SEALING SENSE
Buffer & Barrier Fluids What are the differences between using water or oil as a buffer or barrier fluid? By FSA member Mark Savage
A
s operators o pumping equipment become more ocused on the saety, reliability and environmental impact resulting rom shaf seal leakage, dual mechanical seals have become more prevalent in the industry. A dual mechanical seal offers a second (outer) seal to contain the pumped fluid by creating a cavity or chamber between the inner and outer seal that can be filled with a fluid. When this fluid is unpressurized, it orms a buffer between the pumped fluid and atmosphere and is commonly reerred to as a buffer fluid. When pressurized, it orms a barrier between the pumped fluid and atmosphere and is known as a barrier fluid. Although mechanical seal designs are available in configurations that use either a liquid or a gas as a barrier fluid, the ollowing discussion ocuses on liquid buffer and barrier fluids only. In addition to separating the pumped fluid rom
Figure 1. Simplified schematic of a Plan 52 system
the atmosphere, liquid buffer and barrier fluids lubricate the mechanical seal and transport rictional heat and absorbed heat rom the mechanical seal to a heat exchanger. Tis controls the fluid’s temperature and lubricating properties. BUFFER & BARRIER FLUID SUPPLY
Buffer/barrier fluid can be stored, monitored and delivered using many methods. Each is identified by a piping plan number that describes the minimum requirements o each system. Te most commonly reerenced piping plan originates rom the American Petroleum Institute’s standard API 682. Unpressurized Systems
A Plan 52 system (see Figure 1) provides a reservoir that stores the buffer fluid. Supply and return lines are connected
Figure 2. Simplified schematic of a Plan 53A system
to the mechanical seal and circulation o the buffer fluid is achieved by an internal circulating device (pumping ring) within the mechanical seal. Te vapor space above the buffer fluid in the reservoir is vented to atmospheric pressure typically via a flare or vapor recovery system. Te reservoir can be instrumented to measure the liquid level and pressure in the reservoir. Ports are fitted to the reservoir to acilitate maintenance activities—such as inspection and cleaning or refilling and draining the buffer fluid. Cooling is accomplished using an internal heat exchanger.
pumps pressurize and circulate the fluid. In open-loop systems, a compatible process stream is used as the barrier fluid and is circulated through the mechanical seal and returned to another point downstream in the process. IMPORTANT CHARACTERISTICS OF A BUFFER/BARRIER FLUID
Several critical properties o a buffer or barrier fluid must be considered when making a selection. An ideal buffer or barrier fluid will have the ollowing properties: Pressurized Systems • Safe to use, handle and store Pressurized dual seal systems contain the same essential com- • Compatible with seal materials ponents as an unpressurized system. However, they also con- • Not a volatile organic compound, volatile hazardous tain a way to pressurize the barrier fluid. Te ollowing plans air pollutant or other regulated compound may be used or pressurized dual seal systems: • Good flow qualities at operational temperatures • Plan 53A —A pressurized gas blanket in the reservoir (including very low temperature service) pressurizes the fluid. Nitrogen is normally used and • Nonflammable the pressure is controlled via a pressure regulator (see • A stable liquid at ambient temperatures Figure 2). Te barrier fluid Thermal Pump Temperature Family Fluid Name Viscosity Pour Boiling Specific Heat S.G. is in direct contact with the Conductivity Range Point Point pressurized gas. @ 15 C (60 F) @ 15 C (60 F) Min Max @ 15 C 40 C 100 C (60 F) (104 F) (212 F) °C °C kJ/kg·°K W/m·°K °C °C • Plan 53B—Pressure is (°F) (°F) (BTU/lb·°F) (BTU/hr·ft·°F) (°F) (°F) cSt cSt generated as a nitrogen-filled 0 100 4.2 0.59 5 70 Water 1 0.68 0.29 Water and (32) (212) (1.0) (0.34) (40) (160) bladder is compressed by the glycol -33 106 3.3 0.36 -29 77 Propylene solutions addition of barrier fluid into 1.05 2.6 0.7 Glycol (-28) (222) (0.79) (0.21) (-20) (170) the bladder accumulator. -126 97 2.2 0.16 -124 69 n-PropylAlcohols 0.81 1.5 0.45 Te bladder prevents direct Alcohol (-195) (207) (0.53) (0.09) (-191) (157) contact of the pressurized gas -29 149 1.9 0.149 -23 121 K-1 Kerosene 0.83 1 (-20) (300) (0.46) (0.086) (-10) (250) with the barrier fluid. -34 177 1.9 0.149 -29 132 • Plan 53C—A pressure K-2 Kerosene 0.85 1.4 0.6 Kerosene (-30) (350) (0.46) (0.086) (-20) (270) and diesel amplifying piston uses pres-34 177 1.9 0.149 -29 149 fuels D-1 Diesel 0.83 1.4 0.6 sure from within the pump (-30) (350) (0.46) (0.086) (-20) (300) (typically the seal chamber) -59 182 1.9 0.149 -12 82 D-2 Diesel 0.86 2.7 1.0 (-75) (360) (0.46) (0.086) (10) (180) to amplify the barrier pressure -59 104 ~2.1 ~0.14 -29 76 by the ratio of the area on Lube-1 0.86 13.9 5.1 (-75) (219) (~0.5) (~0.08) (-20) (169) each side of the piston. Te -12 168 ~2.1 ~0.14 -7 140 Lube oils Lube-2 0.85 9.5 2.6 barrier fluid is not exposed to (10) (335) (~0.5) (~0.08) (20) (285) any pressurized gas. -50 >149 ~2.1 ~0.14 -7 135 Lube-3 0.89 9.5 2.3 (-58) (>300) (~0.5) (~0.08) (20) (275) • Plan 54—An external system -62 371 2.3 0.154 -32 221 is used to pressurize and Synth-1 0.80 7 1.5 (-80) (700) (0.56) (0.089) (-25) (430) circulate the barrier fluid. 62 371 2.4 0.147 -4 227 Synthetic Synth-2 0.82 22 3.3 A Plan 54 system can be (-80) (700) (0.57) (0.085) (25) (440) lube oils -50 256 2.2 0.137 17 227 broadly classified into two Synth-3 0.96 37.2 5.35 (-58) (493) (0.53) (0.079) (62) (440) groups: closed- and open-4 343 1.5 0.122 13 316 Heat transfer Aromatic-1 1.01 29 4 loop systems. In closed-loop fluids (25) (650) (0.35) (0.071) (55) (600) systems, the barrier fluid is Table 1. Typical buffer and barrier fluid properties stored in a large reservoir and
SEALING SENSE
• • • • • •
Good lubricity Non-foaming when pressurized Good heat transfer properties Low gas solubility Compatible with process fluid Inexpensive
BUFFER/BARRIER FLUID FAMILIES
Liquids that have characteristics suitable or use as buffer and barrier fluids can be broadly classified into the ollowing: • Water and glycol solutions • Petroleum-based hydraulic and lubricating oils • Alcohols • Synthetic hydraulic oils • Kerosene and diesel fuels • Heat transfer fluids
WATER
Water offers several benefits as a buffer/barrier fluid. Its thermal conductivity is about three times greater than oils and its specific heat is about twice that o oils, so it is good at transporting heat away rom a mechanical seal. Water is inexpensive, easy to handle and store, has ew seal material compatibility issues and is nonflammable. It is also compatible with many aqueous pumped solutions. Its viscosity is generally around 1 centistoke at moderate temperatures which offers low resistance to flow in the barrier system. However, the viscosity becomes low at elevated temperatures limiting its effectiveness as a lubricant or the mechanical seal aces. Water is also susceptible to reezing during the winter months. Tis results in a narrow window o service and environment temperatures in which water can be used. OILS
ypical physical properties o fluids in these categories are listed in able 1 with a suggested service temperature range.
Generally, oils can be used in a much wider range o service temperatures. Compared to water, oils offer greater fluid stability at elevated temperatures and are not susceptible to reezing. Tey also provide excellent lubrication o the
Do you need PDH Credit? Visit PumpSystemsMatter.org today UNMATCHABLE EXPERIENCE IN P RIVATE COM PANY TRANSACTIONS
Jordan, Knauff & Company is a knowledgeable and experienced provider of a comprehensive line of investment banking services to the pump, valve and filtration industries (“Flow Control”). Our lines of business include: selling companies, raising debt and equity capital, and assistance on acquisitions. To learn more about Jordan, Knauff & Company, contact any member of our Flow Control team. Access our Flow Control research at www.jordanknauff.com/flowcontrol. G. Cook Jordan, Jr.
David A. Kakareka
Managing Principal
[email protected] 312.254.5901
Associate
[email protected] 312.254.5907
Pump Systems Matter is the pump industry’s educational resource for pumps, pump accessories and pump systems efficiency. Offerings include: — —
For a complete listing of new webinars, in-person courses, & e-Learning courses visit: www.PumpSystemsMatter.org
MEMBER FINRA, SIPC
circle 181 on card or go to psfreeinfo com
circle 182 on card or go to psfreeinfo com
mechanical seal aces and, thereore, have the potential to offer longer seal lie. Few compatibility issues with mechanical seal materials exist. Oils are available in a wide range o types, compositions and viscosities. raditional oils used in the industry include turbine oils and automatic transmission fluids. However, perormance as buffer and barrier fluids has not been as successul as other oils, primarily because o the complex mixture o additives in these fluids. Good perormance can be achieved rom oils with viscosity below that o ISO Grade 32 oils. High viscosities can result in damage to the mechanical seal aces, particularly when carbon is used as a ace material. Paraffinic-based oils also generally perorm better than naphthenic oils while synthetic oils offer even better perormance. Synthetic lubricants specifically developed or use as a buffer/barrier fluid are now available in the marketplace and offer excellent perormance. However, this perormance is achieved at the sacrifice o cost. P&S NEXT MONTH: What are pressure seals, and how do they work? We invite your suggestions for article topics as well as questions on sealing issues so we can better respond to the needs of the industry. Please direct your suggestions and questions to sealingsensequestions@fluidsealing.com.
circle 178 on card or go to psfreeinfo com
The following members of the Mechanical Seal Division sponsored this “Sealing Sense”: Advanced Sealing International (ASI) A.W. Chesterton Co. Daikin America, Inc. Flex-A-Seal, Inc. Flowserve, Flow Solutions Group Hydro-Ergoseal, Inc. Industrias Vago de Mexico SA de CV John Crane Latty International S.A. Morgan AM&T Nippon Pillar Corp. of America Scenic Precise Element Inc. SEPCO - Sealing Equipment Products Co. SGL Technic Polycarbon Division Turmond SpA
circle 188 on card or go to psfreeinfo com
