Tanks
Not knowing whether you are using vacuum or a blower for discharge is a bit of a problem but I will give you my opinion.
1. In a cement silo the the cement is blown into storage and gravity fed.
2. The tank has a trap door on the top of the silo that must be opened before the cement is blown out of the tanker into the silo. this allows the discharge to atmosphere of the high volume low pressure air coming in to the silo from the cement trailer.
3. During drawdown of cement the tank draws air in through a atmospheric vent elbow which is always open to atmospere, the elbow has a mesh screen in it to keep out insects etc.
The same rule applies for a tank that has a vacuum discharge as air must be allowed to enter the tank from some point to allow movement of material and avoidance of tank rupture due to the negative pressure induced by the blower or vacuum pump.
The forum library bookstore has lots of information for you. ■
Bin Flow
Mark,
There are a number or correlations in the literature that will enable you to calculate the unrestricted mass flow of material out of the bin bottom nozzle.
Convert the above-calculated mass flow to a volume flow based on an aerated bulk density for your specific material.
Determine a reasonable operating underpressure (vacuum) with respect to the design underpressure for the bin. A ratio of 2/3 is commonly used.
Based on this flow and underpressure a vacuum relief valve can be sized. Another possibility is a padding arrangement that utilizes a pressure transmitter and control valve.
Dennis Hauch, PE ■
Re: Venting Of Storage Bins
This is not a difficult problem.
You will surely need a vent filter to separate the dust from the displaced air during conveying/filling. It will be sized according to the design of the pneumatic conveying system, (a typical flow of 0.35 cu.m/s or 750 cfm).
It is unlikely that you will be discharging product at this rate but, in any case, the vent filter will protect the silo from overpressure/underpressure, providing that the unit does not become blinded because of neglect.
Provide a standard rupture disc in the roof of the silo if you need further reassurance.
Michael Reid. ■
Re: Venting Of Storage Bins
Mark,
For bins and silos I always use a vacuum breaker in addition to the vent for discharging the incoming air/gas. These two are sized based on the vacuum and pressure ratings of the silo. The vacuum breaker is nothing but a breather vent that sucks in air/gas as needed when emptying the bin and the venting system protects the bin against over-pressure. Rupture discs or explosion panels or needed only when handling combustible dusts.
Regards,
Amrit Agarwal
Consulting Engineer
Pneumatic Conveying Consulting
Email: polypcc@aol.com
Ph and Fax: 304 346 5125 ■
Re: Venting Of Storage Bins
Thank You all for your replies,
Much food for thought. Could anyone give me the title of the text/article on this subject to cross reference \verify any calculations.
My problem is I am looking for the velocity of the air going into the bin, I know I need a pressure difference to enable this but I am missing this info.
The volumetric flow rate is 800cfm of the product (40#cuft) , gravity fed, tank is 11ft dia , 30 ft height. I know I need to match this for air intake, and dividing by the air velocity would give me the reqd minimum area.
Does anybody have any thoughts on a reasonable or exact method to calculate the velocity.
Regards
Mark ■
Cement Silo
Hello Mark,
Any of the blower manufacturers will have any and all the information you need.
lzaharis ■
Re: Venting Of Storage Bins
Hi Izaharis,
Thanks for replying. Would a blower catalogue only be applicable during filling, I am trying to get the pressure difference during gravity discharge. The discharging of the material creates a lower pressure inside the tank ( does it not? ) . May be I am over complecating the problem. I am after the velocity induced by this pressure difference.
Regards
Mark ■
Venting Of Storage Bins
Mark,
You are over-complicating the problem. All you need to do is provide an adequately sized vent filter. It will protect the silo during unloading as well as loading.
Michael Reid. ■
Cement Help
Dear Mark,
all your going to is get a rush of atmosperic pressure air as the only air that will come into the silo is what will enter the silo through (the open to atmospere vent) to replace the volume of cement that is being removed.
So the possible vacuum is no more than 1 atmospere but it will not harm anything. ■
Venting of storage bins
Dear Lynn,
Do you or any of your fellow professionals know of any guidelines/rules in the design of venting to prevent collasping of the tank/bin walls when discharging. Specifically calculations in calculating required area to allow a sufficent amount of air to replace the discharged product, and perhaps a factor to allow for bridging in the tank.
Regards
Mark Dawson ■