If a steam turbine that is running at full load was to experience a full load trip, it could experience a catastrophic overspeed failure in less than 2 seconds.
Protection Against Overspeeding
In most cases, the turbine control system is the first line of protection in the event of an overspeed condition. If the turbine control system were to fail, there are other “pre-emergency” protective schemes designed as backups to the control system. If these backup systems failed to operate properly, the last line of protection is the overspeed trip protection, which is designed to assure the turbine doesn’t reach a destructive overspeed condition.
All of the above mentioned protective devices use a combination of electrical and mechanical components. Most of these devices are able to be tested with the turbine “on-line” to assure, if an emergency condition should occur, they will provide the necessary protection required for the turbine.
It is the mechanical components of these control and protective systems that are vulnerable to failure due to fluid contamination. More specifically, ultra-fine particle contamination of the hydraulic fluid that is used by these components.
Turbine Overspeed Potential
Based upon the size of the turbine, the potential for a catastrophic overspeed condition could occur in less than 2 seconds. Calculations have shown that a turbine running at rated speed and carrying full load, has the potential to accelerate at a rate of 15% of rated speed per second, if it were to experience an uncontrolled trip at full load. Most turbine manufacturers state that speeds greater than 20% above running speed will cause serious damage to the rotating components of the turbine and/or generator. In other words, in less than 2 seconds, the turbine could experience a catastrophic failure.
In order to assure the mechanical components of the control and protective systems function quick enough to assure the turbine does not exceed 20% of rated speed following a full load trip, it is critical that the fluid that controls these devices is clean.
In spite of the fact that stations try to maintain their hydraulic fluid in a clean condition, the method of maintaining low particle levels in the fluid is provided by mechanical filters located on the discharge of the main hydraulic pumps, or side stream kidney loop filtering systems, and in some cases fluid drain side filters. In most of these cases, the finest filters being used are rated at 3 microns. Many stations are using higher micron rated filters than that.
Fluid Analysis Results
In order to assure the fluid is being maintained within OEM specs, periodic fluid samples are taken and sent to outside laboratories for analysis. One of the variables reported by this analysis is the particle count of the fluid. These reports express the particulate values based upon size and quantity of particulates. In ALL cases the smallest size particle reported is 4 microns. Unfortunately, it is the particles that are smaller than 4 microns that can cause the most problems with critical control and protective devices.
|TYPICAL PARTICLE COUNT/100 ml|
|ISO 4406, >4/>6/>16||21/20/19||16/14/12|
Typical Fluid Analysis Results
Critical Component Clearances
Generally, the clearances between the moving and non-moving parts of these components is 1-4 microns. If particulate nearly the same size as the clearance were to get stuck in these critical components, they could experience either slowed response times or fail to move at all. On some components that are static (non-controlling) such as solenoid or directional valves, silting can occur causing these devices to stick or have slow response times. Most of these static devices are tested periodically to assure that they move freely. In most cases no one checks the response time of these components, they just check whether or not they respond.
Component Response Times
The response time of the mechanical overspeed trip device should be less than 0.2 seconds from initiation of signal to the device, to the point that the device has fully responded. If the response time exceeds this value (<0.2 second), the turbine could experience a catastrophic failure during an actual overspeed condition.
As a result of this information, it is very important that the” on-line” overspeed trip test response time be checked periodically.
Filter Analysis Results (SEM)
As mentioned earlier in this article, it is not possible to quantify whether there are excessive “ultrafine” particulates in the fluid with standard fluid analysis. In order to determine whether your hydraulic fluid system has high levels of “ultrafine” particles, it is necessary to analyze the system’s filters using SEM (Scanning Electron Microscope) technology. The SEM can quantify not only what the particle is (element), but it can also determine its size down to <1 micron.
Electrostatic Fluid Cleaner
If the hydraulic fluid system is shown to have high levels of “ultrafine” particulates, the only proven method of removing them from the system is by using an Electrostatic Fluid Cleaner (EFC).
HOW DOES AN ELECTROSTATIC FLUID CLEANER (EFC) WORK?
1) ALL contaminants in the fluid are either positively (+) or negatively (-) charged. When the fluid is passed through an electric field, positively (+) charged particles are drawn to the negative (-) pole and vice versa.
2) As the fluid is circulated across multiple layers of “Collection Media” contaminates are pulled from the fluid and bond to the collection media.
WHAT IS THE LIMIT TO WHICH FLUID CAN BE CLEANED BY EFCs
This is one of the filter manufacturer’s biggest challenge: the smaller the particle, the more difficult it is to remove by filtration. For fine micron mechanical filters, small particles create problems, such as limiting dP (differential pressure) values faster.
EFCs, on the other hand works on a different principle, and it is easier to remove smaller particles. Because of their small size, the smaller particles tend to have higher “Charge Density”(charge/area) and move faster in the electric field due to their small weight. Hence they are more easily removed by the EFC.
All particles suspended in fluid, magnetic, non-magnetic, organic, inorganic, resinous matter or sludge, can be removed from the fluid by the EFC.
The EFC does not affect the chemistry of the fluid which means that the fluid is maintained at a high level of cleanliness assuring a longer fluid life and improved overall system performance.
To learn more about the Electrostatic Fluid Cleaner (EFC) go to http://sibul.com/shop/catalog/fluid-conditioning-products/electrostatic-ultrafine-particulate-filter/