3.4.6
Maintaining Bulk Plant
Compressors
Knowledge of the processes involved in maintaining bulk
plant compressors is necessary for safe and efficient
plant operations.
In this module you will learn to identify:
(1) Devices basic to compressor operations
(2) Compressor protective devices
(3) Basic compressor operations
(4) Procedures for maintaining compressor systems
3.4.6 Student Book  © 2004 Propane Education & Research Council
Page 1
Devices Basic to Compressor Operations
Propane compressors transfer liquid from one container to another by
creating a [vapor] pressure difference between the supply tank and
the receiver tank.
Figure 1. Transferring Propane with a Compressor
3.4.6 Student Book  © 2004 Propane Education & Research Council
Page 1
Devices Basic to Compressor Operations
The ratio between the absolute intake and absolute exhaust pressure is known
as the compression ratio and is usually monitored by means of pressure gauges
mounted in the inlet and outlet connections to the compressor.
Figure 2. Propane Compressor
3.4.6 Student Book  © 2004 Propane Education & Research Council
Page 2
Devices Basic to Compressor Operations
Valve Assemblies— Most compressors are designed so the valves can
be easily removed for inspection. If found to be worn or damaged, they
are usually replaced as an assembly and not rebuilt.
Piston, Crosshead and Packing Seal— Many propane compressors
are "oil-less" or "dry" compressors, especially designed to prevent the
oil they use for lubrication from being compressed along with the vapor
in the compression chamber.
Lubrication System— At the bottom of the compressor is a sump that
contains the compressor's oil supply and an oil pump or slinger.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages 2 & 3
Devices Basic to Compressor Operations
Cooling System— A fan built into the flywheel at the rear of the
compressor cools the fins by blowing air across them whenever the
compressor is running. Compressors should never be mounted closer
than 18 inches to a wall or other surface that would restrict free
movement of air and should always be 18 inches above grade to
prevent the flywheel fan from blowing dirt and debris into cooling fins.
Drive System — Stationary compressors are usually driven by
explosion-proof electrical motors attached to the compressor drive shaft
with V-belts and pulleys.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages
3&4
Identifying Basic Compressor Protective Devices
Figure 4. Compressor System with Protective Line Devices
3.4.6 Student Book  © 2004 Propane Education & Research Council
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5
Identifying Basic Compressor Protective Devices
Strainer— Compressors can be damaged by dirt, pipe scale, and
other debris in the plant piping system. Strainers are usually ordered
according to the pipe size and their inlet and outlet openings and are
usually at least one pipe size larger than the system vapor piping to
minimize restrictions to flow.
Liquid Trap— To prevent liquid from reaching the compression
chamber, most compressors have a liquid trap installed in the inlet
piping between the strainer and the compressor intake.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages
5&6
Identifying Basic Compressor Protective Devices
Liquid Trap
Figure 5. Liquid Trap
3.4.6 Student Book  © 2004 Propane Education & Research Council
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6
Identifying Basic Compressor Protective Devices
Discharge Relief Valve— Compressor systems must be protected
against damage from excessively high pressure. For this reason, the
discharge line of most compressors is equipped with a discharge relief
valve.
Discharge valves should be inspected regularly for leaks and to be sure
the inside of the valve is free from corrosion and accumulations of
debris that would prevent the valve from opening and closing properly.
CAUTION:
When inspecting relief valves, use an inspection mirror or similar device to
check inside of the valve. Never look directly into the outlet of any valve.
3.4.6 Student Book  © 2004 Propane Education & Research Council
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7
Identifying Basic Compressor Operations
Compressor operations are usually used in unloading operations.
Vapor Recovery Operations
•
Even when a tank appears to be empty of liquid, it is full of propane vapor.
The tank may also contain some liquid (called a "liquid heel") which lies too
low in the tank to be accessible from the liquid withdrawal connection.
•
While such small amounts of the product may seem insignificant, they are
not. For example, an "empty" jumbo railcar may actually contain enough
vapor to make nearly 1,500 gallons of propane liquid. For obvious reasons
then, many dealers find it worthwhile to recover some of the propane vapor
left in railcars and other large tanks.
•
The compressor withdraws vapor from the tank car and forces it into the
liquid space of the plant storage tank, where the cooler temperature
condenses the vapor back into a liquid. The compressor is actually lowering
the pressure in the tank car by withdrawing vapor, the compressor also
removes the liquid heel, which boils off with the drop in pressure.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages
7& 8
Identifying Basic Compressor Operations
Figure 6. Three Valve
Manifold Piping System
Courtesy of Corken, Inc.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Page
8
Identifying Basic Compressor Operations
Some installations stop vapor recovery operations when the inlet pressure at the
compressor reaches some predetermined value.
This value is usually
somewhere between 30 and 50 psig. Others stop when the ratio between the
inlet and outlet pressures become approximately one-to-four.
PROPANE EVACUATION TIME
for 33,000 Gallon W.C.
(124,905 Liter) Capacity Railcar
(Model "B" Size Compressor)
Figure 7. Vapor Recovery Graph
3.4.6 Student Book  © 2004 Propane Education & Research Council
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9
Identifying Basic Compressor Operations
Piping Manifolds and Piping Systems The compressor is always installed
together with a piping manifold, which is located near the compressor and used to
control the flow of vapor through the system during transfer operations.
•
The exact construction of the manifold depends upon the types of
operations for which the compressor will be used.
•
The size of the vapor and liquid piping, which must be large enough
to deliver the rated flow capacity of the compressor is very important.
Undersized piping will result in high differential pressures, lower flow
rates, and increased horsepower requirements for the compressor
drive system.
•
The compressor should be located as close as practicable to the
tank to be unloaded.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages
9 & 10
Identifying Basic Compressor Operations
Four-Way Directional Control Valves
Figure 8. Four-Way Directional Control Valve
3.4.6 Student Book  © 2004 Propane Education & Research Council
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10
Compressor System Maintenance Procedures
Drive System Maintenance — Check V-belts daily for proper alignment
and tension. Like the V-belts on pump drive systems, they should depress 1/2"
to 3/4" under moderate thumb pressure.
Lubrication System Maintenance — Check the crankcase for leaks,
especially around the flywheel-end of the crankshaft, the dipstick, and the
breather valve assembly. When checking the breather valve assembly, be sure
that it is clear and free of any dirt or trash.
Check the oil level in the crankcase. If the oil level is low, check the
manufacturer's instructions for the type of oil to use and the amount to add.
Check the oil pressure gauge. If the oil pressure is unusually high or low, shut
down the compressor.
3.4.6 Student Book  © 2004 Propane Education & Research Council
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Compressor System Maintenance Procedures
Packing Maintenance — Check the packings for leaks on a daily basis.
Liquid Trap Assembly
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages 11 & 12
Time to See If You Got the Key Points of
This Module…
• Complete the Review on pages 13 & 14.
• See if you are ready for the Certification
Exam by checking off the
performance criteria on pages 15 &
16.
3.4.6 Student Book  © 2004 Propane Education & Research Council
Pages 13 - 16