Post by runner42 on Feb 17, 2024 2:30:33 GMT
I have read the precautions applicable to using ordinary PTFE piston valves, the most noteable one being cognisant of the increase in bore diameter due to expansion when cycling from ambient to operating temperature. An adjustment is by reducing the diameter at ambient so that at working temperature expansion increases the diameter to be a tight working fit in the cylinder but not so as to cause binding and locking the running gear up. The recommended method of determing how much the PTFE piston valve needs to be reduced is done by starting off with an optimum size PTFE piston valve and placing them mounted on their bobbin into water which is heated to boiling point and after some time at this temperature measure the diameter to determine the increase. The increase is used to determine how much the diameter needs to be reduced. Measurement of the increased diameter is considered insufficient so putting the hot piston valve into the piston valve liner to determine by feel is recommended. But, are there other factors to consider in achieving a good running fit at operating temperature? Say for example the piston valve liner is cold when undertaking the trial fit but in practice this is also going to increase in diameter when at operating temperature. Also, can the true operating temperature be higher than the boiling point of water?
Methods to minimise the increase in diameter is to keep the PTFE annulus as small as possible and to provide some spacing between the bobbin and PTFE to allow the expansion to occur in the downward direction and limit the outward expansion. The rationale being that volumeric expansion is in all directions so that the foregoing is controlling the direction in which expansion occurs.
If these two factors are important enough to take into account is it better to have a reasonably free PTFE piston valve installed in the piston valve liner and heat the whole cylinder assembly with a heat gun, no flaming heat source, place a temperature probe down the steam intake port and monitor the increase in temperature. When operating temperature is reached move the running gear to test the sealing quality of the piston valve. My assumption is also that the PTFE will adopt some maleable state and follow the exact profile of the piston valve liner at the place of operation.
Brian
Methods to minimise the increase in diameter is to keep the PTFE annulus as small as possible and to provide some spacing between the bobbin and PTFE to allow the expansion to occur in the downward direction and limit the outward expansion. The rationale being that volumeric expansion is in all directions so that the foregoing is controlling the direction in which expansion occurs.
If these two factors are important enough to take into account is it better to have a reasonably free PTFE piston valve installed in the piston valve liner and heat the whole cylinder assembly with a heat gun, no flaming heat source, place a temperature probe down the steam intake port and monitor the increase in temperature. When operating temperature is reached move the running gear to test the sealing quality of the piston valve. My assumption is also that the PTFE will adopt some maleable state and follow the exact profile of the piston valve liner at the place of operation.
Brian
