Increasingly restrictive regulations on leakage and safety have resulted in an increase in the number of multiple seal arrangements. All wet multiple seals use an external fluid in addition to the process fluid that is to be sealed. The terms ‘buffer fluid’ and ‘barrier fluid’ are used to describe these fluids. As defined in API 682, a buffer fluid is used in unpressurized dual seals. A barrier fluid is used in pressurized dual seals to isolate the pump process liquid from the environment. Gases may be used as buffer or barrier fluids depending on the design of the mechanical seals. The following information focuses on liquid buffer or barrier fluids.

Selecting a Buffer/Barrier Fluid

Several critical properties should be considered when selecting a buffer or barrier fluid. An ideal buffer/barrier fluid would have the following properties:

• Safe to use, handle, store
  • Not a VOC, VHAP or other regulated compound
  • Non-flammable
• Good lubricity
• Good heat transfer properties
• Compatible with process fluid
• Compatible with seal materials
• Does not form deposit (“plate out”) on faces
• Good flow qualities at very low temperatures
• Remains a stable liquid at ambient temperatures
• Non-foaming when pressurized
• Low solubility of gas

Safety

A good buffer or barrier fluid should not present any potential danger whether equipment is running or stationary. Safety should be a top priority when selecting a fluid. It should not be a listed VOC (Volatile Organic Compound) or VHAP (Volatile Hazardous Air Pollutants).  It should not be flammable in the considered application.

A process temperature at or above the boiling point of the buffer/barrier fluid would cause the formation of vapor on and around the sealing faces on the atmospheric side. It would not only promote shortened seal life or catastrophic failure, but formed vapor could be a fire risk. The buffer fluid frequently operates near atmospheric pressure and can potentially reach the same temperature as the pump it serves. Therefore the atmospheric boiling point must be considered and should be at least 50°F (28°C) above the process temperature.

Flash point and fire point must be at least 20°F (28°C) above process service temperature to avoid any vapor flammability risks. Vapor pressure and volatility should be checked at ambient temperature and pressure.

Refer to the MSDS sheet for proper handling and storage.

Viscosity

Viscosity is one of the most important properties of a good buffer or barrier fluid and is representative of lubricity. The fluid should be viscous enough to separate surfaces and prevent wear without restricting seal movement or heat transfer. High viscosity can also cause carbon blistering. In general, current guidelines for buffer/barrier fluid viscosity are to use lower viscosities than in the past. The following guidelines are helpful:

• The ISO Grade of an oil is the viscosity in cSt at 104°F (40 °C).
• The most desirable viscosity is between 2 and 10 cSt at the buffer/barrier fluid temperature.
• Low speed seals require higher viscosity than high speed seals.
• Viscosity should be checked over the entire operating temperature range including startup conditions.
• The viscosity at minimum temperature should be less than 500 cSt (2310 SUS).
• At startup conditions, viscosity should not exceed 68 cSt (350 SUS) to avoid carbon
• The lower viscosity limit is 1 cSt (31 SUS).
• Minimum temperature should be at least 5°F (2.8°C) above pour point, and of course above freezing A heater may be necessary to maintain the proper temperature and eliminate cold starts.
• If the fluid is being exposed to a wide temperature range, its viscosity index should be maximized to ensure a stable viscosity.

As an example of the change in viscosity with temperature, an ISO Grade 32 oil with a nominal viscosity of 32 cSt at 104°F (40 °C) will have a viscosity of 100 cSt at 68 °F (20 °C) and 250 cSt at 32°F (0 °C).  This oil might be satisfactory when warm but cause blistering on startups on a cold day.

Heat transfer

The buffer or barrier fluid should be a good heat transfer fluid. The physical properties of a fluid that reflect this ability are thermal conductivity and specific heat. The higher the process temperature, the higher these values should be. Water has very good heat transfer capability. The specific gravity should be at least at 0.7 at operating temperature. A higher specific gravity decreases the required flow rate and allows better heat removal.

Compatibility

The buffer or barrier fluid must be compatible not only with the metallurgy, elastomers and other materials of the sealing system but also with the process fluid.

Lubricants are generally non-corrosive to hardware and faces so attention is focused on the elastomeric parts of the seal that are more susceptible to chemical reaction.  In particular, the compatibility of synthetic rubber with an oil is dependent on the value of the aniline point of the oil. A low aniline point causes swelling and softening of the elastomer which may result in extrusion. On the other hand, if the aniline point of the oil is too high, the elastomer may shrink and harden.

The fluid should also be highly compatible with the process fluid being sealed. This compatibility is desired whether a buffer or barrier fluid is being considered. Situations that tend to cause any reaction are to be avoided.  The formation of gases, particles, high viscosity liquids or vapors as a consequence would disturb the fluid flow, plate the seal faces or cause wear and leakage.  Each fluid must be individually considered based on its chemical compatibility with the process stream.

Some buffer/barrier fluids are relatively clear in color. Sometimes the supplier or end user adds a dye to make the fluid easier to see in reservoir sight glasses. The dye may change the properties or compatibility of the buffer/barrier fluid.

Deposits

Conventional lubricating oils contain various additives according to the intended use of the oil. For example, additives may be used to suppress H₂S, resist corrosion, prevent foaming, reduce wear, etc.  Some of these additives are intended to deposit, or “plate out”, on the surfaces being lubricated.  In the case of mechanical seals, such deposits increase seal leakage and are not desirable.  In particular, oils labeled as “turbine oils”, “gear oils” and “EP” (extreme pressure) creates problems for mechanical seal faces and are not recommended.

Foaming

In Plan 53A pressurized systems, the barrier fluid is pressurized using an inert gas, typically nitrogen, in direct contact with the barrier fluid.  In this case, problems can occur when the gas is absorbed into the barrier fluid. As pressure is relieved or temperature rises, gas may be released from the fluid. This release of gas can cause the pumping to become vapor locked or cause foaming, resulting in loss of lubrication, heat transfer, and circulation.

Buffer fluids (Plan 52) should also have low gas solubility and a low vapor pressure.

Fluid stability

With stable buffer/barrier fluids, maintenance intervals can be increased.  Requirements for fluid change intervals can vary significantly and are beyond the scope of this document; however, monthly monitoring for changes in pH, color, viscosity, consistency and presence of solids is suggested.

Fluids exposed to oxygen must resist oxidation at operating and static conditions. The oxidation of the fluid causes the formation of acids and carbonized by-products. This results in carbon deposit on the faces (coking), viscosity change, and loss of sealing and heat transfer properties. The oxidation resistance of a fluid is indicated by its total acid number. Synthetic oils are more susceptible to acid formation than hydrocarbons.

Unpressurized buffer fluids may lose volatile materials, causing an adverse effect on their original performance characteristics. Highly volatile fluids should not be used. Fluids with low vapor pressure are essential to keep the volume of the lubricant constant.

Intuitively, buffer/barrier fluids operated at high temperatures should be changed more regularly than those operated at lower temperatures. API 682 guidelines for allowable temperature rise in buffer/barrier fluid systems are 15°F (8.3°C) for water based solutions and diesel/kerosene but 30°F (16.6°C) for mineral or synthetic oils. As an example, a system using oil might have the reservoir at an average temperature of 130°F (54.4°C) with an outlet temperature of 115°F (46°C) and inlet of 145°F (62.8°C). API 682 does not provide guidelines for the average temperature. A simple rule of thumb for chemical reactions is that the rate of reaction doubles for every 18°F (10°C) rise in temperature. This rule of thumb can be applied to the decomposition of buffer/barrier fluids. As an example, suppose that a certain barrier fluid performs well for six months at an average temperature of 130°F (54°C). If the reservoir temperature were 148°F (64°C) then the fluid change interval would probably become three months. This guideline can be useful for evaluating heat transfer options during system design.

 

Barrier/Buffer Fluid Classification

To aid in the selection of buffer and barrier fluids, it is helpful to consider six classifications of fluids:

1. Glycol/Water solutions
2. Alcohols
3. Kerosene and diesel fuels
4. Petroleum based hydraulic and lubricating oils
5. Synthetic hydraulic oils
6. F. Heat transfer fluids

Glycol/water solutions

Water can be a good barrier/buffer fluid. Viscosity is generally around 1 cSt (31 SUS) at modest temperatures; however, the viscosity of water is low at 212°F (100°C), which is also the atmospheric boiling point. Also, in many climates, water may freeze at ambient conditions.  Traditional automotive anti-freeze has not been recommended because of the corrosion inhibitors in it; however, this may be changing as discussed below.

Inhibited ethylene glycol is the most used coolant in the automotive world. The traditional recommendations by the auto industry have been as follows:

• 50/50 mixture of inhibited ethylene glycol and water which provided
  • Best compromise between heat transfer, boiling point and freezing point
  • Retains enough fluidity at low temperatures.
• Disadvantage: toxicity which  led to marketing of propylene glycol which has low toxicity.

There are three types of commonly used inhibited ethylene glycol coolants used in the automotive world:

• Silicate inhibited developed in the 1980s
• Organic Acid Technology (OAT) developed in the ‘90s (does not contain nitrate-, phosphate-, silicate-, borate- or amine inhibitors)
• Hybrid Organic Acid Technology (HOAT) developed in the ‘90s (OAT + nitrates or silicates)

The silicate inhibited coolant and also HOAT are somewhat problematic because of leaving film on the sealing faces. The film is believed to form as the coolant boiled/evaporated between the faces due to upset condition in the cooling system and in turn leaving residue (silicates) behind which eventually led to uneven gaps on the faces and then failure of the seal.  The tendency to leave deposits gets worse as the concentration of glycol in water becomes more than about 60%

The OAT coolant is not as problematic and deposits little or no film on the face. Since 1996, this OAT antifreeze has been used in GM cars and is marketed as “Havoline DEX-COOL”.   Although OAT coolants are not widely used in mechanical seal systems these coolants appear to be good candidates for replacing traditional inhibited ethylene glycol coolants.

High levels of dissolved chlorides, sulfates, magnesium, and calcium in tap water also cause scale / sludge deposits and corrosion therefore distilled or de-ionized water is preferred for mixing with the glycol.

Commercial inhibited glycol coolants use the older silicate inhibitor technology; this is why uninhibited glycol solutions were recommended for buffer/barrier fluids with mechanical seals.

Many distributors mix and private label their own coolants.  There are also companies selling inhibitor packages for either making your own inhibited coolant or for replenishing used coolant.  There are various references and articles about color codes for glycols and antifreeze but none of these are true standards and there are many variations.

For commercial use, reference is often made to Dow coolants:

• Dowtherm SR-1 is inhibited ethylene glycol (pink)
• Dowfrost is inhibited propylene glycol (clear)

The DOW inhibitor apparently is the old style silicates, nitrates, etc.

Uninhibited glycol/water solutions apparently are subject to foaming and an anti-foaming agent is recommended.

Because ethyleneglycolisnow classifiedasaVHAP, it isbeingreplacedbypropylene glycol.

Alcohols

Alcohols can have a high rate of evaporation and frequent refills may be required on a Plan 52 (unpressurized buffer fluid). Check the properties, especially vapor pressure, of the alcohol type that is being considered.

Methanol. Although methanol has been used in the past as a buffer fluid for tandem seals in low temperature services, it is a VHAP and is not recommended. In addition to being toxic, methanol has a low boiling point and low viscosity. It is not a good seal face lubricant.

Propanol. 1-Propanol, or n-propyl alcohol, has replaced methanol as a buffer fluid for low temperature services. However, propanol has an exposure limit of 200 ppm (50 mg/m³) OSHA TWA (Total Weight Average).

Kerosene and diesel fuel

The viscosity of diesel fuels and deodorized kerosenes can provide adequate seal face lubrication through a wide temperature range.  However, low sulfur diesel fuel does not appear to be a good seal face lubricant and is not recommended as a buffer/barrier fluid. Although not a flashing hydrocarbon according to API 682, diesel fuels and kerosenes may be classified as volatile organic compounds (VOC), especially at higher temperatures.

Petroleum based hydraulic and lubricating oils

Lube oils. Although turbine oils have been used extensively in the past, experience is that the anti-wear/oxidation resistant additives plate out on the seal faces. The lower viscosity grade (less than ISO Grade 32) provide better performance. Paraffinic based oils seem to be better than naphthenic oils. Blistering of carbon seal faces is common when lube oils are used as buffer/barrier fluids, especially ISO grades 32 and higher. Experience has shown that synthetic oils perform better than conventional turbine oils; this may be partially due to the (generally) lower viscosity of the synthetics. Automatic transmission fluid. Automatic transmission fluid has the proper range of viscosities but is not recommended. Actual experience has generally been that automatic transmission fluid is a poor barrier fluid; the assumption is that the various additives are the problem.

Synthetic Based Hydraulic and Lubricating Oils

Specific purpose buffer/barrier fluids, usually polyalphaolefin (PAO) based synthetic oils, have been increasingly used in the past decade and becoming more and more popular. These synthetics are approximately ISO Grade 5 to 20. The most popular is the lowest viscosity. The higher viscosity oils are used for higher temperatures and/or lower shaft speeds. Unfortunately, synthetics are the most expensive buffer/barrier fluids.

Heat Transfer Fluids

Heat transfer fluids that have the ability to provide adequate lubrication throughout a wide range of temperatures and pressures can be used as buffer or barrier fluids. Heat transfer fluids encompass a broad spectrum of chemical families (water, steam, inorganic salts, certain liquefied metals, organic class fluids …).  This guide addresses the use of organic class heat transfer fluids. They fall into two categories:

• Petroleum based fluids (called ‘hot oils’)
• Synthetic aromatic fluids, such as the various Dowtherm fluids. These offer higher thermal stability, broader working temperatures range and are more effective than petroleum ‘hot oils’.

Dowtherm.  Dowtherm is a family of synthetic heat transfer fluids manufactured by Dow Chemical Company.  Currently, Dowtherm types are A, G, J, MX, Q, T, and RP.  Dowtherm RP has been used extensively in high temperature tests with good results.
A number of other heat transfer fluids such as Therminol, Multitherm, Paratherm, etc., are commonly used as buffer/barrier fluids. Selection should be based on temperature limits for the selected Plan 52, 53, or 54 system based upon experience with the heat transfer fluid.

 

Temperature Guidelines for Buffer/Barrier Fluid Classes

The table below shows general temperature guidelines for Buffer/Barrier Fluid Classes.  Use of a fluid from these  families does not necessarily ensure successful operation of a sealing system.

 

Guidelines for Buffer/Barrier Fluids

Family	Practical Limit (Fluid Temperature)
	Minimum		Maximum
	°F	°C	°F	°C
Glycols	-20	-29	185	85
Alcohols	-191	-124	157	70
Kerosene/Diesel	0	-18	180	82
Lube Oils	-20	-29	300	150
Synthetic Oils	-25	-32	480	250
Heat Transfer Fluids	0	-18	650	340

Buffer and Barrier Fluid Service Life

Although it is generally accepted that buffer/barrier fluid systems for mechanical seals should be drained and the fluid completely replaced at certain intervals, there have been no definitive guidelines established for recommending that interval.  The following recommendations are starting points and are not absolute or all-inclusive. Monitoring the fluid for degradation and contamination is strongly recommended and appropriate adjustments should be made to the interval between fluid changes.

The advantage of using this method as an initial estimate of service life is that it provides a means for optimizing operating costs based on buffer/barrier fluid type, pump and system temperatures, seal arrangement and sealing system. For example, system temperature is determined by the size and design of heat transfer equipment such as heat exchangers and reservoirs.

Intuitively, the buffer/barrier fluid used in high temperature systems should be changed more regularly than the fluid in lower temperature systems.

 

Maximum operating temperature

All buffer/barrier fluid systems can be considered to have the bulk of the fluid at some average operating temperature.   Typically, this bulk fluid temperature is less than 150 F but considerably higher average temperatures are sometimes permitted or even used as a design point.  One advantage of high bulk fluid temperature is that heat soak from the pump is reduced.  Another advantage is that the efficiency of heat transfer to cooling water is increased.  As noted previously, a disadvantage is that the buffer/barrier fluid is expected to degrade more rapidly at high temperatures. Another disadvantage is that many end users and local regulations require personnel protection near high temperature reservoirs and piping.

API 682 4^th edition does not limit the bulk fluid operating temperature for buffer or barrier systems.  Therefore the maximum allowable bulk fluid temperature remains an operating or design detail that might vary significantly.  However, in absence of other information or requirements, a good guideline for the maximum bulk fluid temperature is 150 F.

Maximum allowable temperature rise

API 682 guidelines for allowable temperature rise in buffer/barrier fluid systems are:  15 F for water based solutions; 15 F for diesel/kerosene; 30 F for mineral or synthetic oils.

As an example, a Plan 53A system using a light synthetic oil might have the reservoir at an average temperature of 150 F with an outlet temperature of 135 F and an inlet temperature of 165 F.

Maximum interval between fluid changes

Buffer/barrier fluids should be drained, the system flushed and refilled with clean fluid at intervals not to exceed six calendar months of continuous or standby operation regardless of seal design, operating conditions or details of the piping plan.  The recommended interval may be less than six calendar months based on the bulk fluid temperature, see below.

Recommended interval between fluid changes based on temperature

The recommended time between fluid changes should be based on the average operating temperature of the system as compared to the base case temperature.

A simple rule of thumb for chemical reactions is that the rate of reaction doubles for every 10 C (18 F) rise in temperature.  This rule of thumb is often applied to lubrication systems and, in the absence of more specific data, will be applied to buffer/barrier fluid systems. For lack of data, the base case temperature for all fluids is assumed to be 150 F and the base case change interval is assumed to be six months.

As an example, suppose that mineral oil is used as a barrier fluid. The base case temperature is 150 F and the base interval is six months.  Based on the rate of reaction doubling for every 10 C (18 F) rise in temperature, if the reservoir temperature is 168 F then the recommended fluid change interval is reduced to three months.