Before we jump into the specifics of the maintenance of FRF in Turbine EHC systems, let’s first understand EH systems. Electro-Hydraulic Control System (EHC System or the DEH System) of a Turbine is the most critical equipment in a Power Plant. It is also known as the heart of the turbine that controls steam flow into the turbine. A malfunctioning EHC System can lead to turbine stoppage and thereafter create a domino effect on the synchronized processes of the power plant. Breakdown-free and reliable operation of the EHC System is the biggest challenge of an O&M Manager and Engineers.
Due to a high steam pressure & temperature in the vicinity of a steam turbine, as safety compliance, it is a mandate to use Fire Resistant Fluids (FRF) for Hydraulic control of the EHC System. A specially designed synthetic fluid called Tri-Xylenol Phosphate Ester (TXP) demonstrates the best Fire Resistant properties for the application.
Phosphate Esters are polar fluids with excellent lubricating properties that can operate under extreme conditions. However, Phosphate Esters require strict control in order to extend their useful lifespan. This article covers best practices for strict maintenance required to prevent the untimely destruction of phosphate ester fluids.
Water and Acids de-grade FRF
Phosphate Esters (which are being used as FRF in EHC Systems) are manufactured under controlled environment through the esterification of Phosphoric Acid, where water is a by-product.
Phosphoric Acid + Alcohol → →Phosphate Ester+ Water
Unfortunately, Phosphate Esters are highly hygroscopic (tendency to absorb water) and the esterification process is reversible when Phosphate Esters come in to contact with water. This is called ester hydrolysis. The higher the water content and temperature, the faster the ester will break down by hydrolysis. Phosphate Ester + Water → →Phosphoric Acid or Acid Phosphates + Alcohol Thereby it is recommended to control the water level, temperature and acidity in FRF of the EHC Systems. If not controlled efficiently, the acidity accelerates rapidly.
TAN Value, i.e. Total Acid Number (measured in unit mgKOH/gm) is a parameter to denote acid content accumulation in FRF. A high TAN Value degrades the fluid rapidly, decreases its viscosity and resistivity. Thereby causing acid corrosion of sensitive servo-valves and other system components of an EH System. FRF de-gradation due to Particles Water and acid are not the only contaminants which can degrade the FRF. The dynamic oil film and fine clearances in servo-valves are less than five microns, even the finest silt particles and sludge/varnish deposits from fluid degradation can hinder proper operation. Fine particles get trapped in clearances between the valve plunger and housing. This abrasive wear is known as seizing or grinding. This can result in wear rates that are a thousand times greater than what is anticipated by the valve manufacturer. Therefore, it only makes sense to use very fine filtration (3-5 micron) for maintaining the EHC fluid.
Consequences of FRF de-gradation
- Acid, gel and sludge/varnish formation
- Valve sticking or blocking
- Reduced lubricity and film strength
- Corrosion, erosion and abrasion wear
- Reduced fluid resistivity
- Soot generation (entrained air)
- Short fluid life
Condition Based Monitoring of FRF
It is highly recommended to carry out regular Fluid Analysis of FRF and identify any abnormalities in the trend for further preventive actions.
Recommended Parameter Values for FRF:
| Parameter | Unit | Value |
| Appearance | ASTM Color Code | Clear,<3 |
| Water Content | ppm | 500 |
| Kinematic Viscosity(@40’c) | cSt | 41-45 |
| Acidity(TAN) | mgKOH/gm | <0.15 |
| Particulate Contamination | ISO 4406 | <15/12 |
| Cleanliness Code | NAS 1638 | <5 |
Minimac® EH Oil / FRF Conditioning System
In order to have complete purification of FRF, an external FRF Reconditioning System is recommended. Minimac® Systems - India is a leading manufacturer of such equipment and has been able to demonstrate miraculous results at multiple power stations during the last 5 years. The Minimac® FRF Re-conditioning System comprises 4 Major Technologies, all packaged comprehensively in one single system.
- Solid contamination Super-fine Filtration
- Moisture in oil separation by Vacuum Dehydration technology
- TAN reduction by Ion Exchange technique
- EHC System Tank Moisture Removal by Inert Gas Blanketing technique
Removal of Solid Particles from FRF
The removal of solid particle contamination in FRF fluid is carried out by the usage of stage-wise Superfine Mechanical Filters (up to 1 micron, Beta 1000 rating). Results better than NAS 4/5 are guaranteed.
Moisture Separation from FRF
As FRF is hygroscopic in nature, water is mostly present in dissolved form. OEMs of EHC Systems often provide Water Absorbing filters. However, these failed to absorb the dissolved moisture content. Vacuum Dehydration technology is safe and recommended by FRF manufacturers for efficient separation of moisture/water in FRF.
TAN Reduction in FRF
OEMs of EHC Systems commonly provide Fuller’s Earth and Activated Alumina (Selexorb®) cartridges for maintenance of Low TAN value in FRF. However, TAN values in FRF is frequently reported beyond 0.2 values in most of the cases. Minimac® uses the proven technology of FRF regeneration and re-conditioning using special Weak Base Anion (WBA) resins under Ion Exchange method which has been successful for over 25 years in Europe and America.
TAN Reduction success stories of Minimac®
Multiple Power Plants in India are facing the trouble of maintaining the desired operational parameters of FRF. Below are results of successful cases of Indian Power plants where FRF Re-conditioning System from Minimac® was adopted for FRF Re-generation.
Case 1: A power plant in Northern India using Indo-Japanese Technology Turbine EHC System.
| Date | Moisture | TAN |
| 12.07.2017 | 860 | 1.83 |
| 17.07.2017 | 259 | 1.51 |
| 27.09.2017 | 784.3 | 0.53 |
| 13.10.2017 | <500 | 0.20 |
Case 2: A power plant in Southern India using Chinese Technology Turbine DEH System.
| Date | Moisture | TAN |
| 28.02.2015 | 331 | 0.96 |
| 30.11.2015 | 302 | 0.20 |
Case 3: A power plant in Northern India using Chinese Technology Turbine DEH System
| Date | Moisture | TAN | NAS |
| 26.04.2018 | 333 | 0.57 | 5 |
| 21.05.2018 | 205 | 0.14 | 4 |
| 29.03.2018 | 329 | 0.40 | 5 |
| 24.04.2018 | 371 | 0.13 | 4 |
Case 4: A coastal power plant in Southern India using Chinese Technology Turbine DEH System
| Date | Moisture | TAN |
| 22.09.2018 | 628 | 0.80 |
| 19.01.209 | 432 | 0.15 |
Case 5: A coastal power plant in Western India using Chinese Technology Turbine DEH System
| Date | Moisture | TAN |
| 01.10.2016 | high | 0.81 |
| 10.11.2016 | 572 | 0.56 |
| 10.03.17 | <500 | 0.14 |
The purpose of the article is to sensitize the reader about FRF Maintenance and to adopt best practices from the industry. The data presented in the article has been researched over technical papers, presentations and the field performance results as observed at various power stations. None of the names have been revealed due to NDA compliances.
About the Author: Anshuman Agrawal of Minimac Systems Pvt Ltd holds Engineering Degree in Machinery (Mechanical) from IIT Dhanbad and a Diploma in Management from IIM Indore. He carries 15 years of vast industry experience in the field of lubrication and hydraulics maintenance.
