Q: What are the advantages to using a slow opening valve? I notice that it is an option for the VAS solenoid valves for gas. Are slow opening valves used for air, also?
A: Slow opening solenoid valves (or “dampened” solenoid valves) are available for both gas and air control. VAS solenoid valves are manufactured for use with either gas or air, and slow opening versions are available from the smallest, Size 1 (3/8” – 1” thread connection), all the way up to Size 8 (4” ANSI connection). In addition, the MB..7 solenoid actuator for BVHM (butterfly valve for air and flue gas) is available in two dampened versions: slow opening/fast closing and slow opening/slow closing.
The main reason to use a dampened valve is to allow for a graceful transition from either the off or low-fire state to the high-fire state of a burner, during pulse-fire operation. By slowing this transition, the gas and air required for combustion are much less likely to become imbalanced in relation to each other. By keeping the air and gas at the proper ratio through the transition, problems such as excess carbon, loss of efficiency, and “hard” lighting (think: BANG) are avoided.
Many modern systems incorporate a ratio regulator (GIK) in the piping for each burner (a practice we strongly recommend), and since the ratio regulator controls the gas flow based on the air pressure, the use of dampened valves is most prevalent on the air control side. With this arrangement, a slow opening valve on the gas side should not be necessary.
When looking at air control solenoid valves and solenoid-actuated butterfly valves, there are several options to choose from. VAS solenoid valves may be used for applications where slow opening/fast closing or fast opening/fast closing operation is acceptable. When sizing VAS valves, be sure to follow the charts in the Technical Information, published by the manufacturer and available on our VAS product page, so that you select the correct valve body based on factors such as flow and pressure drop. MB..7 solenoid actuators, paired with BVHM butterfly valves, offer slow opening/fast closing, slow opening/slow closing, and fast opening/fast closing operation. Fast opening valves are used only in very rare cases for air control.
For pulse-fire systems, it is best to use a slow opening air valve. However, the choice of closing speed is dependent upon the type of pulse control you are using, the type of heating you are doing (direct vs. indirect), and the desired behavior of the burner when it transitions from high-fire to either low-fire or off.
Knowing when to use a fast closing valve and when to use a slow closing valve is largely a matter of on/off vs. high/low. A fast closing valve may be used on systems that are pulsing on/off, but make certain that your process can tolerate a very short (about 1 second) excess fuel condition, which appears as a small ball of yellow/orange flame as the burner shuts off.
On systems that are pulsing high/low, a slow closing valve is a must, and for two very important reasons. First, the air/gas ratio should remain constant as the burner transitions from high- to low-fire. Second, gas regulation remains consistent through the transition, which avoids any “bouncing” of the regulator, a condition that could cause the burner to go out.
Special precautions should be observed for applications using indirect heating, such as when the burners on your system fire into radiant tubes. We recommend purging the tube before turning the burner on and, in the event that you are using a slow opening/slow closing valve, you MUST ensure that the air valve is at a low (light-off) position before attempting ignition. This is a good practice on all systems, as failure to set your air valve in the light-off position in preparation for ignition can result in “hard” lighting (as mentioned above). And while this is always exciting, it is never desired. Elster Kromschröder’s BCU series of burner controls can be set to control air valve light-off positions automatically, as part of the ignition sequence.