Let’s Build a Boiler: Part 1
Let’s Build a Boiler: Part 1
By guest blogger, Bruce Grossman
Boilers. Talk about a device that is probably the simplest in concept yet most troublesome in your plant. In addition, it is also the most potentially dangerous. For this reason, NEVER DO ANY MAINTENANCE OR REPAIRS OF ANY KIND ON A BOILER THAT IS HOT OR UNDER PRESSURE.
Now on with the show! As I’m sure you are aware, there are many different types of boilers and many sub-species of tube (water tube, fire tube, tubeless) and other configurations within each type. We are not going to get into the FAQS of each type. Our purpose this issue is to gain an understanding of what a boiler is and how a boiler functions in order to generate steam.
Take a pot of water, put it on a stove and heat it on a low flame. Put a thermometer in the pot of boiling water and if you are at sea level, the thermometer will read 212 degrees Fahrenheit. If you turn the flame up all the way, the water boils more violently and changes to steam at a more rapid rate – but the thermometer still reads 212 F. As long as there is water in the pot, that thermometer will stay at 212 F, or until the cows come home. The steam rises from the surface of the water and disperses into the air without being able to do much in the line of useful work unless you have some vegetables in a steamer above the water in the pot.
If you were able to put a lid on that pot that was tight enough to contain the steam being generated, the pressure in the pot would begin to rise. Since this increase in pressure is also pressing down on the boiling water, it takes more heat energy to force the water to turn into steam. This increase in heat energy is felt as an increase in temperature of the water steam mixture. The higher the pressure inside the pot, the higher the boiling point temperature of the water and steam. There is a direct and unchanging relationship between steam pressure and temperature. It is built into the nature of water. That is why you heat your dry cleaning presses with 80lb steam (324 F) and your laundry machinery with 100lb steam (337 F).
If you had some veggies in the pot they would cook to mush in no time because you have the kitchen appliance called a PRESSURE COOKER with internal temperatures much greater than 212 F. In addition, the space above the water line and the lid is a built-in storage area for the contained steam. At some point, unless you somehow release the steam or turn off the heat source, the pressure will increase until the pot ruptures (explodes). On pressure cookers there is a weighted or spring-loaded relief valve in the lid which controls the internal pressure and also a safety blow-out vent if the relief valve fails.
Let’s start adding components to the PRESSURE COOKER in order to create a boiler. I’m going to use the example of a gas fired boiler for this article.
- Put a solenoid valve in the gas supply line so that we can turn the gas on and off; this would be your gas supply system. At the end is the burner which distributes the gas evenly over the bottom of the PRESSURE COOKER.
- Now since we are dealing with gas, we need to know that when the solenoid valve opens, the gas ignites almost immediately. If not, we shut the solenoid which cuts off the gas flow. This is so we don’t blow up the building or perhaps the entire block. We’ll call this control system the ignition monitoring system.
- Remember I mentioned that as long as water was in the pot, the temperature would not rise above the boiling point of the water. Well, if the all the water boils away there is nothing to adsorb the heat energy. The temperature of the metal of the pot will rise until it glows red. In a boiler this would destroy the boiler. So let’s place a sensor in the PRESSURE COOKER that monitors the level of the water. If the water level drops too low, we will shut off the gas supply to the burner. This is your boiler’s low water control. Having the proper level of water is so critical there is usually a requirement for a second back up low water control for safety.
- At this point we are operating our PRESSURE COOKER without blowing up the plant via a gas explosion or melting the PRESSURE COOKER by running out of water while the heat source on. Doing great so far so let’s not forget about generating too much pressure in the PRESSURE COOKER and taking the roof off the building. In order to control the internal pressure, we need a control that will turn off the gas when the desired pressure is reached. In fact for safety’s sake, let’s add two controls: an operational pressure switch which goes on and off with pressure changes controlling the gas solenoid valve, and a safety pressure switch that trips when the operating pressure is exceeded. Safety pressure switches require a manual reset. The reason for the manual reset is to let you know there is a problem in the pressure control system. Since steam explosions are so devastatingly dangerous, let’s install a fitting in the PRESSURE COOKER lid and screw in a spring-loaded valve which will open at a pressure lower than the bursting point of the PRESSURE COOKER. This allows steam to vent in case all the other controls fail and serves as your pressure relief safety valve.
- Assuming we have some device connected to a fitting at the top of the PRESSURE COOKER that is using the steam generated, we must find a means to add new water to the PRESSURE COOKER to compensate for the steam that has flowed out. How are we going to get water into a hot pressurized pot? We can’t open it and pour in water. How about adding a fitting that connects to a source of water that can be raised to a higher pressure than the pressure inside the PRESSURE COOKER? We’ll do this by using a pump called a FEED PUMP or Return Pump. Let’s turn this pump on when the water level drops below a certain point but not low enough to activate the boilers low water control. It would be real handy to do this automatically so we add another water level sensor and call it the pump controller. As a practical matter, when the pump is not running the pressure in the PRESSURE COOKER, the pump controller would just push the water out of the PRESSURE COOKER through the pump. We can solve this problem by adding at least one check valve between the pump and the PRESSURE COOKER. A check valve allows a fluid, water in this case, to flow in only one direction: into the PRESSURE COOKER and will stop it from flowing back out.
- Just like any container that keeps getting water added and then boiled off, there will be a build up of the “stuff” called scale at the bottom of the PRESSURE COOKER. The easiest way to remove this scale would be to install a fitting at the bottom of the PRESSURE COOKER and use the pressure inside to blow out the scale. So let’s install a valve that we can open at low pressure and let the flow water and steam push the scale out of the boiler. This is your blow down valve or valves. Since it’s not safe to let all this hot water and steam just blow out of a pipe, we’ll place a cylindrical container (or Blowdown Tank) at the end of the pipe with a vent out the top that allows steam to escape with a drain out the bottom for water. Inside the cylinder we’ll put some baffles like those in a car muffler to slow down and damp the escaping steam and water.
Whew! There you have it. Finally I can stop all this PRESSURE COOKER analogy stuff and call this contraption a boiler. Next issue will be devoted to refining this boiler and designing the system that supplies steam to the machinery and handles the returning condensate.
