Why do I need to blow down my boiler?
We try to prevent hardness and other solids from entering a boiler through proper makeup water and feedwater pretreatment, however, it is inevitable some of these will manage to enter the boiler. Blowing down is a process which involves draining a small amount higher hardness and solids water from the boiler and replacing it with lower hardness and solids makeup water to maintain a predetermined concentration of hardness and solids. It is performed so that boiler performance is maximized and maintenance and repair costs on the boiler are kept to a minimum. It is expensive to maintain low hardness and dissolved solids in the boiler because each gallon of blowdown water has already been heated and treated. The amount and frequency of blowdown differs for each boiler application. Various factors affect blowdown including type of boiler, operating pressure, water treatment, amount and quality of makeup water. It is important to find the right balance and frequency for blowing down your system.
What happens if hardness and dissolved solids build up in the boiler?
An excessive amount of solids and sludge can insulate the heating surfaces on the waterside of the boiler. The metal overheats and, as a result, the pressure vessel fails. High hardness and dissolved solids concentrations can also lead to carryover of the boiler water into the steam which can cause water hammer and damage piping and equipment. Low water conditions can also occur.
When boiler water turns to steam, solids are left behind. Unless the condensate return is 100%, the hardness and dissolved solids will cycle up when the boiler takes on makeup water. On hot water systems, no makeup water is generally used. The solids concentration remains the same and no blowdown is needed.
How do I perform a blowdown?
There are actually two different types of boiler blowdown. The first is a bottom blowdown and the second is a surface blowdown. The purpose of the bottom blow down is to remove undissolved solids that can build up and settle to the bottom of the boiler. Surface blowdown is intended to remove the high hardness and dissolved solids from the boiler.
BOTTOM BLOWDOWN is used to remove suspended solids such as those formed by:
- calcium and magnesium salts
SURFACE BLOWDOWN or surface 'skimming' is used to remove dissolved solids that concentrate up in the boiler water as steam is generated such as:
- chloride and soluble calcium
- phosphate and silica
High levels of these dissolved solids can result in boiler water carryover into the steam system which may cause severe damage to equipment that uses steam such as steam turbines, heat exchangers and steam traps. Surface blowdown is the method used to remove these dissolved solids.
How do I Perform a Bottom Blowdown?
Bottom blowdown, the removal of boiler water and sludge from the lower portions of a boiler, is a common practice used to prevent deposition on heat transfer surfaces.
Superior feedwater quality can reduce the frequency of bottom blowdown to as little as once per week while fair to poor feedwater quality may necessitate daily or multiple bottom blows per day.
Piping and Valves
Blowdown piping should be at least the same size as the blowdown tapping on the boiler. Valves should be sized according to ASME code and piped to a safe point of discharge. The system should be equipped with either two slow-opening valves or one quick-opening valve and one slow-opening valve piped in series. If the boiler has more than one bottom blowdown connection, a second quick-opening valve is needed. However, both quick-opening valves can be piped to a common header and slow-opening valve. (A slow-opening valve is defined as one that needs 5 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.) If both types of devices are installed, the quick-opening valve(s) should be placed closest to the boiler. Whenever possible, blowdown valves should be piped on the same side of the boiler as the water column gauge glass.
Steps to bottom blowdown a boiler:
- Open the quick-opening valves (ones closest to the boiler).
- Open the slow-opening valve.
- Blow down the boiler for the amount of time specified by your water management consultant by opening and then closing the slow-opening valve. Pay close attention to the water level in the gauge glass. Some loads require several, short blowdown cycles to maintain the proper water level in the boiler.
- Close the slow-opening valve.
- Close the quick-opening valve(s).
- Open the slow-opening valve again to drain the line between the quick and slow-opening valves.
- Close the slow-opening valve again and double check that the shutoff is tight AFTER the valve has cooled off.
NEVER PUMP A QUICK-OPENING VALVE TO BLOW DOWN THE BOILER. Such action could cause water hammer and damage piping and valves. It could also cause personal injury. NEVER leave an open blowdown valve unattended. Remember that the quick-opening valve(s) (those closest to the boiler) must be opened first and closed last. This procedure protects the valves from the wear associated with blowdown and keeps them more reliable. In addition, maintenance and repair can be performed on the slow-opening valve farthest from the boiler without having to drain the boiler.
How Does a Surface Blowdown System Work?
Surface blowdown systems typically consist of a pipe entering the upper section of the boiler approximately 6 inches below the boiler water surface. This may be in a steam drum in a water tube boiler or the upper steaming portion of a firetube boiler. This pipe needs to remain below the water level at all times when the boiler is operating. The flow of boiler water from the "skimmer" pipe is controlled using a special pressure rated "needle" valve or flow throttling valve designed for this purpose. Some of these "skimmer" valves have built in micrometers so that the valve setting can be monitored and adjusted precisely. Operators simply adjust this valve to maintain the recommended boiler water dissolved solids level. This may be an adequate method of controlling dissolved solids in a boiler that is operated at a steady load. However, because most boilers operate with a fluctuating steam load, controlling dissolved solids with a needle valve becomes impractical because of the frequent adjustments that are needed. In these situations, automatic blowdown controllers are recommended.
Should I change my blowdown schedule or frequency?
If you have questions regarding boiler blow down for your system or want to learn more about our boiler treatment services, visit our website or call 414-425-3339. It is important to work with your water treater make sure you maximize cycles of concentration on your system to help ensure efficiency while protecting your equipment.