Water System Design Manual, December 2009
Document source : www.doh.wa.gov
Page 100
December 2009
Water System Design Manual
9.0.2 Operational Storage
OS is the volume of the reservoir devoted to supplying the water system while, under normal
operating conditions, the sources of supply are in "off" status (WAC 246-290-010). This volume
will vary according to two main factors:
1.
The sensitivity of the water level sensors controlling the source pumps.
2.
The configuration of the tank designed to provide the volume required to prevent
excessive cycling (starting and stopping) of the pump motor(s).
OS is in addition to the other storage components. When the reservoir is full, OS provides a
safety factor beyond that provided by the ES, SB, and FSS.
There are various water level sensors, including float switches, ultrasonic sensors, and pressure
switches. Some can detect water level changes as small as a fraction of an inch. Others require
more than a foot. Tank designers must account for the type of level sensor they used to determine
the vertical dimension needed for proper operation of the device. Manufacturer's specifications
generally govern the determination of this dimension.
After selecting the pump control device, the tank designer can use the vertical dimension to
determine other aspects of tank configuration, such as the width, height, and shape. The OS
volume should be sufficient to avoid pump cycling in excess of the pump motor manufacturer's
recommendation. In general, limit the motor to no more than six starts per hour. However, many
manufacturers warrant more frequent cycling for their pump motors, depending on the size of the
pump.
The OS volume in this situation is comparable to the withdrawal volume required when using
hydropneumatic tanks for pump motor protection. The Recommended Standards for Water
Works recommends that the gross volume of the hydropneumatic tank, in gallons, be at least 10
times the capacity of the largest pump, rated in gpm (Ten State Standards 2007). The withdrawal
volume of a hydropneumatic tank is usually about 25 percent of the gross volume. Using this
relationship, DOH recommends that the OS volume be about 2.5 times the capacity of the largest
pump. Calculating the OS volume will verify that typically, for gravity storage tanks, it is
substantially less than the remaining volume of the tank. The volume associated with the
elevation difference required for the pump level sensors is usually larger than that required for
pump motor protection, so that volume becomes the limiting factor when determining the
required OS volume.
OS does not apply to water systems operating under a continuous pumping mode (see Section
9.0.3). This operational mode protects the pump motor. The designer needs to consider only the
other components of effective storage (ES, SB, and FSS).
Summary :
Page 100 December 2009 Water System Design Manual 9.0.2 Operational Storage OS is the volume of the reservoir devoted to supplying the water system while, under normal operating conditions, the sources of supply are in "off" status (WAC 246-290-010). The volume associated with the elevation difference required for the pump level sensors is usually larger than that required for pump motor protection, so that volume becomes the limiting factor when determining the required OS volume.
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tank,water,required,leel,motor,sensors,storage,hydropneumatic,manufacturers,other,dimension,cycling,factor