In technical papers , information is available about low emissions and how aiming for the best torque on a valve packing gland is essential to optimize economic resources and environmental impact . The correct balance between sealing force and stem friction must be reached . Inadequate torque methods result in several problems , including increased maintenance , higher leakage , and reduced packing life with the consequent waste of money and energy . Reducing energy consumption , being competitive and minimizing costs means being efficient . The best results are needed . So , once the correct packing material , geometry , and design have been chosen , the right torque choice and its controlled application on the bolts are required to reach an optimized compressive load . This is one of the most important considerations to be taken . Appropriate load and an accurate transmission to the packing must be achieved .

At this point , two questions must be addressed : torque value and tightening method .

Emission testing at Babcock Valves installation .

Torque Value & Tightening

Usually , the packing manufacturer will recommend the torque value for their particular packing . The optimum torque value is calculated by considering several parameters : number of bolts , quality , diameter , packing composition , and

friction factors . Regarding the tightening method , some variation occurs in the information given by manufacturers , but in general best practice will involve the following elements :

• Clean and inspect the housing and counter face

• Use hardened flat washers

• Tighten the gland bolts ( often recommended to be done in several stages )

• Use a calibrated torque wrench

• Measure bolts distance

• Readjust gland nuts after a short working period ( sometimes it is recommended at least 5 times with the valve in the closed stroke position ).

• Proper lubrication

These practices will minimize the changes and losses due to variations in the torquing process . All of them affect the sealing system .

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 , and more ) 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 .

Can We Adjust the Packing to Minimize Deviations in the Torquing Process ?

The first contact between the packing and the valve stem is produced by a seating torque value . During assembly , the first torque value required to seat the packing will affect packing behavior during operation . Plastic deformation in the packing set takes place , and all the gaps between the stem / housing and packing gradually 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 .

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

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

• 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