System Selection & Sizing
Selecting
the Proper Type of Priming System
Selecting the proper type of
system depends upon the application requirements of the pump or
pumps that require priming. There are two system types to consider,
On-Demand priming or Automatic priming. Each system type is also
available in either a simplex or duplex version.
On-Demand Priming Systems
On-Demand Priming Systems are used to establish a prime, usually to
a single pump, just prior to the need for it to start. These systems
are usually applied with pumps that are used infrequently or when
priming is required only when water levels drop below than is
normally encountered. A simplex design is usually employed for this
service.
System size is
based on the desired or acceptable amount of time to establish the
prime. Sizing involves determining the airflow necessary to evacuate
the volume of air in the suction pipe within the required time. All
aspects of the system design from layout of pump suction piping that
must be primed, to the priming valve size and type necessary for a
properly functioning system are considered. Figure 1 shows a typical
connection for an On-Demand system. The priming valve isolates the
suction side piping from the vacuum side. Typical priming valves
incorporate a float actuated closure against an orifice seat as
depicted in the system shown in FIG 1 above. These valves
need to be sized for the required flow rate of the system. When flow
rates are very high or when dealing with liquids having a tendency
to foam, a priming chamber such as shown in FIG 2 above is
better suited.
Automatic Priming
Systems
Automatic priming systems are used when one or multiple pumps are
required to start without delay on an as needed basis. The standard
design for these systems is to automatically maintain a prime
through pressure control of the system’s operation. Each
connected pump and any isolated high points in the suction piping
requires a priming valve. The priming valve has a float actuated
seat to isolate the priming system from suction piping. Through
pressure control operation, the priming system maintains a vacuum
between a high and low set point to the connected system’s priming
valves. The low set point is always greater than the vacuum required
to fully prime all pumps connected to the system.
Level controlled systems are also an option. The
application of these systems is suitable when all pumps needing a
prime are located at the same level and take their suctions from the
same level. The system automatically cycles on and off to maintain a
water level range within the receiver, and eliminates the
requirement of priming valves. Variations in the operating
configurations of these two system types are available to meet the
needs of special installations.
FIG 3
above shows a typical layout for a pressure controlled automatic
priming system. With this type of system there are few
restrictions to where pumps are located or whether suctions are
taken at different levels. With proper consideration in the layout
of the interconnected vacuum piping between the priming system and
the pumps, the priming system can be located wherever is most
convenient.
Automatic
priming systems are available in either simplex or duplex
versions. Typically duplex versions are selected for their inherent
back-up capability to serve critical applications, or as a central
system maintaining prime to several pumps. With both vacuum pumps
operating, duplex systems can be sized to provide a relatively rapid
initial prime to all connected pumps. The system will then
automatically alternate between vacuum pumps to maintain prime.
Control of vacuum pump operation in our standard duplex systems can
also be set to a designated lead-lag operation.
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