Tribology Handbook 2E Episode 8 pptx

For high temperature/high pressure steam, moulded rings of expanded graphite foil material are commonly used.. B25 Packed glands PACKING MATERIALS Table 25.1 Materials for use in packed

625 Packed glands

The main applications of packed glands are for sealing the stems of valves, the shafts of rotary pumps and the plungers of reciprocating pumps With a correct choice ofgland design and packing material they can operate for extended periods with the minimum need for adjustment

VALVE STEMS

Valve stem packings use up to 5 rings of packing material

as in Figure 25.1 For high temperature/high pressure

steam, moulded rings of expanded graphite foil material

are commonly used This gives low valve stem friction

T o reduce the risk of extrusion of the lamellar graphite

during frequent valve operation, the end rings of the

packing can be made from graphite/yarn filament

Materials of this type only compress in service by a

small amount and can provide a virtually maintenance

free valve packing if used with live loading as shown in

Figure 25.2

Rotary pump glands commonly use up to 5 rings of

packing material For most applications u p to a PV of

150 bar m/sec (sealed pressure X shaft surface speed) a

simple design as in Figure 25.3 is adequate I n most

pumps the pressure at the gland will be 5 bar or less and

those with pressures over 10 bar will be exceptional

At PV values over 150 bar m/sec direct water cooling or

jacket cooling are usually necessary and typical arrange-

ments are shown in Figure 25.4 and 25.5

When pumping abrasive or toxic fluids there may be a

need to provide a flushing fluid entry at the fluid end of the

glands, as in Figure 25.6, or a high pressure barrier fluid

which is usually injected near the centre of the gland as in

Figure 25.7

Figure 25.1 A typical valve stem packing

DISC SPRING STACK STUD SPACER

S L E E V E

B R A I D E D GRAPHITE EXPANDED GRAPHITE BRAIDED GRAPH IT€

Figure 25.2 A valve stem packing using spring loading to maintain compression of the valve packing and avoid leakage

LIQUID

G L A N D FOLLOWER

Figure 25.3 A general duty rotary pump gland

625.1

Reciprocating pumps also use typically 5 packing rings

However due to the increased risk of extrusion of the

packing due ito the combination of high pressure and

reciprocating movement, an1 i extrusion elements are

usually incorporated in the gland

Self adjusting glands c a n be used on reciprocating

pumps but the spring loading for compression take up

must act in the same direction as the fluid pressure

loading, as shown in Figure 25.10

JACKET COOLING JACKET COOLING

PTF E ANTI-EXTRUSION RING

Figure 25.8 A reciprocating pump gland with PTFE

anti-extrusion washers between the packing rings

Figure 25.10 A reciprocating pump gland with

internal spring loading t o maintain compression of

the packing

Figure 25.9 A reciprocating pump gland with an anti-extrusion moulded hard fabric lip seal

B25.2

B25 Packed glands

PACKING MATERIALS

Table 25.1 Materials for use in packed glands

Material Maximum operating

temperature “C Special properties Typical application5

2500°C in non-oxidising low compression set and

constituents Available as rings

square section lengths

Resistant to extrusion

Valve stems

chemical resistance

Valve stems Pumps a t surface speeds below

10 m/s

speed rotary shafts

Close bush clearances needed

to reduce risk of extrusion

Good resistance to abrasives

Pump shafts for speeds of the order of 25 m/s

Typical gland dimensions are shown in Figure 25.1 1 and packing sizes in Table 25.2

Table 25.2 Typical radial housing widths in relation to shaft diameters All dimensions in mm

All packings except expanded graphite Exponded graphite

Shajl diameter Housing radial width Shaft diameter Housing radial width

8

10 12.5

Packed glands B25

A = 7 W

r FIRST OBSTRUCTION

ROTATING S H A F T (DEPTH 7 W APPLIES WHEN LANTERN GLAND IS USED)

RECIPROCATING SHAFT

Figure 25.11 Typical gland dimensions for rotating and reciprocating shafts

Pump shafts, valve stems and reciprocating rams should have a surface finish of better than 0.4 p m Ra Their hardness should not be less than 250 Brinell

Rings should be cut with square butt joins and each fitted individually with joins staggered at a minimum of 90" After applying a small degree of compression to the complete set, gland nuts must be slackened off to finger tight prior to start up Once running, any excessive leakage can then be gradually reduced by repeated small degrees of adjustment T h e major

cause of packing failure is excessive compression, particularly at the initial fitting stage

Further advice may be obtained from packing manufacturers

B25.4

B26 Mechanical piston rod packinqs

mechanical rod packing assembly T h e packing (sealing)

rings are free to move radially in the cups and are given an

axial clearance appropriate to the materials used (see

Table 26.2) The back clearance is in the range of 1 to

5 mm (& to in) The diametral clearance of the cups is

chosen to prevent contact with the rod; it lies typically in

the range 1 to 5 mm (A to 6 in) T h e sealing faces on the

rings and cups are accurately ground or lapped

T h e case material can be cast iron, carbon steel, stain-

less steel or bronze to suit the chemical conditions It may

be drilled to provide lubricant feed to the packing, to vent

leakage gas or to provide water cooling

The rings are held in contact with the rod by spring

pressure; sealing action however, depends on gas forces

which hold the rings radially in contact with the rod and

axially against the next cup

PRESS END

CONNECTION GASKE

Figure 26.1 General arrangement of a typical mechanical piston rod packing assembly

Description

Three-piece ring with bore matching rod Total circumfe-

rential clearance 0.25 mm Garter spring to ensure contact

with rod

Applications

Used in first one or two compartments next to high

pressure, when sealing pressure above 35 bar (500 p s i )

to reduce pressure and pressure fluctuations on sealing

rings

2 Radial cut/Tangential cut pair

Description

The radial cut ring is mounted on the high pressure side

(Two tangential cut rings can be used when there is a

reversing pressure drop.) The rings are pegged to prevent

the radial slots from lining up Garter springs are fitted to

ensure rod contact Ring bores match the rod

Applications

The standard design of segmental packing Used for both

metallic and filled PTFE packings

_ ~ _ _ _ _ _ _ _

TANGENT 'IAL

OR RADIAL CUT RING

B26.1

Mechanical piston rod packings B26

3 Unequal segment ring

Description

The rings are pegged to prevent the gaps lining up Garter springs are fitted to ensure rod contact The bore of the larger segment matches the rod

of packing has to be assembled over the end of the rod

Applications

Used for both metallic and filled PTFE packings

B26.2

B26 Mechanical piston rod packings

DESIGN OF PACKING ARRANGEMENT

Number of sealing rings

There is no theoretical basis for determining the number of

sealing rings Table 26.1 gives values that are typical of

70-1 50 bar (1OOC-2000 p.s.i.) 8

above 150 bar (2000 p.s.i.) 9-12

Piston rods

Rod material is chosen for strength or chemical resistance Carbon, low alloy and high chromium steels are suitable For the harder packings (lead bronze and cast iron) hardened rods should be used; treatment can be flame or induction hardening, or nitriding Chrome plating or high chromium steel is used for chemical resistance

f0.05 mm (+0.002 in) -0.05 mm (-0.002 in)

Notes: 1 With Type 4 packings increase number of sealing rings

by 50-100%

be adequate

breakers (Type 1) in addition, on the pressure side

‘2 With Type 5 packings four sets of sealing rings should

3 Above 35 bar (500 p s i ) use one or two pressure

(1) Lead-bronze 250 BHN min 0.08-0.12 mm Optimum material with high thermal conductivity and good

(0.003-0.005 in) lubricated bearing properties Used where chemical

conditions allow Suitable for pressures up to 3000 bar (50 000 psi.)

(0.003-0.005 in)

Cheaper alternative to (1); bore may be tin coated to assist

operation

(0.003-0.005 in)

Used where (1) and (2) not suitable because of chemical

chrome-plated rods Max pressure 350 bar (5000 p s i ) Max temperature 120°C

(4) Filled PTFE 400 BHN min 0.4-0.5 mm Suitable for unlubricated and marginally lubricated operation

(0.015-0.020 in) as well as fully lubricated Very good chemical resistance

Above 25 bar (400 p s i ) a lead bronze backing ring

(0.1/0.2 mm) clear of rod should be used to give support and improved heat removal

(5) Reinforced p.f not critical 0.25-0.4 mm Used with sour hydrocarbon gases and where lubricant may

Mechanical piston rod packings B26

FllTilNG AND RUNNING IN

1 Cleanliness is essential so that cups bear squarely

together and to prevent scuffing or damage a t start up

2 Handle segments carefully to avoid damage during

assembly

3 Check packings float freely in cups

4 With lubricated packings, check that plenty of oil is

present before starting to run-in Oil line must have a

check valve between the lubricator and the packing Manually fill the oil lines before starting Use maxi-

m u m lubrication feed rate during run-in

5 If the temperature of the rod rises excessively (say above 100°C) during run-in, stop and allow to cool and then re-start run-in

6 Run in with short no-load period

B26.4

B27 Soft piston seals

SELECTION AND DESIGN

Table 27.1 Guidance on the selection of basic types

l&e name Distributor ‘u’ CUP ‘0’ rinz

External-fitted to piston, sealing in bore

COMPRESSION

Internal-fitted in housing, sealing on piston or rod

double-acting ‘Non-return’ valve action can

Effective but usually used in

Use correct fits and guided piston, etc

Avoid parting line flash on the sealing except under high pressure

If seal too soft for pressure, lip may curl away Unsuitable for

Long lips take up wear better and improve stability but

increase friction Use plastic back-up rings to reduce

extrusion a t high pressures T h e use of a thin oil will

reduce wear but may increase friction For pneumatic

assemblies use light grease which may contain colloidal

graphite or MoS2 Choose light hydraulic oil for mist

lubrication

Avoid metal-to-metal contact due to side loading or

piston weight If seals will not maintain concentricity use

B27.1

VENT TO

ATMOSPHERE

Soft piston seals B27

~~

Double-acting, one-piece, narrow width, but preswre can be trapped between lips and seal may jam Needs composire piston

Similar, but no pressure trap and can be fitted to one-piece piston

Derived from ‘0’ ring Less tendency to roll Improved and multiple sealing surfaces Sealing forces reduced and parting line flash removed from working surface

Multiple sealing lips to obviate leakage due to curl

_ -

~

e~ ‘W’ section Good for hydraulic

applications and high pressures Can

be used internally or externally

Material

Rubber

Dynamic seal on piston

‘\“‘ Register between body Polyethylene sections

Static seal in body sections

Dynamic seal on piston

Piston head seal Polyethylene

SPRING Fits ‘0’ ring groove Usually

Use internally or F’TFE externally Suitable for

rotational movement

Table 27.4 Mating surface materials

Materials ryPe Finish Remarks

0.6 p m max 0.2 to 0.4 pm

(8 to 16 pin) preferred

0.2 p m rnax 0.05 to

0.1 prn (2 to 4 ,pin) preferred

Best untreated materials Improve

Stainless steel Ground

Notes: Anodising and plating can be porous to air causing apparent seal leakage The finish on the seal housing can be 0.8 p m Use rust prevention treatment for mild steel in storage

B27.2

B27 Soft piston seals

INSTALLAT1BN

Table 27.5 Assembly hazards

Problem Suggested solution

Multiple seal grooves

External grooves

Internal grooves

Crossing ports

Crossing threads

Crossing edges and circlip grooves

Fitting piston assemblies to bores

M

plastic blade -use light greasc

Tilt

Deburr, chamfer, use assembly sleeve

or temporary plug in port

Use thin wall sleeve

Deburr or

chamfer

B

B27.3

Soft piston seals 627

Table 27.6 ‘0‘ ring fits

LARGE CLEARANCE

Dimensions t Q BS 1806

No standard available

Small radial clearance

Large clearance possible

Moderate bore a n d housing tolerances Close tolerances on ‘0’ ring dimensions, bore and groove width

-~

Tolerant to material swell and shrinkage

Seals at zero pressure drop

Sensitive to swell and shrinkage

Seals gas at low pressure-under 1 p.s.i with 0.003 in Unsuitable for liquids a t any pressure

clearance on width

FAILURE

Table 27.7 Types and causes of failure

TVpe Usual symptom Cause

Too hot and/or excess friction

Notes: Symptoms ofcontamination by solid particles are similar to channelling but the grooves are less regular Uneven distribution of wear suggests eccentricity or side loading ‘0’ ring rolling produces variation in shape and size of section

B27.4

Selection of lubricant t w e c1

Table 1 P importance of lubricant properties in relation to bearing type

? j p e of component Plain journal Open gears, Clock and Hinges, slides, Lubricant propeirty \ bearing bearing closed gear' ropes, chainr, etc inrtrument piuots latches, etc

~~ ~ ~~

1 Boundary lubricating properties

2 Cooling

3 Friction or torque

4 Ability to remain in bearing

5 Ability to seal out contaminants,

+

+ +

Note: The relative importance of each lubricant property in a particular class of component is indicated on a scale from + + + =

highly important to - = quite unimportant

Speed at bearing contact, mm/s -t

Figure 1.1 SpeedAoad limitations for different types of lubricant

c1.1

c1 Selection of lubricant type

0

LIFE, h

Figure 1.2 Temperature limits for mineral oils

.LPOUR POINT' LIMIT 'FOR SILICONES AND ESTERS I I

Selection of lubricant t w e CI

LIFE, h

Figure 1.4 liemperature limits for greases In many

cases the grease life will be controlled by volatility or

migration This cannot be depicted simply, as it varies

with pressure and the degree of ventilation, but in

general the hnits may be slightly below the oxidation

in the bearing

bearings may - 2 -

Shear rate in

Shear rate in -5 200 - standard test

._ a methods is low I be high

Typical SA€ 20/50 mineral oil at 10OoC

.Typical SA€ 30 I mineral oil at I \I 100°C 1-

CI Selection of lubricant type

Table 7.2 Examples of specific mechanisms and possible lubricants and systems

Lubricating Lubricant system

Journal bearings Oil By hand

Circulating system Ring lubrication Porous bearings

Maintenance Inuestment Rate of heat cost cost removal ty lubricant Remarks

low specific pressures

system

Only for light duty Good pumpahility of grease required if long lines to bearings

Rolling hearings Oil Oil mist Low High Small If compressed air in necessary quantity and

cleanliness available, investment costs are moderate

system

Oil feed jets must be properly designed and positioned to ensure optimum lubrication and heat removal

Careful design and filling required to avoid excessive churning

components (e.g gears) necessary to ensure adequate oil supply

costs are only low if re-lubrication period not too short

Gears Oil Bath Low Low Moderate Careful design of housing required to ensure

adequate oil supply to all gears and to avoid excessive churning

to ensure even oil distribution and heat removal

system

slumping and overheating

C1.4

Mineral oils c2

CLASS1 FI CAT10 N

Mineral oils are basically hydrocarbons, but all contain

thousands of different types of varying structure, molecular

weight and volatility, as well as minor but important

amounts of hydrocarbon derivatives containing one or

more of the elements nitrogen, oxygen and sulphur They

are classified in various ways as follows

'Types of crude petroleum

Parafinic

Naphthenic

Mixed base

Contains significant amounts of waxy hydro-

carbons and has 'wax' pour point (see below)

but little or no asphaltic matter Their naph-

thenes have long side-chains

Contains asphaltic matter in least volatile

fractions, but little or no wax Their naph-

thenes have short side-chains Has 'viscosity'

pour point

Clontains both waxy and asphaltic materials

Their naphthenes have moderate to long side-

chains Has 'wax' pour point

Viscosity index

Lubricating oils are also commonly classified by their

change in kinematic viscosity with temperature, i.e by

their kinematic viscosity index or KVI Formerly, KVIs

ranged between 0 and 100 only, the higher figures repre-

senting lower degrees of viscosity change with temperature,

but nowadays oils may be obtained with KVIs outside

these limits They are generally grouped into high, medium

and low, as in Table 2.1

Table 2,9 Classification by viscosity index

index

Low viscosity index (LVI) Below 35

Medium viscosity index (MVI) 35-80

High viscosity index (HVI) 80-1 10

grades by their typical uses as follows:

Spindle oils Low viscosity oils (e.g below about 0.01

Ns/m2 at 60DC,) suitable for thelubrica- tion of high-speed bearings such as textile spindles

Medium viscosity oils (e.g 0.01-0.02 Ns/mZ) a t 60°C, suitable for machinery running a t moderate speeds

Heavy rnachine oils Higher viscosity oils (e.g 0.02-0.10

Ns/mZ) a t 60DC, suitable for slow-moving machinery

Suitable for the lubrication of steam engine cylinder; viscosities from 0.12 to

0.3 Ns/m2 at 60°C

Light machine oils

Cylinder oils

Hydrocarbon types

T h e various hydrocarbon types are classified as follows:

( a ) Chemically saturated (i.e no doubie valence bonds) straight and branched chain (Paraffins

or alkanes.)

( b ) Saturated 5- and 6-membered rings with attached

side-chains of various lengths up to 20 carbon atoms long (Naphthenes.)

( E ) As ( b ) but also containing 1 , 2 or more 6-membered unsaturated ring groups, i.e containing double valence bonds, e.g mono-aromatics, di-aromatics, polynuclear aromatics, respectively

A typical paraffinic lubricating oil may have these hydrocarbon types in the proportions given in Table 2.2

Table 2.1 Hydrocarbon types in Venezuelan 95

VI solvent extracted and dewaxed distillate

yo Volume Hydrocarbon types

Saturates (KVI = 105)

It should be noted, however, that in Table 2.5 viscosity

index has been determined from dynamic viscosities by

the method of Roelands, Blok and Vlugter,' since this is

a more fundamental system and allows truer comparison

between mineral oils Except for low viscosity oils, when

DVIs are higher than KVIs, there is little difference

between KVI and DVI for mineral oils

Mono-aromatics 18 Di-aromatics 6

Aromatics

T h e V I of the saturates has a predominant influence

o n the VI of the oil I n paraffinic oils the VI of the saturates

m a y be 105-120 and 60-80 in naphthenic oils

c2.1

c2 Mineral oils

Structural group analyses

This is a useful way of accurately characterising mineral

oils and of obtaining a general picture of their structure

which is particularly relevant to physical properties, e.g

increase of viscosity with pressure From certain other

physical properties the statistical distribution of carbon

atoms in aromatic groups (%eA), in naphthenic groups

(% CN), in paraffinic groups (% C p ) , and the total number

(RT) of naphthenic and aromatic rings (RN and R A )

joined together Table 2.3 presents examples on a number

Ns/m2 molecular

at 100°C weighf Oil pppc

~~~~ ~ ~~

~~

R EFl Nl NG

Distillation

Lubricants are produced from crude petroleum by dis-

tillation according to t h e outline scheme given in Figure 2.1

The second distillation is carried out under vacuum

to avoid subjecting the oil to temperatures over about

370°C, which would rapidly crack the oil

T h e vacuum residues of naphthenic crudes are bitu-

mens These are not usually classified as lubricants but

are used as such on some plain bearings subject to high

temperatures and as blending components in oils and

greases to form very viscous lubricants for open gears,

etc

Refining processes

Table 2.4 Refining processes (courtesy:

Institution of Mechanical Engineers)

Proccss Purpose

De-waxing Removes waxy materials from paraf-

finic and mixed-base oils to prevent early solidification when the oil is cooled to low temperatures, i.e to reduce pour point

De-asphal ting Removes asphaltic matter, particu-

lady from mixed-base short residues, which would separate out at hig5

and low temperatures and block oil-ways

Solvent extraction Removes more highly aromatic mat-

erials, chiefly the polyaromatics,

in order to improve oxidation stability

Hydrotreating Reduces sulphur content, and accord-

ing to severity, reduces aromatic content by conversion to naphthenes Acid treatment Now mainly used as additional to

other treatments to produce special qualities such as transformer oils, white oils and medicinal oils Earth treatment Mainly to obtain rapid separation of

oil from water, i.e good demulsi- bility

c2.2

Mineral oils c2

T h e distillates and residues are used to a minor extent

as such, but generally they are treated or refined both

before and after vacuum distillation to fit them for the

more stringent requirement!; The principal processes

listed in Table 2.4 are selected to suit the type of crude oil

and the properties required

Elimination of aromatics increases the VI of a n oil A

lightly refined naphthenic oil may be LVI but M V I if

highly refined Similarly a lightly refined mixed-base

oil may be MVI but HVI if highly refined Elimination of

aromatics also reduces nitrogen, oxygen and sulphur

contents

T h e distillates and residues may be used alone or blended

together Additionally, minor amounts of fatty oils or of

special oil-soluble chemicals (additives) are blended in

to form additive engine oils, cutting oils, gear oils, hydraulic

oils, turbine oils, and so on, with superior properties to

plain oils, as discussed below The tolerance i n blend

viscosity for commercial branded oils is typically *4y0

but official standards usually have wider limits, e.g & 10%

for I S 0 3448

Viscosity-.Temperature

Figure 2.4 illustrates the variation of viscosity with

temperature for a series of oils with kinematic viscosity

index of 95 (dynamic viscosity index 93) Figure 2.2 shows

the difference between 150 Grade I S 0 3448 oils with

KVIs of 0 and 95

Vi scosity P ressu re

T h e viscosity of oils increases significantly under pres-

sure Naphthtenic oils are more affected than paraffinic

but, very roughly, both double their viscosity for every

35 MN/m2 increase of pressure Figure 2 3 gives an

impression of the variation in viscosity of an SAE 20LY

I S 0 3448 or medium machine oil, HVI type, with both

temperature and pressure

I n elastohydrodynamic (e h l ) formulae it is usually

assumed that the viscosity increases exponentially with

pressure Though in fact considerable deviations from a n

exponential increase may occur a t high pressures, the

assumption is valid up to pressures which control ehl

behaviour, i.e about 35 MN/m2 Typical pressure viscosity

coefficients are given in Table 2.5, together with other

physical properties~

Pour point

De-waxed paraffinic oils still contain 1 % or so of waxy

hydrocarbons, whereas naphthenic oils only have traces

of them At about O"C, according to the degree of de-

waxing, the waxes in paraffinic oils crystallise out of

solution and a t about -IOo@ the crystals grow to the

extent that the remaining oil can no longer flow This

temperature, or close to it, when determined under

specified conditions is known as the pour point Naph-

thenic oils, in contrast, simply become so viscous with

decreasing temperature that they fail to flow, although no

wax crystal structure develops Paraffinic oils are therefore

said to have 'wax' pour points while naphthenic oils are

said to have 'viscosity' pour points

Figure 2.2 150 grade IS0 3448 oils of 0 and 95 KW

Figure 2.3 Variation of viscosity with temperature and pressure of an SAE 2OW (HVI) oil (courtesy

Institution of Mechanical Engineers)

C2.3