By Joseph M. Sutula
The overall effects of industrial pollution may be controversial depending upon our personal beliefs, but there is concrete evidence that life on earth can suffer from pollution in air, ocean, or ground water. In light of the increasing emphasis on decreasing leakage of pollutants in the industrial setting, there is a need to become informed about sealing equipment under our control. Because pumps are generally considered to be more expensive than the piping, valves, and related accessories surrounding them, pumps are higher profile when they leak. Seal manufacturers are gearing up to meet the challenge but have thus far focused on centrifugal pump principles because their numbers dominate the market. Positive-displacement (PD) pumps may pose special sealing problems because they have been designed to handle viscous, non-Newtonian, or other problem fluids under a wide range of physical conditions. Many factors must be considered in order to arrive at the proper sealing solution.
Supplying Information To Your Pump Representative
Liquid Name and Synonyms
It is important to give the proper name of the liquid, as well as any synonyms, to your manufacturer's representative. If the name must remain proprietary, then supply at least a family description; for example, acidic or alkaline, starch or salt. Oftentimes furnishing initials alone can be confusing to nonchemists. PVA is interpreted by most to mean polyvinyl alcohol while polyvinyl acetate is generally represented by PVAc.
Time and money can be saved if elastomer and liquid compatibility is known at the outset. Typically, when little is known about the liquid, more expensive components will be recommended for sealing than may be warranted simply as a safety measure. For example, perfluoroelastomers can be applied to liquids pH 2 through 13 at ambient to moderate temperatures and on some heat transfer liquids to 315°C / 600°F. Fluoroelastomers, at approximately one-tenth the cost, can be applied to many acids at moderate temperatures and can handle most heat transfer liquids up to 205°C / 400°F.
Chemical attack on elastomer frequently manifests itself as o-ring swell and/or shrink. O-ring grooves are usually designed with some extra volume built in to compensate for swell, but the physical forces available from excess swell cause the confined o-ring to extrude beyond or break the confining wall. Many flat gaskets contain elastomers as binders which are also subject to modification or failure when they come in contact with the wrong liquid environment.
Newtonian liquid viscosity varies with temperature. In general it will become less viscous as temperature rises and more viscous with decreasing temperature. Viscosity can vary widely depending upon temperature conditions imposed by transport, storage and processing.
Mechanical seal efficiency is affected by liquid viscosity. As a rule of thumb, hard faced (both rubbing faces) should be used when viscosities exceed 5,500 cSt / 25,000 SSU. Performance of other seal components, such as springs and set collars, can also be adversely affected by high viscosity. Low viscosity, on the other hand requires different solutions. Water is typically handled very well with one hard face and one soft. Solvents are less lubricating than water and often require the use of harder faces to maintain seal face integrity.
It is important to know both ambient and operating temperature viscosity from the standpoint of transfer as well as sealing of a liquid.
Liquids which are normally solids at ambient temperature must be maintained within a narrow range of temperature above their solidification points to avoid damaging seal components.
Physical Phase Change
Some chemicals tend to become more viscous, solidify, or precipitate onto seal faces with changes in liquid temperature. The amount of change in viscosity with change in temperature will affect decisions regarding seal design selection, as will the tendency for abrasive precipitates to form.
Pressure within a pumping system is a factor in determining whether a balanced or unbalanced mechanical seal should be used. Single unbalanced friction drive seals are appropriate for use to about 17 BAR / 250 PSI discharge on lubricating liquids 165 cSt / 750 SSU and are limited to about 6.9 BAR / 100 PSI on viscosities as low as one cps. Single balanced seals can be applied with nonlubricating liquids to 35 BAR / 500 PSI. Higher discharge pressures can be attained using positive-drive, single-balanced seals depending upon liquid lubricity, seal face combination, and seal drive mechanics. The manufacturer engineering department should review all the conditions before pumps are put into high-pressure service.
Generally liquids whose specific gravity is 0.6 or lower should be handled with a balanced seal. Liquids in this class are normally extremely volatile and tend to boil as rubbing friction raises the liquid temperature adjacent to the seal faces. Contact surface areas (i.e., seal faces) of balanced seals are mathematically designed to decrease frictional heat, thus increasing application range and seal life.
Vapor pressure at operating temperature is helpful information, especially when the suction side condition on the pump is a vacuum. A mechanical seal operating at pressure near the liquid's vapor pressure, especially at startup, is subject to flashing. This breaks down the lubricant film and leads to premature seal failure. The application of a recirculation tube from pump discharge to the seal chamber is frequently used to alleviate problems of flashing associated with low pressure at the seal faces.
Pumps forced to operate under insufficient NPSHa conditions usually exhibit less than rated flow volume and pressure. They frequently cavitate and vibrate. If a pump does not operate smoothly due to insufficient liquid, there will probably be insufficient lubrication at the seal faces as well. High friction and cavitation, or vibration, at the seal faces can lead to early seal failure.
API Specification 682
If API standards are to be the benchmark for specifications, API Standard 682 is the applicable document to use in specifying mechanical seals for PD pumps. The most recent document is aimed at the purchase of new pumps for refinery service. It defines three generic standard seal arrangements and an engineered seal to be used with an engineered support system for extremes in duty requirements in some services. API 682 specifies that standard seals, regardless of arrangement, will be of the cartridge design.
What has been the user's experience with the mechanical seal currently in service? How long has it been in use? If service life has not been satisfactory, what symptoms have been exhibited? An early seal failure in a new installation may have been caused by choosing the wrong seal materials or seal arrangement but might also be a result of damage caused from construction residue left in the piping.
When there is previous experience, successful or not, the sealing technology should be reviewed and updated if necessary.
Environmental Controls at Site
Are temperature controls required for the process? Above or below ambient temperature? Is steam or other energy available for heating? Water or refrigerant for cooling?
The liquid phase of many tars and asphaltic products can be sealed using low-pressure steam as a quench or blanket across the atmospheric side of the mechanical seal faces. A steam quench flushes away weepage and contributes heat to the seal faces to ensure that product on the faces is in a liquid state while running or at startup.
Products for heat transfer are handled through a wide range of temperatures. Cooling may be required to avoid overheating the pump bearings above 290°C / 550°F .
The existence of mechanical or electronic devices for disposal or detection of weepage or catastrophic leakage should be noted so that any update in sealing can be interfaced with existing technology.
Types of Shaft Seals
A remarkable number of pumps sealed with packing are still in use today. Modern materials used in manufacture of packings include inert substances like PTFE and carbon graphite, metal foils, and flax. Manufacturers will provide recommendations for tailoring the packing type to the application. Keys to successful operation include the use of a hardened or coated shaft through the packing box area, a proper break-in period, and good adjustment of the packing gland during run time.
Packings pick up dried material and other particles and act as an abrasive wheel against the shaft. Once the shafting is grooved, packing failure is imminent. Hardened shafting resists wear and prolongs packing life.
Packings "leak" to allow the product to lubricate the interface between shafting and packing, especially during initial pump startup. Initial adjustment varies with packing type, but generally means to run the pump for 30 minutes with the packing gland loose enough to allow several drops per minute to leak. Every fifteen minutes thereafter, tighten each gland bolt one-sixth of a turn until leakage is minimized and the packing box temperature is stable. Ideally, product should accumulate under the packing gland but should not drip.
Hydraulic gear pumps must be considered to be unidirectional pumps when they use lipseals to retain fluids. The chamber in which the lipseal is mounted is vented to suction port pressure, generally atmospheric modified by the liquid head in the reservoir above or below the pump. When these same pumps are applied to industrial applications, they must also be unidirectional or employ more sophisticated sealing principles.
Recently, multiple lipseals mounted in a gland on a sleeve have been applied against a variety of viscous nonabrasive liquids successfully. Lip design and reinforcement allow operating at pressures to 10 BAR / 150 PSI without supporting fluid pressure behind the lips. Gland ports provide access to interior passages for cooling the lips or to pressurize internally (Figure 1).
Mechanical Face Seals
Friction drive seals can be applied to 17 BAR / 250 PSI on lubricating liquids and are capable of handling viscosity to 3,300 cSt / 15,000 SSU.
Component Double Seals
Classifications of the double seals are the same as for the single seals. They are simply configured with two complete units mounted back to back (Figure 5). Because they are positioned back to back, the annular space internal to the two sets of seal faces must be hydraulically pressurized in order to close both sets of faces. For best results, the pressurization liquid must be a lubricant and a barrier to the product. Internal pressure between seals is normally maintained at .7 to 1.0 BAR / 10 to 15 PSI higher than adjacent atmospheric or product side pressure. Thus, the film on the seal faces necessary for lubrication will be the barrier liquid. This arrangement can be used to seal noxious liquids, hazardous liquids, or those too viscous for a single seal to handle; because all the seal components are rotating in a "friendly" environment. The barrier liquid protects the seal faces while excluding product and atmosphere. Viscosity limits are not known. Friction drive double seals have been successful in 33,000 cSt / 150,000 SSU viscosity liquid, and set screw drive-type doubles can be applied to 55,000 cSt / 250,000 SSU. Pressure on a good lubricating barrier liquid between the seals can be up to 17 BAR / 250 PSI.
Abrasive Liquid Seals
The term "dual seal" is actually the modern description of two single mechanical seals mounted on the same pump shaft and should be qualified by adding the mounting description "double" or "tandem." Tandem seals are oriented in the same direction hydraulically, and double seals are mounted back to back. Both arrangements require the use of barrier/lubricants, but only the double must be pressurized. For tandem dual seals, only the product side seal rotates in the product, but product weepage across the seal faces will eventually contaminate the barrier lubricant (Figure 8). The consequences of this contamination upon the atmospheric side seal or the surrounding environment depend upon the nature of the product.
Cartridge seals are complete single, double, or tandem mechanical seals contained within a gland and built onto a sleeve (Figure 9). In the past, most PD pumps had to be modified to accommodate cartridge seals. Recently, seal vendors have manufactured cartridges to fit the special shaft diameters and specifically designed stuffing boxes of PD pumps.
Single cartridge seals will operate within roughly the same physical parameters as component positive-drive mechanical seals. They can handle viscosities to 7,700 cSt / 35,000 SSU, pressures to 20 BAR / 300 PSI, and temperatures to 205°C / 400°F.
Most dual cartridge seals can be treated as either true double seals or as tandem seals, because each is a mathematically balanced seal (Figure 10). The use of a barrier lubricant between seals is required. Lubricant pressure applied internally dictates whether product or barrier liquid will be on the product side seal faces. Pressure is in general limited to 20 BAR / 300 PSI. It is safe to assume that dual seals can be successfully applied to viscosities approaching 9,900 cSt / 45,000 SSU at reduced speeds and temperatures to 205°C / 400°F. A review of components, secondary seals, and metals may allow application of the cartridge seal at higher temperatures; however, the proximity of antifriction bearings may call for cooling of the barrier/lubricant to assure acceptable bearing life.
API Specification 682
To paraphrase a John Crane International publication, API 682 is a refinery specification. It defines three generic arrangements:
Three types of mechanical seals are available to satisfy these arrangements: a rotating pusher, a rotating metal bellows (i.e., nonpusher), and a stationary metal bellows. For extremes in duty services, reference is made to an engineered seal (ES) and an engineered sealing support system for some services.
Gas Barrier Seals
Gas barrier seals, the latest technology in mechanical face seals, are undergoing extensive lab and field testing. These are cartridge-style dual seals with faces especially designed to be pressurized using an inert gas as a barrier between product and atmosphere. The gas replaces traditional lubricating liquid.
In one version, the seal faces separate as soon as the pump shaft starts to revolve. The faces do not come in contact with each other again until the rotation stops. In another version, seal faces remain in light contact during normal operation. Current gas barrier seal designs allow a small amount of gas to escape in the product and for some to escape to atmosphere.
Mag Drive Coupling Units
Magnetically driven (mag drive) pumps offer positive sealing of hazardous or difficult-to-contain liquids. Magnets mounted radially around the pump drive shaft are surrounded by a close fitting canister which contains the product circulating through the pump. Magnets attached to the inside wall of a hollow cylinder affixed to the drive shaft and situated around the canister set up a magnetic field. When the drive shaft rotates, the field compels the pump shaft to rotate. The canister wall is not penetrated by either shaft and is statically sealed at its interface with the pump housing (Figure 11).
Product must be circulated through the canister to cool the magnets. Thus, consideration must be given to the chemical compatibility of the product with the magnets, canister, and canister gasket. O-rings are normally used as canister gaskets to allow for a wide range of material compatibility. Canisters can be formed from corrosion resistant alloys and coupling magnets are encapsulated with inert materials.
Mag drive pumps are designed to handle corrosive and hard to seal liquids. For example, sulfuric acid and sodium hydroxide are common corrosives handled with magnetically driven pumps. Hard to seal applications include isocyanates.
Match The Seal To The Environment
Mechanical seal metal parts are relatively thin and are more vulnerable to corrosive attack than thicker pump walls. For maximum life cycle performance, use seal components that are as inert as possible in corrosive applications. The seal metals should be at least as noble as the pump, if not more so. Springs or metal bellows are especially susceptible to corrosion because of their thin cross sections and large surface areas. Finally, 316 stainless steel metal parts and Hastelloy-C® springs are becoming the standard for cartridge mounted or component screw drive seals.
The use of silicon carbide as a material for at least one seal face has helped to minimize corrosion damage to that area. Some silicon carbide grades are resistant to pH in a range from 1 to 13 at moderate to fairly high temperatures.
Secondary seals receive the most attention of any seal component because corrosion on them generally leads to failure of other components. Fortunately, secondary seal corrosion is easy to detect, even in early stages.
Much has been published by the major manufacturers of sealing products rating the resistance of elastomers and other components when immersed in various chemicals. Corrosion rating for metals, elastomers, and plastics can be found from NACE, most seal manufacturers, and many pump manufacturers.
Abrasive liquids pose difficult challenges for mechanical seals. The abrasive effect is lessened with small particles or if a viscous matrix surrounds the particles. Abrasive liquids are commonly handled with PD pumps at slow rotational speeds with positively driven, hard-faced mechanical seals. Some applications include caustic precipitates, coatings, inks, and oxide slurries.
Caustic Precipitates: Certain solutions tend to precipitate crystalline solids on the atmospheric side of the seal faces. These crystals then grow near the atmospheric interface on the sealing surfaces eventually forcing the seal faces open and separating the sealing surfaces. For example, sodium hydroxide (i.e., caustic soda) poses this challenge. And because it is also corrosive, weepage across the seal faces can extract a costly toll in pump shafts, bearings, and other parts. A dual seal can be used with low-pressure water flowing through the cartridge, between the seals, and out to a safe drain.
Coatings: Dried paint forms a hard shield because of abrasive whiteners or other tints in adhesive vehicles. Secondary seals are selected based on the chemical formula of the coating and by considering the cleaning solvent. Hard faces coupled with a positive drive allow a single pusher seal to handle coating viscosity up to 16,500 cSt / 75,000 SSU.
Inks: Thousands of gallons of ink per day are spread upon newsprint every day. Abrasive metal tints and carbon black suspended in natural oils or with fast-dry solvents are applied by high-speed presses fed by PD pumps. Abrasive liquid seals similar to those in use in the paint industry are successfully applied in formulating, transporting, and printing with inks. Printing inks are often pseudoplastic; they are quite viscous at rest, but thin out in the pipeline as they begin to move.
Oxide Slurries: Slurries of finely ground metal oxides suspended in fast dry solvents are coated onto mylar tapes to record audio or video signals. Metal bellows seals with hard faces and perfluoroelastomers offer thousands of hours of performance in abrasive-fitted pumps.
Single-friction drive seals can handle viscosities up to 3,300 cSt / 15,000 SSU. The viscosity-handling capability of positive-drive single seals increases to 5,500 cSt / 25,000 SSU by set screw type, double if pin driven. Vendors of cartridge-mounted mechanical seals currently suggest their products be limited to 7,700 cSt / 35,000 SSU.
Component double seals are successfully applied to viscous resins and caulks. A buffer liquid must be present on the sealing faces to exclude the viscous product and provide a compatible environment in which the seals can rotate. Buffer liquid systems can be equipped with devices which will set off an alarm or shut off the pump in case a change occurs in the seal chamber, signaling an incursion of product or a catastrophic leak.
A multiple-lip cartridge seal design is gaining acceptance for handling viscous materials; for example, polyester resin 44,000 cSt / 200,000 SSU at 150°C / 300°F. Several successful clean asphalt applications have also been reported. Many installations have succeeded with no support fluid system necessary. This sealing technology is not recommended for abrasive liquid service.
Many thin liquids, including solvents such as water, can be handled by PD pumps with unbalanced single seals. The secondary seals must be selected with product temperature in mind. Also, mechanical seal manufacturers recommend product temperatures adjacent to the seal faces be maintained at least 10°C / 50°F below the boiling point of the liquid to keep the liquid from flashing across the seal faces.
High temperatures affect seal components. For example, mechanical seal faces become distorted, uneven expansion between different materials allows interference fits to loosen, secondary seals degrade, and springs tend to relax. A variety of properties contribute to the difficulty of sealing hot liquids, including:
Thermoplastics must be maintained at temperatures above their solidification point. Tars and asphaltic compounds can be sealed with cartridge-mounted metal bellows seals using a low-pressure steam quench adjacent to the atmospheric side of the seal faces. If steam is not available, a double seal pressurized with a hot heat transfer liquid is appropriate.
Thermosetting liquids, such as phenolics, polymerize when heated, then become solids. Consideration must be given to the flush cycle and its compatibility with the mechanical seal.
Heat transfer liquids are useful for temperatures ranging from below zero to 450°C / 850°F, depending upon their chemical composition. They vary in makeup from petroleum based to synthetics. Some are solid salts at room temperature and must be thoroughly liquefied before pump startup. All have vapor pressures lower than water at any given temperature, which is the property that most makes them useful.
Stuffing box-mounted component seals, pusher, or metal bellows with fluoroelastomer secondary seals, and a clamped-in stationary face are serviceable to 205°C / 400°F without cooling. The same arrangements for perfluoroelastomers are serviceable to 290°C / 550°F without cooling. Above 290°C / 550°F a cooling device is required to ensure the thrust bearing will not overheat because of its close proximity of the stuffing box. A single component seal, pusher or metal bellows fitted with perfluoroelastomers and a cooling collar can be used to 450°C / 850°F.
Cooked starches used as adhesives are generally handled at temperatures less than 95°C / 200°F and can be handled with a single nonpusher (i.e., rubber bellows) mechanical seal, carbon versus Ni-Resist faces, and with nitrile elastomer secondary seals. Pump shaft speeds should be reduced from motor speeds.
Solvent-borne ureas are used in wood fabrication. These adhesives are hot, tacky, and viscous. Nonpusher metal bellows seals with hard faces perform well at low pump shaft speeds. Perfluoroelastomer secondary seals are required to resist the elevated temperatures and fast-dry solvents. Cartridge-mounted stuffing box dual seals using an unpressurized buffer liquid solvent could also be used, but the bearings and pump shaft need to be hardened.
Component stuffing box seals with PTFE secondary seals and clamped-in stationary faces are applicable to temperatures of -67°C / -90°F in stainless steel PD pumps. The stationary seat is held in place with a gland machined with ports for vent/drain, outboard the seat. A blanket of nitrogen gas, or dry air, in the vent / drain area will prevent ice from forming at the seal interface. O-rings of fluorosilicone elastomer can be used to -73°C / -100°F if liquid is chemically compatible.
Refrigeration ammonia is a noxious nonlubricating gas liquefied under pressure. Pressurized double seals are required for sealing circulating systems. Petroleum oil presents a nearly impermeable barrier because oil and ammonia are immiscible. Select oil viscosity to be compatible at the pumping temperature of the ammonia. Pump gaskets used to seal liquefied gasses should be o-ring design in order to contain vapors during operation. The o-ring design will also contain pressures associated with increase in product temperature. Simple transfer operations can be accomplished with single seals.
HCFC and CFC refrigerants can be transferred with single seals, preferably a balanced design, because of the high vapor pressure of these liquefied gasses. Over time, significant quantities of these solvents can escape through elastomeric gaskets if the correct formulation is not applied. Circulating systems should be designed with this fact in mind.
Most solvents, including water, can be handled and sealed within PD pumps. Remember that vapor pressure and liquid solubility increase as liquid temperature increases.
Underwriter's Laboratory (UL) Requirements
A number of pumps are constructed to handle certain hazardous liquids requiring certification by the Underwriter's Laboratory. Inventories of manufactured and purchased component parts (mechanical seals, o-rings) are regularly checked by UL inspectors. Suppliers of purchased components must submit their product to UL for certification and host random onsite inspections. Each pump assembled must pass a physical test established by Underwriters Laboratory before it can receive the UL Label.
High Vacuum Applications
PD pumps can evacuate still bottoms from a high vacuum if there is enough physical head afforded by the liquid column on the suction side of the pump to overcome pipe friction loss and the system vapor pressure. Pressurized double seals should be used to maintain the integrity of the vacuum. Cartridge dual seals may be applied when suitable for the product viscosity. PD pumps are not designed to produce the system vacuum or draw off the condensate.
Identify Potential Leak Causes
A high pressure differential between the product side of the mechanical seal to the atmospheric forces a thin film of product to spread across the surface of the rotating face. Rubbing friction of the seal faces generates heat. Surround the seal with product to help dissipate that rubbing heat. If heat is not removed or if no lubricating film exists on the seal faces, damage can occur, even in a short period of time. Thermal distortion of either face promotes uneven face wear, since only the "high spots" are in contact. Eventually, openings at the product edges allow entry of any substances in the area, including damaging particles. A rapid change in temperature of 95°C / 200°F can fracture a seal face from thermal shock.
Startup of the pump with no liquid at the inlet port may result in seal damage. Damage is minimized, however, if liquid reaches the mechanical seal within thirty seconds or if a film left over from shutdown exists between the seal faces.
In some applications, the pump must be drained between runs. To avoid seal damage from repeated dry startups, fit the pump with a cartridge dual seal and a reservoir mounted on the pump base pressurized to supply lubricant to the seal faces. This arrangement allows the pump elements and piping to be drained with no damage occurring to the mechanical seal.
Double seals must be pressurized and the normal pressure level recommendation should take into account system relief valve and other downstream pressure variations. Seal chamber pressure should be maintained at a level higher than the pump relief valve setting because an increase in product viscosity or a valve failure can cause dramatic increases in downstream pressure. Moreover, seal faces must remain closed, otherwise barrier liquids may become contaminated or lost.
Handling a volatile liquid at a temperature too close to its boiling point may cause "flashing" of vapor across the seal faces. Vapors do not lubricate the seal faces adequately resulting in diminished seal life. One solution is to direct a flow of discharge into the seal chamber to increase the pressure surrounding the seal. If the pressure increase is high enough, the product's boiling point may change enough to prevent "flashing."
System Test With Improper Fluid
New systems are often tested for joint and seal leaks prior to startup with fluids which are easy to detect. The test fluid must be lubricating as well as chemically compatible with the mechanical seal components if the pump is to be operated. Water is relatively inexpensive and considered to be inert, but it is not a good lubricant and can corrode steel piping. CFCs and ammonia leaks are readily detectable but are not compatible with all sealing elastomers and are not lubricants. Double seals must be pressurized prior to the start of testing as if in an actual production situation.
Avoid tests with nitrogen or steam unless the pump can be bypassed. Neither has lubricating qualities, and elastomer compatibility must be considered with steam.
Liquid containment is a growing concern in the fluid-handling industry. Please remember this is a basic introduction and engineers are strongly encouraged to discuss liquid containment options with the pump manufacturer's representative.
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