Air Systems

Question 1

Reference for air supply for pneumatic control systems?
a) State the 3 contaminants that may be present?
b) Why are these undesirable?

Answer 1

Water Vapor - Leading to corrosion of air receivers, pipework and control systems
downstream.
Particulates - Increased erosion/wear and early fouling of system filters
Emulsifications - Water vapor and oil carried over from compressor. Creates a sticky emulsion
that can gum up components - specifically on fine control system components.

Question 2

Explain the situation under which each of the following devices fitted to an air compressor may
operate, and where they are fitted?
Fusible plug on compressor discharge?
Fusible plug o air receiver ?
Bursting disc?

Answer 2

a) Fusible Plug on compressor discharge
Designed to melt at approximately 120 degrees C. If the compressor loses its cooling medium
and overheats. The final stage must have one or an alarm.
b) Fusible Plug on Air Receiver
If there was a fire in the engine room. The risk of the receiver overheating, rupturing and
providing enormous amounts of oxygen to fuel the fire is a very real one. The fusible link will
be designed to melt and the compresses air pipes out the engine room
c) Bursting Disk
Water cooled heat exchangers must be fitted with a pressure relief within the water jacket.
This protect the compressor if the cooling pipe bursts and the cooling water gets exposed to
full delivery air pressure from the compressor.

Question 3

With reference to a full automatic, water cooled starting air compressor?
what alarms need to be fitted

Answer 3

High Temperature (water + Air)
Water cooling level
Low Oil Pressure
High Pressure trip
Unload valve failure
Low Lube Oil Level
Motor Overload

Question 4

How is damage from over pressure prevented in each of the following?
Intercoolers
waterjackets

Answer 4

Pressure relief valves or fusible links
b. Waterjackets
Pressure relief valves or bursting disks

Question 5

for high pressure systems?
Why the pipes should not be subject to sudden impact?
Why the valves should be open slowly?

Answer 5

Why the pipes should not be subject to sudden impact?
The pipes are designed to withstand the internal pressure. When pressurized and external hit
could lead to pipe failure and the release of high pressure air and energy which can be
dangerous and fatal
b) Why the valves should be open slowly?
To not shock load the system which will create a high level of stress int eh system components
and can lead to component failure. If the pipework was to contain any moisture then this
could cause a shock wave and resulting in a water hammer action which can lead to physical
failure of valves and pipework joints.

Question 6

Reference to air recievers?
list 4 safety features?
state the pressure the relief valve should lift?

Answer 6

a) List 4 safety features?
Pressure relief valve
Fusible Plug
Drain Valve
Quick Closing isolating Valve.
Low Pressure Alarm
b) State pressure at which the relief valve should lift?
The relief valve should be of such a size and set so the maximum accumulation pressure will
not exceed the working pressure by no more than 10%. Should all the discharge valves be shut
and all compressors are filling the receiver at maximum capacity.
Working Pressure = 27
Test Pressure = 45
Design Pressure = 30

Question 7

List 5 fittings to the shell of the air receiver? And each of the purposes?

Answer 7

Manhole Door - Allow access for cleaning and inspection
Calibrated Pressure Gauge - Indicates the stored pressure
Relief Valve - Designed to rapidly dump the air in the case of over pressure and prevent potential
rupture of the air receiver
Drain - Remove Any Moisture, limits corrosion and carry over into equipment.
Fusible Link - Designed to melt at a set point so the air receiver can dump its air out of the engine
room in case of a local fire
Inlet Line (non return) - Feed from the compressors, none return so the air cant go back to the
compressors
Outlet Line (fast closing slow opening) - Quick isolation if required, slow opening to reduce system
stresses.

Question 8

Reference to starting air systems.
Why should they be drained regulary?
Why outlet valves should be open slowly?

Answer 8

a) Why should they be drained regularly?
Removal of water and also emulsion. This reduces the chance of corrosion within the tank and
down the line which can lead to premature failure. Rust can also clog us sensitive equipment
and prematurely block up filters. Emulsions gum up and cause operation failure downstream.
b) Why outlet valves should be opened slowly?
To not shock load the system which will create a high level of stress int eh system components
and can lead to component failure. If the pipework was to contain any moisture then this
could cause a shock wave and resulting in a water hammer action which can lead to physical
failure of valves and pipework joints.

Question 9

Why two stage compression?

Answer 9

Two stage compression reduces the amount of work required to get the same amount of
compressed air.
Refer to the diagram below for the PV diagram for a typical compressed air system.
Including an intercooler reduces the amount of work required (therefore increasing the efficiency of
the generator.