Selection of mechanical seals for high temperature heavy oil pumps
(HY-industry technical centre)
For the petrochemical industry, the selection of mechanical seals for high-temperature heavy oil pumps has been a major problem, such as catalytic cracking slurry pumps, refining oil pumps, atmospheric pressure bottom pumps, primary distillation bottom pumps, vacuum bottom pumps, Delayed coking radiation feed pumps, etc.
The medium of the high temperature heavy oil pump has the following common features:
High temperature: generally at 340 to 400 ° C;
The viscosity of the medium is large: the general kinematic viscosity at temperature is (12-180) X 10-6 m/s;
The medium has particles: such as catalyst, coke, and other impurities containing sand.
Mechanical seal for high temperature heavy oil medium pumps. Every company now uses welded metal bellows for mechanical seals. DBM type, XL-604/606/609 type, YH-604/606/609 type, etc. are better used now. The bellows material adopts eight AM350, INCONEL718, Hastelloy B, C and other stainless steels; corrosion-resistant high-temperature alloys, etc. Some wave plates adopt a double-layer structure, which increases the bearing pressure from 2MPa to 5MPa, which effectively solves the corrugated pipe The problem of bullet loss.
For the inside of the bellows, coke and charcoal, as well as solid particles, etc., the relevant information has been explained. For example, steam purge, friction pair use “hard to hard”, external flushing, etc. To a lesser extent, it is not elaborated here. However, the various methods previously proposed have not been ideally applied due to various factors in practical applications. In order to better improve the service life of mechanical seals, save money and reduce consumption, it is recommended to combine the following measures for various situations:
The metal corrugated liter is designed as a rotary structure, and the rotating bellows mechanical seal has self-cleaning centrifugal action, which can reduce the peripheral deposition and the inner coking of the bellows.
For the pair of friction pairs, it is recommended to use a “hard to hard” structure, generally using tungsten carbide to tungsten carbide (including YG6-YG6) and tungsten carbide to silicon carbide. To choose a “hard to hard” structure, you must pay attention to the following questions:
The cooling system should be guaranteed to prevent the cooling water from being interrupted to prevent the end from rising, and the lubricating film flashes to reduce the lubrication of the sealing end face and aggravate the wear;
Mechanical seal In the installation process, it is necessary to pour some lubricating oil (oil or butter) to the sealing end. To prevent the pump from starting. Dry friction of the sealed end face due to lack of lubrication;
The use of clean external rinsing is one of the more effective methods to solve the accumulation of solvent particles, but this method is wasteful, and the medium, temperature and pressure of various pumps (generally required the rinsing liquid pressure is 0.07 higher than the medium side pressure). —D. 12MPa ) is different, the structure of the external flushing system is more complicated, and the investment of the flushing facilities and the consumption of maintenance costs sometimes cause disadvantages, especially in some small and medium-sized industries. Therefore, many companies’ oil-sealing systems are discarded or there is no such system. For these situations, it is recommended to use a multi-sealed structure with an isolating medium, such as a slurry pump, a refinery pump, etc., with a double-end mechanical seal. , between the two sets of sealing end faces filled with the medium (clean engine oil, etc.), as shown in below picture.
This structure can effectively extend the service life of a long mechanical seal, generally up to 6000-8000h. In addition, the following two points are taken into consideration:
Near a set of sealed ends of the impeller and the material is selected from a “hard-to-hard” structure (such as YG6-YG6); and a set of sealed end faces near the mechanical seal gland can be carbonized by carbon or graphite-impregnated copper or tantalum. Tungsten or silicon carbide;
The isolation medium selected for the high temperature oil pump shall have the characteristics of thermal decomposition temperature, spontaneous ignition point, high flash point (generally above 260 °C), good thermal oxidation stability and low temperature evaporation loss.
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