1. What is a turbine EHC system and why is it important?
A turbine EHC system (Electro-Hydraulic Control System) consists of the high pressure hydraulic system and trip system that are used to position the steam valves on a steam turbine. These systems include all of the control devices and instrumentation that interfaces to the turbine control system (TCS). Most built after the year 1969 were supplied by the original equipment manufacturer (OEM) with an EHC system, turbines built prior to this were typically supplied with a mechanical hydraulic control system (MHC).
Components that are common to EHC systems include servo valves, position feedback devices for valves (LVDTs, or Linear Variable Differential Transformers, are common), valve actuators, hydraulic power units (also known as EH Skids), trip solenoids/trip manifolds, accumulators and filters. A turbine EHC system is important because this is the system that actually operates and positions the steam valves to control turbine speed and generator output (load). This system is also responsible for the reliable and rapid closing of all steam valves in emergency situations such as turbine overspeed, power-load imbalance and many others.
2. What are some key steps to insure reliable operation of a steam turbine EHC system?
- Developing and implementing a preventative maintenance program is an important first step. This includes developing a schedule for preventative maintenance activities on all components in the EHC system that are integrated into maintenance outages or major outages for the turbine.
- Maintaining hydraulic fluid health is a second important step which may also be considered part of a successful preventative maintenance program.
- Identifying critical spare parts for the EHC system and insuring that these parts are stocked and available on-site is another item that is often overlooked. Responding to failures that lead to a forced outage is all too common, and having the correct spare parts available is crucial to response time. These spare parts include obvious items, such as pumps and servo valves, but should also include simple items including replacement hoses, o-rings and special gaskets which may also be challenging to source during a forced outage.
3. What is the most critical item regarding EHC system preventative maintenance?
EHC systems on steam turbines typically use specialized synthetic hydraulic fluids. These fluids are more fire resistant than standard petroleum-based fluids and also have additives to allow high temperature operation. This is because the hydraulic lines and components are typically routed close to the turbine casing and steam chest where surface temperatures can approach 1000 degrees Fahrenheit. Fluid leaks in these area pose severe fire risks which is why fire resistant fluids are employed. The synthetic fluids require special care, not only because of their special properties, but also because of the high operating pressures and precision components in the EHC systems.
The components used in the EHC system have very small internal clearances and particulate contamination in the fluid will cause problems with the operation and response of these devices. Many of the synthetic fluids have the tendency to absorb water from the atmosphere and the water content must be controlled. The pH (acidity) of the fluid is also a critical parameter, if the pH is not within specification this can lead to advanced degradation of the fluid and EHC system components. Regular fluid sampling and analysis is the best way to make sure the EHC fluid is within acceptable limits for particulate count, water content and acidity. The limits for particulate count will be dictated by the components used in the system, typically the servo valves, pumps and other valves. The operating ranges for water content and acidity are provided by the fluid manufacturers. Conducting fluid sampling and analysis on a monthly basis will help to insure that the EHC fluid characteristics are within recommended operating ranges.
Steve Sinka is the mechanical engineering & manufacturing manager at the ABB facility in Natrona Heights, PA. This facility focuses on solutions for turbine controls including mechanical retrofit solutions. Steve has 21 years of experience with mechanical retrofit solutions for turbine controls including the engineering, installation, and commissioning of these systems. Steve earned his Bachelors of Science in Mechanical Engineering from Geneva College in 1995 and has been a registered Professional Engineer since 2001.