What is a multiphase pump?
The multiphase pump is the key equipment for transporting oil and gas multiphase flow. In the process of multiphase flow transportation, the working conditions of multiphase pumps are very harsh. The medium to be conveyed is a mixed flow of crude oil, seawater and natural gas, which also contains solid sand particles. “Gas-liquid-solid” three-phase flow pump.
The exploration and development of oil and gas in offshore (offshore and deep sea) oil fields has given birth to the birth and development of oil and gas multiphase mixed transportation technology. This is because the oil and gas are separated and the investment in laying pipelines separately is too huge. After adopting the multi-phase mixed transportation technology, oil and gas transmission can share a pipeline, thereby greatly reducing the construction investment cost of pipelines and equipment, and can also reduce the pipeline operation and monitoring costs after commissioning. Due to the long-distance transportation involved, the development of this multi-phase mixed transportation technology is of great economic significance.
Although the multi-phase mixed pump technology has been developed for more than 20 years, it is still considered a young technology because of the complex and changeable natural conditions of the subsea structure and the difficulty.
In the field operation of the multiphase pump, even under the moderate gas content of the subsea oil wellhead (the gas content does not exceed 90), it may still encounter the gas phase of 100 for a long time (for example, no more than 2 hours) several times a day, that is, running under dry running, at this time, the internal dynamic and static parts of the pump will be damaged due to high-temperature rise caused by dry friction, resulting in a shutdown. Changes in temperature difference (thermal shock), result in temporary leakage of the sealing end face or even failure of the sealing failure. These seemingly unsolvable technical problems have forced major oil companies to include breakthroughs in oil-gas mixed-flow transportation technology in their development strategies.
What are the different types of multiphase pumps?
There are two main types of mainstream pumps that can be successfully put into industrial production now: one is the screw axial flow multiphase pump belonging to the co-rotating force pump, and the other is the twin-screw multiphase pump belonging to the positive displacement pump. These two basic types of multiphase pumps, which are completely different, have their own advantages and disadvantages, and they need to be analyzed in detail when they are applied.
1. Spiral axial flow multiphase pump
The discharge flow of the screw axial flow multiphase pump is much larger than that of the twin screw multiphase pump, and the structure is compact; it can operate under high sand concentration and is not sensitive to the solid sand contained in the multiphase flow. (that is, the sand content rate) can exceed 5%; the gas content range can reach 10% to 90%, which is not as large as that of the twin-screw multiphase pump (up to 97%); the viscosity range of the medium is not as wide as that of the twin-screw multiphase pump. If the viscosity is too high, the efficiency of the pump will drop sharply, and it may even fail to run. At present, the flow rate of the helical axial flow multiphase pump with the largest displacement in the world can reach 3300m/h, and the single machine power can reach up to 6000kW.
2. Twin-screw multiphase pump
Due to the inherent characteristics of twin-screw pumps, twin-screw multiphase pumps have some incomparable unique advantages:
1) Advantages of twin-screw multiphase pumps
① It has the characteristics of the forced output of gas and liquid, no matter how the gas content changes, it can be forced to discharge, and the gas content of the medium can reach 97%;
② It can transport oil-gas mixed flow with high viscosity, and the dynamic viscosity range of the medium can be from 2 to 2000cP;
③ Since the master and slave screws are not driven by mutual meshing, but are driven by external synchronous gears, the master and slave screws, and the screw and the stator do not contact each other, so they can withstand “dry running”, that is Deliver 100% of the gas in a short time;
④ Twin-screw multiphase pump has high volumetric efficiency.
2) Disadvantages of twin-screw multiphase pumps
① The sand content of the medium is not as large as that of the spiral axial flow multiphase pump, and cannot exceed 5%;
② The maximum flow rate of the twin-screw multiphase pump seen so far is far less than that of the helical axial flow multiphase pump. According to the actual measurement of the US Fusi company, the maximum flow rate of the twin-screw multiphase pump is only 600m3/h.
What is the capacity of multiphase pumps?
In 2008, in the test of a screw axial flow multiphase pump prototype jointly developed by British Shell Oil Company, French Total Company, British Petroleum Company (BP) and Petrobras, it reached 15000kPa (150bar), which is equivalent to 1500m water column of clean water. When the inlet gas content rate is 50%, this head value is a breakthrough, which means that if two pumps are connected in series in the subsea pump station, oil can be produced at a depth of 2000-3000m under the sea. This is the maximum head (pressure difference) achieved by the helical axial flow multiphase pumps in the world.
A newly designed twin-screw multi-phase pump from Bornemann Pump Company in Germany has a lift (pressure difference) of 172001cPa (172 bar) obtained in the test. This is an unprecedented important milestone in the research and development of twin-screw multiphase pumps, which means that the subsea depth of oil production that twin-screw multiphase pumps can now reach has exceeded that of screw axial flow multiphase pumps.
A multi-phase pump unit (including the pump itself, subsea high-voltage motor, high-voltage electrical control system, etc.) installed in the subsea multi-phase pumping station weighs about 20 tons. It can be seen that this subsea technology is very difficult. In this “dual-purpose pumping station” under the sea, multiphase pumps can be connected in series (pumping to a depth of 3000m) or in parallel. If the displacement of the multiphase pump (referring to the screw axial flow multiphase pump) is large, one pump can be connected in parallel to serve several oil wells. Up to one multiphase pump can be seen now, pumping oil and gas mixed flow for four oil wells.