Download Gas Processing I NGT 140

Gas Compression and Flow
Dynamics
NGT 150
Reciprocating Gas Compressors
Chapter 6
Maintenance
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Maintenance Programs and Repair Indicators
• Maintenance Programs
1. Failure – Equipment repaired or replace
when it ceases to perform its assigned
function.
2. Preventive – Based on experience. Repairs
are made at assigned intervals
3. Predictive – Repairs are made as needed
and as predicted by equipment analysis
and operating data.
Four Stroke Engine (p. 6-9)
Repair Indicators - Engine
• Crankcase oil consumption
a. Compare oil consumption to normal
operating records. (abnormal – damaged
rings or liner)
b. Check oil reservoir gauge
c. Check how much oil adding by hand.
• Crankcase pressure
a. Blowby – combustion gas that passes by
piston rings and causes pressure inside the
crankcase.
b. Causes – bad rings, damaged pistons, or
scored liners
Repair Indicators - Engine
• Manifold pressure (four stroke cycle engine) –
indicates the load condition, proper fuel-air
mixture and the mechanical condition of the
valves and piston rings.
• Scavenging Pressure (two stroke cycle engine)
– indicates plugged ports, condition of
scavenging valves, cylinders, rings, packing and
air breather condition.
Two Stroke Engine (p. 5-6)
Repair Indicators - Engine
• Compression pressure
a. Loss in cylinder compression pressure.
Indicates blowby or leaking valves, piston
rings or head gaskets.
b. Not good to operate in this condition due
to increase lubricating oil and fuel
consumption.
c. Also causes the engine to wear very rapidly.
d. Leads to ring, piston, valve, head, liner and
supercharger failure.
Repair Indicators - Engine
• Condition of oil
a. Oil change interval depends on engine type
and manufacturer recommendations.
b. At oil change check for water, sludge,
babitt, metal cuttings and carbon.
Repair Indicators - Engine
• Crankcase Inspection
a. Inspection can be very revealing
b. Inspect crankcase door back for foreign
material from the crankshaft.
c. Bronze cuttings – faulty wristpin bushing
d. Babitt – from bearings
e. Cast iron – from liners or pistons
f. Move piston to top dead center to inspect
liner for scuffing.
g. Look for discolored parts that indicate
heating between mating surfaces (blue is
hot enough to warped or cracked).
Repair Indicators - Engine
•
•
•
•
Sound Check (can use special stethoscope)
Leaking valves
Abnormal knocks, noises and vibrations
Piston hitting end of cylinder, or loose valves or
packing glands
Bearing Failure
1. Fatigue – fine cracks spreading from bearing surface
through to bearing wall that propagate. Also foreign
particles embedded in the load area.
2. Scoring on bearing surface caused by dirt or foreign
particles in the oil film.
3. Seizure from metal to metal rubbing.
4. Erosion – caused by washing away the bearing lining
particle by particle (rare). Induced by high oil velocity
or sudden pressure changes in oil film (foaming)
5. Corrosion – changes in chemical makeup of the
bearing lining leaving a spongy matrix that leads to
bearing failure.
Compressor Valve Failure
• Wear and Fatigue
a. Is a fact of life but can be minimized with
proper lubrication, design and selection of
materials.
b. Wear can occur at the guides.
c. Fatigue is the result of repeated cyclic
stress from operation
Compressor Valve Failure
• Foreign materials
1. Liquid carryover
a. From an upstream process or interstage
cooler.
b. A slug
c. Condensate from poor separation
upstream of the compressor
2. Dirty gas – for example fracking sand.
3. Carbon formation or sludge from certain
oil and gas combinations. Oil additives can
be a problem.
4. Corrosion from corrosive elements in the
gas
Compressor Valve Failure
• Abnormal Mechanical Action
1. Slamming of valve when opening or
closing. Caused by using clearance pockets
or finite differences in valve cycle times.
2. Fluttering due to insufficient pressure drop.
(valves operate on differential pressure)
Pressure drop is a function of velocity,
density, and flow coefficient.
3. Resonance or pulsations can cause late
closing and slamming of valves.
4. Flow pattern – rare but can happen due to
a disturbance within the cylinder passage
leading to a valve.
Engine-Compressor Analyzer
• The analyzer (laptop or oscilloscope) has the
means to provides displays and analysis of:
a. Pressure-volume
b. Pressure-time
c. Ignition
d. Vibration patterns
Engine-Compressor Analyzer
Engine-Compressor Analyzer
Operating and Maintenance Records
• Economical operation of a gas compressor unit
depends to a great extent on routine
operational checks and adequate recording of
data for evaluation
• A satisfactory maintenance program is
dependent upon adequate record keeping of
all checks (eg. clearance & pressures) and
repair work performed (and when) on the unit.
• Preventive maintenance is based upon the
fact that practically all failures involving critical
parts are preceded by certain signs, conditions,
or indications that parts are approaching
failure.
Typical Routine Operating Procedures
1.
2.
3.
4.
5.
6.
7.
8.
Observe critical temperature and pressures on lubrication and
cooling systems.
Note unusual noises.
Check for proper operating levels of fluids.
Check for leaks in lube oil and water lines.
Check for gas leaks (using appropriate test equipment)
Check operation and belts of water pump and cooler fan drive.
Check the temperature of the compressor cylinder suction valve
caps. A hot cap indicates a leaking suction valve.
Check the liquids in the compressor cylinder suction surge drums
(The simplest form of a pulsation dampener is called a surge drum. It is a pressure vessel
without internal baffles and is usually mounted directly to the compressor cylinder
connection, either suction or discharge.)
9.
Check exhaust temperatures to determine if all cylinders are
firing. Or if exhaust temperatures are excessive the unit or
cylinder is overloaded or the intake ports are clogged with
carbon.
Safety Precautions
Serious injury is always a possibility when engaging in repairs. As a
minimum take the following precautions. Safety and LOTO procedures
must be followed.
1.
Pressure should be vented from the compressor cylinders and power cylinders and
the vent valves left open. Flammable gas should be purged with nitrogen.
2. Fuel gas should be turned off and vent valves left open.
3. The ignition switch should be in off position or spark plug wires disconnected.
4. The flywheel locking device should be engaged
5. The jacking bar should be removed from the flywheel after turning the engine
manually.
6. Lifting equipment such as hoists should be in good operating condition and should
not be overloaded.
7. If spilled, oil should be wiped up from the floor immediately to prevent slipping.
(control and contain all fluid spills or leaks)
8. Vapor proof extension lights should be used.
9. Compressor suction and discharge valves should be sealed off by a blind vent valve
left open before work is done in cylinder.
10. Personnel should not stand directly over spark plugs when the engine is operating.
Center portion may blow out.
11. All compressor piping and cylinders in natural gas service should be purged
thoroughly with gas prior to restarting the equipment. A serious explosion can
result from not removing all air completely from the system.