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