Evaluating barrier fluids for mechanical seal applications

Water is readily available. It is environmentally acceptable and has no health and safety restrictions. It also has a high specific gravity and specific heat which aids in heat transfer. For all of its favorable properties however, water has severe limitations as a barrier fluid. Pure water is a poor lubricant. At ambient conditions, water has viscosity that is suitable for lubricating seals with carbon vs hard face combinations. The viscosity decreases rapidly as the temperature increases. By 71°C (160°F), the viscosity is low enough that the fluid film in a standard seal fails to support the face often resulting in higher wear rates. The low viscosity of water can also create problems for Plan 54 systems.

This article is an excerpt from a paper, "Barrier and buffer fluid selection and considerations for mechanical seals" by Michael Huebner of Flowserve Corporation at the 2016 Turbomachinery & Pump Symposium.

Many Plan 54 systems are designed as an open system and use a positive displacement pump to create pressure and circulate the barrier fluid through the system. Many of these positive displacement pumps are designed with rubbing or sliding pump components that are designed to operate on a lubricating fluid. Operating these pumps on water can greatly reduce the reliability of the Plan 54.

Water at low temperatures can introduce different concerns. At 0°C (32°F), water freezes. This can have a severe impact on the condition of the seals and the auxiliary components. When a pump is in operation, the water barrier fluid may be heated by the process or the seal generated heat. In standby service though, the barrier fluid may reach ambient temperature conditions. Water is relatively non-corrosive but it will rust wrought and cast carbon steels. Normal seal piping plans will use stainless for most components but may use carbon steel for the reservoir to reduce cost.

Water barrier or buffer fluid systems must be designed with non-rusting materials. Not all water supply systems have clean, pure water. Contamination in the systems (e.g. rust or dirt) as well as water treatment chemicals (e.g. descalers, rust inhibitors, biocides, etc.) may affect chemical compatibility of the seal components or the lubricating properties between the seal faces. Users must ensure that the water supply is clean and suitable as barrier fluid.

One of the primary reasons why water is selected as a barrier fluid is for compatibility with the process fluids. In normal operation, small amounts of barrier fluid will leak into the process. In some processes, pure water introduced into the system would not be considered as a contaminant in the process. Water, specifically condensate, is the most common barrier fluid in the food processing, pharmaceutical, and biotech industries.

Many of the shortcomings of water can be addressed by mixing the water barrier fluid with other chemicals. The most common mixtures are water with either Ethylene Glycol (EG) or Propylene Glycol (PG). The addition of a glycol to the water depresses the freezing point and elevates the boiling point. It also increases the viscosity of the mixture which can provide better lubrication to the seal faces. It does this while still maintaining the high specific gravity and specific heat required for effective heat transfer. The improved properties made this buffer fluid an industry standard in refineries in light hydrocarbons services for many years.

Ethylene glycol is commonly used as a heat transfer fluid in industrial applications and automobile cooling systems. Automotive anti-freeze is most commonly a mixture of EG and other chemical additives. These other additives provide useful properties to automotive applications including preventing rust and corrosion, descaling metal surfaces, and stopping leaks in the cooling system. While these additives enhance the performance in automotive applications, they can cause high wear on the seal faces and reduce the reliability of the seal. For this reason, automotive anti-freeze should not be used in barrier or buffer fluid systems. Only pure EG or PG should be used. While ethylene glycol improves the properties of the barrier fluid, it has the drawback of being mildly toxic. Casual exposure to the skin is not considered a significant hazard but it must not be ingested and leakage into the environment may be regulated. For these reasons, many users have switched to propylene glycol/water mixtures. The properties of PG/water and EG/water are comparable and they will provide similar performance in most applications.

Propylene glycol is considered non-toxic and is safe for human exposure. Food grade PG is available and can be used in many food handling processes. Propylene glycol should be the first choice for glycol/water barrier and buffer fluids in most applications.

Alcohols are a class of organic compounds which covers a wide range of molecular weights, chemical structures, and physical properties. Simple alcohols, such as methanol, ethanol, and propanol are low viscosity fluids which are liquid under moderate ambient conditions. At these temperatures, the low viscosity and poor lubricating properties make them unsuitable for maintaining a stable fluid film. Their high vapor pressures and tendency to evaporate quickly add to the list of characteristics which would make them seem unsuitable as a barrier fluid. There is however one physical property which is unique compared to other barrier fluids; their ability to remain in a liquid phase and have a moderate viscosity at very low temperatures.

Cryogenic temperatures are not uncommon in some refinery or petrochemical processes. In these applications, common barrier fluids will be very viscous or freeze. Simple alcohols such as methanol however will resist freezing down to approximately -98°C (-143°F). At these temperatures, the viscosity of the fluid can allow dual liquid seals to function acceptably. The seals and seal supports systems must however be designed to operate at these temperatures and prevent the temperature of alcohol barrier or buffer fluid from becoming too high. Historically, methanol was used for the applications. However, propanol provides a broader range of temperature capability and superior lubricating properties at ambient temperatures.

Crude oil based lubricants are one of the major products produced in any modern refinery. Through various distillation processes, the crude oil is separated into products ranging from light hydrocarbons to asphalt. In the middle of this range are products which range from low viscosity lubricating oils to heavy gas oils. Each product is a mixture of various lengths of hydrocarbons chains and their specific composition gives the oil its unique properties. Lubricating oils are used on virtually every piece of rotating or reciprocating equipment in industry.

Because there is a broad scope of equipment with a wide variety of lubrication needs, there is a correspondingly wide range of lubricating oils on the market. While all of these provide lubrication, their specific formulation, viscosity and additive packages are often tailored for the equipment and the operating conditions for a specific piece of equipment. In general, these modifiers negatively impact the unique lubricating requirements of mechanical seals.

The increased use of dual mechanical seals has created a significant demand for specialty barrier and buffer fluids. These fluids have excellent lubricating properties, are available in a range of viscosities, and are free from additives which can negatively impact seal performance. These fluids are marketed from major refineries, lubrication manufacturers, and seal OEMs. In some cases, these fluids are used primarily within the manufacturer’s facilities (in the case of refineries). In other cases, certain formulations have seen wide acceptance across the seal industry. Most successful specialty barrier and buffer fluid are based on synthetically created lubricants. These are commonly based on polyalphaolefin (PAO) synthetics.

These fluids have a higher viscosity index than refined products. They have excellent chemical compatibility with most processes. They have good oxidation resistance and thermal breakdown properties. In addition, they can be formulated so they are safe for applications requiring FDA compliance.

Specialty barrier fluids are available with a wide range of viscosities that allow these fluids to be used over a broad range of operating temperatures. In high temperature applications, some end users will use very high viscosity barrier fluids since the viscosity will be correct while the pump is in operation and the barrier fluid is at operating temperatures. The end user must ensure that the fluid is maintained at a temperature where the barrier fluid viscosity does not become too high or solidify during stand-by conditions. High viscosity barrier fluids also require that the seal use a hard vs. hard face combination (e.g. SiC vs. SiC) to prevent blistering of a carbon face.

The high cost of the base stock and purity of the product make specialty barrier fluids more expensive than traditional industrial lubricants. This cost may make end users reluctant to use this class of fluids especially if they have large Plan 54 systems or a large number of dual seals systems to support in the plant. The high cost however is largely offset by the improved operating performance of the fluids in seal applications. The superior fluid properties of specialty barrier fluids may allow for standardization of a single fluid in most applications in a plant. Finally the stability of the fluid may allow for longer run times between fluid changes resulting in reduced maintenance costs.