Author Topic: Pumps in Parallel  (Read 1989 times)

Cary Austin

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Pumps in Parallel
« on: November 05, 2014, 04:50:36 PM »

 500 GPM and Multi-pump Video

 Pumps in parallel can be tricky to control. As the flow is reduced, whichever pump builds the most head is doing the entire job while the other pump or pumps are being deadheaded. Each pump is having to buck the pressure of all the other pump. If each pump has it’s own Cycle Stop Valve, each pump is only bucking it’s own back pressure, not that of the other pumps. Cycle Stop Valves or CSV’s can never completely close. The seat is designed to allow 5 GPM to pass, even when the valve is in the closed position. This keeps the pump cool without the need for a recirculation line. Since the valve never completely closes, it is also designed to react almost instantaneously. This high-speed reaction, would not be possible with fully closing type valves, without causing water hammer or transients.

 When maintaining a constant head or pressure by throttling a constant speed pump, the energy used by most pumps is almost identical to the energy used when the pump is slowed with a VFD.

 There are several ways to set up controls for multiple pumps. My favorite would be to stagger the pressures so that the smallest pump runs first and the largest last. Say for instance you require a minimum of 60 PSI. I would set the largest pump as pump 3 and have it come on at 60 and go off at 65 with a standard pressure switch. Pump 2 would come on 65 and off at 70. While pump #1 would come on at 70 and only go off at 75 if the demand ever got lower than 5 GPM. Each pump would have it’s own CSV, which would be set to hold a downstream pressure the same as that particular pump started.

 With across the line starters, when the demand increases, these pumps will instantly come on and instantly supply the correct amount of water needed. The demand is instantly met with the correct increase in supply. When the demand decreases, the CSV instantly reduces flow to match the new demand. If the system pressure increases by 5 PSI, the lower pressure pump is shut down as it is no longer needed.

 All this is done while each pump is guaranteed a minimum of 5 GPM for cooling purposes. I have systems with more than 10 pumps in parallel that are controlled this way and do not produce pressure transients on start up or shut down.

Multiple Pumps in Parallel

 Cycle Stop Valves can solve the problems associated with using multiple pumps in parallel. Most of the time one of the pumps will build slightly higher pressure than the other pump or the static water level in one well is slightly higher than in the other well. This causes the pump that builds the most pressure, even if it is only 1 PSI more, to create a dead head situation for the other pump. At a low flow rate the two pumps are working against each other and the pump that builds the least pressure will be destroyed due to a lack of cooling flow. This is one reason we do not recommend using a single Cycle Stop Valve for two or more pumps unless only one of the pumps is operated at any given time.

 When a Cycle Stop Valve is placed on the discharge of each pump before the lines manifold together, backpressure from the second pump does not affect the first pump. Each pump is then working against it's own backpressure. The non-closing feature of the Cycle Stop Valve insures a minimum flow to keep each pump cool, because the backpressure or inlet pressure to the Cycle Stop Valve is higher than the outlet or system pressure.

 Multiple pumps can be the most efficient way of supplying water. Systems with a wide variation in flow can benefit greatly from being able to utilize the pump or pumps that can best meet the particular flow required. Using a Cycle Stop Valve on each pump allows multiple pumps to operate in parallel safely and efficiently. The following are examples of how Cycle Stop Valves can control different type of multiple pump systems.

 Some systems use multiple pumps that are sitting side by side and pumping from the same water source to a common system. Other systems may use multiple pumps located in different locations and pumping from different water sources to a common system. Each pump needs it's own Cycle Stop Valve, check valve, and pressure switch. If the pumps are located in different locations, a small pressure tank is needed for each pump. If the pumps are located together they can use a single pressure tank plumbed to the common discharge of all Cycle Stop Valves. All of the pressure switches should be in a manifold together with the small line that enters the pressure tank. Once installed in this way the only connection between the pumps is that they pump into a common manifold and run on staggered pressure settings.

 For this example we will use a three pump system having a small, medium, and a large pump. A minimum of 40 PSI is required at all times and the large pump is set to come on at 40 PSI and off at 50 PSI. The medium pump comes on at 50 PSI and off at 60 PSI. The small pump will come on at 60 PSI and if the system flow ever gets below 5 GPM this pump will shut off at 70 PSI. Usually these large systems have more than 5 GPM leaking so the small pump will run continuously and it's Cycle Stop Valve will hold the system at a constant 60 PSI. The Cycle Stop Valves on multiple pump systems should be adjusted to hold pressure constant at the same pressure as its' pump starts.

 When flow increases and the small pump is no longer able to keep up, the pressure will drop from 60 PSI to 50 PSI and the medium pump is started. The Cycle Stop Valve on this pump maintains 50 PSI until flow increases beyond the capabilities of the first two pumps. The pressure then drops to 40 PSI and the large pump is started. The Cycle Stop Valve on the large pump will keep the system pressure at 40 PSI as long as the amount of water needed can be produced by the three pumps. When the system flow is decreased to a point that can be supplied by the small and medium pump, the Cycle Stop Valve on the medium pump will bring the pressure up to 50 PSI and the large pump is shut off. If the system flow continues to decrease to a point that can be supplied by the small pump, the Cycle Stop Valve on the small pump will bring the pressure up to 60 PSI and the medium pump is also shut off. The Cycle Stop Valve on the small pump will maintain 60 PSI supplying the leaks and small demands in the system. Only if there is zero flow will the Cycle Stop Valve on the small pump allow the system to increase to 70 PSI and the small pump is shut off. A pressure relief valve set at 75 PSI can be used for a safety. The largest pump runs at the lowest pressure of 40 PSI, therefore the tank should be precharged with air to 35 PSI.

 These pumps can be sitting side by side or they can be miles apart. The staggered pressure settings make all pumps work together when needed. There is no need for wires or radio controls between pumps. Even pumps that are miles from each other operate on system pressure only. If the pump that is running cannot keep up with the demand, the system pressure drops slightly and the next pump required is started by its' own pressure switch. As demand decreases, system pressure increases and pumps that are no longer needed are shut down by there own pressure switch. Cycle Stop Valves allow simple, safe, and efficient use of multiple pumps in parallel.
« Last Edit: November 05, 2014, 04:52:51 PM by Cary Austin »