The engineering group is proposing to mix two streams of 63 bar steam. One stream is at 490 deg C while the other is at 420 deg C. The mixing is being proposed in a steam header directly at the outlet of the boilers. This is being done to get a composite steam temperature of around 450 deg C. What problems can be encountered if we mix these streams? What is the proper method of mixing two streams?

We mix steam temps all the time. We operate 3 boilers into 1 header then on into a turbine generator.
2 of our boilers operate at 1300 psi and @ 955*F with 420 kpph flow each. Then we have another larger boiler that operates at 1300 psi but the superheater outlet temps are 875*F with 400 to 600 kpph flow. These pipes come together about 10ft apart then another 40 ft to the turbine inlet. We operate at every possible combination of the 3, when all 3 are operating we average 912*f at the turbine inlet. When the big unit goes down we operate at 950*f at the turbine inlet. then when one of the other units go down we operate at 880* f. at the turbine inlet. This works well for us and we have no special mixing procedure or guide line. We have operated this way for 20 years. Thanks Doug

I agree with Doug's post. Mixing of steam of different temps is not an issue as long as they are good quality and dry. Also ensure your piping and associated devices is designed properly. Taking into account for flow, temps, pressure, etc.

we have a header is called Aux. Steam Header , this header is to provide steam for ( Gland Steam Sealing , Mazout Oil Atomizing , Steam Coil Air Heater , Mazout Oil Heaters )
what i want to say , that this header takes steam supply from two sources ( Cold reheat , Primary Superheater tubes from boiler ) , those two supplies are operating at different temperature and pressure , and for each supply has a control valve with internal water spray , to justify the temperature and pressure for the AUX. Steam Header

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