STEAM TURBINE HYDRAULIC FLUID SYSTEM PROBLEMS AND SOLUTIONS
This article addresses the ever increasing problems being encountered with steam turbine high pressure hydraulic fluid systems using Fire Resistant Fluids (FRF) and the methods needed to correct them.
In order to determine a potential problem, it is necessary to evaluate the system.
Once a problem is noted it needs to be corrected before a failure occurs.
Ultimately it is necessary to eliminate the root cause of the problem to keep it from returning.
Since the late 1960’s most steam turbine hydraulic fluid systems have incorporated high pressure hydraulics to control the critical components of the turbine’s control and protective systems.
The change from low pressure to high pressure hydraulic systems significantly reduced the size of the hydraulic actuators used to position the steam valves. In addition the response time of the valves improved greatly, making overall operation more reliable.
The pressure requirements of these systems were increased from approximately 200 PSIG (13.8 BAR) to greater than 1600 PSIG (110.3 BAR). This created the potential for catastrophic fires in the event of a hydraulic fluid leak in the area of hot steam pipes. As a result of this dangerous situation Fire Resistant Fluids (FRF) were introduced for use in these systems.
In a majority of the applications the Fire Resistant Fluid (FRF) currently in use is a “phosphate ester” fluid. This fluid provides excellent hydraulic capabilities; while at the same time has good fire resistant characteristics.
For more than 40 years these systems have provided safe and reliable operation, but over the past 5-7 years the high pressure fluid systems have seen a significant increase in problems. In most cases these problems have not caused forced outages, but the trend is an increasing severity and number of problems within these systems.
- Erratic operation of the main high pressure fluid pumps
- Pump discharge filters becoming plugged more quickly than previously experienced
- Erratic operation of the servo valves
- Valves closing for no reason
- During valve test, the valve closes but does not open back up
- Solenoid Valve failure
- Unsuccessful trip system testing
- Inability to maintain proper fluid condition
Evaluation of these systems indicates that most of these issues are the result of reactions between the FRF and the conditioning filters being used.
The FRF creates phosphoric acids that must be continuously conditioned in order to control acid formation in the fluids. Excessive acidity in the fluid can cause corrosion of components throughout the system including the main hydraulic fluid pumps, hydraulic actuators, as well as the solenoids, and servo valves. Corrosion of any of these components can lead to serious mechanical failures.
FULLER’S EARTH FILTERS CAUSING GEL AND VARNISH FORMATION
When initially introduced, FRF acids were controlled by Fuller’s Earth filters. The Fuller’s Earth media is made up of a naturally occurring material called attapulgus clay. Fuller’s Earth absorbs water, which can be helpful in controlling the water content of the fluid however, as a result of the water absorption capabilities of the Fuller’s Earth media, filter life may be reduced.
During their normal use, Fuller’s Earth filters continuously release small quantities of magnesium and calcium in the form of metal salts into the fluid. As the acid levels increase these metal salts continue to be released in greater quantities.
Metal salts, in sufficient quantity, ultimately cause a chemical reaction to occur in the fluid which results in the formation of gels. The gels are simply FRF in a gel form with metal salts present.
Figure 1 EHC Reservoir with Gels
These gels are capable of passing through the smallest of filter media as long as there is sufficient flow through the filter. Once the gels are picked up by the hydraulic pumps they are circulated throughout the entire high pressure fluid system.
Normal fluid sampling does not reveal this formation since the gels are still FRF in a different physical state. Fluid sampling can only expose particulates and chemical anomalies within the system. In addition, a fluid sample is one moment in time and may not be representative of the actual operating fluid in all events. The filters in the system record data over a period of time and can be a better indicator of the system’s condition.
A visual inspection of the reservoir may reveal traces of gels, but the best indication of gels is a visual inspection of the servo valve strainer. This strainer is located in the pilot supply to the servo valve torque motor which has very little flow through it. The gels migrate down the pilot supply line and ultimately, due to the low flow, begin to agglomerate or cake up on the servo valve strainer. If the servo strainer becomes plugged, the servo will fail to the closed position. In some cases this occurs during a valve test where the valve will test closed but will not reopen. In order to fix this problem most stations replace their servo strainers on a regular basis prior to complete plugging of the strainer.
Over a period of time, the gels form as hard deposits (varnish) on the tubes and pipes of the hydraulic fluid system. The varnish does not generally form on the components of the system since they move and can ultimately scrape off any varnish build up. On the other hand, the pipes have no moving parts. The fluid flowing through these pipes is under high pressure but the flow is laminar and there is very little turbulence. This combination allows for easy varnish formation.
Figure 2 FRF Varnish
The majority of the pipes in the system are located downstream of the main pump discharge filters. Over time the varnish formed on these pipes can break off and enter the system entrained in the fluid. This condition can occur as a result of excessive build up, high vibration or shaking of the pipes during a system upset such as a turbine trip or, in some cases simply a normal valve test.
The varnish is very hard, and since the varnish is deposited downstream of the pump discharge filters it can cause the most amount of damage to the system components such as the servo valves, solenoid valves and hydraulic actuators.
SELEXSORB AND ION EXCHANGE FILTERS
In order to eliminate the release of metal salts and ultimately the formation of gels and varnish, Selexsorb filters were introduced. Selexsorb filters are made up of activated alumina. These filters did an equally good job of controlling the FRF acids but did not remove any water.
Experience with Selexsorb filters indicates they have a tendency to release ultra fine particulates into the hydraulic fluid system that cannot be removed with standard filtration. Over time these ultra fine particulates can cause silting to occur on mechanical components that are not routinely tested resulting in a failed test due to a sticking valve.
Recently, Ion Exchange filters have been used in place of Fuller’s Earth and Selexsorb filters in order to control the acids in FRF. Ion Exchange filters use combinations of anionic and cationic resins. The Ion Exchange filters have the ability to reduce very high acid numbers. Both Fuller’s Earth and Selexsorb filters could only reduce acids up to a threshold of 0.25 mg KOH/g. Ion Exchange filters do not release any metal salts therefore do not cause any gel or varnish formation. However, Ion Exchange filters do cause the FRF resistivity (ability to conduct electricity) to be significantly reduced.
Normal FRF fluid has resistivity values >20 x109 ohm-cm. With Ion Exchange filters this value is reduced to <10 x109 ohm-cm. OEM’s require the fluid’s resistivity to be maintained to >5 x109 ohm-cm. High acidity, high water content and high ferrous particulates all contribute to reducing the FRF resistivity. Low resistivity can cause etching of the servo valves. The etching of the servo valve spools will cause increased flows through the servo valve. As these flows increase, the hydraulic fluid pumps work harder and can ultimately experience premature wear of their internals causing pump problems. On some servo valves, the etching can cause problems with the torque motor nozzles causing the servo valve to respond erratically or in some cases fail to the closed position and remain there. Even though the Ion Exchange filters do not cause fluid resistivity to drop below OEM recommendation, they leave lower margin for error with acids, water and particulates.
Figure 3 Etched Servo Spool
Figure 4 New and Etched Servo Nozzles
REGULAR MONITORING OF THE FLUID
No system is immune from problems but careful evaluation can identify a problem before it occurs. In order to determine whether a problem exists and to what extent the problem is affecting the system, fluid samples, filter analysis and visual inspection of components should be done on a regular basis.
A. Take monthly fluid samples and have them analyzed
- Total Acid Number should be < 0.1 mg KOH/g*
- Particulates should be <2,000 particles per 100 ml at 5-10 micron*
- Resistivity should be >5 x109 ohm-cm, min @ 20°C
- Water content in the fluid should be <1000 ppm*
B. Periodically remove and analyze the pump discharge filters
- Identify the size and material of particles on filter elements
C. Periodically visually inspect servo strainers
- Observe if gel and/or varnish buildup are deposited on the strainer. If problems exist corrective action should be taken at the earliest convenient time
D. High Acid Number
- If the acid number is less than 0.25 mg KOH/g then change out the acid control filter (note the acid control filters should be changed whenever (i) the monthly fluid sample indicates acids >0.10 mg KOH/g or (ii) there is a step change of more than 0.05 mg KOH/g from the previous monthly sample.)
* These limits were determined by a study conducted by The Electrical Power Research Institute (EPRI) in 2002
- If the acid number exceeds 0.25 mg KOH/g more drastic measures may need to be employed such as changing out all of the fluid (this is not necessary if Ion Exchange filters are being used. In this case the Ion Exchange filters will need to be changed out more often until the acid number is reduced to > 0.1 mg KOH/g).
E. High Particulate
- Change out both the main pump discharge filter and if equipped its corresponding suction filter. The side stream particulate filter should also be changed at the same time. If unit has Return Line filters, these should also be changed at this time.
- Note: The particulate filters should be sent out and analyzed to determine the nature of the particulate. This may help to identify a potential component failure.
F. Low Resistivity
- Since high acids, high water content or high ferrous particulate all contribute to lowering of the resistivity, check these to see if any of them require action. If the acids, water content and particulates are within acceptable limits it may not be possible to diagnose the problems. In this case it may be necessary to replace all of the remaining fluid with new fluid.
- Gels and varnish may also contribute to the lowering of the resistivity. Check for these as well.
G. High Water content in the fluid
- Removing water from FRF is not an easy task. Generally it requires the use of a vacuum dehydrator.
- Water can be introduced into the fluid by ingression of moist air entering through the reservoir breather. The breather on most systems does not do a very good job at keeping water out of the reservoir. A continuous dry air purge of the reservoir will assure no moisture is admitted into the reservoir.
- Leaky water-to-hydraulic fluid heat exchangers can be a source of water in the hydraulic fluid. The heat exchangers should be inspected periodically in order to avoid this condition.
- Water can also accumulate in the reservoir as a result of condensation forming inside the reservoir. This condition can be eliminated by using the system’s fluid heaters during times when the hydraulic fluid system is shut down. This assures the fluid temperature is maintained at a minimum of 85 degrees F (29.4 C) prior to putting the system back into service thereby reducing the amount of temperature change experienced by the fluid during the start up of the system.
H. Signs of Gels and/or Varnish in the system
- If Gels and/or Varnish are observed anywhere in the system (servo strainers, reservoir walls or floor, and filters) a thorough chemical flush of the entire hydraulic fluid system is needed. This flush will dissolve and/or dislodge any varnish that may be in the system. Once it is in suspension it can be easily removed by a final rinse of the system.
- The chemical flush must be done such that all varnish is dissolved and/or dislodged during the flush, and the chemical used to flush the system (amines) is thoroughly removed prior to adding new operating fluid to the system. Any amines left behind following a flush can cause problems such as increased acidity of the new operating fluid and in some cases rapid degradation of system seals.
- Experience has shown there is no easy way to get rid of the varnish in the system short of a thorough chemical flush using amines.
- If not completely eliminated from the system, the varnish can recontaminate new fluid.
Most of the problems being experienced on hydraulic fluid systems today can be traced back to the reactions between the FRF and the acid control filters being used.
REPLACEMENT OF FRF
Recently some alternative fluids have been introduced as replacements for the existing FRF fluids. These fluids are approved as fire resistant fluids by Factory Mutual (the certifying agency for insurance companies).
The replacement fluids do not create the harmful phosphoric acids of the existing fluids and therefore do not require the use of acid control filters (Fuller’s Earth, Selexsorb or Ion Exchange filters). The replacement of the FRF fluids with these alternate fluids not only eliminates the problems and environmental hazards inherent with handing the phosphate ester fluids but also eliminates the much larger problem of gels and varnish formation mentioned earlier in this article.
The replacement fluids are more lubracitive than the existing FRF fluids which can help prolong the life of most components in the hydraulic fluid system, provided the fluid remains clean.
The replacement fluids also have higher viscosity indices (VI) thereby allowing them to operate over a wider temperature range.
Existing phosphate ester fluids require the use of special sealing materials (teflon, viton, fluorosilicone or EPDM). The replacement fluids can operate with the existing seals mentioned, and most common sealing materials including nitrile and Buna-N. Many outages have been caused by the use of sealing materials that are not compatible with phosphate ester fluids.
Some of the replacement fluids are being offered as “servo clean” which eliminates the need to pre-filter the new fluid prior to adding it to the reservoir. The current fluids are significantly dirtier than OEM recommendations out of the barrel.
It is important to note another significant attribute of the replacement fluids. They are considered biodegradable by US Environmental Protection Agency (EPA) standards. The MSDS for the phosphate ester fluids note that they are a marine pollutant since they are heavier than water and if spilled will sink to the bottom of any water source into which they are introduced. The replacement fluids are lighter than water and will float in water if spilled. This will allow both easier and safer recovery of the fluid from the water source.
In addition the MSDS states that phosphate ester fluids are a primary source of skin and eye irritation. Additionally gastrointestinal irritation and nerve damage can occur if the fluid is ingested. Inhalation of vapors or mists can cause respiratory tract irritation. The replacement fluids don’t create phosphoric acids or harmful vapors; therefore, they are safer to handle.
The replacement fluids are not new; in fact some of these fluids have been successfully used to replace phosphate ester fluids in hydraulic fluid systems in the steel casting industry for nearly 10 years.
Recently the US Electric Utility Industry has begun replacing some of its phosphate ester fluid with much success.
In addition to increasing equipment reliability, personnel safety and environment issues are of utmost importance to the utility industry.
It is vital that the hydraulic fluid systems be carefully and continuously monitored. If any of the areas discussed herein indicate problems they need to be addressed before a major failure occurs.
As pointed out in this article, not only are the hydraulic fluid system failures increasing at an alarming rate, but the handling and disposal of the current FRF is becoming more difficult.
In order to eliminate future problems and assure personnel safety and waste disposal is properly maintained the existing FRF should be replaced.
If ignored, the problems will escalate to “epidemic” state.