SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I'm working as installing my system and here's what I've got connected so far...

A slip Tee from the mainline runs to a shutoff valve, to a rainbird RBY filter, to a Watts 1" Double Check, to a Tee for the first manifold, and then to a temp shutoff valve (to be later extended to the next manifold). The first manifold is a bit of a kludge. It allows for four automatic vavles, two dirrectly from the mainline, the other two AFTER a 40psi regulator (the regulator is the PVC style inline regulator that comes with a rainbird drip kit, also the source of the filter). The two full pressure pipe and one of the "regulated" pressure pipes currently lead to manual shutoff valves. The other "regulated" pipe of the manifold is connected to the valve that came with the drip kit, but the lateral line leading away from the valve is not connected to anything yet.

I pressure test the system by openning the main valve, then one-by-one open and close all manual and automatic valves so that there is water in all of the existing system. I then dry everything off so that I can check the pressurized system for leaks.

Combined with previous tests, I'm satisifed that the system doesn't leak and I get ready to burry what I've got in the ground so far. To keep dirt out of the one lateral line, I slip a cap over the end of the line (so it's just held in with friction, no cement yet).

But then I decide to try one more stress test to the system. I open the valve at the end of the mainline and get max amound of water flowing through existing system. I then try to test for water hammer by closing the shutoff valve at the end of the existing mainline as fast as possible. When I do, I hear a "POP" and think "Oh No!!!" But the only thing wrong is the cap slipped on the end of the existing latteral has popped off. I slide it back on and try again and once again enough pressure builds up to pop this cap back off.

HOW??? / WHY???

How is the "train wreck" caused by quickly shutting off the flow of water allowing enough air pressure to build up in a lateral line of a valve that is already shut off that a cap, held on by friction, is blown off.