ON THE ISSUE OF A SCHEME FOR AUTOMATIC CONTROL OF BOILER UNITS DURING THEIR PARALLEL OPERATION

I. K. LEBEDEV

Remarks on Schemes for Automatic Control of Combustion Processes

As is known, when several boilers operate in parallel on a common steam main, in automating combustion processes one pressure regulator, called the main regulator, is often installed for several or even all boilers. The sensing element of this regulator detects the change in steam pressure in the steam line and, through the fuel regulators, acts on the fuel supply simultaneously to all automated boilers, changing their steam output. Such a scheme has a number of substantial shortcomings.

When the total load of the boiler plant changes, the load on all units changes simultaneously, regardless of their loads at the moment preceding the change in the boiler-plant load. Moreover, even with a constant total load of the boiler plant, any changes in the load on one of the boilers—caused, for example, by changes in the delivery of pulverized fuel by the individual pulverized-fuel feeders owing to the instability of their operating characteristic or by other causes internal to the given boiler—also lead to changes in the load on all boilers. Thus, in most cases, automatic control, owing to the features of the scheme, does not ensure the maintenance of a constant load on the boilers, but forces them to operate all the time under variable conditions even when this is not required by the nature of the operation of the steam consumers. It is not difficult to see that such an operating regime of boiler units causes a reduction in their economy.

In a number of works devoted to the selection of economical distributions of loads among boiler units [1; 2], it is proved that, when different boilers operate in parallel on one steam line—which often occurs at our power stations built in several stages at different times—an economical distribution of loads corresponds to equal relative increments of fuel consumption for all boiler units. It is indicated [2] that distribution of load by this method, in comparison with the widely used method of dividing boilers into regulating and base-load boilers, gives fuel savings of up to 6%, and on average 2–4%. However, with this method of distributing load among boilers, it is often the case that several or all boilers participate in covering the peaks of the boiler-plant load, and the change in load on the boilers is not the same. The above-mentioned automatic-control scheme, in the presence of one common pressure regulator, does not ensure automatic optimum distribution of loads among the individual boilers.

At the same time it must be noted that the presence of one common pressure regulator in the form of a main regulator does not simplify, but complicates

scheme of automatic control, since the presence of a master regulator does not free the control system of each boiler from fuel regulators, while the long impulse lines from the master regulator to the fuel regulators of each boiler complicate maintenance and make the entire scheme as a whole more expensive.

The indicated shortcomings can be eliminated by introducing purely individual schemes of automatic control for each boiler with residual control nonuniformity. In this case, the magnitude of the residual nonuniformity and the setting of the static control characteristics for each boiler must be selected on the basis of the conditions for ensuring automatic optimal distribution of the load among the boiler units when the total load of the boiler plant changes. The method of constructing the static characteristics and determining the control nonuniformity for each boiler can best be shown by the individual examples given below.

Construction of the Static Control Characteristics for Several Boilers

Let us consider, by way of example, the construction of static characteristics and the determination of control nonuniformity for three boilers operating on one steam main.

Suppose, for example, that the optimal distribution of loads among the boilers, determined by the method of equal fuel-consumption increment per unit of load, is shown in Fig. 1(a), as is recommended in [2]. In the giv—

Fig. 1.

In this graph, the total steam load of the boiler house in t/hr is plotted along the abscissa axis, and the loads of the individual boilers are plotted along the ordinate axis. It is understood that the value of the boiler load plotted on the right-hand ordinate axis corresponds to their maximum permissible load, and on the left-hand axis—to their permissible minimum load.

Let us specify the overall nonuniformity of steam-pressure control in the common steam line of the boiler house. Let this nonuniformity be expressed in the form of the rectilinear dependence of the steam pressure in the steam line on the load, plotted in Fig. 1(b); here the pressure in the steam line is plotted downward along the ordinate axis in some scale. This dependence could have been shown in a separate figure, but, for convenience of further constructions, we shall take the axis of the total load of the boiler house common for Figs. 1(a) and 1(b).

We shall call the static control characteristic of each boiler unit the dependence of the steam pressure in the steam line on the steam load of the boiler. To construct the static characteristics of the boilers, we plot, in some scale, along the horizontal to the left, the axis of boiler loads from the origin of the axis of the total load of the boiler house (Fig. 1(c)). In this case, in quadrant 1(c) the static characteristics of the boilers will be represented. The pressure in the steam line of each boiler will always be equal to the pressure in the common steam line; therefore, for constructing the static characteristics of the boilers, for the purpose of facilitating the subsequent construction, the same pressure axis has been adopted as for constructing the overall pressure nonuniformity.

Let us take some total load of the boiler house, for example, \(D'\), and plot it on the axis of total boiler-house loads. Through this point let us draw a vertical straight line, which at point \(a\) will intersect the load curve of boiler No. 1, at point \(b\)—the load line of boiler No. 2, at point \(c\)—the load line of boiler No. 3, and at point \(d\)—the line of nonuniformity of steam pressure in the common steam line of the boiler house. It is not difficult to see that the line \(D'a\) will represent the load of boiler No. 1, denoted in the drawing by \(D'_1\), while the line \(D'd\) represents the pressure in the common steam line and, consequently, also the pressure for boiler No. 1. Let us plot the load \(D'_1\) of boiler No. 1 on the boiler-load axis in the form of a straight-line segment (Fig. 1(c)). Through the end of this segment let us draw a vertical straight line downward, and through point \(d\) draw a horizontal straight line to the left. The intersection of these straight lines in quadrant 1(c) (point \(e\)) will be a point of the static control characteristic of boiler No. 1. By assigning other values to the boiler-house loads \(D''\), \(D'''\), \(D''''\), etc., all the other points of the static control characteristic of boiler No. 1 can be found by the same method. In an analogous way, the static control characteristics of the other two boilers can be constructed. The static control characteristics constructed by this method are shown in Fig. 1(c).

As is evident from the drawing, in order to obtain automatic distribution of loads among the boilers according to the load-distribution schedule (Fig. 1(a)), the static characteristics of the boilers, with a rectilinear dependence of the pressure in the boiler-house steam line on the total boiler-house load, will not be straight lines; moreover, they must not be identical for different boilers. Thus, if initially the static characteristic of boiler No. 1 is flatter, then toward the end it will be steeper. As for the static characteristic of boiler No. 2, here the opposite picture is observed. This means, as is also evident from the load-distribution schedule, that when the total load of the boiler house increases, at first this change in load will be covered to a greater extent by boiler No. 1, with a comparatively smaller change in the load on boiler No. 2; toward the end of the control range, the roles of these boilers in covering the total boiler-house load will be reversed.

Knowing the static control characteristic of the boiler, it is not difficult to determine the nonuniformity of control. For a characteristic of curvilinear form, this nonuniformity will be different at each point of the characteristic and can be determined graphically as the first derivative of the pressure with respect to the boiler load.

If there are several completely identical units in the boiler house, the principle of equality of relative increments leads to the requirement of uniform distribution of the load among the units \[1\]. In this case, the construction of the static control characteristics of the boiler units is shown in Fig. 2. As can be seen from this figure, the static control

Fig. 2.

characteristics of all boilers are identical, rectilinear, and the degree of nonuniformity of control of each boiler is

\[ \delta_1 = \delta_2 = \delta_3 = \delta_k; \]

where \(\delta_1, \delta_2, \delta_3\) are the degrees of nonuniformity of control of the individual boilers, and \(\delta_k\) is the degree of pressure nonuniformity in the boiler-house steam header.

Redistribution of Load Among Boilers in the Presence of Stiff Static Control Characteristics

Stiff static control characteristics of boiler units make it possible not only to maintain a definite load on each unit at definite total loads of the boiler house, but also make it very easy to redistribute loads among the boilers.

up to the designation of individual boilers as base boilers, carrying a constant load, with transfer of regulation to individual units.

Let us assume, for example, that there are two identical boilers with identical static regulation characteristics. Let us depict these static characteristics as shown in Fig. 3. With identical static characteristics and identical adjustment, at a certain pressure in the boiler-house steam line, for example \(p_1\), both boilers will carry identical loads \(D_1\) and \(D_2\). When the pressure in the boiler-house steam line changes, the change in the boiler loads will likewise be identical.

Fig. 3.

By adjusting the sensitive element to different pressures, the static characteristic of any one of the boilers can be shifted upward or downward. In this case, at a certain pressure in the boiler-house steam line, the loads carried by the boilers will no longer be identical.

Let us shift, in the indicated manner, the static characteristic of boiler No. 1 upward to the position denoted by \(1'\). In this case, boiler No. 1, within the range of total loads from \((D_{1\max}+D_{2\min})\) to \((D_{1\max}+D_{2\max})\), will operate as a base boiler with constant load \(D_{1\max}\), and all changes in the boiler-house load will be taken up by boiler No. 2. Within the range of total boiler-house loads from \((D_{1\min}+D_{2\min})\) to \((D_{1\max}+D_{2\min})\), boiler No. 2 will operate at minimum load, while boiler No. 1 will take up all changes in the boiler-house load, that is, it will be the regulating boiler. It is clear that in this case the static regulation characteristics will have vertical portions: boiler No. 1 at the end of the characteristic, and boiler No. 2 at the beginning of the characteristic, as shown in Fig. 3.

It is easy to show that, without changing the static regulation characteristics of the boiler units, it is possible to change the minimum and maximum loads of the boilers, that is, to shift their static regulation characteristics to the right or to the left. When using DC motors as the drive for the dust feeders, as is usually the case, this shift of the static characteristics to the left or to the right is easily accomplished with the aid of the dust-feeder adjusting rheostats.

It is not difficult to imagine that, when there are several boilers in a boiler house having individual automatic regulation systems, a number of boilers may, if economically advantageous and necessary, be assigned, by adjustment of the sensitive elements, to the category of base boilers with constant load, while coverage of changes in load may—

may be assigned to individual regulating boilers. In this case, any changes in the load of the boiler plant, as well as load shedding on individual boilers, will not affect the operation of the boilers with base load, but will be taken up only by the regulating boilers.

Conclusions

1. Individual automatic control systems for boiler units in their parallel operation make it possible to carry out automatically an economically advantageous distribution of loads among the boilers.

2. Individual automatic control systems for boilers operating in parallel must have static control characteristics with a specified control nonuniformity.

3. Individual control systems with a static characteristic make it possible to ensure the operation of a number of boilers at constant loads.

4. Individual control systems are simpler than systems with a master regulator.

As regards the dynamics of boilers operating in parallel with individual automatic control systems, as well as questions related to the choice of the magnitude of the nonuniformity of control of the steam pressure in the boiler-house steam line, these will be considered in separate works.

Literature

1. Gornshtein V. M. The Most Advantageous Distribution of Loads Among Power Stations Operating in Parallel. GEI, 1949.

2. Boiler Installations, vol. II, edited by E. I. Romm. GEI, 1946.