Priming System Sizing
Proper sizing of a vacuum priming system is based on the required volume of air to be removed from the suction piping within the acceptable period of time. Sizing also depends on whether operation of the system is On-Demand, where priming is done before each time a pump needs to start or Automatic priming systems that maintain a constant prime for immediate start-up.
Proper sizing of a vacuum priming system is based on the required volume (V) of air to be removed from the suction side piping and the required lift (h) of the water from the reservoir to the centrifugal pump. The required vacuum (PREQ) to establish a prime is determined from the following simplified formula.
PREQ = PATM – ρ x g x h
at standard S.L. conditions we have
PREQ (“Hg absolute)= 29.92 – 0.88 x Feet of Lift
Sizing for On-Demand Priming Systems
With On-Demand Priming Systems, priming is done before each time a pump needs to start. On-Demand Priming Systems are sized based on being able to establish a prime within a given amount of time (T). To determine the required average system capacity (QAVG) in ACFM we use the following formula.
QAVG = V x PATM x ln(PATM/PREQ) / (T x (PATM-PREQ))
NOTE: Pressure values are to be in absolute units, volume (V) in Ft3,
time (T) in minutes, and ln is the natural log.
Sizing for Automatic Priming Systems
Automatic systems maintain a prime to one or several (Central System) pumps for immediate start-up whenever needed. With automatic priming systems, once the system is primed, minimal vacuum pump capacity is required to maintain the prime. However, the priming system needs to be sized with sufficient capacity to be able to establish the initial prime within an acceptable period of time. Our systems are generally sized to establish the initial prime within 10 to 20 minutes. Once the initial prime has been established, the system automatically maintains a prime state for immediate startup of all connected centrifugal pumps as they are needed.
Proper system sizing for most applications can be determined by using the table below along with the following guidelines:
1.Calculate the total volume of the system including the suction pipe, casing volume of the pumps that needs to be primed, and discharge piping to the stop valve.
2.From the table, select the corresponding model, simplex or duplex, which correlates to your calculated system volume.
3.For priming (5) or more centrifugal pumps, select the next larger primer size.
4.For lifts greater than 15ft. priming times will be longer and you may want to select the next larger primer size. For lifts greater than 25ft. or any special applications, consult with a LYNN engineer for assistance with the proper selection.
There are not any special sizing requirements for systems using liquid ring vacuum pumps. Their use does however require a continuous source of clear water for sealing purposes. If a separate water source is not readily available, the priming system can be furnished as a totally recirculated system. Such systems require more in-depth sizing than is covered here.
1) System models for liquid ring vacuum pumps are based on the use of Gardner Denver pumps. System capacity is approximate and subject to vary with the selection of alternate vacuum pumps.
2) Systems with these larger tanks are furnished with vacuum pump(s) mounted on separate base next to receiver tank. These larger systems typically use vertical receivers for reduced footprint.
Receiver tank size shown is the standard size for a particular model. Depending upon the specifics of the application other receiver sizes may be suitable. For further assistance with design and selection of a vacuum priming system for your application contact an expert at LYNN.