What does a blow down separator do?

September 16, 2014 by Aaron Rhoade

One of the questions we get regularly concerns blowdown separators. What exactly is the function of a blowdown separator? We have listed some information that may be helpful in understanding blowdown separators below.

  • The primary function of a blowdown separator is to allow the boiler operator to safely and legally handle the bottom blowdown water that comes from the boiler.  There are two types of boiler blowdown.  The bottom blowdown is done by opening a set of two valves that drains water out of the bottom of the boiler.  The purpose of the bottom blowdown activity is to clean out solids that accumulate at the bottom of the boiler.  Keeping the inside of the boiler clean improves the efficiency of the boiler.
  • The other type of blowdown is called continuous or skimmer blowdown.  This blowdown also takes hot water from the boiler, but the source of the water is at the top of the boiler.  The purpose of the continuous blowdown is to help control the water quality in the boiler.  Keeping the boiler water within the specification limits improves efficiency and steam quality, and reduces boiler maintenance.  Click here to see the Madden continuous blowdown heat recovery systems that are available for handling the continuous boiler blowdown.
  • The flow rate for the bottom blow down depends on the boiler operating pressure and the size of the valves and piping for the blowdown.  The bottom blowdown is normally done to lower the water level in the boiler 4”, and the time period is very short, less than a minute in some cases.  The flow rate for a 150 psi operating pressure boiler with 1-1/2” valves and piping is around 30,000 lbs of water per hour, so the blowdown system must be engineered to handle a high, instantaneous flow rate of very hot water.
  • Once the blowdown valves are open, and the hot boiler water starts to flow, then it needs to be contained and cooled.  The standard temperature limitation for drainage to a public sewer system for water is a maximum of 140 deg. F.  For example, blowdown water coming from a 150 psi operating pressure boiler is 366 deg. F.  The blowdown separator, with the use of the attached aftercooler, is designed to reduce the temperature of the water to at least 140 deg. F. before it is dumped into the sewer.
  • So, how does the blowdown separator manage that temperature reduction?  The first step is the creation of flash steam in the centrifugal separator.  When the high-pressure hot water comes into the blowdown separator pressure vessel it hits a striking plate and droplets form.  A portion of the water changes to flash steam due to the pressure drop.  10-20% of the water, depending on the initial boiler pressure, can be converted to flash steam which is vented to the atmosphere.  The water that remains after the steam is created will be at 212 deg. F, and it still needs to be cooled to a minimum of 140 deg. F. by the aftercooler before it can be dumped into the drain.  The main issues with creating the flash steam are having enough volume in the pressure vessel to handle the steam without building up pressure and making sure the vent piping is large enough to handle the high flow rate of flash steam.
  • The second phase of the cooling process is accomplished in the aftercooler.  When the hot 212 deg. F. water exits the blowdown separator pressure vessel it enters the aftercooler.  The aftercooler tempers the hot water with the addition of cooling water just like a home faucet does.  An automatic temperature regulating valve opens as soon as the temperature bulb is impacted by the hot water.  Cooling water is mixed with the hot water at a rate to get the temperature of the combined flow to at least 140 deg. F. as it goes to the drain.
  • The relevant information required to select a blowdown separator model is the boiler operating pressure and the size of the blowdown valves and piping.

Take a look at the Madden blowdown separator datasheet and you’ll see that we offer models Click here to handle any boiler operating up to 300 psi.  For boilers operating over 300 psi, we recommend a blowdown tank.