If your house is like mine, there are a lot of pipes running through the basement.
I’ve got cold water, hot water, radiator water feed and return, and natural gas. And it’s not always obvious which way the flow is going: the cold water comes in at the northwest corner of the house and loops around counterclockwise hitting the bathroom, then the kitchen, laundry, and ending at the boiler and hot water heater in the northeast. The hot water pipes parallel the cold, but naturally they’re flowing the opposite direction: from the water heater in the northeast clockwise around to the laundry, kitchen, and then the bathroom. And the gas pipes are more like a tree through the middle.
So unless you really stop and think about what you’re doing, and shut off the valve that’s really upstream of where you need to work, you can easily cause quite a mess.
Industrial settings have this same issue, but they have many more different things that could be in the pipes, and a lot of them are dangerous. “Quite a mess” is one thing, but “lots of people dead” is quite another. So there are labeling standards that have been developed.
In an industrial setting, every pipe is labeled with what’s in it and which way it’s flowing. The labels repeat near every valve, every place the pipes go through a wall, every tee, and periodically along long runs. If it’s done completely, there’s never any doubt. And it starts to look like you’re working in a children’s television program: “Hey, kids! What’s in this pipe? That’s right! High pressure trichloroethane!”
Your house doesn’t need that level of rigor.
But you will benefit from at least marking flow direction near every valve, and contents if there’s any possibility of confusion. Near the hot water heater for instance, where the gas, cold, and hot water pipes are all tangled up together.
This is a project that is on my list. Because I’ve already been caught once by shutting off a valve I thought was upstream that was really downstream. I caused quite a mess. I’ll probably do the labeling at the same time I put in insulation around all the hot pipes to keep my hot water hot by the time it gets to the shower and make as much heat as possible come out of the radiators instead of coming out of the pipes leading to the radiators.