As a general rule of thumb, blowdown helps maintain proper chemistry and remove suspended solids and impurities in a boiler system, but how often should blowdowns occur and by what method?
The primary purpose of boiler blowdowns is to ensure that water chemistry is being maintained within the set parameters issued either by ASME guidelines and/or the specific water treatment program. Blowing-down a boiler refers to the process of manually or automatically removing an amount of water within the pressure vessel, which should then be proportionally replaced with make-up water. The process not only aids in removing precipitated impurities but also maintains steam purity and system efficiency. With traditional steam boilers, there are typically three methods of blowdown: low-water cutoff, surface skimmer, and bottom blowdowns. Not all methods may be readily available but often a combination of blowdown types will provide the most effective results.
Low-Water Cutoff Blowdowns
The low-water cutoff method of manually blowing-down a boiler is the least effective given its position and volume of water removal. However, low-water cutoffs are important as they help avoid boiler explosions or catastrophic failure events. Traditional steam boilers will most certainly include a low-water cutoff mechanism to effectively maintain boiler operation. This is typically accomplished with a float style valve although there are electronic control options. In terms of blowdowns, the primary objective of the low-water cutoff method is to verify that the float valve is in working condition free of debris or sediment. With smaller steam boilers, this may be the only option to collect a water sample and remove water. During wet boiler layup this can also be the best location to collect a representative boiler sample as well.
Figure 1) Low Water Cutoff Blowdown Valve Schematic
Surface Skimming Blowdowns
Historically, the surface blowdown valve was cracked open to continuously bleed off a measured volume of water from the upper water column. Skimming is effective at removing suspended or buoyant oil that may otherwise cause foaming or carry-over. Most boilers are now utilizing automation for intermittent surface blowdowns. The frequency of blowdowns will depend upon the size of the boiler and its water makeup quality. Automating the blowdown process typically requires a solenoid valve tied in with a boiler controller and conductivity probe. The goal of periodic sampling is to collect a representative sample from the boiler. While bottom blowdowns are still required, this alleviates the human-error of manually adjusting the rate of blowdown. Occasionally when boilers are large enough, both methods (intermittent & continuous) are required to maintain the recommended water chemistry. When the surface blowdown is continuous, a needle valve or globe valve is best for precision. And although rarely an issue, the pipe diameter needs to accommodate the volume of blowdown necessary.
Figure 2) Surface Blowdown automation schematic
Suspended solids within the boiler ideally with react with active scale inhibitors to drop from suspension, leading to the accumulation of sludge near the bottom portion of a boiler. If not adequately removed, material can begin to inhibit heat transfer efficiency and contribute to possible failures. Although bottom blowdowns are required, they aren’t necessarily efficient and so the less water needed, the better. Traditional bottom blowdowns will include two separate valves: a gate valve and a knife valve. The ASTM standard when performing bottom blowdowns is to fully open the knife valve and then open the gate valve for a regulated period with the intention of reaching a healthy velocity. As most boilers control TDS based on surface blowdowns, the goal is not to reduce conductivity, but rather the impurity content. Frequent, short blowdown periods are often better then long valve openings. Unfortunately, these variables are not universal for each pressure vessel. Blowdown frequency and length can be determined based on several factors such as percent condensate return, makeup water quality, boiler firing rate, etc. and should be prescribed from a professional water treater. For instance, bottom blowdowns would need to be increased during a de-scaling operation. It is a possibility that the bottom blowdown process be automated although less typical compared with surface blowdowns.
Figure 3) Blowdown Flash Tank Schematic
Blowdown water is quite valuable due to its heat and pressure. Most systems will require a blowoff system to provide a safe means of controlling boiler blowoff water. This typically requires a reduction in both pressure and temperature to limits acceptable for safe discharge into a sewer or drain system (depicted in Figure 2).
A simple blowdown calculation is as follows. Further calculations can be computed to compute the amount of energy lost due to blowdown.
Blowdown = (Feedwater) – (Steam)
where Steam = (Condensate Return) + (Steam Loss)
and Feedwater = (Makeup water) + (Condensate Return)
There is a fine line with blowdowns as although they are necessary, can be costly with the loss of energy. Finding these setpoints and blowdown periods can be better obtained from several calculations. As mentioned earlier that there are several blowdown methods, and typically a combination of them will allow for the most efficient boiler system. Long story short, the amount, length, and frequency of blowdowns will depend.
List of Resources:
 “Boiler Blowdown Tanks.” Madden Engineered Products, www.maddenep.com/boiler-blowdown-equipment/boiler-blowdown-tanks. Accessed 30 Jan. 2022.
 “Water Handbook - Boiler Blowdown Control | SUEZ.” SuezWaterTechnologies, www.suezwatertechnologies.com/handbook/chapter-13-boiler-blowdown-control. Accessed 29 Jan. 2022.
 Editorial Staff. “What Is Boiler Blowdown ?” Inst Tools, 6 July 2019, instrumentationtools.com/what-is-boiler-blowdown.
 Welsh, Matt. “Proper Boiler Blowdown Procedure.” Chardon Labs, 27 Sept. 2021, www.chardonlabs.com/resources/proper-boiler-blowdown-procedure.
 Boilers, Thermodyne. “Blowdown in Boiler | Boiler Blowdown Calculation Formula and Calculator.” Thermodyne Boilers, 10 May 2021, www.thermodyneboilers.com/boiler-blowdown-calculation.