Storage tank outlet valve pressure 1

[MetalGuru]

Hello
We are looking to install some diesel fuel storage tanks of approx 4m diameter and 14m height.
I am trying to calculate the pressure rating on the outlet valve and am having some problems/need some reassurance!
Is the Pressure rating calculated from
P=density x g x height
only? Or is there more to it? This gives a low rating of just over 1 bar, and I would perhaps expect it to be higher.
Could someone please clarify this, tell me the formula or point me in the direction of a good web site please?

Thanks

 

[JStephen]

The pressure at the nozzles in an atmospheric tank is normally low, as you have calculated. Usually, the piping and valves that are used are rated for much higher pressures, as in 150# fittings.

In some cases, it might be possible to generate higher pressures when the valves to the tank are closed than when they are open. In some cases, the piping may be hydrotested at higher pressures.

 

[MetalGuru]

Thanks JStephen

I have calculated the head pressure now, and am satisfied with that.
If this head pressure is present at the suction flange of a transfer pump, what other pressures need to be taken into account on the discharge side?

 

[quark]

MetalGuru,

Your procedure is right and much simpler and approximate formula will be liquid column height*specific gravity of fluid/10. This is the maximum pressure(if we neglect hammering, which is rare in your case) appearing at the suction flange of pump.

At discharge, the maximum pressure will be the sum of pressure at suction and pump shutoff pressure(again neglecting hammering, but there are fair chances here).

Regards,

 

[Montemayor]

MetalGuru:

All the above, especially quark's response, is exactly correct from a theoretical and operability point of view.

HOWEVER, there is a additional and practical point to consider: sooner or later, all storage tanks are submitted to the practical necessity of filling them with water in order to test the integrity of repairs or welds. When this necessity is applied, of course, the specific gravity value in quark's equation immediately increases from an approximate 0.7 to 1.0 - an increase in hydrostatic pressure of approximately 43% at the bottom of the tank.

This is a point that the guys in maintenance will always throw back in your face if you are not prepared with field experience. Sometimes we all tend to forget about those guys that make our lives miserable, but make our designs work in the field. I just thought I'd add this key learning I picked up in the plants over 40 years ago when I was wet behind the ears and thought I knew more than the guys in the maintenance shack.

 

[25362]

Speaking of unforeseen effects. It sometimes happens with offsite piping. If there is a chance that the downstream pipe becomes blocked in and exposed to solar heating, pressures on that side of the outlet valve due to liquid expansion may rise at the rate of about 10 bar/^oC. Some kind of protection may be needed. Do I exaggerate ?

 

[TD2K]

The other thing I would suggest is to design the tank, in addition to being full of water, for some small vapor space pressure/vacuum. This is needed if you are going to install breather valves on the tank. Any breather valve needs to have some positive/negative pressure.

You don't need much pressure, a few inches of pressure/vacuum rating will make your life much easier later if you ever have to install breather valves, a vapor recovery system, etc.

 

[quark]

Mr Montemayor,

That is an excellent suggestion, always overlooked by me, and I appreciate the detailed insight(as usual) of you three guys(You, 25362 and TD2K).

Regards,