POSITIVE-DISPLACEMENT SCREW PUMP

Abstract: A screw displacement pump (10) for comingled material has a body (11) defining a chamber (12′). Inlets are provided for the admission of fluid to the chamber and an outlet is provided for discharge of fluid from the chamber (12′) intermeshing screw members (40) are mounted for rotation within the chamber (12′) for transporting comingled material from the inlet to the outlet, the threads (41,42) of the intermeshing screw members (40) being of opposite hand The pitch of the screws (40) at the outlet end thereof is smaller than the pitch of the screws (40) at the inlet end thereof to cause compression of gaseous material being transported. Clearance is provided between the screws (40) and between the screws (40) and the walls of the chamber (12′) to allow sufficient leakage of the material towards the inlet, when the material is in the liquid phase, to avoid a liquid lock.

Description: The invention relates to positive displacement screw pumps. Positive displacement screw pumps are commonly used for pumping liquids, the screws in the pumps having a constant pitch such that there is no tendency to compress the liquid along the length of the screw and therefore no risk of a liquid lock. A problem arises when using such a constant pitch positive displacement screw pump for pumping comingled flows, such as oil and gas mixtures from an oil well, in that, although pumping of the gaseous phase will be achieved, it will not be achieved efficiently since no compression of the gas is taking place along the length of the screw. Output from an oil well will be a mixture of gas and oil which will vary from time to time and the pump must be able, when passing nearly 100% gas, suddenly to accept 100% oil. According to the invention, there is provided a screw displacement pump for comingled material having a gas part and a liquid part, the pump comprising a body defining a chamber, at least one inlet and at least one outlet for the admission of fluid to and discharge of fluid from the chamber, a plurality of intermeshing screw members mounted for rotation within the chamber for transporting the comingled material from the inlet to the outlet, the threads of the intermeshing screw members being of opposite hand, wherein the pitch of the screws at the outlet end thereof is smaller than the pitch of the screws at the inlet end thereof to cause compression of gaseous material being transported, characterised in that clearance is provided between the screws and between the screws and the walls of the chamber to allow sufficient leakage of the material towards the inlet, when the material is in the liquid phase, to avoid a liquid lock. A screw displacement pump according to the classifying portion of the preceding paragraph is known from United Kingdom Patent No. 448, 235. The pitch of the screws may vary along the length of the screws, or alternatively the pitch of the screws may decrease in discrete steps from the inlet end thereof to the outlet end thereof. There may be breaks between the threads of the screws at one or more of the discrete steps. The chamber preferably has one central outlet and two inlets one at each end of the chamber, or a central inlet and two outlets one at each end of the chamber, and one set of screw members mounted for rotation in the housing on each side of the central outlet or inlet, for providing hydraulic balance to the screw members. By way of example, one embodiment of a pump according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a sectional view of a conventional screw displacement pump of constant screw pitch; Figure 2 is a view illustrating flow of fluid along the screws of the pump; Figure 3 is a view illustrating a screw having two sections of different pitch and its relationship with a chamber wall; (showing one end of pump only). Figure 4 is a view illustrating a screw having two sections of different pitch and a gap between the two sections, and its relationship with a chamber wall. Figure 5 is a pressure-volume diagram for the conventional pump of Figures 1 to 3; and Figure 6 is a pressure-volume diagram for a pump including the screws of Figure 4. Figure 1 shows a known screw displacement pump 10 having a body 11, and a chamber 12 within the body. Operational principles are similar in the conventional pump to the pump according to the invention, and the operational principles of the conventional pump 10 will therefore be described. Within the chamber 12 are mounted two screw shafts 13,14 arranged to intermesh. The screw shafts 13,14 are mounted for rotation in bearings 15,16, and timing gears 17 on the screw shafts intermesh to ensure that the screw shafts 13,14 rotate at the same speed in opposite directions. Each screw shaft 13,14 has two threaded portions one on each side of a central threadless portion, the two threaded portions of the screw shaft 14 having reference numerals 22 and 23 and the two threaded portions of the screw shaft 13 having reference numerals 20 and 21. The two threaded portions of each screw shaft are of opposite hand, and the meshing threads of the screw shafts 13 and 14 are of opposite hand. Inlet to the chamber 12 is at each end thereof from an inlet plenum chamber 30, the screw shafts 13,14 drawing fluid to the centre of the chamber where fluid is discharged through discharge opening 31. Figure 2 illustrates diagrammatically the flow of fluid along the screw shafts 13 and 14. It will be appreciated that this flow arrangement avoids any net axial thrust on the screw shafts 13 and 14. It will be appreciated that seals between the screw shafts and the surrounding body 10 are necessary, but these seals are conventional and will not be described in detail. The pump of Figures 1 and 2 is a conventional screw displacement pump, designed for liquid handling. Where comingled flows are to be pumped, the pump of Figures 1 and 2 has a disadvantage that it effects no compression of the gas phase during passage along the screws. The pressure/volume diagram for the conventional screw pump of fixed pitch when passing fluid at the inlet pressure P₁ and outlet pressure P₂ is illustrated in Figure 5. Fluid enters the pump inlet at the pressure P₁ in the inlet pipe and upon reaching the outlet is suddenly compressed to pressure P₂. The work done by the pump drive is represented by the area abcd, whereas any compression taking place before discharge will clearly reduce the power consumed. Figure 3 illustrates a threaded portion of screw shafts 40 to be used in a pump according to the invention. The body, chamber, drive, bearings and seals of the pump according to the invention will be as already described with reference to Figure 1, but the screw shafts will both carry threaded portions, each threaded portion having a change of pitch along its length. In Figure 3 the screw shafts 40 lie in chamber 12', and there are clearances between the screw shafts and between the chamber wall and the threads of the screw shafts 40. Each screw shaft has a first threaded portion 41 of pitch X at the inlet end of the thread and a second threaded portion 42 of pitch Y smaller than pitch X at the discharge end. When 100% liquid is being pumped, compression at the transition from pitch X to pitch Y cannot take place and the leakage across the clearances between the chamber wall and the screw shafts 40 and between the intermeshing screw shafts must take place to avoid a liquid lock. The output when 100% liquid is being pumped thus corresponds to the swept volume of the second threaded portion 42. Figure 4 illustrates a threaded portion of screw shafts 40 to be used in a pump according to the invention. The body, chamber, drive, bearings and seals of the pump according to the invention will be as already described with reference to Figure 1, but as with the embodiment of Figure 3, the screw shafts 50 will both carry threaded portions, each threaded portion having a change of pitch along its length. In the Figure 4 embodiment, however, there is a break between the threads of different pitch to provide an intermediate plenum chamber 51. The screw shafts 50 lie in a chamber 12" and there are clearances between the screw shafts and between the chamber wall and the threads of the screw shafts 50. Each screw shaft 50 has a first threaded portion 52 of pitch X at the inlet end of the thread and a second threaded portion 53 of pitch Y smaller than pitch X at the discharge end. The intermediate plenum chamber 51 lies between the threaded portions 52 and 53. When 100% liquid is being pumped through a pump having screw shafts of Figure 3 or Figure 4, compression at the transition from pitch X to pitch Y cannot take place and leakage across the clearances between the chamber wall and the screw shafts and between the screw shafts must take place at the portions 41 or 52 of larger pitch to avoid a liquid lock. The output when 100% liquid is being pumped thus corresponds to the swept volume of the second threaded portion 42 (for the Figure 3 embodiment) or 53 (for the Figure 4 embodiment). Once the material being pumped includes some gas, compression of the gas in the comingled flow can take place at the transition between the first and second threaded portions 41 and 42 in Figure 3, or between the threaded portions 52 and 53 in Figure 4. Once there is a proportion of gas in the comingled flow an intermediate pressure (P R ) level determined by the particular dimensions of the pump and the relationship between the pitches X and Y, is attained and leakage across the threaded portions of larger pitch 41 or 52 is reduced, compression of the gas in the comingled flaw allowing sufficient reduction in the volume of the comingled fluid to avoid a liquid lock. The pressure/volume diagram of Figure 6 shows what happens to the gas in the comingled flow where the proportion of gas in the comingled flow has reached the predetermined level. Volume A - B represents inlet volume V1 modified by the volumetric efficiency of pitch X in Figures 3 and 4 against the differential pressure of P ′g - Pl ′ . Volume A - C represents interstage volume Vg modified by volumetric efficiency of pitch Y in Figures 3 and 4 against the differential pressure of P ′2 - Pg ′ to give the final output volume. Pressure Pl ′ represents inlet pressure. Pressure Pg ′ represents interstage pressure, which is dependent on the pitch ration of X : Y and gas to oil ratio. Pressure P ′2 represents outlet pressure (system resistance). The work done is based on the inlet and interstage volumes Vl and Vg respectively. The work done with a two pitch configuration of X and Y is as follows:- For pitch X (inlet pitch) with 100% fluid, work done is as Figure 5, pitch Y having no work input. For pitch X with gas content, the work done will be to raise volume A-B to intermediate pressure P ′g shown on Figure 6, as the area within a ′, b ′, e ′, h ′, the gas content being compressed by pressure ratio of P ′g to Pl ′ at interstage. For pitch Y (outlet pitch) the lesser volume A - C is raised from P ′g to P ′2 and work done is represented as h ′, g ′, f ′, d ′. As a smaller volume is being raised to the outlet pressure P ′2, the work saving over the single stage pump is represented by g ′, e ′, c ′, f ′. It will be appreciated that there must be sufficient liquid phase present to seal the clearances against gas leakage. While screws having two distinct stages have been described, it will be appreciated that the pitch of the threads could be reduced continually along the threaded portions, and that the variation in pitch need not be uniform. Also, there may be more than two distinct pitch changes. By staging within the pump and balancing pitch ratios, hydraulic lock can be avoided, the pump itself compensating for the various flow regimes. A significant advantage of this embodiment of the invention is the reduction in power consumption when handling comingled flow as compared to a conventional screw displacement pump.

Claims: A screw displacement pump (10) for comingled material having a gas part and a liquid part, the pump comprising a body (11) defining a chamber (12), at at least one inlet (30) and at least one outlet (31) for the admission of fluid to and discharge of fluid from the chamber, a plurality of intermeshing screw members (13,14,40,50) mounted for rotation within the chamber for transporting the comingled material from the inlet to the outlet, the threads (20-23,41,42,52,53) of the intermeshing screw members being of opposite hand, wherein the pitch of the screw members at the outlet end thereof (42,53) is smaller than the pitch of the screw members at the inlet end thereof (41,52) to cause compression of gaseous material being transported, characterised in that clearances are provided between the screw members and between the screw members and the walls of the chamber to allow sufficient leakage of the material towards the inlet, when the material is in the liquid phase, to avoid a liquid lock. A screw displacement pump as claimed in Claim 1 wherein the pitch of the screw members varies along the length of the screw members. A screw displacement pump as claimed in Claim 1 wherein the pitch of the screw members decreases in discrete steps from the inlet end thereof to the outlet end thereof. A screw displacement pump as claimed in Claim 3 having a break (51) between threads of the screw members at one or more of said discrete steps. A screw displacement pump as claimed in any preceding Claim having one central outlet and two inlets one at each end of the chamber, the pump comprising one set of screw members mounted for rotation in the housing on each side of the central outlet, for providing hydraulic balance to the screw members. A screw displacement pump as claimed in any one of Claims 1 to 4 wherein the chamber has a central inlet and two outlets one at each end of the chamber, the pump comprising one set of screw members mounted for rotation in the housing on each side of the central inlet, for providing hydraulic balance to the screw members.

Inventors: GOODRIDGE, RAYMOND ANTHONY LEONARD
Applicants: STOTHERT & PITT PLC
Application year: 1985
Main classification: F04C2
IPC codes: F04B19, F04C13, F04C2