Technical
Figure . 16
consist of a trunnion-mounted ball where the inlet seats ( left hand side ) are self-relieving , and the outlet seats ( to the right ) are double piston seats . Now the valve will have SPE on the left side and DPE on the right-hand side . Let us assume that the valve is a 10 ” class 900 with 120 bar pressure . This 10 ” valve will have a seal diameter of 270 mm which equals an area of 572 cm 2 . Let us start with illustration A . When the valve is in closed position with 120 bar in the cavity and outlet , the blue arrows indicate a ball in perfect balance , fully equalized . For the moment there will only be a passive seal , as there is no pressure difference ( PD ) across the valve . Then take away the downstream pressure ( illustration B ) to activate the seal and to test that the downstream seat seals . Now the ball will be out of balance ; the inlet ball side is balanced , but as there is zero pressure in the outlet side the cavity pressure of 120 bar will force the downstream side of the ball towards the zero pressure . This will stretch the ball with a force equal to 120 bar x 572 cm 2 = 68.640 kg ( 68.6 tons ), indicated with the red arrow . This stretching will force the seat some 10 th of millimetre downward . The downstream side of the valve will seal . So far , all good .
Flexing – leak paths
In illustration C , the cavity is depressurized to test the inlet seat seal . Now the outlet side of the ball will be in balance , and the 68 tons DP ( differential pressure ) force will go away . The outlet side of the ball will fall back the 10 th of mm it was stretched . If the seat experiences friction between the seat pocket and the seat that exceeds the force of the coil springs , the springs will be unable to force the seat back in contact with the ball . Now there will be a leak path between the seat and the ball which nobody for the moment will be aware of .
Figure . 17
At the same time , when reducing the cavity pressure , the inlet ball side will face the same challenge as the inlet pressure of 120 bar ( 68.6 tons ) will force the ball away from the inlet seat . The result : if the friction in the seat pocket is too high , the inlet seat may leak as it doesn ’ t follow the ball back . Let us assume that the inlet seat is in good condition and working correctly , sealing 100 % as the inlet seat follows the ball back towards the right . There will now be zero pressure in the cavity . To confirm the double isolation ( as this is a DIB-2 valve ) the cavity will now be pressurised with 20 bar nitrogen and activate the downstream seat .
Flexing - consequences
The last Illustration ( D ) illustrates this injection of 20 bar nitrogen into the cavity . This can result in two situations . If the friction in the seat pocket is too high , the seat will not follow the ball back as indicated in illustration C . The nitrogen will now leak between the seat and the ball . The seat that earlier was fine with full DP across , now does have a leak . Secondly , the friction in the seat pockets is less than the spring force and the seat follows the ball back and the downstream seat seals . I have witnessed several situations where after depressurizing the downstream side and the cavity we ended up with a leaky valve . This left us unable to decide what to do as there was no way to troubleshoot the valve . As a consequence , the only solution was to replace the valve . If Axy valves had been installed we would have had more opportunity to determine the source of the leak .
Take control of your valves
We must go back to the iSmart system , and you MUST have a Axy valve installed . With the Axy valve ( illustrated in Figure 15 ) installed in the cavity of the valve you can pre-test the valve before establishing a barrier . Start with illustration A in Figure 17 . With the valve in closed position and 120 bar all around , both sides of the ball are balanced and if the seal area of the seat is in good condition and the seat is in contact with the ball the valve is fine .
Increasing valve knowledge and maintenance
In the last 8 years I have been working together with Téchne in developing special valves to be used in testing , maintaining and isolation on major ball and gate valves . These are the system iSmart and the auxiliary valves known as Axy valves . One of the goals in my work is trying to increase the general knowledge and maintenance on valves . In this work Téchne have been a perfect partner in producing the high quality products needed to perform the iSmart valve maintenance .
In illustration B , the service valve and the gauge are installed in the Axy valve ( as in Figure 5 ) and the valve is ready to be tested . Start with the service valve in closed position and open the Axy valve . Read the cavity pressure on the gauge . Reduce the cavity pressure with 10 bar by opening the service valve . The cavity pressure will now be 110 bar with DP to both sides . If the cavity pressure is stabile the seat seals . Reduce the cavity pressure with 20 bar , creating a DP of 30 bar . Let it stay for five minutes . If the pressure is stable , again reduce the cavity pressure with 20 bar . Do this until the cavity is fully depressurized . If the cavity is zero with 120 bar DP from both sides the valve is perfect . There will now be 68 tons from both sides squeezing the ball towards the centre . This is how the valve works if everything is okay . But if the valve starts to leak when the DP is 60 bar and the leak rate increases with 70 bar DP , then the friction between the seat and the seat pocket is too high and the coil springs are unable to force the seat against the ball . Now the valve is telling you that it needs maintenance . You don ’ t have to replace the valve , it only needs maintenance . How to maintain the valve , that is another story . But you do need Axy valves to perform that maintenance . By installing Axy valves TM and performing iSmart TM maintenance you may end up saving hundreds of millions as you are in full control of the valves .
46 Valve World September 2022 www . valve-world . net