Damper control of Gas Turbine Exhaust to WHRU or HRSG is always a tricky thing. I would like to know what would be the best control scheme for controlling the HOT OIL temperature at the WHRU outlet by means of controlling the turbine exhaust gas flow. What kind of dampers are recommended for optimum control, single blade/louvers etc? Is it ok to have temperature control output on both the dampers (1 damper on Bypass & 1 damper to WHRU)

 

You don't say what the exhaust arrangement is. If you haven't designed it yet and you really need to control via dampers, I think separate WHRU and bypass stack dampers are a slightly better alternative than a single blade diverter damper. However, either way does a mediocre job at best. Remember, trying to control temperature via dampers is actually a flow control. The temperature of the exhaust gas from the gas turbine is not materially affected by damper position - you need to change the load (or maybe IGV angle) on the gas turbine to change the exhaust temperature. Also note that the exhaust gas from the gas turbine is rotating, and unless you put in straightening vanes, when you open the bypass damper you may get some really bad flow distribution in your WHRU. I do not have any experience with hot oil systems, but I have a lot of experience with HRSG's, and for those systems it is much better to attemperate the steam than to try to control it with exhaust gas dampers.

 

Dear Otised,

We have designed a system which has two dampers 1 each on bypass and whru. The bypass line is in straight line with the Turbine exhaust and the whru will be in the 0ffset line ( 90 deg).the basic aim to control the temperature of teh HOT oil at the outlet of the whru by controlling air flow via the bypass damper. We have given manual loader on the damper on whru damper and only temperature controller output to the bypass damper.By restricting the flow via the bypass damper we will be pushing more hot air to the whru and thereby controlling the hot oil temp.

My worry is,

1. how well the damper control will work if i use single blade damper or louvers (multiple blades) is preferable.

2. whether this scheme will produce the desired results,

3. Fall back is that we control both the dampers with the temperature controller output.

 

Dear Sanjeev Vijayan,

Where we have used separate bypass and HRSG dampers we have had better control with louvers than single blade. However, we generally used them only during startup -once on line, we were full open to the HRSG and full closed to the bypass stack. The louvers gave us a tighter closure on the bypass than single blade. We never tried to move both dampers simultaneously - we always had a protective interlock for the gas turbine that required one damper to be fully open.

More recently, on those few projects with bypass stacks, we have used a single diverter damper, but those are not at all suitable for modulating control.

I don't think I have anything more to offer you on this since I don't know your process. I would guess that your oil flow is controlled by pumps and valves and is relatively independent of the exhaust gas flow through the WHRU, unlike an HRSG.