Multiphase pumps are used when oil, gas, water and sometimes solid particles need to be transported through the same gathering system. They are especially useful when separating the fluids at every wellhead is impractical or too expensive.
The idea is straightforward, but the operating conditions are not. A multiphase pump may receive a fluid whose gas content, viscosity, pressure and solids concentration change continuously. It may also have to deal with sand, corrosion, wax, hydrates, slug flow and large temperature changes. These conditions are the reason that a pump that works well in a clean-liquid test may perform very differently in an oilfield.
The following issues are usually the most important ones to consider when designing, selecting and operating a multiphase pumping system.
1. The pumped fluid is not a fixed product
In a laboratory, a fluid can be described by a single set of properties. At a well site, the composition may change with the well, the season, the production rate and the pressure in the gathering line.
The mixture may contain crude oil, produced water, natural gas, dissolved salts, carbon dioxide, hydrogen sulfide, wax, hydrates and sand. The amount of each component may also change over time. As a result, the pump has to work with a range of conditions rather than one design point.
This matters because each component creates a different problem. Water and dissolved gases can contribute to corrosion. Sand can wear the screws, bushings and seal faces. Wax can increase viscosity or deposit on cold surfaces. Hydrates can restrict the line if the pressure and temperature are suitable for their formation. Hydrogen sulfide also creates material and safety requirements that cannot be treated as ordinary corrosion concerns.
For this reason, pump selection should begin with actual field data. At a minimum, the supplier or design engineer needs the oil, water and gas rates; pressure and temperature ranges; water cut; gas composition; viscosity; expected sand concentration; particle size; and any known wax, hydrate or sour-service conditions.
2. Gas content can change the way the pump operates
A conventional liquid pump is normally selected on the assumption that its pumping chambers remain filled with liquid. A multiphase pump does not have that luxury. The gas volume fraction can vary from a relatively small amount to a gas-dominant condition, and the flow pattern can change as the mixture travels through the system.
At low gas content, the gas may be dispersed in the liquid. At higher gas content, the flow can become intermittent or form large gas pockets. Slug flow is particularly important because the pump may receive alternating volumes of gas and liquid rather than a stable mixture.
These changes affect several parts of the system at once:
- the amount of fluid entering each pumping chamber;
- the pressure rise and delivered flow;
- the torque required by the driver;
- the temperature produced by gas compression;
- vibration and pressure fluctuation; and
- the load on valves, couplings and the pipeline.
A gas-volume figure is meaningful only when its basis is clear. Gas volume changes with pressure and temperature, so the design should state whether the value refers to inlet conditions, standard conditions or another reference state. It is also important to distinguish between a continuous operating limit and a short-duration capability.
Using an average gas content for sizing can be misleading. The cases that determine reliability are often the high-gas, low-pressure or slugging conditions rather than the average production condition.
3. Gas compression produces heat
When gas is compressed inside the pump, some of the input energy becomes heat. Friction, leakage and mixing can add to this heat. The temperature increase depends on the gas fraction, pressure ratio, rotational speed, clearances, cooling arrangement and properties of the mixture.
The problem is not only the average temperature of the pump. A local hot spot can be more damaging than a moderate overall temperature. Excessive heat may cause the following problems:
- seals and elastomers lose their properties;
- lubricant temperature rises and service life is reduced;
- components expand and clearances become smaller;
- wax or other deposits change their behavior;
- the rotor and stationary parts may begin to contact; and
- the pump may trip or seize.
Cooling should therefore be considered during the initial design, not added after a temperature problem appears. Depending on the application, heat may be managed through a casing cooling jacket, external cooling, a controlled seal-support system, lubrication or a reduction in operating speed.
Temperature sensors should be placed where they can reveal the actual failure mechanism. Monitoring only the motor temperature, for example, may not show that the seal area or pump casing is overheating.
4. Mechanical seals have a difficult job
Mechanical seals in oil and gas multiphase service may be exposed to gas, pressure fluctuations, solids, corrosion and high temperature. They can also experience periods in which the liquid film needed to lubricate the seal faces becomes unstable.
There is no single seal arrangement that is correct for every application. A relatively clean and moderate-duty service may use a simpler seal arrangement. A service with high gas content, high pressure, high temperature, abrasive solids or hazardous fluids may require a dual seal and a controlled barrier or buffer system.
The seal review should include more than the seal-face materials. It should also cover:
- pressure on each side of the seal;
- temperature during normal operation and start-up;
- the possibility of dry running;
- gas exposure and pressure fluctuations;
- solids and abrasive particles;
- compatibility of O-rings and other elastomers;
- support-fluid pressure, temperature and flow; and
- what happens if the support system loses pressure.
Seal support is part of the safety and reliability system. It needs its own instruments, alarms, maintenance procedure and shutdown logic. Simply specifying a seal type without defining its operating conditions leaves an important part of the design unfinished.
5. High gas content may require liquid replenishment
The internal clearances of a screw-type multiphase pump have to be filled sufficiently for the pump to separate the high- and low-pressure sides and produce a useful pressure rise. If the pump becomes almost entirely gas-filled, the normal liquid circulation may no longer be enough.
This is why some systems include gas–liquid separation, liquid recirculation or automatic liquid replenishment. These arrangements are intended to return enough liquid to the pump so that it can continue operating or recover safely when the gas content falls again.
The need for such equipment depends on the pump design and the production conditions. It becomes more important when the system may experience very high gas content, large gas slugs, unstable well flow or temporary loss of liquid supply.
A published gas-content limit should always be read with its conditions. The limit may depend on inlet pressure, speed, temperature, duration, cooling, liquid replenishment and the required output. A statement that a pump can handle a certain gas fraction is not, by itself, a complete operating specification.
6. Sand causes both erosion and secondary damage
Sand is one of the most common causes of wear in oilfield pumping equipment. The damage depends not only on the amount of sand, but also on particle size, hardness, shape and velocity.
Erosion can affect screws, bushings, liners, valve seats and seal faces. As clearances increase, internal leakage may also increase. The pump then needs more power to produce the same pressure, while its capacity and efficiency may decline.
Sand can also damage the seal indirectly. Particles that enter the seal area may scratch the faces or prevent them from closing properly. This can lead to leakage even when the seal materials are chemically compatible with the pumped fluid.
The design response may include lower pump speed, wear-resistant materials, surface hardening, coatings, upstream sand management or a maintenance plan based on measured wear. No material is universally best; a hard surface that resists abrasion may not be the best choice for a corrosive service or a design with tight thermal clearances.
7. Corrosion requires a complete materials review
Choosing stainless steel for the casing does not automatically make the pump corrosion-resistant. Corrosion depends on the composition of the water phase, temperature, pressure, flow velocity, dissolved gases, chlorides, deposits and local surface conditions.
Every wetted component should be reviewed separately. This includes the casing, screws, shafts, bushings, liners, coatings, gaskets, O-rings and seal faces. The materials used for the metal components may be suitable while an elastomer swells or hardens in the same fluid.
For sour service, the material review should also follow the project’s applicable requirements for hydrogen sulfide exposure. The result should be documented rather than based on a general statement such as “oilfield grade” or “stainless steel construction.”
Where the fluid composition is uncertain, it is safer to identify the uncertainty and obtain additional samples or laboratory compatibility data than to select a material based on the fluid name alone.
8. Slug flow creates a system problem, not only a pump problem
Slug flow can produce rapid changes in density, pressure, flow rate and torque. The pump may be able to tolerate some variation, but the complete system still has to deal with the resulting transients.
The following items may need to be reviewed together:
- the expected size, frequency and velocity of liquid slugs;
- the suction line and any upstream separator or slug catcher;
- the pressure rating of the pump and pipeline;
- the response of the motor, gearbox and variable-speed drive;
- check valves and automatic isolation valves;
- pressure-relief and emergency-shutdown arrangements; and
- the control system’s response to sudden changes in inlet pressure.
Changing pump speed can help match the output to the inlet condition, but it is not a substitute for understanding the flow regime. A control system needs reliable pressure, temperature, vibration and speed measurements, along with clear limits for alarm, speed reduction and shutdown.
9. Instrumentation should answer practical operating questions
Instrumentation is useful when it helps the operator answer a specific question: Is the pump receiving enough liquid? Is the seal still supported? Is the pump overheating? Is the discharge blocked? Is the driver overloaded? Is the pump beginning to wear?
Depending on the application, useful measurements may include suction and discharge pressure, casing and seal temperature, bearing vibration, motor current or power, rotational speed, seal-support pressure and level, and process flow or an inferred flow value.
The system should also define the response to abnormal conditions. For example, loss of seal-support pressure may require an alarm followed by a controlled shutdown. A blocked discharge may require immediate pressure protection. Low suction pressure may require speed reduction or isolation. These actions should be established during design and tested during commissioning.
Automatic valves can protect the pump and pipeline, but their closing time and sequence matter. Poorly coordinated valve action can create pressure surges instead of preventing them.
10. Start-up and shutdown are often harder than normal operation
A pump may operate satisfactorily once the well fluid has reached its normal temperature, but the same pump may have difficulty starting when the fluid is cold and more viscous. Wax or hydrates may also be more likely to deposit during a shutdown.
The operating procedure should answer the following questions:
- Is the pump supplied with enough liquid before starting?
- Does the fluid require heating or circulation?
- What is the maximum allowable start-up torque?
- What happens if the pump is filled with gas?
- How is the line depressurized or isolated during maintenance?
- How long can the pump remain stopped before deposits form?
- What checks are required before an automatic restart?
Many failures attributed to pump design are actually related to start-up, flushing, isolation or restart procedures. These procedures should be written around the real fluid and the real installation rather than copied from a clean-water application.
A practical checklist for reviewing a multiphase pump system
Before finalizing a pump or package, confirm that the project has answers to the following questions:
- What are the minimum, normal and maximum oil, water and gas rates?
- How is gas volume fraction defined, and how much can it vary?
- What pressure and temperature ranges will occur at the pump inlet and outlet?
- Is slug flow expected? If so, what are its size and frequency?
- How much sand is present, and what are the particle size and hardness?
- Are hydrogen sulfide, carbon dioxide, chlorides, wax or hydrates present?
- Which component is most likely to fail from corrosion, erosion or heat?
- What seal arrangement and seal-support conditions are required?
- How will the system supply liquid if the pump becomes gas-dominant?
- What measurements will detect overheating, loss of seal support, vibration or overload?
- What will the controls do during low suction pressure, blocked discharge or reverse flow?
- Can the pump start, stop and restart safely under the actual field conditions?
Conclusion
Multiphase pumping is difficult because the pump does not handle one stable liquid. It handles a changing mixture whose gas content, pressure, temperature, viscosity and solids load may all vary during production.
The most reliable approach is to design around the complete operating envelope. Material selection, seal design, cooling, liquid replenishment, sand management, slug-flow analysis, instrumentation and start-up procedures should be considered together. A pump’s advertised maximum gas content or pressure is useful only when the associated conditions are understood.
Good field data is the starting point. If the gas, liquid, solids and transient conditions are not known, the first task is not to choose a larger pump. It is to improve the process information and identify the conditions that will control reliability.
