Pumps – Condition responsive control of drive transmission or pump... – Plural pumps with individual or relative control
Reexamination Certificate
2002-08-20
2004-08-31
Yu, Justine (Department: 3746)
Pumps
Condition responsive control of drive transmission or pump...
Plural pumps with individual or relative control
C417S212000, C417S244000, C417S266000, C417S426000, C417S429000, C417S053000
Reexamination Certificate
active
06783331
ABSTRACT:
BACKGROUND OF THIS INVENTION
The present invention relates to a pumping system for multiple-phase fluids. More specifically, it relates to a multi-phase pumping system that includes multiple-phase pumps with mechanical differential units, which are able to pump liquids only, gases only, or liquids and gases simultaneously in any ratio, eliminating the recirculation of fluids. The system of this invention is particularly useful in the oil industry. The invention also refers to the method used by the system put forward here for pumping multi-phase fluids.
PRIOR ART
In industry, particularly the oil industry, there are many situations in which liquids and gases are found together or mixed together, and need to be supplied with power for transporting through pipelines.
There are two distinct types of conventional equipment used to do this: pumps and compressors.
Pumps work efficiently with liquid, though not when gas is present; when gas is present the pump may cease to function, depending on the percentage of gas.
The same behaviour is seen in reverse with compressors.
Thus, if for example energy is to be transfered into a multi-phase flow in order to facilitate long-distance transport, it becomes necessary to separate the constituents into liquid and gas flows. For this op ration one uses the liquid- and gas-phase separators. In this way, following separation, the liquid flow will be directed to a pump, there to be supplied with energy and transported, while the flow of gas will be directed to a compressor for the same reason.
Generally, to work with flows of fluids at high-pressure, the separators are heavy, bulky vessels which are fitted with control and safety systems in order to maintain the correct liquid level for operation. Besides being expensive they overload the production system, especially in applications where there are limitations on space, weight or the complexity of the components installed (for example, off-shore oil-production rigs and/or sea-bed oil-production systems).
In order to do away with use of separators, industry has set about using, adapting and developing mono-phase liquid pumps and mono-phase gas compressors which can function as multi-phase pumps, pumping in two phases, liquid and gas.
Many types of multi-phase pump are under development such as: piston pumps, diaphragm pumps, single and/or multiple screw Moineau, spiral-axial, or centrifugal pumps. However, until now, none of these designs has yet reached the stage of large-scale application in industry. Those that attained the most widespread application were multi-phase twin-screw pumps and the rotary-dynamic pumps of the spiral-axial type.
A basic problem of the multi-phase fluid pump is the Circulatory Flow (C.F) of fluids, to be explained in detail later herein.
An Oscillating Chamber String (OCS), disclosed by Sulzer Pumps (Germany) in 1989-1990, is a multi-phase pump with variable capacity of the pistons which solves the C.F. problem, though in a more complex way than that adopted by the present invention. The OCS piston pump has a positive displacement action. The pistons and connected sheaths connected in a set produce a multiple-stage pump. The travel of each piston is variable. A control-system and motors connected to each piston reset the piston's travel, so as to maintain equal pressure increments in the component stages of this design.
A twin-screw pump is normally used to pump liquids, at which it gives good performance, and it has been adapted to serve as a multi-phase pump. This is also a positive-displacement pump, made up of two metal screws and two metal sheaths, producing cavities of equal volume, which move by suction to discharge the pump, in order to drive the fluids. The screws and sheaths form metal seals between the cavities; in other words, each cavity demarcates a stage of the pump.
The twin-screw pump displays the following disadvantages, brought about by the phenomenon of Circulatory Flow (C.F.):
1. durability is reduced through the increased proportion of gas;
2. energy-efficiency is reduced through the increased proportion of gas, possibly declining to zero;
3. it is unable to pump high proportions (for example, over 95%) of gas, or gas alone.
There follows a description of C.F. and its effects.
The mono-phase gas compressor has variable volumes at each stage, being unable to pump liquid, hence an Excessive Rise in Pressure (E.R.P.) would arise at each of its stages. In order that the liquid will be pumped while avoiding E.R.P, the twin-screw pump exhibits stages or cavities at constant volumes. Hence, there would be no reduction in the volume of gas entering the cavity, so pressure would not rise. Thus, suction pressure would be maintained in all pumping stages, and discharge pressure would increase only at the final stage, when the cavity communicates with the pump discharge. This is certainly not what actually happens, since the final stage does not resist any increase in the required pressure. If it did resist, there would be no need for an extra stage.
Single-stage pumps do not present this problem, since the single stage resists any increase in the pressure required.
Under the law of conservation of mass, the flow-mass must be constant at all stages of the twin-screw pump. Fluid pressure rises while the cavity is moving; in other words, pressure rises from one stage to another. With that rise in pressure, the volumetric flow of fluids declines, allowing a state of gas equilibration, and as a result it cannot succeed in completely filling the cavity. Thus, it is filled up with fluids which will not normally drain away. Those fluids that remain in place, occupying cavity spaces that pass through them, represent the C.F. of the fluids.
By way of illustration, let us suppose a twin-screw pump with several stages, compressing gas with a pressure value of 1 Absolute Unit of Pressure (AUP) of suction and 10 AUP of discharge. The volumetric suction gas-flow is at its maximum whereas, when discharged, this flow declines to {fraction (1/10)} of the volumetric gas flow in the first cavity. Consequently, {fraction (9/10)} of this volumetric flow will need to be supplemented by fluid originating in C.F: in other words, this {fraction (9/10)} of the fluid continues to occupy the cavity, moving to the previous cavity, as long as this continues to occupy the position of the last cavity.
This same phenomenon occurs with the remaining cavities; however, the C.F. will be lower, since it depends on the relationship between the cavity and pump suction.
The return, or greater C.F., occurs at the final cavity where there is greater pressure, and the smaller C.F. at the first where there is less pressure. However, linear distribution of pressure does not occur, because the return flow, being much greater in the higher stages, is impeded by the clearances found between the cavities. Therefore, in the presence of gas, the higher stages function with a greater Rise in Pressure, or greater E.R.P.
The twin-screw pumps are installed with a minimum clearance between the screws and sheaths, when they function as virtually non-compressible liquids. These pumps are not multi-phase and do not compress gas, because an E.R.P. would arise at the stages. In order to make them multi-phase, designers reduce the E.R.P. increasing the clearance between screws and sheaths so that the remaining stages will function.
Supposing a discharge of a liquid pump should be linked to its own suction by means of a choke or control valve, so that 90% of the pumped flow returns. If the hydraulic power of the pump were 10 Units of Power, 9 of those units would be dissipated at the choke in the form of heat. If the choke did not exchange heat with the environment (considering that this is an adiabatic process), the result is equivalent to installing a heater with the same 9 units of pump-suction power, in order to heat just 10% of the flow passing through the pump. However, the return of fluids at the twin-screw pumping stages causes overheating, similar to the overheating caused by the choke when working under
Filho Elisio Caetano
Lopes Divonsir
Nixon & Vanderhye PC
Petroleo Brasileiro S.A. - Petrobras
Solak Timothy P.
Yu Justine
LandOfFree
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