Level control boilers

Another method for dealing with high reactor temperatures is to generate steam, as shown in Fig. 4.19. Here we allow the coolant to boil and thereby provide a constant jacket temperature. The secondary loop controls pressure in the boiler drum by venting steam. Fresh boiler feed water is added by level control. A potential problem W ith this arrangement is the possibility for boiler swell that results in an increase in the level due to increased vaporization in the jacket. The increased level due to swell reduces the intake of boiler feed water when in reality it should be increased. This problem can be overcome by providing a ratio controller between the steam flow and the feed water with the ratio reset by the steam drum level controller. Boiler feed water flow will now change in the correct direction in response to load. 

There is also another limitation of controllers that is often overlooked. A controller must be given only its one Job to do, not several. For example, a level controller might be designed to deliver feed to a process having a varying demand. A boiler level controller is such an example. This controller can be designed... 

The boilers are normally dispatched to site as a packaged unit with the shell and smokeboxes fully insulated, painted, and mounted on a base frame. The combustion appliance and control panel will be fitted together with the feedwater pump, water-level controls, gauges, and a full complement of boiler valves. Additional equipment may be specified and incorporated during construction. Larger boilers may have to have certain items fitted at site due to site restriction or weight. 

Water-level controls continuously monitor the level of water in a steam boiler in order to control the flow of feedwater into the boiler and to protect against a low water condition which may expose the heating surfaces with consequent damage. The controls may be either float operated or conductivity probes. 

With water-level controls, it is important to check they are functioning correctly, and they will be operated daily to simulate low-water condition. On shell boilers, this is invariably a manual operation but may be motorized on watertube boilers. 

Shell boilers may be fitted with internal level controls. Here controls are mounted on the crown of the boiler with the floats or probes extending to the water surface through the steam space. To check the operating function of these, it is necessary to drop the water level in the boiler, or, alternatively, a separate electronic testing device can be fitted. With fully flooded hot water boilers, a single level control or switch is fitted to protect against low-water condition. 

Automatic water-level controls arranged to positively control the flow of water into the boiler and maintain a set level between predetermined limits ... 

An independent overriding second low-water control. This will be set below the level control in (2). It will give an audible alarm on shutting off the fuel and air supply and require manual reset before the boiler may be brought back into operation ... 

For hot water boilers the burner controls will be similar but controlled by a combination of pressure and temperature signals. A single overriding level control will be fitted to the flooded boiler to protect against any accidental low-water condition. 

When a boiler may not be shut down for maintenance of the level control chambers isolating valves can be fitted between the water-level control and the steam space. In this instance, the valves must be capable of being locked in the open position and the key retained by a responsible person. When these valves are closed during maintenance periods the boiler must be under manual attendance. Fitting of these valves should only be with the agreement of the insurance company responsible for the boiler. Drains from the water-level controls and level gauges should be collected at a manifold or sealed tundish before running to the blowdown vessel. 

Sometimes it is not practical to blowdown the level controls and shut down the incinerator. In this case, the situation should be discussed with the insurance company and the boiler supplier. It is possible to include for an extra high working water level giving a safety margin above the heating surfaces. The controls may then be blown down and checked for satisfactory operation with a predetermined time delay before it shuts down the incinerator or operates a bypass in the event of a fault. 

Where the MU water supply to cast-iron boilers does not precisely keep up with steam generation demands, the water level can quickly decrease and the problems become even more acute. Conversely, where MU does precisely keep up with steaming rates and is supplied to a common condensate receiver-FW tank via automatic level control, the tank can easily overfill when condensate finally drains back under on-off operating conditions. This gives rise to a loss of valuable hot, treated water from the system and the start of another chain of cause and effect problems. 

NOTE For lower pressure boilers, the maximum permitted level of boiler water silica (as ppm Si02), typically is 0.4 x caustic alkalinity (as ppm CaCOf). This method of control is satisfactory as far as it goes, but it does not solve the problem of the risk of silica deposits in the pre-boiler section. 

These features may indeed provide cost-effectiveness and other tangible benefits however, unless the boiler plant operates constantly and FW quality remains consistent, these benefits are unlikely to be fully realized. Where operating conditions vary, the use of multiblend programs exposes the potential for constant readjustment, poor waterside treatment (scales, deposits, oxygen pitting, etc.), and water-level control problems. 

More formal inspections also verily the operation of safety and other controls such as low water and fuel cutoffs, level controls, fusible plugs, pressure gauges, water glasses, gage cocks, stop valves, safety/relief valves, and BD valves and lines. Also, FW pumps, flue and damper arrangements, combustion safeguards, name plate specifications, set pressures, boiler connections, floats, mercury switches, bellows, and other components may be inspected. 

Power plant boilers are either of the once-through or dmm-type design. Once-through boilers operate under supercritical conditions and have no wastewater streams directly associated with their operation. Drum-type boilers operate under subcritical conditions where steam generated in the drum-type units is in equilibrium with the boiler water. Boiler water impurities are concentrated in the liquid phase. Boiler blowdown serves to maintain concentrations of dissolved and suspended solids at acceptable levels for boiler operation. The sources of impurities in the blowdown are the intake water, internal corrosion of the boiler, and chemicals added to the boiler. Phosphate is added to the boiler to control solids deposition. 

Boiler controls have already been described in Section 2.2, so their discussion here will be limited and oriented toward power generation. The level control of the steam drum of an HRSG is very similar to that of fired boilers, except that up to 30% of nominal steam flow, a single-element controller is usually used. Above 30%, the loop is bumplessly transferred to a three-element control (Figure 2.116). 

The collected condensate is supplemented by fresh feedwater under level control (FC-11) and is sent to the boiler feedwater preheater (HE-1). 

FIGURE 15.54 Boiler drum level control, (a) Feedback, (b) feedforward, and (c) feedback and feedforward combined. 

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

Waste heat recovery systems. WHRB 1 and WHRB 2, economiserfs), boiler feed water tank and pumps, raw water storage and water treatment plant for soft water/DM water, level controllers for WHRB, etc. 

Keep a check on boiler water level and the working of the level controller. 

Quality of water fed to the boilers and working of instrumentation provided on the boilers for level control, etc. 

Boiler mountings (make, installation dates, specs)—all valves, level controllers Ferrules used for boiler tubes (nos. and specs)... 

Stmctural saddle supports and woik platforms may be obtained from the vendor. Vendor shall supply all standard fittings and valves for feed water, blowdown, vent, safety, as well as steam stop valves, instmmentation, level controllers with low-level alarms, and boiler feed water pumps with motors. All of these shall be supplied with test certificates and approval of statutory authorities. 

Instrumentation level controller, pressure gauge, conductivity meter for boiler feed water (optional), etc. ... 

Confirm water-level controller for boiler is working correctly. 

Waste heat recovery boiler (WHRB) Heat generated during oxidation of ammonia is recovered to generate steam If water flow gets intermpted the boiler will be in danger Water-level controller is a must stop gas feed if water level is too low... 

Low water level in waste Chance of leak of water tubes of boiler Automatic level controller and audio Air blower and sulphur pump should trip at very low level of water... 

Trouble-free generation of steam requires water of high quality. The characteristics of boiler feed water and methods for its production are covered below in Section 12.4.3.3. This water is fed to the boiler(s) on demand. The simplest way to control the addition of water is from a level instrument in the steam drum. This iqrproach has a certain amount of inherent instability. When (cold) water is added to the drum in response to a demand from the level controller, some of the bubbles present in the liquid will collapse. Hie level will go down, and the controller will call for even more water. In the opposite case, when the level rises and the water flow is reduced, more bubbles will form and the level will increase, causing the controller to call for less water. The process itself is supplying positive feedback. 

Final control elements In rare instances the final control elements can be duplicated, in cases when the erosive/corrosive or sticking characteristics of the fluid could cause unacceptable downtime or in cases of critical controls (viz, boiler drum level control with control valves in medium-sized power plants). The major cases are as follows ... 

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