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Boiler Blowdown Procedures

We have noticed in our web site statistics that many are searching for “boiler blowdown procedure”. We don’t think we have adequately addressed that on our site, and thought we would address it here in our Blog/Forum.
BOILER BLOWDOWN PROCEDURES
There are two types of boiler blowdown: Skimmer Blowdown and Bottom Blowdown (sometimes referred to as Manual Blowdown). They have different purposes.
SKIMMER BLOWDOWN
Skimmer blowdown gets its name from the fact that water is skimmed from the top 2 – 3 inches of the boiler operating level. It is in this area that the bulk of steam generation occurs and that the greatest concentration of dissolved minerals also occurs. Removing water from this area is one of the most efficient ways to maintain the cycles of concentration (boiler solids divided by feed water solids - OR -feed water volume divided by blowdown volume). When water is skimmed from this area continuously flow rate is constant with constant pressure. A continuous draw only maintains a constant cycles of concentration if steam generation is also a constant. It seldom is. Thus to maintain the maximum cycles of concentration to maximize efficiency while preventing scale and carry over it is often necessary to modulate skimmer blowdown flow with steam flow. Fortunately, when steam is formed it leaves behind dissolved minerals in the boiler water which increase in direct proportion to steam generation. And also fortunately, these minerals increase the conductivity of the water allowing us to use conductivity as a control mechanism to modulate skimmer blowdown flow in proportion to steam generation.
Skimmer blow down is generally to reduce dissolved solids and alkalinity in the boiler primarily for the purpose of preventing carryover of boiler water into the steam. In some special situations skimmer blow down can be used to reduce the silica concentration in the boiler. Skimmer blow down can either be continuous or modulated (intermittent).
If boiler water is allowed to enter the steam the result can be steam with a reduced BTU content per pound (reduced energy efficiency), steam traps that are over worked (higher maintenance costs), insulating deposits formed in the steam lines (reduced energy efficiency), water hammers (both higher maintenance costs and reduced energy efficiency), corrosion of brass valves due to excessively alkaline water in the steam and condensate lines (higher maintenance costs). If your steam system includes turbines they can be damaged by the water in the steam (increasing both your maintenance costs and reducing your electrical energy production).
Because dissolved solids, alkalinity, and silica increase in the boiler with the evaporation of pure distilled water, they can be monitored by an increase in the conductivity of the boiler water. Thus it is often desirable to monitor and control their accumulation with a Boiler Conductivity Controller. Such controllers offer a significant improvement over continuous blow down by being able to modulate the amount of boiler blow down according to changing boiler loads.
BOTTOM BLOW DOWN
The purpose of bottom blow down is to remove suspended material from the boiler water. This could be iron that has come back to the boiler from condensate corrosion or it could be calcium carbonate which entered the boiler during a softener upset. It could be any number of minerals that have entered the boiler and now exceed their limits of solubility due to cycling up. The exact mineral composition does not matter for the purpose of this discussion. If the material does not get removed it can settle on top of the boiler tubes and bake on. If this happens, heat transfer will be lost and energy consumption will rise as surely as if the material deposited on the tubes in the first place.
Since this is suspended material, it will settle during period of low fire and settle even faster during periods when the boiler is off. The circulation of the boiler during high fire and to a lesser degree during low fire keep it suspended. Thus, you should bottom blow the boiler only when the boiler is on low fire or turned off (keeping in mind that you still need to have the boiler under pressure). To bottom blow the boiler under high fire conditions gives you nothing more than what a skimmer blowdown gives you since the suspended material is dispersed and not concentrated near the bottom of the boiler. It is desirable to remove as much sludge as possible using as little boiler water as possible to do that. This conserves the energy that you paid for by keeping as much heated water in the boiler as possible.
In the bottom blowdown line there should be two blow down valves in series. One is fast acting and the other is slow acting. There are two schools of thought with regard to which one to open first. The water treater would say to open the slow one first and use the fast valve second to maximize the draw of sludges from the bottom of the boiler since the high velocity water would remove more sludge. The boiler manufacturer, in many cases, says the opposite pointing out the stress that will occur on the boiler if you open the fast one second. But then the scale in the boiler is not the boiler manufacturer’s concern, is it? You the boiler owner has to decide which way is best for you. No matter how you do it the bottom blowdown should end when you can no longer notice sludge being removed to drain. His of course implies that you should actually be able to observe the boiler water as it goes into the floor drain. If this is continuously piped all the way to the floor, you will have no way to assess when you have achieved maximum sludge removal.
Many smaller boilers cannot be equipped with skimmer blowdown. In these cases bottom blowdown must serve the purpose of both suspended solids removal and dissolved solids removal. Under these conditions a bottom blowdown could be done several times per day and should be monitored by using the desired conductivity of the boiler water as your determinant regarding whether enough blowdown is being done or not.
These are my thought on the subject. What are you comments or questions?
To read more articles that we have written about boilers, go to: http://www.richardhouriganinc.com/resource_center.html
To view a selection of Boiler Conductivity Controllers that we sell, go to: http://www.richardhouriganinc.com/sunshop/catalog/controllers-boiler-13-1.html

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