GE D11, LP Inlet Pressure Control SP?

So, I operate a 2x1 plant, self learned, migrated from coal. We have two GE7FA.05 CT's on NE HRSG's, to a GE D11 steamer. The HRSG's are three drum, LP, IP (with reheat), & HP. GE Mark6, and ovation for the BOP. The D11 has a low pressure steam inlet / reg valve that receives additional low pressure steam from the back end of the HRSG's. The SP on the Mark6 HMI is adjustable from 42~72 psi. The pressure reference is taken from the HRSG, or upstream side of the valve. The SP can be changed at any load, willy-nilly. (The LP steam is not used for any other process). We have no reference in the manuals that speak to its function, why it's there, when to use it, adjust it, but only that it exists where it is. Nothing more. Changing the SP does nothing for load. I can only guess what it's for but I don't want to bias any response. I've been asking the question for three years with no response. (Yes, I've asked ALL of THEM).

Even if someone has some "play time" experience they would like to share, I'm all ears!

Thanks
 
Kurths,

This is an older thread but I figured the answer may help anyone who finds it in the future. Our facility very closely match your set-up, except we're 4x2 with Aalborg HRSGs.

We place the Admission steam flow setpoint at 100% and the pressure setpoint at 55psi in order to control the LP drum pressure. The reason we control the LP drum pressure to keep the drum operating under its moisture carryover limit. You should be able to find a moisture carry-over curve for each of your drums which depicts the maximum steam flow for a given drum pressure which keeps the drum operating its designed carry-over limit (assuming all your drum internals are in good shape).
 
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