Our October 21
blog from last week dealt with reasons for blowing down boilers. Today's blog will get into the actual process. Keep in mind that 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.
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.
The following steps should be taken to blow down a boiler:
- Open the quick-opening valves (ones closed 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.