Achieving optimum service lives with hydrodynamic shaft seal

Shaft protected by whirlwind

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The horizontal and vertical pumps by Bungartz cope with complicated, difficult or dangerous process conditions since the company’s very first days. For each and any specific application, a technically and economically perfect solution has been developed. The key component is the hydrodynamic shaft seal, which operates like a whirlwind without wear and tear.

More than 70 years ago, the sturdy slurry pump MOR conveyed the difficult to handle medium corn mash for Maizena (now Cargill). The “M” in the name remained as well as the numerous benefits characteristically representing today’s durable horizontal pumps MOS and UMOS, which are capable of dry running. One of the fundamental prerequisites for the success of these centrifugal pumps is the hydrodynamic shaft seal. This seal basically consists of a distinctive blading on the impeller’s rear side. Thus, the pumping medium is carried away from the shaft opening protecting the downstream shaft seal system. Depending on the application case, many of the pump ranges by Bungartz combine the hydrodynamic seal as primary seal with a downstream secondary seal (gland, mechanical or lip seal, or magnetic coupling). No sealing (no gland, no mechanical seal) is required during the operation of the horizontal centrifugal pumps MOS and UMOS. The hydrodynamic shaft seal operates without any friction and wear. It is well known that it is suitable for solids: “Some time ago a MOR was removed. It had been working since 1948 and was fully functional all the time,” tells Frank Bungartz, General Manager of the company of the same name. This pump type still has its merits. An example for this represents the big circulating pump M-UMOR (900 m3/h), which is, for instance, used to convey ammonium nitrate.

The pumps of the ranges MOS/UMOS and MOR/UMOR (designs for high/low suction heads) achieve best results for pumping media which are corrosive, abrasive, viscid, gas-laden, crystallising or gelling. Thanks to their high reliability, very high availability and low maintenance costs, they are used in a wide variety of plants. They are used globally to convey ammonium nitrate, ammonia, urea solutions or phosphoric acid.

Toxic, outgassing and hazardous

Toxic waste water, hazardous or outgassing liquids, or solid-laden, sludgy, crystallising or corrosive media require leak-free pumps with hermetic seals. Conventional magnet-coupled centrifugal pumps or canned motor pumps do not meet these specific requirements. Usually, they are equipped with liquid-lubricated slide bearings, which are not safe to run dry and are subject to high wear. They require a clean, solid-free liquid for bearing lubrication. The pumping medium is typically used for this purpose.

The design of MPCV-AN follows a different approach. The vertical pumps function without bearings in the pumping liquid. In this case the shaft seal concept of the permanently dry-running pump is also based on the completely hydrodynamic relief of the bearing and seal unit. The concept is based on permanently grease-lubricated roller bearings. They are protected against the product vapours by a gas barrier. An eddy current-free magnetic coupling seals the pump hermetically from the surrounding environment. Roller bearing service lives of more than five years continuous operation are the norm. Also thanks to the vertical orientation of the pump, the bearing and sealing unit runs entirely without product contact, even in the event of a sealing gas failure. Even high volumes of solids and gases are permitted. Like all pumps in the V-AN range, the MPCV-AN works with special control characteristics, i. e. the pumps adapt automatically to variable feed rates – without any mechanical or electrical regulation equipment. And as with all pumps of the V-AN range, considerable cost advantages can be achieved through the correct pump selection. Through early planning, there is no need for the construction of pits, scaffolding or vessels, while regulator valves, signals, etc. can be dispensed with completely.

On-site operation

Substances are heated to temperatures of up to 1000 °C and blended at a company. The resulting highly hazardous medium that forms hydrochloric acid vapours upon contact with humidity, ought rather be a job for the already described pump type UMOR with hydrodynamic seal. The disadvantage: This pump type, equipped with mechanical seals, would have to struggle with chlorine ions diffusing through the sliding clearances of the mechanical seals. This type of seal would reach a service life of approx. two years. This is why Bungartz counts on another pump type: the magnetically coupled pump MPCV-AN was installed in October 2008. According to the operating manual, the pump should undergo a maintenance service after two years. The operator carried out vibration measurements each month. No malfunctions on the bearing were found until that time. This is why the operator ignored this specification. Even in the following years no malfunctions or failures occurred. The reserve pump, which was acquired at the same time as the MPCV-AN, has not been put into operation at all. It is still waiting to be used. In the meantime, the installed MPCV-AN is running for more than 50,000 h – permanently and without any incidents. Eventually, the operator asks Bungartz what is about to happen with the necessary reserve pump.

After extensive discussion with the team at the centrifugal pump manufacturer, the responsible engineer at the operator’s decides: The reserve pump is to be activated so that the used pump could undergo the recommended maintenance after more than six years of operation. In the meantime, the bearings and seals of the reserve pump are replaced. Thus optimally prepared it takes over its duties – just as smooth as the removed pump. Meanwhile, the removed MPCV-AN, which did not have to experience an inspection since its installation, is maintained at the production site in the Eifel region. Here also, it shows no anomalies.

www.prozesstechnik-online.de

Suchwort: cpp0218bungartz

Hall 8.0, Booth C1


Author: Annette van Dorp

Freelancer

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