James R. Brennan
Imo Industries Inc.
Monroe, N.C.
In petroleum production, screw pumps fulfill the requirements for pumping crude oil with an API gravity of 20 or less (0.93 sp gr or greater).
In many cases, these pumps provide more efficiency than centrifugal and reciprocating pumps.
Heavy crude oils typically have viscosities at pumping temperatures in excess of 500 SSU (100 cSt).
While screw pumps can handle viscosities ranging from fractional centistokes to millions of centistokes, the normal range for crude oil, from cost and efficiency viewpoints, is above 100 SSU (20 cSt). Bitumens can also be handled by screw pumps.
Invented in 1922, rotary, positive-displacement multiple-screw pumps have been used for pumping fuel for burning, lubricating oil, fluid power, process feed, food, chemicals, synthetic-fiber slurries, general transfer, and many other applications.
Screw pumps have successfully pumped heavy crude oil for more than 40 years.
Worldwide there are about 1,500 screw pumps transporting crude oil in gathering lines and pipelines. Major installations can be found in Canada, Colombia, Indonesia, Venezuela, and the U.S. Additional screw pumps are operating on crude oil service in China, Egypt, Kuwait, Mexico, Peru, and Saudi Arabia.
Services include gathering lines, pipelines, tank farms, and loading and unloading barges and ships.
PUMP DESIGN
Even though centrifugal and reciprocating pumps have been preferred in petroleum production, the pressure capabilities of 1505,000 psi (10-345 bar) and flow capabilities of 1-15,000 gpm (0.25-3,400 cu m/hr) make screw pumps also suitable for an extensive range of services.
CENTRIFUGAL PUMPS
Single and multistage oil field centrifugal pumps are usually of the impeller-between-bearing design, horizontally split. The pumps have two bearing and two shaft seals, one each at each end of the pump shaft.
Because the pumps normally are tested with water, actual field performance with viscous hydrocarbons is estimated from water test data and empirical consider- ations.
With viscous liquids, centrifugal pumps exhibit a marked input power increase while at the same time delivering a reduced capacity and lower head capability. This performance degradation becomes severe at about 500 SSu (100 cSt),
A centrifugal pump's annual energy costs, even at modest power levels, can be high for pumping medium and heavy oils.
Screw pumps, however, provide much better efficiencies compared to centrifugal pumps when pumping these medium and high-viscosity crude oils.
Fig. 1 illustrates the rate at which centrifugal pump efficiency drops with increasing viscosity and also shows the comparable effect for a typical screw pump (see box).
When pumps are properly sized for the same medium and especially heavy crude oils, screw pumps with their drivers are competitively priced with centrifugal pumps and their drivers.
Where modest amounts of gas must be handled, a centrifugal pump normally will lose prime and stop pumping (become air bound).
A two-screw pump can handle up to 40% gas, by volume at inlet conditions, and continue to operate without difficulty.
Because of needed high tip speeds, centrifugal pumps produce high shear rates on the fluids pumped.
These high shear rates tend to increase the strength and degree of oil/water emulsification. The later separation of the water is more difficult and time consuming, requiring more energy and thus greater cost.
High pour-point crude oils, even if low in viscosity, are frequently transported by screw pumps. The constant flow rate of screw pumps provides a scouring action that minimizes wax buildup on pipe inside diameters.
Centrifugal pumps, in contrast, reduce flow rate with increasing pressure. Thus they provide little or no increase in pipeline flow velocity and cleaning action.
RECIPROCATING PUMPS
Reciprocating pumps will lose suction because of crude oil gelling in the suction pipelines.
Reciprocating pumps produce flow with extreme pressure/flow pulses that can damage piping, valves, instruments, and other flow system components by exposure to excessive pressure spikes and cyclic fatigue.
On lightly constructed, unmanned lake production platforms, the pressure pulsations and heavy vibration can jeopardize the platform itself.
Fig. 2 qualitatively illustrates the output flow and pressure from a triplex (three plunger) single-acting reciprocating pump. Depending on the pump configuration, the flow/pressure variations can exceed 25% of the mean pressure.
Reciprocating pumps, while reasonably efficient, may not self prime, are expensive to purchase, require a large amount of space, and are costly to install and maintain.
Because these pumps operate at only low speeds, a speed-reducing gear box or other mechanism is normally needed. This increases attendant cost, maintenance, and complexity.
Reciprocating pumps have inlet and outlet valves that are pushed open and closed by the flow. Thus, 100% of the pumped fluid is forced through these valves. This results in very high rates of fluid shear.
Again, this high shear rate increases the strength of oil/water emulsions that make breaking or separating out the water more expensive.
Driving reciprocating pumps is hard on prime movers such as electric motors or engines. This is because of cyclic torque fluctuations imposed by a reciprocating pump. To limit amperage variations, motor overheating and speed surging, NEMA Design C or D electric motors may be required instead of the more standard Design B.
On start-up, pump suction valve unloading devices may be required to avoid prolonged acceleration times or even to start accelerating the pump and fluid.
For all but small units, reciprocating pumps with their drivers should be installed on reinforced concrete foundations that rest on firm soil or pilings. These foundations should be entirely independent of walls or footings, building supports, or floor structures.
Like screw pumps, reciprocating pumps require discharge pressure relief valves to protect the pump, drive, drive train, and system from over pressure. To minimize relief valve seat damage from pump pressure pulsations, unlike screw pumps, the relief valve in reciprocating pumps must be set about 25% higher than design pressure.
Reciprocating pumps frequently need discharge pressure pulsation dampeners to limit the destructive damage caused by pulsations. These dampeners usually take the form of accumulators that consist of high-pressure vessels having an inert gas precharge separated from the pumped fluid by an elastomeric barrier.
Besides the purchase and installation cost, these pulsation dampeners require regular maintenance and can pose serious personal injury hazards if not operated and maintained adequately,
To absorb cyclic suction flow variations, dampeners are also frequently recommended for reciprocating pump suction lines. The dampeners help minimize the continuous acceleration pressure losses inherent in reciprocating pump suction.
Table 1 shows the recommended reductions in basic reciprocating pump operating speed on oils above 300 SSU (65 cSt). Intermediate values may be interpolated. The pump manufacturer should be consulted for operating recommendations above these values.
Further speed reduction is appropriate in the presence of abrasives, dissolved gas, high liquid or ambient temperatures, unattended operation, poor maintenance, no spare parts, or no standby pump.
SCREW PUMP
Screw pumps produce flow in a smooth continuous manner resulting in virtually no pressure or flow pulsation. Table 2 indicates the best pump to use for crude oil transport.
For surface pumping of crude oil, screw pumps have two basic designs. The most common is the three-screw type that requires no timing gears (Fig. 3). These are normally the least costly type and the most readily field repairable.
Construction normally includes case-hardened alloy steel screws for maximum abrasion resistance and thread ground forms for complete rotor interchangeability. Each wrap of the screw forms a separate chamber effectively acting like stages.
Pressure loads and wear are distributed along the entire length of the pumping screws. This design maximizes pump life,
Three-screw pumps should be limited to crude oils relatively free of gas and containing little continuous free water. These pumps rely upon a fluid film to support internal pressure forces.
Emulsions of crude oil and water, however, do not present problems for three-screw pumps. In fact, manufactured emulsions of bitumen and water such as those in Colombia and Venezuela are readily handled by screw pumps.
Three-screw pumps are very easy to repair to like new condition, have only one shaft seal and one bearing and require less space than almost any other type of pump. The pumps operate at relatively high speed for positive displacement.
Four or six-pole electric motors or direct-drive diesel engines are normally used in the 1,000-1,800 rpm range.
Modest foundation requirements make screw pumps relatively simple and inexpensive to install. No pressure/flow pulsation dampeners are necessary, and normal torque standard motors are more than adequate for drivers.
Fig. 4 shows a 1,000 hp (745 kw) three-screw pipeline pump handling 31,000 b/d (200 cu m/hr) of heavy crude at 1,400 psi (95 bar). This pump, in Alberta, is believed to be the highest power three-screw pump ever built. The unit has over 12,000 hr of continuous operation without maintenance. A second duplicate unit is awaiting start-up at an adjacent site.
A less common and somewhat more expensive screw pump design is the two-screw pump (Fig. 5). These pumps use timing gears to control the mesh of the screws. The timing gears, one on each shaft and shaft support bearing (normally four) are outside of the product being pumped and are separately lubricated.
The pumps have four shaft seals to prevent pumped fluid from contacting the gears and bearing. There is no internal metal-to-metal contact within the pump.
The two-screw pump is capable of pumping liquids with high gas percentages as well as pumping 100% water.
To better resist abrasive wear from the sand content in crude oils, screws frequently have hard-face coated outside diameters and hard-face coated bore inside diameters.
Some two-screw pump designs are fully field repairable, even to the extent that the pumping elements are assembled into a back pullout cartridge that is loaded into the pump casing much like a barrel-type centrifugal pump.
Fig. 6 shows such an 18,000 b/d (120 cu m/hr) pump. Fig. 7 illustrates cartridge loading.
In this design, neither the pump piping connections nor the driver need to be moved during routine repairs.
Both two and three-screw pumps expose the pumped fluid to low internal velocities and contain no valves. Thus the pumps produce little fluid shear and do not tend to increase the strength or degree of oil/water emulsification.
Operating speeds, in the 1,000-1,800 rpm range, are normally those of four to six-pole motors or direct-drive diesel engines.
Pipeline pressures to 1,500 psi (100 bar) are practical for either type screw pump. Units are operating in mountainous regions of South America at pressures exceeding 1,800 psi (125 bar).
Screw pumps are available for flow rates of 20-1,000 gpm (10-225 cu m/hr) at 1,500 psi and to 15,000 gpm (3,400 cu m/hr) at reduced pressures of 150-225 psi (1015 bar).
Fig. 8 illustrates the capability of two and three-screw crude oil transport pumps. Power inputs to screw pumps range from fractional to over 1,250 hp (from a few kilowatts to over 1 megawatt).
The pumps are relatively simple machines and will provide cost effective, durable, competitive pumping of medium and heavy crude oils at reasonable-to-excellent efficiencies without damaging pressure pulsations.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
