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Natural Rubber Lined Tank Inspections

Natural Rubber Lined Tank Inspections

(OP)
Hi, I am x-posting this from the storage tank engineering section as I have not received any responses there in the last week. I can delete that thread if I get some activity here.

http://www.eng-tips.com/viewthread.cfm?qid=405624

________________________

From that post:

Hi All,

I've got a few rubber lined storage tanks in aqueous HCl service (~32-36%) at ambient pressure/temp. In reviewing some PM requirements, I came across some notes about maintaining a minimum acid concentration to protect the rubber. Liner is PG-70 natural rubber.

Problem: Low concentration acid / water can be absorbed by the rubber causing it to swell. The risk of the liner swelling due to water absorption increases the lower the acid concentration and the longer the exposure time.

I understand that as the rubber ages in service, it develops a "crust" or more brittle outer layer. And when the rubber swells, it cracks the brittle outer layer, shortening the life, and it stresses the seams. However I'm finding some conflicting information in the outreach. Comments from various rubber/liner experts:

GUY 1:
• After 3 months of service the rubber will form ~15 mil thick crust. Once the crust forms water won’t swell the liner. For the initial system test, water will make the liner swell. Pure water is the worst and it is important to avoid heat with the water.
• The liner will swell 25% in water in 4 weeks
• 7-10% swell in 1 week
• 2-5% swell in 1 day

GUY 2:
• PG-70 forms the crust over 9 months
• Washing it out by filling and draining could cause the rubber behind the rigid crust to swell, causing cracking and damaging rubber. He would rather just empty the tank and hose it
• • Water will cause the liner to swell
• They’d recommend not getting in the tank. Entry into the tank will damage the rubber; we should inspect remotely
• Just rinse the walls and get water out of the bottom of the tank
• Walking on crust even when using a 4x8 sheet of plywood would damage it and shorten the service life

GUY 3
• Not worried about water swelling rubber or crust cracking
• Get it back into service ASAP since PG-70 doesn’t hold up well to elements like atmospheric ozone when not in service. Told a story about a tank left open for 3 months and the rubber was garbage.
• Tanks are routinely washed out and stepped on.


So my question anyone having history/experience with rubber-lined acid tanks?
1. Do you physically inspect (walk on rubber?)
2. Do you set or track acid concentration minimum limits?
3. How do you prep for inspections? How long is the acid out? Dry or keep wet?


Thanks

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