Steam Flow or Evaporation or Boiler Output

This is the amount of steam generated from the FW supplied at a certain temperature. It is the capacity or the rating of the boiler expressed in pounds per hour (lb/h), kilograms per hour (kg/h), or kilograms per second (kg/s).

In a reheater (RH) boiler, heat is added to steam to raise its temperature from inlet to outlet condition. This also forms a part of evaporation.

As the feed temperature varies a good deal and reheating is invariably present in util­ity boilers, boiler ratings are better designated by the heat duty than evaporation. This is expressed in million British thermal units per hour (MBtu/h), million kilocalories per hour (Mkcal/h), or megawatt thermals (MWth).

Heat duty of a non-RH boiler = Ws(Hs — Hw)

Heat duty of an RH boiler = Ws(Hs — Hw) + Wsrh (Hro — Hri) (11)

(MBtu/h, Mkcal/h, or MWth)


Ws = main steam flow Wsrh = RH flow Hs = enthalpy of SH outlet steam Hw = enthalpy of FW at inlet Hro and Hri = RH steam enthalpies at RH outlet and inlet, respectively

Maximum Continuous Rating

Maximum continuous rating (MCR) is the ability of the boiler to generate and supply the declared amount of steam continuously and effortlessly without any kind of short­fall or side effects (such as overheating or slagging or overloading) on the main boiler or auxiliaries.

It is, in fact, the minimum assured evaporation under specified conditions. With the test block margins available in the various auxiliaries, it is possible to obtain 8-10% more steam from the boiler when it is new. Of course, with aging, a substantial portion of this overcapacity would be lost, mainly as the surfaces foul up and parts wear out. If the fuels, for which the boiler is designed, do not deteriorate and the boiler does not suffer any seri­ous accidents like explosions, and the O&M practices are good, a well-designed boiler with proper margins meets its MCR rating even at the end of its designed life of usually 30 years.

At the time of selection and ordering of the boiler plant if the various scenarios on load and fuel are clearly developed, adding further capacity to avoid a possible derat­ing over time is unnecessary and wasteful. As the resulting oversized boiler would operate at less than the rated condition, the running costs (mainly the auxiliary power costs of fans and pumps and milling) are all going to be higher, in addition to the ini­tial cost.

Peak Rating

Peak rating is the evaporation that can be sustained by the boiler for a specified period of, for example, 2 or 4 h in a day, to meet an increased need in either the process or the power plant.

• Peak duty is always met at a fractionally reduced efficiency, as the final exit tem­perature of the gas from the unit would be more than that at an MCR condition (as the fuel flow is higher), leading to higher stack losses.

• Usually the peak duty does not exceed 110% MCR and 4 h in a day, and it is mostly met by making use of the design and test block margins of the boiler and the aux­iliaries without having to oversize the equipment.

• However, if the peak rating or duration is higher, the boiler plant has to be resized suitably with the efficiency optimized at MCR or normal continuous rating (NCR) and the equipment sized to suit the peak rating. Often in a power plant the peak duty is also to meet the turbine valve wide open (VWO) condition, which is usually —5-10% of the turbine MCR flow. This is the maximum flow when all the control valves in the turbine inlet chest are fully open. This condition can give additional power with a new turbine and can protect against a possible loss of power in the later years due to the aging factors.

• Running the boiler continuously at its peak rating is best avoided because the life of the boiler is affected due to sustained higher temperatures in furnace, SH, and RH areas, causing slagging, fouling, and erosion problems besides a higher loss.

The concept of peak rating does not apply to HRSGs. Gas turbine (GT) ratings are given at their optimum conditions and GTs are scarcely operated at temperatures higher than the design loads for the fear of severe shortening of life of hot parts.

Normal Continuous Rating

Normal continuous rating is slightly lower than MCR and represents the condition at which the boiler would be operated most of the time; and naturally it is the condition at which the efficiency needs to be optimized. This is typically at 90% MCR. In power plant application, NCR usually corresponds to the MCR of the turbine and the MCR to the VWO condition or more.

1.3.1 Types of Pressures

SH Outlet Pressure

Superheater outlet pressure is the pressure at which steam is available for further consumption.

• Usually it is —5% higher than process or turbine inlet pressure to account for the piping loss, that is, pressure losses in piping, bends, flow nozzle, and valves.

• In fixed-pressure boilers, this pressure is held constant as the turbine inlet valves vary the steam pressure to the inlet nozzles. In variable-pressure boilers, where steam is supplied directly to the turbine inlet nozzles, the SOP varies with load.

Drum Pressure

Drum pressure is the pressure at which the circulation of steam and water takes place in evaporator circuits. Drum pressure is —10% higher than SOP to account for steam pressure loss in SH and attemperator.

Design Pressure

In principle, the boiler design pressure is usually the lifting pressure of the second safety valve (or the highest set safety valve lifting pressure if there are more than two safety valves) on the drum and it forms the basis for the strength calculations of the boiler pres­sure parts (PPs) in drum-type boilers. The design pressure limits the maximum operating pressure of a boiler.

Reheater design pressure is the highest safety valve lift pressure (SVLP) on the inlet side of the RH. Boiler codes may vary slightly in defining the design pressures for boiler and RH.

RH Inlet and Outlet Pressures

These pressures are turbine-dependent based on the power cycle design. Unlike SOP, which is held constant in drum-type boilers, RH pressure slides downward at lower loads.

Calculation Pressure

Calculation pressure is the design pressure, adjusted for factors such as the static head and used for calculating the strength of individual components. There may be a single boiler design pressure, whereas the calculating pressures can vary for different boiler parts.

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