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Millwright Level 3 > Recprocating Compressors > Flashcards

Recprocating Compressors Flashcards

1
Q

What is a piston?

A

A cylinder or disc that fits snugly into the larger cylinder and displaces or compressed the gases

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2
Q

What helps to create the seal in the cylinder?

A

Piston

Piston rings

Cylinder wall

All work together to create the seal

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3
Q

What is the difference between single acting compressors and double acting?

A

Single acting only compress on one side of the piston. Usually splash lubricated

Double acting compresses gas on both ends of the piston.

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4
Q

What material is a piston usually used for higher pressure applications?

A

Forged

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5
Q

Aluminum pistons are usually use on higher speed reciprocating compressors to help with vibration caused by the inertia of reciprocating parts. What characteristics does aluminum have?

A

Weighs one third as much as cast iron

Transmits heat three times fast but twice the thermal expansion rate

When installing they require more clearance when installed cold

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6
Q

How do larger compressors with higher compression ratios have oil delivered?

A

Through drilled passages in the con rod and spraying on the underside of the piston.

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7
Q

What are the two methods of pistons pins?

A

Full float - free to move inside the con rod bushing and the piston bushing

Semi floating - can either be locked onto the con rod or the piston

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8
Q

How come most compressors use a double acting cylinder?

A

Increased efficiency because cylinder is compressing gas on both sides of the piston

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9
Q

When the cylinder reaches TDC the all the valves are _________

A

Closed

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10
Q

Typical double acted pistons are _______?

A

Hollow and made in three pieces

2 rider bands to support weight of piston and half the weight of the rod

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11
Q

What is the purpose of the piston rod ?

A

Joins the piston and crosshead into one synchronous assembly

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12
Q

If the locknut were to loosen on the piston rod what would happen?

A

The piston and cylinder components would be damaged

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13
Q

How come piston rods are typically plated in with chrome?

A

Hardness

Low coefficient of friction

And smoothness of finish

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14
Q

What is rod run out ?

A

Rod run out means the piston rod is not running totally straight though the packing.

Centrelines of the piston, piston rod, and crosshead must be all in the same horizontal and vertical plane at operating temperature

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15
Q

What is rod loading

A

As the piston moves toward the head end, the force of discharge pressure on the piston head tends to buckle or bend the rod. At the stoke the rod is in compression

As the piston moves toward the crank end of the return stroke the net force of the suction and discharge pressure results in a tension load

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16
Q

Why is excessive rod loading bad

A

Creates wear on components

Usually either the crosshead pin and con rod bushing

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17
Q

What is rod reversal?

Why is it important?

A

Refers to the reversal of the rod load from compression to tension.

Allows lubrication to flow around the pins

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18
Q

Tailrods can be used in smaller compressors to compensate for the lack of rod reversal.

True or false

A

True

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19
Q

When working on piston rods it is important not to damage the rod because?

A

It has to be prefect to seal through the packing.

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20
Q

How do you check for rod runout?

A

Place rod in two v blocks or between lath centres. Make sure to place paper under the blocks to prevent scoring the rod. Rotate rod with dial indicator touching the rod and record measurements

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21
Q

What types of threads are typically on piston rods and can they be repaired if damaged by a file or die nut?

A

Rolled threads

No they cannot be repaired in the field

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22
Q

What is a common field method for checking cracks in the piston?

A

Adding heat to the piston will open up small cracks and make them visible to the eye. May also push oil out through the cracks

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23
Q

Why must you be careful when cleaning aluminum pistons?

A

Very easily damaged and scored

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24
Q

What are the functions of the piston rings

A

Provide a dynamic seal between the piston and cylinder wall (compression rings)

Prevent compression gases from leaking through to the crankcase on single acting compressors

Prevent compression gases from leaking through to the other side of the piston on double acting compressors

Prevent lubricating oil from entering the compression chamber on single acting compressors and cools the piston by transferring heat to cylinder walls

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25
Q

What are the materials for piston rings

A

Cast iron - low cost light duty

Lead bronze - high pressure over 2500 psi very common in most compressors

Tin bronze - commonly used for one piece piston rings

Synthetics - ring of choice for most compressor applications outlast metalic rings

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26
Q

What is the purpose of the compression ring?

A

Form a dynamic seal that traps the compressed gasses and prevents them from leaking through to the other side of the piston

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27
Q

What is the purpose of the oil control ring?

A

Controls the flow of oil on the cylinder walls by providing oil flow paths with groove around and holes in the ring.

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28
Q

What are the three types of piston ring joints?

A

Butt joint - most common and used with compression and oil control rings

Angle joint

Step joint

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29
Q

What does excess clearance in piston rings cause?

What does inadequate clearance cause

A

Blow by

Inadequate clearance will bind when compressor warms up

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30
Q

How much do you stagger piston ring gaps

A

180 degrees

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31
Q

How can piston ring grooves be repaired?

A

Machining the groove larger and installing a spacer ring above the ring

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32
Q

What is the purpose of the rider band?

A

Prevent the piston from touching the cylinder during start up and operation

Support the weight of the piston and half the weight of the piston rod

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33
Q

Most rider bands are made of?

A

Teflon based products in order to have low coefficient of friction and the ability to last.

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34
Q

What are the different types and uses for each rider band type?

A

Uncut - pass over valve ports in into a counter bore of a cylinder

Angle cut - easy to instal but cannot pass over a port

Double twin - designed where piston is to small to have separate grooves for the piston rings. Piston rings fit with rider band in same groove

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35
Q

After installing rider bands make sure to _______ and record for future reference.

A

Measure

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36
Q

When clamping piston rods in vices what is ver important?

A

To use PVC to protect rod surface

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37
Q

Torque multipliers must be used to attain final torque when tightening piston to rod.

A

True

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38
Q

How do you set the piston to cylinder head clearance?

A

By turning the piston rod into or out of the crosshead.

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39
Q

Ideally when the compressor is hot and running the clearance between the piston and crank end head at BDC should be equal to?

A

The clearance between the piston and the head end at TDC

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40
Q

To estimate how much rod must be turned Example if there are 8 threads per inch on the rod.

One complete turn the piston moves .125”
1/2 turn moves .0625”
1/4 turn moves _______

A

.03125

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41
Q

What is the purpose of a balance nut?

A

To fine tune the weight balancing of opposing multistage reciprocating compressor parts

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42
Q

What information is often included on the cylinder name plate?

A

Bore diameter
Piston stroke
Rated discharge pressure
Maximum allowable working pressure
Maximum allowable discharge gas temp
Displacement

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43
Q

What is the function of a cylinder?

A

A cylindrical tube that confines the compressed gasses as the piston pushes on them

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44
Q

How are cylinders cooled?

A

Heat may be removed by air or water (glycol)

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45
Q

What is more the simplest way to cool the cylinders?

A

Air because it just needs fins cast into the outside of the cylinder

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46
Q

If heat temps are above 88C (190F) but below 121C (250F) what type of liquid cooling can be used?

A

Thermo-syphon cooling

Moves strictly by convection

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47
Q

When discharge gas temps are over 121C (250F) what type of liquid cooling must you use?

A

Circulated (pumped) cooling

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48
Q

Using to cold of water for cooling can result in?

A

Condensation inside the cylinder

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49
Q

What are the simplest cylinder bores?

A

Cast integral - typically used for smaller compressors

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50
Q

What is the difference between wet and dry cylinder bore liners?

A

Dry liners do not come in contact with the coolant and wet liners actually form part of the water passage

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51
Q

How do oil free compressors work?

A

They use special Teflon rings. Cylinder bore is machined to a finer surface finish.

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52
Q

Oil compressors use what to lubricate?

A

Pressurized oil system that injects a certain amount of drops per minute into the cylinder. Oil pressure must be slightly higher then the compressed gas

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53
Q

What is a common way to control the compressor output?

A

Clearance pockets

Fixed

Or variable

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54
Q

A clearance pocket increases the size of the clearance volume.(the space that prevents the piston from hitting the head) if the size of the clearance volume increases it ________ the amount of compressed gas leaving the cylinder:

A

Reduces

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55
Q

When capacity of the compressor must be changed by varying amounts on a regular basis what clearance pocket would be the best choice?

A

Variable volume clearance pocket VVCP

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56
Q

By cutting the clearance plug to calculated lengths you can give the desired ________ in capacity?

A

Reduction

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57
Q

Is it safe to adjust the VVCP while compressor is running?

A

No, the pulsations could damage the threads on the control rod and the piston rod could be pushed through the housing. This could cause injury or death

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58
Q

A compressor needs to be shut down and looked at if what symptoms are present?

A

High temps

Strange or unusual noises

Pressure falling or rising out of normal operating range

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59
Q

What causes scores or scratched cylinder walls?

A

Cylinder rings and cylinder getting to hot allowing small spots on the rings to momentarily weld to the cylinder wall

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60
Q

When you see discolouration on the cylinder walls is a sign of?

A

Overhearing

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61
Q

Liner cracks are usually caused by:
Hot spots
Overheating
And

A

Piston seizures

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62
Q

Premature or excessive wear is often attributed to?

A

Dust or contamination coming through the air filter. Or leaks coming through the intake piping

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63
Q

Proper performance of a cylinder relies on the cylinder bore being straight round and within the proper __________?

A

Dimensions

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64
Q

On a small cylinder wall less than 6 inches what is considered to much wear?

A

A ridge near the top of the piston travel the width of a fingernail

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65
Q

What is the best tool to measure is a cylinder bore is within acceptable limits?

A

Micrometer or dial bore gauge

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66
Q

Cylinder bore must be checked in a minimum of ________ positions to determine if the cylinder is out of round.

A

4

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67
Q

Hydro testing is done to see if there are any _______ ?

A

Cracks

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68
Q

NDT checks are done to detect______?

A

Defects invisible to the naked eye

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69
Q

Why are the cylinder bores hard to remove?

A

Installed with an interference fit

70
Q

If dry liners cannot be removed by force what is the next option?

A

Grinding along the length of the sleeve so it can collapse

71
Q

How do you install new liners?

A

Cooled and shrunk with dry ice until it slips into the bore

Sometimes heating the cylinder bore is necessary as well

72
Q

If the cylinder does not have a sleeve what can be be if there is excessive wear?

A

The cylinder assembly can be sent to a machine shop to be bored out. You can either install a sleeve or oversized piston

73
Q

There is a maximum safe limit a cylinder can be bored. Who do you consult to find that information?

A

Manufacture

74
Q

If cylinder bore measurements are in spec what is done?

A

Honed or reglazed and reused

75
Q

What can be used to remove ridges
On cylinders?

A

Small cylinders under 6 inches can use a ridge reamer

Larger cylinders rotary bur or hone is used

Do not cut below ring travel

76
Q

How is deglazing done?

A

Flexible hone or ball deglazer

77
Q

What degree should the cross hatch section be in a perfectly honed cylinder?

A

Between 40 and 60 degrees

78
Q

If the cylinder is slightly out of straight and round and the taper is not to excessive it can be repaired with_____?

A

Ridged/fixed hone

79
Q

After a cylinder has been honed or deglazed what is used to remove the grit and iron particles?

A

Hot soapy water than wiped with a lint free oily rag to prevent oxidation

80
Q

Before doing any work on clearance pockets what must be done?

A

Vent all pressure

Failure to do so can result in injury or death

81
Q

Why must a respirator be used when removing clearance pockets?

A

Toxic gases could still be trapped under pressure in the clearance pocket.

82
Q

After reassembling a compressors and it has ran long enough to reach normal operating temperatures what must be done?

A

Shut down, bled and retighten bolts to recommended torque

Never attempt to tighten bolts on a operating or pressurized cylinder

83
Q

How often should piston rod runout be checked and why?

A

Every year

Misalignment due to settling foundations

84
Q

What is a major cause of horizontal runout?

A

Pipe strain

85
Q

What factors influence vertical rod runout?

A

Piston cylinder bore clearances

Crosshead to crosshead guide clearances

86
Q

When setting piston centreline cold it is normal to set it _______ than the crosshead centre line. Why?

A

Lower

Because when the compressor is at operating temp it will be in line due to thermal expansion

Especially important with large diameter aluminum pistons

87
Q

Factors that affect rod runout

A
  • Improperly shimmed cross heads
  • worn cross heads or guides
  • cylinder to frame joint imperfections or loose bolts
  • piston rod or balance nuts not seating squarely against the cross head face
  • excessive cylinder bore or piston wear
  • worn rider bands
  • twisted or settled main frame
  • lack of support on the outboard end of the cylinders
  • excessive piping strain or distorted supports.
88
Q

Follow this procedure for checking rod runout?

A
  1. Roll compressor to BDC and zero dial
  2. Bar the compressor to TDC and read dial
    3: if the reading is positive, the piston is low or crosshead high. If the reading is negative the piston high or crosshead is low
  3. Use the formula to make adjustments
    R/S=X/L
    x = shim move needed at the crosshead or cylinder support
    L = length of piston rod from centre of piston to centre of crosshead
    S=length of piston stroke
    R = initial rod run out reading
89
Q

Why must packing and oil wipers be used and maintained in good working order on reciprocating compressors?

A

To prevent excess seal oil consumption and to minimize contamination of the gas being compressed

90
Q

Cylinder packing is located on the ______end and the oil wipers are located at the other side of the ________ piece to keep oil inside the main frame

A

Crank end

Distance piece

91
Q

What is free-floating packing mechanical packing?

A

Conists of several cups with sets of segmental sealing rings inside each cup.

The rings are free to move inside the cups in all directions.

Whole assembly is bolted into the crank end cylinder head with piston rod running through it

92
Q

What is full-floating packing?

A

A precision mechanical seal with ground or lapped surfaces.

Must be handled with extreme care and is designed to meet the challenges of sealing effectively around the rod under conditions of

Alternating vacuum
High pressure between suction and discharge
Different types of gases
Temp swings
Different rod materials and finishes

93
Q

Different types of sealing rings are:

A

Pressure breaker
Radial cut
Tangent cut
Anti extrusion or backup rings

94
Q

What influences proper packing material?

A

Lubrication and gas pressures

95
Q

Metallics rings

A

High pressure applications where strength and ridgity are important

Bronze is found in the highest pressure

96
Q

Non metallic

A

Proffered when high degree of conformability is required

Have pressure limitation but need no break in period, self lubricating, and chemically inert

97
Q

What is the purpose of the pressure breaker?

A

The single, radial, cut segmental ring in the first cup on the cylinder side. It’s purpose is to throttle the pressure allowed to enter the packing case

98
Q

Pressure breakers are extremely effective on high speed compressors or on compressors above _______ psi?

A

300

99
Q

_______ breakers are better then Teflon

A

Metal

100
Q

The arrangement of packing rings has the pressure breaker found in the first cup from the cylinder and is the only packing ring used singly. All other cups have a ________ of packing rings either 2-3 sets

A

Sets

101
Q

The radial ring is doweled with the ________ and together they work to seal the rod and packing case effectively

A

Tangent

102
Q

Radial ring must be installed on the _______ with the markings (dots or letters) on both rings facing toward the cylinder.

A

Pressure side

103
Q

What is the packing case

A

Contains the rings and provide connection points for accessories

104
Q

The application, pressure, and compressor speed determine:

A

Number of cups required
- the type of sealing needed between the cups
- whether lubrication is needed
- whether cooling is needed

105
Q

Cast iron packing casings can withsatanf pressure about _______psi

A

5000

106
Q

For pressures up to 15000 psi steel is alloyed with many elements and is ______\ treated to give desired characteristics of strength, wear, fatigue and corrosion resistance.

A

Heat

Stainless steel is often used when corrosion is a problem

107
Q

Bronze packing cases are found in _________ situations?

A

Corrosion resistance with superior heat conductivity

Non sparking applications such as in oxygen services

108
Q

Cups are designed to have a clearance between the and and internal bore of the packing cups to prevent contact under normal operating conditions of rod runout and wear.

A

True

109
Q

Improper orientation of a full-floating packing case assembly will result in ?

A

Inadequate sealing

110
Q

Most packing cases are equipped with a vent connection at the bottom for toxic gases and as a mean of draining access oil . To prevent reverse flow what should be installed

A

Check valve

111
Q

Unlike packing rings, which use the gas pressure to seal them onto the rod, the wiper packing relies on garter springs around the outside diameter of the rod to proved the sealing pressure. Markings should be oriented in which way?

A

To face the oil being removed (toward the crankcase)

112
Q

The distance piece connects the frame to the cylinder and serves the following purposes:

  • prevents the cylinders leaking compression gasses from pressurizing the crankcase or main frame
  • it prevents the oil from the crankcase or main frame from travelling along the piston rod into the compression cylinder
  • it allows access to the compression packing and oil wipers for maintenance and replacement

And?

A

Provides access to the piston rod for runout readings

113
Q

The distance price is a solid piece of ______steal that acts as a spacer in order to separate the frame and the cylinder.

A

Cast

114
Q

Distance applications are used where ?

A

No problems with exists with oil mixing with the compressed gasses

Or

Where there is not an explosion hazard

115
Q

Long distance pieces are typically used for non lubricated systems such as oil free air. How does it work

A

The extra distance with an oils baffle makes it almost impossible for oil to reach the cylinder

116
Q

When do you need a two compartment distance piece?

A

Where a complete sealing from of an explosive gas from the compression chamber must exist

117
Q

A bent flange or a metal bur on a distance piece affects what?

A

Runout

118
Q

What is the purpose of the crosshead guide? T

A

To maintain alignment

119
Q

Where is the crosshead and guide located?

A

In a double acting compressors between the connecting rod and the piston rod

120
Q

Cross head and guides can be bolted onto the frame as Esperanto units or as part of the ?

A

Main frame

121
Q

Manufacturers make cross heads of different weights because?

A

The crosshead, piston rod, and piston graven back and fourth as a unit. To properly balance you will need correct weight crosshead

122
Q

Cross head guides as one piece or with removable shoes, what is the purpose of the removable shoe crosshead

A

To adjust clearance between the crosshead and guides

123
Q

One piece crosshead a have a babitted or ________ sliding surface?

A

Machined

124
Q

Adjustable cross heads are advantageous because:

A

Allow for a faster and less expensive overhaul.

Spare shoes can be swapped out and can adjust clearances by adding or removing shims

125
Q

Before pulling the Crosshead you will want to _________\

A

Check clearances

126
Q

If you cannot fit a _____ “ feeler gauge into the bottom it means what?

A

.015”
Most crosshead style compressors are horizontal gravity keeps the crosshead tight to the bottom guide. If it does not fit that means the alignment is good and you can take all other measurements

127
Q

Top crosshead clearances are between _______ and .010” but check with manufacturers specs

A

.007

128
Q

After measuring cross head to guid clearance you need to measure?

A

OD of pin ID of pin then subtract the two. This give you the clearance of pin and bushing

129
Q

Any uneven wear in the crosshead shoes indicate?

A

Misalignment

130
Q

A visual inspection of the pin and bushing may indicate lubrication problems, if all is well may be a_______?

A

Excessive rod loading

131
Q

Connecting rods for the link between?

A

Crankshaft and piston on single acting

Crankshaft and the crosshead on double acting

132
Q

On operation what forces are applied to the connecting rod?

A

Compression
Tension
Bending
Twisting

133
Q

For good ductility and fatigue resistance these rods are made of?

A

Heat treated alloy steel

134
Q

Three main styles of connecting rods?

A

Straight cut - parting line is 90 degrees to the rod beam (most common)

Bias cut - used when you just pull the connecting rod out through the cylinder bore

Articulating - used when you want to place two pistons (one power one compression) on one camshaft

135
Q

Always keep _______ marks matched when assembling connecting rods

A

Mate

136
Q

Connecting rod bearings must be _____\ shell to allow crankshaft to fit over the crankshaft journal area?

A

Split

137
Q

Inspecting connecting rods consists of?

A

Visual inspection - connecting rod bores, bushings, and split shells for size and signs fatigue and corrosion.

NDT testing - check for cracks around bolt holes and along beam and shank

Straightness

138
Q

If clearance is incorrect when reassembling connecting rods check for?

A

Insert caught on tab or dowel
Dirt under insert
Distorted rod bore
Wrong size insert

139
Q

_________ is used to check connecting rod cap clearance?

A

Plastigage

140
Q

Other then the obvious not lining up match marks what is another common installation error when inserting journal bearing to connecting rods??

A

Not lining up oil hole

141
Q

The compressor frame is the _______ support for all components. It is precision made surfaces to allow manufactures to assemble the components accurately and correctly with minimum misalignment.

A

Central

142
Q

Frames are usually made of cast iron and sometimes ________ to relieve stresses.

A

Heat treated

143
Q

The bottom of the interior forms the oil sump for the main frame lubrication system. How is the oil pump driven?

A

Oil pump(usually driven off the crankshaft) picks up the oil from the sump and send it to the main bearings, crankshaft, con-rod shells, bushings, crosshead and cross head guides

144
Q

The frame does not require much maintenance other then?

A

Cleaning and making sure covers and attatchments are secure to prevent leaked and component failure and ensure hold down bolts are secure

145
Q

What are the two fundamental concerns when dealing with design of a proper foundation for an industrial compressor application?

A

Keep the frame from distorting

Properly align the compressor to the driver

146
Q

To reduce problems with settling or vibrations permanent foundations should have _______ and should be separated from the building with some sort of insulator

A

Sufficient mass and should rest on solid soil or bearing surfaces

147
Q

What can help stop the penetration of liquids into the compressor foundations?

A

A good quality paint or sealer

148
Q

Which conditions would occur if there was a issue with the main frame?

A

Main bearings heating up or wearing out pre-maturely (signs of twisted frame or settling foundation

Fatigued crankshaft

Valve or piston parts dropping into the compression chamber causing a bent con rod or piston rod

149
Q

To check for frame warpage you would use the _________ or precision laser

A

Wire alignment

150
Q

What is the purpose of the crankshaft

A

Convert rotary motion into reciprocating motion

151
Q

Larger compressors crankshafts above 150hp usually have cranks shafts made of?

A

Heat treated forged steel alloys

152
Q

Main bearing allow the crankshaft to rotate _______ cause the crank pin journals to rotate in a larger Circle.

A

Throws

153
Q

Stoke is equal to ________ the throw

A

Two times

154
Q

What are the two different ways of balancing?

A

Opposed throws

Counterweights

155
Q

You can balance with opposed throws when you have an ______ number of cylinders?

A

Even

156
Q

Counter weights are used when ?

A

You only have one cylinder

157
Q

Larger compressors use _______ and counter weights to balance rotational forces.

A

Opposed tbrow

158
Q

When inspecting journal surfaces measure cranks shaft journal areas in at least__________ positions

A

2 positions 90 degrees apart but three checks 60 degrees is better

159
Q

What are the different ways of of reconditioning a crank shaft?

A

Grinding and rebuilding

Chroming

Straightening

160
Q

What should you check for before installing crankshaft?

A

Bearing inserts are installed correctly

Tangs or dowels are lined up

Oil holes are not blocked

Shells and shaft journals are clean and apply a suitable amount of oil

161
Q

Before fully tightening the cap bolts you should align the _______\ bearings? Then tighten caps to torque starting at the middle bearing working alternately toward either end of the frame

A

Thrust bearings or washers

162
Q

Main bearing on large compressors are usually ______ type bearings

A

Shell

163
Q

Main bearing failure is usually from:

A

Loss of lubrication
- corrosion
- acids formed from compressed gases
- misalignment
- improper installation
- excessive rod loading
- failure to run a pre lube

164
Q

Compressor bearing in service may show signs of failure such as:

A

Lubricating pressure decrease due to increased clearances from wear

Excessive heat in bearing area

Excessive vibration in bearing area

Excessive amperage draw from the electric motor

165
Q

What is one method to check for bearing wear?

A

Bump check

166
Q

What is the larger factor in compressor efficiency?

A

Valves

167
Q

Because valves are directly in contact with the gas stream they are subject to contaminants such as:

A

Entrained liquids

Dirt

Corrosive gases

168
Q

With every compressive stroke valves are at risk of many types of destructive forces such as:

A

Abrasion

Erosion

Bending

Twisting

Flexing

Temperature extremes

169
Q

If a compressor operates at 1000 rpm how many times does the valve open and close?

A

1000 times a minute

525 million times over the course of a year

170
Q

What 4 components are required for a valve to do it’s job?

A

A sealing element that opens and closes valve

A valve seat

A valve guard to act as a backstop

Assembly elements

171
Q

What are the basic things all valves need?

A

Effective seal under adverse conditions
- high reliability and reasonably long life
- low noise level
- quick response to small pressure differentials
- sealing elements of low weights
- large unrestricted flow paths
- small physical dimensions
- maintenance and service case

172
Q

What are the different valve types l?

A

Concentric ring and plate - can be customized for non lubricated or lubricated service

Dampened disc valve - meant to reduce flutter

Poppet valve - well suited for applications where there is condensate or other liquids in the gas stream

Feather valves -

Channel valves - offers good combination of effective gas cushioning, flow and replacement of worn parts

Millwright Level 3 (15 decks)

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