Metering pump

A Metering pump is a pump used to pump liquidsPumps used in high pressure chromatography
at adjustable flow rates which are precise whenPumps used in high pressure chromatography
averaged over time. Delivery of fluids in precisesuch as HPLC and ion chromatography are much
adjustable flow rates is sometimes calledlike small piston metering pumps. For wear
metering. The term "metering pump" is based onresistance and chemical resistance to solvents,
the application or use rather than the exact kindetc., typically the pistons are made of artificial
of pump used, although a couple types of pumpssapphire and the ball check valves have ruby balls
are far more suitable than most other types ofand sapphire seats. To produce good
pumps.chromatograms, it is desirable to have a pumping
Although metering pumps can pump water, theyflow rate as constant as possible. Either a single
are often used to pump chemicals, solutions, orpiston pump with a quick refill is used or a double
other liquids. Many metering pumps are rated topump 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 ratespossible.
which are practically constant (when averagedDiaphragm and peristaltic pumps
over time) within a wide range of dischargeIn order to avoid leakage at the packing or seal
(outlet) pressure. Manufacturers provide each ofparticularly when a liquid is dangerous, toxic, or
their models of metering pumps with a maximumnoxious, diaphragm pumps are used for metering.
discharge pressure rating against which eachDiaphragm 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 thatmotion is transmitted. The liquid does not
the pressure and temperature ratings and wettedpenetrate through the diaphragm, so the liquid
pump materials are compatible for the applicationinside 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 athe pump head so that liquid enters through an
motor. The liquid being pumped goes through theinlet check valve during decompression and exits
pump head, entering through an inlet line andthrough an outlet check valve during compression,
leaving through an outlet line. The motor isin a manner similar to piston pumps. Diaphragm
commonly an electric motor which drives thepumps can also be made which discharge at fairly
pump head.high pressure. Diaphragm metering pumps are
Piston pumpscommonly hydraulically driven.
Many metering pumps are piston-driven. PistonPeristaltic pumps use motor-driven rollers to roll
pumps are positive displacement pumps which canalong flexible tubing, compressing it to push
be designed to pump at practically constant flowforward a liquid inside. Although peristaltic pumps
rates (averaged over time) against a wide rangecan be used to meter at lower pressures, the
of discharge pressure, including high dischargeflexible tubing is limited in the level of pressure it
pressures of thousands of psi.can withstand.
Piston-driven metering pumps commonly work asPossible problems
follows: There is a piston (sometimes calledThe maximum pressure rating of a metering
plunger), typically cylindrical, which can go in andpump is actually the top of the discharge pressure
out of a correspondingly shaped chamber in therange the pump is guaranteed to pump against at
pump head. The inlet and outlet lines are joined toa 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 pumpexceeding its pressure rating, although not
head, one at the inlet line and the other at theguaranteed to. For this reason, if there is any
outlet line. The inlet valve allows flow from thestop valve downstream of the pump, a pressure
inlet line to the piston chamber, but not in therelief valve should be placed in between to
reverse direction. The outlet valve allows flowprevent overpressuring of the tubing or piping line
from the chamber to the outlet line, but not inin case the stop valve is inadvertently shut while
reverse. The motor repeatedly moves the pistonthe pump is running. The relief valve setting should
into and out of the piston chamber, causing thebe below the maximum pressure rating that the
volume of the chamber to repeatedly becomepiping, tubing, or any other components there
smaller and larger. When the piston moves out, acould withstand.
vacuum is created. Low pressure in the chamberLiquids are only very slightly compressible. This
causes liquid to enter and fill the chamber throughproperty of liquids lets metering pumps discharge
the inlet check valve, but higher pressure at theliquids at high pressure. Since a liquid can be only
outlet causes the outlet valve to shut. Then whenslightly compressed during a discharge stroke, it is
the piston moves in, it pressurizes the liquid in theforced out of the pump head. Gases are much
chamber. High pressure in the chamber causesmore compressible. Metering pumps are not good
the inlet valve to shut and forces the outlet valveat pumping gases. Sometimes, a metering or
to open, forcing liquid out at the outlet. Thesesimilar pump has to be primed before operation, i.
alternating suction and discharge strokes aree. the pump head filled with the liquid to be
repeated over and over to meter the liquid. Inpumped. When gas bubbles enter a pump head,
back of the chamber, there is packing around thethe compression motion compresses the gas but
piston or a doughnut-shaped seal with ahas a hard time forcing it out of the pump head.
toroid-shaped sphincter-like spring insideThe pump may stop pumping liquid with gas
compressing the seal around the piston. This holdsbubbles in the pump head even though
the fluid pressure when the piston slides in andmechanically the pump is going through the
out and makes the pump leak-tight. The packingmotions, repeatedly compressing and
or seals can wear out after prolonged use anddecompressing the bubbles. To prevent this type
can be replaced. The metering rate can beof "vapor lock", chromatography solvents are
adjusted by varying the strokelength by whichoften degassed before pumping.
the piston moves back and forth or varying theIf 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 outletspite of the pumping, then this pressure
only during the discharge stroke. If the piston'sdifference opens both check valves
suction and discharge strokes occur at the samesimultaneously and the liquid flows through the
speed and liquid is metered out half the time thepump head uncontrollably from inlet to outlet. This
pump is working, then the overall metering ratecan happen whether the pump is working or not.
averaged over time equals half the average flowThis situation can avoided by placing a
rate during the discharge stroke. Somecorrectly-rated positive pressure differential check
single-piston pumps may have a constant slowvalve downstream of the pump. Such a valve will
piston motion for discharge and a quick retractonly 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 themost high pressure metering pumps can easily
pumping rate during the discharge stroke.exceed.