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

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?

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.

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.

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thanks everybody for your replies.

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

Yes.

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

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

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

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.

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"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

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.