Calculation of the thrust required for the valve in the selection calculation for control valves
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This post was last edited by Sammy_Wang on 2015-12-27 20:12. When calculating the thrust required for valves recently, I encountered a problem: I don’t know how to calculate the friction force between the packing and the stem (or between the seal and the plug) According to the formulas of classical physics, it should be f=pi()*Dstem*Lpacking*μ*F. How is this F calculated, and what is its relationship with the pressure difference △P at the sealing point of the seal? I’m seeking advice from those with more experience. The following information comes from the user on floor 2: Sorry, there’s an issue with the previous formula; it should be f=pi()*Dstem*Lpacking*μ*P’. What is the relationship between this P’ and the pressure difference ΔP acting at the seal site of the seal? How should the friction force at seals such as gaskets be calculated? Are there any materials I can refer to? Thank you!
Sorry, there was an issue with the previous formula; it should be f=pi()*Dstem*Lpacking*μ*P'. What is the relationship between this P' and the pressure difference ΔP acting at the seal site of the seal? How should the friction force at seals such as gaskets be calculated? Are there any materials I can refer to? Thank you!
Hello, the friction of the filler is generally quite low, so there’s no need to worry too much about it. You can refer to FISHER’s control valve manual, which provides the packing friction force for various valve stem diameters; you can use those values.
Well, that’s probably just for reference; isn’t calculation not necessary during the actual design and selection process?
It’s nonsense up there – the friction caused by the packing is closely related to the diameter of the valve stem as well as the pressure difference. Try using a valve stem with a diameter of 50 mm under conditions of 5 MPa; especially with graphite packing, which accounts for a large portion of the thrust required to operate the actuator. FISHER valves have lower packing friction because their control valve design features pressure balance in the valve core, and the driving force is primarily used to overcome the friction of the packing. As a result, the diameter of the valve stems in these valves is relatively small, which helps reduce packing friction and thus the driving force needed to operate the valve. In fact, the formulas used to calculate packing friction are mainly applicable to high-pressure conditions; they are not suitable for low-pressure conditions, as the tighter the packing gland is tightened, the greater the friction generated by the packing
You’re right; is there a corresponding formula for the relationship between this friction force and the pressure difference?
I agree, especially with manual valves. When the packing is compressed too tightly, it becomes difficult to open the valve; if it isn’t compressed tightly enough, it can lead to leakage from the valve’s packing
Yes, that’s why it’s still very important to calculate the thrust at the sealing area of the valve