Improving performance for pumps handling high viscosity fluids
McWilliams, Carter C
The label “high viscosity fluids” can be applied to an incredible range of liquids. Providing a seal system to fit this varying range of fluids has posed a dilemma to plant maintenance engineers. Because viscosity directly relates to temperature, a single pump in a batch process can encounter a broad spectrum of viscosity. Improper application of a sealing system can lead to unacceptable seal life or a packing which leaks valuable product to the atmosphere. In a niche where mechanical seals provide one solution, and mechanical packing another, mechanical lip seals satisfy a middle ground that has previously been scarcely found.
This paper examines the differences between three sealing solutions on typical chemical process pumps. The discussion will include design concepts that demonstrate the theories of mechanical lip seals manifested in improved properties present at sealing surface areas. Use of case studies as well as test data illustrates the performance of polytetrafluoroethylene (PTFE) in high viscosity situations. The investigation of temperature gradients, leakage rates, and cost profiles is aimed at finding the optimal sealing solution.
Lip Seal; Mechanical Seal; Mechanical Packing; Polytetrafluoroethylene; Viscosity; Batch Process; Crystallization; Thermal Distortion; Dry Running; Conduction; Newtonian
The performance of rotary pumps using PTFE lip seals is compared to mechanical face seals in high viscosity fluid service. Full description of the two sealing systems are mentioned with graphic illustrations showing performance history and test results. The relationship between fluid viscosity and temperature rise at sealing surfaces in each sealing system is discussed. Cost analysis is examined, by paralleling sealing solutions for two separate options geared specifically for high viscosity fluids in batch processes. The design theory, followed by direct application of circumferential lip seal technology is fully discussed and analyzed for rotary pump services that handle viscous fluids.
HIGH VISCOSITY LIQUID
Viscosity is defined as a fluids resistance to flow. Most people know from experience what viscosity means and associate it with the ability of a fluid to flow freely. For instance, heavy oil flows very slowly from a can. Light oil flows out quickly. It is an intriguing fact that flow of a viscous fluid from the bottom of a container is a difficult program for which no analytical solution exists at present. A more fundamental approach to viscosity shows that this property of a fluid relates applied stress to the resulting strain rate. In the United States the most widely used instrument for measuring viscosity is the Saybolt Universal Viscosimeter. This instrument, adapted by the American Society for Testing and Material (ASTM), measures the time required for a given quality of fluid to flow through a capillary tube in terms of Seconds Saybolt Universal (ssu). For high viscosity, a Saybolt Furol Viscosimeter is used, generating a result in terms of Second Saybolt Furol (ssf). Because the viscosity of a liquid is a measure of the tendency to resist an external shearing force, a fluid that requires high shear stress has a high viscosity (4). Figure I shows a viscosity comparison chart for two common viscous fluids, corn syrup and sugar syrup.
Compared with other non-Newtonian fluids, such as dilatent, plastic, pseudo-plastic, thixotropic, and rheopectic, experimentation has shown that rotary pumps are better equipped to handle high viscosity fluids. Most importantly, when handling high viscosity fluids the capacity of a rotary pump varies directly with relative speed, and is independent of pressure within the operating limits. Pump efficiency is grossly affected by fluid viscosity as illustrated in Fig. 2.
On high viscosity liquids, the reduction of pump speed will reduce frictional head in addition to allowing a greater static head. The approximated maximum viscosity rating for rotary pumps is 2,000,000 ssu, compared with centrifugal pumps at 3,000 ssu, and reciprocating pumps at 5,000 ssu (4).
END FACE MECHANICAL SEALS
End face mechanical seals, also known as shaft seals are available in various configurations. These arrangements include single coil spring mechanical face seals, which employ a single spring to drive the sealing surface; multiple coil springs, which use more than one coil spring; bellows seals, which use metal, elastomer or PTFE bellows in place of springs. Single or multiple wave springs are also use in certain applications. See Fig. 3.
Selecting a mechanical end face seal for a high viscosity fluid requires an intensive evaluation of all available process conditions, including information on single vs. double seal, inboard vs. outboard mounting, environmental control devices, balance ratio, and secondary components. For pumps handling high viscosity fluids, a balanced mechanical face seal is recommended. Fluids with viscosity ranging from 100 ssu to 50,000 ssu require combinations of mating seal faces. For instance, viscosity above 15,000 ssu requires a hard face combination to create a seal at the interface. See Fig. 4.
Due to the high shear force created at the seal face as a result of the fluid viscosity, frictional heat will be generated at the face. This heat, combined with fluid temperature, can result in an extremely high seal face temperature. If the heat is not dissipated thermal distortion of the seal face can occur, potentially resulting in seal leakage and failure of the sealing system. Figure 5 illustrates seal face thermal distortion contrasted by a normal operating geometry. The primary ring is flexed to the exaggerated position shown when excessive heat at the interface is coupled with process pressure.
EFFECT OF VISCOSITY ON PUMPS AND MECHANICAL SEALS
The viscosity of a pumped fluid generally affects pump ratings as follows: The net positive inlet pressure required (NPIPR) increases with increasing viscosity. Moreover, the required pump input power increases with increasing viscosity. Another factor that effects pump performance is the increase in power loss at the seal face due to the high torque required to rotate the seal face, as illustrated in Fig. 6. Here, a 3.5 inch diameter seal will have a power loss of 4 hp when pumping heavy hydrocarbons at 1800 rpm and a pressure of 200 psig. The same seal has a power loss of 2.75 hp in water.
The maximum allowable pump speed decreases with increasing velocity, as shown in Fig. 7. Pump slip also decreases with increasing velocity. Since energy put into a fluid to overcome resistance to shear causes a finite temperature rise of the fluid, the above general affect on pump rating may change with non-Newtonian fluids as the viscosity may change within the pump due to shearing forces (1).
As far as mating materials are concerned, lubricity and viscosity, which decrease with rising temperature, must be determined. A high viscosity fluid (above 50,000 ssu,) has to maintain flow characteristics at certain temperatures. Mating faces selected for such services are usually hard faces. These hard faces generate significant heat at the sealing interface. Overheating will adversely impact secondary elastomeric sealing components.
CIRCUMFERENTIAL RADIAL LIP SEALS FOR ROTARY SHAFTS
Circumferential lip seals represent a type of seal configuration for rotary shafts by using components that provide a highly flexible leg. The lips are formed in a radial direction against the surface of the rotating shaft. Its advantages are low cost, excellent performance characteristics, persistently durable materials, simple design, high degree of reliability at minimal nearly predictable leakage rate, and easy replacement. Circumferential lip seals are suited for a wide variety of sealing requirements. Such seals can be operated with almost all of the industrially used oils, hydraulic fluids, and chemicals from atmospheric to medium pressure across a considerable range of temperature. Lip seals are less sensitive to moderate shaft misalignment, dynamic shaft runout, or even variations in shaft speed. The sealing function is a result of interference fit between a flexible sealing element (rubber, PTFE, synthetic rubber or polyimide) and the rotating shaft. Fluid retention requires a precise amount of lip contact pressure in conjunction with a minimum interference fit. Proper seal function and long seal life expectancy are obtained when contact pressure controls the thickness of lubricating film between the lip and the shaft. Experience shows that the best film thickness is found to be around 0.0001 inch. Leakage will occur at greater film thickness. If the film thickness becomes thinner than 0.0001 inch, the lip is subject to wear caused by increased friction. An adequate film thickness is critical. When the contact pressure of the lip increases, the film thickness decreases. With rising shaft speed the fluid temperature increases and the viscosity decreases, in turn, causing the film to become thinner. As film thickness decreases, lubricant weakens and friction increases causing the temperature to rise at the interface between the seal lip and the shaft (1).
PTFE RADIAL LIP SEALS FOR ROTARY SHAFTS
PTFE radial lip seals generally incorporate a uniformly thin element called a waffle. The purpose of this thin cross section is to compensate for the high flexural modulus of PTFE specifically on an application in which severe shaft run out is encountered. The thin section also minimizes the degree of thermal expansion and compressive “creep” and their effect on maintaining a controlled contact pattern on the shaft. See Fig. 8.
SEALING SOLUTION DILEMAS
As the numerous rotating surfaces in a mechanical seal turn with the shaft, each surface must shear the fluid present in the seal chamber. The higher the shear stress of the fluid, the more heat that is generated when components move through the fluid or agitate the fluid. This translates to increased heat at the sealing interface as well as in the seal chamber. When heat is built up around the sealing surface of a mechanical seal, several potential downfalls are introduced. Seal faces tend to blister under extreme heat, introducing inconsistencies on the sealing surfaces. Once any inconsistency is found on the faces, wear rates increase exponentially, causing rapid failure. Seal faces can also see inconsistencies when product seeps between sealing surfaces. In batch processes, most often seen by positive displacement pumps, seepage to the faces with high viscosity fluids often solidifies, or crystallizes, between batches. This introduces enough product to separate the faces when shear force creates heat and an end result of fluid expansion. Another potential downfall is presented when crystallization on the faces creates a slight physical bond with the face. Crystallization changes the properties of a fluid to a solid, and in turn, creates an abrasive medium bonded to the sealing surface. Upon pump start-up, this can remove small chips from the faces leading to increased wear rates. In the past, these problems have been addressed using hard vs. hard, seal face combinations.
Mechanical Packing has provided much more versatility based on the properties of materials used in manufacturing. Packing can be manufactured using a much wider variety of materials than mechanical seals. Most notably, polytetrafluoroethylene (PTFE,) brings several advantages to the sealing area. PTFE, commonly known to be very lubricious, presents a non-stick surface. The lubricity of the material, among other properties, will not allow crystallized product to create a physical bond. PTFE is also completely chemically inert, ruling out the potential for a chemical bond with the product. Unfortunately, packing is designed to leak in order to keep the sealing surface cool. Product loss and packing usually go hand in hand.
LIP SEAL BACKGROUND
Lip seals present an excellent alternative to the aforementioned solutions. The design of a lip seal yields a pressure actuated, membrane-like body, that can be manufactured from PTFE. Pressure differential between the atmospheric and process sides of the lip seal generate a mechanical force, creating an effective seal. When operating within given viscosity ranges, leakage rates should be comparable to end face mechanical seals. The viscosity range of fluids for lip seals in dynamic applications is roughly 30-800,000 ssu. As viscosity increases, the seal-ability generally increases, based on the premise that the functionality of the sealing surface is not 100% air tight. At the microscopic level, the surface finish of the lip seal has void spaces, that consequently allow small particle size gases and liquids to flow at lower actuating pressures. When operating in the proper viscosity range, fluids can flow only one direction (membrane-like,) from back to front of a lip seal, due to flexibility of the lip seal material. Flushing of the sealing surfaces can take place during shut-down, or while the pump is in service.
Several different materials have been used for lip seals, including elastomers and PTFE. Properties of PTFE, most importantly the coefficient of friction, yield greater versatility than elastomers due to the heat generated at the sealing surface. The root of PTFE is a Carbon-Fluorine bond. Because this bond is extremely strong, the potential for chemical attack is eliminated, creating an inert, tough, non-flammable, lubricious compound after polymerization. When using filled PTFE (see Fig. 11 for test results using three different filler materials), properties can be achieved that will not only enhance the lubricious surface, but also increase heat transfer from the sealing area. These properties also allow dry-running situations to persist without generating detrimental heat at the sealing surface. Unlike a mechanical seal, there are no rotating parts in lip seals. The shearing forces from high viscosity fluids are kept to a minimum, in turn, yielding the lowest possible heat in the seal chamber.
As the length of the heat transfer area increases, so does the amount of heat generated. The larger the temperature gradient between T1, and T2, the greater the heat transfer rate will be. Therefore, as the length increases, the ability to remove heat is less efficient, and in turn, the sealing surface area remains hotter. Heat removal is then based on the distance through the seal chamber to a heat sink coupled with the properties of the fluid present. Figure 10 shows test data using varying sealing areas, and their respective seal chamber temperatures. Using a thermocouple in the seal chamber to quantify temperature gradients, all other variables including pump rpm and suction pressure were held constant.
where b reflects lip seal concentricity; AP is the pressure differential across the lip seal; P^sub i^ is the pressure due to seal and sleeve interference and V^sub m^ is the velocity at the sealing surface diameter. As pressure and velocity increase, frictional forces create increasing heat at the sealing surface (5). It is important to use a lip seal material that has a low coefficient of friction (see Fig. 12).
A series of tests were performed on a lip seal arrangement aimed at verifying design theory as well as filler material selection for PTFE lip seals. The test rig consisted of the following:
Goulds 3196 pump Variable Frequency Drive (speeds from 400-1750 rpm) 1.75 inch shaft 2.125 sleeve OD Hard coated sleeve (70 Rockwell) Process fluid used for all tests: H^sub 2^O, 70 deg F
In order to test the heat transfer capabilities of several materials, tests were performed at 400 rpm and 100 psi stuffing box pressure (pressure on the lip seals). Figure 11 shows the seal chamber temperatures of three materials used as fillers in the PTFE.
Wear rate of the lip seal surface is also dependent on material. Due to pressure-velocity effects, the coefficient of friction impacts seal life. Figure 12 shows the effect of filler material on leakage.
Testing of the lip seal can be summarized by the operating limitations curve, Fig. 13. During the gamut of experiments, pressure was used as a variable while holding all other operating parameters constant. Likewise, rpm was varied in order to understand the limitations of fpm. Figure 13 also focuses on parameter combinations where dry-running was achieved successfully.
The performance of a pump can be evaluated by combining the result of several different quantifiable indicators. Mean time between failures and the associated cost required to keep a pump running usually constitute the crux of decision making. Heat generation at the seal surfaces in addition to generation in the seal chamber both have a profound impact on the efficiency of the seal, and in turn, the performance of pumps handling high viscosity fluids. PTFE lip seals not only generate less heat but also have fewer pitfalls associated with high heat. Conversely, mechanical seals can experience blistering, heat check, and thermal distortion in adverse situations. PTFE lip seals also sidestep potential mechanical seal failures caused by product seepage or physical bonding to sealing surfaces. PTFE lip seals, when applied correctly, have negligible product loss and are maintenance free. By correctly applying seals to pumps in high viscosity applications, operating costs as well as maintenance hassle can be minimized.
Presented at the 56th Annual Meeting in Orlando, Florida
May 20-24, 2001
Final manuscript approved March 29, 2001 Review led by Harold Greiner
(1) Buchler, H. H., Industrial Sealing Technology, John Wiley and Son Inc., pp 122-187 and pp 235-272, (1979).
(2) White, F. M., Viscous Fluid Flow, New York, pp 6-84, (1991).
(3) American National Standard for Rotary Pumps, The Hydraulic Institute ANSI/HI, pp 3.1-3.5, (1994).
(4) Viking Pump Inc., “Engineering Data,” Rotary Pump Fundamentals, p 510.23, (1996).
(5) Johnson, R. L. and Schoenherr, K., “Seal Wear,” Wear Control Handbook, Peterson, M. B. and Winer, W. O., Eds., ASME, New York, pp 737-753, (1980).
(6) Perry, R. H., Chemical Engineers Handbook, 5th Ed., McGraw-Hill Book Co., New York, pp 10, 6-10, 50, (1973). E
CARTER C. McWILLIAMS (Member, STLE) and ALFRED W. COOPER (Member, STLE)
John Crane, Inc.
Morton Grove, Illinois 60053-0805
Copyright Society of Tribologists and Lubrication Engineers Aug 2001
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