Sizing and Capacity Split
Once the pump differential pressure and system capacity have been calculated it is necessary to determine the number, size, and horsepower of the pumps and other components that will best suit the application.
Water usage studies indicate that for typical installations, water demand is less than 20% of peak demand for more than 70% of the time. Because of this system characteristic, a multiple pump system or a multiple pump and hydro-pneumatic tank system can considerably diminish operating expense. These systems select the smallest suitable pump horsepower to deliver the demand required.
For reasons of first cost, floor space, maintenance and efficiency the industry standard for booster systems has evolved to include two or three constant speed pumps, automatically sequenced on and off in accordance with the building water demand with constant pressure being achieved by means of pressure reducing valves.
In addition, booster systems can be designed to operate continuously or on an intermittent basis. Most applications require little or no water during some part of each day, depending on the type of building; and therefore, it is most economical to automatically turn off the booster during these periods. A hydro-pneumatic tank must be installed in the system when the booster system will be operated intermittently. The tank will provide low system demand and will maintain system pressure while the pumps are off preventing pump short cycling, reducing pump wear and providing energy savings.
Canariis offers three basic designs (simplex, duplex or triplex) with each design having the option of a hydro-pneumatic tank for system shut down during low or no flow conditions.
When deciding which design best suits your needs, the peak demand, purchase price, installation expense, operating cost, type of building and designer preference must all be considered.
When asked for our recommendations we offer the following guidelines:
- Duplex systems are commonly used for system capacities up to 300 GPM and horsepowers up to 15 HP per pump; however, larger flows can be considered depending on the demand cycle, split selected, and required reliability.
- Triplex systems are recommended where system capacities are 300 GPM or greater or where operating efficiency, reserve capacity and utmost reliability are important considerations. Triplex systems should be considered at lower system capacities if the system is continuous run or has infrequent low or no flow periods.
- Duplex or triplex systems with a system capacity of 500 GPM or less, that have frequent periods of low or no flow demand should be installed with the shutdown feature using a hydro-pneumatic tank in the system. Low flow and no flow conditions can prevail up to 80% of the time for schools, 65% for office buildings, 40% for apartments and dorms, and 30% for motels etc.
- The industry standards for capacity splits for duplex and triplex systems are as follows, and are shown as a percent of system capacity for each pump. If flush valve fixtures are used, the lead pump should be sized for a minimum capacity of 50 GPM.
Duplex System Capacity
| P1 |
P2 |
REASONS FOR CHOICE |
| 33% |
67% |
This split is usually the most economical duplex to operate. It is especially suited for applications that have long periods of low demand. |
| 50% |
50% |
This is the most common duplex arrangement. It allows for flow greater than 33% with one pump running as well as allowing pump alternation to equalize usage between pumps. Parts are interchangeable between the pumps. |
| 65% |
65% |
This arrangement provides benefits similar to the 50%-50% split as well as allowing one pump to run most of the time as very seldom do system demands exceed 65%. Because of this, the operating pump usually can provide an adequate supply of water should either pump require service. Parts are interchangeable between pumps. |
| 100% |
100% |
This arrangement provides 100% standby and can be considered on small systems up to 5 HP per pump. Pumps can be alternated and parts are interchangeable between pumps. |
Triplex System Capacity
| P1 |
P2 |
P3 |
REASONS FOR CHOICE |
| 20% |
40% |
40% |
This is the most common triplex arrangement and offers increased operating efficiency over a similar capacity duplex, as well as providing a minimum of 60% peak demand even if one pump is not operational. Alternation on P2 & P3 equalizes usage between these two pumps and provides for parts interchangeability. |
| 30% |
40% |
40% |
This arrangement provides the benefits above as well as providing a minimum of 70% peak demand with two pumps running and a peak capacity of up to 110% with all pumps running. |
| 50% |
50% |
50% |
This arrangement provides a minimum of 100% peak demand with two pumps running, alternation between all three pumps, one complete operational standby or a peak capacity of up to 150% with all pumps running. |
| 30% |
70% |
70% |
Provides a minimum of 100% peak with two pumps running and a peak capacity of up to 170% with all pumps running. Alternation on P2 & P3 equalizes usage between these two pumps and provides for parts interchangeability. |