Improving Machinery Reliability 3 Episode 14 docx

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. Seal Classification All sealing devices can be classified into two generic classifications: static and dynamic (Figure 13-35). Static seals include such products as gaskets, sealants, and direct-contact sealing devices. Dynamic seals, which will be the broader point of discussion in this seg- ment, are classified into two broad categories: rotating seals and reciprocating seals. Our focus will primarily be on rotating sealing devices. 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). 560 Improving Machinery Reliability I SealingPevices I I Gaskets I I Sealants I I Direct Contact 1 Figure 13-35. General classifications of seals. n I Semi-dynamic\secondary seal Figure 13-36. Typical inside pusher seal. Providing Safety and Reliability Through Modern Sealing Technology 561 Solids Build Up / Leakage 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 Solids Bulld Up ' Figure 13-38. Non-pusher seal. 562 Improving Machinery Reliability life, reduced power consumption, and reduced emissions to the atmosphere can be achieved. The four basic modes of lubrication include full-fluid film lubrication with either liquids or gases and boundary lubricated seals with either liquids or gases (Figure 13-39). Full-Fluid Film Gas and Liquid Lubrication. This terminology is applied to seals that for all practical purposes are constantly separated by a full-fluid film. Because there is no contact between the mating faces, there is virtually no wear, and seal designers are able to predict long life and stable seal performance. The pressure pro- file generated across the seal faces is a function of the seal face geometry and seal face deflections that occur under pressure and temperature transient conditions in the seal cavity. Evaluation of these seals defies older methods that assumed a linear pressure drop across the seal faces. This technology is applied to a new family of 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. Full Fluid Film Boundary Lubrication Figure 13-39. Lubrication modes. 563 fldalive vdwiiy at Seal Ccnlact (IeeVmin.) Figure 13-40. Operating limits of boundary and full-fluid film seals. Dollars 2,000 1.530 1.000 50 0 HP Consumption 5 4 3 2 1 '0 1.000 2.000 3.000 Seal Size, inch 4.000 Figure 13-41. Annual energy cost and power consumption for oil vs. gas lubricated seals. 564 Improving Machinery Reliability Basic Seal Geometry Another factor having significant impact on seal selection for various applications is our understanding of basic seal geometry and the advantages and disadvantages that various seal geometries offer. In general, most commercial seal designs can be classified by one of four basic seal geometries (Figures 13-42 through 13-45). None of these arrangements is considered a superior design for all applications. But, under given operating conditions, each design has recognized advantages and disad- vantages that should be understood. Table 13-4 summarizes the four basic seal geome- tries and what are recognized as some of their basic advantages and disadvantages. 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 Design Varieties -0 DEE. na Shafl Shaft Shaft Shan auwuons asistlng to seal 1. Flexible Rolor 2. Flexlble Rolor 3. Flexlble Stator 4. Flexible Stator psrlommnce Wear Fam 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 SBal Yes No Yes Yes Yes Yes Yes No No Yes NO No Yes Yes No No Yes No NO No 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 566 Improving Machinery Reliability 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. 568 Improving Machinery Reliability Axial Equipment Movement Vibration Mixer Compressor Centrifuge Steam Turbine Table 13-5 Specialty Equipment Performance Characteristics High Speed High I I I I Mixers 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. [...]... Contam 2nd 1st 3rd na $12,000 na na na na $18,980 $20,878 $22,966 $31 , 536 $33 ,1 13 $34 ,768 $ 130 ,086 $ 130 ,086 $ 130 ,086 $25,000 $31 ,250 $39 ,0 63 $15,000 $15,000 $15,000 $0 $0 $0 $220,602 $242 ,32 7 $241.8 83 $220,602 $462,929 $704,812 Gas Seals 5th 4th $12,000 na $25,262 $36 ,507 $ 130 ,086 $48,828 $15,000 $0 $267,6 83 $972,495 na na $27,789 $38 ,33 2 $ 130 ,086 $61, 035 $15,000 $0 $272.242 $1,244, 737 Years of Operation... Shutdown na $0 - $12,000 na $20,878 $33 ,1 13 $ 130 ,086 $31 ,250 $15,000 $0 na na $22,966 $34 ,768 $ 130 ,086 $39 ,0 63 $15,000 $0 $12,000 na $25,262 $36 ,507 $ 130 ,086 $48,828 $15,000 $0 Annual Total Cost $174,000 $220,602 $242 ,32 7 $241,8 83 $267,6 83 na na $27,789 $38 ,33 2 $ 130 ,086 $61, 035 $15,000 $0 $272,242 CumulativeTotal Cost $174,000 $39 4,602 $ 636 ,929 $878,812 $1 ,146 ,495 $1,418, 737 Item Startup ~ Gas Seals Years... 2nd 3rd 5th 4th na na na na na na na $4,205 $6, 938 $5,000 na na $4,205 $6, 938 $5,000 na na $4,205 $6, 938 $5,000 na na $4,205 $6, 938 $5,000 na na $4,205 $6, 938 $5,000 $195,000 $16,1 43 $16,1 43 $16,1 43 $16,1 43 $16,1 43 $195,000 $211,1 43 $227,285 $2 43, 428 $259,571 $275, 714 Cost of Ownership Gas Seals vs.Oi1 Seals $1,400,000 1 I $1,200,000 + 8 - $1,000,000 ' $ $800,000 $600,000 $400,000 $200,000 $0 0 1 2 3. .. 2nd na na 3rd na na na na na na na na na na na na $4,205 $4,205 $4,205 $4,205 $6, 938 $6, 938 $6, 938 $6. 938 $5,000 $5,000 $5,000 $5,000 $11,1 43 $11,1 43 $11,1 43 $11,1 43 na na $4,205 $6, 938 $5,000 $11,1 43 $196,1 43 na na 5th 4th $207,285 na na $218,428 $229,571 $240, 714 Cost of Ownership Gas Seals vs.0il Seals $1,400,000 c g !? m E $1,200,000 $1,000,000 $800,000 $600,000 $400,000 $200,000 $0 0 1 2 3 Years... elastomers and more flexible keys, have also been found suitable Table 13- 13 summarizes compressor gas seal reliability issues and recommended solutions as a quick reference Figure 13- 75 Soft drive of rotor Providing Safety and Reliability Through Modern SeaEing’Technology 5 93 Table 13- 13 Rotating Equipment Reliability Problems COMPRESSOR Reliability issue Safety-high pressure oil leaks are a potential... centrifuge that must be properly application-engineered in order to achieve adequate centrifuge performance These sealing points include: 574 Improving Machinery Reliability Figure 13- 58 Split seal technology Table 13- 6 Rotating Equipment Reliability Problems Reliability Issue Recommended Solution Shaft orbiting Mixers Make sure basic seal design and geometry accommodateorbiting and runout Avoid clamp-style... bracket 9 Governor 10 Internal oil reservoir 11 Instrument port 578 Improving Machinery Reliability 12 Hand-operated nozzle control 13 Water cooling jackets 14 Overspeed trip adjustment port Table 13- 8 summarizes the typical characteristics of the predominant seal types that are used in general-purpose, single-stage, steam turbines with a 3- inch shaft size The initial capital cost represents the cost of... steam, the mechanical seals applied to steam turbines are effectively dry running seals and consequently employ gas seal technology Figure 13- 63 shows cross-sectional views o f Figure 13- 63 Typical flexible rotor steam turbine seal designs 580 Improving Machinery Reliability a typical steam turbine mechanical seal using both elastomer and spring energized secondary seals Because of the high operating... this configuration of tolerance Providing Safety and Reliability Through Modern Sealing Technology Figure 13- 72 Rotor design with high internal stresses Figure 13- 73 Seal rotor design minimizing internal stresses Figure 13- 74 Tolerance ring centering at OD versus ID 591 592 Improving Machinery Reliability rings occurs when the ring slips up underneath the rotating seal face and gets trapped between... deflections at the seal faces of 50 to 30 10 lightbands still occur This can cause unstable startup problems with the mechanical seals in 570 Improving Machinery Reliability 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 Figure 13- 55, is to isolate their inboard . and hydraulic forces (Figure 13- 37). 560 Improving Machinery Reliability I SealingPevices I I Gaskets I I Sealants I I Direct Contact 1 Figure 13- 35. General classifications. sealing points include: 574 Improving Machinery Reliability Figure 13- 58. Split seal technology. Table 13- 6 Rotating Equipment Reliability Problems Mixers Reliability Issue Recommended. shown in Figure 13- 56. Providing Safety and Reliability Through Modern Sealing Technology 5 73 Figure 13- 53. Mixer seal with increased runout capabilities. Figure 13- 54. Clamped-style