Cause of thermal expansion inside cavity:

 
Cause of thermal expansion inside cavity:
For the floating type/soft seat ball valve, and liquid fluid
1) In case of temperature of fluid and atmosphere has some increasing(for example""water: 30°C"", depends on fluid), and valve not operated for the time of increasing temperature,"
2) In case of valve position is "Full Open" for the time of increasing temperature,
3) In case of valve position is ""Full Close"" for the time of increasing temperature and differential pressure between upstream and downstream is below 0.98MPa,
4) In case of operation frequency is little and ball seat is new,
The above conditions are happened more than 2subjects, fluid inside body cavity(inside pocket) is increased pressure by thermal expansion. This condition is called "Thermal Expansion Inside Cavity".
 
Thermal Expansion inside Cavity is happened other problems as followings.
1) Operation torque is increased suddenly, and happened operation problem for on-off valve and manual valve.
2)There is possibility to get damage for seal parts.
3)There is a possibility to get damage for valve body. (especially for cast iron body.)

 
In order to avoid thermal expansion in cavity:
1)Making a hole (bypass) body cavity to valve port. (For the time of increasing temperature and valve position ""Full Open"", body cavity and port is same pressure condition.) Standard Spec. for HF5 model."
2) In case of increased temperature, valve position "full open" and fluid pressurizing direction 1-way,
1. make a relief groove on ball seat at upstream side.
2. make a hole on ball face at upstream side pass to port.
3) In case of increasing tempertaure, valve position ""full close"" and fluid pressurizing direction non-fixed, valve should be used Trunnion type ball valve."


 
Reference:
http://www.hisaka.co.jp/english/valve/techDoc/techDoc08.html