Wet Tap

[dundee]

We just had a problem doing (2) wet taps on a 16" line here in Virginia. The tap was on the bottom of the 16" pipe 20 feet in the air. We installed the cast iron gate valves and then installed the hot tap machine on the bottom of one of the valves. After doing the tap, the valve would not close. We damaged the valve and had to shut the line down in order to install a new valve. Before performing the next tap, we pulled the valve down, tested the valve, and reinstalled that valve. After performing the tap, again we had problems closing the valve. We had to use a cheater-bar to close the valve. We are not sure what we did wrong. Should we have supported the tap machine instead of hanging it off the valve? Could we have stretched the valve by hanging the machine off the bottom? Could shavings get in disc area preventing it from closing?

This problem took three days to fix. We would appreciate any information you could provide to prevent this from happening again.

 

[kstaylor]

We sometimes need to perform wet taps (or as we call them, "hot taps&quot to make connections on live piping or equipment. Over the years we have learned from negative hot tap events.

Our standard practice is to hydrotest the hot tap valve in the closed position on both sides, one at a time. This is done to be sure the valve will hold pressure in either direction when closed. We also check to make sure the hot tap machine will physically pass through the open valve. This is to assure that no internal interferences exist that will prevent the tap from being made or the valve from closing properly afterward.

Also, we usually connect on the top side of the pipe to prevent metal shavings from the cutting operation from dropping into the valve and preventing it from sealing properly.

We normally avoid using cast iron or ductile iron valves on hot taps just because of all the extra bolting, unbolting, torquing, cutting, etc. that occurs that during these activities and the very low ductility that these materials usually exhibit. I don't believe that your iron valve "stretched" because it probably would have broken first! I suspect that the problem lies in some internal interferrence or damage that was caused by the cutter passing through the valve.

Good luck!

 

[way]

I hope that you were using full port valves? When we do hot taps, we like to use full port ball valves...ball valves are hard to beat for positive instantaneous shutoff.
I think kstaylor covered just about everything else. Better luck next time.