Anaerobic Digester Vacuum relief valve

• Jul 10, 2018

• #2

bimr

Civil/Environmental

Feb 25, 2003

9,385

US

Not sure what the question is, but anaerobic digesters are typically fitted with Pressure/Vacuum Valve & Flame Arrester combination units that are designed to protect the tank from damage created by over- pressure or excessive vacuum, at the same time that they provide protection from externally caused sources of heat and ignition. The result is increased fire protection and safety.


Pressure/Vacuum Valve & Flame Arrester combination unit

 

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• Jul 10, 2018

• Thread starter
• #3

Sam1390

Chemical

Jul 10, 2018

18

AU

Thanks Bimr for your reply , actually my question is how it is worth to put a vacuum relief valve on digester when entering air into the tank could be so dangerous?

 

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• Jul 11, 2018

• #4

bimr

Civil/Environmental

Feb 25, 2003

9,385

US

Normally the anaerobic digester is under a slight positive pressure so the vacuum breaker should not be operating. Adding air will also dilute the digester gas making it less valuable for fuel. The vacuum breaker is necessary to protect the tank.

The digester gas is an explosion hazard whether diluted or not. Because of the explosion hazard, no open flames are allowed near a digester. Also, equipment located in the electrical classification zone around the digester are required by the NEC to be rated explosion-proof.

 

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• Jul 11, 2018

• #5

LittleInch

Petroleum

Mar 27, 2013

23,042

GB

Sam 1390,

Well neither operation is ideal, but as bimr rightly states adding air into a digestor should be a non standard operation. However if you have no vacuum relief valve it is likely that the digestor would implode and collapse / buckle. The ability of large vertical cylindrical tanks to withstand any appreciable vacuum level is very low ( inches of water) before they collapse.

In relative terms a tank has a thinner wall than a coke can or maybe even a large plastic bottle. you can collapse those with just a small in breath.

Better a potential explosive mixture inside where there are no ignition sources than a collapsed tank.



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

 

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• Jul 11, 2018

• Thread starter
• #6

Sam1390

Chemical

Jul 10, 2018

18

AU

thanks everybody for your replies.

 

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• Jul 16, 2018

• Thread starter
• #7

Sam1390

Chemical

Jul 10, 2018

18

AU

if the digester is a concrete type , vacuum relied is still needed?

 

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• Jul 16, 2018

• #8

bimr

Civil/Environmental

Feb 25, 2003

9,385

US

Yes.

 

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• Jul 16, 2018

• Thread starter
• #9

Sam1390

Chemical

Jul 10, 2018

18

AU

you say that vacuum could collapse the reinforced concrete digester? how?

 

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• Jul 17, 2018

• #10

itsmoked

Electrical

Feb 18, 2005

19,114

US

Pounds per square inch.
There are a LOT of square inches.
Is the roof concrete?
What if a pump goes haywire and throws several PSI of vacuum on the system?

Keith Cress
kcress -

http://www.flaminsystems.com

 

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• Jul 17, 2018

• #11

bimr

Civil/Environmental

Feb 25, 2003

9,385

US

Large tanks are not typically designed for vacuum conditions, the tanks are designed to hold liquid. The roof will probably fail first.

"The second concern, would be that the vacuum relief valve could fail which could cause a vacuum in the digester and the roof could collapse."

To prevent vacuum conditions from occurring, the digested sludge should be withdrawn slowly and raw sludge, sewage, digester gas (if gas storage is
available), or water should be pumped into the digester at the same or higher rate than the digested sludge is being removed.

Study Link

 

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• Jul 17, 2018

• #12

LittleInch

Petroleum

Mar 27, 2013

23,042

GB

sam

You need to look at the design limits of your concrete digestor.

A perfect concrete sphere might be able to withstand a large vacuum load, but if this is a standard vertical tank like structure with a cone roof, then it won't.



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

 

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• Jul 17, 2018

• #13

bimr

Civil/Environmental

Feb 25, 2003

9,385

US

"The commissioning and maintenance of a biogas plant is a period when the plant is particularly at risk. During the draining of liquids from tanks, digesters and storage vessels, it is essential that air admission vents are opened before draining starts, so to prevent a vacuum developing in the tank or vessel. Tanks and vessels can implode if subjected to an internal vacuum. It should also be recognised that allowing air into a vessel containing biogas will result in the biogas passing through the LEL and UEL range at some point; creating the risk of a gas explosion. Draining and venting of vessels and tanks should be controlled by a written procedure which has been risk assessed by a competent engineer."

Risk Statement - Insurance Company Statement

 

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• Jul 17, 2018

• #14

georgeverghese

Chemical

Mar 19, 2015

5,115

AU

Admitting air into a tank with mostly methane in the vapor space will result in a flammable mix only when the mix is below the UEL limit for methane which is approx 15% by volume. This requires a significant vacuum pressure in the tank for the mix to then reach this UEL. So a flammable condition is not likely in most cases.
Additional precautionary measures one could implement would be
a) Install a control N2 (or inert gas) line feed to this tank to keep tank pressure within the vacuum pressure limits. Setting for this regulator would be few inches of water higher than the VRV that admits air into the tank.
b)Add a low pressure alarm at CCR.
c)Have the digestor electrically grounded to prevent static buildup. Arrange to have this grounding tested every year.

 

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