Depending on the material of the sealing sub of the globe valve, metal seals and non-metallic seals can be used for the globe valve. When using metal seal and non-metallic ceramic seal, not only high sealing specific pressure is required, but also uniformity all around is needed to achieve the required sealing. According to the above requirements, there are many kinds of structural designs of sealing subs, and the sealing principle and the calculation of sealing force are different.
1. Flat Seal
The advantage of the flat seal is that the flap has a certain amount of shaking during assembly, the flap can automatically find the right and the seat sealing surface, so the requirements for the guidance of the flap are not important; the flap is not rotated when it falls on the seat, there is no friction between the sealing sub, so the requirements for the sealing surface material anti-scuffing is not strict. At the same time, when the bore of the valve seat is deformed due to pipeline stress, the sealing performance will not be affected. The disadvantage is that the solid particles and sediment in the medium tend to damage the sealing surface. The sealing principle is that when the medium flows in from below the valve flap, the sealing force applied must be equal to or greater than the sum of the required specific pressure generated on the sealing surface and the upward force of the medium. See the following equation.
QMZ≧QMF＋QMJ
QMF=π(DMN+bM)bMqMF
QMJ=π/4(DMN+bM)²P
where QMZ – the total force applied on the sealing surface, N.
QMF – the sealing force on the sealing surface, N
QMJ – medium force on the sealing surface, N.
DMN – inner diameter of the sealing surface, mm.
bM – width of the sealing surface, mm.
qMF – required specific pressure at the sealing surface, MPa.
p – calculated pressure, usually take the nominal pressure, MPa.
When the medium flows in from above the valve flap, the applied sealing force only needs to be equal to or greater than the difference between the required specific pressure on the sealing surface and the force of the medium, i.e.
QMZ ≥ QMF – QMJ
2. Tapered seal
Tapered sealing is to make the sealing surface conical so that the contact surface becomes narrower. In this kind of seal, under the action of a certain sealing force, its sealing specific pressure increases greatly and it is easier to achieve sealing, and the sealing force applied is smaller compared with the flat sealing structure. Due to the narrow sealing surface, it is not easy to make the valve flap pressed to the seat surface correctly when closing. In order to improve the sealing performance, the valve flap must be guided. When the valve flap is guided in the valve body, the lateral thrust of the valve flap by the fluid is borne by the valve body instead of the valve stem, which further enhances the sealing performance and the reliability of the packing seal. When the conical valve flap is used in large diameter valves, it is not easy to achieve sealing because of the piping stress, which causes a certain amount of deformation in the roundness of the valve seat hole.
On the other hand, the conical seal is anastomosed with friction between the two sealing surfaces, so the sealing material must be able to resist abrasion. Compared with the flat seal, the conical seal is relatively less damaged by solid particles and media deposits, but it is also not suitable for use in media containing solid particles and media deposits. The sealing principle is that when the medium flows from below the valve flap, the sealing force applied must be equal to or slightly greater than the sum of the necessary specific pressure generated on the sealing surface and the upward force of the medium, see the following equation.
QMZ≧QMF+QMJ
QMF=π/4(DMW²-DMN²)(1+fM/tanα)qMF
QMJ=π/4(DMN+bMsinα)²p
where QMZ – the total force applied on the sealing surface, N.
QMF – the sealing force on the sealing surface, N
QMJ – medium force on the sealing surface, N.
DMW – outer diameter of the sealing surface, mm.
DMN – inner diameter of the sealing surface, mm.
FM – friction factor of the sealing surface.
α – cone half angle of the sealing surface, (°).
qMF – required specific pressure of the sealing surface, MPa.
bM – width of the sealing surface, mm.
p – calculated pressure, usually take the nominal pressure, MPa.
When the medium flows from above the valve flap, the applied sealing force is equal to or greater than the difference between the required specific pressure on the sealing surface and the force of the medium.
In order to improve the strength of the conical seal without sacrificing its sealing stress, the conical half angle of the sealing surface is made 15°, which provides a wider sealing surface so that the valve flap can more easily fit with the valve seat. In order to achieve higher sealing stress, the seat sealing surface begins to contact the valve part is narrower, about 3mm, and the rest of the tapered part left can be slightly longer. When the sealing load increases, the valve slides deeper into the seat, thus increasing the width of the sealing surface. This design of the sealing surface is not as susceptible to erosion damage as the narrow sealing surface. In addition, due to the long tapered surface, the throttling characteristics of the valve are improved.
3. Spherical seal
Make the valve flap spherical and the valve seat conical. The sphere of the valve can rotate freely in the hole of the valve stem. Therefore, the valve can be adjusted by making a certain range of rotation on the valve seat. Since the contact between the two sealing surfaces is almost a line, i.e., a line seal, the sealing stress is high and it is easier to achieve sealing. The valve sphere can also use carbide or ceramic materials with a hardness of 40 to 60 HRC and can withstand very high temperatures, so it can be applied to high-temperature globe valves. The disadvantage is that the sealing surface linear contact is easily damaged by erosion, so the valve seat should choose erosion-resistant materials. A spherical seal globe valve can be applied to gas or liquid with tiny solid particles in the medium. The sealing principle is that when the medium flows from below the valve flap, the sealing force applied must be equal to or slightly greater than the sum of the necessary specific pressure generated on the sealing surface and the force on the medium.
QMZ≧QMF+QMJ
QMF=πDMNqMF
QMJ=π/4DMN²P
where QMZ – the total force applied on the sealing surface, N.
QMF – the sealing force on the sealing surface, N
QMJ – medium force on the sealing surface, N.
DMN – internal diameter of the sealing surface, mm.
qMF – required specific pressure on the sealing surface, MPa.
p – calculated pressure, usually take the nominal pressure, MPa.
When the medium flows from above the valve flap, the applied sealing force must be equal to or slightly greater than the difference between the required specific pressure on the sealing surface and the downward force of the medium.
TAG: globe valve, globe valve flaps, Three sealing types, wesdom, wesdom valve
