Recprocating Compressors Flashcards
What is a piston?
A cylinder or disc that fits snugly into the larger cylinder and displaces or compressed the gases
What helps to create the seal in the cylinder?
Piston
Piston rings
Cylinder wall
All work together to create the seal
What is the difference between single acting compressors and double acting?
Single acting only compress on one side of the piston. Usually splash lubricated
Double acting compresses gas on both ends of the piston.
What material is a piston usually used for higher pressure applications?
Forged
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?
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
How do larger compressors with higher compression ratios have oil delivered?
Through drilled passages in the con rod and spraying on the underside of the piston.
What are the two methods of pistons pins?
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
How come most compressors use a double acting cylinder?
Increased efficiency because cylinder is compressing gas on both sides of the piston
When the cylinder reaches TDC the all the valves are _________
Closed
Typical double acted pistons are _______?
Hollow and made in three pieces
2 rider bands to support weight of piston and half the weight of the rod
What is the purpose of the piston rod ?
Joins the piston and crosshead into one synchronous assembly
If the locknut were to loosen on the piston rod what would happen?
The piston and cylinder components would be damaged
How come piston rods are typically plated in with chrome?
Hardness
Low coefficient of friction
And smoothness of finish
What is rod run out ?
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
What is rod loading
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
Why is excessive rod loading bad
Creates wear on components
Usually either the crosshead pin and con rod bushing
What is rod reversal?
Why is it important?
Refers to the reversal of the rod load from compression to tension.
Allows lubrication to flow around the pins
Tailrods can be used in smaller compressors to compensate for the lack of rod reversal.
True or false
True
When working on piston rods it is important not to damage the rod because?
It has to be prefect to seal through the packing.
How do you check for rod runout?
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
What types of threads are typically on piston rods and can they be repaired if damaged by a file or die nut?
Rolled threads
No they cannot be repaired in the field
What is a common field method for checking cracks in the piston?
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
Why must you be careful when cleaning aluminum pistons?
Very easily damaged and scored
What are the functions of the piston rings
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
What are the materials for piston rings
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
What is the purpose of the compression ring?
Form a dynamic seal that traps the compressed gasses and prevents them from leaking through to the other side of the piston
What is the purpose of the oil control ring?
Controls the flow of oil on the cylinder walls by providing oil flow paths with groove around and holes in the ring.
What are the three types of piston ring joints?
Butt joint - most common and used with compression and oil control rings
Angle joint
Step joint
What does excess clearance in piston rings cause?
What does inadequate clearance cause
Blow by
Inadequate clearance will bind when compressor warms up
How much do you stagger piston ring gaps
180 degrees
How can piston ring grooves be repaired?
Machining the groove larger and installing a spacer ring above the ring
What is the purpose of the rider band?
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
Most rider bands are made of?
Teflon based products in order to have low coefficient of friction and the ability to last.
What are the different types and uses for each rider band type?
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
After installing rider bands make sure to _______ and record for future reference.
Measure
When clamping piston rods in vices what is ver important?
To use PVC to protect rod surface
Torque multipliers must be used to attain final torque when tightening piston to rod.
True
How do you set the piston to cylinder head clearance?
By turning the piston rod into or out of the crosshead.
Ideally when the compressor is hot and running the clearance between the piston and crank end head at BDC should be equal to?
The clearance between the piston and the head end at TDC
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 _______
.03125
What is the purpose of a balance nut?
To fine tune the weight balancing of opposing multistage reciprocating compressor parts
What information is often included on the cylinder name plate?
Bore diameter
Piston stroke
Rated discharge pressure
Maximum allowable working pressure
Maximum allowable discharge gas temp
Displacement
What is the function of a cylinder?
A cylindrical tube that confines the compressed gasses as the piston pushes on them
How are cylinders cooled?
Heat may be removed by air or water (glycol)
What is more the simplest way to cool the cylinders?
Air because it just needs fins cast into the outside of the cylinder
If heat temps are above 88C (190F) but below 121C (250F) what type of liquid cooling can be used?
Thermo-syphon cooling
Moves strictly by convection
When discharge gas temps are over 121C (250F) what type of liquid cooling must you use?
Circulated (pumped) cooling
Using to cold of water for cooling can result in?
Condensation inside the cylinder
What are the simplest cylinder bores?
Cast integral - typically used for smaller compressors
What is the difference between wet and dry cylinder bore liners?
Dry liners do not come in contact with the coolant and wet liners actually form part of the water passage
How do oil free compressors work?
They use special Teflon rings. Cylinder bore is machined to a finer surface finish.
Oil compressors use what to lubricate?
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
What is a common way to control the compressor output?
Clearance pockets
Fixed
Or variable
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:
Reduces
When capacity of the compressor must be changed by varying amounts on a regular basis what clearance pocket would be the best choice?
Variable volume clearance pocket VVCP
By cutting the clearance plug to calculated lengths you can give the desired ________ in capacity?
Reduction
Is it safe to adjust the VVCP while compressor is running?
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
A compressor needs to be shut down and looked at if what symptoms are present?
High temps
Strange or unusual noises
Pressure falling or rising out of normal operating range
What causes scores or scratched cylinder walls?
Cylinder rings and cylinder getting to hot allowing small spots on the rings to momentarily weld to the cylinder wall
When you see discolouration on the cylinder walls is a sign of?
Overhearing
Liner cracks are usually caused by:
Hot spots
Overheating
And
Piston seizures
Premature or excessive wear is often attributed to?
Dust or contamination coming through the air filter. Or leaks coming through the intake piping
Proper performance of a cylinder relies on the cylinder bore being straight round and within the proper __________?
Dimensions
On a small cylinder wall less than 6 inches what is considered to much wear?
A ridge near the top of the piston travel the width of a fingernail
What is the best tool to measure is a cylinder bore is within acceptable limits?
Micrometer or dial bore gauge
Cylinder bore must be checked in a minimum of ________ positions to determine if the cylinder is out of round.
4
Hydro testing is done to see if there are any _______ ?
Cracks
NDT checks are done to detect______?
Defects invisible to the naked eye
Why are the cylinder bores hard to remove?
Installed with an interference fit
If dry liners cannot be removed by force what is the next option?
Grinding along the length of the sleeve so it can collapse
How do you install new liners?
Cooled and shrunk with dry ice until it slips into the bore
Sometimes heating the cylinder bore is necessary as well
If the cylinder does not have a sleeve what can be be if there is excessive wear?
The cylinder assembly can be sent to a machine shop to be bored out. You can either install a sleeve or oversized piston
There is a maximum safe limit a cylinder can be bored. Who do you consult to find that information?
Manufacture
If cylinder bore measurements are in spec what is done?
Honed or reglazed and reused
What can be used to remove ridges
On cylinders?
Small cylinders under 6 inches can use a ridge reamer
Larger cylinders rotary bur or hone is used
Do not cut below ring travel
How is deglazing done?
Flexible hone or ball deglazer
What degree should the cross hatch section be in a perfectly honed cylinder?
Between 40 and 60 degrees
If the cylinder is slightly out of straight and round and the taper is not to excessive it can be repaired with_____?
Ridged/fixed hone
After a cylinder has been honed or deglazed what is used to remove the grit and iron particles?
Hot soapy water than wiped with a lint free oily rag to prevent oxidation
Before doing any work on clearance pockets what must be done?
Vent all pressure
Failure to do so can result in injury or death
Why must a respirator be used when removing clearance pockets?
Toxic gases could still be trapped under pressure in the clearance pocket.
After reassembling a compressors and it has ran long enough to reach normal operating temperatures what must be done?
Shut down, bled and retighten bolts to recommended torque
Never attempt to tighten bolts on a operating or pressurized cylinder
How often should piston rod runout be checked and why?
Every year
Misalignment due to settling foundations
What is a major cause of horizontal runout?
Pipe strain
What factors influence vertical rod runout?
Piston cylinder bore clearances
Crosshead to crosshead guide clearances
When setting piston centreline cold it is normal to set it _______ than the crosshead centre line. Why?
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
Factors that affect rod runout
- 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.
Follow this procedure for checking rod runout?
- Roll compressor to BDC and zero dial
- 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 - 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
Why must packing and oil wipers be used and maintained in good working order on reciprocating compressors?
To prevent excess seal oil consumption and to minimize contamination of the gas being compressed
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
Crank end
Distance piece
What is free-floating packing mechanical packing?
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
What is full-floating packing?
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
Different types of sealing rings are:
Pressure breaker
Radial cut
Tangent cut
Anti extrusion or backup rings
What influences proper packing material?
Lubrication and gas pressures
Metallics rings
High pressure applications where strength and ridgity are important
Bronze is found in the highest pressure
Non metallic
Proffered when high degree of conformability is required
Have pressure limitation but need no break in period, self lubricating, and chemically inert
What is the purpose of the pressure breaker?
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
Pressure breakers are extremely effective on high speed compressors or on compressors above _______ psi?
300
_______ breakers are better then Teflon
Metal
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
Sets
The radial ring is doweled with the ________ and together they work to seal the rod and packing case effectively
Tangent
Radial ring must be installed on the _______ with the markings (dots or letters) on both rings facing toward the cylinder.
Pressure side
What is the packing case
Contains the rings and provide connection points for accessories
The application, pressure, and compressor speed determine:
Number of cups required
- the type of sealing needed between the cups
- whether lubrication is needed
- whether cooling is needed
Cast iron packing casings can withsatanf pressure about _______psi
5000
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.
Heat
Stainless steel is often used when corrosion is a problem
Bronze packing cases are found in _________ situations?
Corrosion resistance with superior heat conductivity
Non sparking applications such as in oxygen services
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.
True
Improper orientation of a full-floating packing case assembly will result in ?
Inadequate sealing
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
Check valve
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?
To face the oil being removed (toward the crankcase)
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?
Provides access to the piston rod for runout readings
The distance price is a solid piece of ______steal that acts as a spacer in order to separate the frame and the cylinder.
Cast
Distance applications are used where ?
No problems with exists with oil mixing with the compressed gasses
Or
Where there is not an explosion hazard
Long distance pieces are typically used for non lubricated systems such as oil free air. How does it work
The extra distance with an oils baffle makes it almost impossible for oil to reach the cylinder
When do you need a two compartment distance piece?
Where a complete sealing from of an explosive gas from the compression chamber must exist
A bent flange or a metal bur on a distance piece affects what?
Runout
What is the purpose of the crosshead guide? T
To maintain alignment
Where is the crosshead and guide located?
In a double acting compressors between the connecting rod and the piston rod
Cross head and guides can be bolted onto the frame as Esperanto units or as part of the ?
Main frame
Manufacturers make cross heads of different weights because?
The crosshead, piston rod, and piston graven back and fourth as a unit. To properly balance you will need correct weight crosshead
Cross head guides as one piece or with removable shoes, what is the purpose of the removable shoe crosshead
To adjust clearance between the crosshead and guides
One piece crosshead a have a babitted or ________ sliding surface?
Machined
Adjustable cross heads are advantageous because:
Allow for a faster and less expensive overhaul.
Spare shoes can be swapped out and can adjust clearances by adding or removing shims
Before pulling the Crosshead you will want to _________\
Check clearances
If you cannot fit a _____ “ feeler gauge into the bottom it means what?
.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
Top crosshead clearances are between _______ and .010” but check with manufacturers specs
.007
After measuring cross head to guid clearance you need to measure?
OD of pin ID of pin then subtract the two. This give you the clearance of pin and bushing
Any uneven wear in the crosshead shoes indicate?
Misalignment
A visual inspection of the pin and bushing may indicate lubrication problems, if all is well may be a_______?
Excessive rod loading
Connecting rods for the link between?
Crankshaft and piston on single acting
Crankshaft and the crosshead on double acting
On operation what forces are applied to the connecting rod?
Compression
Tension
Bending
Twisting
For good ductility and fatigue resistance these rods are made of?
Heat treated alloy steel
Three main styles of connecting rods?
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
Always keep _______ marks matched when assembling connecting rods
Mate
Connecting rod bearings must be _____\ shell to allow crankshaft to fit over the crankshaft journal area?
Split
Inspecting connecting rods consists of?
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
If clearance is incorrect when reassembling connecting rods check for?
Insert caught on tab or dowel
Dirt under insert
Distorted rod bore
Wrong size insert
_________ is used to check connecting rod cap clearance?
Plastigage
Other then the obvious not lining up match marks what is another common installation error when inserting journal bearing to connecting rods??
Not lining up oil hole
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.
Central
Frames are usually made of cast iron and sometimes ________ to relieve stresses.
Heat treated
The bottom of the interior forms the oil sump for the main frame lubrication system. How is the oil pump driven?
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
The frame does not require much maintenance other then?
Cleaning and making sure covers and attatchments are secure to prevent leaked and component failure and ensure hold down bolts are secure
What are the two fundamental concerns when dealing with design of a proper foundation for an industrial compressor application?
Keep the frame from distorting
Properly align the compressor to the driver
To reduce problems with settling or vibrations permanent foundations should have _______ and should be separated from the building with some sort of insulator
Sufficient mass and should rest on solid soil or bearing surfaces
What can help stop the penetration of liquids into the compressor foundations?
A good quality paint or sealer
Which conditions would occur if there was a issue with the main frame?
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
To check for frame warpage you would use the _________ or precision laser
Wire alignment
What is the purpose of the crankshaft
Convert rotary motion into reciprocating motion
Larger compressors crankshafts above 150hp usually have cranks shafts made of?
Heat treated forged steel alloys
Main bearing allow the crankshaft to rotate _______ cause the crank pin journals to rotate in a larger Circle.
Throws
Stoke is equal to ________ the throw
Two times
What are the two different ways of balancing?
Opposed throws
Counterweights
You can balance with opposed throws when you have an ______ number of cylinders?
Even
Counter weights are used when ?
You only have one cylinder
Larger compressors use _______ and counter weights to balance rotational forces.
Opposed tbrow
When inspecting journal surfaces measure cranks shaft journal areas in at least__________ positions
2 positions 90 degrees apart but three checks 60 degrees is better
What are the different ways of of reconditioning a crank shaft?
Grinding and rebuilding
Chroming
Straightening
What should you check for before installing crankshaft?
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
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
Thrust bearings or washers
Main bearing on large compressors are usually ______ type bearings
Shell
Main bearing failure is usually from:
Loss of lubrication
- corrosion
- acids formed from compressed gases
- misalignment
- improper installation
- excessive rod loading
- failure to run a pre lube
Compressor bearing in service may show signs of failure such as:
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
What is one method to check for bearing wear?
Bump check
What is the larger factor in compressor efficiency?
Valves
Because valves are directly in contact with the gas stream they are subject to contaminants such as:
Entrained liquids
Dirt
Corrosive gases
With every compressive stroke valves are at risk of many types of destructive forces such as:
Abrasion
Erosion
Bending
Twisting
Flexing
Temperature extremes
If a compressor operates at 1000 rpm how many times does the valve open and close?
1000 times a minute
525 million times over the course of a year
What 4 components are required for a valve to do it’s job?
A sealing element that opens and closes valve
A valve seat
A valve guard to act as a backstop
Assembly elements
What are the basic things all valves need?
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
What are the different valve types l?
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
