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Among the technologies developed for oil and gas production over the past two decades, heterogeneous pumping is one of the most promising technologies. Significant reductions in capital investment at sea and in the coast coupled with higher productivity have placed higher demands on installed pumping systems.
In heterogeneous pumping, the biggest challenge rotary equipment to overcome is the change in process fluid, which changes from 100% of the liquid to 100% of the gas. This change creates extremely difficult operating conditions for the pump, creating more stringent conditions for the mechanical seal of the pump's core component.
Mechanical seals can be used in liquid or gas applications. However, in multi-phase pumps, mechanical seals experience transients. The transient nature of the wellhead airflow creates pressure fluctuations and air locks. Multiphase pumps and their mechanical seals must be able to withstand all these harsh working conditions. Therefore, we must develop a special mechanical seal.
Figure 1: Pressurized double-ended mechanical seal (API610, plan 53 or
54), suitable for multi-phase pump SH
Type seal
Pump type
Multi-phase pump can be divided into two categories, each has its own advantages and limitations. These two types of pumps are rotary power pumps and positive displacement pumps.
Rotary power pump based on the concept of helical axial flow hydraulics. Multistage pumps can have up to 15 compression units on a single shaft. Each compression unit includes a rotating helical axial flow impeller and a stationary vane.
Thus, the effluent produced can be pressurized over longer distances without prior separation, and there is no limit to the gas volume ratio. This type of pump is small and compact, with mechanical simplicity and reliability, allowing solid particles to exist at high flow rates.
As is well known, positive displacement pumps or twin-screw pumps have been able to handle gas volume ratios up to 70%. In this respect, only minor improvements to the existing pump design are required. Double-rotor screw pump includes two reverse screw.
The gap between the screw and the liner requires a certain amount of liquid to seal. This pump offers true dry running performance and can handle a wide range of specific and gas volumetric ratios. Their reliability has been demonstrated in some practical multiphase booster applications.
Mission problems
Unlike multi-phase pumps, mechanical seals are not specifically designed for use in liquid or gaseous media, but operate with ever-changing gas and liquid flow rates. Therefore, the mechanical seal must deal with the following situations: Figure 2: Rotary power pump based on screw axial flow mechanics Figure 3: Twin-screw pump with two counter-rotating screws Figure 4: Typical face-to-face double-ended high-pressure seal
â—† unpredictable process media composition â—† density and viscosity changes â—† temperature changes â—† erosion, mainly caused by the sand â—† high and low working pressure â—† seal the end of the lubrication and lack of cooling
Chemical parameters
Depending on the field and / or location, the chemical analysis of the processing medium is not consistent. This means that the crude oil may contain:
â—† Wax â—† Salt â—† Water â—† Acid gas (H2S) and / or carbon dioxide (CO2)
â—† cracked hydrocarbons â—† solid particles
All of these affect the performance of the seal and can cause erosion and corrosion of the mechanical seal.
Sealing principle
The pressure of the fluid being sealed is reduced radially along the sealing surface. Friction of the seal face causes the temperature of the fluid at the seal face to increase. A decrease in pressure, and an increase in temperature of the fluid at the sealing face will result in the vaporization of volatile elements within the sealed fluid.
This can result in insufficient lubrication and cooling of the seal face. The gas content in multiphase process media poses the additional risk of insufficient lubrication of the seal face, which can cause failure of the seal face.
In the optimized parallel clearance, the axial opening force and the closing force are balanced. In this case, the contact force (Fc) is evenly distributed along the width of the sealing surface. The pressure gradient (Fs) of the sealed fluid is linear.
According to the mechanical seal surface hardness, there will be two other forms of gap.
In the case of the A-shaped gap, a high contact force is formed at the outer diameter and the pressure gradient becomes steep. Resulting in a sealed unstable operating condition.
V-shaped gap, the sealing surface of the inner diameter of the formation of low contact force, the pressure gradient is gentle, the mechanical seal running in steady state.
Here are two examples of rotary seal solutions.
Single-End Mechanical Seal (API 610, plan 32) Single-end mechanical seals are used in multi-phase twin-screw pumps. Single sealed lubrication and cooling comes from external pressure source flushing (API 610-plan 32). Flushing must be continuous and reliable - even during pump start-up and shut-down. Throttle bushings located in front of the seal with a small gap control the flow of flushing fluid into the pump, isolating the seal from the process fluid.
Pressurized double-ended mechanical seals (API 610, plan 53 or 54) Pressurized double-ended seals are mainly used in multi-phase pumps. This example shows a typical double-sided, high-pressure seal in face-to-face format. Pressurized double-ended seals are commonly used when the use of single-ended seals flush with the outside of plan 32 (continuous loss) is limited. Double seal is lubricated by an external closed-loop supply system. Figure 5: Clearance rigidity of a mechanical seal
The mechanical seal is specially designed for heavy duty applications. The sealing surface is double balanced; the seal remains tightly closed in the event of a drop of isolated hydraulic pressure or high pressure transients in the sealed chamber, allowing the pump to stop without risk of seal surface damage or product contamination of the atmosphere. The same dead-ring assembly has the best operating characteristics when operating conditions change. The seal is designed as an integrated structure so it is compact, rugged and easy to maintain.
Select the appropriate sealing surface materials and structural materials to avoid corrosion. Silicon carbide is often used as the sealing surface material because of its high hardness and excellent thermal conductivity. Use duplex and duplex stainless steel as the material for metal parts.
Conclusion: Mechanical seals have proven their reliability in sealing multiphase mixtures in hundreds of applications worldwide, under a wide range of operating conditions and in harsh environments.
Today, mechanical seal types and sliding materials are capable of meeting any technical requirement of multiphase pumping. In order to adapt the sealing system to the specific operating conditions, close cooperation between the sealing manufacturer, the pump manufacturer and the end user is a basic precondition. Proper application, installation and operation are key factors in achieving a long "MTBF." Figure 1: Pressurized double-ended mechanical seal (API610, plan 53 or
54), suitable for multi-phase pump SH
Type seal Figure 2: Rotary power pump based on screw axial flow mechanics Figure 3: Twin-screw pump with two counter-rotating screws Figure 4: Typical face-to-face double-ended high-pressure seal
Figure 5: Clearance rigidity of a mechanical seal
Mechanical seal technology used in heterogeneous pumping
Nikolaus Necker of Burgmann Industries GmbH & CoKG explains the benefits of using mechanical seal technology in heterogeneous pumping.