steam carry over in hrsg

[Coalhouse]

hope you are all well,
thanks for your answer

one of the specification I am working on states that the minimum carry over from steam will be
0.04% for HP
0.03% for LP
0.02% for IP.

my questions are below:
where does this figure comes from (empirical or first principle)
What drive this figure?
is there a standard which covers this, even indirectly

on do you measure that carry over on site.
Many thanks
Regards,
GTE

 

[marty007]

I am not familiar with those exact numbers, but in general carryover can have a big impact on the operation of some HRSG systems. The big key is understanding how the steam will be used.

If the steam is going to be superheated, then carryover is very important. When the steam reaches the superheater, any remaining water droplets/mist will be completely vaporized, and any dissolved solids will come out of solution and cake onto the superheater tubes, which will reduce its performance. To reduce the need for cleaning of the superheater, a maximum allowable limit for carryover is typically specified.

If the steam is not going to be superheater, but will instead be used in condensing service to heat other process streams, carryover is less important. Any remaining droplets/mist would either be caught in a low-point steam-trap in the piping, or enter a condensing heat exchanger and be discharged with the liquid.

Hope that helps,
Marty

 

[racookpe1978]

Where are these points being measured? (What is the carry-over standard being applied?)

The only HP, IP, LP terms I have ever seen are the turbine sections (high pressure, intermediate pressure turbine, low pressure turbine; never as sections of a boiler (heat recovery or straight-fired boiler). Almost by definition, a boiler is at the same pressure all the way through - minus a tiny bit of tube wall friction loss from boiler feed pump to boiler steam stop.

Carry-over from stage to stage inside a turbine is critical because of blade erosion and cutting damage on the blades.

Carry-over as a function of boiler outlet is defined to prevent impingement of solids (or saturated water particles) from the boiler downstream into the throttle inlet and cutout valves (to prevent seat erosion) and to prevent subsequent stage blade erosion - particularly in the last LP blades after saturation above the condenser.

Particles also create/exaggerate foaming in the condenser as one final hazard. But that too is not a boiler "stage" issue, but a LP turbine last stage outlet measurement.

 

[davefitz]

The steam turbine vendor will provide max permitted levels of silica, sodium, copper etc that are permitted in the final steam while retaining the steam turbine warranty. If you exceed those levels the warranty is kaput.

These constituents find there way into the steam from carryover in the steam drum separators, chevron driers and demisters. The concentration of these elements in the drum /boiler water is about 100 times higher than that in the feedwater , since there is only about a 1% boiler water blowdown rate. There is also a contribution due to spray water- that water has the same concentration of contaminants as the feedwater ( since that is where it is sourced) .

Silica carryover is difficult to prevent as it is volatile therefore it must be removed upstream of the dearator. Rea B+W's book "Steam " chapter 24 to understand water chemistry and carryover.

The bottom line is: the steam turbine vendor defines the amount of allowable contaminants, the possible carryover due to drum internals is defined by the mfr of the driers or demisters, and the resultant level permitted in the feedwater is related to the blowdown rate and the prior two physical limit.

"Whom the gods would destroy, they first make mad "