6 replies to this topic

[Rush123]

Hi,

 

We have a Diesel storage tank. whose design conditions are stated as "ATM @ AMB." (See Note 1 below.)

This tank is open to atmosphere via two 3" vent lines.

 

A concern was raised that the hydraulic head of diesel in the tank (let's say approx 25 ft) could possiblly over pressure the tank and consideration shpould be given to including a relief valve to protect against this overpressure.

 

To me, this is almost a nonsensical type of comment. Below ar ethe details I know / have gathered so far:

1.) This tank is actually the pedestal of a crane, which is being used for diesel storage. This is not uncommon and I have seen this on many facilites, especially where space is an issue.

2.) The tank material is carbon steel and has a vessel trim equivalent to 150# piping.

3.) API 650 states that the "Design Pressure and Temperature conditions of vessel are in terms of vapor space" i.e the tank is not designed to be have any pressure vapor in it, but should be able to withstand full load of liquid.

4.) The wall thickness of the tank (which I do not know at the moment) would probably be good enough for pressures greater than atmospheric.

 

While i try to gather more information in order to close out this seemingly nonsensical tpye of problem, i though i would share this and see what others may come up with.

 

Note: Thsi tank also has a liquid overfill line, (3") which may be undersized. I'll post a question abt that in a different forum and provide the link here.

 

 

[ColinR33]

I have never heard of a relief device to protect a vessel from hydrostatic head of the liquid it holds!  Did the person suggesting this want to put the relief valve on the top of the vessel as well?  I agree it is a nonsensical statement.  As per API-650 (and common sense!), storage tanks should always designed to handle the hydrostatic head of the liquid in the vessel in addition to any design pressure, heck, we do this for pressure vessels.  The only issue I could see is if the liquid in the tank is of a significantly higher density than what the tank was designed for, and you cannot install a relief valve for that.  If you have the original drawings/calcs you should be able to confirm the wall thickness required for hydrostatic head design, or get someone else (maybe the original designer/manufacturer) to do so.

 

Cheers,

[Sharma Varun]

I may sound offtrack here but in such cases the philosphy we adapt is to specify tank full of water as water may be administered for flushing and generally its on higher side w.r.t. gravity. 

 

However I do agree that idea of providing PSV at top for hysdrostatic head sounds nonsensical    but such a psv if envisaged shall be definilty located on shell closed to bottom or on Locked open inlet/out line (assuming same at bottom of tank.

 

 

varun

Edited by Sharma Varun, 09 October 2013 - 11:36 PM.

[fallah]

Rush,

 

A PSV might be used in an atmospheric tank to protect the tank against overpressure, due to liquid static head more than the value on which the tank to be designed, just when the tank hasn't been designed full of liquid....

In such rare case the tank's shell courses (especially bottom course) haven't adequate thickness to withstand the pressure more than corresponding value of a prespecified liquid static head inside the tank...Then a PSV might be installed near the tank bottom to be able to release a load equal to maximum tank filling rate...

[Rush123]

Hi,

 

Just following up on this topic to say how I am going to close the issue.

 

Firstly, the tank wall is 1.6". Which suggests that it should be more than capable of handling hydrostatic head (based on wall thickness of pressure vessels.) There are alos a number of layers of protection / control to ensure that liquid overfill does not occur. (Eg: level control alarms, level control shutdown.)

 

This may be a very flimsy way of closing this off for the time being, but the risk associated with it is too low to warrant much more attention given some of the other issues on the facility.

 

P.S: Your suggestions abt putting a PSV on the bottom of the tank to prevent overpressure due to static head would be valid. We aren't pursuing this at the moment due to the reasons stated aboove.

[S.AHMAD]

1. PSV at the top of an atmospheric tank?.... a waste of money!

2. Static head of a tank is used to determine the side wall thickness of the tank... we can see some tank top thickness is thinner as compared to the bottom side thickness

3. In determining the thickness need to consider the maximum possible fluid density... mostly water since density of water is always higher than diesel...

Edited by S.AHMAD, 16 November 2013 - 12:50 AM.

[kmb]

Have you ever review API 2000 regarding overpressure due to fire exposure?