De-superheater 1

[PMCap]

I'm reviewing a high pressure feedwater heater for a fossil plant design that has an internal de-superheater located in the skirt region of the shell side to knock down the pressure and temperature of the steam.

Skirt is designed for inlet steam temperature but a lower pressure than the inlet steam. Shell is design for the same reduced pressure as the skirt and for a lower temperature than the inlet steam.

Design Data
De-superheater - 500 psig @ 770 F
Skirt - 250 psig @ 770 f
Shell - 250 psig @ 480 F

Questions -

What is the standard practice in selecting the skirt and shell design pressures and temperatures?

Shouldn't the skirt and shell be at least designed for the same pressure as the inlet steam?

Thanks

 

[davefitz]

The design you described would need to add steam relief valves ( and vent stack) on the shell side of the heater to account for a failure mode of the inlet PRV valve.

Normally a power plant that uses regenerative feedwater heaters woud strive to maximize the efficiency of the cyle- this means it is desireable to minimize the pressure drop between the extraction steam source and the heater shell . If the optimum heater feedwater outlet temperature is equal to the saturation temperature at 250 psig, then the extraction source should be roughly 10% greater than 250psig ( 275 psig) . Instead of using a 500psig source, one would use a 275-300psig source of extraction steam and avoid the inlet pressure reduction and steam releif valves .

 

[stgrme]

I think that you are confused about the purpose the de-superheater. Most heat exchangers only take advantage of the heat of vaporization (condensation). The de-superheater allows the heater to use a portion of the superheat in the extraction steam.

As far as pressure is concerned, there is very little pressure loss through the heater on the shell side. Assuming there are no pressure reducing valves between the turbine extraction nozzle and the heater inlet, the minimum design pressure for the entire shell side should be the pressure at the turbine nozzle. I suggest adding a margin of 5% for normal variation in extraction pressure and perhaps rounding up to the next 50 psi increment for off-design operating conditions.

For the skirt design temperature, I suggest following a constant entropy line for the steam conditions at the turbine extraction nozzle up to the design pressure (described in the paragraph above) and reading the corresponding temperature. For the shell design temperature, I suggest you use the saturation temperature at the design pressure plus 25 to 30 degrees-F.

Best of luck!

 

[muld0020]

I suggest you grab a copy of HEI for closed feedwater heaters.

See HEI 3.2 for design pressure and 3.3 for Design Temp.

The purchaser should specify the design pressure, but it is typically something above the Steam Inlet (50 psig). The skirt and shell typically have the same design pressure. The typical pressure drop limit though the desuperheating zone (DSH) is 5 psi and 5 psi in the subcooling zone (DC)if you have one.

In a feedwater heater the Skirt design temp you follow the Mollier Diagram at normal operating temp and pressure and follow a constant entropy line to the max operating pressure and round to next higher 10deg F. The Shell design temp must be equal to or greater than Tsat.

This difference in design temp from skirt to shell is because in the main zone of a feedwater heater (condensing zone) you are condensing the steam and are at saturation temp. The skirt has the higher temp, due to the "dry" steam condition. Once you get over 750F for design temp, you will end up with a CrMoly Skirt instead of CS which adds cost, so you want to get the skirt design temp right.

When evaluating designs you want to look at the dry wall safety margin of the DSH (in all load cases) as well as overload vibration in the zone. There are certain designs that meet the overload conditions better than others. You may want to put a premium on designs that exceed the HEI minimum requirements thermal performance and vibration.

 

[rmw]

Wow, Muld0020, a star for you. You know your stuff. Would you mind if I added the words "outlet pass" between DSH and (in all cases) in the last paragraph about dry wall margin?

PMcap, what is the operating perssure of this DSH? Is this heater a single heater for this point or one of an A/B pair? Makes a big diffrence on the overload condition analysis.

rmw