Safe Over Pressure
In our last blog, we explored the circulator pump and its role in a hydronic heating system. One of the major concepts was building up a supply of hot water to gradually use throughout the day. What happens when you have too much water? This week we’re going to look at a key piece of safety hardware that keeps your home or facility in one piece when things go wrong.
Let’s start with the scary answer.
If your heating system had no safety mechanisms, and continuously pumped water into the hot water tank, the tank would eventually explode. This all comes down to a fundamental property of water: it doesn’t compress. As water is crammed into the tank, it starts to exert force on the tank. In regular operation, this force might simply push water through the pipes. In a complete-failure scenario, too much force will cause something to break. A minor manufacturing defect may cause part of the tank to crack and spew out boiling hot water with enough force to cut steel or the whole thing could go up in a ball of metal shrapnel and steam.
In more elaborate systems, the same problem can occur in the pipes. A closed off valve or a blockage connected to a source of high pressure will exert the same force against the pipe. At some point, the pipe, it’s joints, or hardware on the pipe will fail just like the hot water storage tank. The failure will likely occur at the weakest point, with the lowest-rated piece of hardware.
Failures like this have occurred since hot water heating was first created. Steam engines in the 1800s could build up an excess of pressure while sitting, before filling the countryside with a deafening boom once they failed. The lessons learned from these accidents have made modern hydronic systems safer than ever. You may notice that your hot water storage tank is made of some pretty sturdy material or that it costs a fortune in shipping charges for larger valves. Almost everything is built to withstand harsher stresses than they’re rated for. Everything has safety built-in to handle excursions to unsafe territory and prevent deadly situations. When sensors fail, controllers shut things down. When pressure builds up, the system handles it. In some cases, multiple safeties can cover the same source problem. Your heating system was designed to fail harmlessly rather than fail deadly.
How does the system handle failure? With special, pressure-activated valves. There are generally two types of relief valves: Pressure Relief Valves and Safety Valves. There are many variations to them, but in their most basic form they do the same thing. Relief Valves open proportionally to the pressure exerted as they near a threshold, relieving pressure as it builds up. The valve may be rated for 100 PSI. It may open at 90 PSI and start a slow relief. By 100 PSI, it’s fully open and venting at its maximum capacity. It remains open until the pressure dips below 90 PSI. Safety Valves open quickly and remain open at full capacity until the pressure dips below a safe threshold. One rated for 100 PSI could remain closed up to 100 PSI, then blow at full power until the pressure reaches 94 PSI, when it slams shut.
Depending on your industry, there may be further concepts involved. In some industries, safety valves can capture and divert their vented material. Consider something like radioactive water in a nuclear plant. You’d want that safety valve to capture and store the over-pressure liquid somewhere safe. In other industries, relief valves vent their material into the open air regularly. They could be used on a large water supply system where a pump causes a pressure surge at startup, before dipping to its regular operating output. A small bit of vented material is harmless but necessary in that situation.
With that in mind, what happens when these valves fail? If you run a large facility, your local laws may have mandatory replacement periods where new valves must be installed at regular intervals to prevent any failures from ever happening. Safety officials would rather prevent hardware from ever being at risk of failure than depending on it to fail safely. Home owners are generally exempt from this requirement. In either case, pressure and safety valves generally fail safe. I like to think they also fail-messy. If the valve hooked up to your furnace fails, it should fail in the venting position. That is, it fails in the safe position so that if an over-pressure occurs, nothing will happen while it’s out of action. You may hear a loud whistle as pressurized water spews everywhere, walk into your basement, and find you’re the proud owner of a basement pond.
Once you’ve shutdown the furnace, cut off the water supply, drained the pipes, and mopped up Lake Disgusting, you can start the repair process. If you’re lucky, everything is threaded nicely together, so it’s mostly a matter of heating and twisting pipes. In other circumstances, it may involve more grumbling, pipe cutting, and working in confined spaces. It’s probably going to be a real pain to work on, no matter what. It’s not a simple case of “oh yea, grab your screw driver, take out four screws, swap the motor, and reverse those steps.”
Like all repairs however, once it’s done, it’s done. These valves are designed with years-long lifespans in mind. You don’t have to do the work and start dreading doing it again next year like cleaning a furnace.
Now for our mandatory sales plug, brought to you by my boss!
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