GE Oil & Gas
Heavy Duty Gas Turbine
Maintenance planning
Maintenance Planning
Is maintenance only a cost or could be an
opportunity?
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HDGT Maintenance planning
June 2013
Maintenance Planning
An appropriate planning:
•
optimizes the
maintenance costs
•
maximizes equipment
availability & reliability
Availability
probability for a machine
of being available within
a period, indifferently
whether the unit is
requested to run or not
Maintenance
Planning
probability for a
machine of
performing when
it was intended to
do so, evaluated in
a certain period
Reliability
Maintenance
Costs
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HDGT Maintenance planning
June 2013
Maintenance Planning
Cost of
Downtime
Reliability
Needs
Environment
On-site
maintenance
capability
Maintenance
Planning
Design
Features
Operating
Conditions
Type of Fuel
Spare Parts
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HDGT Maintenance planning
June 2013
Maintenance Planning
To facilitate on-site maintenance practices, GE heavy duty gas turbines are provided
with appropriate design features, such as:
•
horizontally split casings for ease of access to the internal parts
•
stator vanes can be slid circumferentially into the casing grooves for removal and
replacement without rotor removal
•
rotor buckets are moment-weighed and computer charted so that they may be replaced
without the need to remove or rebalance the rotor
•
bearing housing are horizontally split to be easily inspected and replaced
•
flow path surfaces can also be visually inspected by means of a borescope
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HDGT Maintenance planning
June 2013
Maintenance Planning
Baseline Condition
•
continuous duty at base load
•
natural gas fuel
•
no steam / water injection
sets the maximum recommended maintenance intervals, which are based on:
•
Hours of continuous duty
•
Number of Starts
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HDGT Maintenance planning
June 2013
Maintenance Planning
Continuous duty
Cyclic duty
Low number of starts
High percentage of cold starts
Units starts daily with week-end shutdowns
12 ->16 hrs per start
•
Corrosion and Oxidation
•
Thermal/mechanical fatigue
•
Creep
•
Rubs/Wear
•
Erosion
•
FOD
•
Rubs/Wear
•
Foreign Objects Damages
Gas turbine components
deterioration differs depending
on its service duty cycle
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HDGT Maintenance planning
June 2013
Maintenance Planning
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HDGT Maintenance planning
June 2013
Maintenance Planning
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HDGT Maintenance planning
June 2013
Maintenance Planning
How does GE approach deal with operating conditions differing from the baseline?
fuel type
firing temperature setting
steam or water injection
produce effects similar to those induced by
Continuous duty (hours-like MF to be applied)
emergency shutdowns
start-up rate
starts with liquid fuel
sudden load changes
produce effects similar to those induced by
Cyclic Duty (starts-like MF to be applied)
Appropriate hours-base or starts-based Maintenance Factors are applied to
reduce the recommended maintenance intervals.
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HDGT Maintenance planning
June 2013
Maintenance Planning
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HDGT Maintenance planning
June 2013
Maintenance Planning
°F
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HDGT Maintenance planning
June 2013
Maintenance Planning
Water or steam injection affects parts lives and maintenance intervals, as it
changes the hot gas heat transport properties.
Higher gas
conductivity
Increased
heat transfer
to hot gas
path parts
Higher metal
temperatures
Reduced
parts lives
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HDGT Maintenance planning
June 2013
Maintenance Planning
The Maintenance Factor may be
calculated as follows:
MF = K + M x I
The parameter I represents the
percentage of steam/water injection
with reference to the inlet air flow.
The values of M and K depend on:
•
the value of the parameter I
•
the dry or wet control of the GT
•
the turbine nozzles material
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HDGT Maintenance planning
June 2013
Maintenance Planning
If overspeed operation represents
a small fraction of the turbine’s
operating profile, this effect on
parts life can sometimes be
ignored.
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HDGT Maintenance planning
June 2013
Maintenance Planning
Hot Gas Parts temperature chart
against time during a usual cycle
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HDGT Maintenance planning
June 2013
Compression stress
Tensile stress
Maintenance Planning
These gradients produce
thermal stresses that, when
cycled, could lead to
cracking.
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HDGT Maintenance planning
June 2013
Maintenance Planning
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HDGT Maintenance planning
June 2013
Maintenance Planning
Any operating condition that
significantly increases the strain
range acts to reduce the fatigue
life and increase the startsbased maintenance factor.
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HDGT Maintenance planning
June 2013
Maintenance Planning
Similarly to trips from load, emergency and fast loading starts affect the startsbased maintenance interval.
 Maintenance Factor for emergency start-up = 20
 Maintenance Factor for fast-loading start-up = 2
Emergency start-up are those that bring the unit from standstill to full load with the fastest sequence
Fast loading start-up means that the unit is brought from standstill to full speed no load with the normal
sequence and then submitted to a fast loading sequence
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HDGT Maintenance planning
June 2013
Maintenance Planning
Starts with liquid fuel normally cause higher thermal effects if compared to natural
gas fuel.

Maintenance factor for start-up with No2 distillate = 1.5
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HDGT Maintenance planning
June 2013
Maintenance Planning
Partial load operating cycles would allow
for an extension of the maintenance
interval.
Factored starts calculation is based upon
the maximum load achieved during
operation.
Note:
If a unit is operated at part load for three weeks
and then ramped up to base load for the last ten
minutes, then the unit’s total operation would be
described as a base load start/stop cycle.
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HDGT Maintenance planning
June 2013
Maintenance Planning
GE maintenance approach define a
sudden load change when both of the
following conditions occur:
•
the load variation rate exceeds 6% of
the turbine base load per second
•
the total load change exceeds 8% of
the turbine base load
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HDGT Maintenance planning
June 2013
Maintenance Planning
How are maintenance
intervals changed?
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HDGT Maintenance planning
June 2013
Maintenance Planning
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HDGT Maintenance planning
June 2013
Maintenance Planning
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HDGT Maintenance planning
June 2013
Maintenance Planning
According to a MS7001 operating data, recorded since the last hot gas path
inspection, we would like to estimate when the next one should be scheduled.
According to GE publications:
• the recommended HGPI interval is set at 24000 hours, if operating at baseline
condition (natural gas fuel, without water/steam injection, continuous duty at base
load).
• the nominal HGPI starts interval is set at 1200, based on normal startups, no trips,
no emergency starts.
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HDGT Maintenance planning
June 2013
Maintenance Planning
Hours-based effects
Starts-base effects
Annual hours with gas fuel, base load
Normal start-stop cycles N = 140/yr
G = 3200 hr/yr
Part load start-stop cycles NA = 40/yr
Annual hours with light distillate fuel
Base load start-stop cycles NB = 100/yr
D = 350 hr/yr
Peak load start-stop cycles NP = 0/yr
Annual hours at peak load
P = 120 hr/yr
Emergency Starts to base load E = 2/yr
Steam injection rate
Fast loads to base load F = 5/yr
I = 2.4%
Trips from base load T = 20/yr
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HDGT Maintenance planning
June 2013
Maintenance Planning
Assume that the nozzles are made of FSX-414
material.
Since the unit operates on dry control curve
and at a steam injection rate of 2.4%, the
value of “M” is 0.18 and “K” is 0.6.
The maintenance factor is:
MF =
[K + M(I)] x [G + 1.5(D) + Af(H) + 6(P)]
(G + D + H + P)
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HDGT Maintenance planning
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Maintenance Planning
MF =
[0.6 + 0.18·(2.4)] x [3200 + 1.5·(350) + 0 + 6·(120)]
(3200 + 350 + 0 + 120)
= 1.25
The hours-based adjusted inspection interval is therefore:
Hrs = 24'000/1.25 = 19'200 hours
Since the total annual operating hours are 3'670, the estimated time to reach
19'200 hours is 5.24 years (19'200/3'670).
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HDGT Maintenance planning
June 2013
Maintenance Planning
Remember that:
NA = 40/yr
NB = 100
NP = 0/yr
E = 2/yr
F = 5/yr
T = 20/yr
and that the starts-based standard
maintenance interval is:
S = 1‘200
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HDGT Maintenance planning
June 2013
Maintenance Planning
The starts-based maintenance factor is:
MF =
MF =
0.5 (NA)+(NB)+1.3(NP)+20(E)+2(F) +  (an-1)Ti
NA + NB + NP
0.5·(40)+(127)+1.3·(0)+20·(2)+2·(5)+(8-1)·20
40 + 127 + 0
=2
The adjusted inspection interval based on starts is
S = 1200/2.0 = 600 starts
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HDGT Maintenance planning
June 2013
Maintenance Planning
Since the total annual number of starts is 167, the estimated time to reach 600
starts is 600/167 = 3.6 years.
starts-based
interval
<
hours-based
interval
HGPI interval set
by STARTS
The unit Hot Gas Path Inspection interval is set at 600 starts
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HDGT Maintenance planning
June 2013
Thank you.
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Introduction & General Overview
maggio 2013