What is the most reliable way for control of Waste Heat Recovery units on Gas Turbine Outlet. Single blade damper with a spring return actuator is what is generally acceptable. But when you want temperature control with these actuators & single blade assembly the lag in the system will be high, so to go for multiple blades/vanes will be more effective? What else can be an option with due consideration of safe shutdown.

 

Trying to control gas turbine exhaust heat input to a waste heat recovery boiler with dampers is pretty much an inherently unreliable method to begin with.

You're talking about moving "vanes" which are large "sails" in a very hot environment against high air flows which cannot be reliably lubricated and which require great forces to be able to control. I've seen motor-operated cable mechanisms, motor-operated gear mechanisms (rack and pinion style and worm gear style), and none of them work very well for very long, especially in cyclic loading or load-following duty where the units are started and stopped regularly and/or frequently.

Best to have a properly designed boiler which can accommodate the shrink and swell experienced during starting and shutdown. Those kinds of boilers are physically larger and more expensive, but damper and damper actuator/operator repairs are much more expensive over the life of the unit, and much more frustrating when they don't work.

And they don't work well for very long, whether they're new or have just had maintenance or repairs. They're just inherently difficult to keep working properly, what with cyclic temperature stresses, the lack of lubrication, and the usual lack of maintenance.

I've seen cables snap, cable drums spin free when the shear pins shear, worm gears stripped, and racks with broken teeth. Maybe not in the first few months, but in the first couple of years or less, especially on units which are started and stopped frequently and regularly.

That's not to say there aren't some good mechanisms out there, there are just damned few of them and they require constant maintenance and attention to keep them in working order and keep them from failing at inopportune times (actually, any time they fail is bad, but it just seems they fail most often when it's critical to get the unit online quickly!).

Hopefully others will report better luck with their damper systems, but this is a very unscientific sampling/poll.

Spend the money for a quality boiler and you'll never look back and wish you had. Guaranteed.

Now if you need to operate occasionally in simple cycle mode and just need a bypass stack isolator, simple "gate" or "sluice" type mechanisms work best, rather than some kind of swinging damper. But, they, too, require maintenance and can't be ignored until they fail if they are used frequently.

So, if you're going to be using any kind of damper or gate, maintenance is key. And don't scrimp on the actuator/operator. You'll be sorry if you do, or if the supplier does.