How to select mechanical seals of twin screw pump for typical application conditions

mechanical seals of twin screw pump

1. Selection of mechanical seals for oil pumps in inland rivers, lakes, canals and other oil tanker applications

Oil tankers shuttle through inland rivers, canals, etc., generally have a length of no more than 120 meters and a width of no more than 10 meters. The deadweight tonnage is generally within 4,000 tons and is equipped with 8 to 10 cargo oil tanks. Due to the limitation of the draft of inland rivers, the general design of the hull is relatively flat, and the depth of the cabin is generally not more than 5 meters. Therefore, the screw pump for this application is generally installed on the deck in the middle of the ship. The conveyed medium is usually heavy fuel oil, such as (IFO380, IFO500, IFO750).

Since the pump is installed on the deck, the inlet pressure of the pump will basically reach -0.3bar when the pump is turned on, or when unloading oil from a distant tank; Since the suction height is about 4.5 meters, the viscosity of the medium is relatively high, and the pressure loss caused by resistance such as pipeline valves, the inlet pressure of the pump can reach -0.8bar. At this time, cavitation will generally be caused, accompanied by abnormal vibration and noise.

Generally, when the oil level in the tank drops to a low level during use, the screw pump will choose to run at a reduced speed controlled by the frequency converter. The general rotation speed is 1500rpm, and 750rpm is generally selected when the liquid level is low. In the final stage of stripping, manually control the opening of the valve and increase the flow rate of the medium to increase the suction capacity of the pump to achieve the purpose of stripping and clearing the tank. Usually, only a few litres of the medium remain in the final oil-collecting pan.

For this typical application condition, the selection of the mechanical seal of the twin-screw pump needs to pay attention to the fact that the pump often needs to deal with the working conditions of evacuation and negative pressure, and the friction surface of the mechanical seal may not be well lubricated and cooled, so the selection of mechanical seal material is required There must be a combination of materials with good self-lubricating properties.

In addition, to deal with self-priming conditions, the static ring may cause axial movement, resulting in bump damage before the dynamic and static rings. In order to avoid this potential risk, the static ring of the mechanical seal needs to choose a G13 static ring instead of a conventional G9 static ring, and a static ring gland needs to be installed to avoid the axial movement damage.

In addition, in this working condition, the heating and heat preservation of the general pump is not done or rarely used, especially in winter, it is easy to cause the mechanical seal to be overloaded due to the high viscosity of the medium. In order to avoid damage and failure of the secondary seal O-ring caused by the rotation of the stationary ring and the moving ring when the pump is started, when the oil viscosity exceeds 380cSt under rated operating conditions, the static ring of the mechanical seal needs a positioning pin to prevent the static ring from rotating.


2. Selection of mechanical seals for other medium conveying conditions such as chemicals in inland rivers, lakes, canals and other ship applications

In addition to crude oil and fuel oil, ships used in inland rivers often encounter the application of transporting the following media, generally with a viscosity of (1-10 cSt): gasoline, diesel, kerosene, naphtha, light fuel oil, machine oil, aviation kerosene, methanol, ethanol, light chemicals, seawater, bilge water, etc.; when general inland ships transport chemicals or multi-purpose ships, there will be working conditions in which the transport medium changes with the transport tasks. Progressive cavity pumps may need to cope with changing media and changing conditions.

In view of this changeable medium and changeable working conditions, in addition to the above-mentioned dynamic and static ring friction pairs that need to have good self-lubricating performance and good thermal conductivity, the most important thing to pay attention to is the secondary seal. That is the selection of O-rings.

Many chemicals will have a swelling effect on fluoro rubber (Viton), and the general nitrile rubber and fluoro rubber cannot be used. Therefore, for such application conditions, the secondary sealing O-ring should be selected from full chemical resistance. Fluorocarbon rubber, such as Kalrez6375, Chemraz 505, etc., especially when naphtha, methanol, methyl ether and other chemicals are transported, perfluoro ether sealing rings must be used.

In addition, the material selection of the static ring of the mechanical seal needs to choose a pressureless sintered graphite ring, that is, the cooperative resin-impregnated carbon graphite; the static ring needs to choose a G13 structure, and the static ring gland needs to be installed to prevent the axial movement of the static ring caused by air suction. Mechanical seal position misalignment failure.


3. Selection of mechanical seals for ocean tanker applications

The main difference between ocean-going ships and inland ships is that ocean-going ships have a relatively large tonnage, generally ranging from more than 5,000 tons to 300,000 tons. The draft of the ocean-going ship is large, that is, the depth of the oil tank is generally 6-10 meters; because the suction is too high, the oil product will not be sucked out, and the cargo oil pump cannot be directly installed on the deck, so the cargo oil pump is generally installed in a special pump room.

When it comes to explosion-proof requirements, the driving device such as a motor or a diesel engine is in the engine room, and the drive is connected to the transmission through an airtight compartment transmission. It can also be driven by the hydraulic motor of the hydraulic pump station; similarly, the hydraulic pump station needs to be driven by a motor, so the hydraulic pump station and control system are generally installed on the upper floor or deck, and the hydraulic motor is driven through the high-pressure pipeline to drive the cargo oil pump.

Since the cargo oil pump is installed at the bilge, the inlet pipeline of the pump generally has a positive pressure of 0.6-1 bar due to the liquid level of the cargo oil tank at the initial stage. When the liquid level is high, in order to ensure delivery efficiency, the pump generally runs at full speed, and when the liquid level is low, it needs to slow down. Generally, in order to clear or strip the tank, that is to completely empty the bilge cargo oil, it is generally necessary to be equipped with a small cargo oil stripping pump such as W5 or W6. Inlet negative pressure can often reach -0.8 bar.


4. Selection and precautions of mechanical seals in applications such as oil depots, tank farms, ports, and power plants

In oil depots, tank farms, ports, offshore drilling platforms, power plants and other applications, twin-screw pumps mainly have two uses:

1) It is used in the process of loading, unloading, loading, unloading, pouring, etc.; the unloading pump of the train tank car is connected to the unloading through the common header, and the pump inlet needs to have good suction performance. Or complete unloading through hoses, pick-up pipes, oil delivery arms, etc. The pump can be fixed or mobile installed.

2) Booster pump, which pressurizes the transmission medium through the screw pump, so as to transport it to different oil tanks or higher or farther end users, such as power plants, chemical plants, etc.

This kind of pump is generally installed in the open air or outdoors, and the conveying medium is changeable. It should be noted that if the pump does not have good heating and heat preservation, especially if this kind of pump generally runs intermittently, it is easy to cause the pump to be cold in winter. Moreover, even in spring and summer, when the ambient temperature is 20-30°C and the temperature of the conveying medium is at ambient temperature, the viscosity may be particularly high. For example heavy fuel oil IFO500, at 20°C, the viscosity can exceed 2500cSt. Under such working conditions, the mechanical seal of the pump is particularly prone to overload damage.

Therefore, for this application condition, if the conveying medium is relatively fixed, the pump is installed indoors, the pump has good heating and heat preservation measures, and the operating conditions are relatively friendly, the mechanical seal can be selected: M7NSF, material combination: Q1AVGG;

If it is to deal with relatively uncertain working conditions, outdoor or open-air installation, heating and insulation measures are not available or the customer does not use it, the selection principle of the mechanical seal is to choose a high-strength alloy material; the mechanical seal can be selected: M7NSF, material combination: Q1U2VGG;


mechanical seals of twin screw pump


5. Selection and precautions of mechanical seals in oil refineries, refineries, and high-temperature asphalt transportation applications

In refineries and refineries, typical twin-screw pumps are used to transport asphalt, coal tar, bitumen, residual oil, etc. The temperature range of the medium is generally 100-400°C; it depends on the type, variety and production of the medium. process. Especially in the negative pressure distillation process of residual oil, the temperature is relatively high, accompanied by high viscosity, negative pressure and other working conditions, the requirements for pumps and mechanical seals are particularly high.

The screw pump itself is a double-layer shell, and the whole pump body has a welded structure for heating and heat preservation. For mechanical seals, choose high-temperature-resistant metal bellows double-ended cartridge mechanical seals. According to the requirements of specific media and working conditions, choose to match the flushing scheme of API Plan 53A or API Plan 54.

For high-temperature, high-viscosity applications, the mechanical seal should use the double-end cartridge type plus pressure flushing scheme, so that continuous sealing can achieve good lubrication and cooling, and ensure the service life of the pump and mechanical seal. If you only consider cost reduction and can accept leakage, you can choose a metal bellows single-end cartridge mechanical seal. Mechanical seals require independent heating and heat preservation devices.


6. Selection and precautions of mechanical seals in high-viscosity polymer conveying applications in petrochemical plants

The properties of various polymers are very different, mainly in the properties of viscosity and density, including the influence of moving shear speed on viscosity, etc. The actual viscosity range is between 100-200,000 cSt, even It is possible to be higher. It depends on whether the medium is a Newtonian medium or a thixotropic medium, that is, the viscosity of the medium is very high before starting the pump, but as the pump starts, the viscosity of the medium decreases greatly after the medium flows. Such characteristics greatly affect the design and selection of the pump. .

In view of this, pumps for conveying high-viscosity polymers must be driven by frequency conversion in order to adjust the speed of the pump, adjust the displacement of the pump, etc. Moreover, the pressure range of the inlet and outlet pressure of the pump may also be very large. The inlet is sometimes close to the negative pressure state of the vacuum, or the inlet pressure from the reactor may reach 8.5Mpa (Celanese process pump); the process of chemical reaction may last for several hours or even several days; after the reaction is over, the pump and pipeline will be flushed with a specific solvent, and this process may also last for several hours. Generally, the viscosity of the solvent is very low, around 0.5-1 cSt; all The process pump needs to be suitable for high viscosity as well as low viscosity operation.

For the selection of mechanical seals in the above working conditions, in principle, double-end cartridge mechanical seals are selected, and the configuration is only the flushing scheme of API Plan 53C or API Plan 54. Due to the different properties of high-viscosity polymers, the selection of sealing materials generally needs to be specially selected according to the material properties. It is recommended to confirm the combination of mechanical seal materials according to the properties of the medium and the seal manufacturer.


7. Selection and precautions of mechanical seals in the application of twin-screw pumps in land or seawater oilfields

There are several typical applications of screw pumps in oil fields, such as crude oil inversion pumps, ship loading or unloading pumps, pipeline booster pumps, production water transfer pumps, oily sewage transfer pumps, oil and gas mixed transfer pumps, etc.

Due to the large difference in oil products in different production areas of crude oil, the viscosity, components, etc. are different, the tax rate is different, and the viscosity is also very different. In addition, the water-containing waste oil or production water in general oilfields is similar to brine or seawater, containing respective salts, chloride ions, and even sulfur dioxide. This kind of water is very corrosive at a slightly higher temperature.

Therefore, for the transportation of production water, water-containing sewage oil and other media, it is necessary to understand the media composition, temperature, and viscosity, and correctly select the material combination of the pump body flow parts and mechanical seals to ensure the normal operation of the pump.


1) Transport pump for clean crude oil that has been separated

For a clean crude oil transfer pump, the choice of mechanical seal is relatively simple. According to the given working conditions, confirm the medium temperature and viscosity range, and generally choose the specification model: M7NSF, material combination: SAVGG or Q1AVGG;

2) Mechanical seal selection of production water (formation water) delivery pump

Corresponding to the choice of mechanical seal and material combination for this application, it is necessary to confirm with the customer the temperature of the production water, the specific components, especially the content of chloride ions, sulfides, etc., to determine whether it belongs to the application range of strong corrosion. The typical mechanical seal model and material combination are: M7NSF, and the material combination: Q1Q12KMM; the metal cage and spring are made of Hastelloy.

3) Selection case of mechanical seal for oil-gas mixed pump under complex working conditions

In the working condition of the oil-gas mixed pump, the pumping medium contains oil, water, and natural gas, and the composition is complex, and even contains a small amount of solid particles such as sand. The mixed medium is pumped out from the oil well, and it needs to be mixed and transported without treatment. The mechanical seal needs to deal with the possibly extremely corrosive and abrasive medium, and the gas content ratio is not fixed, and sometimes even 100% gas content occurs. If it is an ordinary mechanical seal, it will fail soon if it is not well lubricated and cooled. Moreover, it is generally necessary to solve the working conditions of large displacement and high gas content ratio. Sometimes the mechanical seal is installed at the outlet end of the pump, that is, the high-pressure end. The mechanical seal also needs to solve the problem of high pressure. Therefore, the mechanical seal should at least choose a balanced mechanical seal with API52 flushing scheme, or choose a special double-ended cartridge mechanical seal with API53/54 flushing scheme.


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