While working with power plant owners and operators over the past ten years, Parker Instrumentation has become increasingly aware of a prevalent reliability issue encountered at dual fuel gas turbine sites. It seems that regardless of a gas turbine’s age, frame size, or manufacturer, they virtually all have issues with the check valves that are commonly deployed in the fuel oil, water injection, or purge airlines close to the turbine combustor cans. Even the newer gas turbine models equipped with NOx reduction technology and the ‘latest and greatest’ equipment can have issues associated with fuel supply and purge systems.
Having analyzed its experiences with this type of equipment, there seemed to Parker to be two possible paths of corrective action:
Redesign the fuel and purge systems from scratch
Greatly improve the performance of the check valves in the existing systems
Since most plant owners and operators were not interested in the high cost and extended downtime of a re-engineered fuel system, Parker Instrumentation has chosen to commit its resources to engineer a new type of check valve called the CB Series Check Valve for the demanding conditions that could be encountered in this application.
There is a long history of performance issues with all manufacturers’ check valve products used for purge air, liquid fuel, and water injection applications. It was discovered that this was primarily due to the fact that most people considered a check valve to be a check valve, and would historically use any standard instrumentation product and expect them to perform in this difficult turbine environment.
A myriad of failures has been encountered on many gas turbine styles due to this philosophy, including seat leaks, short life, and coking problems – which lead to bigger issues of turbine trip, unplanned outages, and a costly repair. Dual fuel gas turbine operators had learned to live with this and accept that they would have to change-out valves regularly. While individual plants operate under different conditions (for example, peaker vs baseload, fuel oil vs gas hours) the problems encountered are consistent and persistent when it comes to check valves. Most difficulties though, seem to arise from switching fuels after prolonged run hours; peaker units also tend to have more issues than baseload units due to heat-up and cool-down thermal effects.
Based on input from users at power plants in both US and international locations, our engineers established what the main design criteria and performance goals should be. Some of the key items were:
Reliable bubble-tight shut-off
Resistance to failure from coking (coking is caused when fuel oil mixes with air at elevated temperatures over time)
Ease of repair
Other considerations included temperature and media compatibility and flow characteristics.
Initially, installation issues were overlooked, as the key focus of the program was on design and function, but eventually, it became apparent that very few operators were willing to modify their existing gas turbine plumbing arrangements, and having a replacement that was a dimensional ‘drop-in’ was an essential element of the new design.
The starting point for the new design was an internal sealing concept based on our existing floating ball valve – the B-series ball valve – which has been used in virtually every fluid system imaginable. This valve employs a self-centering ball that compensates for misalignments; in contrast, all other poppet-style check valves the authors have seen can be adversely affected by seat leakage when in a misaligned condition. A key design feature was to separate a traditional check valve poppet into an independent ball and cage, thereby allowing the use of a micro-finished ball that has optimal sealing capabilities compared to traditional machined surfaces.
Utilizing PTFE copolymer seats also proved invaluable due to the soft and forgiving nature of the material. It forms a nearly perfect fit around the ball for an excellent bubble-tight seal and improves during gas turbine operating conditions. PTFE coating internal parts of the check valve have proved valuable in reducing coke deposits and build-up, which leads to premature failure of other designs.
Large flow passages also allow the internal cavities to drain easier, which further reduces coking effects. The valve seats and seals are designed to withstand continuous operating conditions in excess of 260°C (500°F). Parker has participated in a number of temperature studies of check valves on gas turbines and found that most operate between 121-176°C (250-350°F) and rarely approach 204°C (400°F).
Utilizing actual gas turbine temperature and operating data was integral to the design process. Considering that some gas turbine OEM specifications apply criteria that were found to be unrealistic, Parker chose to base all the performance aspects of the valve (temperature, pressure, and flow) on empirically measured field conditions (see Figure 3). Most importantly here, the seat material options of Parker fill and Parker Carbon (carbon graphite and carbon-reinforced PTFE) are designed and optimized specifically for the demands of fuel oil, purge air, and water injection applications on turbines. These two compounds are chemically inert, highly resistant to ‘hot flow’, resistant to coke sticking, and most importantly provide an exceptional bubble-tight shutoff. Interestingly, seal performance actually improves as the turbine heats up.
Internal testing of these components involved extensive flow and oven bake time in our valve test laboratory. The seats and seals were thermally cycled and thermally soaked under pressure over a long period to simulate actual gas turbine field conditions. Fine-tuning the valves’ flow characteristics for different gas turbine flow requirements was also essential, to effectively negate chatter across the given flow range for each check valve type.
After extensive lab testing on the individual components and full assemblies, the valves were installed on a field unit for beta testing. Slight plumbing modifications were required on the beta unit as the dimensions had yet to be fine-tuned for ‘drop-in’ installation. The original gas turbine plumbed with the CB Series valves used 20 water injection and 20 fuel oil check valves. Inspections of the seats, balls, and cages were performed after a six-month time span involving dozens of starts, a few fuel switches, and over a hundred run hours. Our engineers and the plant operators were very pleased with the results. The original valves installed on that unit three years ago are still in operation today.
Field history over the past three years has demonstrated the reliability of the new valve design on a wide range of gas turbine types and frame sizes, including systems from the two largest gas turbine OEMs, as well as aero derivatives.
These sites have accumulated thousands of gas turbine run hours in addition to hundreds of fuel switches and startups. The plant operators have found that the product’s true value lies in reducing the costly gas turbine downtime and maintenance issues, and most are looking to expand and continue usage of the valve design within their gas turbine and/or fleets.
The CB Series check valve design is also available with inline filter/strainers, typically for use in fuel oil and water applications with heavy particulates used in dual fuel gas turbines. These serve to protect the check valve seats and more importantly the gas turbine fuel nozzles. Additionally, it can be provided in a variety of tee configurations for virtually all frame sizes.
Download the Power Engineering International article, “New Check Valve from Parker Eliminates Downtime and Maintenance in Dual Fuel Gas Turbines” to read the field-proven test results.
Article contributed by Linwood (Wood) Tenney, business development, Instrumentation Group, Parker Hannifin.
Mike Doutt, senior engineer, Instrumentation Products Division, Parker Hannifin
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