The high temperature conditions here refer to the conditions when the medium temperatures are equal to or higher than the starting creep temperatures of the metallic materials. The creep temperature of carbon steel is about 400℃; chromium molybdenum alloy steel 450℃; austenitic iron-based high-temperature alloy steel 540℃; nickel-based high-temperature alloy 650℃; aluminum alloy 200℃; titanium alloy 310℃.
Under high temperature conditions, thermal expansion and contraction will happen on both the metal and non-metallic materials, which will have an impact on the sealing performance of the valves. The thermal expansion compensation structures of the butterfly valves and the treatment of the gate valves' clacks to prevent clipping tightly are to eliminate the adverse effects of thermal expansion and contraction.
When we choose the valves used for high temperature conditions, we should give consideration to the following principles.
The types of valves
We should first choose valves which have the valve discs with good thermal expansion compensation performance. The thermal expansion compensation capacity of the commonly used shut-off valves from high to low is: globe valves, gate valves, ball valves, mechanically balanced plug valves, metal sealed butterfly valves.
When the medium temperatures exceed the starting creep temperatures of the bolts' materials, it is not recommended to use the wafer butterfly valves and the check valves.
When the non-metallic seal is adopted, attention should be paid to the operation temperatures of the non-metallic materials. We can consult the valve manufacturers about the specific operation temperature range of the materials.
The structures of valves
When the gate valves are selected, we should consider the possibility of the valve clacks being clipped tightly. Therefore, we should select gate valves with elastic valve clacks when the gate valves of DN50 and above DN50 are decided to be used.
Welding connections are not emphasized to be adopted in the simple high temperature environments and they will be recommended if the high temperature conditions are combined with the pressure and then causes the nominal pressure rating of valves higher than or equal to Class600 (for Class series) or PN100 (for PN series). Welding connections will also be recommended if the high temperature conditions are combined with the flammable media and the temperature of the media exceeds the spontaneous ignition points and flash points of media.
The bypass should be set on the valves used for high temperature steam (3.5 Mpa and above) pipelines or other pipelines considered by the engineering designers that they can not withstand thermal shocks.
The materials of valves
For the pressure components of valves, manufacturers should be required to have creep tests on materials, to provide the data of type tests, and to improve their quality control of materials, including the control of surface defects, internal defects and non-metallic inclusions.
When the temperatures of the media are higher than the starting creep temperatures of the bolts' materials, the adverse effects of possible stress relaxation of the bolts on the sealing performance of the valve bonnets should be evaluated. Replace the materials with those of higher creep temperatures, or take other measures if necessary.
As the non-synchronized thermal expansion of the valve components may create additional force on the sealing parts, the hardening treatment should be had on the sealing surfaces of the valve clacks and the valve seats.
The effects of high temperature conditions on the accelerated aging of the stem packing should be evaluated. If necessary, replace the materials with better ones, or take appropriate measures.
