4 replies to this topic

[chem55]

Dear Experts,

In my application I am designing new facility which consist indoor ATM Storage Tank with Steam Heated coil. Steam flow is controlled by Temperature controlled valve. The steam flow for the startup will be 2500 lb/hr and then to maintain the temperature it will have 150 lb/hr.

I would like to know the required size for emergency vent for coil rupture case. My understanding is that I have to assume that when coil ruptures, Control valve is completely open, and that steam flow is going through the emergency vent. However, I don’t have vendor data for the Control valve (Trim size).

-Is it possible to size the emergency vent without vendor data?
- Is it necessary to design the tank for Full Vacuum? I believe it is not required as far as vent is properly sized and also as it is indoor tank.

Thanks in advance!

[Art Montemayor]

Chem55:

You should size the emergency vent with the vendor data that you obtain from the CV’s manufacturer. Simply read the name plate on the CV (if you are trying to use an existing CV) and call the manufacturer. You are in Canada, so this is an easy and routine task. If you don’t have the name plate and you can’t identify the manufacturer, then you are in deep poo-poo by using the CV. Trash it and design a new, identifiable CV with a known maximum flow through it. If you are designing a new installation, I don’t understand your dilemma. All you need is a Fisher Controls catalog or simply sit down with your CV representative and get his recommendation based on your specification sheet for the CV. Again, you are in Canada, so this should be a no-brainer.

It certainly should not be necessary to design the tank for Full Vacuum. Simply do what everyone else does: install a conservation vent with pressure and vacuum relief. Or, if you only want vacuum protection, install a vacuum relief valve. Once again, you are in a fortunate location. Simply call your local or regional rep for pressure/vacuum relief valves and get their recommendations based on your specification sheets. In fact, you can do this via the web:

THE PROTECTOSEAL COMPANY
Headquarters, Manufacturing
and Warehouse Facility
225 W. Foster Avenue
Bensenville, IL 60106
Phone: 630/595-0800
Toll-Free: 800/323-2268
Fax: 630/595-8059
E-mail: info@protectoseal.com

I always install a restriction orifice on the steam supply line to the CV on these types of operations. The orifice should be sized to yield a maximum flow rate that corresponds to the capacity of the emergency vent. This is normal procedure to ensure that you don’t exceed the vent capacity.

[ankur2061]

chem55,

Normally atmospheric storage tanks are not designed for full vacuum. If the tank undergoes a steam-out operation i.e. steam is actually used to clean or flush the process side then due to condensation of the steam vacuum would occur. I have not heard of atm storge tanks designed for full vacuum just because it has a steam coil in it. It is important to remember that the steam coil needs to remain submerged in the liquid and good engineering requires that an instrument interlock be provided to cut-off the steam control valve in case a low level occurs in the tank.

Assuming that a coil leak does occur, then the steam will mix with the stored liquid in the tank forming an aqueous phase of the stored liquid. Thus there is no need to size your emergency vent for steam leak from the coil.

Also note that a scenario considering failure of the control valve to close on low level and a coil leak occuring simultaneously is not a valid scenario. It constitutes a double jeopardy and as per API STD 521 two contigencies occuring simultaneously is not a valid case for sizing any overpressure protection or relief device.

Hope this helps.

Regards,
Ankur.

[fallah]

-Is it possible to size the emergency vent without vendor data?
Normally,EV would be sized for fire risk and covers the coil rupture case
- Is it necessary to design the tank for Full Vacuum? I believe it is not required as far as vent is properly sized and also as it is indoor tank.
As Ankur mentioned normally you don't need to design the storage tank for full vacuum

[djack77494]

In almost every case, designing an atmospheric tank for full vacuum is not practical. These tanks are generally large-oftentimes very large. Even a small amount of vacuum acting over the large available surface would collapse any tank not made of very thick steel. Typically, they can withstand very little vacuum-of the order of a couple inches of water column. I like to design them for up to 6 inches W.C., but only if they are not particularly large. You need a good tank guy to give you a "reality check" if you get carried away.