emission standards require sha# sed that not leak ...infohouse.p2ric.org/ref/31/30397.pdf · i f...

Strict emission standards require sha# s e d that do not leak. Adyzing failures preuents seal maintenance probhns.

IOSEPH L. FOSZCZ. P E K P E . Senior Editor

hen shaft seals consisted of packing rings in a stuffing box, failure was typically a gradual increase in leak-

age. Repairs could be put off by tightening the stuffing box gland nuts. A certain amount of leakage was tolerated and ac- cepted. For example, a typical packing set wouici ieak an average of 90 drops a minute or 833 gal./yr.

With the scheduled implementation of Phase I of the Clean Air Act Amendments of 1990 due after October 1992 (Fig. I ) , packing-type leakage is no longer accept- able and plants are looking for ways to re- duce leakage and be in compliance. A me- chanical face seal averages 5 drops an hour or 0.8 gal./yr.

Mechanical face seals provide a dramatic reduction in leakage around a shaft and place no wear on the shaft. When the seal is new, leakage is negligible and can be con-

(Photo courtesy John Crane, Inc.)

sidered nonexistent. An understanding of how these seals function, types available, and materials of construction helps in their selection and troubleshooting.

Although designs differ, all mechanical seals basically function the same way using two or four contacting faces or sealing sur- faces (Fig. 2 ) . These sealing surfaces, of dis- similar materials, are perpendicular to the shaft. One is stationary and the other rotat- es. The surfaces ride on, and are lubricated and cooled by, a film of fluid. The seal faces are held together by spring forces and, in some designs, by fluid pressure.

The variety of seal designs available re- sults from the various mounting methods used, the flexibility provided, and the needs of the applications (Fig. 3). The most common designs are pusher and nonpusher seals.

Pusher seals require axial movement of a

FILE 4099/5540 46 * PLANT ENGINEERING SEPTEMBER 3,1992

dynamic secondary seal, such as an O-ring, to compensate for face wear and keep the seal faces in contact. These seals use single, multiple, or wave-washer springs to main- tain a seal closing force.

Nonpusher seals include elastomeric or metal bellows secondary seals. These sec- ondary seals are static and do not require axial movement past them to maintain seal face contact.

The flexible portion of the seal that pro- vides closing force to the seal faces can be rotary or stationary. The stationary portion is called the seat or insert.

Seal designs are also divided by the way hydraulic: pressure is appiied io the seai faces. If the seal is designed to reduce hy- draulic forces in the seal chamber acting to close the seal faces, it is balanced. Unbal- anced seals allow increasing hydraulic pres- sure to increase the forces on the seal faces.

Seal Types Single inside seals are usually mounted in

stuffing box chambers where fluid pressure assists sealing and promotes face closing. The materials of construction are selected to withstand corrosive liquids in the stuff-

I General Emission Requirements

m

0 m 2 w

Refinery SOCMA Quarterly Quarterly Quarterly/ Annual Uncon- Uncon- LDAR LDAR Semiannual Monitoring trolled trolled (10,000 (500 ppm) LDAR

PPm) (500 ppm); Monthly

Monitoring

Regulatory Program SOCMA - Synthetic Organic Chemical Manufacturers Association LDAR - Leak Detectlon And Repalr

ing box. These seais are easiiy modified io accommodate environmental controls and can be balanced. Adjustments require dis- mantling equipment unless the seal is car- tridge mounted.

Single inside seals represent about 75% of all installations. They are the most eco- nomical sealing systems available to indus- try. This type of seal uses the liquid to be sealed for lubrication.

Single outside seals handle extremely cor- rosive liquids that have satisfactory lubri- cating properties. Expensive metallurgies

Fig. i. Emissions to trje at- mosphere will eventual4 have to be lowered from 10,000 to IOOOppm for pumps. Mechanical seals are capable of meeting these emission standards.

FILE 4099/5540 SEPTEMBER 3, 1992 PLANT ENGINEERING * 47

i f

Seal d e s i w re@ct the nee& of the application. Ald desi= are avaihble in cartridge form.

Mechanical Seal Classifications

I

Inside ? Double Face-To-Face Outside

Tandem 1

I Staged

I ' I 1 Unbalanced I Pusher Type I Non-Pusher Type 1 Balanced I SecondaG Seal

I

Secondary Seal b 7 Single Spring Bellows Seal L I Flexible Rotor Metal

V-Rings I I I

Face On Rotor I I I I I Elastomer

I Wedge Ring I

Fig2 Mechanical seals are classified by arrangement and design. m i l e appearances may differ, all seals f i t into one of the classifications shown.

FILE 4099/55d 48 PLANT ENGINEERING SEPTEMBER 3,1992

Fig2 Mechanical seals are classified by arrangement and design. m i l e appearances may differ, all seals f i t into one of the classifications shown.

I

TFE I

FILE 4099/55d 48 PLANT ENGINEERING SEPTEMBER 3,1992

are avoided. Outside seals are used with equipment that will not accommodate in- side seals. They are easy to access for ad- justment and troubleshooting. Since the seal is exposed, it is vulnerable to damage by impact. All outside seals are limited to applications having moderate pressures.

Double seals handle toxic liquids whose leakage would be hazardous, liquids with suspended abrasives that would rapidly wear faces, and corrosive liquids requiring costly materials. All multiple seals require a barrier fluid system. In double back-to- back seals, barrier fluid prevents the pumped product from contacting inner portions of the seal and lubricates both sets of seal faces. The life of a double back-to- back seal can be up to five times that of a single seal in certain severe environments.

A double face-to-face seal is usually car- tridge mounted. One seal is inside the stuff- ing box and one seal outside. The main lim- itation of face-to-face arrangements is exposure of the inner seal to the product. Viscous, abrasive, thermosetting, or corro- sive products will damage the inner seal and cause leakage.

Tandem seals are used for volatile, toxic, carcinogenic, and hazardous fluids. They eliminate icing and freezing of light hydro- carbons and other fluids that fall below the freezing point of water in air. A tandem seal increases online reliability. If a prima- ry seal fails on a critical piece of equip- ment, the outboard seal will take over and function until maintenance is performed.

All seal designs are available in car- tridges that are assembled with relative ease. Only bolts on the gland plate and set screws in a collar need to be fastened to the pump and shaft. After spacers are removed and piping checked, the pump is ready to operate (Fig. 4).

Split seals are another design growing in popularity. This feature, available in some designs, eliminates the need for complete disassembly and realignment of pump and driver.

* Seal Failure Analysis Seal failure is a po-weif.uj tool for

diagnosing, correcting, and preventing ma- jor problems in seal maintenance. There is an important difference between seal fail- ure analysis and troubleshooting. In trou- bleshooting, the focus is on immediate problems. In seal failure analysis, the focus is on specific symptoms within a failed seal.

The purpose of seal failure analysis is not only to correct a specific failure, but to cor- rect conditions that caused the failure. There are four basic questions that should

3 4

Application ciuiae I O Gontroi tmissions

Tandem or double seals acceptable Double seals required

0 ' 1 05 506 100% 10,050 PPM

I Chart Area I Acceptabre Technology

be asked in seal failure analysis: What does the damage look like? Is it

chemical, mechanical, or thermal? How does the damage affect seal levels.

Fig. 3. S e d selection guide lines are based onfluid spc cific gravity and emission

- performance?

What does the ,damage indicate about a seal's past history?

What corrective steps will be taken to pre- vent damage from recurring?

Overall chemical attack leaves parts ap- pearing dull, honeycombed, flaky, or start- ing to crumble or break up. Corrosion is caused by using the wrong materials of con- struction, failure of the pressurizing sys- tem, or contaminated barrier fluid.

Remedial action should include obtain- ing a complete chemical analysis of the product being sealed and upgrading the materials of construction to meet chemical requirements. The corrosive environment can be neutralized by using double seals or flushing a single seal from an external source with a clean, compatible fluid.

Fretting corrosion causes leakage at sec- ondary seals and damages the sleeve direct- ly under the secondary seal. This area will appear pitted or shiny bright. This type of corrosion results from constant back-and- I U l l l l I l l U V G l l l G I l i 0 1 > G c u l l u a l y > G a l > UI Lul l -

stant vibration of the shaft packing over a shaft sleeve.

Causes of excessive vibration of second- ary seals are eliminated by making sure shaft runout, deflection, and endplay are within recommended limits. Protective hard-faced alloys can be applied directly under the area where secondary seals make contact, or the shaft or sleeve material can be upgraded. TFE V-rings, wedge rings, or taper rings can be replaced with O-rings that absorb minor axial shaft movement.

C _ - L L c ^ _ _ _ A ^ _ . . - - - l " _ _ ^^-

General Pump Checks for

Extending Seal Life

J Bearing lubrication

J Driver alignment with

J Rotor dynamic balance

J Piping strains due to misalignment or temperature

J Base plate rigidity and grouting

J Equipment-to-baseplate mounting

J Pump condition, bearings, shaft, and seal housing, including internal alignment

Pump

FILE 4099/5540 SEPTEMBER 3,1992 PLANT ENGINEERING 49

Common sed failures inclzue chemical attack, fretting corrosion, damaged elastomers, and leaching

Cartridge Single Seal (Inside)

A L / Drive Collar

1

' Drive Sleeve Stationary Face (Seat or Insert)

' Flush Port

'Gland Plate

Fig. 4. Seal components are assembled in a cartridge ready for mounting. (Cour- tesy A. W. Chesterton Co.) .

Pusher-type seals can be replaced with non- pusher-type seals having a static secondary seal.

Damaged elastomers that are swollen, hardened, bubbled, broken, or blistered are the result of chemical attack or excessive temperatures. The most likely causes are incorrect material selection or barrier fluid contamination or loss.

The fluid being pumped should be chem- ically analyzed and elastomer material se- lection re-evaluated. Frequently, the pres- ence of trace e!ements, arlgix!!y overlooked when specifying seals, is at fault. If suitable material cannot be found, the seal should be flushed by an external source or replaced with a seal design using a static secondary seal.

O-rings that appear to be cut or peeled may have been extruded. Causes may be excessive pressure or temperature, chemi- cal softening, or improper shaft and groove sizing. The cure can be a reduction in pres- sure or temperature; installation of a back- up ring; a change in seal design, O-ring ma- terial, or durometer; or proper sizing of

shaft and groove on the equipment. Leaching normally causes a minor in-

crease in seal leakage and a large increase in wear of the carbon faces. Ceramic and tungsten carbide faces that have been leached will appear dull and matted. Hard- ness readings on such seal faces will indi- cate a decrease from original values of 5 points or more on the Rockwell A-scale.

To correct the problem, the seal base ma- terial should be upgraded. .A seal arrange- ment should be set up that provides a buff- er fluid at the seal faces, such as a single seal with a flush from an external source or a double seal with a suitable barrier fluid.

Face distortion frequently causes exces- sive leakage and shows a nonuniform wear pattern. Improper assembly or uneven gland nut torquing causes nonuniform face loading. Improper cooling induces thermal stresses and distortions. Poor surface finish at the face of the seal housing, due to corro- sion or mechanical damage, also causes face distortion. Misalignment is another cause of an uneven wear pattern.

Seal faces could be relapped to remove distortions. Other corrrective actions in- clude using flexibly mounted stationary seal faces to compensate for gland distor- tion, checking alignment of the pump shaft and drive motor, and retorquing the gland nuts to recommended values.

Seal face deflection is indicated by un- even wear or a narrow wear pattern on the wide face. Wear can be either concave or convex. A convexed seal face usually has abnormally high leakage rates, while a con- vexed face usually results in excessive seal face torque and heat. Seals with either con- dition are not stable under cyclic pressure conditions. Causes may be improper sta- tionary seal face support, swelling of sec- ondary seals, excessive pressure, or inade- quate balancing of hydraulic and mechanical loads on primary seal faces.

Corrective and preventive measures in- clude checking the seal design's operating limits and consulting with the manufactur- er about other seal designs, considering a

and replacing carbon seal faces with seal faces having a higher modulus of elasticity (such as bronze, silicon carbide, or tungsten carbide).

Tracking creates a wear pattern on the larger seal face that is wider than the thin- ner face. This condition is caused by mis- alignment, pipe strain, cavitation, equip- ment in poor operating condition, or excessive pressure.

The problem is often corrected by check- ing and adjusting the centering of the seal in its housing as well as equipment and op-

i

>.

Bexil?!e "Et fa: the s:a:ionaiy sea: face,

10 PLANT ENGINEERING * SEPTEMBER 3,1992 FILE 4099/5540

Mechanical Face Seal Reference Guide Shaft Pressure, Temp Face

Circle Company Size, in. Speed, Max. Psig Material" Cartridge Range, F Single, Inside

52 A. W. Chesterton Co. 5/ , to 6 4000 fpm 700 -65 to 500 C, SC, TC Y & SPLIT

54 John Crane, Inc. 1 t o7 7200 rpm 3000 700 C, SC, TC, CE Y & SPLIT 55 Durametallic Corp. 1 t o 4 & 3600 rpm 300 300 C, SC, CE Y & SPLIT

51 BW/IP International, Inc. l/z to 12 250 fpm 150 800 C, SC, TC, CE Y

53 C. LeeCook 6% 5500 rpm 1500 30 to 400 SC, CG Y

56 EG&G Sealol 3h to 6 4500 fpm 1000 100 to 800 C, SC Y 57 Garlock, Inc. 1/2 to 8 2500 fpm 400 400 C, SC, TC, CE Y 58 Pac-Seal Inc. 1h to 3 5000 fpm 200 unbalanced, -40 to 400 C, T, S, P, NI Y

ANSI sizes

600 balanced

700 C, SC, TC, CE Y 69 Durametallic Corp. 400 C, SC,TC Y 70 Garlock, Inc. 400 C, SC, TC. CE Y 71 Pac-Seal Inc. -40 to 400 C. T, S. P. NI N Double, Face-to-Face 72 BW/IP International, Inc. r/, to 12 250 fpm 150 800 C, SC, TC, CE Y 73 A. W. Chesterton Co. 1 to 4% 4000 fpm 700 -65 to 500 C, SC, TC Y 74 C. LeeCook 6% 5500 rpm 1500 30 to 400 SC, CG Y 75 John Crane, Inc. 1 t o 6 18,000 rpm 1200 700 C, SC, TC, CE Y 76 Garlock, Inc. 3h to 5 3500 fpm 200 400 C, SC, TC Y

77 BW/IP International, Inc. 112 to 12 250 fpm 3000 800 C, SC, TC, CE Y 78 A. W. Chesterton Co. 1 % to 2% 4000 fpm 300 -65 to 500 C, SC Y 79 John Crane, Inc. 1 to13 25,000 rpm 3500 700 C, SC, TC, CE Y 80 Durametallic Corp. ANSI sizes 3600 rpm 225 400 C, SC,TC Y 81 EG&G Sealol 1 % to 1 % 4500 fpm 300 -100 to 400 C, SC, TC Y 82 Garlock, Inc. 3h to 5 3500 fpm 200 400 C, SC,TC Y 83 Pac-Seal Inc. 1h to 3 5000 fpm 200 -40 to 400 C, T. S, P, NI N

Double, Tandem

'AC = Alumina Ceramic; C = Carbon; CE = Ceramic; CG = Carbon Graphite; NI = Ni-Resist P = Purebide; S = Silicon; SC = Silicon Carbide; T = Tungsten; TC = Tungsten Carbide; TE = Teflon

erating conditions. In instances where alignment is not possible, a design with a

face material, such as tungsten carbide or bronze, for carbon.

static secondary seal may provi&e enough compensation.

Heat checking is indicated by the pres- ence of fine-to-large cracks that radiate from the center of the seal ring. These cracks act as a series of cutting and scraping edges against the opposite seal face. Heat causes this condition by a lack of cooling, vaporization at the seal faces, or excessive pressures and velocities.

Temperatures can be reduced by flushing or cooling the seal faces and eliminating overloading. Often, a thrust collar may have become damaged or inoperative, cre- ating excessive seal face loads.

Blistering is characterized by small circu- lar sections that appear raised on carbon seal faces. Blisters separate the seal faces during operation and cause high leakage rates. Blistering commonly occurs in seals that are started and stopped frequently in applications involving high viscosity fluids.

The viscosity of the fluid in the seal cavi- ty can be reduced by increasing tempera- ture or by substituting another fluid. Other actions include eliminating frequent starts and stops or substituting a nonporous seal

For more information. . . A previous PLANT ENGINEERING article pre- sented information on seal failure. "Avoiding Mechanical Pump Seal Problems" (1/19/89, p 98, File 4010) offered detailed information on checking a pump's mechanical condition to locate potential causes of premature seal failure.

Dura Seal Manual, ninth edition, published by the Durametallic Corp., is a source of in- formation on all aspects of seal technology. Contact the company at 2104 Factory St.,

dering and pricing information. Mechanical Seal Handbook, published by

the Fluid Sealing Association, discusses the basics of mechanical seals, design, selec- tion, and troubleshooting. Contact the asso- ciation at 2017 Walnut St., Philadelphia, PA 191 03, (21 5) 569-3650 for ordering and pric- ing information. . . . Joseph L. Foszcz, Senior Editor, 708-

Ka!mazee, ?A! 49001, (616) 381 -2650 tor or-

390-2699.

For information on how to order copies of this article circle 10 on post card

FILE 4099/5540 SEPTEMBER 3,1992 * PLANT ENGINEERING 51