A duplex or a system with two pumps is designed to have a backup or spare pump. When an alternator is in the system, you effectively do not have a spare pump. As the person who has been called to repair these pumps over many years, I have learned that there is very little time to repair pump #1. The alternator has made sure that pump #2 has exactly the same number of cycles and the same number of running hours as pump #1. Usually before pump #1 is repaired, pump #2 has also failed.
Alternators were designed to split the number of cycles in half for each pump by switching back and forth between the primary and the secondary pump. This means that a pump that would normally cycle 100 times per day only sees 50 cycles per day because the "spare" pump also cycles 50 times a day. Water towers and hydro tanks should never be used for water storage but only to reduce the cycling of the pump or pumps. Even with very large tanks cycling can still be a problem.
More modern pump control systems such as the Cycle Stop Valve completely eliminate the cycling that has been associated with hydro tanks and water towers. The pump produces exactly the same amount of water as is being used by maintaining a constant pressure. (see What Are Cycle Stop Valves and Constant Pressure Valves) When cycling has been eliminated, alternators are no longer needed. The primary pump will last much longer running continuously (24/7) than if allowed to cycle 50 or any number of times per day. The secondary pump does need to be exercised occasionally to prevent bearing failure and to circulate the water in the pump and related piping. By exercising this secondary pump only as much as needed, say five minutes once a week, this secondary pump will still be like brand new if the primary pump fails. Now the secondary pump is a reliable back up. When pump #1 is out for repair, there is no need to worry that pump #2 is working on its last leg. Plenty of time is available to repair pump #1 because, pump #2 has been kept in like new condition instead of matching pump #1 cycle for cycle and hour for hour.
When used with a Cycle Stop Valve the secondary pump is simply staggered on at a slightly lower pressure than pump #1. An exercise clock can be used to turn on pump #2 for a few minutes once a week. The Cycle Stop Valve on pump #2 having been set at a lower pressure than the Cycle Stop Valve on pump #1, will only allow this pump to produce 5 GPM for the 5 minutes of run time required by the exercise clock. When this extra 5 GPM is forced into the system from pump #2, the Cycle Stop Valve on pump #1 will simply restrict the output of pump #1 by 5 GPM. When the exercise clock turns off pump #2, the Cycle Stop Valve on pump #1 will automatically increase flow by 5 GPM to again exactly match the demand required by the system.
The pressures should be staggered to make pump #2 come on at a slightly lower pressure than pump #1. This insures that if for any reason pump #1 fails, the drop in pressure will automatically start up pump #2. This staggered pressure also insures that during normal operation, if more water is needed than can be supplied by pump #1, the pressure will drop and pump #2 will come on and automatically supply the extra amount of water needed. In other words, if pump #1 is a 100 GPM pump and the system is requiring 110 GPM, pump #2 will be started and the Cycle Stop Valve will make it add 10 GPM to the 100 GPM already coming from pump #1. As long as both pumps are working, this system would be capable of supplying 200 GPM if needed.
Alternators were a band-aid to help reduce cycling from old style systems that used hydro tanks and water towers. Constant pressure systems do not have rapid cycle problems. The secondary pump can now be a reliable backup because of the elimination of the alternator.
Alternate Between Two Pumps Without an Alternator
Two well system for a subdivision.
Pump #1 is a 3 HP 30 GPM submersible.
Pump #2 is a 5 HP 50 GPM submersible.
We want pump #1 to be shut off anytime pump #2 is required and can handle the demand by itself.
Cycle Stop Valve set 50 PSI.
Pressure Switch ON 50 PSI.
Pressure Switch OFF 60 PSI.
Cycle Stop Valve set 60 PSI.
Pressure Switch ON 45 PSI.
Pressure Switch OFF 70 PSI.
Small demands will be met by pump #1. Each pump needs it's own pressure tank. When a tap in the system is opened the tanks will release water as pressure drops from 60 PSI to 50 PSI. At 50 PSI pump #1 is started and the Cycle Stop Valve on this pump will maintain 50 PSI on the system as long as demand is between 5 GPM and 30 GPM.
If demand goes to 0 GPM the Cycle Stop Valve on pump #1 will allow 5 GPM to fill the pressure tanks in the system to 60 PSI and the pump will be shut off. If the flow required increases above 30 GPM, pump #1 is no longer able to keep up with demand. The pressure in the system will drop to 45 PSI and pump #2 will be started. The Cycle Stop Valve on pump #2 will bring the system up to 60 PSI and pump #1 is shut off. The Cycle Stop Valve on pump #2 will maintain 60 PSI on the system as flow rates vary from 5 GPM to 50 GPM.
If the flow required increases above 50 GPM, pump #2 will no longer be able to keep up and the pressure will drop back to 50 PSI starting pump #1 again. If for instance 65 GPM is being used in the system, pump #2 will be pumping 50 GPM and the Cycle Stop Valve on pump #1 will maintain 50 PSI on the system by providing exactly 15 GPM extra. With both pumps running there will be up to 80 GPM available at 50 PSI for peak demands on the system.
When the flow required falls to less than 50 GPM, the Cycle Stop Valves on both pumps will allow the tanks to slowly fill to 60 PSI and pump #1 will be shut off. When demand is reduced to less than 5 GPM the Cycle Stop Valve on pump #2 will allow the system pressure to increase to 70 PSI and pump #2 will also be shut off. The next time water is required in the system, pressure from the tanks will fall to 50 PSI and pump #1 is again started. The above process is then repeated as required.
There are two possible problems with this application. The first problem could arise if the flow required never exceeds 30 GPM. If this is the case, pump #2 will never be used unless a timer is used to shut down pump #1 occasionally. Pump #1 could also be turned off manually. Any way pump #1 is shut off, pump #2 would automatically start and take care of the system.
The second problem could arise if the demand never decreases below 5 GPM. When demand increases above 30 GPM pump #2 is started which shuts down pump #1. If flow never decreases below 5 GPM, pump #2 will never be able to build the system pressure up to 70 PSI so that pump #2 will shut down and switch the system back to operating on pump #1. This can be easily solved by installing a bypass line around the Cycle Stop Valve on pump #2. This bypass can be adjusted to allow 10 to 20 GPM to go around the Cycle Stop Valve. When 20 GPM is bypassing the Cycle Stop Valve, anytime the system is using less than 20 GPM the system pressure increases to 70 PSI and pump #2 is shut down. This bypass can be adjusted as needed to make sure that pump #2 will be able to shut off.