Sizing a steam accumulator
A steam accumulator in the steam system gives increased storage capacity. Proper design of the steam accumulator ensures that any flowrate can be catered for. There are no theoretical limits to the size of a steam accumulator, but of course practical considerations will impose restrictions.
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In practice the steam accumulator volume is based on the storage required to meet a peak demand, with an allowable pressure drop, whilst still supplying clean dry steam at a suitable steam release velocity from the water surface. Example 3.22.2 below, is used to calculate the potential of steam capacity in a horizontal steam accumulator.
Example 3.22.2
Boiler:
Maximum continuous rating = 5 000 kg/h
Normal working pressure = 10 bar g (hf = 781 kJ/kg, from steam tables)
Burner switching differential = 1 bar (0.5 bar either side of 10 bar g)
Plant requirements:
Maximum instantaneous overload = 12 000 kg/h
Distribution pressure = 5 bar g
Although the maximum instantaneous overload is 12 000 kg/h, the mean value of the overload should be used to size the accumulator.
This prevents unnecessary oversizing of the accumulator. Equally, it is necessary to determine and use the mean &#;off-peak&#; load in the sizing calculation. Off-peak load is any load below the boiler MCR.
Finding the mean value of the overload and off-peak load
There are three possible methods to establish the mean loads for existing boiler plant:
- To guestimate, based on experience.
- To interrogate the existing boiler steam output charts to establish the mean loads and the time periods over which they occur.
- To program a steam meter&#;s computer to integrate the steam load over both the overload and off-peak load periods.
Method 1 could prove to be rather reckless, if an expensive accumulator ended up too small.
However, if the boiler plant is still at the design stage, an educated guess will be the only option. From the designer&#;s knowledge of the installation, it should be possible to give a reasonable estimate of the maximum plant load, the load diversity, and the times over which they occur.
Method 2 is quite easy to expedite, and should give a reasonably accurate result.
Method 3 would provide the most accurate results, and the cost of the steam meter is small relative to the overall cost of an accumulator project.
The following procedure shows how to determine the mean steam loads from an existing chart recording the load pattern. The procedure is built up from Figure 3.22.4, which shows the flow pattern for Example 3.22.2.
From Figure 3.22.4, it can be seen that the off-peak loads have been divided up into the following mean loads and time periods. From this data, the mean surplus load for each off-peak period can be determined.
The mean surplus flow is calculated in the following way:
1st off-peak load
2nd off-peak load
A similar exercise is carried out for the overload periods from Figure 3.22.4.
1st overload
2nd overload
The accumulator design pressure needs to be chosen, and it is usual to choose a pressure 1 bar higher than the distribution pressure. This gives a reasonable flash steam capacity, without unduly oversizing the downstream PRV.
In this example the distribution pressure is 5 bar g, so the accumulator design pressure can initially be considered at 6 bar g (Note: the water mass is taken at boiler working pressure).
From this information, an accumulator may now be sized.
Steam accumulator:
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Note that this 2 797 kg of flash steam will be released in the time taken for the pressure to drop. If this has been an hour, the steaming rate is 2 797 kg/h; if it were over 30 minutes, then the steaming rate would be:
If the steam accumulator is connected to a boiler rated at 5 000 kg/h, and supplying an average demand within its capacity, the combined boiler and accumulator outputs could meet average overload conditions of 5 594 + 5 000 = 10 594 kg/h for 30 minutes. The alternative is an additional combination of boilers capable of generating 10 594 kg/h for 30 minutes with the limitations previously noted.
It is now possible to check the accumulator size.
The figures as used in Example 3.22.2 are used below to facilitate checking.
Boiler
Maximum continuous rating = 5 000 kg/h
Normal working pressure = 10 bar g
Plant requirements
Largest mean overload = 10 300 kg/h for 30 minutes every 95 minutes
Pressure = 5 bar g
Required steam storage = 10 300 kg/h - 5 000 kg/h steam supplied by the boiler
Required steam storage = 5 300 kg/h
However, steam is only required for 30 minutes every hour, so the steam storage required must be:
The amount of water required to release 2 650 kg of steam is a function of the proportion of flash steam released due to the drop in pressure.
This satifies the criterion of having enough water to produce the required amount of flash steam. It can be seen that the storage capacity of 2 797 kg is greater than the storage required of 2 650 kg of steam.
If the steam accumulator will be charged at 10 bar g by the boiler, and discharged at 6 bar g to the plant, the proportion of flash steam can be calculated as follows:
The vessel capacity is larger at 87.9 m³, so the vessel satisfies this criterion.
Using the vessel dimensions given earlier, the water surface area is approximately 20.53 m² when fully charged, at a volume of 90% of the vessel capacity.
The maximum steaming rate from the accumulator is given as 5 300 kg/h, therefore:
Empirical test work shows that the rate at which dry steam can be released from the surface of water is a function of pressure. A working approximation suggests:
Maximum release rate without steam entrainment (kg/m² h) = 220 x pressure (bar a)
The steam accumulator in Example 3.22.2 is operating at 6 bar g (7 bar a). The maximum release rate without steam entrainment will be:
220 x 7 bar a = 1 540 kg/m² h
This is shown graphically in Figure 3.22.5.
The example at 258 kg/m² h is well below the maximum value, and dry steam can be expected. Had the steam release rate been too high, different diameters and lengths giving the same vessel volume would need to be considered.
It must be emphasised that this is only an indication, and design details should always be delegated to specialist manufacturers.
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