Sealless multiphase screw-pump-and-motor package

Details Sealless multiphase screw-pump-and-motor package Sealless multiphase screw-pump-and-motor package

2000-05-04

2002-10-01

Freay, Charles G. (Department: 3746)

Pumps

Motor driven

Including means utilizing pump fluid for augmenting cooling,...

C418S102000

Reexamination Certificate

active

06457950

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to positive displacement pumps and more particularly to sealless screw pump/motor packages especially for pumping multi-phase fluids in subsea applications.
As remote subsea wells deplete, boosting is not cost effective if the pump requires mostly liquid in order to function; because such wells produce a large volume fraction of dirty water and gas along with a small amount of oil. The small amounts of oil involved, 1000 barrels per day (bpd) or less, cannot be economically recovered unless a multiphase pump is located in the vicinity of the well. To improve economics, multiple wells can be manifolded together to feed a single pump, the piping arrangement providing for a flow check of each pump individually. This concept is illustrated in FIG.
1
. Several of such multiphase pumps delivering product to centrally located separation equipment on a surface platform or onshore appears to be a practical way to extend the life of wells that would otherwise have to be abandoned. These wells normally produce mixtures of gas, oil and water in varying proportions that can vary considerably at the pump inlet over time. Gas void fractions (GVF) of 0.95 (i.e., 95% gas by volume)—and higher—are fairly typical. GVF is related to the more frequently quoted gas-oil ratio (GOR) or the mass of gas in standard cubic feet per barrel of oil (scf/bbl) as follows:
GVF=GLR/(1+GLR)  (1)
where GLR is the volume flowrate ratio of gas Q
G
to liquid Q
L
and is given by
GLR=(GOR)(T/T
std
)(P
std
/P)/(5.615 cu ft per bbl)  (2)
where T is absolute temperature and p is pressure. Standard temperature and pressure are 15° C. and 14.7 psia respectively; so that T
std
=(273.15+15)°K. This mixture must be pumped to as much as 50 bar or 700 psi.
To date, practically all multiphase pumps have been located on the surface and generally onshore, where the installation costs are smaller and the frequent maintenance needed for new concepts can be carried out with relative ease. To install and maintain a pump subsea requires a considerable infusion of deepwater technology, which is as sophisticated as the design of the pump package itself. As more success is achieved in dealing with the technical and reliability issues encountered in the multiphase pumps located on the surface, there is now more impetus to place them subsea.
For pumping multiphase fluids, two quite different types of multiphase pump are employed, namely, a) rotodynamic and b) rotary positive displacement. Type (a) creates pressure dynamically; i.e., shaft torque is converted into fluid angular momentum. The pressure rise then depends on the product of average fluid density and velocity change. The helico-axial configuration is the rotodynamic concept that is used for multiphase pumping, because it has many axial-flow stages that do not vapor-lock; i.e., they do not separate the gas and liquid phases by the centrifuging—as can occur, e.g., in a single-stage centrifugal pump (also a rotodynamic machine). This machine depends on speed and fluid density to develop pressure. Sudden changes in fluid density, as would occur in slugging, produce sudden changes in torque. Type (b) develops pressure hydrostatically and so does not depend on the pump speed or fluid density. The inlet of the pump is walled off from the discharge, e.g. in the case of the popular two-screw configuration, by the meshing of the screws. As with a reciprocating pump, the shaft power is simply the displacement volume rate Q
d
times the pressure difference &Dgr;p across the pump; and the shaft torque is this power divided by the angular speed &ohgr; of the drive shaft. Thus if slugging occurs and the &Dgr;p remains constant, this slugging has a relatively small effect on shaft torque.
In both cases, the intake volume flowrate capability increases with speed. A rotodynamic pump needs to speed up at high GVF (low average fluid density) in order to maintain &Dgr;p at the same level that a lower speed produces at lower GVF; while a positive displacement pump can run at constant speed; albeit with reduced liquid output.
The efficiency of multistage pumping is the ideal power P
i
divided by the pump shaft power P
s
. In the presence of typical amounts of liquid, the process tends to be isothermal, in which case P
i
=P
isoth
, where
P
isoth
=m
RT
1
1n(p
2
/p
1
)+Q
L
&Dgr;p  (3)
whereas, for no liquid flow Q
L
present, the process tends to be adiabatic, in which case P
i
=P
ad
, where
P
ad
=mc
p
JT
1
{[p
2
/p
1
]exp[&ggr;−1)/&ggr;]−1}  (4)
In these equations, m is the mass flowrate, R is the gas constant, c
p
is the specific heat of the gas at constant pressure, J is the mechanical equivalent of heat, &ggr; is the ratio of specific heats of the gas, and subscripts 1 and 2 denote pump inlet and discharge respectively.
Multistaging minimizes the shaft power for a given ideal power, especially for high pressure ratios p
2
/p
1
. Such multistaging is necessary for helico-axial pumps to work; however, a single stage is the normal embodiment of a screw pump. Screw pumps tend to be smaller; so that efficiency may not then be an issue. In view of this, screw pumps are preferable for subsea applications because the small sizes needed for the low flowing remote wells are relatively inexpensive. Further economies are to be had in that they can be driven subsea by correspondingly small, constant-speed, submersible electric motors; thereby eliminating the need for VFD's or subsea deployment of hydraulic lines to run variable-speed turbines. Also, torque shock does not occur with slugging, thereby simplifying the mechanical design of the rotors.
The mechanical design of a two-screw pump is relatively simple, because a double-suction configuration is utilized. Each rotor ingests the fluid from both ends and conveys it to the center, where it is discharged, providing an axial balance that insures long bearing life. The screws do not touch each other, and clearance is provided between the screws and the surrounding bores in the body. The two rotors are kept clear of each other by a set of timing gears that are lubricated by clean oil, along with the adjacent bearings, seals being required to isolate this oil from the pumpage. The total diametral clearances and those between the meshing screw threads do not vary with axial position; so, when pumping 100% liquid, the leakage across each land and through each portion of the mesh is the same and produces a linear development of pressure vs. axial length.
Multiphase pumps depend on the liquid sealing of these clearances to produce a net positive flowrate vs. what would otherwise be a massive leakage from discharge back to inlet. In the case of 100% gas (GVF=1), this liquid sealing is maintained by recirculating liquid that was previously captured by a phase-separation plate in the discharge zone at the center of the rotors. The reservoir for this captured liquid is the special feature of a multiphase screw pump that makes possible sustained operation at GVF=1 and which can be seen in FIG.
2
. In fact, the liquid sealing is so effective at high GVF that no gas leaks back to the inlet or suction cavities at the ends of the screws. This is illustrated in the laboratory test data of
FIG. 3
for the total intake volume flowrate Q
1
vs. the pressure difference &Dgr;p across the pump. Except for the very small leakage of sealing liquid, usually less than 1% of Q
1
, the volumetric efficiency &eegr;
v
, where &eegr;
v
=Q
1
/Q
d
, is therefore 100%.
The development of pressure along the screws at high GVF is not linear with axial position as it would be for pure liquid (GVF=0). This is because gas leaks (along with the sealing liquid) across the higher-pressure screw lands near the center of the pump in order to compress the gas in the neighboring. “lock” or trapped volume between successive mesh points along the length. The pressure d

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