Q: What special considerations are required for mechanical seals in water applications?
A: Water is one of the most common fluids pumped by centrifugal pumps. It is easy to consider water as simply drinking water or municipal water, but water applications reach into virtually every industry and chemical process. While these applications all fundamentally pump water, the nature of the water and the application conditions can vary significantly. The expectations for seal performance and reliability can also differ between industries and end users.
Typical Water Applications
Municipal potable water – Water supply systems provide drinking water to municipal water systems and industrial users. The quality of drinking water is tightly controlled and generally free from impurities and other chemicals. Many drinking water systems have a low concentration of chlorine and fluorine, but at normal levels, these do not impact sealing options. Most potable water systems are at ambient temperature and low pressure.
Food processing – Food processing plants use water in services ranging from cleaning and cooking to dilution and packaging. Water systems in plants will be tightly controlled and may be subject to cleaning or sterilization cycles. It is essential to understand all applications conditions when selecting materials of construction for these applications. There are some seal models which are specifically designed to minimize cavities and crevices to allow for more thorough sterilization and cleaning in place. When selecting seal materials, the requirements and regulations for equipment in direct contact with food or drinking water must be considered.
Waste water and sewage – Water carrying a significant concentration of solids or contamination requires seal solutions which will not be impacted by debris collecting around the seal. Seals designed for slurry applications, or bellows seals, are commonly used to prevent the seal from hanging up in operation. Depending upon the nature of the contamination, hard face material combinations may be required to minimize face damage. The design of the seal chamber and the appropriate selection of the piping plan can significantly improve seal reliability.
Cooling water – Cooling water systems are used through many manufacturing, refining, and chemical process plants. The cooling water is used to control the temperature in the equipment and processes and is recirculated throughout the plant. Cooling water can vary in quality and can contain rust, dirt, and organic materials. Most plants have an adequate water treatment program to prevent this from impacting seal reliability. Cooling water systems generally operate at relatively moderate temperatures and low pressures.
Figure 1: Example of split seal
Boiler systems – Water is used in steam generation systems to transfer heat and generate electricity. Since water is converted to steam, the water quality is tightly controlled to prevent fouling and corrosion in the water system. Boiler feed pumps and circulation pumps can operate at high pressures, speeds, and temperatures. For these reasons, the seals are often highly engineered designs and require piping plans which reduce the temperature in the seal chamber (i.e., Plan 23).
Water injection/seawater injection – Oil recovery in established fields often rely on water injection to maintain the formation pressure and increase the amount of oil production from the well. These applications often involve pumps operating at very high pressures and speeds. Water quality can also be challenging since the injected water may be produced water, seawater, or river water.
Challenges With Water
In many ways, water is an excellent fluid for pumping and sealing. It is chemically compatible with a wide range of metals and elastomers. It is generally a low safety risk and it is easy to decontaminate exposed components. Leakage from many water applications is not an environmental hazard and only represents a housekeeping concern.
As a sealed fluid, though, water can present some significant challenges. Water is a poor lubricant. At lower temperatures, water has an adequate viscosity to lubricate the seal faces and allow for a flexible selection in seal face materials. As the temperature increases, the viscosity decreases and sealing challenges become more significant. Above 70-82°C (160-180°F), it is a common practice to apply piping plans to reduce the temperature in the seal chamber to improve reliability.
Water also has a relatively small range of temperatures where it is suitable for sealing. Water freezes at 0°C (32°F), so pumping and sealing systems must be protected from damage caused by low-temperatures. This concern is valid not only for normal operating conditions, but also standby conditions where the equipment may reach ambient temperatures. At higher temperature, the vapor pressure of water may cause flashing or boiling in the pump or between the seal faces. While it is possible to suppress vaporization in the seal chamber through pressurization, the water will still flash as the pressure drops across the seal faces.
Water is often not just water. Contaminants and solids in the form of abrasives, rust, process contamination, and water treatment chemicals can all impact the quality of the water and mechanical seal performance. In most cases, seal material selection is the primary consideration, but in some cases, more specialized piping plans may be required.
Read Part 2 of the article on October 21st!