Boiler blowdown is a common occurrence in every boiler room in America. This is done regularly to remove the sediment, solids and chemical concentrations in the boiler system. Ultimately this ensures high quality steam and prevents carryover and corrosion in the system. However, when you are blowing down your boiler the water released is under pressure and at a very high temperature. When this high pressure and high temperature water hits a low pressure area it will flash to steam. That is where a blowdown separator comes in. In this article we cover the basic information you should know about these pieces of equipment.
Basics Of Operation
If your boiler is over 15psi., then you likely have a blowdown separator. The blowdown separator provides a safe place for blowdown water to flash to steam. The water enters the separator at the top and runs over a spiral shaped striking plate. As the water runs over the striking plate it separates the steam and condensate. The vent located at the top of the separator allows gasses to escape to the atmosphere.
The remaining condensate makes its way to the bottom of the vessel where it enters the drain system.
Blowdown After Coolers
The heat of the water after running through the blowdown separator can sometimes be a problem. If drains are not rated for the high temperature you can end up causing all sorts of issues. Most state codes also require water to be less than 140 degrees Fahrenheit before entering the sewer system. Blowdown aftercoolers use a thermostat and temperature regulator valve to release cold feedwater to mix with the hot condensate. This reduces the condensate temperature before draining.
Blowdown separators must meet requirements of Section VIII of the ASME Code and stamped by the National Board of Pressure Vessel Inspectors. This is because the separator must be rated properly for the pressure of your boiler system.
This also means that an ASME stamped service provider is needed when repairs are necessary.
Depending on the system installation may vary. Penn Separators does a good job of explaining the basic installation process:
“The inlet should be piped from the boiler blowdown valves, using sch. 80 min. code rated materials. The piping pressure rating to the inlet should be rated to the design pressure of the boiler. A dead boiler drain should be provided off the inlet piping.
… Drainage from the unit should be as fast as the blowdown flow is separated. The floor drain should be steel, cast iron, or concrete of the same size or larger than the separator drain and aftercooler.
Drains should be sloped a minimum of 1’ per 100’. More slope or larger size should be used on obstructed or angled drains. The vent should also be as direct as possible to atmosphere to limit pressure drop… The vent should be extended above buildings, work areas, and personnel to a safe point of discharge. Flashing should be used when vent is through a composition roof.”