Valve sealing surface/inner leakage and deposited welding material

We all know that there are three major factors that affect valve life:

1 valve inner leakage: The main problem is the quality of the sealing surface
2 valve outside leakage: The main problem is the quality of the casting, but also packing problem
3 valve structure: The main problems in the design and processing of pieces

Here we discuss only the inner steel middle high pressure valve leakage problem, the requiement of valve sealing surfaces.

Valve sealing surface, at a predetermined pressure, temperature, medium, installation location,  ensure safe and reliable seal; while in the manufacturing process, the sealing surface materials and structures, welding, machining, assembly and grinding process shoudl be good; sealing surface should also to be ease of maintenance, specifically requires the following:
1 Anti-corrosion
2  Anti-scratch
3 cavitation erosion resistance
4 should have a certain strength to withstand the pressure seal formed by itself
5 should have a certain hardness
6 should be a certain resistance to high temperature enviroment
7 surfacing base metal and sealing surface have similar linear expansion coefficient
8 good processing performance

Through extensive testing by other organisation we believe the valve sealing surface wear properties are mainly metal wear, with part abrasive wear. We have also seen the destruction of the valve sealing surface are three kinds of failure modes: abrasions, scratches, wounds. Anti-abrasion performance surfacing material is good or bad depends on the composition of deposited metal and metal structure, resistance to scratches and pad injury performance depends on the level surfacing material hardness, therefore surfacing material composition and the metal structure is most important, the surfacing material hardness is second. The idea that the higher the hardness of the material sealing surface have better scratch resistance is not accurate.
In our opinion, deposited metal sealing surface hardness can’t be too low, because the valve working condition media are not very pure, contain a certain amount of sediment and hard particles particle, low sealing surface hardness will be unable to resist scratches and wound. The general sealing surface hardness should be HRC ≥ 30, more suitable hardness HRC = 32-40. When high hardness HRC ≥ 40,  processing and welding will face rather considerable difficulties.

Various types of valve sealing surface characteristics and failure type

valve type

deposite weld places

sealing surfaces type

failure type

gate valve

seat,disc

flat faces

main abraion, erosion

check valve

seat, disc

flat faces

shork, erosion

high temperature ball valve

seat

cone faces

main abraion, erosion

butterfly valve

seat

cone faces

ersion

globe valve

seat disc

cone face

main abraion, erosion

throttle valve

seat disc

cone face

erosion

safety valve

seat disc

flat faces

main abraion, erosion

pressure reduce valve

seat disc

flat or cone face

main abraion, erosion

Goodness of fit affect abrasion performance
Goodness of fit  is one of the important factors affecting the ratio of pressure that have a direct confrontation abrasion performance impact. In the case of the same medium pressure, ratio of pressure and inversely proportional to the degree of matching, matching degree is smaller than the pressure, the more easily scratched, so to improve the life of the valve sealing surface, the valve sealing surface to improve the degree of matching to lower than the pressure valve standards. Abroad anastomosis valve sealing surface is 100%, means the valve is closed when the gate valve seat sealing surface all been covered. China requires valve sealing surface goodness of fit should be 60% -70%.

Surfacing materials are selected based on following:
Select the valve sealing surface material, the valve should first meet the requirements for use, long life, good sealing performance, should be based on the main failure valve sealing surface shaped test, note overall performance of materials, processes, workability, economy heap selection solder alloys.
According to temperature, pressure,  selcect sealing surface material.
According to the applicability of welding process of surfacing materials select sealing surface material, therefore the crack resistance of surfacing material is extremely important indicator. Surfacing material according to the workability of the sealing surface material selected, should consider roughness and hardness and the goodness of fit, adopt strict requirement due to the high hardness of surfacing material, difficult to process. So the sealing surface hardness is generally not too high.

Now several using surfacing materials economic and technical analysis:
Manual welding stainless steel 18-8 18-8 electrode compared to 2cr13 are more expensive, the hardness is too low, so except in special circumstances, 18-8 stainless steel valves should not be used as surfacing materials should be thoroughly eliminated. Large number of experimental studies have shown that manual welding 2Cr13, 1Cr13, due to process complexity, life skills and Economic effects are poor. So in the temperature and pressure valve should selected SF-3T, 85 # alloy electrode manual welding, SF-3J, 85 number of automatic surfacing alloy flux, temperature and high pressure carbon steel valves suggested the use of Alloy 137 SF-4T electrode manual welding and solder alloy 137 and F326 and other automatic welding, plasma powder surfacing.
85, No. 137 alloy can achieve automatic welding, a variety of boron-containing iron-based alloy powder plasma surfacing than manual welding production efficiency 3-6 times longer life than 2Cr13 improved sealing surface several times, using the submerged arc automatic welding processes, plasma powder surfacing process can save a lot of hours of work, saving, saving materials and greatly reduce the manufacturing cost of the sealing surface.
D507Mo paired with the D577 two kinds of welding rod sealing surface also have a higher life, you can also surfacing in the temperature and pressure carbon steel valves, the election must first pay attention to the two electrodes paired, and secondly to pay attention to these two electrodes higher price .
Cobalt-based alloy is world High corrosion resistant valves commonly used surfacing material, due to lack of resources of cobalt, the domestic developed  four cobalt alternative materials D547Mo, NDG-2 #, SF-5T, SF-6T. The four cobalt-alternative materials have used our factory, our view is: Because D547Mo crack is bad, should not be used, NDG-2 # material can be used, SF-5T materials can be used, SF-5T electrode containing ring acid oil medium heat uniform corrosion and high-temperature gas containing H2S stress corrosion better than cobalt-based alloys. SF-6T electrode plant in Shen valve 2 “65Y3500 pound power station valve surfacing successfully replaced the cobalt-based alloys, we believe cobalt-based alloy high-temperature performance is better, but it’s poor corrosion resistance, domestic cobalt-based Alloy priced at 380-500 yuan / kg fluctuated between; NDG-2 priced at 150-170 yuan / kg fluctuations; SF-5T, SF-6T two kinds of electrodes priced at 100 yuan / kg.

Leave a Reply