Valve World Magazine August 2022 - Sample | Page 29

PACKING
Figure 8 : Leakage diagram ( reference B ) for 40 MPa initial load
• The dynamic friction coefficient at the stem / packing interface ( under stem linear or rotational movement )
• The ring axial thermal expansion
• The ring load / thickness curve
• The elastic modulus of the ring at unloading
To identify these material parameters , dedicated testing facilities and procedures have been developed . As shown
Figure 9 : Mechanical parameters ( test result example obtained on die-formed graphite ring ) in Figure 3 , the test bench involves the following elements :
• A stem actuator ( rotary or linear )
• A friction measurement system positioned on the stem ( force transducer for linear movement or torque-meter for rotational movement )
• A compression press applying the desired force sequence to the packing through the gland .
• A test cell in which the packing is positioned . Two test cells have been developed and can be installed alternatively in the test rig frame . The dimensions , clearance tolerances and surface finish of the test cells are those defined in the test specification API622 - ‘ Type Testing of Process Valve Packing for Fugitive Emissions ’:
• A sealing test cell : allowing definition of the relationship between initial / residual packing load and leakage rate . ( Figure 4 )
• A mechanical test cell : allowing definition of the packing mechanical behaviour parameters described above ( Figure 5 ). Tests up to 150 ° C can be performed on this test cell using heating elements as shown in Figure 6 .
Figure 7 and Figure 8 show examples of sealing test results for two types of graphite packings ( comprising 4 sealing rings and two anti-extrusion rings ). From the target leakage rate ( 2 E-04 [ mbar . l / s / mm ]) and the operating fluid pressure ( 40 bar ), the required residual axial pressure ( 32 MPa ) can be obtained from the diagram of Figure 7 for example . But the leakage behaviour can also be shown in tables in the same way as typical EN 13555 test results presentation . In addition to these leakage results , Figure 9 shows an example of typical test results regarding mechanical parameters .
Conclusion
Test devices to determine the leakage and mechanical parameters of a packing set have been manufactured and tested . A calculation method to determine the necessary gland load for a packing set has been developed . The next step is a validation program of the proposed method involving both Finite Element Analysis and experiments on two valves , which is currently in progress . www . valve-world . net Valve World August 2022
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