Steam Seal Pressure Variation

How old is the system and what is the typical Si content of the boiler make-up water?

system is 20 years old and Si content of the boiler make up water is less than 10ppm.

My initial instinct is to say your regulator is gummed up. My first course of action would be to disassemble it, clean each part, and check to make sure they're all free to move as designed.

The valve can have a gumming issue, or buildup.

You can also try to modify the control parameter once the valve has been checkd to make it act a bit like a PD controller.

Loss of vacuum an obvious answer due to too much air being sucked in at the LP end. What is or where is source of steam to header? Any impediment there?

Generally running source is from inlet end of LP or from inter-stage tap to the exhaust end with start-up steam supplied from system. Is this your arrangement? Normally additional supply not required while operating. The HP should provide it's own outflow and should need only a supply when starting up or at idle.

The high number says that you have plenty of supply (0.9). The low number would be my concern. Perhaps your LP labyrinths are worn and need replacement.

Normally sealing steam is required during start-up to maintain the vacuume and it is cut off after normal loading of turbines. Is it mendatory in your turbine to keep sealing steam on? High sealing steam pressure cause lot of steam comming out of of seals and can enter in the bearing housing causing condensate contamination problem in oil.

If you are drawing sealing steam from a high pressure steam header, control valve may not be able to control the out let pressure as flow remains very low. You can try putting some orifice at up stream of control valve so that limited flow comes to control valve.