when working on valves?

A

high pressures, high temperatures, reactive agents, flammable liquids or gases, poisonous substances

67
Q

What should be done to the splits on new packing when installing?

A

they should be staggered

68
Q

What is the purpose of a balance valve?

A

smaller actuators may be used to operate the valve because less force is required

4th Year Millwright (11 decks)

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