Horizontal Atmospheric Water storage tanks

[msnoddon031]

I am trying to find out how much internal pressure is acceptable for a water tank classified as 'atmospheric'. I assume there must be some leeway (i.e +/- 3 psig). So far I have found some references that mention +/- 0.5 psig, but I am looking to confirm this with a concrete code reference (I can`t get my hands on the ASME Pressure Vessels handbook to find a ref.)

Mathew

 

[EGT01]

Mathew,

I hate to disappoint you but I'll be surprised if you get a straight answer for what you want. You might as well be asking how much internal pressure is acceptable for a tank classified as a pressure vessel.

As you have assumed, there are limitations to the internal pressures of a tank but they are specific to the codes and customer requirements to which the tank was built.

I'm not sure which ASME Pressure Vessel handbook that you are referring to but the ASME Pressure Vessel codes are intended for vessels with an internal pressure greater than 15 psi so they are not likely a suitable reference for your tank if it is just an "atmospheric" tank but there are exceptions. If it is a horizontal cylindrical shell with heads, it could fall under a number of other codes or standards such as UL, AWWA, etc.

If you are looking for a code to reference, the first place you should be looking is at a name plate attached to the vessel or in the vessels equipment file. The information you seek regarding acceptable internal pressures may be located there as well.

Try posting more details about your tank. This is not my area of expertise but maybe someone else can assist you better.

 

[msnoddon031]

The scenario is as follows:

I have a HDPE water storage tank that is gravity fed. The specs req`d an 'atmospheric tank', however the overflow was placed on top and as such when the tank filled to capacity the pressure exerted by the column of water the rose above the top of the tank to the overflow caused the manhole to rupture. Therefore I am trying to find out whether the fault lay in the design of the supplier's tank (even though classed as atmospheric should it nonetheless have been able to withstand the pressure - 3 kPa - caused by an 8" column of water?) or does the fault lay in the fact that the overflow was poorly placed as per the specs?

I assume there must be some code that covers this gray area between atmospheric and low-pressure tanks?

Any help is appreciated.

Mathew

 

[MJCronin]

msnoddon031,

In my opinion there are a couple of things to note here:

1) ASME Section VIII and the API 650/620 tank and pressure vessel Codes and standards do not cover HDPE...they are for the specific metals that are described within

2) Since your system developed a head of water that caused material failure of a tank component, it appears that your overflow was undersized for the application

3) When a steel, vertical axis, flat bottomed, "atmospheric tank" is designed to API-650, the purchaser decides upon the actual design pressure and vacuum. +10 IWC and -5 IWC are values commonly used (....again, Steve....in my opinion)

4) When an atmospheric, horizontal axis, metallic tank is desired, the UL-142 code is commonly used.....I do not know of any HDPE/plastic tank code that addresses the unique design requirements of horizontal axis tanks (can anyone out there help ???..??)

-MJC

 

[jdarco]

I doubt you will get any HDPE tank manufacturer to allow any internal pressure above atmospheric. Virtually all HDPE tanks are atmoshperic design. I suspect your manway (which was likely threaded) failed by blowing off the tank. This is pretty typical with HDPE tanks that are overpressurized. You can probably pressurize a HDPE tank to 2-3psi, but the typical manways will continue to blow off! Also, if you were able to seal the manway (by bolting and gasketing for example), you will stress the tank itself. Although not intended to, the "cheapness" of the manway acts as a pressure relief of sorts. ASTM D-1998 covers vertical poly tank design(atmospheric), I'm not aware of any codes or specs for horizontal tanks!

 

[msnoddon031]

So this means that ultimately the failure lies with having designed the overflow coming out of the top of the tank and thus causing a pressure head, and does not mean that there was a flaw in the tank design itself. Ultimately I am trying to resolve whether the tank was poorly designed (it was not supplied with the positioning of the overflow) or if the flaw lay in the engineering of the hookups (by who the decision was taken to place to overflow on top of the tank).

I realize right off the bat that placing the overflow on top was was bad design practice, however I must find some part of the code that will back this theory up. Which is why I am trying to prove that specifying an 'atmospheric' tank does imply a margin of error of +/- 5 kPa psig.

Thanks for the help so far. Please keep the info coming if you have any other.

Regards,

Mathew

 

[EGT01]

Mathew,

Unlike the others, I can't help you regarding mechanical integrity but you still haven't supplied a critical piece of information..... To what code or standard was your tank constructed?

Based on the others response, maybe there is a problem with the basic design of the tank.

 

[msnoddon031]

Unfortunately, I can`t attest to the design of the tank, I am stuck between the engineer who designed the plans for the installation and the supplier. So I`m trying to find out who is at fault.

Mathew