Boiler blowdowns are integral to your operation. A blowdown removes unwanted impurities from the boiler water, which causes unwanted operating conditions. These impurities, when not removed, also contribute to the deterioration of popping, values, instruments, and other components, which are needed for safe operation and control.
Ultimately, the purpose of boiler blowdown is to maintain proper water chemistry, so that you can run a safe and efficient operation. To help you incorporate this into your preventative maintenance plan, we explain the following:
Let’s start with the basics! Boiler blowdown is the process of removing water from the system. This helps to eliminate impurities, suspended solids, and sludge, in an effort to control the water parameters. The amount of blowdown required is directly related to the quality of the water being fed into the boiler. So remember, the better the water quality, the less a blowdown is required.
A local chemical expert and knowledgeable boiler service personnel can best advise on the proper blowdown procedure and timing for each boiler. Contact us with any questions, and we're happy to help!
We want to mention that there are two different methods to conduct a boiler blowdown: manual and automatic. Most operators are familiar with the manual method of blowdown, which requires the manual operation of valves and controls to remove water from the boiler or system. However, there are also systems that can be put in place to automatically perform these tasks at desired intervals. Powerhouse offers a comprehensive boiler room monitoring system, in which this automated blowdown method can be added as a custom feature in order to meet your requirements. Learn more about our Boiler Link system here.
There are three main types of boiler blowdowns: a surface blowdown, a bottom blowdown., and a water column blowdown We will explain the differences between the three types along with the advantages and disadvantages of each one. It’s important to know these distinctions, so you can determine the proper procedures and frequency that make the most sense for your operation.
Surface blowdown is commonly referred to as “continuous blowdown.” since this type of blowdown is accomplished by a slow, continuous removal off the top of the boiler water level. A manually controlled needle valve can be utilized to control the flow of continuous blowdown or a more sophisticated automated system may be used. The conductivity level of the boiler water is used to monitor the amount of continuous blowdown required. Surface blowdown is typically only used on larger boilers (~250 HP and above).
Bottom blowdown is the most widely known type of blowdown. Typically, this is what operators are referring to when they mention “boiler blowdown” or “intermittent blowdown”. Bottom blowdown utilizes connections on the lowest point of the boiler to remove the sludge and sediment that accumulates at the bottom of the boiler.
Water column blowdown is the most forgotten type of blowdown. However, the importance of a regularly scheduled water column blowdown cannot be overemphasized. When talking about the water column blowdown, we are referring to the blowdown of the water gage class (or site glass), the water column, and the associated piping and valves. The water gage glass is the visual indication for the boiler operator of the water level in the boiler so maintaining a clean glass, piping, and valving is crucial for safe operation.
There are also several types of boiler blowdown systems, including blowdown separators, blowdown tanks, and blowdown heat recovery systems. Again, we will help explain the blowdown system and provide the benefits and downsides of each system.
A blowdown separator is a vessel that is designed to reduce the pressurized blowdown from the boiler to atmospheric conditions by allowing steam flashing to occur. Since blowdown of a boiler is done while the boiler is under pressure, the water removed is still at the boiler’s operating pressure and temperature. Since it is inherently full of impurities, this water needs to be directed to a drain but first must be conditioned to do so. The first task is to bring the pressure down from the boiler pressure to atmospheric pressure, which will cause the high temperature water to flash a percentage off to steam immediately. Typically, an aftercooler is also included as part of the blowdown separator package to further cool the water down to < 140° F, which is the standard maximum for the sanitary drain.
A blowdown tank is inherently doing the same thing as a blowdown separator but in a different way. A blowdown tank is a vessel meant to collect and store the blowdown water and allow cooling to happen over time through the vessel wall. After sufficient cooling, the tank may be drained via a valve or the water may be removed via a pump. Similar to the blowdown separator, the resulting flash steam typically reduces the incoming water pressure to atmospheric pressure. The created flash steam is then vented to the atmosphere, as required.
As blowdown water enters the blowdown tank, it is immediately reduced to atmospheric pressure and the flash steam is vented to the atmosphere. The remaining water is then allowed to cool through natural convection. Now that there is cooler water in the tank, subsequent blowdown water to enter the tank that is not flashed into steam is combined with the existing water and cooled down to an equilibrium state. When the tank reaches the high level set point, it is drained as prescribed in the operating instructions, as required.
A heat recovery system is a general title used to describe any system or device used to recover some of the wasted energy of a particular process. For example, an economizer is a heat recovery device installed in the boiler exhaust to recover some of the lost heat energy in the exhaust and transfer it into the feedwater entering the boiler.
A blowdown heat recovery system utilizes the heat available in the blowdown to preheat the feedwater before entering the boiler. Typically, this is accomplished in the deaerator utilizing the flash steam created in the blowdown vessel. Another method of heat recovery would be to use a heat exchanger to transfer the heat to another process medium.
Here are some of your frequently asked questions about boiler blowdowns!
The boiler blowdown rate is the quantity of water removed from the boiler for the purpose of controlling water parameters by periodic blowdown. Typically this value is expressed as a percentage of the feedwater input to the boiler. 5% is the normally specified value for blowdown rate but highly depends on the boiler feedwater quality.
Blowdown rate can be calculated as:
qBD = qS fc / (bc - fc)
qBD = blowdown rate (lb/hr)
qS = steam consumption (lb/hr)
fc = Total Dissolved Solids (TDS) in the feed water (ppm)
bc = maximum allowable TDS in the boiler water (ppm)
The most common recommendation for the frequency of bottom blowdown of a steam boiler is at least once every 8-hour shift. However, this guideline is dependent upon the boiler operating conditions and water quality. A local chemical expert is the best resource for developing the required blowdown rate and frequency to maintain proper water quality.
Most blowdown lines are piped with two valves, each with a different design and purpose. The first valve (closest to the boiler) is a quick-acting valve followed by a slow-acting valve. The purpose of this configuration is that the quick-acting valve is opened first while the slow-acting valve is then opened slowly to allow the blowdown line time to acclimate to the high temperature and high pressure of the water being removed from the boiler. The slow-acting valve prevents the downstream piping and fittings from being thermally shocked by a large flow of hot blowdown water. Then, to ensure the blowdown is stopped with quick precision, the quick-acting valve is also closed first. The slow-acting valve can then be shut slowly since the flow of water out of the boiler has already been stopped.