The electric solenoid valve can be used to correct deficiencies in the combustion process
by George Lanthier
There are two critical areas for any airplane pilot in flying from one point to the next — the takeoff and the landing. Most accidents still occur during these two procedures. Put another way, this is like the startup and shutdown of the flight. After takeoff and before landing flying can be quite boring if no equipment failures occur, and thankfully most don't, which is a good thing — a very good thing.
There are also two critical points to correct oilburner operation - startup and shutdown. Getting an oilburner running correctly during operation is easy compared to lighting them off and shutting them down. Some things happen during these two phases that don't happen during operation, such as a smoke spike. The production of a smoke spike will not only occur at startup, but also at shutdown and this may be normal for the age and design of the burner in use.
However, these poor startups and shutdowns can be improved through the use of a good understanding of combustion and burner operation. In addition, the use of oil valves, pre-heaters, purging controls, fuel additives and other options will make any burner perform better and reduce these smoke spikes as shown in Figure 1 from Beckett.
Although the pre-heater can be a great help in startup, its effect adds virtually nothing to the shutdown sequence. Purging controls can have an enormous impact of startup and shutdown, but they require something to allow them to do their job, 'the magic valve.' This is a solenoid valve manufactured by several companies.
The electric solenoid valve, Figure 2, can be used to not only correct problems that develop on the pump, but also to correct deficiencies in other parts of the combustion process. It has long been known that a solenoid valve can be used as a permanent repair for the after-drip that is caused by a faulty fuel cutoff valve within the pump, but it can also be used to enhance operation. If any of the following are noted, a solenoid valve should be considered:
These symptoms may indicate problems with air in the pump, oil system, nozzle lines or adapter. They may also indicate problems with the draft conditions that are present. The benefits of paused and proper ignition of oil have been recognized for a long time. This is the purpose of a pre-purge phase. Pre-purging allows the draft flow to be established in the unit and the purging of the heat exchanger of the last combustion sequences' gasses. This will help ensure controlled and smooth ignition of the oil.
- puffbacks or noisy starts
- rumbling and pulsating starts
- rumbling and noisy shutdowns
Many burner manufacturers not only recommend a pre-purge cycle, but the use of a post-purge is highly recommended whenever there is a large amount of latent heat left in the combustion area on shutdown. In fact, one burner manufacturer sells all of their products with a pre-purge, and most control manufacturers now have a primary control that puts both pre-purge and post-purge features on a residential burner at a very reasonable cost.
The benefits of a post-purge is to clear the unit of combustion gasses and to reduce problems with thermal residue and expansion in the burner. These problems with thermal expansion and residue can lead to after-drip from the nozzle, carboning of the head and electrodes and damage to cad-cell detectors and other parts such as ignition transformers and burner mounted controls and components.
The use of an oil valve with either a purge control or solid-state timer to ensure the proper delay every time is highly recommended. With the use of an oil valve, a short delay will allow the burner to start more smoothly since the delay allows the air or draft in the unit to establish itself and find proper direction and flow before ignition of the fuel. By adding a pre-purge cycle you separate a good installation from a great one.
The use of a pre-purge cycle is always used on commercial and forced-draft installations. Think about that and ask yourself why. The purge doesn't have to be long, even a few seconds is better than nothing. Did you know that with the burner design that came before the retention head, the Shell Head, no burner was ever installed without a delayed oil valve? Also, have you ever seen a commercial burner or industrial burner without one? The reason commercial, industrial and better forced-draft burners always use a pre-purged oil valve is simple. It allows the burner to establish draft flow, volume and direction. In English, it just works better.
As for Shell Heads, here’s the official word from the Shell Oil Co., who developed it: “The purpose therefore is to allow the fan to come up to operating speed and to give direction to the draft before oil is introduced for burning.” Interesting, huh? So this isn’t really anything new, it’s just going back to what those old-timers, I call the ‘Legend Legion,’ always knew.
Today many new pumps either include a bypass-type valve or blocking oil valve into their design. A blocking- type valve pump is one in which the valve is normally closed and the oil valve coil is not energized on start. When the valve coil is energized, the valve opens. This concept is the one closest to a separate oil valve in operation.
With the bypass-type valve pump, Figure 3, the valve mechanism is normally open and the oil valve coil is also not energized on start. The bypass-type pump relies on the internal porting of the pump to bypass the bypass valve seat and circulate the oil within, causing a lower pressure within the pump. This reduced pressure is not enough to open the fuel cutoff and also allow the oil to go to the nozzle. When the valve is energized as shown in Figure 4, the bypass closes and an increased pressure occurs. With the bypass closed, oil pressure pushes the fuel cutoff off of the valve seat and allows oil to travel to the nozzle port.
In practically every case I've worked on or heard about, a noticeable improvement in burner operation will occur due to the addition of a delayed oil valve, so give one a try on that next job with the 'rough start' or 'hard shutdown.'
Finally, when all of your efforts to improve an oilburner have failed, the burner and possibly the appliance it burns into may have to be replaced. Since we may have a smoke spike on startup and shutdown again, it only makes sense that a higher than normal carbon monoxide (CO) level may appear. Again, verifying the correct nozzle, pump pressure and ensuring the correct air handling parts are essential to good burner operation. Don't go chasing fictitious ailments — stick with proven technology and techniques because, quite simply, they work, and you'll find that whatever brand of burner you use they will all work better with 'the magic valve'.