Category Archives: Technical Information

Rainwater Catchment, Technical Information

When are Diaphragm Pumps Used in a Rainwater Collection System?

Diaphragm PumpAlthough centrifugal pumps are the most common type of pump used in rainwater collection systems for both sump pumps and pressure pumps, diaphragm pumps could be used in some applications.

The diaphragm pump, or positive displacement pump, uses a flexible membrane that separates the pump housing into two separate chambers. The membrane is pushed or pulled by mechanical means to enlarge or collapse a chamber, forcing the fluid to discharge. Non return valves are used on both sides of the diaphragm to prevent back flow. Electrical, mechanical, or manual can be the power supply for a diaphragm pump. A hand pump is a piston type diaphragm pump and used throughout the world, where there is no source of a power supply.

Air, chemical, and liquid can be pumped with a diaphragm pump with high efficiency. However, because of the reciprocating mechanical action or “pulsating” that occurs, a diaphragm pump would not be suited where a “steady flow” is desired .  Diaphragm pumps are very efficient in suction lift and, in some cases, could effectively be used to transfer water from a sump well below the the desired cistern location.

A more common use for a diaphragm pump in a rainwater collection system is a small air compressor used to keep large amounts of stored water from going anaerobic or with chemical treatment such as chlorine, if required.

Rainwater Catchment, Technical Information

Should You Use an External or Submersible Pump in Your Rainwater Collection System?

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submersible-pumpAs mentioned in a previous post, centrifugal pumps are the most common type used in rainwater collection. Sizing of the pump is essential in finding the right pump to meet the needs for which the system was designed. A water cooled centrifugal pump is recommended for potable application. Friction, static head, lift,PSI, and GPM are factors that need to be considered in sizing your pump. Pumps are either submersible or external, and depending on the application, one could be better suited than the other.

Sump tanks are generally smaller than the cistern and their purpose is to convey water collected from the roof to a central collection tank, then pumped to storage. A submersible pump with a float switch is best suited for this application. As the sump tank becomes full of collected water from the down spouts, the float switch turns the pump on to transfer water to storage. When the water level in the sump drops the float switch to the down position, the pump turns off. Submersible pumps for this application “MUST” be water cooled. The use of an oil cooled submersible pump is not considered NSF for potable use. The water cooled, submersible centrifugal pump needs to lift to the top of the cistern, whether the plumbing is external or internal. A larger pump will be required if the pump is not able to meet the lift requirements. Unlike external centrifugal pumps, which need to be primed and maintain that prime during and after pump cycle, submersible pumps, if completely submerged, will maintain a constant prime for operation.

Submersible pumps can be 120 or 220 volt and can be AC or DC. Solar power with battery backup can be used for sump pumps allowing an “off grid” application. Care must be taken to insure that the pump’s electrical needs are met from the power supply in order to protect the pump from damage.

External centrifugal pumps are generally used to transfer collected water from storage (cisterns) to use. This can be a “on demand” booster pump or a dynamic pump with control of suction flow to a pressure tank. A booster pump needs to be primed before operation and able to maintain that prime after the pump shuts off. With an external centrifugal pump, suction lift is limited. This type of pump should not be higher than the stored water low level, allowing head pressure to feed the pump, rather than the pump pulling water up to its level. AC, DC, or solar can be used for pouring the pump. This style of pump is generally water, air cooled and should be protected from freezing.

Whether you are looking into a submersible or a external pump, centrifugal pumps offer a wide range of horse power, GPM and PSI. Sizing your pump will determine its efficiency and longevity.

Rainwater Catchment, Technical Information

What Pumps Work Best for a Rainwater Collection System?

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Depending on the desired use of your rainwater collection system, and tank configuration there are a variety of types of pumps that are best suited to meet the needs.

There are three classifications of pumps for conveyance of water: direct lift, displacement, and gravity pumps. Pumps can be operated by electricity, wind, or solar. Mechanical pumps can be submersible, or external. Centrifugal pumps are the most often applied in rainwater collection systems and are referred to as a “dynamic pump”.  A centrifugal pump is a dynamic pump and uses an impeller with back sweeping veins or arms and can be operated against a closed valve, allowing for an “on demand” or continuous energy.

Positive displacement pumps will deliver a constant amount of discharge no matter what the discharge pressure is and must not run against a closed valve on the discharge side. Damage can be a result if flow is restricted on the discharge side. Pump operation must be controlled by the suction side of the pump, stopping the flow to the pump. An electrical interruption of the pump operation is the method typically used.

Pumps used to move water for potable applications must meet “safe drinking water” standards and be lead free, and should also be water cooled. 220 or 120 volt pumps are available for what suits your electrical requirements. Solar powered pumps are becoming increasingly popular, combining rainwater collection and solar for a sustainable system

Sizing the pump by gallons per minute (GPM) and pounds per square inch (PSI) required for optimal operation of the pump is necessary. Volume of water required and what pressure is desired will determine the “size” of your pump. Diameter of pipe, both suction and discharge, friction, (turns & length) and restrictions all need to be considered in sizing a pump. Manufacturer’s pump curves are a “must” when selecting a pump.

The pump selected for your rainwater collection system is the heart of your system and careful consideration of intended use and requirements will ensure optimum performance, reliability, and life span.