Application Considerations

7/28/2019 Application Considerations

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APPLICATION CONSIDERATIONS TABLE OF CONTENTS

Fluid Services

Crystallizing Fluids

Abrasive Fluids

Fluids that Solidify or Harden

High Vapor Pressure Fluids

Hazardous Fluids

Hot Fluids

Low Temperature Services

Cryogenic Fluids

Corrosive Fluids

Hot Water

Buffer/Barrier Fluids

Pump Considerations

Single Stage Horizontal Overhung Pumps

Vertical In-Line Pumps

Double Ended Pumps

Vertical Can-type Pumps

Close Coupled Pumps

Rotary Pumps

Cavitation

Vibration

Vacuum Conditions

Seal Considerations

Pressure (Maximum Allowable Working Pressure)

Pressure (Maximum Dynamic Sealing Pressure)

Pressure (Maximum Static Sealing Pressure)

Material Limits

Axial Travel

Eccentricity / Radial Movement

Angular Misalignment

High Speed


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APPLICATION CONSIDERATIONS FLUID SERVICES

Crystallizing Fluids Fluids that crystallize or leave behind a solid crystalline

substance on the atmospheric side of the seal after passing across the seal faceare generally salts, caustics, and other soluble mixtures. Providing a low sealchamber temperature is important. There are a variety of piping plans that can beused including those that include heat exchangers. In addition a suitable quenchfluid helps maintain the crystals in solution to help prevent abrasive damage andpotential seal hang-up. Quench fluids should be compatible so that the crystallineformation will remain in solution. Depending upon the concentration of thepumpage the quench can range from a static quench to a minimal flow to maintaina liquid volume in the quench area, to a steady stream. The throttle bushing typedepends upon the available space and allowance for leakage to atmosphere.

Abrasive Fluids Abrasive fluids exist in all industries and can range from a few

PPM of particles in suspension to a heavy slurry. Sealing can range from a singleseal with a periodic to a continuous quench to dual pressurized seals. For singleseals the arrangement can be a rotating non-pusher type seal with a single springto a stationary pusher type seal that has the springs on the atmospheric side of theseal. Various piping plans with abrasive separators to an injection (Plan 32) canalso be used. When a Plan 32 is used a suitable throat bushing is required to keepabrasives out of the seal chamber. The recommended velocity through thebushing is 15 fps. The abrasive laden flush should not be directed at the seal asthis can cause erosion of seal hardware or faces. Back-to-back dual pressurizedseals should be avoided as they can suffer hang-up underneath the inboard sealclose clearances.

Fluids that Solidify or Harden Thermosensitive or viscous fluids that can

harden in the seal chamber if heated or cooled beyond their limits include resins,asphalt, sulfur, heavy crude, and molasses. Either or both the pump and glandshould include a jacket to maintain the proper seal chamber temperature andadequate time, up to 4+ hours, may be required to acquire the proper seal

chamber temperature prior to pump operation. Steam quenches are typically used(if appropriate) and are particularly useful with a steam baffle to wash awayleakage and maintain the proper temperature. Drain connections should be aslarge as practical. Seal faces should be a hard vs hard combination and the drivemechanism must be adequate for high torque conditions. Dual gas seals thatproduce minimal seal generated heat can also be appropriate for fluids that easilypolymerize.


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High Vapor Pressure Fluids Fluids with a vapor pressure at pumping

temperature over a few atmospheres require a properly balanced seal that bothminimizes heat generation and will remain closed as the fluid transforms from aliquid to a vapor across the seal faces. A flush plan that cools the pumpage toobtain a minimum of VP margin of 25 psi is recommended. Alternately, a closeclearance throat bushing with an adequate flush rate can increase the sealchamber pressure to obtain a proper VP margin. Knowledge of the seal chamberpressure is paramount. A flush directed at the seal faces, such as a DistributedFlush, is recommended to prevent vapor bubbles at the outer diameter of the sealfaces. Dual unpressurized seals with a liquid buffer may not be able to beadequately cooled and may result in a lower VP margin. Dry running containmentseals should be considered where appropriate.

Hazardous Fluids Toxic, highly flammable, carcinogens, and other fluids with

similar characteristics should be sealed with a dual pressurized seal arrangementto prevent a product leakage to atmosphere. The barrier fluid should bemaintained at a minimum of 25 psi or 10% over the seal chamber pressure. In

some cases the inboard seal should be designed to withstand reverse pressures.The barrier fluid must be compatible with the product.

Hot Fluids (> 400F) These fluids are generally present in the petroleum

industries or where heat transfer fluids are used. Non-pusher metal bellows sealsare the default selection. Common piping plans are 02/62, 23, or 32. With Plan 02systems, fouling of the pump cooling jacket is common leading to poor heattransfer over time. In addition a steam quench is required to retard the formationof coke on the atmospheric side of the seal. The use of a stationary seal with asteam baffle is the best arrangement. While nitrogen can be used it is not as goodas steam and is more costly. Hard vs hard faces are recommended above

temperatures over 500F.

Low Temperature Services (


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In some cases back-to-back dual pressurized seals with no wetted metal hardwareis recommended. This can be the safe arrangement for unknown fluids alongwith PTFE or Perfluoroelastomers and (alpha) SiC vs SiC faces.

Hot Water (> 180F) Hot water applications are similar to high vapor pressure

fluids in that the pumpage can vaporize at the seal faces. In addition the elevatedtemperature reduces the wear rate of the carbon primary ring. Within guidelinelimits both non-pusher and pusher seals can be used in uncooled services, seeTechnology Bulletin #25. Alternately, Plan 21 can be used or more commonly Plan23 can be used to lower the temperature. Water can contain any number ofchemicals that can cause scaling and be abrasive when it undergoes a phasechange (from liquid to vapor) at the seal interface.

Buffer/Barrier Fluids

o Watercan be used in a variety of applications, but has a limited temperature

range and may freeze in colder climates. Tap water can be used, butdepending upon the treatment levels can scale or plate out at elevatedtemperatures. Demineralized or distilled water is a better buffer/barrier fluidthan tap water. Deionized or high purity water has been used inapplications where any contamination of the barrier fluid can be detrimentalto the process fluid. Depending upon the purity level of this water it can becorrosive and can attack metals and seal face materials.

o Glycols: Ethylene glycol has been successfully used in the past, but is a

listed VHAP (volatile hazardous air pollutants) and has been replaced bypropylene glycol. Typical mixtures are in the 40 to 60 percent range.

Automotive anti-freeze is not recommended as it contains a number ofadditives that can be harmful to the seal. Triethylene glycol has a verylimited usage as a buffer/barrier fluid, mainly in the resin and plastic

industries. Triethylene glycol is useful in a temperature range of 0C (32F)

to 270C (520F).

o Propanolor n-propyl alcohol has replaced methanol as a buffer/barrier fluid

as the latter is a listed VHAP. It is useful for applications involving very lowtemperatures, but has a limited upper temperature range.

o Kerosene or diesel fuels come in a variety of viscosities and are widely used

in petrochemical and refinery applications. Local regulations should bechecked to ensure that these are not restricted in use.

o Lube Oils: Lube oils come in a wide variety of viscosity ranges and can

contain numerous additives, some of which can be detrimental to sealperformance. Anti-wear additives can plate out on the seal faces causingleakage and on other exposed areas the film created can restricting the heattransfer characteristics of the face materials. Oils above an ISO 32 can


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created blistering problems with carbon seal faces and can lower pumpingring performance due to the additional resistance or pressure drop in thepiping circuit.

o Synthetic Oils: There are a number of synthetic oils on the market, some of

which are geared specifically as mechanical seal buffer/barrier fluids. As agroup they have better performance properties than petroleum based lubeoils and have resolved some problems when replacing lube oils. It isrecommended to consult the seal manufacturer regarding the use of specificsynthetic oils.

o Heat Transfer Fluids: Heat transfer fluids come in a wide variety of

viscosities with varying temperature ranges. They can be petroleum based,glycols, or synthetic aromatic fluids in addition to others. Most have theability to provide adequate to excellent lubrication for seals within theirrespective temperature ranges and are good for hot applications typicallyfound in refineries.

APPLICATION CONSIDERATIONS PUMP CONSIDERATIONS

Single Stage Horizontal Overhung Pumps Available in a wide variety of

designs; centrifugal, rotary, progressive cavity, etc. that can a variety of sealchamber (stuffing box) designs. The seal chamber pressure can range from nearsuction to discharge pressure depending upon the impeller design. Seal chamberscan range from large bore or tapered bore designs to small bore seal chambers orstuffing boxes. Bolting can range from standard 4 bolt designs, to two swing bolts,to multiple bolt API designs. These typically have the best combination ofaxial/radial movement and runout conditions. See MMTC 901-2 Seal ChamberPressure for various pump types.


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Vertical In-Line Pumps A vertical version of the horizontal centrifugal overhung

pump that should not be confused with a vertical can or vertical turbine pump.There are a variety of designs; close coupled, rigid coupled, and integral bearingflexibly-coupled mountings. Close coupled designs are difficult to service andrigid models have long unsupported shafts that are subject to higher runouts, moreshaft deflection, and imbalance leading to a variety of seal life challenges.

Double Ended Pumps There are a wide variety of double ended pump designs;

single and multiple stages, single and double suction, radially and horizontally split,

along with a variety of (API) edition seal chambers. The access to seal glandsvaries dramatically over the range of pumps involved. In most cases access to thelower section of the gland is restrictive, if not totally inaccessible, as is the case forinstallation and removal of cartridge seal spacers. Horizontally split pumps musthave the gland O-ring on the face of the seal chamber and the upper and lowerhalves may be misaligned so stationary seals are less prone to problems, likegland distortion. On long multi-stage pumps the shaft/casing has the potential ofsignificant transient growth due to differences in temperature between the shaftand the pump casing, especially on hot services. Some customers prefer glandsthat are uni-directional while others prefer bi-directional glands with outlets for bothshaft rotations. See MMTC 901-2 Seal Chamber Pressure to determine the sealchamber pressure.


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Vertical Can-Type Pumps Unlike vertical In-Line pumps, the seal chambers in

these pumps are subject to pump discharge pressure. The shafts can be quitelong and generally have sleeve type bearings within the pump casing. The lengthof the seal cartridge is restricted to the length of the spacer coupling. Alignment ofthe pump and motor shaft is difficult to achieve, so angular misalignment can bequite large. It is important that the gland porting allows full venting of theentrapped air and must be located above the seal faces. With the use of a Plan 13flush, the flow past the seal faces is not optimum so the use of a Distributed Flushto draw the flow near the faces is recommended.


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Close Coupled Pumps Typically use High Volume elastomeric bellows seals to

maintain a low cost design. The seal chamber & gland are usually an integral partof the pump and has a limited number of flush plans available. Typical sealingproblems can arise from out-of-square mating rings (cup mounted) or very limitedstartup torque from the fractional HP motors used.

Rotary Pumps There are a variety of rotary pump types; internal and external

gear, lobes, vane, screw, and others like progressive cavity pumps. Seal chamber

size is typically limited in axial and radial space, and some utilize 2 bolt glands thatcan lead to mating ring distortion if not installed properly. While operating at lowershaft speeds than centrifugal pumps they are generally used on higher viscosityfluids that can pose problems with drive mechanisms and potentially limited facelubrication. Twin shaft screw pumps have 4 seals per pump and require specialglands with limited space between shafts.

Cavitation can obstruct the pump, impair performance and flow capacity, damage

the impeller and other sensitive components, and create large vibrations. In short,the pump has a heart attack called cavitation.

Classical cavitation occurs when the absolute pressure of a moving liquid isreduced to, or even below, the vapor pressure of the liquid in the impeller eye.Bubbles are formed as a result of this pressure drop. Lower pressures in theimpeller eye are caused by variations in velocity of the fluid and friction losses as


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the fluid enters the impeller. The bubbles are caught up and swept outward alongthe impeller vane. Somewhere along the non-visible side of the impeller vane, thepressure may once again exceed the vapor pressure and cause the bubbles tocollapse. Implosions of these vapor pockets can be so rapid that arumbling/cracking noise is produced. The hydraulic impacts caused by thecollapsing bubbles are strong enough to cause minute areas of fatigue on themetal impeller surfaces. Depending on the severity of the cavitation, a decrease inpump performance may also be noted.

Vibration Radial or lateral vibration is the most frequent type of pump vibration

where the displacement is predominantly in the plane perpendicular to the pumpaxis. There are a number of limits that vary within industry standards (HydraulicInstitute HI 1.1-1.4 vs API 610). For Overhung, Vertical In-Line, and BetweenBearing pumps, at normal pump speeds ranging from 1200 to 3600 rpm the levelscan vary from 0.001 to 0.004 peak-to-peak and Vertical suspended pumps thevalues can reach 0.002 to 0.008 peak-to-peak (Unfiltered). More detailedinformation is available within the above standards. Machinery vibration severitycharts, available from a variety of sources, should not be applied to pumpsindiscriminately.

Vacuum Conditions Hydraulically balanced seals can perform well in vacuum

services. Dual pressurized seals can also be used. When single seals are used,the seal chamber pressure must be increased through the use of an injection orby-pass flush in conjunction with a close clearance throat bushing. Retention ofthe mating ring must be taken into account. Depending upon size and vacuumconditions, reverse pressure compatible seal designs may be required.

APPLICATION CONSIDERATIONS SEAL CONSIDERATIONS

Pressure (definitions):

Maximum allowable working pressure (MAWP) maximum continuouspressure for which the (pump) manufacturer has designed the equipmentwhen handling the specified fluid at the specified maximum operatingtemperature.

Maximum dynamic sealing pressure (MDSP) highest pressure expected atthe seal(s) during any specified operating condition and during start-up and

shutdown. In seal design must consider face pressure, pressure andthermal distortions of the faces, stresses and deformations, secondary sealextrusion, primary and mating ring drive, piping plans, thrust loads and PVlimits as a minimum.

Maximum static sealing pressure (MSSP) highest pressure, excludingpressures encountered during hydrostatic testing, to which the seal(s) canbe subjected to while the pump is shut down. In seal design considerationthat the seal may be exposed to this pressure for extended periods of time


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at minimum/maximum temperature when the pump is a stand-by spare.

Material Limits these can be based upon a variety of conditions: chemical

compatibility, corrosion rate less than 0.002/year, stress crack corrosion, pitting,minimum and maximum temperature limits, thermal stresses, PV limits, facecontact pressure limits, wear rate at elevated temperatures, secondary sealcompression set / swell / shrinkage / explosive decompression / thermal expansionrate, material toughness, fluid viscosity limits (minimum/maximum).

Axial travel The equipment may exhibit axial travel due to thrust loads, shaft

shuttling, or due to temperature changes. On pumps during warm-up, the shaft willmost likely expand faster than the pump case, causing a temporaryexpansion/contraction of the seal. This is especially true of multi-stage doubleended pumps and large mixers. The seal springs/bellows needs to be able tocompensate for this. Axial shuttling of the shaft can cause excessive secondaryseal wear on pusher seals and should be corrected. Coated shafts/sleeves with an8-20 Ra finish will reduce the wear, but is only a temporary fix.

Eccentricity / Radial Movement Eccentricity of the seal chamber or gland plate

to the shaft must be considered especially when there are multiple pilots. Radialmovement of the shaft can be due to parallel misalignment or from the type ofbearings used on various types of equipment such as vertically suspended shafts.The primary ring face should not run off the mating ring face in operation. Size-on-size faces should be considered for abrasive applications to avoid grooving thatcould result in chipping of the seal faces. It is also advisable to ensure that themating ring or other stationary components have sufficient clearance so as not tohave contact with the equipment shaft. See UK Technical Policy UKTP027.

Angular Misalignment Angular misalignment or runout limits differ based uponthe type of secondary seal and shaft speed. Non-pusher seals can withstand moremisalignment than pusher type seals, but can still suffer from drive mechanismwear or fatigue associated with welded metal bellows. Angular misalignment ismeasured by mounting a dial indicator on the shaft and rotating it against the sealchamber face. Rotating seals must compensate for the runout with each revolutionof the shaft. Stationary seals eliminate this condition. Dynamic shaft deflections,piping strain, thermal distortions, and inadequate equipment mounting can alsocreate angular misalignment in addition to what is measured above. The tablebelow can be used as a general guideline, which accounts for the measurablemisalignment that can be accounted for when equipment is checked. It does not

account for dynamic shaft deflections, thermal distortions, or fluid considerationsthat can affect seal life that will tend to accelerate drive mechanism wear. Forvalues larger than those listed calculations need to be run to see if the seal canwithstand the runout due to internal clearances.

Angular Misalignment Limits for Seals

Shaft Speed(rpm)

Pusher & MetalBellows (mm)

Pusher & MetalBellows (in)

ElastomerBellows (mm)

ElastomerBellows (in)


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500 0.152 0.006 0.279 0.011

1000 0.127 0.005 0.254 0.010

2000 0.089 0.0035 0.191 0.0075

3000 0.064 0.0025 0.152 0.006

4000 0.051 0.002 0.127 0.005

5000 0.038 0.0015 0.089 0.00356000 0.025 0.001 0.051 0.002

High Speed Shaft speeds in excess of 5,000 fpm or alternately shaft speeds in

excess of 6,000+ rpm (for smaller shaft diameters) can create unbalance forces inthe seal. Above these limits stationary seals must be used. Radial containment ofthe mating ring should also be considered when using brittle materials such assilicon carbide.

Application Considerations

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