The use of auxiliary valves ( Axy ) on DIB-2 : Part 2
Auxiliary valves
( Axy ) are needed to conduct in-service maintenance and repair on pressurized gate or ball valves in a safe manner .
Axy valves should be used in combination with service valve for throttling . This is part 2 of the article , the first part was published in Valve World July .
By Ingolf Holmslet
Padlock for locking valve in open position
Figure . 13 Figure . 14
There are many valves where there may be a risk of overpressure due to thermic overpressure . To prevent overpressure , those valves are secured by a TRV ( thermic relief valve ) also called a TSV ( thermic safety valve ). Normally the TRV will be installed directly in the cavity of the valve without possibility of testing or replacing those without depressurizing the valve . To avoid depressurizing the entire system , an Axy valve with three outlets can be used . Figures 13 and 14 illustrate this combination valve . This Axy valve is installed with a padlock locking device ( Figure 13 ) to secure the valve in its open position . This combination valve will be equipped with three outlets after the ball , and under normal operation the valve will be installed with one TRV where the blue plug is illustrated in Figure 14 .
No pressure on the ball
If there is a need for replacing the TRV , the Axy valve will be closed and the bleed plug will be opened to ensure there is no pressure after the ball . The TRV will be replaced , the bleed plug closed and the Axy valve will be opened and locked in its open position . This without any operation on the main valve . If this combination valve is to be used in maintenance work ( on the main valve ), the TRV should be taken off while performing this maintenance . And then reinstalled when maintenance is completed . To prepare the Axy valve for maintenance , close the Axy valve and open the bleed plug . Disconnect the TRV and replace it with a blind plug . Take off the red plug and install the gauge in this 1 ⁄ 4 ” connection . Then disconnect the orange bleed plug and install the service valve and connect the bleed to safe location , as illustrated in Figure 5 . You are now ready for performing maintenance on the valve . have a relatively large volume with several other valves connected to the downstream pipework . It can be hard to tell which valve is leaking if you have 8 to 10 valves connected to the downstream pipework and you have a pressure build-up . If you have Axy valves connected to the cavity of the main valves ( EDV – XV - HV ) like in Figure 15 , you can use those Axy valves to verify the leak rate in that valve . The procedure is as follows . First ensure the Axy valve is in the closed position . Open the bleed , checking that the Axy valve seals . Take off the 1 ⁄ 4 ” plug and install the gauge , take away the bleed plug and install the service valve . Close the service valve and open the Axy valve . You will now have full control of the cavity pressure by looking at the gauge . If the downstream pressure increases without increasing cavity pressure that indicates that this particular valve is not the source of the leak , which must be elsewhere . I have been involved in several situations involving large valves sized 24 ” and up where the valve was apparently leaking . But after a lot of trouble , we discovered that the leak was somewhere else . Having the Axy valve in the cavity enables you to test and control the leak through the main valve .
Flexing – the causes
Figure 16 illustrates how a DIB-2 valves can flex and cause a leak . All the four illustrations ( A to D )
Using the Axy valve for barrier setting
When setting a barrier on a major gate or ball valve , you normally close the valve , take away the downstream pressure and hope for the best . Without the Axy valve , you are working in the blind as you may
Figure . 15 π www . valve-world . net Valve World September 2022
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