Fuel Gas Firing – Minimum-Fire Bypasses
There are two basic types of minimum-fire bypasses: one based on fuel flow, the other based on fuel pressure. Each type has its place. Select the type that best meets the plant’s needs. They are frequently sized to provide 10% of the full load heat release in order to remain lit should the main fires go out. Experience has shown that with heat releases below 10%, the minimum fires can be blown out.
Minimum Flow Bypasses. These bypasses deliver a fixed flow of fuel to the burners when the main firing valve is closed. Typically, they consist of a manually set valve locked in place after it’s set on startup.
The flow rate is set experimentally on initial startup. All burners should be in operation at design draft for this to be set properly. However, note that with several burners off (such as for startup) the minimum flow could deliver too much fuel to the remaining burners.
Alternatively, a minimum flow bypass can be designed with block valves plus a union-blind restriction orifice. To check the bypass, block in the orifice for inspection and cleaning, then replace it, and unblock the blind.
The flow tends to be constant regardless of how many burners are operating. Therefore, as the number of burners in service is reduced, the remaining flames get bigger. The operating procedures should define the minimum number of burners that can be safely fired with the manual flow bypass sized to fire all burners at minimum fires. A minimum pressure bypass may be a better choice for some furnaces, especially those with a large number of burners, and/or large turndowns.
Minimum Pressure Bypasses. These bypasses prevent the burner header pressure from dropping below a preset minimum. Typically such bypasses are self contained, pilot-operated pressure control valves. The pilot capacity should be sufficient to open the bypass fast enough to maintain the minimum pressure to the burner even when the main fuel control valve is closed as fast as possible. The adequacy of the pilot’s capacity should be verified by field test before the system is released for operation.
Such bypasses automatically adjust their flow to suit operating conditions. They are tightly closed during normal operation of the firebox. When the main firing valve closes down, the burner header pressure drops. When it drops to the preset minimum pressure, the bypass opens just enough to maintain the header pressure, thus maintaining minimum fires regardless of how many burners are in service. Minimum pressure bypasses have the following advantages over minimum flow bypasses:
• They can better adjust for heater turndown, during normal operation, or for startup conditions where only a few burners may be lit.
• They can be tested more positively, and with no impact on the heater, if properly installed. (See the following information on “Testing.”)
• They lessen the potential problem with minimum flow devices that the metered flow rate will change due to partial plugging, or that the flow rate may be inadvertently set incorrectly.