Originally published in the December 2012 issue of Pumps & Systems.
As end users face increasingly restrictive leakage and safety regulations, a growing number are turning to multiple seal arrangements. Multiple sealing arrangements require a liquid or gas buffer or barrier fluid to operate, introducing a new factor that end users must monitor.
Below are some best practices for liquid buffer and barrier fluids. By selecting an appropriate fluid and following proper maintenance procedures, end users can promote system reliability and extend their systems’ operating lives.
Buffer Versus Barrier Fluids
In addition to process fluid, all multiple seals use an external fluid. Depending on the sealing arrangement, this external fluid is called buffer fluid or barrier fluid. American Petroleum Institute (API) Standard 682 specifies that unpressurized dual seals, also known as traditional tandem seal arrangements, use buffer fluid. Pressurized dual seals, on the other hand, use barrier fluid, which isolates the pump process liquid from the rest of the system.
Ideal Fluid Characteristics
When choosing appropriate buffer or barrier fluids, end users must consider many factors. An ideal buffer or barrier fluid:
- Is safe to use, handle and store
- Is not a volatile organic compound, volatile hazardous air pollutant or other regulated compound
- Is not flammable
- Lubricates well
- Transfers heat well
- Is compatible with the process fluid and sealing materials
- Flows well at low temperatures
- Remains stable at ambient temperatures
- Does not foam in a pressurized environment
- Has low solubility levels for gases
- Is inexpensive
When selecting a fluid, end users should weigh the pros and cons of their applications. Some common barrier and buffer fluids, which have benefits and potential risks, are:
- Glycol solutions — These, which usually contain 50 percent ethylene glycol and 50 percent water, are the simplest and most common barrier or buffer fluids. Because inhibitors can come out of the solution and damage the seal faces, end users should use uninhibited glycol in these solutions instead of glycol with inhibitors, such as antifreeze. Since some areas restrict the use of ethylene glycol, end users may need to use propylene glycol instead.
- Alcohol — This is best-suited for low-temperature services.
- Kerosene or diesel fuel — Either works well with seal faces but is flammable.
- Petroleum-based hydraulic, conventional gear and bearing lubricating oils — Because they are widely available, they are popular choices. However, the viscosity of these oils can cause carbon seal face blistering, particularly with oils that are Grade 32 or higher on the International Organization for Standardization (ISO) scale.
- Synthetic oils specifically formulated for barrier or buffer use — These have grown increasingly popular during the last decade. These synthetics are typically polyalphaolefin-based, and they range between ISO Grade 5 and Grade 20 (approximately). Lower-viscosity synthetics are the most popular option among end users. Higher-viscosity synthetics, which are more expensive, are the best choice for applications with high temperatures or low shaft speeds.
- Heat transfer fluids — Although useful in extremely high-temperature services, these fluids can pose problems because they tend to decompose and form coke, or hard carbon formations.
Maintaining Buffer and Barrier Fluids
After choosing the correct fluid for their system, end users must maintain the fluid and the sealing environment properly to ensure peak performance. Checking the fluid each month for changes in pH, color, viscosity, consistency and the presence of solids is a good way to help promote system reliability.
While users should change their barrier and buffer fluids regularly, service life can vary dramatically. In general, fluids operating at high temperatures need changing more frequently than those at lower temperatures.
According to API 682, the allowable temperature rise is 15° F for systems with buffer or barrier fluids that are water-based, diesel or kerosene, and 30° F for systems that use mineral or synthetic oil as buffer or barrier fluids. For example, a system using oil could have an average reservoir temperature of 130° F with an outlet temperature of 115° F and an inlet temperature of 145° F.
When monitoring the decomposition of buffer or barrier fluids, end users should consider this rule of thumb for chemical reactions — the rate of reaction doubles for every 18 F rise in temperature. For example, if a barrier fluid needs changing every six months at an average reservoir temperature of 130° F, the same fluid would need to be changed every three months if the reservoir temperature is 148° F. This simple guideline can be useful for evaluating heat transfer options during system design.
In addition to temperature considerations, buffer fluids can become contaminated by the process liquid and may require more frequent changing than barrier fluids.
Other Reliability Considerations
While selecting the proper buffer and barrier fluid is an important first step toward sealing system reliability, the following considerations also have a big effect on performance:
- Appropriate piping — Systems should include short runs with minimal bends and fittings.
- Appropriate sizing — End users should use pipe or tubing that is no smaller than half an inch in diameter. Three-quarters of an inch is preferable.
- Reservoir selection — Volume, gas entrainment and cooling are all critical factors in choosing the right reservoir.
- Alarm systems — End users should consider the systems’ low and high levels, pressures, and temperatures.
- Venting — Inadequate venting causes a large percentage of sealing system issues. Without proper venting, the pumping ring will not circulate fluid.
API 682 provides useful guidelines and default selections for standardized dual seal systems — including Piping Plans 52, 53A/B/C and 54. By combining the API guidelines with the best practices for buffer and barrier fluids described in this article, end users can keep their systems operating smoothly.