The sealing performance of the gate valve is affected by multi-dimensional factors such as materials, structural design, working conditions and maintenance, and the interaction of these factors may lead to seal failure (e.g., internal leakage, external leakage). The following is an analysis of the core influencing factors, mechanisms and typical cases:
I. The decisive influence of material selection on sealing performance
1. Matching of sealing surface materials
Metal hard seal (high pressure and high temperature scenario):
Common combination: valve valve valve surfacing Stellite 6 (cobalt-based alloy) + valve seat surfacing Inconel 718 (nickel-based alloy), the hardness difference is controlled at HRC5~10 (e.g. Stellite 6 hardness HRC45~50, Inconel 718 hardness HRC35~40), which can fill the surface microscopic defects through micro-plastic deformation to enhance sealing reliability.
Risk of failure: if the difference in hardness is too small (e.g. both are HRC45), the sealing surface is prone to ‘biting’, which will scratch the surface and lead to leakage when opening and closing; if the material is not sufficiently corrosion resistant (e.g. ordinary carbon steel in seawater media), the sealing surface will be pitted due to galvanic corrosion.
Soft seal (low pressure and normal temperature scene):
Common materials: valve flap lined with PTFE (polytetrafluoroethylene) or EPDM (ethylene propylene rubber), the use of elastic deformation of the material to fill the gap between the sealing surface (such as DN100 soft sealing gate valve sealing pressure needs to be ≥ 2MPa to achieve zero leakage).
Failure risk: high temperature (such as PTFE temperature ≤ 260 ° C) under the material softening deformation, or medium swelling (such as EPDM in gasoline volume expansion rate of > 10%) leading to sealing surface failure.
2. Valve body and stem material effects
Valve body strength is not enough: such as cast iron valve body at high pressure (> 1.6MPa) under the micro-cracks, resulting in leakage of media from the valve wall;
Stem corrosion: seawater pipelines do not use stainless steel (such as 316L) stem, chloride ions will trigger stress corrosion cracking, resulting in leakage of the stem and gate connection.
II, The key role of structural design on sealing performance
1. sealing surface structure form
Wedge gate (the most common):
Structure: gate sealing surface and vertical surface into 3 ° ~ 6 ° wedge angle, relying on the axial force of the valve stem to squeeze the valve seat to achieve sealing (such as DN200 wedge gate valve sealing pressure needs to be ≥ 5MPa).
Influencing factors: wedge angle deviation of more than ± 0.5 ° will lead to uneven force on both sides of the sealing surface, one side of the excessive wear and leakage; gate guide wear (such as guide groove clearance > 0.3mm) will make the gate opening and closing skewed, the sealing surface of the local contact pressure is insufficient.
Parallel type gate:
Structure: spring loading mechanism is provided between the double gates to compensate for sealing surface wear by spring force (e.g., the spring preload of parallel gate valve for natural gas pipeline needs to be ≥ 1.2 times the medium pressure).
Risk of failure: spring fatigue fracture under long-term pressure (such as use more than 5 years), resulting in insufficient gate thrust, increased leakage of the sealing surface.
2. stem seal structure
Stuffing box design:
Insufficient packing layers (such as DN100 gate valve packing layer <5 layers) will lead to leakage; packing box depth is too shallow (<1.5 times the diameter of the valve stem), the medium pressure is easy to top out the packing.
Typical case: a steam pipeline gate valve because the depth of the stuffing box is only 1 times the stem diameter, after six months of operation, high-temperature steam carbonisation of the packing, leakage amounted to 10 drops / minute.
Bellows seal (high-end applications):
Bellows layers (such as 3 layers) or wall thickness (<0.3mm) is insufficient, the bellows rupture under high pressure, resulting in external leakage (such as 10MPa conditions require the use of 5 layers of stainless steel bellows).

III. The dynamic impact of working conditions on sealing performance
1. Pressure and temperature alternating effects
High pressure difference scouring:
When the pressure difference between the front and rear of the gate valve> 3MPa, the medium through a small leakage gap to produce high-speed jet (flow rate> 50m/s), erosion of the sealing surface metal (such as oil pipelines in the sand-containing media will be in the sealing surface to form grooves).
Dramatic changes in temperature:
High temperature (> 425 ° C) under the valve body and gate thermal expansion coefficient differences (such as carbon steel valve body and bronze gate) resulting in increased sealing surface gap; low temperature (<-40 ° C) under the contraction of the metal sealing surface resulting in micro-cracks (such as LNG pipeline gate valve is not used in the low-temperature steel, the leakage of the sealing surface can be increased by 30%).
2. the erosive effect of media properties
Medium containing particles:
Such as slurry pipeline, particle size > 0.1mm will be embedded in the soft sealing surface (such as rubber), or scratch the hard sealing surface (such as every time the opening and closing of the particles extrusion sealing surface, resulting in the sealing surface roughness from Ra1.6μm deterioration to Ra6.3μm).
Corrosive media:
Acidic gases (such as H₂S) will electrochemically react with the sealing surface metal, forming corrosion pits on the sealing surface (e.g., gate valves not overlaid with corrosion-resistant alloys, sealing surface perforation within 1 year in natural gas containing H₂S).

Yantai Valtone Valve Co., Ltd.

Email: 

liujun@valtone-valve.com

carrie@valtone-valve.com

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+86-535-6391338
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+86-18660599666

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