Sizing a steam accumulator
Steam accumulators play a crucial role in steam systems by enhancing storage capacity and enabling efficient management of steam demand. These devices store and release steam when necessary, ensuring a steady supply. It's important to design the steam accumulator correctly to accommodate varying flow rates without encountering theoretical limitations, although practical constraints may limit its size.
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The practical volume of a steam accumulator is determined by the storage needed to meet peak demands, accounting for acceptable pressure drops while delivering clean, dry steam at an appropriate release velocity. The calculation for steam capacity in a horizontal steam accumulator is illustrated in Example 3.22.2.
Example 3.22.2
Boiler:
Maximum continuous rating = 5,000 kg/h
Normal working pressure = 10 bar g (hf = 781 kJ/kg, according to steam tables)
Burner switching differential = 1 bar (0.5 bar on either side of 10 bar g)
Plant requirements:
Maximum instantaneous overload = 12,000 kg/h
Distribution pressure = 5 bar g
To size the accumulator, even though the maximum instantaneous overload is 12,000 kg/h, the mean overload should be used to prevent unnecessary oversizing. Additionally, identifying the mean 'off-peak' load is essential for the calculations, as this pertains to any load below the boiler's maximum continuous rating (MCR).
Finding the mean value of the overload and off-peak load
Three methods exist for establishing mean loads in existing boiler systems:
- Estimating based on experience.
- Reviewing existing boiler steam output charts to determine mean loads and their occurrence times.
- Using a steam meter's computer to integrate steam load across both overload and off-peak periods.
While the first method may lead to oversizing if the accumulator proves insufficient, an educated guess is often necessary during the design stage, based on the knowledge of the system to produce an estimated maximum load and load diversity.
The second method is straightforward and typically results in reasonably accurate values. The third method, although more complex, provides the best accuracy and is cost-effective in relation to the overall investment for an accumulator project.
The following procedure details how to deduce mean steam loads using load pattern data. Referencing Figure 3.22.4 will aid understanding of Example 3.22.2.
From Figure 3.22.4, we note the off-peak loads are segmented into specific mean loads and time frames, which helps in calculating the mean surplus load for each off-peak period.
The mean surplus flow can be computed as outlined.
1st off-peak load
2nd off-peak load
A parallel calculation should be performed for overload durations referencing Figure 3.22.4.
1st overload
2nd overload
When designing, the accumulator's pressure is usually set 1 bar above the distribution pressure, ensuring adequate flash steam generation without excessively oversizing the downstream pressure reduction valve (PRV). In this example, with a distribution pressure of 5 bar g, the design pressure of the accumulator could be set at 6 bar g, assuming the water mass remains at boiler working pressure.
With this data, we can proceed to size the accumulator.
Steam accumulator:
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Consider that 2,797 kg of flash steam will be released as the pressure drops. Should this process occur within one hour, the steaming rate is 2,797 kg/h; over 30 minutes, the steaming rate would be:
If the steam accumulator links to a 5,000 kg/h rated boiler, fulfilling average demands, the total output from the combined systems could sustain average overload conditions of 10,594 kg/h for 30 minutes. As an alternative, additional boiler setups could generate 10,594 kg/h for half an hour, considering previous limitations.
Verification of the accumulator size is now feasible.
Utilizing the numbers from Example 3.22.2 to facilitate this check:
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 provided by the boiler
Required steam storage = 5,300 kg/h
However, since steam is needed only for 30 minutes each hour, the actual steam storage needed is:
The volume of water required to generate 2,650 kg of steam relates to the amount of flash steam produced from pressure drop.
This satisfies the need for sufficient water to create the required flash steam since the accumulator's storage capacity of 2,797 kg exceeds the 2,650 kg of steam storage needed.
If the steam accumulator operates at a charged pressure of 10 bar g and discharges at 6 bar g, the proportion of flash steam results can be determined as follows:
The vessel’s capacity is 87.9 m3, confirming its adequacy for this criterion.
Using earlier specified dimensions, water surface area achieves approximately 20.53 m2 when fully charged at 90% of total vessel volume.
The maximum steaming rate from the accumulator measures at 5,300 kg/h, thus:
Empirical testing illustrates that dry steam release rate is pressure dependent. A useful approximation states:
Maximum release rate without steam entrainment (kg/m2 h) = 220 x pressure (bar a)
Following this model, the steam accumulator in Example 3.22.2 operates at 6 bar g (7 bar a). Thus, the maximum release rate without steam entrainment is calculated as:
220 x 7 bar a = 1,540 kg/m2 h
This is visually represented in Figure 3.22.5.
The example's rate of 258 kg/m2 h is comfortably beneath the maximum threshold, ensuring the expectation of dry steam delivery. Should the steam release rate exceed limits, re-evaluating vessel dimensions for equivalent volumes would be necessary.
It's critical to underscore that these insights serve only as guidelines; specific design nuances should be entrusted to specialized manufacturers.
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