Calderas

Shell boilers are made for capacities up to around 27 000 kg / h of steam. When loads in excess of this are required, two or more boilers are connected in parallel, with an installation of four or more boilers not being uncommon.
The design of the interconnecting steam header is highly important.

Figure 3.8.1 shows a common method of connecting four boilers: a method that is frequently asource of problems.
Fig. 3.8.1
Common four boiler layout - not recommended
Referring to Figure 3.8.1, with all boilers operating at the same pressure, the pressure at point A has to be less than that at point B for steam to flow from point B to the plant. Consequently, there must be a greater pressure drop between boiler number 4 and point A than boiler number 3 and point B, whilst the differencein these two pressure drops takes place between A and B.

Flow depends on pressure drop, it follows then, that boiler number 4 will discharge more steam into the common header than boiler number 3. Likewise, boiler number 3 will discharge more than number 2, and so on. The net effect is that if boiler number 1 is fully loaded, the other boilers are progressively overloaded, the effect worseningnearer to the final off-take.

It can be shown that, typically, if boiler number 1 is fully loaded, number 2 will be around 1% overloaded, number 3 around 6%, and number 4 around 15% overloaded. Whilst shell boilers are able to cope with occasional overload conditions of 5%, an overload of 15% is undesirable. The increased steam outlet velocity from the boiler creates an extremely volatilewater surface, and the level control system might fail to control.

At high loads, in this example, boiler number 4 would lock-out, throwing an already unstable system onto the three remaining boilers, which would soon also lock-out.

The main observation is that this design of distribution header does not allow the boilers to share the load equally.

The aim should be that the pressure dropsbetween each boiler outlet and the header off-take to the plant should be within 0.1 bars. This will minimize carryover and help to prevent overload and lockout of boilers.

The layout shown in Figure 3.8.2 shows an improved design of a new header.

Fig. 3.8.2
Four boiler header design - improved layout

The header is arranged to discharge from the centre, rather than at one end. In thisway, no boiler will be overloaded by the header by more than 1%, providing the header pipe work is properly sized.

A better arrangement is shown in Figure 3.8.3 for an installation of four or more boilers, rather like a family tree, where the load on each boiler is spread equally. This arrangement is recommended for heavily loaded boilers, with sequencing control where one or more is regularlyoff-line.

It is emphasised that correct header design will save much trouble and expense later.

Correct boiler header design on multi-boiler applications will always result in a well-balanced operation.

Fig. 3.8.3
Four boiler header design - recommended layout
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Steam off-takes
Havingconsidered the general arrangement of the steam header, the following conditions need to be ensured:
* That dry steam is exported to the plant.
* That the warm-up operation is properly controlled.
* That steam is properly distributed to the plant.
* That one boiler cannot accidentally pressurise another.
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Water carryover
When awell-designed boiler generates steam under steady load conditions, the dryness fraction of the steam will be high, approximately 96 to 99%. Changes in load that occur faster than the boiler can respond will adversely affect the dryness fraction. Poor control of boiler water TDS, or contamination of boiler feedwater, will result in wet steam being discharged from the boiler.

A number of problems...