Boiler pressures

The American Society of Mechanical Engineers (ASME) United Engineering Center 345 East 47th Street New York, NY 10017 The ASME Boiler and Pressure Vessel Code, under the cognisance of the ASME PoHcy Board, Codes, and Standards, considers the interdependence of design procedures, material selection, fabrication procedures, inspection, and test methods that affect the safety of boilers, pressure vessels, and nuclear-plant components, whose failures could endanger the operators or the pubHc (see Nuclearreactors). It does not cover other aspects of these topics that affect operation, maintenance, or nonha2ardous deterioration. 

Rules for Nuclear Power piping are in Section 3 of the ASME Boiler Pressure Vessel Code. 

In dmm boilers sodium hydroxide (caustic), sodium phosphate, or both may be added for pH and scale control. Sodium hydroxide is used more in Europe than in the United States, where sodium phosphate treatment is usually preferred. In boilers operating above 4 MPa (580 psia), caustic concentrations must be carefully controlled to prevent highly corrosive deposits from forming. In the lowest pressure boilers, phosphate treatment may be used to compensate for lower purity feedwater. As the boiler pressure increases, the allowable phosphate concentration decreases, and at 16.5 MPa (2400 psia) or above, equiUbrium phosphate treatment may be used. In this treatment, caustic is added to a low phosphate concentration in the boiler to maintain the proper pH (20). 

Boiler Pressure (psig) Total Solids (ppm) Alkalinity (ppm) Solids (ppm) Silica (ppm)... 

Figure 1. Relationships between boiler pressure, boiler water silica content, and silica solubility in steam.

When the heat load even locally exceeds 230 kW m - the target values for drum pressure, 160 bar (except for Si02), should be used for all boiler pressures. For feedwater, the recommended values for >67 bar should be... 

Some of Rufli s calculations for (tjo)cp> for a single boiler pressure p, are shown in Fig. 7.7a. There are two important features here ... 

Kessel-niederscUag, m. deposit in a kettle, boiler or reservoir, -ofen, m. pot furnace crucible furnace. -rohr, n. boiler tube, -spaonung,/. boiler pressure, -speisewasser, n. boiler feed water, -speisimg, /. boiler feeding, boiler feed. 

Having ascertained the process steam flow and developed some ideas on the boiler pressure, the following step is to analyze the power available. Figure 15.23 provides a ready means of determining the approximate relationship between power available and process steam for specific steam conditions. Use of this and similar charts will allow an assessment to be made of the potential of a CHP scheme with a backpressure turbine. The conditions can be changed to give the required balance for heat and power. 

In anything but the smallest installations the flow of steam from the boiler into the cold pipes at start-up (while the boiler pressure is still very low) will lead to excessive carry-over of boiler water with the steam. Such carryover may be enough to overload a separator at the boiler take-off point or its steam trap (Figure 22.5). Great care... 

Statutory inspections of boilers/pressure vessels not required. 

No maintenance task can be issued without the person responsible for controlling the workforce establishing that the work is to be carried out safely. Areas where this is essential would include boilers, pressure systems, electrical systems, confined spaces, hot work, deep sumps/shafts, tanks, and lone working. 

As has been noted, scaling due to inadequate water treatment may, in the short term, lead to efficiency losses but, eventually, could result in overheating and possibly to associated corrosion. Awareness of this has led to an increasing use of appropriate water treatment with increasing boiler pressure. Scaling due to hardness salts is therefore less common now than formerly, but deposits caused by other means may occur with similarly undesirable consequences. 

For both coordinated and congruent control, the pH depends upon the phosphate concentration and the sodium to phosphate ratio. Generally, however, phosphates are unsuitable for use at boiler pressures above 100 bar as their low solubility and high concentration factors developed lead to corrosive conditions. 

At boiler pressures above about 85 bar, the tolerable level of total dissolved solids is so low that all the parameters specified are necessarily low in value. Several examples of recent specifications are given in Table 17.8 (feed) and 17.9 (boiler). 

NOTE The commonly assumed zero enthalpy (internal energy) for air is approximately 14.7 pounds per square inch pressure absolute (psia), equivalent to 1.01 bar at 80 °F (27 °C). However, boiler pressure is commonly... 

Table 1.1 Saturated steam temperatures at various boiler pressures...

Appurtenances are primarily the fittings that enable boilers to operate safely. They control the FW supply, boiler pressure, BW level, and steam supply. Appurtenances include ... 

The primary reason for installing a cooling coil is for operator safety when sampling, As the boiler pressure rating increases, it becomes increasingly important to use a cooling coil. 

Flash steam and heat recovery systems perform more efficiently if a continuous source of blowdown is provided. Depending on boiler pressure, the potential BW blowdown recovery is up to 25% of the blowdown volume recovered as flash steam and up to 75% of the heat content recovered. The flash steam can be passed to a LP steam line or sent back to a deaerator or feed tank, where it provides both FW heating and a replacement for MU water. 

The percentage of BW blowdown steam flashing off is a function of the boiler pressure, as shown in Table 3.1. An example of FSHR calculation is provided at the end of this section. 

Boiler pressure psig Percentage (%) flash steam... 

Inspection of boiler pressure vessels is generally required every 12 months, but this period may be extended to 15 to 18 months under certain specific circumstances. 

In addition to external conditioning processes and the need to provide internal chemical treatments to some or all steam-water circuits within the steam cycle, the scope of boiler water treatment includes, as mentioned earlier, the provision of suitable technical resources sufficient to control the steam-water chemistry within defined limits appropriate for the boiler plant under consideration. Because these steam-water control limits tend to narrow considerably with increase in boiler pressures (and heat-flux densities), suitable monitoring and control procedures may require implementing actions with knife-edge precision. 

The potential for hardness salts to represent a dominant part of any deposit usually is reduced with an increase in boiler pressure. This is because higher pressure boilers generally have additional and progressively more sophisticated pre-boiler treatment processes installed,... 

If BW silica levels increase above 180 to 200 ppm, it may not, in fact, be possible to totally control silica deposition by water chemistry means alone. Also, as boiler pressures, temperatures, and heat-flux densities increases, so does the need for external silica removal equipment. 

Operational boiler pressure and rates of water circulation... 

Strict limitations on the salinity content of BW become increasingly important as boiler pressure or heat-flux density increases. This is primarily effected by ... 

Although the risk of scale deposition and fouling in the boiler section is related to several factors such as the FW volume demands, boiler pressure, and heat flux density at various boiler surfaces, it is equally a function of the level of FW contaminants such as residual hardness, sulfates, silica, and iron. Thus, as a generality, the higher the quality of FW (reduced levels of contaminants), the lower the risk of deposition on boiler surfaces. 

Where considerable boiler pressure fluctuations occur... 

NOTE Some recirculation of ammonia and amine takes place within the overall boiler plant system, although at higher pH much of the ammonia is lost at the deaerator vent. In practice, this recirculation coupled with low er-than-theoretical C02 liberation (as a result of the incomplete breakdown of sodium bicarbonate when present in the boiler) typically results in a reduced amine-demand for any particular boiler pressure. This reduced demand, compared with the apparent demand, results in real cost savings. 

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