3.4 Boiler steam temperature control

Aim

There are boilers whose production is saturated steam, so the temperature of the steam is determined by the operating pressure, as they are directly related variables.

There are also boilers in which the steam is slightly superheated for better flow or consumption, but these are sized so that changes in steam temperature are not significant for the consumer. However, many boilers require the generated steam to be at a specific temperature to ensure proper operation for their users. In these cases, where the degree of superheating is typically higher, the impossibility of designing the boiler to produce steam at the desired temperature under all required loads necessitates temperature control to maintain it within the desired operating range.

Philosophy of control

There are different methods for controlling steam temperature, some based on gas control mechanisms and others on water control mechanisms in the boiler. The former are typical of thermal power plant boilers and are rarely used in industrial boilers, so we will not discuss them further. The latter, more widespread, consist of tempering the steam either by direct water injection (Figure 3-30) or by using a water-steam heat exchanger (Figure 3-31). From a control standpoint, the first method will have a shorter time constant and a faster response. However, introducing water into the steam will alter its quality. In the second case, this potential steam contamination does not exist, but the system's response, and therefore its time constant, will be much longer, with the drawbacks this can cause for the process and the adjustment of the control system.

Figure 3-30. Spray tempering

Figure 3-31. Temperation by heat exchanger

Control of an element

In small boilers where the load remains constant or changes slowly, or in those where the amount of superheat does not change too noticeably with the load, the use of a simple PI controller such as that in Figure 3-32 can give satisfactory results.

Figure 3-32. Steam temperature control to an element

If the temperature rises, the valve opens to supply more water, and vice versa.

However, in most facilities this type of control will not be sufficient to maintain the steam temperature within the required ranges.

Two-element control

This type of design is based on a feedforward control system with feedback. The combustion air flow rate indicates the magnitude of the fire demand in the boiler, as well as the volume of gases sent to the superheater, making it a good indicator for anticipating the spray valve position. The steam temperature will logically be the feedback variable (Figure 3-33), and any errors in this temperature will correct the required position.

Figure 3-33. Two-element steam temperature control

If for any reason the air flow rate cannot be used as an index, the steam flow rate may be used instead, although in transient situations it may not be as indicative for steam temperature control.

An alternative to this two-element control is based on a cascade control like the one in Figure 3-34.

Figure 3-34. Two-element steam temperature cascade control

In this control system, the final steam temperature is used as the process variable for the primary controller, and a measurement of the spray outlet temperature is used as the process variable for the secondary controller. With this new measurement, changes in the desaturation flow rate will have a more immediate impact on the control, allowing the secondary controller to be adjusted to provide a fairly rapid response. As with any cascaded loop, the adjustment of the primary controller should be slower to allow time for proper regulation in the secondary controller and to prevent saturation.

Since detempering is not normally required until a certain boiler load, it is common to have a shut-off valve upstream of the control valve to prevent it from operating as such, especially if the pressure... Generally speaking, the choice of the first strategy will depend on the availability of a suitable index for the correct positioning of the control valve. In installations where this index is not available or where detempering is similar throughout the operating range, the second option is usually advisable.

Three-element control

As we have discussed, a drawback to using cascade control can be the significant difference in control valve positions for different boiler loads. This problem can be mitigated by including the combustion air flow rate as a third element in this loop (Figure 3-35).

Figure 3-35. Three-element steam temperature control

The derivative of the air flow rate will be used as an anticipatory signal to modify the valve position in response to load changes, while in steady-state situations it will have no influence.

Another possibility for the three-element design is that of Figure 3-36.

Figure 3-36. Three-element steam temperature control alternative

This configuration combines the two designs presented for control with two elements, so that the airflow is used as an index of the valve position and is corrected for temperature errors, whether at the end or at the spray outlet.

The considerations given for adjusting the two-element control are clearly valid for this one.

When the spray flow rate measurement is available, it can be used as an additional element for flow control (Figure 3-37) that ensures the correct functioning of the indices regardless of the repeatability of the control element, as already seen in level control (see section 3.3)

Figure 3-37. Steam temperature control with spray flow rate

Obviously, cascading three regulators involves adjustment complications, as well as the inclusion of unwanted delays in the loop, which makes this design advisable only in particular situations.

Interlocks

In cascaded control systems, when the secondary controller is in manual or local mode (if one exists), the primary controller must also be in manual mode. Alternatively, the primary controller can be kept in follow-up mode so that its output matches the secondary controller's process variable. This way, switching the secondary controller to automatic or remote setpoint mode will directly bring the primary controller into control.

If the primary is in manual mode, the secondary will switch to manual mode or not depending on whether or not local setpoint control is allowed on the secondary.

The opening and closing order of the shut-off valve will be given when the demand exceeds 3% and falls below 1% respectively.