American Oil and Gas Reporter - June 2019 - 66

SpecialReport: Artificial Lift Technology
profiles are turning. As shown in Figure 3, the chambers transfer
the volume of gas and liquids that are contained in each chamber
from the inlet to discharge side of the rotors by the two opposing
sets of screw profiles. This configuration neutralizes the hydraulic
axial load and eliminates the need for balance drums or thrust
bearings.
Handling Liquid, Gas Phases
The liquid phase of the multiphase flow plays a deciding
role for compressing the gas. As a result of the screw rotation,
the gravity difference between gas and liquid will force the
liquid phase to separate from the gas phase. The liquids will
remain in the annulus formed between the liner and the tip of
the screw profile, while the gas will accumulate along the
center at the root of the screw profile. When the flow stream
reaches the end of the screw profile, it becomes exposed to the
pump backpressure and an almost solid liquid phase develops.
Because of the pump discharge or backpressure, the liquid
will flow as an annulus slipstream in the opposite direction of
the main flow and subsequently backfill one chamber after the
other. The gas in each chamber becomes compressed by the
backflow and the gas volume is reduced significantly as the
pressure increases at the pump discharge.
In addition, the liquid in the flow stream plays an important
role for sealing the screws, compressing the gas and removing
the heat of compression. At a very high GVF (98-99 percent)
the stability of the liquid phase is no longer assured; the liquid

can foam and the backflow can be undefined and turbulent, or
simply disappear. If the liquid flow is disrupted, the pump will
lose the ability to compress gas and could vapor lock. This may
result in overheating, which can damage the pump.
The liquid fraction at the pump suction needs to be kept at a
minimum of 3-5% of inlet flow. However, adding a liquid recirculation system (Figure 4) as an integral part of the pump
package allows the unit to run on 100% gas at the skid inlet for
an extended time. The liquid recirculation system includes a
liquid knockout boot or separator, and is designed to retain
adequate liquid to compensate for long gas slugs and unstable
inlet conditions. It is important for starting up and flowing shutin, gas-capped wells, where the gas is the initial flow and little
or no liquids are produced.
In wet-gas applications, the liquid knockout also can work
in conjunction with a gathering compressor where a reduced
sized inlet separator becomes the knockout boot, and combining
with liquids from the scrubber, the multiphase pump transports
all liquids plus carry under gas to a re-combiner downstream of
the compressor. The advantage is that only one pipeline is
needed for bringing the total production to the gas processing
plant and all liquids handling is eliminated at the production
site.
Production Challenges
The increasing use of downhole pumps has become necessary,
often right from the start of producing a reservoir as operators

FIGURE 4
Liquid Recirculation System
Bypass Line
Bypass Drain
PSV
100

TE TE
101 102
TE TE
103 104

DPI
101

PI
100

PIT TI
100 100

Strainer

Multiphase
Pump

PI
101

PIT TI
101 101

TE
100

Vent

Inlet

Outlet
Pump Suction Flange
Drain

Pump Discharge Flange
Pump
Casing Drains

Seal Drain Collection Lines
Sight
Glass

LSL
100

Drain
Seal Drain
Collection Tank

TE
105

TE
106

TE
107

TE
108

TE
109

TE
110

Winding RTDs
(2) Per Phase

66 THE AMERICAN OIL & GAS REPORTER

Motor

Process Seal Flush

Liquid
Knockout
Boot

FE
100
FIT
100

Drains

LG
100

Blind Flange,
Clean Out Access

American Oil and Gas Reporter - June 2019

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