Approaching compressor challenge from the side finds a solution.
Specialists from Hoerbiger Engineering Services and Anderson Consulting Testing and Training (ACTT) helped find a successful approach. A study showed that the engines needed more air and a way to control air during off peak operations. A control strategy that treated the unit as a large inter-related system guided the solution.
The units had proven to be a handful operationally, with frequent and substantial failures of power side components., including not only the common scored cylinder, broken rings and wrist pin failure, but also broken cylinder bolts and cracked heads.
Hoerbiger Engineering Services (HES) introduced Randy Anderson of Anderson Consulting Testing and Training (ACTT) to Questar as part of a Hydrocom upgrade project. Questar retained ACTT to investigate the likely cause of the frequent failures. The probe found there were issues with oil and water temperature control, air manifold temperature control, a complete lack of any air manifold pressure control and frequent detonation.
HES was asked to offer control strategies that could address the multiple deficiencies. As part of the process, HES undertook a basic site evaluation and mapping program to determine the operational state of the existing installation. The result found that the engines needed more air, a way to control the air during off peak operation and a control strategy that treated the unit as one large inter-related system.
Recognizing that emissions control is really combustion control, HES employed the same design approach used for emissions projects to solve the combustion problem for Questar. By attacking the problem from an emissions standpoint (the side entrance), they designed an effective system solution.
As with any engine performance upgrade, local personnel were key in the planning and execution of the project. Questar personnel took responsibility for physical unit modifications and the turbocharger removal and replacement efforts. Cory Gale, Lead Mechanic Specialist for Questar, explained the historical problems and directly participated in the data collection process during commissioning.
Development And Diagnosis
Questar wanted to resolve the long-running operational issues that were faced at the Kastler plant. Among the problems noted were frequent failures of cylinder tie down bolts and a failure where a cylinder was actually torn from the frame.
Anderson quickly determined that the running peak pressures were approaching 1,000 psi, well beyond the manufacturer's recommended limits. He also noted pressures as high as 1,800 psi, indicating detonation. He diagnosed the lack of an effective air fuel ratio control algorithm as a primary driver for the detonation and noted that there were issues with the cooling water system allowing an inversion of the oil and water inlet temperatures. Based on these observations, he recommended that Questar contact HES to discuss employing the HyperLogic[TM] control strategy as a solution.
After an initial presentation and meeting between the three parties, Questar agreed that HyperLogic[TM] would provide the necessary control. They further agreed to begin the effort with a field study to determine where they were and how best to proceed.
As presented in the "Engine Aspiration" short course (2004 GMRC, Beshouri and Mathews), NOx can be used as surrogate for determining air fuel ratio. HES and ACTT conducted a complete emissions performance test (map) and let the engine report its effective operating air fuel ratio. A 14-point map which included torque, timing, AMP and speed changes was conducted on unit 1. The results were startling, as they showed that while still a lean burn, the engine was running much richer than expected.
There is a relationship between air-fuel ratio, NOx, misfire and detonation. Based on the measured data in the pre-test, it was clear that the primary driver for combustion damage was because the unit's operating air-fuel ratio tended toward the rich side of the operating window.
Why the engines ran so rich was simply a lack of air. The station is located at an elevation of approximately 6,500 feet. The elevation reduces the turbocharger performance while requiring additional boost to make up for the altitude. In practice, the turbochargers were making ~6 psi of boost. A bit low, but adequate for a machine operating at sea level. However, when this boost pressure is corrected for altitude it is equivalent to ~2.5 psi at sea level, and the problem becomes obvious. The machines needed more air.
[FIGURE 1 OMITTED]
Other issues identified were related to inadequate ignition systems and the aforementioned cooling water issues. These coupled with the lack of available air are what led to a difficult and failure prone operation.
HES recommended that Questar implement the following design elements to resolve the operational issues:
Turbocharger upgrades--sufficient to produce an operating NOx of 12-15 g/bhp-hr.
Cold side dump valve for air fuel ratio control.
CPU-2000 ignition system with independent primary wiring.
HyperLogic[TM]--automation upgrade to manage ignition, water and air fuel ratio.
The turbocharger upgrade was specified to deliver an operating NOx of 12-15 g/bhp-hr. The units at the station are not currently facing a low emissions permit, so any operating point could be chosen as long as the emissions did not increase. This allowed HES to choose an operating point that was optimized for fuel economy and operational reliability. It is the experience of HES that stock units operating in the 12-15 g/bhp-hr NOx range have good heat rates and few reliability problems. The specified range is safely lean of detonation, without being so lean as to require changes to the fuel injection system or the addition of pre-chambers.
HES determined that meeting the design targets would require a boost level of 12-14 psi, a substantial upgrade for the existing Elliott H-50 configuration. Cameron compression now maintains the Elliott line and was contacted concerning the upgrade. Cameron reported that there was no known upgrade for H-50, but committed to designing a cost-effective solution and so became a key partner in the operational success. As part of the turbo upgrade, HES had Cameron add speed pickups to each turbo for monitoring in the PLC. This proved to be helpful in the testing phase.
Cold Air Dump
A cold air dump valve is used on engines that do not have access to install a waste gate valve. The valve is used to control the air manifold pressure by dumping excess air around the engine and turbocharger.
The cold air dump valve specified was a 4 inch, high-temperature butterfly valve with an integrated I/P and a visual position indicator (picture). An existing 3 inch blind flanged pipe in the center of the air manifold was enlarged to a 4 inch pipe to meet the air flow specification. The outlet of the 4 inch valve is routed into the exhaust, downstream of the turbocharger exhaust outlet.
[FIGURE 2 OMITTED]
A CPU-2000 ignition system was installed to provide the necessary ignition energy for reliable open chamber combustion. Reliable ignition is a key element for consistent engine operation. As a crank referenced capacitive discharge ignition, the CPU-2000 provides a consistent ignition location and a high energy spark to the plugs. HES recommended that the ignition be set up with independent primary circuits on the KVSR (HES recommends this configuration whenever possible).
HyperLogic is an HES control system based on the "Advanced Two Stroke Cycle Control" algorithms. The HyperLogic control algorithms were installed into Questar's existing Allen Bradley PLC-5 control platform, greatly reducing the cost of the controls upgrade. Some of the existing temperature inputs were moved from a multiplexer to direct analog inputs to improve update rates for critical control loop signals.
HyperLogic is an adaptive control system that works to maintain a constant combustion process, regardless of the engine speed, load or ambient conditions. It takes controls of all key engine parameters including cooling water for AMT and engine control, waste gate (air dump) for air manifold pressure control, ignition timing, speed, load control and jet assist (when needed).
Air fuel ratio control is maintained by calculating a speed and timing corrected mass air fuel ratio set point. The system then optimizes the engine systems to provide the desired result. In the case that one engine system reaches its limit, the system then seeks an alternate solution using the remaining control variables.
As an example of this operation, consider an engine running full torque at minimum speed on a hot day. In this condition the engine may run out of waste gate margin (air). HyperLogic will recognize the onset of this condition and try to make up the missing air pressure by lowering the air manifold temperature, in an attempt to maintain constant air density. As the cooling water system typically has a large thermal mass and responds slowly, HyperLogic may use ignition retard on a temporary basis to improve the available turbo energy and reduce the air demand.
If the cooling water system cannot succeed in lowering the AMT, then the system will speed up and/or unload the machine. The system does whatever is required to maintain constant combustion, ensuring compliance with the desired combustion target and, as a result, any emissions target.
Several new control screens were developed to facility the new functionality. The focus was on simple, but informative screens that were user friendly for users of all levels.
Installation was performed in two phases. The short lead time efforts, including the ignition system and control upgrade, were carried out at the end of 2005 and early 2006. The turbocharger upgrade and installation of the air dump valve were carried out on the first unit in February 2006.
The CPU 2000's require a rather straight forward installation effort. For this phase of the upgrade, the focus was on ensuring that all conduit and wiring was run so that their position would not interfere with maintenance. All coils were wired with independent primaries to ensure maximum energy to the spark plugs. A side benefit is that this setup allows plug by plug diagnostics to be performed by the CPU-2000's diagnostic module.
As there was no excess air to control at this point, the control programming focused on addressing the cooling water system function. The Jacket Water and Aux water are common in the cooler but they split once they get to the engine. To facilitate the upgrade, two RTDs were added to the existing system, one on the lube oil inlet and one on the jacket water inlet. The existing control strategy was changed from controlling the outlet temperatures, to controlling the inlet temperatures of each and monitoring the outlet temperatures.
The lube oil already had a thermostatic valve that held the inlet temp constant. This control was augmented by the addition of overrides on the Aux water that are set in motion by the Oil inlet and JW inlet differential temps getting to close. This helps to ensure that the proper differential between lube oil and JW are maintained at all times, thus eliminating this problem as a cause for scored liners.
The control algorithm for auxiliary water out of the cooler was changed to make it bump-less and the control loop was tuned for tighter outlet temperature control. Overrides were added to the auxiliary water outlet control that considered Oil--JW inlet temperature differential when driving for a cooling water set point. Control of the two cooling fans was brought into the new cooling system algorithm so that one task takes care of all cooling water needs.
These initial efforts were completed in early 2006. The benefits of the ignition upgrade and the improved cooling control were immediate. Rigorous commissioning testing was postponed until the turbocharger upgrade was complete, but the units sounded better and the water and oil temperature inversion problem was resolved.
In preparation for the installation, Questar sent a pair of "spare" turbochargers to Cameron for retrofit. This effort led the on-engine installation work by about four weeks. Cameron re-configured the turbine side of the H-50 turbocharger to meet the new air specification. Once the turbochargers were completed, one turbo was fully mapped at Cameron's test cell and the second turbo was run to verify its design point performance. After verifying performance, the turbochargers were shipped to the site for installation.
The first pair of turbochargers was ready for installation in February 2006. Turbocharger installation was handled by the Kastler station crew as were the piping changes to facilitate installation of the air dump. Since the original frames were re-used, there were no modifications needed to the turbocharger mount, air or exhaust piping. This made the upgrade a simple bolt in swap.
Emissions testing was used to verify the new operating air-fuel ratio. This testing showed that NOx had been reduced by about 40% at full speed, indicating a leaner combustion event.
The new-found abundance of air and the ability to control the AFR made a great deal of difference in the combustion performance of the engine. After modifications were made, the average peak firing pressures at full load averaged less than 700 psi. The unit was easy to balance, no detonation was noted and the engine stayed balanced over a variety of load conditions. Testing also showed that the ignition timing could be run at 15 degrees of advance (as compared to 12) without detonation.
[FIGURE 3 OMITTED]
Unit heat rate was relatively unchanged, with the negative impact of lean operation being offset by the positive impact of more ignition timing.
HyperLogic showed that it could control the combustion event to the desired set point. More importantly, it showed it could make the necessary control tradeoffs to maintain a constant combustion event regardless of ambient conditions.
The only setback encountered in the application was that the turbochargers surged at high (> 13 Psig) air manifold pressure settings. The turbocharger was modified in late 2006. Subsequent testing showed that the surge problem was solved and the turbocharger easily met the performance requirements.
It is fair to say that "souping up" the old turbochargers proved to be a greater challenge than anyone anticipated. However, all parties were committed to finding a solution and success was achieved. The greatest success is the lack of power side failures. Since being retrofit, the trial unit has run nearly 4,000 hours and has not failed any power side components.
This article is based on a presentation at the 2006 GMRC conference in Oklahoma City.
Todd Rose began his career with Pacific Power and Light Company, near Rock Springs WY. He joined Mountain Fuel Supply Company (later to become Questar) in 1979. Todd is Supervisor of Operations and his responsibilities include six compressor stations, Clay Basin storage field, two hydrocarbon processing plants and nearly 800 miles of pipeline with associated meter and regulator interconnects.
Randy Anderson began his career at Panhandle Eastern Pipeline in 1974 as a casual laborer. He later earned his BSME from Kennedy Western University and ultimately became Senior Engineer for Plant Operations and Maintenance at Panhandle Eastern, Texas Eastern and Trunkline. He left Panhandle to found Anderson Consulting Testing and Training, which he later sold to CECO. Today, Randy serves as the Principal Consultant for ACTT.
Gavin Goolsbee is the Automation Supervisor for Hoerbiger Engineering Services. He graduated from the University of Houston in 1992 with a BSMET and began his career at Tennessee Gas Pipeline. His career has included experience at ASSET, ACES and AETC. He joined Hoerbiger in 2005 as part of the HyperLogic acquisition and rollout.
Hans Mathews is General Manager of the Hoerbiger Engineering Services group. He graduated from Texas A&M University in 1991 with a BSME degree and began his career with Tennessee Gas Pipeline. He joined Hoerbiger in 2000 where he established the Gas Engine Systems group (now Hoerbiger Engineering Services) as a path to market for Hoerbiger engine technologies and services.
|Printer friendly Cite/link Email Feedback|
|Author:||Rose, Todd; Goolsbee, Gavin; Mathews, Hans; Anderson, Randy|
|Publication:||Pipeline & Gas Journal|
|Date:||Jun 1, 2007|
|Previous Article:||Aerial lidar scanning system seeks gas leaks.|
|Next Article:||A state-of-the-art look at compression research.|