Most power plant operators do not realize what they are risking if they allow seal wear in hydraulic actuators to simply take its course. In this practical guide you will find out why this aspect is so important and how to cost-effectively prevent damage and production downtime with predictive planning.
In order for gas and steam turbines to generate electricity with the right output and rotational speed, the supply of media must be precisely controlled. Hydraulic actuators ensure exact control of the valves. In the event of a malfunction, they also carry out an important safety function by mechanically disrupting the flow of medium and redirecting it. In order to ensure this in the long term, it is essential that the hydraulic actuators for gas and steam turbines are inspected and maintained at regular intervals in accordance with the relevant legal regulations and manufacturer's recommendations. The reason: each hydraulic actuator – no matter how high-quality it is – exhibits wear due to operational reasons. The wear starts on the seals and, in the absence of maintenance, may extend to other components. Anyone who disregards this fact, and waits until an obvious failure occurs, risks high repair costs and long waiting periods until unrestricted production can be resumed.
Imminent loss of the SIL capability
If the specifications from the operating instructions and the safety manual are not complied with – maintenance requirements and proof-test intervals, for example – or if the actuators are not properly used, under certain circumstances the drive loses its SIL conformity and thus its suitability for safety-related applications. Improper usage includes the use of dirty or contaminated hydraulic oil. This can lead to failure of the solenoid valves and cartridge valves, as well as to increased wear on the piston rod seal and the piston rod. In addition, unsuitable placement in the escaping steam can cause damage due to overheating (cf. 1.3), which can subsequently result in strong corrosion on safety-related components – such as magnetic and cartridge valves, piston rod, and spring assembly. Last but not least, a lack of regulation or control poses a risk to the SIL capability, because inadmissible oscillations could cause significant wear in short time.
- Damage causes
1.1 Ongoing seal wear
In particular, if the piston and rod seals are not replaced according to the schedule for the actuator – at the latest after six years – the wear will lead to functional impairments. First, leakage occurs; later, there is damage to the piston and the piston housing. Depending on the circumstances, this leads directly to failure. On the one hand, the wear ensues regardless of the product quality due to the aging process of the sealing material, which gradually becomes brittle and porous, so that the sealing effect decreases. On the other hand, the frequency of the changes in movement, in combination with the stroke length, plays a role. The strokes are therefore to be determined by the control system so as to ensure that an adequate lubricating film and thus low-wear operation for the seals are ensured. Constant operation in the short-stroke range, on the other hand, leads to increased seal wear and damage to the mating surfaces. Especially in the case of short strokes at high-frequency, the replacement intervals for seals recommended in the maintenance schedule can be significantly reduced. Regular monitoring is essential.
In the case of actuators that only extend to short strokes or remain in one position over a longer period of time – depending on the application, this could be several weeks or months – separated seal particles mostly remain in the chamber, as there is hardly any exchange of the oil. Thus, contamination on the inside of the cylinder builds up and the sealing rings wear down more quickly.
Not recognizable from the outside: The natural aging process makes the seals inside become brittle and porous. Permanent operation in shortstroke range additionally limits the sealing effect.
1.2 Incorrect maintenance or assembly
Apart from the wear, maintenance or assembly errors could also be the cause of permanent damage to the actuators. A common source of error is the alignment to the steam or gas valve. If a valve and piston rod are not properly aligned, lateral forces acting on the coupling place stress on the guide belts. Consequently, the guide belts and the seals wear excessively. A further cause of damage resulting from faulty handling is the filling of the hydraulic system with an incorrect fluid. This can trigger a chemical reaction between the seal and the fluid, which in turn accelerates the process of wear and tear.
1.3 Special risks associated with steam valves
Power plant operators who generate electricity by means of steam should also observe the leak-tightness of the steam valves in the direction of the cylinder. If the more than 500 ºC hot medium flows long enough over the outer coating of the actuator, the coating can “burn off” there so that the surface becomes corroded. Apart from this obvious damage, the spring assembly inside the cylinder can also be damaged, which, in the event of a malfunction, subjects the valve purely mechanically to up to 400 kN force in the locked position.
At the interface to the armature – the coupling – the ambient temperature must not exceed 200 °C. As a result of the effects of excessive heat, the existing anticorrosive coating – consisting of a layer of wax on the Belleville spring washer system – can be damaged. If a rusty spring breaks, the actuator loses its safety function. In this case, the component requires an immediate overhaul in a manufacturer’s certified service center.
In addition, uncontrolled steam discharge can damage the actuator due to overheating of the hydraulic fluid, because the fluid temperature must not exceed 70 ºC in circulation. If the system can not compensate for the influx of heat, the hydraulic oil will be “burnt”. This means that it becomes viscous and sticky. The seal wear increases, valves and other components lose their freedom of movement and function.
Safety-relevant component: The spring assembly inside the cylinder brings the valve into the safety position purely mechanically in the event of a malfunction. If it breaks due to corrosion, the actuator loses its safety function.
2 Consequences of inadequate maintenance
2.1 Reduced control quality
Even before the seals lose their originally intended function through continued wear, increasing internal or external leakage leads to energy losses. The accompanying temperature rise in turn promotes wear and tear in other areas of the hydraulic system. Another consequence of the increased leakage is diminished control quality. Therefore, under certain circumstances, the system would no longer be able to adjust the required performance of the turbine with sufficient accuracy.
2.2 Mechanical damage (worst case)
If the damage of the seal system has deteriorated to such an extent that it leads to mechanical contact between the piston rod and cylinder housing, the main components suffer permanent damage. Due to the leakage that occurs, the actuator can no longer properly perform its regulatory function. Although this is not safety-relevant with the Belleville spring washer system intact, the damage is however much greater. Because in comparison to preventive seal replacement, an extraordinary repair is now due which, in comparison with a normal overhaul, is significantly more complicated and more expensive. The reason: the main components of the actuator, such as the cylinder housing and piston, are generally not stockable items for the manufacturer due to the large variance. Some parts must even be cast and then machined. In the worst case, this could take weeks or months. If the waiting period occurs during the high season, operators must possibly accept painful constraints to production.
3 Recommendations for practical use
3.1 Preventing the worst-case scenario
Anyone who wishes to avoid the worst-case scenario in a cost-efficient manner, should not only comply with the maintenance rules and instructions, but also schedule major inspections in a timely manner. …
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Author: Volker Tenhaeff
Job title: Head of Global Service Support
Volker is the head of Global Service Support for Industrial Hydraulics at Bosch Rexroth. He has more than 8 years of global experience in sales, service and application engineering in hydraulic drive systems for various industry sectors like metallurgy, marine & power generation. He holds a Master of Engineering in mechanical engineering from Technical University Munich, Germany.
Author: Ralf Bentfeldt
Job title: Head of project management and maintenance, service hydraulics
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