AERODERIVATIVE TURBINE OPERATIONS & MAINTENANCE

WESTERN TURBINE USER GROUP ADDRESSES LM5000 END OF LIFE, CONTROL SYSTEMS, HRSG SUPERHEATERS, THE RENEWABLE INFLUX AND GT SALES TRENDS

One CTOTF presentation examined best practices for HRSG superheater maintenance[/caption]

Aeroderivatives are the bread and butter of Western Turbine Users Inc. (WTUI). The user group brings together close to 1,000 users of various GE aeroderivative gas turbines (GTs) for an annual conference, this time in March, in Palm Springs, California. The user group addressed the rapid change within the power generation industry. Coal and nuclear plants are being decommissioned, the Clean Power Plan’s mandate for greater penetration of renewables is demanding aeroderivatives start faster and more often while operating with lower emissions and higher output. It’s a major challenge to satisfy these needs while remaining reliable, available and under-budget, said WTUI President Chuck Casey, Utility Generation Manager for Riverside Public Utilities in California.

The conference program brought together a collection of user, OEM, consultant and aftermarket representatives from companies, such as GE Water and Power, Woodward, Sulzer, Air New Zealand Gas Turbines (ANZ), MTU Maintenance, Trans Canada Turbines (TCT), Axford Consulting, IHI Japan and HRST. They delivered the latest on LM turbomachinery operations, maintenance and upgrades, as well as briefings on LM5000 end of life plans, the basics of control systems, Heat Recovery Steam Generator (HRSG) superheater repair, and sales trends in gas turbines.

Setting the tone Casey addressed the rise of simple cycle and combined cycle natural gas-fired plants. Recently, renewables have flooded the grid, coal plants are being retired, and battery storage is beginning to pick up, he said. Referring to the Duck Curve of California generation (Turbomachinery International, p. 24 January/February, 2016), he said the ramp up and down due to renewables is getting ever more pronounced. The fallout for generators is significant. “In 13 out of the last 30 days, Independent System Operator (ISO) prices went negative for at least one hour,” said Casey. “In other words, they are paying you to take the power.”

He urged WTUI members to become more flexible with their operations. His plant, for example, has nine GTs and most start a couple of times per day and run for only an hour or so. “We need to know more about synchronous condensing, ramp rates, fast starts, dual fuel, improving emissions and fast start,” said Casey. “In this new world, maintenance schedules are changing: Instead of worrying about filters clogging, we now need to watch out for filter cages rusting as we don’t run them enough.”

GT trends

Each year, WTUI invites Mark Axford of Axford Consulting to brief attendees on ongoing trends impacting gas turbines. Last year he predicted U.S. orders would fall 10% while worldwide orders would rise by 10%. He was close on the worldwide side where orders increased by 11%. But the U.S. actually saw a jump of 6% in 2015. Each year, anywhere from about 40,000 MW to 80,000 MW of GTs come onto the grid. “We have seen steady growth in GT orders worldwide of about 5% per year for decades,” said Axford. “That has not been the case in the U.S., although there has been stronger growth here in the past two years.”

The Eurozone continues to be in dire straits as regards new GT orders, which have crashed from 8,000 MW per year to zero since 2008. Governmental policy is a major factor in that decline. Meanwhile, in the U.S., shale gas exploitation has led pipelines that used to flow north to be reversed. Mexico is another area where policy is making a big difference (Turbomachinery International, p.17 March/April 2016). That nation’s GT orders have soared from 500 MW per year to 4,000 MW as a result of the breaking up of the monopoly of state-owned CFE and PEMEX. “There are now six new pipelines under construction to the Mexico- Texas border,” said Axford.

Further afield, Africa remains a booming market for GT sales. In recent times, Algeria has been the star, deploying scores of mobile GTs to cope with burgeoning demand. Now Egypt has taken the lead with a number of large orders. Hotly contested race On the OEM front, GE has held the lead for some time. But Siemens is threatening that domination, coming in a close second in a hotly contested race. Axford added that Siemens is off to a fast start this year and could end up number one for 2016. Specific to aeroderivative turbines, Axford said GE accounted for almost all international orders.

While some continue to opt for the LM2500 for pipeline duty, the LM6000 and LMS100 have the lion’s share of orders. There were 14 LM6000 units ordered last year (only two in the U.S.). This makes it the lowest order rate for LM6000s since 1992. With the LM6000 orders in decline, it looks like the LMS100 may be finally finding its feet. After several rocky years, LMS100 aeroderivative orders rebounded to 17 last year, which represents a record year. That takes the fleet size up to nearly 100. “The faster you can achieve a large fleet, the more orders you are going to accumulate,” said Axford. “Once a large user base develops, you see the development of best practices.”

The last few years had also witnessed the development of a market preference toward combined cycle. The ratio has averaged about 75% combined cycle and 25% simple cycle for a few years. But last year, it moved back close to 50%- 50%. Part of the reason for that was the surge in LMS100 orders. But overall, aeroderivatives now have a much stronger presence in North America where a picture is emerging of aeroderivatives dominating for simple cycle plants while heavy frames have the upper hand in combined cycle power plants. Axford expects GT orders to be hurt by lower oil prices as well as renewables taking more GT orders away. “We predict U.S. and worldwide orders to be down 10% this year.”

LM5000 end of life

GE introduced the LM5000 in 1978 and over 100 of these turbines were produced. Based on the CF6-50 aircraft engine, this workhorse machine has an output of 38 MW and has a twin-spool generator driving a free power turbine. However, GE no longer makes these machines and has been phasing out of their support for several years.

Perry Leslie, Power Plant Technician at Wellhead Services of Yuba City, CA and chair of the LM5000 track at WTUI, led a session where GE officially announced it had ended support of the machine and that the LM5000 lease program is no longer being offered by GE. GE, however, will supply parts for the LM5000 through its authorized depot, ANZ. Lead times and minimum quantities will increase. As well as ANZ, GE has also licensed MTU to help with support until the 30 or so remaining GTs reach end of life. ANZ is partnering with Sulzer to manage the power turbine side. ANZ’s user survey found that 25 LM5000s are still in operation and five more are in disposal.

Only a few users plan to operate the machine in five years’ time. Users offered end dates ranging from 2017 out as far as 2025. Parts availability was the biggest concern voiced by operators. ANZ is developing a plan for every part. The process starts with a demand forecast and creating supply solutions for each item. This is being done in close cooperation with MTU.

“Both of us carry a lot of inventory and we know what we each have new and used,” said Alex Robins, Strategic Business Development Manager for ANZ. “We try to reorder together to meet GE’s minimum order quantity requirements.” He said about 90% of material and part needs can be satisfied by their joint inventories and over time that percentage is going to fall. So the ongoing effort is to focus on the 10% missing items and devise a plan for each part. ANZ plans to widen the scope of its inspections when engines are in for depot maintenance to spot issues early and maximize the remaining lifespan.

MTU Maintenance Director Eva Maria Meijnen said her company was working to improve repairs so new parts would not be needed so often. Other strategies include maintaining existing LM5000 repair sources and incorporating technical improvements from other LM engines. “A better stationary oil seal can replace a Teflon seal on the LM5000 to prevent oil leakage and increase durability,” said Meijnen. “Plans are also afoot for a dovetail coating refurbishment of HPC blade stages 3 to 5 instead of having to conduct a blade replacement. “This is a high cost item so the potential savings are major.”

Further tracks at the show covered the many aspects of the LM2500, LM6000 and LMS100. The LM2500 track, for example, touched upon such things as issues with the High Pressure Compressor (HPC) rotor. GE is investigating the location of fracture initiation on Stage 16 blades due to local stresses occurring above high-cycle fatigue limits at certain operating conditions. As a result, a modification had been introduced that crops the blade platform corner. Another issue concerned the Vespel strip debonding from the HPC rotor air duct. Particles of Vespel strip had been found in the air duct and inside the Stage 2 disk. This can lead to HPC vibration changes. The best advice was to monitor vibration levels. GE is also looking at changing the epoxy used with a higher-temperature alternative. The briefing led to many user questions on the implications and how to get around it.

Control basics

Mike Toll, Application Engineer for Industrial Turbomachinery Systems at Woodward, followed with a lecture covering basic aeroderivative GT control fundamentals. Typically, there are two or three rings of control with the core at the center that is specified and protected by the OEM. It contains controls for fuel use and metering, for example. GE Aviation specifies the core for all LM engines. The core is modular, consisting of various subsystems, so you can change one module and not have to retest the whole core. The core is surrounded by the controls ring, which deals with packager-specific needs, such as auxiliary systems, fans and oil pumps. There might also be site-specific logic as a third ring. Data is transmitted back and forth among the rings.

“By separating each ring, it is easier to make changes to them without affecting the operability of the engine,” said Toll. “Upgrades to the core can be performed as the core is independent of the package application logic.”

But each LM aeroderivative turbine has specialized controls to fit with their individual designs. The LM6000, for example, has a low-pressure compressor (LPC), highpressure compressor (HPC), combustor, high-pressure turbine (HPT) and low-pressure turbine (LPT). “Turbine speed, pressure, and temperature information is used to keep the turbine operating in a safe condition from lights off to maximum power and all conditions in between,” said Toll. “Inputs are used to schedule fuel, water or steam, SPRINT injection and to control turbine variable geometry.” A different control set up is used on the LM2500 as it only has an HPC, combustor HPT and LPT. The LMS100 has an LPC, HPC, combustor, HPT, Intermediate Power turbine (IPT) and LPT.

The key element of most controls is the proportional–integral–derivative (PID) controller. There are multiple PID controllers. For fuel metering, for example, there are speed, temperature, pressure, acceleration, load, deceleration, minimum fuel and maximum fuel. “The maximum fuel-regulator limits fuel based on air measured entering the engine to maintain the air-fuel ratio that supports combustion,” said Toll. “Regulation of minimum fuel prevents flameouts and provides light off-flow.”

Various temperature regulators provide limiting temperature to protect materials and extend the operational hours of the turbine. There are also regulators for MW output. In general, for low-speed operation, the inlet guide vanes (IGVs) and the variable stator vanes (VSVs) are closed and the variable bleed valves (VBVs) are opened. For high speed, the valves are in the opposite position. There are also two types of control — asynchronous and synchronous. For asynchronous, there is some delay as the data comes in and is processed. This might be delays of 5ms or 10ms here and there within the controls, and once they are coordinated, this can add up to as much as 40ms of delay. Synchronous controls, on the other hand, ensure that all actions are constant for each part of the control hardware and software resulting in repeatable system performance.

“Jitter in the controls can result in inconsistent dynamic results, excessive ware on actuators and tuning issues,” said Toll. The control system and associated data logs make it possible to troubleshoot before, during and after an event, said Toll. He advised users to review any changes recently made to the engine, package, fuel delivery or process, as well as historical data. “One of the best troubleshooting tools is high-speed data logs,” said Toll. “When you have a trip, the data log will stop and all event data is stored for later analysis”

HRSG superheaters

Ned Congdon, a systems engineer for HRST, delved into the intricacies of HRSG superheaters. The superheater adds energy to saturated steam from the HRSG evaporators. Reheaters add energy to energydepleted steam coming back for the HP steam turbine. “The basic idea is to superheat steam to increase efficiency,” said Congdon “The more you can superheat, the higher the efficiency.”

Superheater tubes rely on steam flow for cooling. They are not designed to run stagnant or dry at full steam turbine exhaust temperatures. The steam temperature is usually about 100°F less than the exhaust temperature.

Superheaters can have different designs. The panelized design involves upper and lower headers with the tubes in the middle, and is the most popular among manufacturers. It has full venting and draining capability, and has mechanical design and constructability advantages. But it has limited flexibility. The rigid connections on top and bottom can result in high stress if neighboring tubes have large temperature differentials. The single row panel design is one alternative, which has no bends. Another possibility is the single row harp design.

There is also a mixed design that takes elements from each. One variant has one bent tube row and one straight tube row. The downside is that the bent tube experiences axial and bending stresses which can lead to failures. “These types of superheaters have an elevated risk of failure especially during cycling operation,” said Congdon.

A common problem with superheaters is water quench of certain tubes and failure to drain. But the most usual issue is tube-totube temperature differences. Water can separate out of the steam and lead to water formation, stresses and bowing. But there is no reason to remove bowed tubes, added Congdon. A better approach is to locate the overspray or drainage issue, and find the actual cause of bowed tubes.

Header restraints or spring supports can minimize stress on tubes while also correcting drain movements. To resolve drainage problems, he recommended separating the drains, making sure the blowdown tank is below the gravity drain. An elevated drain can lead to water in the tubes and cause damage. “Condensate should be purged from lower heads before every start, which is difficult to do without automatic valves,” said Congdon. “Superheaters must be dry when steam flow begins and drain size does matter.”

WTUI 2017 will be held in Las Vegas from the 19th to the 22nd of March of 2017. For more information, visit www.WTUI.com