Which of the following is considered the device that directly controls the manipulated variable?

11.3.2.1.1 HPBP Pressure Control

The control strategy is based on a single-element control concept. The selected signal of the main steam pressure near the turbine outlet becomes the measured/process variable. For a fixed main steam pressure control system, the HPBP set point is also a fixed value, which is normally higher than the master pressure control set point with a slight margin (~ 5.0 kg/cm2). For a variable set point for a sliding pressure controlled system, the set point is derived as characterized from the load index, such as the main steam flow or the first-stage pressure, and then the bias for the margin is added. After the selection, the set point signal is passed through a maximum and a minimum selector. The steam pressure set point is thus limited within a minimum and maximum value and then passed through a tracking integrator (TRI) to have a ramped and smooth output to become the final set point so as to prevent process upsets. The maximum value of the integrating gradient is limited. In fact, the operating gradient is a very low value so that the instantaneous set value becomes lower than the main steam pressure in case of a sudden upset. For example, when there is a large load rejection, the steam pressure starts rising and if it becomes higher than the set point (with margin), the bypass valve opens as per the controller output command and tries to reduce the steam pressure. The set value for a variable pressure system would readjust its value as per the new load and the bypass valve would remain open up to a certain value until the steam pressure matches. The firing rate is then modified as per the load and the steam pressure is also readjusted so as to gradually close the bypass valve to bring back the normal operating condition.

There is another control loop strategy applicable to both fixed and variable pressure systems that is also shown in the above drawing as an alternative. Here, the main steam pressure itself is utilized as the set point after adding the bias for margin DP and passes through TRI to generate the final set point. The tracking integrator plays the vital role as described above.

The TRI would change its output at a very slow rate. Whenever there would be large load throw off, the MS pressure would shoot up at a very fast rate while the set point increased at a slower rate, which causes the HPBP valve to open to arrest the high pressure excursion. When the load and heat input stabilize, the bypass valve would again close. In a turbine trip, the bypass valve would open and the pressure control set point would be the minimum value, for example, the pressure required for a hot start-up. The rest of the system is the same as before.

11.3.2.1.2 HPBP Temperature Control (by Spray Water Valve)

The control strategy is based on a very simple single-element control concept. The selected signal of steam temperature at the HP turbine bypass outlet becomes the measured/process variable with a fixed set point. The difference between the two signals form the error signal of the control loop and the controller (PID) output is supplemented with a feed forward signal from the HPBP valve position to get an advanced sense of the process condition. If measured separately, the bypass steam flow itself may be used as a feed forward signal for the temperature control system. Otherwise, the suitably characterized bypass valve position indexed by the steam pressure may be taken as an indicative signal of this steam flow. The combined signal is used to regulate the high-pressure desuperheater spray water control valve.

11.3.2.1.3 HPBP Spray Water Pressure Control

The header pressure of the spray water should be maintained at a desired value. For this, there is a separate pressure control loop, as shown in Fig. 8.49. Here, the error generated by the set point and the measured variable (pressure at the spray water valve inlet) is fed to a controller, the output of which regulates the opening of the HPBP spray water pressure-control valve.

There are certain interlocked conditions other than the fast opening and fast closing criteria indicated in clause no. 11.3.2.3.1; they are indicated as follows:

Valve is not permitted to open at the HPBP spray water pressure low.

Valve is not permitted to open when the HPBP spray block valve is not fully open.

11.3.2.2.1 LPBP Pressure Control

The control strategy is based on a single-element control concept. The selected signal of the reheater outlet steam pressure at the turbine inlet (HRH) becomes the measured/process variable. The set point is of variable value, which is obtained from the characterized turbine first-stage pressure. The relation between the turbine first-stage pressure and the HP turbine exhaust, that is, the cold reheat (CRH) steam pressure, is exploited with some consideration of the line pressure loss in the reheater circuit determines the HRH set value.

When the turbine trips and/or during start-up, there will not be any first-stage pressure to act as the set point. So, the HPBP valve position (in case of more than one HPBP pressure control valve, the average valve positions) is taken as the set point through a selector circuit, as shown in the loop.

The difference between the two signals forms the error signal of the control loop and the controller (PID) output is used to regulate the high-pressure control valve.

11.3.2.2.2 LPBP Temperature Control (by Spray Water Valve)

Efficient operation of any desuperheater calls for the injection of hot water, essentially to be at a temperature near the saturation temperature of the steam being cooled. It ensures that mainly the latent heat is extracted from the steam to evaporate the injected water. This design criteria assumes minimum suspension time experienced by the water particles in the steam path so as to make certain that the injection water is completely evaporated. Complete evaporation must be attained before the first pipe bend to avoid the impinging of water particles on the inside walls of the pipes. The desuperheater performance depends on the degree of atomization of the injected water and the mixing with the steam. Proper and speedy evaporation depends on the proper location and direction of the spray water jet as well.

The control strategy (Fig. 8.50A) for LP bypass temperature control is based on the ISA recommended simple single-element control concept. The selected signal of the steam temperature at the outlet of the LP turbine bypass becomes the measured/process variable. The set point is calculated essentially from the steam mass flow and steam condition. Normally, the steam mass flow online measurement is not done in this large diameter pipe, but in turn is calculated as a function of the bypass valve position duly characterized to the steam flow and the corresponding steam conditions. The difference between the two signals forms the error signal of the control loop and the controller (PID) output regulates the desuperheater spray water control valve lift position. This determines the requisite quantity of injection water flow that provides the guideline for the LPBP downstream steam condition.

An alternative loop (Fig. 8.50B) suggests making the calculated unit enthalpy from the LPBP valve outlet temperature and the pressure multiplied by the calculated steam mass flow as the measured variable. The set point is the manually adjustable enthalpy, as desired. This loop has been developed to take into consideration that steam conditions after the LP bypass desuperheater spray are very close to or at the saturation condition; the temperature after the desuperheater is not recommended to be used as a control signal. The feed forward signal is incorporated from changes in the LPBP valve position.

Another alternative loop (Fig. 8.50C) avoiding LPBP temperature measurement incorporates the heat balance method to determine the requisite quantity of spray or cooling water to maintain the enthalpy in relation tothe condenser outlet temperature. The heat loss by the steam is equal to the heat gained by the water when mixed to form a saturated condition. Heat lost by steam is the product of steam quantity and the difference between enthalpy at the LPBP inlet and the condenser inlet; heat gained by water is the product of water quantity and the difference between enthalpy at the condenser inlet and the spray water. The enthalpy is taken from the steam table at process temperature and pressure. The desired water quantity is the ratio of total heat lost by steam and enthalpy difference from the water side, which becomes the set point. Water quantity is the measured variable and the controller output regulates the spray water valve lift.

As has already been discussed, the LP bypass is basically provided for the protection of the condenser. As such, some condenser manufacturers stipulate the following guidelines in addition to the above control philosophy for start-up as well as intermittent and continuous control to maintain the LP bypass downstream steam temperature conditioning:

Enthalpy restriction

Steam entering the condenser with enthalpy > 670–680 kcal/kg is restricted; in the case of sudden high flow steam dumps, the enthalpy is recommended to be restricted to 660 kcal/kg. In certain cases, steam admission to the condenser with enthalpy > 680 kcal/kg is also considered where the specific conditions of unit operation suggest it.

Pressure restriction

The maximum pressure of steam admission to the condenser is restricted to a limit value of 16 kg/cm2 (g), mainly applicable for the dump condenser.

To accommodate the above guidelines, temperature and pressure at a suitable point are measured to calculate the enthalpy of the entering steam; this is considered as a measured/process variable. An appropriate set value as depicted above is provided for generating the control error. Also, a controller output is added to the conditioned steam flow derived from the LP bypass pressure control valve position to form the demand for the LPBP spray control valve position.

Interlocked conditions other than the fast opening/closing criteria (indicated in clause no. 11.3.2.3.2) are as follows:

The valve is not permitted to open for any of the following conditions.

Insufficient desuperheater spray water pressure.

Block valve is not fully open.

High condenser pressure.

High condenser temperature.

High condenser hot well level.

11.3.2.3.1 HP Bypass Interlock (Fig. 8.51A)

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Fast opening criteria: Under these severe conditions, it is required that the HPBP valve and the associated spray valve and block valves shall open fully for a period to allow the HP bypass to dump the steam. During this time, the spray control pressure valve shall be auto so that suitable pressure is maintained. Suitable signals are sent to the LPBP system to enable its fast opening (Typically, these conditions are shown as signal output and the initiating criteria listed below as the input signal—applicable for all cases):

Load shedding relay is operated.

Generator circuit breaker is open.

Turbine trip acted.

High load dumping.

MS pressure at/near turbine is very high.

HPBP valve is almost open (to ensure full open).

Force/fast closing criteria:

The PBP valve shall close under any of the following conditions:

Fast closing criteria from LPBP (discussed in clause no. 11.3.2.3.2).

HPBP spray water pressure low.

HPBP spray block valve not fully open.

Steam temperature after HPBP valve is The

The HPBP spray control valve shall close under all the above conditions, provided the associated HPBP valve is closed (< 2% open).

The HPBP spray pressure control valve shall close when the HPBP spray control valve is closed, provided fast opening criteria do not exist.

11.3.2.3.2 LP Bypass Interlock (Fig. 8.51B)

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Fast closing criteria: It is necessary for condenser protection against conditions that would tend to increase condenser temperature.

The LPBP valve shall be closed under following condition:

Condenser temperature is very high.

Condenser pressure is very high.

LPBP spray water temperature is very high.

LPBP spray water pressure is low.

LPBP spray block valve is not fully open.

The LPBP spray valve shall be closed under the conditions stated above, provided the associated LPBP valve is closed (< 2% open).

Force/fast closing criteria:

The HPBP valve shall close under any of the following conditions:

Fast closing criteria from LPBP, as discussed in clause no. 11.3.2.3.2.

HPBP spray water pressure is low.

HPBP spray water pressure is low.

HPBP spray block valve is not fully open.

HPBP valve outlet steam temperature is very high.

The HPBP spray control valve shall close under all the above conditions, provided the associated HPBP valve is closed (< 2% open)

The HPBP spray pressure control valve shall close when the HPBP spray control valve is closed and no fast opening criteria exist.

Fast opening criteria: Under these severe conditions, it is required that the LPBP shall open fast under fast opening conditions of the HPBP, provided there is no fast closing criteria or very high steam temperature after the LPBP valve. These will cause all LPBP pressure and spray valves and associated block valves to open.