Had a very similar situation with a friend/customer, we were working on his NH3 applicator and the shut-off valve was giving problems, not exactly what caused it to start blowing hoses off of the manifolds and we could not shut it off, two full tanks behind and no rope on the manual shut-off valve, and to top it off almost no wind to blow the NH3 away, We drove around for twenty minutes just staying ahead of the vapor cloud, finally the electric valve decided to close, I went right back to town and got him a rope and did not give him an option of not putting it on.