Understanding Mechanical Seals: What Is a Split Pump Seal?
A split mechanical seal is a device that contains fluid within a vessel wherein a rotating shaft moves through housing or sometimes where housing rotates around a shaft. These vessels are often pumps, mixers, agitators, etc. The mechanical seal allows the shaft to rotate freely without allowing huge amounts of fluid to escape.
What is a split pump seal? This type of seal has two separate pieces. Unlike ordinary cartridge mechanical seals, these parts can be removed or installed from around the shaft without needing to disassemble the equipment. Once joined, the sealing elements are mated to create a proper seal around the shaft.
What Is a Split Pump Seal?
It is called a split seal because it comes in two separate pieces. These parts can be installed or removed from around the shaft. In the split seal, each component is cut into half and assembled on the pump without removing the bearings or motor.
You need not disassemble the equipment. Once you join the two faces, it’s essential to mate the sealing elements to establish proper sealing around the shaft.
Use Of The Split Mechanical Seal
When do you utilize a split mechanical seal? It’s widely used in equipment with large shafts, double-suction pumps, vertical shaft pumps, side-entry mixers, and agitators.
If a common component seal is present, you must remove the end bearings, motor coupling, and motor to replace them. This requires more effort, time, and money.
A split mechanical seal doesn’t include such hassles. Seal replacement is a matter of a couple of minutes in this case.
Benefits Of Split Pump Seals
Once split seals are installed, the applications benefit from reduced installation time, reduced sleeve wear, and increased reliability.
You save money on maintenance and installation activities. Though the initial cost of installing a split mechanical seal is high, the long-term operational benefits compensate for the cost.
The savings are higher than the expenses. Hence, it’s an economically feasible and suitable option. You require less labor, and the downtime also drops significantly.
The advantages of split mechanical seals make them popular compared to other seals. However, a word of caution: Split seals aren’t recommended for applications where leakage or emission threatens the company’s employees.
History of Split Seals
Since their introduction in 1986, off-the-shelf split mechanical seals have been used worldwide in process industries to simplify maintenance activities and reduce associated costs. Convinced of the advantages of split seals on larger rotating equipment, the power generation industry quickly adopted the technology for large pump applications.
Since the first generation split seal was introduced, numerous technological improvements have been made and integrated into the latest generation. These improvements have significantly 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, split seals can be found in more demanding applications across many industries, ranging from boiler feed pumps to 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: there is no need for equipment disassembly. Split seals eliminate the need to remove anything from the pump except the seal. As split seals can be installed in place and typically by one installer without removing the motor, pump, or coupling, they simplify the repair process and take out 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 and correspondingly large savings in time, material, and personnel.
Split Seals in the Power Industry
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.
While acknowledging the usefulness of the 442 seal design, the power industry needed a seal that could operate at the temperature extremes of hot water (ranging from 150ºC to 180ºC). High-temperature water is a difficult sealing medium, so engineers designed a split seal that utilizes an integral pumping ring. This design allows for 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 sealed cooler and back to the seal chamber, ensuring a 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, high-pressure versions can be used in higher pressures commonly found in the power industry.
High-pressure versions typically use a positive drive and standard captured gasket that allows the operation to a 30 bar pressure limit, as much as four times higher than most other split seals.
With thousands of installations worldwide, split seals have become the preferred choice in the power industry. They have successfully sealed fly ash pumps, boiler feed and circulation pumps, heater drain pumps, fly ash and bottom ash pumps, and flue gas desulphurization (FGD) agitators.
Size Does Matter
Split seals’ savings 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, the main turbine shafts are considerably larger in hydroelectric power generation.
These large shafts have traditionally been sealed by compression packing, rubber sealing elements, or segmented seals because there is no true split-face seal technology in this size range.
Conventional compression packing may initially appear inexpensive, but it is expensive due to leakage, sleeve wear, and frequent maintenance.
Segmented seals consist of several carbon or polymer/fabric face segments installed around the shaft. Seal operation depends on an external system that provides high-purity pressurized water for hydraulic loading and flushing. This external system is often more of a reliability concern when not maintained and operated properly.
Rubber seals come in various designs, all 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 greatly expanded 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 a 900 mm shaft size, providing full mechanical seal reliability without requiring extensive external environmental control systems. Since 2002, 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 then, many more split seals in a similar size range have been installed on hydroelectric turbine shafts and cooling water pumps worldwide.
Split seals in this size range are normally manufactured specifically for an application, allowing the solution to be optimized for each application.
The Complete Solution is the Key
Split mechanical seals provide power plant operators with a proven and reliable solution for sealing all their pumps, including their largest and most critical ones.
Whether they are condensate pumps, boiler feed pumps, cooling water pumps, or turbine shafts, split seals have a proven track record in all these large pumping applications.
The ultimate result is lower operating costs without disassembling through improved reliability and better maintainability.
Tips for Installing Mechanical Split Seals
- Pre-Assembly Shaft Inspection
Ensure the shaft is free and clear of any burrs, debris, metal from the previous seal installation, and any defects to the shaft, particularly in the area wherein the O-ring seals against the shaft. Shaft nicks needs to be smoothed out to bring it back to the best condition feasible. Measure shaft runout to guarantee it is within the tolerance of the seal. The seal will only perform as designed if the equipment is within its operating specs.
- Check the Rotor Assembly of the New Split Seal
Inspect the new split seal to guarantee there are no chips or burrs on any of the mating ring half faces that might have become damaged during shipping. Check to guarantee a consistent gap between the ID of the mating-ring adapter piece or the OD of the mating-ring face. During shipping, the mating ring might become dislodged from the sealing strip that’s located behind the mating ring. If there’s a gap, put pressure with your thumbs up and towards the gap, and the mating ring will pop back into the proper position. Avoiding this fix will lead to excessive seal leaking.
- Setting the Seal’s Operating Height.
To set the seal’s proper operating height, the rotor assembly comes with little plastic spacers. Put them on the rear of the mating-ring adapter of the rotor assembly. Press the spacers up lightly against the seal chamber face. If you push excessively, the spacers will affect and bend the rotor assembly’s proper setting height. Tighten the screws and take out the spacers.
- Go Easy on O-Ring Lubricant
Often, installers in the field get excessively liberal with O-ring lubricant on the elastomers; you only require very little O-ring lubricant to do the assembly. The lubricant is only there for easy assembly, not to help the seal operate. If too much lubricant is utilized, it can move to hot spots where the split faces make contact, leading to blistering, chipping, seal damage, and leakage.
- Inspect the Position of the Rotor Assembly on the Shaf
When the rotor assembly is assembled on the shaft, where the split joint joins, drag your fingernail across the split, ensuring the joint is smooth. Your fingernail shouldn’t snag on anything. If any raised edge catches, the parts haven’t gone together correctly, and the seal must be disassembled and checked to find out why the seal faces aren’t fitting tightly together.
- Primary Ring Installation
Whether a carbon or silicon carbide primary ring is split into halves, the retaining ring holds the halves together while you assemble. Lay the primary ring on the rotor assembly’s face, center it, and turn it to ensure smoothness. If it doesn’t mesh right, it will not operate properly.
- Final Split-Seal Assembly and Gland Plate Installation
Installing two gland plate halves will finish the split-seal assembly. Place the bottom half of the gland plate beneath the primary ring so it engages and centers the plate over the shaft with a different set of little plastic spacers. Bring in the upper gland plate, match the captive screws with the lower plate’s holes, and tighten. Make sure to remove the plastic spacers. Finally, the three assemblies are evenly torqued together and flushed up against the seal chamber face of the pump with no gaps.
Do you need help determining which mechanical seal configuration is right for your application? Ask us! We gladly provide technical assistance to businesses in Central Texas.
