The PPs of a boiler consist essentially of rounds of three types categorized as (1) drums, (2) headers, and (3) tubes. The PP design of boiler components is primarily built around a thin shell of components, where the thickness is derived from the formula

T = + c (1.15)

2f

Where

P = calculation pressure of the component

D = mean diameter

F = allowable stress at the design temperature

C = design allowance to cover for effects such as corrosion and erosion

Pressure. Design pressure of the boiler is generally the highest safety valve lifting pres­sure. Some codes demand that all the components be designed uniformly to a single pressure, namely the boiler design pressure. In such a case the design and calculation pressures are the same.

Some codes permit adjustments to the design pressure to take into account the pressure drops in the steam pipes and tubes and also the effects of the static head. The design pres­sure in such case will be adjusted to arrive at the calculation pressure.

The pressure that a component can safely withstand without exceeding the safe permis­sible limits at the specified design temperature is the maximum working pressure.

Temperature. The design temperature of the boiler component is the maximum tempera­ture to which it can be subjected under upset conditions. This can occur typically when there are load rampups, peak loads, unbalance in steam and gas flows and temperatures, operation with slagging and fouling, and so on. For different components the margins to be added for arriving at the design temperature are different and the code specifies the values.

Allowable stresses. The stress values to be used in the design are temperature-dependent.

• Tensile strength governs the stress values up to a temperature of ~300-350°C (~570-660°F). Drums and all headers, except for SH and RH, are therefore decided by tensile strength considerations. ECON and evaporator tubes also fall in this category in most cases. ASME specified, since 1998, a safety factor of 3.5 over the room-temperature tensile strength. European codes are less conservative, as they adopt a lower factor in the range of 2.5-2.7.

• Yield strength governs the stress values between —300 and 500°C (~570-930°F) depending on the metallurgy. The safety factor adopted by ASME is 1.5, which is the same as that for the European practice. Bulk of the lower-end SH and RH tubes and headers fall in this range.

• Creep strength or stress to rupture, whichever is lower, governs the stress values to be adopted for high-end tubes and headers of SH and RH.

• Creep strength is the average stress to produce 0.01% creep rate in 1000 h as per ASME with a safety factor of 1.

• Stress to rupture is considered for 100,000 h. It is the lower of (1) 80% of the mini­mum or (2) 67% of the average stress to rupture.

ASME specifies directly the allowable stress values to be adopted at various tempera­tures based on the aforementioned considerations. Many other codes do not specify the allowable stresses but provide the ultimate strength values and the formulae from which the stress values are to be derived.

Unlike the thermal design, the PP design to the codes is simpler in concept and content. There are design rules to be followed and a set of manufacturing standards to be observed to achieve safe and acceptable results in PP making because the PP design affects the safety.

The thermal design defines the performance of the boiler and sizes the surfaces, whereas the PP design defines the metallurgy, weight, manufacturing, and quality. The build quality of the boiler is defined and it has a major bearing on the final cost.

The PP calculations are relatively simple, but the practices adopted are continuously refined and upgraded to improve the overall PP reliability and lower the manufacturing cost, besides addressing all the issues of ever-escalating pressures and temperatures.

Table 1.13 tabulates the allowable stress values for some of the popularly permitted materials as per ASME BPVC of 2007 and these are converted into a graph as shown in Figure 1.7. A few interesting points to note are as follows:

Although ASME presents a wide range, the boiler manufacturers narrow their choices to a few select materials, typically as shown in Table 1.13, for practical reasons.

Refer to Chapter 5 that deals with PP materials in detail.

B. Structural design of the main boiler (hot and cold structures) to arrive at load data for foundations

C. Design of flues and ducts, hoppers, casing, and bunkers

D. BRIL design

E. Drafting

Process and basic engineering forms the essence of boiler technology and is mainly thermal design (combustion, heat transfer, and fluid flow), as opposed to the detailed engi­neering, which is primarily mechanical and structural design. Because a boiler is a custom — engineered product, design is calculation — and drawing-intensive. Even if two boilers were of identical construction, fuel characteristics would usually be different, calling for a fresh set of performance calculations. The boiler calculations can be broadly classified in the following manner.

1. Process calculations: calculations for each load and each fuel or any given fuel combination:

A. Boiler heat duty calculation

B. Combustion calculations for each fuel

C. Unit air and gas weights

D. Preliminary heat balance or boiler efficiency

E. Fuel fired and fuel burnt

F. Heat input and heat available

G. Total air and gas weights

2. Selection and sizing of firing equipment including coal pipes for PF firing

3. Calculations for sizing of heating surfaces

A. Furnace, division wall, and platen/wing wall SH

B. Hot surfaces—SH and RH

C. Evaporator surfaces such as boiler bank

D. Back end—ECON, AH, SCAPH

4. Sizing of auxiliaries

A. Air and gas circuits such as fans, electrostatic precipitator (ESP), flues, and ducts

B. Water and steam circuit such as pumps, valves, and piping

5. Structural calculations

6. Strength or PP calculations to codes

7. Any other case-specific calculations

Of these calculations,

• Items 1 and 4 are of interest to the boiler users and consultants as well as the boilermakers.

• Items 2 and 3 are proprietary in nature and are of interest only to the boilermaker.

• Items 5 and 6 are boiler-specific, of use to the boilermaker and the concerned inspection agencies and insurance companies.

Calculations 1 and 4 are featured in Appendix A for a typical industrial coal-fired boil­ers equipped with spreader stokers.

It is essential that all the important parameters of basic engineering be captured for boiler commissioning and for future reference and further development by all concerned— boilermaker, owner, and consultant based on the following documents:

• Contact data sheets (CDSs)

• Process and instrumentation diagrams (P&IDs)

• Boiler main and auxiliary arrangement and layout drawings

• PP arrangement

• Firing equipment arrangement and layout

CDS and P&IDs are most frequently consulted for commissioning and future designing.

Contact data sheets are comprehensive documents prepared at the end of the basic design and updated a few times during the contract period to incorporate the finalized details of mainly the auxiliaries. A typical CDS consists of the following data:

• Details of client, consultant, and inspection agents, and existing plant details as applicable

• Steaming, fuel, ambient conditions, electrical, instrument, site, wind, soil, and other related data

• Construction material, safety codes, and any other mandatory design requirements

• Guarantees for boiler performance, auxiliary power, emissions and noise, and any other parameters

• Construction notes of all HSs such as furnace, SH, RH, ECON, AH, and SCAPH

• Specifications of auxiliaries such as fans, motors, valves and mountings, and attemperators

• Specifications of all common auxiliaries such as feed pumps, deaerators, feed tanks, and turbines

• Sizing and design limits of all PPs such as drums, tubes (coils, panels, and loose), headers, and pipes

• Sizing of firing equipment—mills and burners, grate and feeders, air and ash noz­zles as applicable

• Soot blower selection and piping schemes

• Anticipated performance

• Any other data relevant to the project

The expected performance from the boiler at various loads and fuels is captured in these formats, which evolved over several decades. These data are given to the commissioning and operating engineers to provide a benchmark for tuning the operations to obtain opti­mum performance from the boiler.

Process and Instrumentation Diagrams. The following P&IDs are usually prepared based on the type of boiler and firing:

• Steam and water

• Air and gas

• Fuel and ash

• Chemical dosing

• Milling and feeding

• Deaerator and feed pumps

Does the P in P&ID stand for process or piping? The debate is inconclusive but the pro­cess is converted into piping when all the details are incorporated.

Further Readings

Acceptance testing of steam generators—DIN 1942.

Acceptance tests for industrial boilers and steam generators—BS 845.

ASME Boiler and Pressure Vessel Code Section 2—Materials.

ASME Boiler and Pressure Vessel Code Section 1—Power Boilers.

Bender, R. J. Special report on steam generation, Power Magazine, June 1964.

Codes for acceptance tests on stationary steam generators of the power station type—BS 2885. Michael, H., 1990, Comparison between American and German code to power boilers, ASME Transactions.

Power Test Codes—Fired steam generators—ASME PTC 4—1998.

Power Test Codes—Performance test code on overall plant performance—ASME PTC46.

Power Test Codes—Steam generating units—ASME PTC 4—1998.

Power Test Codes—Steam generating units—ASME PTC 4.1—1964 (reaffirmed in 1991). Specification for water tube steam generating plant—BS 1113.

VGB directives for ordering high capacity boilers.

VGB directives for the construction and inspection of high capacity steam boilers.

Woodroff, E. B., Lammers H. B., Lammers T. B. Steam Plant Operations, 8th edition. 2004. McGraw-Hill, New York.