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How Does a Metering Pump Work?

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How Does a Metering Pump Work?

Metering pumps, also called dosing pumps, are pumps that are designed to dispense specific amounts of fluid and measured flow control. They use expanding and contracting chambers to move the liquids. Metering pumps also have a high level of accuracy over time and can pump a wide range of liquids including corrosives, acids, and bases, as well as slurries and viscous liquids. They are used in various industries like manufacturing, agriculture, and medicine. There are a variety of types of metering pumps that work in different ways. For the purposes of this post, we’ll look at diaphragm and peristaltic metering pumps.

How Diaphragm and Peristaltic Metering Pumps Work

Both types of metering pumps – diaphragm pumps and peristaltic – are very useful and will typically provide many years of reliable, efficient operation.

Diaphragm Metering Pumps

Diaphragm pumps are positive displacement pumps that move liquids using a reciprocating diaphragm. They are found to be very reliable because they don’t have internal parts that rub together, creating friction and leading to wear and tear. Additionally, because they don’t require seals or lubrication in the pump head, there isn’t a chance of oil vapor contamination or leakage of the media being pumped.

Simple diaphragm pumps have a diaphragm, two valves, a displacement chamber, and a driving mechanism. The diaphragm is a flexible membrane that vibrates to create suction to move fluid in and out of the pumping chamber. It is located between the side of the displacement chamber and an attached flange. The two valves are usually flapper valves or spring-loaded ball valves that are made of the same material as the diaphragm. They operate by admitting the liquid in and out of the chamber. The driving mechanism is what activates the diaphragm into operation. There are a number of different driving mechanisms that diaphragm pumps may use. The two most common are air operated and motor driven.

Air operated diaphragm metering pumps use compressed air to drive a double diaphragm (two diaphragms) alternatively. A shuttle valve alternates the air flow between the two diaphragms. The flow of the media that is being pumped is adjusted by how much air pressure is supplied to the pump.

Motor driven diaphragm metering pumps uses the rotary motion of a motor, which is converted to a reciprocating movement via a cam mechanism, to cause a displacement in the volume of the liquid, transferring it at a consistent rate.

Peristaltic Metering Pumps

Peristaltic metering pumps, like diaphragm metering pumps, are positive displacement pumps. However, they operate quite differently. Peristaltic pumps use rotating rollers to squeeze a flexible tube to move the liquid in a pressurized flow. As the tube is constricted and the low-pressure volume increases, it creates a vacuum that pulls the liquid into the tube. The liquid is then pushed through the tubing as the tubing is constricted at several points by the rollers. With each oscillating or rotating motion, the fluid flows through the tubing. Peristaltic metering pumps are designed as either circular (rotary) or linear.

Benefits of Metering Pumps

Metering pumps, whether diaphragm or peristaltic, provide many benefits to the industries where they are used. They are reliable for dispersing the exact amount of liquid that is needed accurately and consistently. Additionally, you will find the following advantages when using metering pumps:

They commonly move low amounts of liquid – Because metering pumps are so accurate and precise, they are often used to move low amounts of fluid. They are typically measured by their capability to pump gallons per minute, instead of gallons per hour, which is an industry standard.

They can pump various types of liquid – Metering pumps are able to move a variety of fluids, from thin to thick, and even hazardous or corrosive chemicals.

They can be used for many different applications – Metering pumps are used in many different industries including medicine, food processing, agriculture, and manufacturing.

They prevent contamination – Both diaphragm and peristaltic metering pumps are effective in preventing the media being pumped from contaminating the pump and the workspace.

While metering pumps work effectively for many applications and different liquids, it isn’t recommended that they be used for moving most types of gases.

Pressure and back pressure

High pressures are no problem in metering systems as long as there is something to counter them. ProMinent hydraulic diaphragm metering pumps therefore use a hydraulic fluid to create back pressure. The benefits

posted Apr 2 by Bop02mo

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If you live in a town or city, you probably don't give much thought to how the water you use each day gets to your house. Even small villages often provide a network of supply pipes that transport water to each home in the neighborhood. All you need to know is how to open the tap at the sink.

 

Move a few miles out of town and the picture can change. While the inner workings are still—thankfully—invisible, your water supply is independent from the neighbor's down the road. Each home has its own well from which to draw water. More than that, each home has its own electromechanical system for getting the water from the well to the house. At the heart of each system is a pump, and the most common types are jet pumps and submersible pumps.

 

Well types

 

In many areas of the country, finding potable water is as easy as getting out a shovel and digging a hole in the ground. Okay, maybe "easy" isn't the right word, but wherever the water table is only several feet below the surface of the ground, part of the battle may already be over. In such a shallow-well situation, lifting the water up to the house is going to be a little easier, if only because the distance you have to move it is modest.

 

If your area doesn't have a high water table, or if it lacks a stable supply of potable water near the surface, you must dig deeper to achieve the same result. And because a deep well means that the water has to be lifted farther, the strategies for moving it change.

 

Shallow-well pumps

 

These days, the most common pump for a shallow well is a jet pump. Jet pumps are mounted above the well, either in the home or in a well house, and draw the water up from the well through suction (see Single-Drop Jet-Pump System diagram on next page). Because suction is involved, atmospheric pressure is what's really doing the work. Think of the system as a long straw. As you suck on the straw, you create a vacuum in the straw above the water. Once the vacuum is there, the weight of the air, or atmospheric pressure, pushes the water up the straw. Consequently, the height that you can lift the water with a shallow-well jet pump relates to the weight of the air. While air pressure varies with elevation, it's common to limit the depth of a jet-pump-operated shallow well to about 25 ft. There is also deep well pump.

 

Jet pumps create suction in a rather novel way. The pump is powered by an electric motor that drives an impeller, or centrifugal pump. The impeller moves water, called drive water, from the well through a narrow orifice, or jet, mounted in the housing in front of the impeller. This constriction at the jet causes the speed of the moving water to increase, much like the nozzle on a garden hose. As the water leaves the jet, a partial vacuum is created that sucks additional water from the well. Directly behind the jet is a Venturi tube that increases in diameter. Its function is to slow down the water and increase the pressure. The pumped water–new water that's drawn from the well by the suction at the jet–then combines with the drive water to discharge into the plumbing system at high pressure.

 

Because shallow-well jet pumps and deep well water pump use water to draw water, they generally need to be primed–filled with water–before they'll work. To keep water in the pump and plumbing system from flowing back down into the well, a 1-way check valve is installed in the feed line to the pump.

 

Solar Powered Water Pumping

 

Any renewable energy source can make the electricity needed to power various appliances, including pumps. Solar electric power in particular is a reliable and economic choice for powering remote water pumping. Solar water pumping systems are in common use for garden fountains, livestock watering, and large-scale watering needs for commercial installations. 

 

Cattle ranchers all over the world are enthusiastic solar pump users because their water sources may be spread over large areas of rangeland that lack utility power and where generator use would be expensive and impractical. Photovoltaic (PV) panels are therefore in widespread use for reliably producing electricity directly from sunlight to power livestock and irrigation watering systems. When properly designed, PV-powered pumping systems can result in significant long-term cost savings and a smaller environmental footprint compared to conventional power systems.

 

System Design Considerations

A typical solar-powered water pump system, which includes a solar array, controller, pump, and storage tank. (Source: “The Montana Agsolar Project – Expanding the Agricultural Uses of Solar Energy in Montana.”)

 

A solar-powered water pumping system consists of four parts:  the actual pump which moves the water, the controller which adjusts the pump speed and output power as the solar panel input varies, the engine, and the solar panels. The specifics of the system design are determined by the following considerations:

 

The site-specific available solar energy (or insolation).

The volume of water required in a given period of time for the application at hand. This may include additional water to be stored for periods when the PV is not operating or has diminished output.

The total dynamic head (TDH) for the pump (the equivalent height that water must be raised, taking friction losses in the pipes into account.)

The quantity and quality of available water.

The system’s proposed layout and hydraulic criteria.

Pumps for Solar Pumping Systems

There are two major types of solar pumps, direct current (DC) and alternating current (AC). DC solar pumps are generally suitable for small applications (garden fountains, landscaping, etc.) and are relatively low-priced, particularly because they do not require inverters to produce AC power from the solar panels. These pumps are generally designed to operate with minimum electrical power, so they have rather low flow rates. Such pumps often find use in submersible deep wells where a slow but steady pump rate is acceptable.

 

AC solar pumps are driven by inverters producing AC power from PV panels. They are suitable for all kinds of applications from landscaping to irrigation, particularly large-scale applications such as farmland irrigation, desert control, and so forth. AC solar pumps are available in power output ranges from 150W to 55kW.

 

Solar-powered pumps are characterized as either positive displacement pumps (e.g. diaphragm, piston, or helical rotor) or centrifugal pumps. Positive displacement pumps are typically used when the TDH is high and the flow rate (measured in gallons per minute) required is low. Conversely, centrifugal pumps are typically used for low TDH and high flow rates.

 

There are many resources available that provide specific information and advice for implementing solar water pumps in a variety of situations. These should be consulted to meet the needs of specific applications.

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