Blowdown of steam boilers is very often a highly neglected or abused aspect of routine boiler room maintenance. The purpose of boiler blowdown is to control solids in the boiler water. Blowdown protects boiler surfaces from severe scaling or corrosion problems that can result otherwise.
There are two types of boiler blowdowns: ‘‘continuous’’ and ‘‘manual’’.
A continuous blowdown uses a calibrated valve and a blowdown tap near the boiler water surface. As the name implies, it continuously takes water from the top of the boiler at a predetermined rate. A continuous blowdown is an optional feature and may not be included on your steam boiler; however, all steam boilers should include a means for manual blowdown as standard equipment.
Manual blowdowns are accomplished through tapings at the bottom of the boiler. These openings allow for the removal of solids that settle at the bottom of the boiler. Manual blowdown is also used to keep water level control devices and cutoffs clean of any solids that would interfere with their operation. All steam boilers require manual blowdown whether or not they are supplied with continuous blowdowns.
Frequency of Manual Blowdown
When continuous blowdown is used, manual blowdown is primarily used to remove suspended solids or sludge. The continuous blowdown removes sediment and oil from the surface of the water along with a prescribed amount of dissolved solids.
When surface or continuous blowdown is not used, manual blowdown is used to control the dissolved or suspended solids in addition to the sludge.
In practice, the valves of the bottom blowdown are opened periodically in accordance with an operating schedule and/or chemical control tests. From the standpoint of control, economy, and results, frequent short blows are preferred to infrequent lengthy blows. The length and frequency of the blowdown is particularly important when the suspended solids content of the water is high. With the use of frequent short blows, a more uniform concentration of the pressure vessel water is maintained.
In cases where the feedwater is exceptionally pure, or where there is a high percentage of return condensate, blowdown may be employed less frequently since less sludge accumulates in the pressure vessel. When dissolved and/or suspended solids approach or exceed predetermined limits, manual blowdown to lower the concentrations is required.
It is generally recommended that a steam boiler be blown down at least once in every eight-hour period, but frequency may vary depending upon water and operating conditions. The blowdown amounts and schedule should be recommended by your local Cleaver-Brooks authorized representative.
A hot water boiler does not normally include openings for surface blowdown and bottom blowdown since blowdowns are seldom practiced. Always be alert to system water losses and corresponding amount of raw water makeup. A water meter is recommended for water makeup lines.
Proper blowdown is performed as follows:
- Blowdown should be done with the boiler under a light load.
- Open the blowdown valve nearest the boiler first. This should be a quick-opening valve.
- Crack open the downstream valve until the line is warm. Then open the valve at a steady rate to drop the water level in the sight glass 1/2 inch. Then close it quickly being sure that the handwheel is backed off slightly from full close to relieve strain on the valve packing.
- Close the valve nearest the boiler.
Repeat the above steps if the boiler has a second blowdown tapping. Water columns should be blown down at least once a shift to keep the bowls clean. Care should be taken to prevent low water shutdown if this will affect process load.
Please keep in mind that all blowdown piping should be checked once a year for obstructions.
Blow Down Purpose
The purpose of blow down is to control the amount of solids and sludge in the boiler water. The blow down process involves partially draining the boiler to remove sludge and to maintain pre-determined concentration levels of solids.
As the water is turned into steam, the solids remain behind. Unless there is 100% condensate return, the solid content tends to build up when the boiler takes on make-up water. On hot water systems, there is generally no make-up water. Therefore, the solid concentration remains constant and no blow down is needed.
The amount and frequency of blow down differs for each boiler application and should be determined by your water management consultant. Blow down is affected by the type of boiler, operating pressure, water treatment, and the amount and quality of make-up water.
Blow down piping should be at least the same size as the blow down tapping on the boiler. Blow down valves should be sized in accordance with the ASME code and piped to a safe point of discharge. There should be either two slow opening valves or one quick opening valve and one slow opening valve piped in series. A slow opening valve is defined as needing five complete 360 degree turns to go from fully closed to fully open. A quick opening valve goes from fully closed to fully open in one complete motion. In the case of one quick and one slow opening valve, the quick opening valve should be located closest to the boiler. If possible, the blow down valves should be piped on the same side of the boiler as the water column gauge glass.
To blow down the boiler:
- Open the quick opening valve (valve closest to the boiler) first.
- Open the slow opening valve last.
- Blow down the boiler for the required amount of time, per your water management consultant, by opening and then closing the slow opening valve.
‘’‘Remember’’’: Pay close attention to the water level in the gauge glass. Certain loads may require several blow down cycles of short duration to maintain proper water level in the boiler.
- Close the slow opening valve first.
- Close the quick opening valve (the valve closest to the boiler) last.
- Open the slow opening valve again to drain the line between the quick and slow opening valve.
- Close the slow opening valve again and double-check for tight shutoff after the valve has cooled off.
‘’‘NEVER’’’ pump the quick opening valve to blow down the boiler! This may cause water hammer, which could damage piping and valves and may cause personal injury. Also, NEVER leave an open blow down valve unattended!
‘’'Remember: The quick opening valve (the valve closest to the boiler) is opened first and closed last, which ensures its protection from the wear associated with blow down. This will make this valve more reliable so maintenance and repair can be performed on the slow opening valve furthest from the boiler, without draining the boiler.