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