Use of purge media for special valves in catalytic cracking
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In catalytic cracking specialty valves, purge air (or steam) is commonly used for the valve stem to prevent sticking, but this also causes the purge medium to cause certain damage to the valve stem. To avoid this, the instrument requires that the purging be changed from continuous to scheduled and quantified purging per shift. This approach is theoretically feasible, but its performance in actual production is not very satisfactory. Excuse me, friend Hai Chuan, has anyone done this? What are your thoughts?
The purging of special valves in catalytic units generally involves three aspects (two for valve passages and one for the packing); it is essential to ensure a continuous supply of the purging medium, as intermittent purging is not recommended. As for the damage to the valve stem you mentioned, it is likely referring to the situation where steam is used as the purging medium. As required, air is used as the loosening medium at points near the regenerator, while steam is used as the loosening medium at points near the settler. However, I have reviewed the designs of two newly built refineries; for purging the regenerative slide valves, purified air is used. It is likely that the design team conducted calculations and determined that the volume of air entering through the orifice plates is small enough as not to pose a risk to production safety. I suggest that you also consult the designers to see if it’s possible to change the special valves that use steam as a purging medium to use purified air instead; this way, your concerns will be resolved, as there will be no problems using air as the purging medium.
Intermittent purging is not recommended; if there is concern about damage to the valve stem, the amount of steam used for purging can be reduced by using a orifice plate! The valve stem and valve passage are usually not damaged by blowing!
The purge medium for the single-acting slide valve and plug valve in our device is air, while the purge medium for the double-acting slide valve is steam! ! We believe there should be no problem with using the purging medium for a long time, as they all have orifice plates! Besides, during maintenance, the purging of the valve passage didn’t seem that thorough either! !
Steam is used to feed the lift pipe, and air is used to feed the regenerator; installing orifice plates will not cause any problems
The steam must be kept in a superheated state (with a flow restrictor, the flow rate will not be very high). It won’t get damaged without condensed water.)
In our facility, the slide valves in the regeneration and standby lines of the special valves are purged with steam, while those in other areas are purged with unpurified air； However, many special valves only have packing purging, with valve channel purging being omitted; flow-limiting orifice plates are installed on all purging media, the flow rate is low, and the purging is continuous rather than intermittent.
I took a close look; what the friend upstairs said is very good. Generally, if conditions permit, it’s better to use air rather than steam, as using steam can cause thermal degradation of the catalyst as well as dew point corrosion issues
Both steam and air can be used as blowing media for the valve track and packing, but there have been cases where the valve stem was broken, probably due to an increased gap between the stem and its surroundings. However, I have been using this device for over a decade without such incidents; it is best to minimize such use if possible. I’ve also seen cases where it worked for a long time without purging. With the modified valve track structure, it is no longer necessary to purge the valve track. To protect the slide valve, it is necessary to properly adjust the material level control; especially when the inclined tube conveyance is stable, the movement of the slide valve is reduced. This post was last edited by chengkang on 2009-4-22 18:53]