Since the first hydropower turbine was designed and installed, sealing the turbine’s main shaft has presented significant design challenges to turbine designers and operations and maintenance headaches for hydropower plant personnel.
By W. Alan Evans and John Sousa
The sealing conditions a water turbine operates in, the size of the shaft and the type of sealing technology used all have a significant impact in maintaining a sealing system that exhibits controllable leakage rates and long, reliable life. When water turbines were first developed, compression packing was used to seal the turbine’s shaft. Packing offered adjustability for leakage control with the opportunity to renew the packing from time to time as required.
However, early packing required a significant amount of water to keep cool. Flush water was also used to keep particulate material from contaminating the packing, which if not abated, would result in abrasive materials embedding in the packing and accelerating the wear of the turbine shaft sleeve. As stated above, packing also requires periodic adjustments to limit the amount of leakage, which can overwhelm the station’s drainage system, resulting in water contamination of other turbine components including oil lubricated bearings as well as overall flooding of the station.
As compression packing technologies have evolved, newer materials, lubricants and blocking agents have been developed that reduce the amount of cooling water required to maintain acceptable packing life. However, the issue of particulate contamination is still ever present and must be dealt with to keep the compression packing usable for a long period of time and shaft sleeve wear to a minimum.
Due to the inherent issues associated with compression packing, designers and operators have looked for other methods of sealing the turbine’s main shaft. Various technologies have been developed over the years, including carbon segmented rings, elastomeric radial sealing elements and variations on these technologies. However, wear and high leakage rates have been significant issues that must be routinely dealt with while maintaining turbine operational reliability from application to application.
Axial face seal development
Axial face mechanical seals have been gaining wider acceptance as a viable long-term sealing solution for water turbine sealing. The ability to offer designers and operators highly reliable shaft sealing capabilities with zero or significantly reduced seal leakage, while mitigating the effects of sedimentation on the sealing elements has become a reality. With continuing advancements in materials and manufacturing processes, the ability to offer fully split mechanical face seals which exhibit little to no visible leakage with abrasion resistant face materials has been achieved.
The term “axial face seal” relates to the orientation of the primary sealing rings (or faces) with respect to the turbine shaft’s main axis. The primary sealing faces are aligned along the axis of the turbine’s shaft. One of these faces is stationary, while the other rotates with the turbine shaft.
The interface where these two faces come together is known as the primary sealing interface and where the “sealing” occurs.
To function as a low or no leakage seal, the running surfaces of these two faces must be extremely flat and remain as close to parallel as possible during operation over the course of the years the turbine is in operation under various hydraulic, mechanical, thermal and dynamic loadings found in every operating machine.
In order to seal effectively, the seal faces are lapped to within 2 to 4 helium light bands of flatness when manufactured, (a helium light band is equivalent to 0.000011 inches/0.3 microns). In order to continue to provide an effective seal over time, these faces must maintain a relative flatness of less than 8 helium light bands. Maintaining this degree of flatness and leakage control has been achieved through extensive engineer analysis in which the seal face geometry is optimized utilizing the latest in finite elemental analysis (FEA) modeling so the seal faces can tolerate the various loads and stresses placed on them while in-service.
This is quite an accomplishment for solid seal faces; however it is even more of an accomplishment when you consider that in a split mechanical seal, the faces are split in half and then goes back to this degree of flatness once the two halves are brought back together.
While this level of analysis may appear to be some of the latest available, it fact the technology has been used for well over 20 years.
Sealing material technology advances
There have been many advances in material science which have expanded axial mechanical sealing capabilities that can be applied to turbine main shaft sealing.
Traditionally, a soft carbon graphite material was used for one of the seal faces and was paired against a hard face material such as alumina ceramic. These materials are well established in their use with early mechanical seals and they could be split due to the inherent brittleness of the materials.
However, the application window for this face pair was limited due to the tribological properties of the materials as well as the poor thermal capabilities of the alumina ceramic, which acts as a very good insulator. A standard parameter used to evaluate the application window for a seal face pair is the Pressure – Velocity (PV) limit. The sealing interface pressure of the application is multiplied by the rotational velocity of the mean face diameter of the mechanical seal. If this value exceeds the limit placed on the seal face pair, high wear and heat generation will occur, significantly shortening the operating life of the face pair.
Newer materials like silicon carbide have been developed which offers significant improvements in PV limits. Compared to the carbon/alumina face pair, a carbon/silicon carbide pair has a PV limit that is 2.3X greater. Additionally, the carbon/silicon carbide face pair will generate 60% less heat which greatly reduces the amount of cooling water required for the seal faces; thus significantly expanding the application envelope for mechanical seals.
An additional characteristic of silicon carbide is its hardness, diamond being the only material that is harder. Therefore, it has great abrasion resistance for those applications where particulates and silt may be entrained.
The final characteristic of silicon carbide that makes it such a good choice for mechanical seal faces is its ability to run against itself with very good tribological properties. As compared to carbon/ceramic face pair, a silicon carbide/silicon carbide face pair has a PV limit 33% higher while generating 50% less heat. Therefore, in those hydro applications were particulates and silt are a concern, such a face pair offers a very abrasion resistant option and has proven to be a superior choice for all but the most demanding applications.
Seal life time analysis
The final area of advancement in the development of sealing systems for water turbines is in analytical tools and models to help the seal designer better understand how well a specific seal will operate in a given application, including FEA methodologies.
The biggest question any facility will have is: how long will a split axial face mechanical seal last in my application? To answer this question, the first issue to deal with is: what is the actual failure mechanism involved? As prior work has demonstrated, most mechanical seal failures are caused by outside influences that result in a mechanical seal exhibiting greater than anticipated or desired leak rates. The failure of bearings and other turbine components associated with the rotating element are typically what cause a mechanical seal difficulty. Therefore, predicting actual life is not possible.
However, if we look only at the mechanical seal, the major consideration for failure is “wear out” of the seal face pair. Unfortunately, there is no way to accurately calculate the wear out of a seal face pair at this time. However, we can look at empirical data to gain a higher degree of confidence that the mechanical seal will exhibit a long reliable life in a specific application. With literally thousands of split axial face seals operating, in a variety of applications around the world, a very large data set from which to make comparisons exists. To this end, we have developed two parameters to estimate the viability of a seal in a specific application and the degree of confidence in the success of a split seal in the application.
The first parameter is simply the Total Sliding Distance (TSD) the seal face pair will travel throughout their anticipated life. Once this value is known, it is compared to other similar applications that have run for at least 10 years without maintenance intervention or failure. If the TDS for the application in question is within the scope of TSD’s calculated for the documented applications, there is a high degree of certainty that the application in question should last at least as long as the applications in the comparative data set.
The second parameter used is a more complex formula taking into account not only the Total Sliding Distance (TDS) of the application but also the loading and tribological operating regime of the face pair to get a more comprehensive view of the application in question as compared to the data set of known applications. This second parameter is known as the Work Life Index©.
The Work Life Index (WLI) for the target application is calculated based on a projected continuous operating life of 10 or 20 years. This value is then compared to the Work Life Index’s calculated for the data set of known applications that have been in continuous service for at least 10 years maintenance free.
As with the TSD value, the WLI value for the target application is compared to the WLI’s calculated for the data set to determine how the severity of the target application compares. If the WLI for the target application is within the scope of the documented applications, there is a high degree of confidence that the target application will exhibit similar life.
A target application well within the scope of both the TSD and WLI gives a high degree of confidence.
A small list of successful applications where split axial face mechanical seals have proven successful in hydropower turbines as well as in other more severe pumping applications shows the average life is 12.4 years of continued maintenance-free service. To date, several hundred split axial face mechanical seals have been installed on hydropower turbines around the world with sizes ranging from 6″ (150 mm)to 29.25″ (743 mm) with new applications added each year.
As has been demonstrated, development of split axial face mechanical seal technology continues to evolve, expanding the capabilities and scope of applications in the hydropower industry. Both turbine designers and owner/operators are benefiting from this technology’s enhanced performance with limited to no leakage capabilities. Over a 20-year span, the use of split axial face mechanical seal technology has been proven, establishing this sealing device choice for turbine main shafts as a viable option for consideration.
W. Alan Evans is the Global Business Development Manager for A.W. Chesterton Company. John Sousa is the Market Research & Support Manager and has been the project leader responsible for Split Mechanical Seal development at A.W. Chesterton for the past 12 years.
Mobile ring sealing: An alternative to D-ring seals for low pressure applications
Is a D-ring the optimal answer to all mobile ring sealing issues? Not necessarily.
PXL Seals share their experience on low pressures applications. As required by a hydro power customer, PXL proceeded to a mobile ring sealing of a spherical valve initially equipped with O-ring seals. The customer requirement was to optimize movable ring sealing by using D-ring profiles. The D-ring profile is designed to increase the seal stability in a gland (with its squared base) preventing twisting failures.
Constraints included a low working pressure, no differential chamber, use of dam raw water and a temperature of 1 to 20 °C.
Further to the maintenance, the customer had movable ring mobility issues: operating time was too long compared to initial values. Another valve (security valve) had to be closed to allow ring mobility (with over pressure created by closing the security valve)!
It was critical to find a solution to allow the mobile rings quicker movement and ensure security. Moreover sealing failures were not due to mobility but to twisting.
After analyzing the seal behavior using FEA simulation, it was determined that the D-ring generated high friction with a higher friction value than O-rings at low pressure. This explained the slow mobility of the equipment.
Friction was caused by the D-ring design. Due to its squared base, a D-ring generates more contact pressure than an O-ring at low pressure. And more contact pressure inevitably equals more friction when using a similar material.
Consequently, an alternative solution was proposed, an X-ring profile, that guarantees the sealing stability in the housing and significantly reduces friction at low pressure. X-ring has the same behavior (friction, pressure tightening) as the O-ring especially at low pressures levels. Furthermore, the solution suits existing housings without any modification.
— By Fanny Champlon, PXL Seals