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Accumulator

An accumulator is a vessel which is partly filled with liquid and partly with gas (often air); its internal pressure is generally higher than atmospheric pressure. Accumulators store fluids to be handled under increased pressure (e. g. in pressure booster systems) in order to attenuate surge pressures and serve as energy storage devices to prolong the run-down time of centrifugal pumps. A transient flow analysis determines the accumulators' size and the valves, compressed air supply connections and instrumentation used.

Accumulators for automatic pressure control in water supply systems (see Pressure booster system) are usually installed vertically; horizontal installations are rare. See Fig. 1 Accumulator

Fig.1 Accumulator: Automatic pressure control in water supply systems                                                                                                                   Fig.1 Accumulator: Automatic pressure control in water supply systems

Accumulator size is determined by the pump set's number of starts per hour (Z). The number of start-ups depends on a variety of factors; information on the frequency of starts should be obtained from the electric motor suppliers (see Frequency of starts).

At start-up pressure (pe), the lowest water level selected must ensure that air can under no circumstances enter the discharge line. The accumulator volume (V) should therefore be selected so that it is 25 to 40 % larger than the effective accumulator volume (J) required. A compressed air shut-off valve may be provided as an additional component. Its purpose is to prevent compressed air entering the discharge line. In the case of unfavourable piping layouts (e. g. in domestic water supply systems) and horizontal vessels, the water level must be checked; if necessary, the connection must be placed at a lower level (e. g. dome).

A safety allowance of 25 % is included in the equation given below for accumulator sizing.


Druckbehälter_Formel_1

Accumulator: correction value K Fig. 2 Accumulator: correction value K


The proportion of usable water volume (S) in relation to total volume (V) depends solely on the start-up and stop pressures and can be calculated as follows:
Druckbehälter_Formel_2

In set-ups with more than one of the same pump, increasing the number of starts and stops by periodically switching between the pumps allows a reduction of accumulator size. Membrane-type accumulators are often provided for smaller units; these eliminate the need for a compressed air shut-off valve or a compressor. In this case, an extra 25 to 40 % of volume in addition to the effective volume (J) is not required.

The number of pumps in a pressure booster system has no bearing on the calculation of the accumulator volume. If several pumps with different flow rates are employed, the mean flow rate of the largest pump should be used in the equation. For systems in which several pumps are flow-controlled, and only the base load pump is started and stopped as a function of pressure, the accumulator size should be calculated in relation to this base load pump.

A sub-division of the calculated accumulator volume between several accumulators is desirable if such smaller vessels can be accommodated more easily in the available space, and the system costs are thereby reduced. When dividing the volume between two accumulators, the pressure settings for pump start-up and stopping can be set in such a way that the second accumulator is filled with air only.

If the volume is divided between more than two accumulators, these must be connected via the gas (air) side to ensure that each accumulator is evenly used. See Fig. 3 Accumulator


Accumulator: Schematic for a water supply system as pressure booster system                                                    Fig. 3 Accumulator: Schematic for a water supply system as pressure booster system

As a proportion of the accumulator's air content is gradually absorbed by the water under pressure, the compressed air in the vessel must be topped up from time to time, usually by means of a compressor. The compressor size is determined by its suction capacity (Qk). Compressor selection depends on the time (T) required to fill the whole accumulator volume. It is assumed that only two thirds of the accumulator volume (which corresponds to the water level at stop pressure) must be filled with compressed air. The filling time should not exceed eight hours.
The suction capacity in m3/h is:
Druckbehälter_Formel_3

The compressor's operating pressure should as a minimum correspond to the pump's maximum stop pressure. The safety valve on the compressor must be pre-set so that the maximum permissible operating pressure of the accumulator is not exceeded.

In accordance with the accident prevention regulations for pressure vessels (German Gas and Waterworks Professional Association, Düsseldorf), fitting a safety valve on accumulators for centrifugal pumps is not mandatory as long as the H/Q curves (see Characteristic curve) of the pumps do not exceed 1.1 times the maximum permissible operating pressure for the vessel, and steps are taken to prevent critical overspeeding of the pumps.

The accumulators are welded, cast, riveted and, occasionally, finished in strip-wound construction (for very high pressures and temperatures in the chemical industry). The materials used are steel plate (boiler plate), non-ferrous metal plate, cast steel and plastic. The design and operating data of commonly used accumulators are standardised.

Principal standards, directives and regulations applicable to accumulators

  • American Petroleum Institute: API 610
  • American Society of Mechanical Engineers:
    ASME-Boiler and Pressure Vessel Code Section I-X
  • German Pressure Vessel Society:
    AD regulations
  • Federal Ministry of Economics: Protection of Labour Act (Federal Bulletin 4/1980) and Steam Boiler and Pressure Vessel Act
  • DIN 3171, DIN 4661, DIN 4810 and EN 962
  • German Organisation for Technical Standards in the Gas and Water Industries (DVGW): DVGW Worksheet W 314
  • TRD Technical Rules for Steam Boilers
  • German Federation of Technical Supervision Associations
  • Regulations of shipbuilding classification societies, e. g. German Lloyd (GL)