| A Metering pump is a pump used to pump liquids | | | | Pumps used in high pressure chromatography |
| at adjustable flow rates which are precise when | | | | Pumps used in high pressure chromatography |
| averaged over time. Delivery of fluids in precise | | | | such as HPLC and ion chromatography are much |
| adjustable flow rates is sometimes called | | | | like small piston metering pumps. For wear |
| metering. The term "metering pump" is based on | | | | resistance and chemical resistance to solvents, |
| the application or use rather than the exact kind | | | | etc., typically the pistons are made of artificial |
| of pump used, although a couple types of pumps | | | | sapphire and the ball check valves have ruby balls |
| are far more suitable than most other types of | | | | and sapphire seats. To produce good |
| pumps. | | | | chromatograms, it is desirable to have a pumping |
| Although metering pumps can pump water, they | | | | flow rate as constant as possible. Either a single |
| are often used to pump chemicals, solutions, or | | | | piston pump with a quick refill is used or a double |
| other liquids. Many metering pumps are rated to | | | | pump head with coordinated piston strokes is |
| be able to pump into a high discharge pressure. | | | | used to provide as constant a pumping rate as |
| They are typically made to meter at flow rates | | | | possible. |
| which are practically constant (when averaged | | | | Diaphragm and peristaltic pumps |
| over time) within a wide range of discharge | | | | In order to avoid leakage at the packing or seal |
| (outlet) pressure. Manufacturers provide each of | | | | particularly when a liquid is dangerous, toxic, or |
| their models of metering pumps with a maximum | | | | noxious, diaphragm pumps are used for metering. |
| discharge pressure rating against which each | | | | Diaphragm pumps have a diaphragm through |
| model is guaranteed to be able to pump against. | | | | which repeated compression/decompression |
| An engineer, designer, or user should ensure that | | | | motion is transmitted. The liquid does not |
| the pressure and temperature ratings and wetted | | | | penetrate through the diaphragm, so the liquid |
| pump materials are compatible for the application | | | | inside the pump is sealed off from the outside. |
| and the type of liquid being pumped. | | | | Such motion changes the volume of a chamber in |
| Most metering pumps have a pump head and a | | | | the pump head so that liquid enters through an |
| motor. The liquid being pumped goes through the | | | | inlet check valve during decompression and exits |
| pump head, entering through an inlet line and | | | | through an outlet check valve during compression, |
| leaving through an outlet line. The motor is | | | | in a manner similar to piston pumps. Diaphragm |
| commonly an electric motor which drives the | | | | pumps can also be made which discharge at fairly |
| pump head. | | | | high pressure. Diaphragm metering pumps are |
| Piston pumps | | | | commonly hydraulically driven. |
| Many metering pumps are piston-driven. Piston | | | | Peristaltic pumps use motor-driven rollers to roll |
| pumps are positive displacement pumps which can | | | | along flexible tubing, compressing it to push |
| be designed to pump at practically constant flow | | | | forward a liquid inside. Although peristaltic pumps |
| rates (averaged over time) against a wide range | | | | can be used to meter at lower pressures, the |
| of discharge pressure, including high discharge | | | | flexible tubing is limited in the level of pressure it |
| pressures of thousands of psi. | | | | can withstand. |
| Piston-driven metering pumps commonly work as | | | | Possible problems |
| follows: There is a piston (sometimes called | | | | The maximum pressure rating of a metering |
| plunger), typically cylindrical, which can go in and | | | | pump is actually the top of the discharge pressure |
| out of a correspondingly shaped chamber in the | | | | range the pump is guaranteed to pump against at |
| pump head. The inlet and outlet lines are joined to | | | | a reasonably controllable flow rate. The pump |
| the piston chamber. There are two check valves, | | | | itself is a pressurizing device often capable of |
| often ball check valves, attached to the pump | | | | exceeding its pressure rating, although not |
| head, one at the inlet line and the other at the | | | | guaranteed to. For this reason, if there is any |
| outlet line. The inlet valve allows flow from the | | | | stop valve downstream of the pump, a pressure |
| inlet line to the piston chamber, but not in the | | | | relief valve should be placed in between to |
| reverse direction. The outlet valve allows flow | | | | prevent overpressuring of the tubing or piping line |
| from the chamber to the outlet line, but not in | | | | in case the stop valve is inadvertently shut while |
| reverse. The motor repeatedly moves the piston | | | | the pump is running. The relief valve setting should |
| into and out of the piston chamber, causing the | | | | be below the maximum pressure rating that the |
| volume of the chamber to repeatedly become | | | | piping, tubing, or any other components there |
| smaller and larger. When the piston moves out, a | | | | could withstand. |
| vacuum is created. Low pressure in the chamber | | | | Liquids are only very slightly compressible. This |
| causes liquid to enter and fill the chamber through | | | | property of liquids lets metering pumps discharge |
| the inlet check valve, but higher pressure at the | | | | liquids at high pressure. Since a liquid can be only |
| outlet causes the outlet valve to shut. Then when | | | | slightly compressed during a discharge stroke, it is |
| the piston moves in, it pressurizes the liquid in the | | | | forced out of the pump head. Gases are much |
| chamber. High pressure in the chamber causes | | | | more compressible. Metering pumps are not good |
| the inlet valve to shut and forces the outlet valve | | | | at pumping gases. Sometimes, a metering or |
| to open, forcing liquid out at the outlet. These | | | | similar pump has to be primed before operation, i. |
| alternating suction and discharge strokes are | | | | e. the pump head filled with the liquid to be |
| repeated over and over to meter the liquid. In | | | | pumped. When gas bubbles enter a pump head, |
| back of the chamber, there is packing around the | | | | the compression motion compresses the gas but |
| piston or a doughnut-shaped seal with a | | | | has a hard time forcing it out of the pump head. |
| toroid-shaped sphincter-like spring inside | | | | The pump may stop pumping liquid with gas |
| compressing the seal around the piston. This holds | | | | bubbles in the pump head even though |
| the fluid pressure when the piston slides in and | | | | mechanically the pump is going through the |
| out and makes the pump leak-tight. The packing | | | | motions, repeatedly compressing and |
| or seals can wear out after prolonged use and | | | | decompressing the bubbles. To prevent this type |
| can be replaced. The metering rate can be | | | | of "vapor lock", chromatography solvents are |
| adjusted by varying the strokelength by which | | | | often degassed before pumping. |
| the piston moves back and forth or varying the | | | | If the pressure at the outlet is lower than the |
| speed of the piston motion. | | | | pressure at the inlet and remains that way in |
| A single-piston pump delivers liquid to the outlet | | | | spite of the pumping, then this pressure |
| only during the discharge stroke. If the piston's | | | | difference opens both check valves |
| suction and discharge strokes occur at the same | | | | simultaneously and the liquid flows through the |
| speed and liquid is metered out half the time the | | | | pump head uncontrollably from inlet to outlet. This |
| pump is working, then the overall metering rate | | | | can happen whether the pump is working or not. |
| averaged over time equals half the average flow | | | | This situation can avoided by placing a |
| rate during the discharge stroke. Some | | | | correctly-rated positive pressure differential check |
| single-piston pumps may have a constant slow | | | | valve downstream of the pump. Such a valve will |
| piston motion for discharge and a quick retract | | | | only open if a minimum rated pressure differential |
| motion for refilling the pump head. In such cases, | | | | across the valve is exceeded, something which |
| the overall metering rate is practically equal to the | | | | most high pressure metering pumps can easily |
| pumping rate during the discharge stroke. | | | | exceed. |