Air Compressors and Pumps Flashcards
Piece of machinery that draws in ambient air, pressurises this air, and delivers in to a holding tank (receiver) or to its end user
Air compressor
7 basic components of an air compressor system
Lubricating system Driving connections Cooling system Compressor element Driving power Control components Air receiver
Three types of driving power
Electrical motors
Internal combustion engine
Reduction gear
Three types of drive connections
Close-coupled
V-belts and pulleys
Reduction gears
When the compressor and the driving unit are mounted on the same shaft, they are ___
Close-coupled
Close-coupling is usually restricted to ___
Small capacity compressors that are driven by electric motors
Sometimes used to join the driving unit to the compressor when the speed of the compressor and the speed of the driving unit are the same
Flexible couplings
Drive connection commonly used with small, low pressure, and some medium-pressure motor driven compressors
V-belts and pulleys
Three types of compressing elements
Centrifugal
Rotary compressors/blowers
Reciprocating
Continuous flow compressor in which the rotating bladed wheels impart a high velocity to the air; the velocity energy developed raises the static pressure of the air without use of valves as it is passed through a diffuser
Centrifugal
Positive displacement compressor in which the displacing and compressing elements are interacting lobes, screws, or eccentrically oriented vanes rotating within housings fitted with suction and discharge ports
Rotary compressors/blowers
Positive displacement compressor in which the displacing and compressing elements are pistons moving back and forth in a cylinder fitted with suction and discharge valves
Reciprocating
Most widely used compressors
Reciprocating compressors
Low operating pressure
150 psi or less
Medium operating pressure
151 to 1000 psi
High operating pressure
Above 1000 psi
Ship’s Service Air
150 psi or less
Start Air, or Diesel Start Air
151 to 1000 psi
SSA
Ship’s Service Air
SA
Start Air
DSA
Diesel Start Air
Total compression form atmospheric pressure to the final discharge pressure, is accomplished in one stroke of the piston
Single-stage compressor
In a two-stage compressor, in ___ of the crankshaft, a compression cycle is completed
One revolution
Cylinder designs very widely depending upon ___
The number of stages of compression required
Two types of pistons
Trunk pistons
Differential pistons
Modified trunk pistons having two or more diametres
Differential pistons
Arranged so that more than one stage of compression is achieved by one piston
Differential pistons
With a differential piston, compression for one stage takes place over the ___, compression for the other stage takes place in the ___
Piston crown
Annular space
The opening and closing of the valves is caused by the difference between ___
The pressure of the air in the cylinder
The pressure of the external air on the intake valve or the pressure of the discharge air on the discharge valve
Valves are either ___
Strip (feather) or disk type
Used for suction and discharge valves of higher pressure stages. These valves are spring-loaded
Disk-type valve
Commonly referred to as a flask or tank
Air receiver
Acts as a storage tank during intervals when the demand for air exceeds the capacity of the compressor
Air receiver
Enables the compressor to shut down during periods of light load
Air receiver
The volume of existing air receivers for low-pressure and high-pressure compressors may be as low as ___ the combined free air capacity per minute of the compressors located in the respective compartment
1/5
Helps to eliminate pulsations in the discharge line of the compressors, and provide a stable pressure sensing station for compressor control
Air receiver
Two types of cooling devices
Intercoolers
Aftercoolers
Devices that remove the heat of compression from the air or gas between consecutive stages of multistage compressor
Intercoolers
Why have a cooling device?
Increase the efficiency of compressing the air, and removing the heat caused by compression
Devices that remove the heat of compression from the air after the last stage of compression prior to its arrival at the air receiver
Aftercoolers
Two types of lubrication systems
Pressurised
Splash
An oil pump is connected to the compressor crankshaft supplying lubrication to the moving parts within the crankcase
Pressurised system
Lubrication system usually found on small, low pressure air compressors
Splash system
Used to provide control of the automatic operation of the air compressor, removal of the compression load, modes of compressor control, and protection capabilities
Components
Designed to remove all but the friction loads on the compressors
Unloading system
Automatically removes the compression load from the compressor while the unit is starting and automatically applies the load again after the unit is up to operating speed
Unloading system
Used on some oil-pressurized compressors in the event of a potentially damaging oil pressure drop
Hydraulic unloaders
Allows the compressor to start in the unloaded state, meaning that the compressor starts, but does not begin to create compressed air until oil pressure is established
Hydraulic unloader
Used in conjunction with unloader towers when the compressor is to be run continuously and an operating pressure range is to be maintained
Pilot valve
Whenever there is a demand for air, the ___ closes, allowing the unloader in the unloader tower to actuate
Pilot valve
As soon as the demand for compressed air is met, the pilot valve ___, allowing air pressure to deactuate in the unloader. The compressor continues to run but does not compress air
Opens
Used to control the method of operation
Manual/Off/Auto switch
Designed to change air pressure without changing air compressors
Pressure-reducing stations
All-in-one unit lubricates the air system, reduces the moisture vapour concentration, and prevents water formation in compressed air lines which, if allowed to accumulate, causes rust in the lines as well as system components
Lubricator/Regulator/Filter
A valve connected to accumulators, separators, filters, receivers, air flasks or other components to remove water, oil and sediment from the component
Blowdown
Usually a small ball valve or petcock which may be attached to a hose and which will vent the sediment away from the operator
Blowdown
Safety precautions for compressors are described in ___
NSTM, Chapter 551
Do not install a ___ between the compressor and receiver unless the compressor is fitted with a relief valve
Stop or check valve
Moisture occurs naturally in air lines as a result of ___
Compression
Recommended companion to filters, aftercoolers, and automatic drains for improving the productivity of compressed air systems
Separators
The area at the compressor intake is constantly under a ___ while the compressor is in operation
Vacuum
Performed on all accumulators, separators, filters, receivers, and air flasks to remove deposits of water, oil, and sediment
Blowdowns
(Blowdown) If double valved, start with ___
Both the upstream and downstream valve closed
(Blowdown) If double valved, open the ___ valve first
Downstream
(Blowdown) Open the upstream valve to the bleed position for ___
10 - 20 seconds
Moisture separators downstream of the compressor shall be blown down ___
Once per hour
Manually drained compressor separators shall be drained ___
Once per watch, or every four hours, whichever comes first
Air filters will be checked ___
Weekly or per PMS schedule
Improper pulley/sheave alignment and belt tension are causes for ___
Motor overloading
Excessive vibration
Premature belt and/or bearing failure
Clean air cleaning element by ___
Blowing LP air from the inside to the outside of the element
Where should you not clean the air cleaning element?
Lazarette
Check belt for:
Cracks
Wear
Alignment
Proper tension
Belt tension shall be ___ drop from middle
7/16”
Test compressor relief valve by ___
Lifting valve by hand and release
Most commonly found displacement type of pumps in the CG
Centrifugal pump
Pump large volumes of flow at low pressure
Centrifugal pump
A centrifugal pump is of what type
Nonpositive displacement type pump which delivers a variable flow rate depending upon system lineup
The discharge flow rate from a nonpositive displacement pump is inversely proportional to ___
The discharge pressure it faces
As discharge pressure rises, flow rate from the pump ___
Drops
The maximum pressure that a pump can produce
Shutoff head
The fluid being pumped also acts as a ___ for the pump internals
Cooling medium
If the discharge pressure of a nonpositive displacement pump approaches shutoff head, the flow approaches ___ and the pump ___
0
Overheats
The force that tends to throw an object outward from a center of rotation
Centrifugal force
The impeller creates a ___ at its center, and is discharged to ___
The center
Outer rim
The liquid can be slowed down by being led through a ___
Volute or a series of diffusing passages
As the velocity of the liquid decreases, the pressure ___
Increases
The pump casing is sometimes referred to as the ___
Housing
The part of the pump that, by design, directs the flow of liquid
Pump casing
Two basic designs of pump casing
Volute
Diffuser
This type of casing uses a gradually widening spiral channel which changes velocity into pressure
Volute
This type of casing serves the same purpose as the volute, but is more efficient. It directs more fluid to the outlet by the use of stationary vanes
Diffuser
Two classifications of centrifugal pumps
Single-stage
Multi-stage
Transfers the motion from the prime mover to the impeller
Shaft
Protects the shaft from excessive wear and corrosion from the seal or packing
Shaft sleeve
The sleeve is often made of ___ and is ___
Softer metal
Inexpensive to replace
Using a sleeve avoids ___
Shaft replacement and is cost effective
Most important part of the pump
Impeller
Transmits centrifugal force to the fluid by spinning at a high number of RPMs
Impeller
An impeller must be ___ to avoid vibrations and wear
Balanced
Impellers can be classified as ___
Closed
Open
Semi-closed
Collar-type rings that protect the impeller and housing from wear
Wearing rings
When the maximum allowable clearance has been exceeded, wearing rings must be ___
Replaced
Prevent stiffing box water from entering the bearing housing
Water flinger rings
Fitting pumps with ___ eliminates excessive stuffing box leakage
Mechanical seals
Prevent air or fluid from entering or exiting the pump housing, and they are installed on the shaft behind the impeller
Mechanical seals
Mechanical seal consists of two parts
Stationary seal
Rotary seal
Composed of a ceramic face held in the pump housing by a rubber cup or seal seat
Stationary seal
Composed of a carbon face, a rubber grommet or o-ring, a tension spring, and a driving band
Rotary seal
Rides on the shaft sleeve and rotates with the shaft and impeller
Rotary seal
The ___ is held against the stationary ceramic seal by the force applied by the tension spring
Carbon face
The rotary seal head is sealed to the pump shaft or shaft sleeve by the ___
Rubber grommet
The carbon face and the ceramic seal are ___
Lap-mated
When replacing the mechanical seal, you must ___
Replace the whole seal
Care must be taken not to touch the mated faces with your hands, because ___
The acid in the body oils of the fingers will break down the carbon face
Mechanical seals should be ___ whenever the seal is removed for any reason, or if the leakage exceeds ____ for new seals, or ___ for used seals
Replaced
5 drops per minute
60 drops per minute
All stuffing boxes are designed to leak for ___
Shaft cooling
Cylindrical space in the pump casing surrounding the shaft
Stuffing box
___ are placed in the stuffing box to form a seal, controlling the rate of leakage along the shaft
Rings of packing material
