Instrumentation and Controls for a Steam Desuperheater

A steam generator is an item of plant equipment that is found in almost every manufacturing site, and its output of steam has many critical uses in a variety of production processes.

Steam has been assigned a measurement quality and a scale based on what is known as its dryness, which indicates how much “free” water it contains. The scale range of dryness is 0 to 1 where 0 corresponds to being completely wet (i.e. hot water) and 1 represents completely dry (i.e. no entrained water). As we can note, the drier the steam is, the greater its ability to do useful work, because it has absorbed more heat i.e. extra heat in the vaporous component, which is given up when work is done. It is hard to produce steam of the exact quality directly from the steam generator; hence, what is done to meet a specific need is to produce steam of higher quality, i.e. with excess heat, and then add a controlled amount of water to arrive at the quality required. The control is based on the temperature of the steam.

Related: The Instrumentation for Furnace Control

Let’s consider the control schemes in figures 1 and 2 below:

In both control schemes above, the temperature controllers TIC-1 have a set point that is manually adjusted by the process operator to a value that is required to give the steam quality desired. However in Figure 1, it is a simple feedback control loop; the steam quality is adjusted after the spray water has been added and therefore some of the steam quality will be “about” the required temperature. The average value, nevertheless, will be within the range required. The control scheme of Figure 2 approximates a feedforward control loop. The average value will be within a much narrower band of acceptable quality because with temperature controller TIC-2 the operator initially adjusts the set point and predicts what the outlet temperature should be, and the controller adjusts the spray water accordingly. When TIC-2 is placed with its set point in remote, then temperature controller TIC-1 monitors and adjusts the outlet steam temperature much closer to that required i.e. feedback stabilization. This type of control demonstrated by Figure 2 is called cascade control. Cascade control systems are very useful means of maintaining control between different loops and are widely applied across a wide range of industries for example for process tank level controls, which can be cascaded onto the inlet material flow control loop to the tank (the level controller output forming the set point for the flow controller), to bring a much more refined means of level control.

You can also read: The Operation of a Temperature Control System for lubricating oil