Metering pump

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A metering pump moves the appropriate volume of fluid over a period of time that provides an accurate volumetric flow rate. Delivery of fluids in a properly adjusted flow velocity is sometimes called metering . The term "metering pump" is based on application or use rather than the correct type of pump used, although some types of pumps are much more suitable than most other types of pumps.

Although metering pumps can pump water, they are often used to pump chemicals, solutions, or other liquids. Many rated metering pumps can pump into high discharge pressures. They are usually made to measure at constant practical flow rates (when averaged over time) in various outlet pressures. Manufacturers provide their respective metering pump models with a maximum discharge pressure rating on each model that is guaranteed to be pumped. An engineer, designer, or user shall ensure that the pressure and rating of the temperature and the wetted pump material are compatible for the application and type of fluid being pumped.

Most metering pumps have pump heads and motors . The pumped liquid flows through the pump head, entering through the inlet and out through the outlet. Motor is generally an electric motor that drives the pump head.

Video Metering pump

Piston pumps

Many metering pumps are driven by pistons. Piston pumps are positive displacement pumps that can be designed to be pumped at constant flow rate (on average) over time to various exhaust pressures, including high discharge pressure of thousands of psi.

The piston-driven metering pump generally works as follows: There is a (sometimes called a thruster), usually a cylinder, which can enter and exit from the corresponding chamber at the pump head. The incoming and outgoing channels join the piston chamber. There are two check valves , often ball valve check valves, attached to the pump head, one in the inlet and the other in the outlet. The inlet valve allows the flow from the inlet line to the piston chamber, but not in the reverse direction. The outlet valve allows the flow from the chamber to the outlet chute, but not vice versa. The motor repeatedly moves the piston into and out of the piston chamber, causing the volume of space to become repeatedly smaller and larger. When the piston moves out, a vacuum is created. The low pressure in the room causes the liquid to enter and fill the space through the inlet check valve, but higher pressure at the outlet causes the outlet valve to close. Then when the piston moves in, it presses the liquid in the room. The high pressure in the chamber causes the inlet valve to close and force the outlet valve to open, forcing the liquid out at the outlet. These repeated suction and discharge punctures repeatedly to measure the fluid. At the back of the room, there is a packing around a piston or a donut-shaped seal with a spoon-shaped sphincter spring inside compressing the seal around the piston. It withstands the fluid pressure when the piston slips in and out and makes the pump leak. Packing or seals can wear out after long and replaceable usage. Metering speed can be adjusted by varying the strokelength where the piston moves back and forth or varies the piston movement speed.

A single piston pump sends liquid to the outlet only during stroke discharge. If the suction piston and stroke discharge occur at the same speed and the liquid is measured at half the pumping time, the average overall metering rate over time equals half the average flow rate during the discharge stroke. Some single piston pumps may have constant slow piston movement for disposal and rapid retraction movements to recharge pump heads. In such cases, the overall measurement rate is almost equal to the pumping rate during stroke discharge.

Maps Metering pump

Pumps used in high-pressure chromatography

Pumps used in high-pressure chromatography such as HPLC and ion chromatography are similar to small piston pumps. For wear resistance and chemical resistance to solvents, etc., Usually pistons are made of artificial sapphire and check ball valves have ruby ​​balls and sapphire chairs. To produce a good chromatogram, it is desirable to have the pumping flow rate as constant as possible. Either a single piston pump with fast rechargeable use or a dual pump head with coordinated piston stroke is used to provide as much pumping rate as possible.

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Diaphragm and peristaltic pump

To avoid leakage on the packaging or seal especially when the liquid is hazardous, toxic, or hazardous, diaphragm pump is used for measurement. The diaphragm pump has a diaphragm in which repetitive compression/decompression movements are transmitted. The liquid does not penetrate through the diaphragm, so the liquid inside the pump is closed from the outside. Such motion changes the volume of space inside the pump head so that the liquid enters through the check valve to enter during decompression and out through the outlet check valve during compression, in a manner similar to the piston pump. Diaphragm pumps can also be made with discharge at sufficiently high pressures. Diaphragm metering pumps are generally hydraulically driven.

The peristaltic pump uses a motor-driven roller to roll along a flexible pipe, compressing it to push the liquid inside. Although peristaltic pumps can be used to measure at lower pressures, flexible tubes are confined to pressure levels that can be retained.

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Possible problems

The maximum pressure rating of the metering pump is actually the peak of the pump discharge pressure range that is assured to be pumped at a reasonably controlled flow rate. The pump itself is a pressurized device that is often able to exceed its pressure rating, although it is not guaranteed. For this reason, if any valve stops downstream of the pump, the pressure relief valve should be placed in between to prevent overloading of pipes or pipes in case the valve stops accidentally closes while the pump is running. The relief valve setting should be below the maximum pressure rating that can be retained by pipes, tubes, or other components.

The liquid is only very little compressible. The properties of this fluid allows the metering to pump the liquid out at high pressure. Since the liquid can only be slightly compressed during the stroke discharge, it is forced out of the pump head. Gas is much more compressible. Metering pumps are not good at pumping gas. Occasionally, metering or similar pumps must be prepared prior to operation, i. e. pump head filled with liquid to be pumped. When the gas bubble enters the pump head, the compression movement presses the gas but has a difficult time forcing it out of the pump head. Pumps can stop pumping liquids with gas bubbles in the pump head even though mechanically pumps will pass through motion, repeatedly compressing and decompressing bubbles. To prevent this type of "steam lock", chromatographic solvents are often released before pumping.

If the pressure at the outlet is lower than the pressure on the inlet and remains that way regardless of pumping, then this pressure difference opens both check valves simultaneously and the liquid flows through the pump head uncontrollably from the inlet to the outlet. This can happen whether the pump works or not. This situation can be avoided by placing the correct positive pressure differential pressure valve at the downstream of the pump. Such a valve will only open if the minimum rated pressure difference in the valve is exceeded, something that most high-pressure metering pumps can easily exceed.

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References

US Patent # 5358000.

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External links

• Flow control with metering pump. Accuracy is critical when using controlled volume control pumps.
• Hydraulic Volume Pressure Metering Pump Institute Requires accurate, repeatable, and adjustable flow rates.

Siphon pump metering can now be repeated with a high degree of accuracy and control with flow control on the crown or buckling siphon channels. A controlled volume is ejected through an inline measuring chamber by cyclically initiating and stopping the siphon stream to fill the master canister. Siphon can be used in this way to supply various liquids above or far from the source. See Siphon Siphon's pump article.

Source of the article : Wikipedia