Let’s Build A Boiler: Part 2
Let’s Build A Boiler: Part 2
By guest blogger, Bruce Grossman, Chief of R&D for EZtimers Manufacturing
In June, PROS Parts post Bruce Grossman’s Let’s Build A Boiler: Part 1 blog. This month we share Bruce’s Part 2 information that we think you will find interesting and helpful.
Bruce’s Part 1 article took us through the necessary components and cobbled them together into a configuration that resembles a boiler. In this installment, we’ll learn how the different parts are organized into systems and how the systems work together to provide you with steam at the proper volume and pressure. The following is Bruce’s additional information on the key functions of a boiler.
When you first peruse the illustration, I suspect you’ll be a bit confused by all the lines, arrows, and labels. Don’t despair; we’ll take each system part by part, and by the time we’re finished, you’ll be diagramming every system in your plant.
For openers, keep in mind that all of the controls on your boiler serve only two functions:
- To turn a pump on or off.
- To turn the fire on or off.
That’s it! Simple, just like a computer, simple; only 1’s or 0’s. When a simple thing is applied with imagination, you’ve got a formidable engine, everything from CT Scans to the internet.
Part 1: Controlling the pump
Look at the boiler. Hanging somewhere off the side is a cylinder with a sight glass in it. This cylinder is the Water Column which provides a convenient place to monitor the water level in the Pressure Vessel. It is attached to the Pressure Vessel at two points; above the desired water level in the steam storage area and well below the desired water level in the water storage area of the Pressure Vessel so that the water level in the column is the same as the water level in the Pressure Vessel. Screwed into the top of this cylinder are three probes that look like spark plugs. When removed from the boiler, these “spark plugs” have metal rods at the opposite ends. These rods are actually electrodes (metal conductors that easily allow electric current to pass through them). When the water level rises high enough to touch the rod on the end of the probe, an electric current passes from the rod, through the water, to the metal of the of the Water Column, signaling that the water is at the level of that probe. The opposite holds true when the water level drops below the rod. Three probes control two separate operations. Two probes A and B control the pump contactor (a contactor is an electrical switch that uses a small amount of power to turn on or off a large amount of power). This contactor turns the return pump on or off. Bada Bing, Bada Bang, Bada Boom- believe it or not, that’s it for controlling the pump. However, controlling the fire is another story. To continue, probe number 3 checks to see that the water is above the lowest acceptable level inside the pressure vessel, to which the water can drop before the boiler shuts off. This continuous monitoring of the lowest water level is so important (you can ruin a boiler if it is heated with insufficient water) that there is usually another sensing probe installed in the Pressure Vessel but more about that in the following paragraph. In addition, the Water Column also contains a sight glass to visually monitor the water level in the column; remember this is also the level in the Pressure Vessel.
Part 2: Controlling the fire
(The circled numbers in the ignition and burner controls illustration correspond to the following paragraph numbers)
The fire is controlled by simply turning a gas solenoid (an electrically operated valve) on and off. Sounds simple, but many things need to happen to accomplish this safely. An Ignition Controller coordinates the activities of the different sensors used to open and close the gas solenoid. These may be called a Honeywell or Fire Eye (and other brand names depending on the boiler manufacturer). An Ignition Controller usually does six things:
- The output from the 1st low water sensor located in the Water Column is monitored- when the level drops below the probe, the gas solenoid is shut off, and an audible alarm is sounded. When the level is at or above the sensor, the gas solenoid circuit is enabled, and if other conditions are met, the flame will go back on, and the alarm will stop.
- The output from the 2nd low water sensor located in the Pressure Vessel is monitored. When the level drops below the probe, the gas solenoid is shut off, and an audible alarm is sounded. The alarm condition will continue until the level is at or above the sensor and a manually operated switch is pressed, clearing the alarm condition. The gas solenoid circuit is then enabled, and if other conditions are met, the flame will go back on, and the alarm will stop.
- The output from the high water level sensor in the Pressure Vessel is monitored. When the level is at or above the probe, the gas solenoid is shut off, and an audible alarm is sounded. The alarm condition will continue until the level is below the sensor. The gas solenoid circuit is then enabled, and if other conditions are met, the flame will go back on, and the alarm will stop.
- The condition of the operating pressure control switch (see section 3 in the illustration) is monitored. This switch has normally closed contacts which open when a preset pressure in the Pressure Vessel is reached and close again when the pressure drops a preset amount. The difference between this switch’s opening and closing pressure setting is called the differential. Opening the gas solenoid valve can only occur if the operating pressure control switch is closed, which means the upper limit of the boiler operating pressure has not been reached.
- The condition of the high-pressure safety limit switch is (see section 3 on the illustration) monitored. The contacts of this switch are normally closed and will open when a preset pressure in the Pressure Vessel is exceeded. When excessive pressure opens this switch, a manual reset is required to close it. The gas solenoid valve can open only if the high-pressure safety limit switch is closed. Usually, there is no alarm indicating this condition.
- To prevent a gas explosion, the ignition of the gas fueling the boiler is monitored. Before the main gas solenoid valve is opened, a smaller pilot solenoid valve is opened, and an electric spark is used to ignite a small burner to act as an ignition source for the larger volume of gas to be released by the gas solenoid valve (see section B on the illustration). If a flame is not sensed after a short preset time, the pilot solenoid valve is shut. Ordinarily, the Ignition Controller will try a few times for a successful pilot ignition, then go to a visual alarm condition requiring a manual reset.
Here are a couple of ideas to ruminate over:
- On almost all controls for water level, indicator lights tell you whether the probe is in contact with water. Open the boiler control panel access door (on some doors, there is an interlock switch that must be depressed or pulled out when the door is opened for the boiler to operate) and observe the operation of the indicator lights on a properly running boiler.
- Make a chart that shows the location of water level control and note whether the light is on or off during boiler operations. That way, you will at least know if a level sensor is detecting a problem if the boiler fails to fire.
- Also, most Ignition Controllers will have visible indicator lights. Take a video of these controls while the boiler is operating correctly to enable you to troubleshoot, perhaps saving an expensive, unnecessary service call.
There are few things sweeter in life than fixing it yourself without making it worse.
