High Pressure Meter Pump

There is a need in liquid chromatography for solvent pump systems which can deliver fluids in accurately metered amounts. It is important in such systems that solvent flow be precisely controlled independently of the load pressure. Liquid chromatography systems and particularly systems utilizing gradient elution require high accuracy at extremely low flow rates. For example, in micro-bore chromatography it is desirable to resolve flow rates as low as one microliter per minute. It has been a problem with most prior art devices that the compressibility of the fluid and the mechanical compliance of the pump combine to cause a severe drop in flow rate as load pressure increases, this phenomenon being commonly referred to as "roll off". A two pump system overcomes inaccuracies due to roll off by utilizing a low pressure meter pump which injects controlled amounts of solvent into a high pressure slave pump. The meter pump is synchronized with the slave pump to insure that the meter pump delivers its metered charge into a low pressure portion of the slave pump cycle. Because the meter pump always operates into a low pressure load, roll off is not a problem affecting meter pump accuracy. Such a pump system is disclosed in U.S. Pat. No. 4,003,679 HIGH PRESSURE PUMP WITH METERING by Douglas McManigill and assigned to the assignee of the present application.

The McManigill specification describes a system in which the meter pump/slave pump pair are mechanically coupled to cycle at the same fixed rate. Flow rate is controlled by mechanically adjusting meter pump displacement. However, mechanical and fluid compliance in the meter pump introduces some error in each meter pump cycle. This error tends to increase as a function of increasing meter pump cycle rate. As flow rate and hence meter pump displacement is reduced this error and others inherent in scaling become an increasingly greater percentage of the volume of fluid pumped. Any such error is reproduced in each cycle of the meter pump/slave pump pair and reduces system flow rate accuracy, particularly at low flow rates. Hence, the range of flow rates over which a given meter pump can operate is limited by degradation of accuracy. Difficulty in self-priming may also become a problem at low flow rates in pumps that adjust flow rate by controlling displacement because larger displacement pumps tend to self-prime better than small displacement pumps.

The present invention overcomes problems with inaccuracy associated with roll off by utilizing a meter pump/slave pump pair as in the McManigill patent. However, further improvements in accuracy are attained by using a servo-driven syringe-type meter pump delivering a charge into a diaphragm slave pump. The volume of the meter pump syringe cylinder is designed to be many times greater than the capacity of the slave pump, so that the slave pump cycles many times for each meter pump cycle. Error in the syringe-type meter pump is very low when expressed as a percentage of the volume of solvent pumped in each meter pump cycle. Because the meter pump is driven by a servomotor at a much lower cycle rate than the slave pump, the small meter pump error is distributed over many slave pump cycles. Consequently, flow rate accuracy is independent of the volume of fluid being pumped. Thus the proposed system is able to resolve very low flow rates such as one microliter per minute. The servomotor may be synchronized to drive the meter pump only during the low pressure input portion of the slave pump cycle. Alternately, a compliance may be inserted between the meter pump and the slave pump allowing the meter pump to operate independently from the slave pump. Because of the nature of this meter pump/slave pump pair the system maintains high accuracy over a relatively broad range of flow rates. Additionally, the ability to self-prime is improved.

FIG. 1 illustrates a pumping system according to a preferred embodiment of the invention.

FIG. 2 shows several filling profiles for the system of FIG. 1.

In FIG. 1, a servomotor 10 drives a pair of syringe-type meter pumps 12 and 14 connected in a push-pull mode. A piston 30 of meter pump 12 is raised and lowered relative to a piston cylinder 32 by engagement of a lead screw 34 with an interior threaded portion of a piston carrier 36. Similarly, a piston 38 in pump 14 is raised and lowered in a piston cylinder 40 by the engagement of a lead screw 42 with an interior threaded portion of a piston carrier 44. Pumps 12 and 14 always move equally in opposite directions in a push-pull mode because a gear 18 attached to threaded shaft 34 of pump 12 drives gear 20 attached to threaded shaft 42 of pump 14, and because gears 18 and 20 and thread pairs 34-36 and 42-44 are matched. Hence, the action of servomotor 10 connected to a gear 16 drives gears 18 and 20 equally in opposite directions. This in turn moves pistons 30 and 38 equal distances in opposite directions.

The flow pattern of the push-pull pair of meter pumps 12 and 14 is reversed by a 90 degree rotation of a four-way rotary valve 22 actuated by servomotor 24, which actuation is synchronized with a reversal of direction of servomotor 10. This sequence of changes may be controlled by a microprocessor 26, such as an Intel 8085 microprocessor, to insure that one of the meter pumps 12 or 14 is pumping while the other is refilling from a reservoir of solvent 28.

At any instant the meter pump which is then pumping, e.g., pump 14 as shown in FIG. 1, delivers its metered charge through a ball valve 46 into a diaphragm pump chamber 48 of a high pressure slave pump 50. A pump motor 52 raises a piston 54, drawing oil from a reservoir 56 through a check valve 58 into a piston cylinder 60. Diaphragm pump chamber 48 is seperated from piston cylinder 60 by a flexible diaphragm 49 such that solvent from meter pump 12 or 14 is not able to mix with oil from slave pump 50. The flexible diaphragm 49 transmits pressure between the oil in piston cylinder 60 and any solvent in diaphragm pump chamber 48. As piston 54 is driven down by motor 52, check valve 58 closes. A check valve 62 opens when the pressure in piston cylinder 60 exceeds load pressure. A spring-loaded ball valve 64, used as a back pressure regulator, together with a compliance 66 and a check valve 68 functions as a high pressure hydraulic override; it is adjusted to open at a pressure above the maximum load pressure. As piston 54 continues its downward movement, oil pressure builds up in piston cylinder 60. The pressure is transmitted across the flexible diaphragm 49 into the solvent in diaphragm pump chamber 48. If ball valve 46 is not already closed, it closes under the increased pressure in diaphragm pump chamber 48. As pressure in diaphragm pump chamber 48 reaches load pressure, check valve 62 opens discharging the solvent at load pressure through tube 70. After all solvent has been forced out of diaphragm pump chamber 48, oil pressure in piston cylinder 60 again rises, opening check valve 68 for the remainder of the downward stroke of piston 54. In this way slave pump 50 delivers the same volume of fluid (both solvent and oil) in every cycle. Excess capacity of slave pump 50 over the amount of solvent delivered by meter pump 12 or 14 to diaphragm pump chamber 48 is expended by pumping oil through check valve 64 into compliance 66 which is held at a pressure above maximum load pressure by adjustment of spring loaded ball valve 64. When the pressure in piston cylinder 60 decreases check valve 68 is closed by the pressure retained in compliance 66. Ball valve 64, however, will remain open as long as its adjusted opening pressure is below the pressure in compliance 66. In this way the high pressure hydraulic override is able to gradually release any retained excess pressure.

The pressure at which ball valve 46 is adjusted to open is such that it will not open solely as a result of the suction pressure caused by the upward motion of piston 54. This is to insure that only solvent actively delivered by meter pump 12 or 14 enters diaphragm pump chamber 48.

In operation, motor 52 drives slave pump 50 at a continuous rate while servomotor 10 incrementally subdivides the volume of meter pump piston cylinder 32 or 40 into subvolumes, delivering one such subvolume for each slave pump cycle. In this way the meter pump cycles much more slowly than the slave pump. Flow rate is controlled by regulating the size of the subvolumes. In a meter pump/slave pump pair as in preferred embodiment, for example, assume motor 52 is of the constant speed type and drives slave pump 50 through six hundred complete pump cycles per minute, and meter pump piston cylinders 32 and 40 have combined volumes of one-fifth of a milliliter, and require eight rotations (four clockwise plus four counterclockwise) (2,880 degrees) of threads 34 and 42 to deliver a volume of one-fifth of a milliliter. Then, at a first flow rate of twelve milliliters per minute, meter pump pair 12 and 14 must deliver one-fifth of a milliliter per second. To accomplish this, servomotor 10 must rotate gears 18 and 20 through six hundred equal incremental rotations of 288 degrees each, per minute. In contrast, a second flow rate of one microliter per minute (or one meter pump cycle in every 200 minutes) requires six hundred incremental rotations of 0.024 degrees each, per minute of gears 18 and 20. In effect the twelve milliliter flow rate subdivides the combined one-fifth milliliter volume of meter pump piston cylinders 32 and 40 into ten subvolumes averaging 20 microliters each. At the one microliter flow rate each one-fifth milliliter volume is subdivided into 120,000 subvolumes averaging 0.001667 microliters each. Any error in the volume of a particular subvolume is compensated for through the integration of many subvolumes over time.

Microprocessor 26 can be programmed to synchronize servomotor 10 with synchronous motor 52 such that meter pump 12 or 14 only delivers solvent through ball valve 46 during the portion of the cycle of slave pump 50 when piston 54 is moving upwards. This insures that meter pump 12 or 14 always experiences minimal roll off. Alternatively, a compliance 72 would allow servomotor 10 to operate independently of synchronous motor 52 by storing solvent during the high pressure portion of the slave pump cycle. Another method of coordinating the action of slave pump 50 with the amount of solvent delivered by meter pump 12 or 14 is to combine a "smart" compliance 72 with a variable speed or variable displacement slave pump 50. In this way slave pump 50 would change the volume of fluid pumped in each cycle in response to the solvent delivered by meter pump 12 or 14 as communicated by "smart" compliance 72 to microprocessor 26. Finally, in liquid chromotography systems utilizing gradient elution the output from multiple meter pump pairs could be mixed and delivered to a single slave pump.

FIG. 2 shows a curve 74 representing the harmonic motion of the piston 54 during one cycle of operation of slave pump 52. Three curves 76a, 76b and 76c illustrate different possible filling profiles for filling of slave pump 52 by meter pumps 12 and 14. Curves 78a, 78b and 78c illustrate the corresponding delivery profiles for slave pump 50 for each of the filling profiles 76a, 76b and 76c. As discussed above, any excess capacity of slave pump 50 over the amount of solvent delivered to the slave pump is expended by pumping of oil by slave pump 50; in FIG. 2, this volume is indicated by refill area 81 and delivery area 80 both corresponding to solvent profiles 76c, 78c.