The centerpiece of Dow Olefinverbund in Böhlen is its ethylene plant. The end product, ethylene, has a purity of >99 % and is conveyed to the next plant for refining by means of a transfer pump. Because of the high safety standard involved, the shaft seal of this transfer pump was designed as a triple seal.
Bernhard Fröhlich, Reinhard Iffland, Kerstin Bechtel
Ethylene is a colorless, combustible and slightly sweet-smelling gas with a boiling point of -104 °C. When combined with air in a wide range, ethylene forms an explosive mixture (explosion limits in air 2.7 to 28.6 %), which is why safety standards in the production of ethylene are very high. Ethylene is used primarily in the manufacture of polyethylene and other plastics. Today ethylene is produced largely from crude oil or natural gas. Naphtha supplied from the refinery is converted into ethylene and propylene through thermal cracking. The actual cracking process must be followed by extensive cleaning and gas separation processes until the ethylene has the purity required for further processing.
High standards
To ensure that the high safety standard is met and to avoid the possibility of product contamination in the event of damage to the seal, a triple mechanical seal is installed in a tandem arrangement in the ethylene transfer pump. One of the challenges faced by the mechanical seal is the low temperature of as little as -100 °C, which is why a metal bellows-type seal is used. In addition, significant icing occurs on the atmosphere side of the pump. To ensure that the bearing supports and mechanical seals do not ice up, the bearing supports are heated electrically. If the heat output is configured too low, the icing that occurs can lead to damage to the bearings and to the seal on the atmosphere side. Since such applications are often operated very close to the vaporization point, suitable measures must be taken in order to prevent vaporization in the sealing gap and hence stop the seal from running dry.
Special seal solution
The triple seal consists of the following Burgmann components: MFLCT80 on the product side, H75VK1 as the second seal and MFL85F with fluid circulation screw as the third seal on the atmosphere side (safety seal). The elastomers are made from a special fluorosilicone rubber that is suitable for low temperatures. The buffer medium flow is split into several partial flows (multipoint injection) which are aimed at the slide face due to the relatively high flow rates. This produces what is known as a jet effect, which means that the rotating liquid ring is pierced and thus removal of the friction heat is intensified and any gas bubbles that may arise are flushed away. The operation of the third seal in accordance with the adjusted plan 52/53 means that, rather than a pressureless quench, what is at work here is a pres- surized quench that is operated at approx. 5 bar, although the pressure is still below the product pressure. The reason for this is to avoid vaporization in the sealing gap of the second seal caused by an increase in pressure. In addition, further structural measures were taken in order to prevent damage to the seal caused when starting the pump by the formation of ice in various places between the pump shaft and the seal on the atmosphere side. These included a heating chamber in the cover of the seal that is heated with low-pressure steam. The steam condenses in the area of the heating chamber and is discharged by means of a channel system. The flow rate should not be reduced through attachments as otherwise there is a danger that the condensate will freeze. Care must also be taken to ensure that the temperature of the quench medium in stand-by operation does not exceed 30 °C. This guarantees that the quench medium does not vaporize in the sealing gap during the start-up process and that the seal does not run dry. The heater for the cover is switched off shortly before the pump is started up as otherwise the quench medium – and hence the seal as well – could overheat.
Successfully in use
The seal concept introduced here has been used successfully for several years and achieves good service lives. The seal variant fitted with heating chamber has likewise been operated successfully since June 2005 without any failures whatsoever. The use of a triple seal enabled the high safety requirements to be met to the complete satisfaction of the customer. Even the challenges presented by the low-temperature application were overcome through the use of a metal bellows-type seal, special ma- terials and other specific measures.
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