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Designing Steam Generators for both High and Low  Steam Pressure Operation    -  V.Ganapathy

Designing a steam generator to operate at both high and low pressures is a challenging task,particularly when a superheater is present.The reason is the large variation in specific volume of steam which affects the steam velocity and steam side pressure drop,which in turn affects steam flow distribution inside the tubes at low loads and consequently the tube wall temperatures. The main steam line velocity and the turn down capability of the check valves is also affected. As an example,if a boiler is designed to operate at 650 psig as well as at 150 psig,the specific volume ratio is about 4. Hence if the superheater is designed for normal steam velocity at 650 psig of say 70 to 100 ft/s inside the tubes and a pressure drop of say 50 psi,it will have a  velocity of 280 to 400 ft/s at the low pressure case of 150 psig and a pressure drop of about 200 psi,which is not feasible. One solution(when the steam generator operates at both high and low pressures on and off) is to derate the steam generator so that in the low pressure mode,the steam generation is also reduced to say 30-40 % of the capacity at the high pressure mode.This would mimize the effect of the specific volume. However if the user needs the same steam generation at both high and low steam pressures,the designer has a challenging task ahead of him.
Presented below is the design of a superheater for a packaged steam generator,which had to operate at 150 psig for the first few years and then at 650 psig later at the same capacity of 175,000 lb/h. The existing superheater bundle was reused with minimum changes when the decision to operate at high pressure was made.Only minor changes to the steam piping at the inlet and exit of the superheater were required.
                                Table: Steam Parameters
  item  low pressure  high pressure(future)
 max steam flow,lb/h  175,000  175,000
 steam temperature,F   680   760
 steam pressure,psig  150   650
 pressure drop,psi  23   46
[feed water=230 F,excess air=15 % flue gas recirculation=17 %,nat gas fuel, Design pressure=800 psig]

Design of Superheater
The steam generator is a packaged D type boiler with a buried superheater design,located behind several rows of screen tubes. This convective design has several advantages over a radiant design as discussed in the article Convective Superheaters vs Radiant. Initially when low pressure steam is generated,the steam flow is split up into two paths,one stream entering the superheater at one end and another at the other,in mixed flow fashion as shown. There are 24 streams (parallel paths) obtained by baffling the headers and this is unchanged at high pressure operation also. The flow in each path balances out depedning on the pressure drops in each section. The final steam mixes and exits at the middle as shown.The steam inlet,outlet have 2-10 in pipes. Note also that the path traversed by the steam is much smaller compared to that in the high pressure case and hence the lower pressure drop.In the future case of high pressure operation,1-10 in pipe for inlet and outelet and the steam enters and leaves the superheater in counter flow fashion as shown. The number of superheater streams is still 24  and due to the lower specific volume,twice the flow passes through the same streams.The pressure drops,tube wall temperatures were all checked at the various modes, low loads and turn down conditions.  
The steam check valve size was also changed to 1- 8 in from 2-10 in pipes.Since the specific volume of water does not change much with pressure,the feed line sizes,valves were unchanged.The steam generator has been in operation for several years.

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