What is a steam accumulator?
Steam Accumulator is an equipment which accumulates energy in form of pressurized hot water which can be used later when the steam demand of the process goes above the boiler capacity. The accumulator stores energy in the form of pressurized hot water which flashes to produce steam at lower pressure for the process. The accumulator is constructed in form of a horizontal cylindrical steel tank with dish-ends and control instrumentation to control the charging and discharging cycle with level controls.
Why steam accumulators are used?
Many processes demand peak steam loads, which are three to four times the average loads. These peak loads last for a few minutes. To size and select the boiler to cater to these peak loads will lead to gross over-sizing of the boiler and result in highly inefficient operation for most operating cycles.
The modern boilers are small in size, efficient and highly responsive to change in loads. However, for them to operate at an optimum efficiency, it is advisable to operate them close to the rated capacity.
Boilers can cater to fluctuations in steam demand to some extent. Energy is stored in boilers in form of hot water at its saturation temperature. If the process steam demand suddenly increases, the pressure inside the boiler drops and this hot water will flash off generating the steam to meet the high steam demand.
However modern boilers have limited water holding capacity, which in turn means that they are not capable of coping up with very high or peak steam demand of the plant (which generally exists for a short duration of time).
As a result of this, the boiler pressure drops down and it may drop below the recommended operating pressure range of the boiler, which leads to problems like
The problem of boiler pressure drop due to peak steam demand can be solved by installing surplussing valve on steam main. This valve maintains the boiler pressure when load increases by throttling down the valve which reduces the flow rate of outgoing steam.
- Water carryover (wet contaminated steam) resulting in poor quality steam supply
- Increased maintenance/ operating costs
- Reduced boiler life and efficiency
- Increased process time and reduced product quality
- In critical cases, boiler may lock out due reduced level of water inside the boiler.
However this only solves issue of pressure drop at boiler plant and this method is not useful in meeting the peak steam demand of the plant.
Also to overcome the problem of meeting peak steam demand, one may think of installing an over-sized boiler or multiple boilers. This results in low operational efficiency and high costs. The main reason for low efficiency in such cases is the radiation loss which is inevitable with over-sized boilers.
If the boiler is designed for peak steam demand (over-sized boiler), which is generally for duration of time, then it means that during off-peak time, the full capacity of boiler will not be used which in turn leads to unnecessarily high radiation losses, this is due to the fact that radiation losses will be constant irrespective of the steam flow rate.
So the best solution to the issues of meeting peak steam demand and also at the same time maintaining proper operating pressure at the boiler is to use the Steam Accumulator along with the boiler.
Working principle of steam accumulators
When the saturated water at high pressure is exposed to low pressure, some amount of this water will flash off into steam by using excess sensible heat in the water. The proportion of flash steam (kg of steam/ kg of water) depends on the pressure difference to which the hot water is exposed.
When steam demand from the plant is low, and the boiler is capable of generating more steam than required (i.e. at maximum continuous rating of boiler), the surplus steam available is injected into a mass of water stored under pressure in the accumulator.
After a while, the temperature of stored water will increase to saturation temperature with respect to the pressure at which the boiler is operating. And when steam demand is high such that it exceeds the maximum capacity of the boiler, then this leads to drop in pressure at accumulator which in turn leads to flashing of some proportion of water into steam, thus fulfilling the high steam demand without affecting the normal operation of the boiler.
Charging of steam accumulators
Accumulator is a cylindrical pressure vessel partially filled with water. The surplus steam during off-peak is charged into the accumulator below the surface of water by a distribution manifold on which a series of steam injectors are fitted. The charging continues until the entire water reaches the required pressure and temperature. The charging can be done by either saturated steam (wet steam) or supersaturated steam.
If the accumulator is charged by saturated steam then there is possibility of slight increase in level over the desired level due to the condensation of steam caused by radiation losses. Here a steam trap can be used as a level limiter by fitting it at the maximum allowed level. And if the charging is done by super-heated steam then it results into evaporation of water leading to reduction in level of water, and in this case to compensate the reduction in water level, a feed pump along with the water level sensor would be required
Discharging of steam accumulators
If the load demand is higher than the boiler capacity, then it leads to pressure drop in the steam accumulator where the water is stored at saturation temperature. So due to the pressure drop at accumulator, flash steam will be generated compensating the high load requirement without effecting the normal operation of boiler.
Once overload situation ends, then it is followed by off-peak load which allows surplus steam to be charged into the accumulator so that accumulator will be ready to handle the next overload situation. So in this way the accumulator allows the boiler to work with its desired operating pressure and maximum efficiency.