Improving Machinery Reliability 3 Episode 14 docx

Providing Safety and Reliability Through Modern Sealing Technology 565 Table 13-4 Basic Seal Geometry auwuons asistlng to seal 1.. Flexible Stator psrlommnce Wear F a m on Stator Wear

Providing Safety and Reliability Through Modern Sealing Technology 559

being pushed toward a five-year goal in refineries and chemical plants in the Unit-

ed States and around the world

e Increased emphasis on economics and energy conservation is forcing seal manu- facturers and end users to select new seal designs that will run at higher tempera- tures, pressures, and speeds than ever before In some of these instances, the requirements are met by simple changes in materials, but more frequently, radical changes in design and fundamental modes of operation are required in order to achieve the performance advantages sought by the end user

Another factor adding to the changes in seal design and sealing systems is the basic understanding of seal technology The basic way we do things has been changing Fundamental modes of lubrication, seal materials, and analytical tools have advanced significantly, greatly expanding the range of application and enhancing the performance of mechanical seals

A broad overview of fundamental seal technology will help end users understand

how fundamental changes may result in more economical and reliable sealing solu- tions in the future

Types of Seals

Under the broad classification of axial end-face mechanical seals, there are two types of seals: pusher and non-pusher seals, which can be found in four different arrangements: single, double, tandem, and staged

Pusher-type seals refer to axial end-face mechanical seals with a semi-dynamic secondary sealing device (Figure 13-36)

The term semi-dynamic secondary sealing device is used to describe the O-ring or

other secondary sealing device that must move backward and forward to accommodate wear at the seal faces and to accommodate vibration and axial run-out of the seal faces Pusher-type seals with elastomers or PTFE secondary seals are fundamentally sus-

ceptible to hang-up and fretting damage, which are characteristics of this seal design Hang-up is a seal term used to define the failure of components to move axially

along the shaft under applied spring loads and hydraulic forces (Figure 13-37)

Figure 13-35 General classifications of seals

Figure 13-36 Typical inside pusher seal

Providing Safety and Reliability Through Modern Sealing Technology 561

Figure 13-37 Pusher seal hung up

Secondary seal hang-up may result from deposits that form on the atmospheric side of the seal as shown in Figure 13-37 Other sources of secondary seal hang-up are internal friction between the secondary sealing device and the shaft sleeve This may be the result of a rough surface finish, lack of lubrication, or swelling of the sec- ondary sealing device due to temperature or chemical attack

Non-pusher-type seals include seals such as metal bellows, PTFE bellows, and elastomer bellows seals that do not require the semi-dynamic secondary seal to accommodate axial movement due to wear, vibration, and run out (Figure 13-38)

Axial movement is accommodated internally in the bellows portion of the seal

Bellows seals, however, are generally less commercially available for wide ranges of pressures and materials of construction

Modes of Lubrication

Recent technology advancements have provided seals that operate in one of four basic modes of lubrication for various pieces of rotating equipment One of these four basic modes of lubrication will provide the lubrication that is necessary between the seal faces Knowledgeable seal manufacturers and end users quite frequently choose between two or more of these modes of lubrication in selecting seals and sealing systems for a given application In doing so, reduced leakage, longer seal

Figure 13-38 Non-pusher seal

high-performance seals for both liquids and gases Until a few years ago, most seal manufacturers were not seriously involved in applying full-fluid film lubrication to axial end-face mechanical seals Today, most seal manufacturers work with the four

fundamental modes of lubrication to achieve higher performance levels in their seal-

ing devices Figure 13-40 shows the relative performance that is anticipated for full- fluid film liquid seals in comparison with conventional sealing devices In general,

an order of magnitude higher in pressure and speed capability is expected over con- ventional boundary lubricated seals Also, full-fluid film seals typically consume 95% less power than do double contacted liquid lubricated seals (Figure 13-41)

Boundary Lubricated Gas and Liquid Seals This terminology refers to the

lubrication that occurs between two seal faces that rub under light or moderate loads

In general, closing forces vary from a few pounds per square inch to several hundred pounds per square inch Lubrication that occurs between the faces is the result of surface waviness, porosity, and surface roughness The pressure profile of the seal faces closely approximates the linear pressure drop that has been proposed in many commercial publications over the past years In the hydrocarbon processing industry, this has been the primary mode of lubrication for the past 30 years Boundary lubri- cated seals provide minimal single-seal leakage rates and allow for substantial spring loads to overcome pusher-type seal secondary seal hang-up

Figure 13-39 Lubrication modes

563

fldalive vdwiiy at Seal Ccnlact (IeeVmin.)

Figure 13-40 Operating limits of boundary and full-fluid film seals

Figure 13-42 A flexible rotor with the wear face on the stator

Figure 13-43 A flexible rotor with the wear face on the rotor

Figure 13-44 A flexible stator with the wear face on the rotor

Figure 13-45 A flexible stator with the wear face on the stator

Providing Safety and Reliability Through Modern Sealing Technology 565

Table 13-4 Basic Seal Geometry

auwuons asistlng

to seal 1 Flexible Rolor 2 Flexlble Rolor 3 Flexlble Stator 4 Flexible Stator

psrlommnce Wear F a m on Stator Wear Faca on Rotor Wear Face on Rotor Wear Face on Stator

Semndary seal lrening

due to slalor misalignment

Secondary seal frening

by non-wearing lace belng

0ul-of.Sqwre

Secnndarq seal lrening

by weanng lace being

oul-o1.sqL!am

Parallel mlsallgnrnent 01 soal

faces resuns in hydraulic

load Imbalance, lilt slability

Seal deslgn IimHed speed

wise due to radlal load

support of the semndary

As a result of industry’s understanding of basic seal geometry, some trends are

expected over the next five to ten years away from the more conventional flexible

rotor design (with the wear face on either the stator or the rotor) to a flexible stator

design with the wear face on the stator The potential advantage of this seal geometry

has been recognized on critical high-pressure and high-speed applications This seal

geometry offers these two advantages

I Parallel misalignment of the seal faces with respect to the shaft or seal housing

will not cause hydraulic load imbalance in the flexible stator design with the

wear face on the stator This is not the case with the flexible rotor design when

the wear face is on the stator (Figures 13-46 and 13-47)

2 The flexible stator design eliminates fretting damage due to out-of-perpendicu-

larity between the gland or seal flange and the shaft axis This is not the case

with the flexible rotor design with the seal face mounted on either the stator or

rotor portion of the seal (Figures 13-48 and 13-49)

Specialty Seals for Non-Pump Applications

Nowhere is it more important to apply the fundamentals of basic seal geometry,

seal types, and modes of lubrication than in the application of mechanical seals to

Figure 13-46 Flexible rotor with wear face on the stator

Figure 13-47 Flexible stator with wear face on stator

specialty non-pump equipment such as compressors, mixers, centrifuges, and steam turbines

Traditional mechanical seals that have been designed for pumps simply won’t work on many of these specialty pieces of equipment Table 13-5 shows some of the predominant operating conditions that must be considered when designing seals for these pieces of equipment

The remainder of this section of this chapter will address the application of spe- cialty mechanical seals to these non-pump applications and the impact that can be achieved on the reliability of the rotating equipment

Providing Safety and Reliability Throzrgh Modern Sealing Technology 567

Figure 13-48 Flexible rotor with wear face on stator

Figure 13-49 Flexible stator with wear face on stator

Approximately 30% of the 1.5 million mixers installed in the United States are

sealed with some type of mechanical seal More will undoubtedly be sealed in the future as reliability, safety, and environmental issues become greater factors Exam- ples of common mixer configurations are shown in Figure 13-50

Mixer Seal Reliability

Depending on the style and type of mixer and whether the mixer was originally designed for packing or mechanical seals, the operating conditions for the seal may vary widely Since the mid-I970s, major reliability problems have been found in six major areas that include:

Excessive shaft orbiting

Stationary seal face warpage

Pressure reversals on double seals

Wear debris contaminating the product

Barrier fluid leakage into the vessel

Non-coupled equipment designs resulting in costly maintenance cycles

The following paragraphs discuss each one of these reliability issues and how they can be overcome using current seal technology

Excessive Shaft Orbiting Shaft run-out or orbiting is measured by using a dial indicator and measuring the F.I.M (full indicator movement) runout at the O.D of

the shaft at the face of the seal chamber (Figure 13-51)

Many mechanical seals are installed on mixers with a bearing support in the seal

canister that limits shaft deflection at the seal faces In other instances, especially

when retrofitting packed mixers to mechanical seals, the bearing may not be present and shaft deflection or orbiting can occur in the seal chamber area to levels that will cause contact between the shaft and stationary components of the mechanical seal Conventional seals designed for mixer canisters with integral bearing support can only tolerate small runouts, less than 0.062 inch When the packing is removed, orbiting of the shaft in the stuffing box area may be as much as 0.150 inch F.I.M One should be aware of these runout conditions before selecting a seal for a mixer

Providing Safety and Reliability Through Modern Sealing Technology 569

Figure 13-50 Various types of mixers

New mixer seal technology is available making tolerances up to 0.250 inch F.I.M at

the seal chamber possible These seals have larger radial clearances between the shaft and wider seal faces to prevent over-wipe of the seal faces during standard operation Figures 13-52 shows an example of conventional mixer seal technology Figure 13-53 shows the same seal design with greater runout capabilities

Stationary Seal Face Warpage One major problem found to cause excessive leakage on mechanical seals adapted to mixers is warpage of the stationary seal face

on either the inboard or the outboard seals Clamping loads on the stationary seal face between the seal housing and upper mixer seal flanges as shown in Figure 13-54 should be avoided Some manufacturers choose to clamp the stationary seat with the

seal housing against the flange using a gasket to cushion these loads While clamp- ing loads are somewhat reduced, deflections at the seal faces of 50 to 3010 lightbands still occur This can cause unstable startup problems with the mechanical seals in

Figure 13-51 Mixer shaft “orbiting,” i.e., operating with excessive runout

Figure 13-52 Conventional mixer seal clearances

terms of leakage to atmosphere and into the vessel A better solution, shown in Fig-

ure 13-55, is to isolate their inboard stator face from these large clamping loads and thermal and mechanical distortion of the vessel flange

Pressure Reversals Unseat Seal Faces Pressure reversals or loss of barrier fluid pressure can result in a reversal of hydraulic loads on the inboard seal hardware If the seal hardware is not properly retained using a lock ring design for the inboard insert and reverse pressure capabilities for the rotating seal ring, the seal faces can blow open and unseat the stationary seal face Pressure reversals on mixer seals is a common failure mode in industry This can be prevented by designing for and order- ing mixer seals with full pressure reversal capabilities as is typical of the seal shown

in Figure 13-56

Providing Safety and Reliability Through Modern Sealing Technology 573

Figure 13-53 Mixer seal with increased runout capabilities

Figure 13-54 Clamped-style stationaty seal face

Wear Debris Contaminates Product Typical wear that occurs at the seal face

may over time accumulate and drop into the mixer vessel This is normally not objectionable because only minute amounts of carbon-wear debris are actually occurring If the product, however, is an injectable drug or has extremely tight purity specifications, this wear debris may become objectionable Objectionable wear debris can be eliminated fmm falling into the mixer vessel by use of the sanitary

Figure 13-55 Non-clamped-style stationary seal face

Figure 13-56

seal face Retained stationary

design feature shown in Figure 13-57 Another solution to objectionable wear debris

is full-fluid film gas or liquid seal technology

Barrier Fluid Leakage Into Vessel Similar to wear-debris contamination of the

product is product contamination due to seal barrier fluid leakage into the process stream Quite often this is assumed to be a norm and processes are designed to clean

up the purity of the product later in the cycle This is not necessarily the only solu- tion Both the sanitary design feature shown in Figure 13-57 and gas barrier sealing technology can be considered as well Double gas seal technology for mixers operat- ing at almost any pressure is available today

Non-Coupled Equipment A problem that often occurs when installing mechanical

seals on older equipment is that the large non-coupled mixer shaft does not have a

Providing Safety and Reliability Through Modern Sealing Technology 573

Figure 13-57 Mechanical seal with debris catcher

coupling either inside or outside the tank to facilitate the installation of a mechanical seal This kind of installation usually requires pulling the entire mixer drive and reworking the equipment, which can result in hiring large cranes and maintenance crews and spending literally days of process time to retrofit the technology In these instances, split seal technology may be a wise choice in order to avoid costly mainte- nance problems while providing near zero leakage characteristics from the mechani- cal seal (Figure 13-58) Both liquid and dry running split seal technologies are avail- able for mixers

Table 13-6 summarizes the most common mixer rotating equipment reliability issues and their solutions

designed into the differential gear box on the equipment The differential speed is

usually between 10 and 100 rpm, either faster or slower than the conveyor speed

There are three distinctly different sealing locations on a typical centrifuge that must

be properly application-engineered in order to achieve adequate centrifuge perfor- mance These sealing points include:

Figure 13-58 Split seal technology

Table 13-6 Rotating Equipment Reliability Problems

Shaft orbiting

Stationary seal face warpage

Pressure reversal unseats seal faces

Wear debris contaminate product

Barrier fluid leakage into vessel

Make sure basic seal design and geometry accommodate orbiting and runout

Avoid clamp-style stationary seal face Make sure seal design offers pressure reversal Investigate stationary design feature

Consider gas seal versus liquid seal technology

The case seal that seals between the centrifuge casing and the rotating shaft con- The feed pipe seal that seals between the static feed pipe and the rotating bowl Internal seals that seal the bearing internal to the centrifuge

veyors

head shaft

Pressurized centrifuges typically use one of three different types of sealing devices depending on the temperature, pressure, speed, and level of emission control that is desired for the application These sealing devices include:

Dynamic seals-usually a repeller or expeller type liquid ring seal

Segmented bushing seals

Axial end face mechanical seals

Centrifugal Seal Reliability

Major reliability factors found over the years relative to these seal selections have included:

Providing Safety and Reliability Through Modern Sealing Technology 575

e Concentricity and perpendicularity and axial shaft growth problems with casing

e Dynamic seal leakage under static conditions

e Product contamination by the buffer fluid

O-ring retention problems

seals

Concentricity, Perpendicularity, and Axial Growth Case seal designs assure

good concentricity, perpendicularity, and axial growth capabilities up to +_OS00 inches by mounting the casing seals perpendicular and concentric with the bearing housing Case seals are typically piloted and registered off the bearing housing to

assure concentricity and perpendicularity with the axis of the shaft Thermal growth differences between the shaft and the centrifuge casing are accommodated through

the use of an expansion joint between the casing seal housing and the centrifuge housing This joint may be sealed with packing, O-rings, or a flexible metal bellows (Figure 13-59)

Dynamic Seal Leakage Upgrades from dynamic seals to either mechanical end

face seals or segmented bushing seal housings with a constant barrier fluid purge are often required in order to achieve true leak-free performance while centrifuges are

static Proper consideration of environmental legislation concerning the process fluid

being handled should address these conditions Retrofit packages ranging from

dy narnic seals to segmented bushing seals or mechanical seals to achieve lower emissions under static standby conditions are available

Figure 13-59 Seal canister secured to bearing housing

Figure 13-60 Rectangular versus dovetail O-ring groove

Table 13-7 Rotating Equipment Reliability Problems

with axial expansion accommodated in seal housing design

Dynamic expeller seal leaks static Upgrade to mechanical seals or segmented

bushing with constant barrier fluid pressure Product contamination by buffer fluid Segmented bushing seal or mechanical seal

with gas barrier technology

O-ring retention problem Use dovetail O-ring groove to aid installation

Steam Turbines

Steam turbines are used in many industries as mechanical drives where electric power is either unreliable or unavailable Figure 13-61 shows the components of a sin- gle-stage steam turbine Sealing devices are used to prevent steam from leaking past the shaft where it passes through the turbine casing The majority of general-purpose, single-stage steam turbines are sealed with a series of segmented carbon bushings, (Figure 13-62) Labyrinth seals are also used as sealing devices Mechanical seals

using gas sealing technology were used at an Exxon plant in 1982 and are now gain-

ing acceptance as a superior sealing alternative for many steam turbine appli~ations.2~

Providing Safety and Reliability Through Modern Sealing Technology 577

Figure 13-61 Single-stage steam turbine showing carbon ring seals, Item 5 (Courtesy

of Dresser-Rand, Wellsville, New York.)

1 Bolt-type overspeed trip

repair costs for these two seal types

The cost of the segmented carbon bushings is based on between 8 and 12 bushings per steam turbine at $50 per bushing The mechanical seals are estimated at $5,000 per seal The expected life of the labyrinth seal is not listed because, assuming prop-

er material selection and sizing, the labyrinth should not wear or corrode and should last indefinitely Although the mechanical seals that are applied in steam turbines are typically non-contacting, the secondary seals are subject to degradation in the high temperature steam, thus generating the expected life of five years A cost analysis of the three sealing options reveals that, in spite of the high initial cost of a mechanical seal, its payback period as compared to either segmented carbon bushings or labyrinth seals is less than six months due to the reduction in operating costs (pri- marily due to significantly lower steam leakage) and lower repair costs

Steam turbines offer a harsh environment in which seals must operate This envi- ronment creates seal reliability issues that are dependent on the seal design These issues are discussed in the following paragraphs for the three seal designs most com- monly applied to steam turbines

Segmented Carbon Bushings

The most significant problem associated with the segmented carbon bushings shown in Figure 13-62 is the high rate of steam leakage In addition to the value asso-

ciated with lost steam, the escaping steam enters the bearing lubricating oil and caus-

Table 13-8 Steam Seal Characteristics

10,000 7,500

Providing Safety and Reliability Through Modern Sealing Technology 579

es premature bearing failure The corrosiveness of the steam also shortens the life of other components in the vicinity of the steam turbine The life of the carbon bushings

themselves is relatively short, ranging from six months to two years The manner in

which the carbon bushings are broken in can significantly affect bushing life Although the bushings are designed to maintain a clearance over the turbine shaft, thermal expansion rates and shaft misalignment can cause contact between the bush- ing and the shaft For this reason, special treatments are often applied to the shafts under the bushings Depending on the degree of contact between shaft and bushings, turbine maintenance procedures may require the shaft to be repaired and recoated

Labyrinth Seals

The labyrinth seals that are used in steam turbines may include features such as

steps or active loading mechanisms As is the case with segmented carbon bushings,

labyrinth seals also have high steam leakage rates that can damage bearings and other components Clearances between the shaft and the labyrinth seal vary as a function of operating temperature due to the different rates of thermal expansion of the materials for these two components The labyrinth seal leakage is proportional to the cube of the clearance so any variations in clearance can have a significant impact

on the steam leakage The potential for contact between the labyrinth seal and the shaft may require the application of special treatments to the shaft

Mechanical Seals

When properly designed and installed, mechanical seals provide a greatly reduced leakage rate as compared to the segmented carbon bushings and labyrinth seals The leakage rates o f the mechanical seal are on the order of 500 times less than the alter- native sealing technologies Due to the poor lubricating properties of steam, the mechanical seals applied to steam turbines are effectively dry running seals and con- sequently employ gas seal technology Figure 13-63 shows cross-sectional views o f

Figure 13-63 Typical flexible rotor steam turbine seal designs

ditions When proper preheating procedures are not followed, mechanical seals may start up in water that requires them to be able to operate with liquid lubrication Con- taminants in the steam can act to clog the face patterns on non-contacting gas seals, diminishing the available load support and possibly resulting in contact between the seal faces If mechanical end face seals are adapted to a steam turbine, quality and cleanliness of the steam must be carefully reviewed

Table 13-9 summarizes major steam turbine seal reliability issues and potential solutions

Table 13-9 Rotating Equipment Reliability Problems STEAM TURBINE Reliability Issue Recommended Solution

Axial end face mechanical seal Bearing closure device upgrade (See Figures 11-8 and 12-3)

Seal sleeve and housing must

be of material with same thermal expansion rate as shaft and casing

Short life and high leakage of segmented Bearing housing oil contamination Dissimilar material of construction

carbon rings with water

Conclusion

Specialty mechanical seals will have a major impact on the reliability of rotating equipment in the future The increased application of mechanical seals to non-pump rotating equipment is expected to require larger diameter seals that operate at higher pressures, speeds, and temperatures than ever before Achieving these goals will require making practical use of our basic understanding of seal geometries and the var-

ious modes of lubrication available to us today Technological advances to convention-

al seal designs will carry us beyond the simple mechanical seal with two lapped faces that operate under boundary lubrication with liquid or gas Future seals will incorporate multifaceted seal face geometries that will operate in one of the four known modes of