Sealing types and applications of Roots blowers

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I. Introduction The sealing of the Roots blower includes sealing at three locations: the shaft extension of the driving shaft, the oil seal at the rear of the bearing housing, and the shaft seal in the wall panel section. The shaft sealing in the wall panel section is primarily used to prevent gas leakage, while the other two sections, in addition to preventing oil leakage, sometimes also require gas sealing. II. Gas-sealing device: A structure that primarily uses sealing gas is usually referred to as a gas-sealing device, while one that functions solely for oil sealing is called an oil-sealing device. Common shaft sealing devices for the wall panel section of Roots blowers include labyrinth seals, packing seals, expansion ring seals, and mechanical seals ； Common sealing devices for the shaft extension of the drive shaft include skeleton oil seal assemblies or mechanical seals. The oil seal at the rear of the bearing housing usually adopts a return groove structure or a skeleton oil seal assembly for sealing. Illustration of the seal system for the Turbocyclone blower: 1. Auxiliary oil tank; 2. Seal at the drive shaft extension; 3. Oil seal at the rear of the bearing housing; 4. Shaft seal in the wall panel section; 5. Main oil tank; 6. Wall panel; 7. Housing. 1. Labyrinth seal: A non-contact dynamic seal that utilizes fluid flow through a series of throttling gaps and expansion chambers to create a throttling effect, thereby preventing leakage. Labyrinth seals are widely used for the shaft seal in the wall section of Roots blowers. Its sealing mechanism can be simply explained as follows: the fluid passes through many tortuous channels, undergoing multiple throttling processes that create significant resistance, thereby making it difficult for the fluid to leak and thus achieving the purpose of sealing. Its advantages are that it is not restricted by speed or temperature, has a simple structure, is easy to maintain, and has a long service life. Increasing the number of sealing teeth appropriately can be used for sealing high-pressure gases ； If the seals are properly processed and assembled, they generally will not fail during operation, nor is frequent replacement necessary. Its main drawback is the high precision requirements for processing the sealing parts. Due to the small sealing gap, frictional wear occurs when the machine is not operating properly, thereby reducing the sealing performance. This type of seal is generally suitable for transporting air, and its use is restricted when transporting toxic, flammable, and explosive gases. This type of seal is generally used for the sealing of the wall panel section in standard Roots blowers. 2. Packing seal: A packing seal is a type of seal that uses packing to block the leakage path and prevent leaks. Packing seals have a simple structure, are easy to install and remove, and are cost-effective, which has led to their widespread use. In the packing seal structures of Roots blowers, commonly used types include ring packing, molded polytetrafluoroethylene packing, and skeleton oil seal assembly packing. Since this type of seal is a contact-type seal, it tends to cause wear on the shaft (at the areas in contact with the oil seal lip or the PTFE packing). Due to its simple structure, ease of installation and disassembly, low cost, good sealing performance, and low requirements regarding the working environment and maintenance, it has been widely used in industries such as fertilizer plants. 3. Expansion ring sealing: In expansion ring sealing, several notched elastic rings are placed in the sealing groove; due to the elasticity of the rings themselves, their outer surfaces press tightly against the housing, with no relative rotation occurring between the outer diameter of the rings and the housing. Gas can only leak downstream through the gap between the expansion ring and the annular groove. During operation, the expansion ring presses tightly against the downstream end face of the annular groove under the action of the gas pressure difference on both sides. The high-pressure gas that enters the gap fills the back-gap space, further enhancing the seal between the outer circumference of the expansion ring and the inner bore of the sealing seat; the only remaining leakage path is the cut in the expansion ring. Due to the very small gap at the cut, and the slices being installed out of alignment during setup, only a small amount escapes after several expansion rings are used for sealing. If a sealing gas seal device is added, the sealing effect will be even better. Since this type of seal is a contact seal, it provides good sealing performance, but grease lubrication is required during use. The expansion rings made from the newly developed special material can be used without the need for lubricant. The shaft sealing for the wall panel section uses the three sealing methods mentioned above. When the wall panel structure is of the open type, the gas being transported will not leak into the oil tank, but it may leak into the atmosphere. Lubricant will not also leak into the casing. When a closed wall panel structure is used, the gas being transported may leak into the fuel tank. When used as a vacuum pump to create a vacuum, lubricant may leak into the casing. III. Mechanical Seal A mechanical seal, also known as an end-face seal, is a device that prevents the working fluid from leaking along the shaft. It usually consists of components such as a stationary ring, a rotating ring, elastic or magnetic elements, rotating ring drive components, and auxiliary sealing rings. During operation, its sealing function is achieved through the two mating surfaces where the rotating ring and the stationary ring rotate relative to each other, as well as an auxiliary seal. The proper functioning of a mechanical seal depends on the ability to establish a stable and reliable lubricant film between the two mating surfaces, as well as on the timely removal of frictional heat. Due to its reliable operation, low leakage rate, long service life, automatic compensation for end-face wear, reduced need for frequent maintenance, low power loss, and no wear on the shaft, mechanical seals are widely used in various rotating machinery in industries such as the machinery industry, oil, and chemical industries. Mechanical seal types are more complex in structure, require higher precision, are more expensive, and are not easy to maintain. It is generally used only in applications with high sealing requirements that cannot be met by other sealing methods. The mechanical seals used in Roots blowers are further divided into Type ! mechanical seals and Type “ mechanical seals. The ! type mechanical seal involves installing a mechanical seal at the shaft extension of the driving shaft, while the shaft seal in the wall panel section is implemented using a labyrinth seal or a packing seal. Since gas is allowed to enter the wall panels or fuel tank, the wall panels must be sealed, and it is required that the gas not have a deteriorating effect on the fuel. With this type of sealing, the gas being transported may leak into the oil tank, but very little leaks into the atmosphere. When evacuating, oil may leak into the casing. It is generally used to transport clean industrial gases such as nitrogen, but fans with this type of sealing cannot be used for gases that degrade lubricants. The shaft seal in the wall panel section of the Roots blower uses four mechanical seals, namely the so-called “” type mechanical seals. It uses a mechanical seal of this type, resulting in extremely low gas leakage, and can practically achieve “zero leakage”. Fans with this sealing type perform better, especially against gases that cause oil degradation. In practical work, there have also been cases where the fan failed to meet the required operating conditions due to an inappropriate choice of fan sealing method. For example, a company in Shandong selected a blower for use in blast furnaces; taking into account specific requirements such as the composition of the medium being transported and the operating conditions, a blower equipped with a single mechanical seal was chosen for it. At that time, the equipment was purchased through a bidding process, with four or five manufacturers of Roots blowers submitting bids; in the end, the manufacturer chose a regular labyrinth-sealed blower. During the installation and commissioning of the equipment, numerous problems arose, and ultimately it was necessary to choose fans with a different sealing mechanism. IV. Conclusion: Since rotary vane blowers come in various sealing structures, and each type of seal has its own characteristics, when selecting a rotary vane blower, in addition to choosing the appropriate model, it is also necessary to select a suitable sealing type based on the properties of the medium to be transported. Generally, a labyrinth seal is used for transporting air ； When fertilizer plants transport CO2 or semi-water gas, due to the impure nature of these fluids and their high content of coal tar, fans with packing seals are generally used, such as the L series packing-sealed fans or the RP series packing-sealed fans ； For Roots blowers used in industries such as petroleum and chemicals to transport special gases that are clean, flammable, explosive, or toxic, mechanical-sealed blowers should be selected. If it is an inert gas such as N2 that has no adverse effect on oil quality and is non-corrosive to components such as bearings and gears, a K-type mechanical seal fan (with a single mechanical seal) can be used; otherwise, an N-type mechanical seal fan (with four mechanical seals) must be selected.