Marco Hanzon, A. W. Chesterton Company, USA
Since their introduction in 1986, off-the-shelf split mechanical seals have been used throughout the process industries around the world to simplify maintenance activities and reduce their associated costs. Convinced of the advantages of split seals on larger rotating equipment, the power generation industry also quickly adopted the technology on large pump applications.
Since the first generation split seal was introduced, many technological improvements have been made and integrated into the latest generation. These improvements have greatly expanded the window of operation and application of split seals.
While initially used for sealing only the easiest applications such as general service water pumps, today split seals can be found in more demanding applications across many industries, ranging from boiler feed pumps, rotary dryers to pulpers. The pressure envelope of today’s second generation split seal has been pushed upward to 35 barg.
Vacuum conditions present no problem either and easily match the vacuum performance of non-split seals. Extended motion capabilities of modern split seals generally exceed those of conventional non-split seals, allowing them to be installed safely on large equipment where radial motion and low-frequency vibrations may be present.
However, the biggest advantage of split seal technology remains the same as before; no need for equipment disassembly. Split seals eliminate the need for removing anything from the pump except the seal. As split seals can be installed, in place and typically by one installer, without removing the pump, motor or coupling, they simplify the repair process and eliminate the associated costs with typical solid seal replacement.
In addition, split seals can commonly be installed in the conversion from packing to seals right on the existing packing sleeve regardless of sleeve condition. Thus it allows upgrading from packing without the necessity and expense of replacing the existing packing sleeve. These advantages result in huge cost savings as well as correspondingly large savings in time, material and personnel.
Split Seals in Power
The power industry has come to know split seals and the benefits derived from their simplicity of installation and field repairability. The second generation split seals, for instance, are commercially available in several variants to meet specific customer needs or demands.
The power industry, while acknowledging the usefulness of the 442 seal design, needed a seal that could operate at the temperature extremes of hot water (ranging from 150 ºC to 180 ºC). High temperature water, being a very difficult sealing medium, engineers designed a split seal that utilizes an integral pumping ring. This design allows for the use of the proper environmental controls and seal flush arrangement necessary for these operating duty extremes.
This ‘pumping ring’ version of the standard split seal is designed to be used with an API Plan 23, cooled seal recirculation in which the seal itself, pumps hot water from the seal chamber through a seal cooler and back to the seal chamber, ensuring an seal chamber that can be easily a hundred degrees Celsius cooler than the product bulk temperature. All arrangements for removing and re-introducing hot and cooled fluid to the seal chamber are integral in the pumping ring variant.
Although split seals are generally considered for low-pressure services there are high-pressure versions available that allow use in higher pressures commonly found in the power industry.
High-pressure versions typically use a positive drive and standard captured gasket that allows operation to 30 bar pressure limit, as much as four times higher than most other split seals.
With thousands of installations operating worldwide, split seals have certainly become the preferred choice in the power industry. From fly ash pumps, boiler feed and boiler circulation pumps, heater drain pumps, fly ash and bottom ash pumps to flue gas desulphurization (FGD) agitators; they have all been successfully sealed with split seals.
Size does Matter
It is evident that the savings that split seals bring become more significant as the shaft size increases. While split seals are commonly available up to a 300 mm shaft size, nuclear and fossil power plants often have larger water intake pumps and cooling water pumps on site. Further, in hydroelectric power generation the main turbine shafts are considerably larger too.
These large shafts have traditionally been sealed by compression packing, rubber sealing elements or segmented seals because of a lack of availability of true split face seal technology in this size range.
Conventional compression packing may at first appear to be an inexpensive solution but it comes at the expense of leakage, sleeve wear and the need for frequent maintenance.
Segmented seals consist of several carbon or polymer/fabric face segments that are installed around the shaft. Seal operation is dependant on an external system that provides high-purity pressurized water for hydraulic loading and flushing. It is this external system that is often more of a reliability concern when not maintained and operated properly.
Rubber seals come in a variety of designs all of them featuring a contacting elastomeric seal. As wear of the elastomeric lips over time is a reliability concern, the more developed versions again feature external systems for hydraulic load control and lubrication.
All these traditional sealing methods are expected to experience some leakage thus requiring additional flood pumps in the seal well. In recent years, designers of split mechanical seals have been able to greatly expand their size capability. Developments in both the design and manufacturing capabilities of large sealing rings have made this possible.
Today, split seals can be manufactured to fit up to a 900 mm shaft size providing full mechanical seal reliability without the need for extensive external environmental control systems. The first 610 mm split seal has been successfully installed and operated at a 12 MW turbine at a hydroelectric power station in the United States since 2002 (Table 1). Since then, many more split seals in a similar size range have been installed on hydroelectric turbine shafts and cooling water pumps around the world.
Split seals in this size range are normally manufactured specific to an application allowing the solution to be optimized for each application. In many installations a SpiralTracTM1 environmental control bushing is installed as part of the sealing solution. The SpiralTrac device features a unique spiral groove design that actively evacuates particles away from the sealing area thereby eliminating the need for high-purity and filtrated water for flushing.
The Complete Solution is Key
Split mechanical seals provide operators of power plants a proven and reliable solution for sealing all of their pumps up to and including their largest and most critical pumps.
Whether they are condensate pumps, boiler feed pumps, cooling water pumps or turbine shafts; split seals have a proven track record on all these large pumping applications. Through improved reliability and better maintainability the ultimate result is lower operating costs – without the need to disassemble.
1 SpiralTrac is a registered trademark of Enviroseal Engineering Limited