Introduction To Basic Knowledge Of Valve Packing

The packing of Lined Valve is the filling material of dynamic seal, which is used to fill the space of the stuffing box to prevent the medium from leaking through the valve stem and the space of the stuffing box.
Packing seal is the requirement of the medium working conditions of valve products. On the other hand, reasonable packing installation method and consideration of the structure of the stuffing box are used to ensure reliable sealing.
1 Requirements for packing itself
(1) Reduce the friction of packing on the valve stem;
(2) Prevent packing from corroding the valve stem and packing;
(3) Appropriate requirements for medium working conditions.
2 Commonly used packing varieties
Foreign data introduces more than 40 types of packing used in various working conditions. This section only introduces the most commonly used packing materials in general valves.
(1) Packing type
a. Rubber asbestos packing: XS250F, XS350F, XS450F, XS550F
b. Oil-impregnated asbestos packing: YS450F, YS350F, YS450F
c. Polytetrafluoroethylene-impregnated asbestos packing:
d. Flexible graphite braided packing: Depending on the reinforcing material, it can be resistant to temperatures of 300°, 450°, 600°, 650°, 850°
e. Polytetrafluoroethylene braided packing, expanded polytetrafluoroethylene square braided packing
f. Semi-metal braided packing: asbestos with stainless steel wire and copper as the core, and asbestos wire with copper wire, stainless steel wire, monel wire, and inconel wire as the outer surface. The surface is treated with graphite, mica, and molybdenum disulfide lubricants according to the purpose. Some are made of asbestos as the core and twisted with lubricated graphite-coated copper foil.
(2) Molded fillers
Molded fillers are press-molded fillers, and the varieties are:
a. Flexible graphite rings
b. Nylon
c. Polytetrafluoroethylene
d. Filled polytetrafluoroethylene (reinforced polytetrafluoroethylene) The reinforcing material is glass fiber, generally 8-15% glass fiber. The filled polytetrafluoroethylene specified in the JB/T 1714 standard is: polytetrafluoroethylene + 20% MoS2 polytetrafluoroethylene + 20% + glass fiber 20% + 5% asbestos
e. Rubber

3 Precautions
(1) When cutting the packing type packing, use a 45° incision. When installing, each circle of incisions should be staggered 180°.
(2) When using polytetrafluoroethylene molded fillers under high pressure, pay attention to its cold flow characteristics.
(3) The sealing effect of flexible graphite rings alone is not good. They should be used in combination with flexible graphite braided packing or YS450F (depending on the temperature). Flexible graphite rings are installed in the middle of the stuffing box and braided packings are installed at both ends. It can also be assembled in layers, that is, one layer of flexible graphite and one layer of braided packing. A spacer ring can also be placed in the middle of the stuffing box, and the spacer rings can be placed on the upper and lower parts to form two groups of packings assembled together.
(4) Graphite corrodes the valve stem and the wall of the stuffing box. When using it, packing with corrosion inhibitors should be selected.
(5) Flexible graphite is not suitable for media such as aqua regia, concentrated sulfuric acid, and concentrated nitric acid.
(6) The dimensional accuracy, surface roughness of the stuffing box, and the dimensional accuracy and surface roughness of the valve stem are the key factors affecting the sealing of the molded packing.
(7) The API 6D standard mentions the important information on the use of asbestos or its substitutes: Since asbestos is related to some diseases that seriously endanger health, some of which are even fatal. Therefore, many substitute materials are currently used and developed to replace asbestos in certain occasions.