Control.com - Nerds in Control

What is the difference between an unload and a runback in a 7FA unit?

The general differentiation is that a runback is initiated by some condition/event and the unit is automatically unloaded, usually to some present value. Some units will go into a runback when one of the exhaust frame blowers is not running, and the unit will usually unload to something like 40% or 50% load and then maintain that load until the exhaust frame blower is repaired. (Not all units have this sequencing; it is just being used as an example.)

Unloading can be initiated by an operator or by some condition/event. An operator initiating a STOP will unload the unit. Some units will unload and continue to a fired shutdown on conditions such as high-high inlet filter differential pressure. (Not all units have this sequencing; it is just being used as an example.)

An operator can also unload a unit by clicking on LOWER SPD/LOAD or by inputting a Pre-Select Load Control setpoint that is lower than the current operating load.

Unloading the unit does not always mean shutdown (opening of the generator breaker and deceleration to cooldown). Runback generally does not result in opening of the generator breaker, and the strict definiton of runback does not include a fired shutdown.

Some units have inputs (either discrete or via an electronic communication link) from a DCS or some other control system to initiate runbacks or shutdowns (both of which unload the unit) for various conditions, such as low boiler drum level or gas compressor trouble, etc.

Having said all of the above, some engineers mistakenly used the terms "unload" and "runback" in signal name descriptions. (Isn't GE wonderful?) But the above descriptions are the intents of the terms. They are related, but should be different.

Hope this helps!