COLUMN
5.00E-04 4.50E-04 4.00E-04 3.50E-04 3.00E-04 2.50E-04 2.00E-04 1.50E-04 1.00E-04 5.00E-05 0.00E + 00
Final meassurement mbar l / s vs seating torque
-6. 345
Y = 75333x R 2 = 0.9955
0 5 10 15 20 25 30 35 40 45
Pre-deformation : 20 Nm x 2 Working load : 30 Nm Measurement : 4.5 10 -4 mbar l / s
Pre-deformation : 30 Nm x 2 Working load : 30 Nm Measurement : 2.7 10 -5 mbar l / s
Pre-deformation : 40 Nm x 2 Working load : 30 Nm Measurement : 5.7 10 -6 mbar l / s
Figure 1 . PTFE Packing test
6.00E-06 5.00E-06 4.00E-06 3.00E-06 2.00E-06 1.00E-06
Final meassurement mbar l / s vs seating torque
y = -4E-08x + 6E-06 R 2 = 0.9957
Pre-deformation : 20 Nm x 2 Working load : 50 Nm Measurement : 5.2 10 -6 mbar l / s
Pre-deformation : 40 Nm x 2 Working load : 50 Nm Measurement : 4.5 10 -6 mbar l / s
Pre-deformation : 50 Nm x 2 Working load : 50 Nm Measurement : 4.05 10 -6 mbar l / s
0.00E + 00
0 10 20 30 40 50 60
Figure 2 . Graphite packing test
disappear as the load increases . This is the pre-deformation or seating process , and this is the torque value that needs place the packing set in the best condition to perform its work . After this , the tightening torque is applied . The aim of pre-deformation is to ensure that the tightening torque value is maintained throughout the life of the packing . Therefore , pre-deformation must optimise the sealing system , which requires us to balance two factors : achieving optimum tightness to ensure low emissions whilst minimising friction . This seating value determines how the packing will adapt to the sealing system . If this seating stress is adequate , the behaviour of the whole system will improve with time . A good seating torque value , in addition to an appropriate working torque value , could optimise packing performance . It is logical to think that this first contact will be decisive in getting a reliable packing performance . To corroborate this idea , we have performed a series of tests . Several types of packings with the same geometry , design and working conditions ( torque values , temperature , cycles , internal pressure , etc ) but with different seating torque values at the beginning , on assembly , were tested . The tests were conducted at one of our valve manufacturing customers , and we thank them for their cooperation . The tests were performed in a 3 ” ball valve , pressure class 300 . The stuffing box dimensions were 28.3 x 44 x 52 mm . A sniffer performed leakage measurement with a helium mass spectrometer . Packings
• Six ring set with 28,3x44x8 mm packing dimension , manufactured from three different packing materials :
• Braided PTFE packing with pure PTFE yarns and PTFE impregnation , density 1.5 g / cm 3 ,
• Braided expanded graphite packing made from Inconel wire over knitted expanded graphite yarn ( C > 99 % carbon content ), density 1.7 g / cm 3 ,
• Braided carbon fibre packing made from carbon yarn with graphite impregnation , density 1.1 g / cm 3 .
Seating process
• Anti-Friction component ( Molykote ) on bolts
• Pre-deformation two times to 20 , 30 and 40 Nm
• Working load 30 Nm
• 3 cycles
• 20 / 10 bar Helium
Conclusions
The pre-deformation or seating torque values of the packing sets impact the final packing sealability when the other conditions are fixed and maintained . In all cases , the packing set sealing behaviour
24 Valve World February 2023 www . valve-world . net