Spiral Guidelines for Valve Bonnets

8/20/2019 Spiral Guidelines for Valve Bonnets

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Basic Spiral Wound Design Guidelines for Valve Bonnets

a) Suggested Clearances

As a spiral-wound gasket is compressed, it tightens-up within itself (think of pulling a tape

measure tight for example). As the element is compressed from the axial forces, it tries tospread both inwards and outwards generating a radial force, as well as having a certain

amount of “folding” inwards of the V-profile. As the windings contact the outer wall of

the recess they will deform from a “V” to a “flattened C” shape, and this support helps to

densify the gasket within itself as well as supporting the windings. If the clearance is too

great or the compression too high there could be a risk of bursting the welds on the outside

of the element. Often, we will tighten the clearance a little on a high pressure gasket

compared to a low pressure one.

The I/D clearance is typically double that of the O/D clearance to allow more room for the

inward folding of the windings.

If the housing is open to the bore, we would typically allow double the I/D clearance

compared to the closed recess, to avoid the winding entering the bore, as of course any

unsupported element under compression would buckle and fail. Open to the bore recesses

can often use a C/IR type gasket having a solid metal inner ring.

Tongue & Groove Housing

2-3X X

Spigot & Recess Housing

X5-6X



Diameter Suggested clearance “x”

(graphite)

<100 0.6-0.8

100-300 0.8-1.0

300-600 1.0-1.2600-1000 1.2-1.5

b) Compression:

Nominal Thickness

mm

Compressed

thickness mm

Typical diameter

range

2.5 1.9 / 2.1 22 – 300

3.2 2.4 / 2.6 10 – 760

4.5 3.2 / 3.45 12 – 1520

7.3 5.0 / 5.25 60 - 3550

c) Tolerances (type C)

Tolerances are generally +ve on I/D and –ve on O/D thus:

Diameter I/D tolerance O/D tolerance

<300 +0.5 / -0.0 -0.5 / +0.0

300-600 +0.8 / -0.0 -0.8 / +0.0

600-1500 +1.5 / -0.0 -1.5 / +0.0

d) Suggested element widths

There are no precise guidelines or definite “rules” to prove width of element vs. system

pressure etc., but the following table gives typical width guidelines that have proven to be

reasonable over a number of years. Again, things such as clearance in the recess can have

an effect and there is the need for the gasket to be practical to manufacture and handle.

(For example if less than “square” in section there are almost no turns of filler as the inner

and outer plain windings lose too much of the overall width.) The element widths might

be varied a little in practice, but these should be a reasonable starting point for a design:

Gasket I/D <40 bar 40-60 bar 60-80 bar 80-100 bar >100 bar

<50 5 5.5 6.5 7.5 8.5

50-100 6 7 8 9 11

100-150 6.5 8 9.5 11.5 13.5

150-200 8 9.5 11.5 13 15.5

200-250 9.5 11 12.5 15 17

250-300 11 12.5 14 16 19

300-450 12 13.5 15.5 18 20.5

450-750 14 16 18 20 22.5750-1250 16 18 20 22 24



e) Flange finish

Typically for spiral-wound gaskets, a traditional light gramophone finish in the 3.2-

6.3µmRa range is considered appropriate. Both opposing flange faces should have a

similar finish wherever possible. We find that more problems tend to arise through poor

application of bolt load than through incorrect flange finish. However, be aware that somemore ductile metals can leave too deep a groove i.e. if the machining tool plucks out a

deep chip of metal. The typical finish when machined as above is perhaps only 0.002”

deep. We have seen problems when a finish has been cut too deep, although at the right

pitch of serrations, as of course the graphite only protrudes above the steel windings by a

fraction of a mm, so cannot fill-in a particularly deep surface finish.

f) Load – compression – leakage

We are presently undertaking a range of gasket tests on various gaskets for EN13555 to

characterise gaskets for the EN1591 design method. However, the type C Metaflex are notcurrently part of our test programme. We have done some work on Type SG/IR with inner

and outer support rings, for use on regular raised face flanges. Obviously the addition of

the support rings will harden the gasket in terms of load-compression characteristics to

some degree, as there is less room for the sealing element to spread under compression.

Below is a typical load-leak graph of an SG/IR with helium. The sample is 1.1/2” N.B..

Leakage curve

Metaflex 66.0x54.5x5.16 mm

Test number: 07-067

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

1.00E+00

0 20 40 60 80 100 120 140 160 180

gasket stress [MPa]

l e a k r a t e [ m g / m / s ]

p = 40 bar

T = 20 °C

He

The next is a load-compression-recovery graph of an SG/IR from another test of several

years ago, but of 3” N.B. :-



Below is another from rather a long time ago of a 4” type SG (outer ring only), which

compresses a little easier than the SG/IR.

Unfortunately these are the only graphs I have readily to hand at present, however, it

should be noted that in the case of type C in a recess they will differ also.

Compression Curve:-

Metaflex Type SG - graphite filled

3

3.2

3.43.6

3.8

4

4.2

4.4

4.6

4.8

5

0 10 20 30 40 50 60 70

Stress (MPa)

T

h i c k n e s s ( m m )



The major things which will influence the load-compression characteristics are:

a) The tightness of fit in the recess

b) The diameter and therefore the hoop strength of the wound element, along with the

winding tension used. (Note that it is possible to “tune” gaskets a little in this way,

though we do have a standard tolerance of number of turns per unit width etc.).

c) The element width in relation to the diameter.

Thus, we would suggest that as an initial estimate, to compress a standard graphite filled

type C from 4.5 mm into the working zone of 3.2-3.45mm might take something in the

region of 50-100 MPa depending on clearances and sizes. Note that a typical load-

compression graph does tend to take an exponential format, so getting from 3.45 to 3.2mm

thick can take quite a bit of additional load.

(As an example of a slightly softer gasket example, I found one I tested many years ago in

my files – 250 x 266 for a relatively low pressure nuclear job (17 bar duty) where the

customer wanted compression between 38 and 58 MPa. The recess was 247 x 267 and

thus slightly greater clearance than the guidelines given above, and on average thesegaskets compressed to 3.45mm at around 40 MPa and 3.3mm at around 49 MPa.)

For the SG/IR tested above, we also have typical unloading modulus values, but again these may

differ on a type C which is in a recess with clearances.

Unloading modulus values (typical) (MPa)

Gasket stress [MPa] ambient temperature

20 1273

30 1657

40 2524

50 2650

60 3403

80 3665

100 4601

120 5038

140 5682

160 6866

180 8753

200 7848

There is of course the problem of actually achieving the theoretical load in practice. Note

that torque-tightening of bolts is relatively inaccurate, as is hydraulic tensioning. Oneparticular problem we encounter with hydraulic tensioning of studs in pump and valve

casings, is that of short bolts with low strains e.g.:-



Thus, where hydraulic tensioners are used, it should be noted that they have a certain

degree of load-transfer relaxation when their oil pressure is removed. Whilst an overload

is used to try and compensate for this it is based on assumed deflections based on the

length to diameter ratio of the bolts. Thus, when short bolts are employed the assumptions

become even more inaccurate and the available overload is insufficient to compensate for

this, resulting in under-tensioned bolts.

Therefore, despite many calculations in theory regarding gasket load-compression-

leakage, we still find occasional problems with the actual measurement of the installed

fastener tension.

I hope this is at least a start in terms of answering some of your questions.

Regards

Dennie Huisman

Sales Executive Business Development

Regular pipeline flanges

Strained length

Pump or valve body

Strained length

Documents

Spiral Guidelines for Valve Bonnets