Pressure Seal Anchor Darling Info

8/14/2019 Pressure Seal Anchor Darling Info

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Surface ConditionsSealing is affected not only by surface dimensions and finish, but

also by material characteristics. The more homogeneous the mating

surfaces, the easier it will be to obtain a tight seal. The finer grain

structure exhibited by forged and welded materials produces a muchbetter sealing surface than the coarser grain structure common to

cast materials. To take advantage of this phenomenon, the bon-

net and pressure seal gasket should be manufactured from forged

materials.

Since other considerations make it desirable to produce bodies made

of cast material, providing a good sealing surface at a gasket-body

neck interface is more difficult. This problem is solved by welding a

stainless inlay in the sealing area of the body (figure 2). This not only

provides a fine-grain structure but prevents problems due to corro-

sion and electrolytic reactions in the same area.

Figure 2 – Stainless inlay

Unit LoadingOf the sealing interfaces in a pressure seal joint, the gasket-bonnet

seal is the easiest to achieve. The major component of the internal

pressure force acts on this interface. This direct line of force is more

than sufficient to produce a tight seal at this joint.

The most frequent leakage problem occurs between the gasket-body

neck interface. This is understandable when examined in the context

of unit loading. The pressure force available is exactly perpendicu-

lar to the direction in which the unit loading is required. There is

no direct component of the pressure force available for sealing

gasket-body interface. The only force available is that component of

the pressure force that can be converted to a radial force by virture

of the gasket angle. The effectiveness of the gasket-body seal is

mainly determined by the efficiency with which gasket angle is able

to convert the vertical pressure force to a radial force. As the gasket

angle decreases, the portion of the pressure force that is converted

to radial unit loading increases. To put it simply, the sharper the

taper of the gasket, the more tightly it will become wedged between

the bonnet and the body and the more reliable the seal becomes.

A joint designed with a gasket angle of 25° (figure 3), will convert

the line pressure force into a large radial force sealing at a far lower

pressure than a joint using a gasket angle of 45° (figure 4). In

addition to maximizing the force, unit loading can be increased by

decreasing the area over which the force acts. However, in adjusting

unit loading by this means, care must be taken to ensure that there

is sufficient gasket area to support the load without crushing.

Figure 3 – 25° Angle

Figure 4 – 45° Angle

Where a pressure seal valve bonnet must seal over a wide range of

pressures, there is a problem. A gasket that is small enough to seal

reliably at 500 psi will not support 2500 psi. A solution to this prob-

lem was to design the gasket and bonnet mating surfaces so that

prior to assembly there is a 1° difference in the surface angle (figure

5). Before the joint is pressurized, only the knife edge of the gasket

is in contact with the bonnet (figure 6). Under pressure, a portion of

the malleable gasket deforms to the bonnet (figure 7). However, the

deformation and actual contact is limited to that necessary to sup-

port the load and still provide a tight seal.



Figure 5 – Differential Angle

Figure 6 – Zero Line Pressure

Figure 7 – High Line Pressure

Careful design of the gasket angle not only increases seal reliability,

it eliminates a major headache found in pressure seal joints with

large (30–45°) gasket angles: having to retighten bonnet studs after

installation. By virture of the narrow (25°) gasket angle, a tight seal

is achieved with only a small upward force on the bonnet. Once the

seal is created, the joint will remain leak-tight regardless of pressure

conditions in the valve. Some experiments with extremely narrow

gasket angles (15–20°) actually wedged the bonnet so tightly that

it became impossible to disassemble the joint. A gasket angle of

approximately 25° was found to provide both a very reliable seal and

ease of disassembly.



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Pressure Seal Anchor Darling Info