The plant can be damaged if the maximum setpoint is exceeded. Such situations can occur, for example, if the amount of energy removed by the boiler fluctuates a lot. To prevent this from happening, a safety shut-off is usually triggered.

However, an early controlled shutdown can be configured in order to prevent both triggering of the safety shut-off and damage to the plant.

In addition, a setpoint shutdown can also be triggered by an external signal

Internal controlled shutdown

Use the dx-STOP and dx-START fields to define the limits for the controlled shutdown, with reference to the current setpoint:

Burner STOPP (dx-STOP)

With this value, you define the limit value for automatic controlled shutdown.

Burner START (dx-START)

With this value, you define the limit value for subsequent burner restart.

If the current actual value is more than or less than the setpoint by these values, the burner is automatically shut down or started up as appropriate.

The limit values apply in conjunction with the current setpoint W of the load controller.

• Burner stop:

  Actual value X > setpoint W + burner stop threshold dx-STOP = automatic burner stop

• Burner start:

  Actual value X < setpoint W + burner start threshold dx-START = automatic burner start

To save energy, the limit for the controlled startup should be above the controller setpoint W (dx-START = positive value).

This prevents unnecessary burner stops that would result in the boiler cooling down due to the purge when restarting. The prevention of unnecessary burner stops reduces strain on the boiler and saves fuel (see following example and figure "Burner capacity and boiler pressure optimum").

W

Setpoint

↗

Burner START

↘

Burner STOP

A

Controlled shutdown

B

Controlled startup (burner start)

C

Boiler purge phase (approx. 3 – 5 minutes)

D

Burner fires

E

Setpoint reached, burner at low load

KD

Boiler pressure (bar)

BB

Burner operation (0 = off, 1 = firing)

BL

Burner load (percent)

0 = Low load

100 = Full load

Controlled Shutdown via External Digital Input Signal

The controlled shutdown can be triggered by way of an external signal. The external controlled shutdown is triggered when the associated channel provides a "FALSE" value.