How to determine the max/min allowable working pressure

[rika kose]

Client has a polyethylene scrubber been in operation since 1979. A blower is downstream the scrubber. outlet pressure in normal operation is -5 ~ -7 mbarg.
The blower max. suction pressure -20 mbarg. (when suction line is blocked)

Now client wants to have a new blower whose max. suction pressure is -35 mbarg.

Then we get a problem, if the inlet line of scrubber is blocked, the scrubber may face -35 mbarg suction pressure. There is no design condition of the scrubber. is there a simple way that we can determine the max./min. allowable working condition?

 

[LittleInch]

"There is no design condition of the scrubber."

Oh.

There must be something? Nameplate? P&ID, or is it 50 years old?

I assume what you're asking is is there a simple way to know if the scrubber will implode.

Answer. Not in my opinion, no.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[shvet]

Hire a contractor to provide:
- inspect scrubber actual design and dimensions
- measure PE properties at points of interest
- forecast PE properties degradation rate for scrubber inspections planning
- provide stress analysis for min / max design P@T combinations
Or you have no means&background for a such work and you are going to provide those on your own?

 

[RVAmeche]

Agree with LI, this is a risky task and if the original vendor can't tell you, I doubt you'll find a vendor/contractor who will take some measurements and give you a report (making them liable) saying the design vacuum is X.

To me, if you want to increase vacuum, you need a new scrubber to go with the new fan.

 

[rika kose]

@littleInch
The only thing that we have is a mechanical arrangement drawing. Height, diameter, height/locations of a nozzles, size of the nozzles, thickness of the wall. etc. No process condition.

yes, you are absolutely right. I want to know if the scrubber will implode. I think it won't. but I have to prove it to the safety department.

@shvet
we can have a mechanical engineer to perform those works, then client has to pay.....

 

[shvet]

How long has it gone since the scrubber was tested for design vacuum conditions and PE has been tested for wearing&degradation rate? If you have those data why not to transmit those to a mech engineer in charge to provide rough calculations? Why do you want to provide those on your own?

 

[rika kose]

As far as I know, it's never been tested.

Thanks for the replies.
It is clear that there is no shortcut.

 

[TiCl4]

Rika,

Normally a full engineering FFS review is required if you want it to be able to withstand the -35 mbar condition. However, I know how stingy corporate management can be, and money for such an analysis may not be approved in a timely fashion, if approved at all. In lieu of that, you can look at historical operational data to determine the maximum demonstrated vacuum in the column. You could then use that demonstrated pressure to size and set a vacuum relief device on the column. There is a time-based caveat to that, however, as polyethylene will tend to degrade over time, especially if exposed to UV light.

Relevant Questions:
Has the column ever experienced the current blower's -20 mbarg suction pressure?
If not, what is the maximum demonstrated vacuum? How long ago was that condition?
Is the scrubber outside or otherwise exposed to an environment that is known to slowly embrittle PE?
Is the chemical being handled known to react with PE, embrittle PE, etc?
Is there vacuum relief on the current column? If so, what is the set pressure?

 

[LittleInch]

Ok, I'd missed that (reading on phone). So this thing is 45 years old made from PE?? I thought it was scrubbing PE....

What's the worst thing that could happen?

Scrubber starts to bend a bit? PE is a pretty forgiving material and will only deform slowly (though it can creep) and 35 mbarg is pretty low.

But any dimensions / diameter to thickness ratio?

You can get vacuum / external pressure data from PE vendors. I have an extract for vacuum which even for 50 years at SDR 32.5 is 115 mbar negative pressure.

Note temperature has a huge impact on the strength of PE. Anything above 20C starts to de-rate and nothing more than 50C.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rika kose]

Thanks a lot! I appreciate it.