Steam drum levels control

Another method for dealing with high reactor temperatures is to generate steam, as shown in Fig. 4.19. Here we allow the coolant to boil and thereby provide a constant jacket temperature. The secondary loop controls pressure in the boiler drum by venting steam. Fresh boiler feed water is added by level control. A potential problem W ith this arrangement is the possibility for boiler swell that results in an increase in the level due to increased vaporization in the jacket. The increased level due to swell reduces the intake of boiler feed water when in reality it should be increased. This problem can be overcome by providing a ratio controller between the steam flow and the feed water with the ratio reset by the steam drum level controller. Boiler feed water flow will now change in the correct direction in response to load. 

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

Condensate makeup to the steam drum is ratioed to the lo-psig steam flow rate from the steam drum. This ratio is then reset by the steam drum level controller. Pressure in the 50-psig steam header is controlled by adding 100-psig steam. 

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

Feedforward is commonly applied to level control in a drum boiler. Because of the low time constant of the drum, level control is subject to rapid load changes. In addition, constant turbulence prevents the use of a narrow proportional band, because this would cause unacceptable variations in feedwater flow. The feedforward system simply manipulates feedwater flow to equal the rate of steam being withdrawn, since this rqiresents the load on drum level. The system is shown in Fig. 8.2. 

Since the drum-level control system admits feedwater at a rate equal to the flow of steam, the pressure-control system is left to manipulate the input of thermal power. To achieve high performance control, a feedforward loop should be used to set firing rate proportional to steam flow. 

Since there is a dead time involved—the time for liquid to flow fi-om the feed tray to the column base—the base level controller settings should be determined by the method of Chapter 16, Section 6. This section also gives a complete design, including overrides for bottom product and steam flows. The reflux drum level controller settings (if a PI controller is used) should be determined by the method of Chapter 16, Section 2. 

In calculating reflux drum level controller settings, whether proportional only or PI, one should take care to account for reflux subcooling (see Chapter 16, Section 4). Overhead composition may be controlled by trimming the distillate/bottom-produrt ratio with perhaps a feedforward compensator connected into the overhead level control loop. Base composition may be controlled by trimming the steam/bottom-product ratio control. 

Determining settings for the reflux drum level controller is, in this case, difficult unless a large reflux drum holdup is available. Preferably one should make 5 minutes level controller tuning will require a dynamic analysis of overall column material balance such as discussed in Chapter 14. If steam flow is metered by an orifice, it should be linearized with a square root extractor. 

High base-level overrides on steam have also given trouble for the reasons mentioned. To a considerable extent, however, these problems can be mitigated by using proportional-reset instead of proportional-only overrides (Chapter 9). A high base-level override on steam shovdd not be used on the same column with either reflux drum level control on reflux or a high reflux drum level override on reflux. 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

At 0119 10, the operator began to increase the rate of feedwater return to reduce the recirculation flow to increase the water level in the steam drums. At 0119 45, the reduced inlet water. stopped water from boiling in the core. The absence of the steam voids reduced the reactivity, and control rods were withdrawn, such that only 6 to 8 rods were in the reactor, rather than the required 30. Then, to avoid reactor trip from steam drum or feedwater signals, their scram circuils v ere locked out (a safety regulation violation). 

A distillalion column is used to separate two close-boiling components that have a relative volatility close to one. The reflux ratio is quite high (IS) and many trays are required (150). To control the compositions of both products the flow rates of the product streams (distillate D and bottoms B) an manipulated. Gas chromatographs are used to measure the product compositions. Base level is controlled by steam flow rate to the icboiler and reflux drum level is controlled by reflux flow rate. 

The statement that the mass, or weight flow of vapor through the trays, increases as the refluxed rate is raised is based on the reboiler being on automatic temperature control. If the reboiler were on manual control, then the flow of steam and the reboiler heat duty would remain constant as the reflux rate was increased, and the weight flow of vapor up the tower would remain constant as the top reflux rate was increased. But the liquid level in the reflux drum would begin to drop. The reflux drum level recorder controller (LRC) would close off to catch to falling level, and the overhead product rate would drop, in proportion to the increase in reflux rate. We can now draw some conclusions from the foregoing discussion ... 

On the surface, this story sounds crazy. But, let s see what happened. This deaerator had been designed for a much smaller flow of 160°F BFW, and a much larger flow of hot-steam condensate, than are current operations. The cold BFW feed line had been oversized, but the steam line was of marginal size. As the demand for hot BFW increased, the cold-BFW level-control valve opened. This reduced the temperature and pressure in the deaerator drum. In response, the steam pressure-control valve also opened. But when the cold-BFW level-control valve was 40 percent open, the steam pressure-control valve was 100 percent. Steam flow was now maxed out. 

Boiler controls have already been described in Section 2.2, so their discussion here will be limited and oriented toward power generation. The level control of the steam drum of an HRSG is very similar to that of fired boilers, except that up to 30% of nominal steam flow, a single-element controller is usually used. Above 30%, the loop is bumplessly transferred to a three-element control (Figure 2.116). 

The flowTate of the purge stream from the base of the purge column is quite small, so it would not do a good job in controlling base level. This is especially true when the large steam flow has been selected to control the reflux drum level. Base level in the purge column can. however, be controlled by manipulating the bottoms flowrate from the DIB column. 

Seven liquid levels are in the process vaporizer, reactor steam drum, separator, absorber, column base, and two decanter layers. Control of the decanter levels is straightforward. The organic product flow controls the organic phase inventory the aqueous product flow controls the... 

Figure S-27a and b shows variations in the response of a distributed lag to a step change in load for different combinations of proportional and integral settings of a PI controller. The maximum deviation is the most important criterion for variables that could exceed safe operating levels, such as steam pressure, drum level, and steam temperature in a boiler. The same rule can apply to product quality if violating specifications causes it to be rejected. However, if the product can oe accumulated in a downstream storage tank, its average quality is more important, and this is a function of the deviation integrated over the residence time of the tank. Deviation in the other direction, where the product is better than specification, is safe but increases production costs in proportion to the integrated deviation because quality is given away.

Figure 15.54c shows a combined feedforward and feedback controller for the control of the level in the steam drum. Note that the feedforward controller provides most of the required control action by responding to the measured steam usage. The feedback controller can be a relatively low-gain controller, since it needs to compensate only for measurement errors and unmeasured disturbances. 

The relatively simple, lumped-parameter system model described above has been tested against and used in earnest to analyse the behaviour of the boiler recirculation loops of a number of power stations. It has been found to give excellent quantitative predictions of all the variables whose trends are important for control engineering purposes, namely steam drum pressure and temperature, feedwater flow, steam production, downcomer flow and, very important, drum water level. 

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