Steam Boiler: Control System

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Installation of Drum Level Control

Installation of drum level control should be carried out properly. Drum level control was previously known that the level of boiler water in a steam drum differs significantly due to circulation of water and changes in load of steam boiler.

These conditions merge to generate it quite hard to control and observe precision level of boiler water. What exactly is needed is a quiet surface of water is which can be representative of exact level of boiler water.
Installation of drum level control can be equipped with probe type or float type and it can be performed through two methods as follow:

Inside protection tubes.
Outside drum level indicator.

- Inside protection tubes (one on one installed level settings)

Inside protection tubes occasionally known as one on one installed level settings, and so they need protection tubes for being mounted inside steam drum shell as demonstrated in Figure 1. The 1st and 2nd low level equipment should be installed in independent protection tubes, therefore that they're fully separate of one another.

Figure 1: Arrangement of Inside Installation of Drum Level Control
(Source: book-The Boiler House-Spirax Sarco)

The protection tubes independently may not be common products, and will probably be exclusively fabricated per specific steam boiler. On the other hand, for the reason that protection tubes design provide a real significant impact on the effective function of drum level control, the examples below give some instruction for installation of drum level control:

· Length:

Inside protection tubes must move as far down among the tubes seeing that actually doable.

· Diameter:
To make sure constant circumstances and give adequate clearance pertaining to probe centering, bore protection tube is made with size 80 mm. In which 2 probes (such as, level control probe additionally self-supervising probe for low level) can be mounted in an individual bore protection tube is needed with size 100 mm.

- Outside drum level indicator

Drum level control is installed on the outside of drum in a steam boiler system. Drum level control is commonly, although not often, installed by using float settings. Several common placements are demonstrated in Figure 2.

Figure 1: Arrangement of Outside Installation of Drum Level Control
(Source: book-The Boiler House-Spirax Sarco)

Two outside drum level indicator are required due to:

· One outside drum level indicator for detects low level.

· Another outside drum level indicator for detects low level and high level of boiler water in steam drum.

This particular means that the 2 low sensors will be in separate drum level indicator. The outside drum level indicator can be installed by using “sequencing valve' along with isolating valve.

Alarm on Drum Water Level

Alarm on drum water level is required to control level of boiler water in the steam drum in which steam boilers are run with no continuous supervision that involves the vast majority of steam boilers especially for industries. Alarm on drum water level is needed to give signal either on low water level or high water level condition. The steam boiler will be shut down if alarm shows that water level in steam drum is low level. The condition of boiler water level on low can be triggered by:

Inability to maintain procedure of drum level control system.
There is leakage in feedwater piping system.
Boiler feed pump cannot work properly.

The codes, regulations, and standards such as ASME, ABMA, NFPA and the others are made to protect steam boiler and make sure them can being operating securely and safety for many years. Furthermore, the explosion, damage, and life loss related to steam boiler operation can be prevented. However, human error and other possibilities which lead to boiler damage may be occur when the low water level is not avoided.

The impacts of low water level in a steam boiler are as follow:

The water wall tubes or furnace wall tube will be over heating because these tubes are not anymore cooled by water due to the lack of water in steam drum.
If the tubes is overheating, the temperature of the tube metal will quickly increases so based on ASME BPV Section II D, the maximum stress will be reduced along with increasing its temperature.
The final impact is the tubes will be crack, rupture and damage.

The following below is the steps of alarm on drum water level based on international regulations due to low level of boiler water:

First alarm on low level of boiler water: shut down or turn off the burner when this alarm is sounded, but enables the burner is turn on when the level of boiler water is recovered.
Second alarm on low level of boiler water (generally known as lockout): this alarm type is same with first alarm, the burner will be shut down when low level is reached but it keep on being 'locked out’ although the level of boiler water is recovered and almost any errors are already rectified. The lockout needs to be by hand reset to enable the burner turn on and working back.

Along with the exclusion of 1 or 2 working standards, the hazards coming from high level of boiler water in steam drum are treated quite without due consideration, otherwise ignored entirely. The hazard of an exceedingly high level of boiler water in a steam drum involves:

Greater water which is carried over into steam may lead to bad function and also failure of steam process elements, because of dirt.
Overfilling the steam boiler may result in water hammer within the steam process, risking destruction to the power plant and also harm to workers.
Unclean and wet steam may contaminate the solution in which it's employed instantly. Wet steam may maximize the water film depth of the heat transfer area, reduced control temperatures, probably interfering by using appropriate sterilization of the product. Ideally, reduced course of action and generation productivity will enhance course of action time and also unit prices.

On-Off Drum Level Control

On-off drum level control is one of methods to control drum level in steam boiler and power plant. Most of drum level control methods explained until now could be utilized to generate an on-off indication pertaining to level control. The many typical drum level control method is basically to get started the feedwater pump on low stage and enable it to operate till a higher water stage is achieved in steam boiler.

The devices which are used at on-off drum level control as following below:

- For perform flexible on-off transitioning drum levels, a steam boiler can uses capacitance probe.

- For perform unchanging transitioning drum levels steam boiler can uses conductivity probes.

- Steam boiler can uses magnetic transition using a developed-in hysteresis to control the level of float.

The method of on-off drum level control is most used commonly on steam boiler which has capacity of steam production under 4500 kg/h for the reason that it is the lowest costly choice. For steam boiler which has capacity of steam production above exceeding 4500 kg/h or 3 MW, the method of modulating drum level control should be installed.

It could be argued, on the other hand, in which the method of on-off drum level control isn't perfect for control boiler drum level, simply because the fairly high rate of feedwater flow any time the pump is on decreases the pressure of steam boiler. This condition leads to the firing rate of burner to frequently fluctuate since the on and off condition of pump is switched.

Using a common case, it could be found by calculations which also using feedwater at temperature of 80°C, the firing rate of burner might must be 40% greater by using feed pump is on, when compared with feed pump is off. This specific constant deviation leads to:

- Decreased performance.

- Temperature of steam boiler will be changes.

- Use on the burner settings.

When high load of steam occur, the parameter of flow rate of steam can usually maximize water is carried over into steam, in addition to can usually create the levels of boiler water significantly unpredictable with the related hazard of small amount of water lockout, especially on multi-boiler constructions. Then again, the reality is still that on-off drum level control is really commonly utilized on medium or small scale boilers.

There are advantages and disadvantages the use on-off drum level control method. The advantages of this method are: low cost and uncomplicated, while the disadvantages of this method are: carry-over of water into steam tend to be more than other method, flow rate and pressure of steam can be changes rapidly, and every steam boiler needs its individual feed pump in the on-off drum level control method.

Boiler Drum Level Control Method

Boiler drum level control method is one thing should be considered in the operation of steam boiler. The steam drum is a crucial component of steam boiler. This vessel’s main purpose is to supply volume and area close to the top of steam boiler in which separation of steam out of water could happen. Steam drum as well gives a place for recirculation water, add-on feedwater, chemical water treatment and also boiler blowdown system which eliminates deposits and keeps a specific amount of impurity to minimize scale creation. Since these capabilities require the continuous inclusion a loss of substance, the amount of steam-water interface is crucial.

Lower level impacts the water recirculation to the tubes of steam boiler and also decreases circulation of water to the tubes as well as decreases the performance of water treatment. High level decreases the area, along with result in dissolved solids and water going into the supply system of steam. The purpose of the boiler drum level control method is to keep the interface of steam-water on the specific level plus present a constant mass/amount balance simply by changing each single pound of steam along with water eliminated with a single pound of feedwater.

The interface level is susceptible to numerous troubles, the pressure of steam becoming a significant one. When the pressure of steam changes because of need, there's transient adjust in level because of the impact of on steam bubbles which is entrained under the interface level of steam. Since pressure declines, an increase in level, known as swell, takes place mainly because the captured bubbles expand. Since pressure increases, a decrease in level happens. This is known as shrink.

There are actually 3 fundamental kinds of boiler drum level control methods as following below and their practical application will depend on the certain boiler dimensions and load alterations:

Boiler drum level control ideally should be equipped with alarm as signal or indicator of drum level. Too high or too low level of boiler water in steam will lead to damage and deterioration in steam boiler. Furthermore, alarm is an important device to maintain proper drum level control.

Three Element Drum Level Control

Three element drum level control is designed to correct the concerns of phasing even now found in the two element drum level control and single element drum level control method, a 3rd element on feedwater circulation is included to control method of drum level. In that method the instructional math result of the controller on two element drum level control is streamed down to a 2nd controller of feedwater flow to react as a remote set level.

Figure 1: Three Element Drum Level Control
(Source: Courtesy of Honeywell)

The controller of feedwater is a rapidly performing circulation controller that utilizes feedwater flow as the procedure parameter and steam flow as the set level. Therefore for each pound of steam pass leaving steam boiler single pound of feedwater is put in. This cycle provides finalized control on the valve of feedwater. Since the remote set level via the two element drum level control variations by using drum level changes and steam flow because of blowdown or various other small losses, the controller of feedwater sets the output to control the required feedwater flow to maintain the drum level especially about level condition, heat balance and mass/amount balance.

In three element drum level control, unmounted tuning of every controller permits for quite accurate drum level control no matter of steam need and also feedwater affects. Then again it truly is pretty necessary to get the thermodynamic of the steam boiler’s recuperation rate into consideration if tuning the controller of feedwater. In case the operator fail to perform this and arrange the controller totally reset too quick on the controller feedwater the operator may accidentally sub-cool the steam drum and also begin a cyclic phasing steps similar to which found in two element drum level control method. Perform the instructional math it will make the steam boiler more secure.

The three element drum level control method could simply manage significant and fast load alterations mainly because it's related the mass/amount balance among the feedwater flow to this method and also the steam flow coming from steam boiler. This method is a must on several boilers giving the similar supply process and feedwater header, because of the changes in the offered feedwater flow to anyone steam boiler, although several boilers are running. This can be regardless to steam boiler dimensions.

Furthermore in case the steam boiler is put through to immediate or unstable need variations like in a batching procedure, the method of three element drum level control has the ability of matching this requirement with no operator trim guidance and correction.

Two Element Drum Level Control

Two element drum level control is one of methods to control boiler especially on drum level. There are 3 methods to control drum level as follow:

- Single element drum level control

- Two element drum level control

- Three element drum level control

The method of two element drum level control is appropriate for operations with average load shifts and rates, and also it could be applied on almost any sizing of steam boiler. This method employs the two parameters, steam flow and drum level to amount balance the requirement of feedwater.

Figure 1: Two Element Drum Level Control
(Source: Courtesy of Honeywell)

Drum level is calculated and the malfunction among the preferred set level along with the real control level is delivered to a instructional math as one among two procedure parameters. Steam flow is calculated and included to the instructional math as the following procedure parameter. The outcome of the instructional math is the manage output to the control valve of feedwater.

Considering that steam flow can be quite dynamic, the outcome of this method is it's going to sense the increase or drop in load requirement previous to the drum level starts to alter. The method subsequently provides or takes away control output to strengthen the response of controller of drum level on the control valve of feedwater. Along with because steam flow is generally greater adjustable it might very easily ignore the trim impact of measurement of drum level on small load alterations, covering a accurate reaction to the requirement change. Throughout steady load the controller of drum level affects the control valve of feedwater and operates to trim the level to the wanted set level.

This method provides 2 side effects that must be considered. First similar to the single element drum level control method, the two element drum level control is unable to adjust load trouble or pressure within the feedwater source, since it is not a calculated in this method. The second consideration is the two element drum level control is unable to remove phasing connections among drum level and feedwater flow since simply the reasonably slow procedure of the drum level can be managed. That second concern can bring on sub-cooled water drum on a huge raise in need by permitting too much feedwater to get into the drum with no thought to the abilities of thermal dynamic on steam boiler.

Single Element Drum Level Control

Single element drum level control is one of methods to control drum or boiler level control. The level control of water/steam is commonly discussed in one among 3 main methods such as single element drum level control, two element drum level control and also three element drum level control. The use of one of these methods is dependent on the certain boiler dimensions (economical) and load changes (design).

Single element drum level control can be probably the most basic method. In this method drum level is calculated employing a single measurement tool as well as offers a control transmission to the feedwater regulator with immediate connection to the existing running drum level. This method is applied in each feedwater control and ON/OFF methods.

Figure 1: Single Element Drum Level Control
(Source: Courtesy of Honeywell)

The single element drum level control method is exclusively efficient for small steam boilers having reasonably constant requirements and also gradual to quite small load alterations. This is mainly because the impact of swell and shrink which leads to an inappropriate preliminary control response, that can bring on under filling down or over filling up of steam drum.

Because steam need grows, there's a preliminary decreasing of steam drum pressure leading to an unnatural increase inside drum level since the steam bubbles increase and enlarge the water level of drum. This happening directs a wrong control transmission to decrease feedwater flow, if actually the feedwater flow must be greater to manage amount balance.

On the other hand, on a reduction of steam need, there's a preliminary increasing of pressure of steam drum that serves to decrease the drum level through decreasing steam bubbles and also diminishing the level of drum water. This directs a wrong transmission to maximize feedwater flow if actually it must be reducing to manage amount balance.

Operations encountering immediate or significant load alterations, can certainly lead to ‘phasing’ impact of swell and shrink leading to the controller of water level to get rid of command of drum level along with cause annoyance small water trips or even excessive carry-over and also water priming.

Types of Control Systems

The types of control systems can generally be grouped into several types as follow:

Control system type with closed and open network.

Control system type with closed network is control system that has output value give effect to desired value through an indicator or recording device (recorder). Then the difference value between controlled value and recording device is used as correction which in turn will be target control. Control system type with open network is control system that has output value does not give effect to the magnitude input. Therefore, the controlled variable cannot be compared to desired output value.

Manual and automatic control system type.

Manual control system type is control systems that the control is done by human beings who act as operator, so it is often called control system with operator. While automatic control system is control systems that the control is done by machines or equipment that works automatically and the operation is supervised directly by human.

Servo and regulator control system type

A regulator is another form of servo. This term is used to indicate system where there is constant steady state for constant input signal. The main difference is that the regulator is given an additional signal (signal interference) so that it will produce different output with servo.

Control system type according to driving source.

This control system is control based on electrical that can be form of electrical circuit, pneumatic control which utilizes air pressure or wind, hydraulic control and mechanical control.

Analog and digital control system type

Analog control system or continuous control type is a control system that the control is performed by using analog components such as Proportional, Integrator and Differentiator which the whole process works analogue. While digital control system or discontinuous control type is a control system that the control is performed by digital components that typically consists of algorithm composition programs in an analog control system.