Improved level control of a feed water valve

Process control authority Bela Liptak answers a reader's question regarding improved level control for a feed water valve that is too far from a boiler drum.

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Ask the ExpertsQUESTION:

 

I'M A NOVEL instrument engineer, and I need some help: I’m trying to improve the level control of a 150,000 pounds/hr boiler, but I think the feed water valve is too far from the drum (100 feet away, there is a lot of dead time), Do you think the control will be better if the valve is relocated closer to the drum?

Alejandro Macías Hernández, Ingeniería de Mantenimiento


ANSWER:

WATER BEING incompressible, as soon as the valve opens to pass more water, more water will enter the drum. Because boiler feedwater and drum-level control are very important topics, I will discuss them in more detail below.
 
Feedwater control is the regulation of water to the boiler drum. This water is admitted to the steam drum and it generates the steam produced by the boiler. Proper boiler operation requires that the level of water in the steam drum be maintained within a certain band. A decrease in this level may uncover boiler tubes, allowing them to become overheated. An increase in this level may interfere with the operation of the internal devices in the drum that separate the moisture from the steam and may cause liquid carryover that can damage the steam turbine. 

The water level in the steam drum is related to, but is not a direct indicator of, the quantity (mass) of water in the drum. At each boiler load there is a different amount of steam bubbles in the water. Thus, as load is increased there are more steam bubbles, and this causes the water to “swell,” or rise, rather than fall, because of the added water usage (See Figure 1 below). Therefore, if the drum volume is kept constant, the corresponding mass of water is minimum at high boiler loads and maximum at low boiler loads. The feedwater controls therefore need to respond to load changes and to maintain the mass of water, which requires to constantly adjust the water volume stored in the system.

FIGURE 1
False water level indication
Partial vaporization in the evaporating tubes causes drum level to shrink when feedwater flow increases and when pressure rises. On the other hand, an increase in the demand for steam causes the level to “swell”.


Feedwater is always colder than the saturated water in the drum. Therefore, some steam is condensed when contacted by the feedwater. As a consequence, a sudden increase in feedwater flow tends to collapse some steam bubbles in the drum and temporarily reduce their formation in the evaporating tubes. Then, although the mass of liquid in the system has increased, the apparent liquid level in the drum falls. This is referred to as the “shrink” effect. Equilibrium is restored within seconds, and the level will begin to rise. 

Nonetheless, the initial reaction to a change in feedwater flow tends to be in the wrong direction. This property, called “inverse response”, causes an effective delay in control action, making control more difficult. Liquid level in general can typically be controlled with a controller proportional band of 10 % or less. By contrast, the steam drum-level controller needs a controller gain closer to 1 (proportional band of 100 %) to maintain stability. Integral action is then necessary, which was not necessary, when  narrow proportional band settings are used.

Control of feedwater on total drum level alone tends to be self-defeating, because during load changes, its action is the opposite of what is needed (a load increase causes it to decrease water feed when it should be increasing it). Boilers are designed for constant level operation, however.

For small boilers having relatively high storage volumes and slow-changing loads, a simple proportional control may suffice, imprecise as it is. Integral action should not be used, because of the instability that is a result of integration of the swell and shrink on load changes that must later be removed. Control of this type therefore involves the addition of feedwater on straight proportional level control.

For medium size boilers and particularly when there is a consistent relationship between valve position and flow, a two-element system can do an adequate job under most operating conditions. Two-element control involves adding the steam flow as a feedforward signal to the feedwater valve. Total level control is undesirable when it is detected by sensors that are insensitive to density variations, such as the conductivity type sensors. Displacement and d/p cell type sensors are preferred from this perspective because they respond to hydrostatic head. Smaller boilers, in which load changes may be rapid, frequent, or of large magnitude, will also require the two-element system.

In the two-element configuration, field testing, characterization, and adjustment of the control valve are required so that the relationship of control signal to feedwater valve flow matches that of the steam flow to the flow transmitter output. Any deviations in this matching will cause a permanent level offset at the particular capacity and less than optimal control.

Three Element Feedwater Systems
As boilers become greater in capacity, economic considerations make it highly desirable to reduce drum sizes and increase velocities in the water and steam systems. Under these conditions the boiler is less able to act as an integrator to absorb disturbances. A three-element system is used on such large boilers to arrest disturbances and react to load changes more rapidly.

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