Blanket Gas Design Line sizing 3

[mohtogh]

Hello all,
As far as I Know, design basis for putting blanket gas on a atmospheric tank is based on rated capacity of downstream pump.(to prevent of vacuum during blocked inlets).With the mentioned flowrate,I want to size blanket gas line. My question is, what is the velocity and pressure loss criteria for this system. I think I cannot use general criteria for gases (e.g 20 to 60 fts ) because this not a continuous flow.What is your idea guys?

Thanks

 

[Montemayor]

Mohtogh:

Establishing the proper design blanket gas flowrate for a storage tank is dependent on a variety of reasons – most of which rely on your specific application, fluid, thermal conditions, etc. besides the pump-out rate. Please note that I am referring to the normal “design” operating flowrate – not to the maximum flowrate. This is what you mention as your design point and I would advise you not to design on that basis. I will give you one example: A normal and valid question during a routine Hazop review on your proposed installation would query what would be the credible, maximum product flowrate out of the tank. If you are using a conventional centrifugal pump, the correct answer is that the maximum pump-out flowrate could be realized if the pump’s head is suddenly diminished by an action downstream – such as a sudden opening of a throttle valve, a ruptured hose or pipe, etc. These are all credible circumstances that could overload the blanket gas supply and force the opening of your vacuum breaker – which you may not want to accept as a mitigation to prevent tank collapse due to partial vacuum being produced in the tank. You may want, under these circumstances, to design the capacity of the blanket gas delivery system to handle the worse credible partial vacuum case. But since we don’t know your basic data or Scope of Work, I can only speculate. You are in the best position to judge and decide on that.

For sizing of the blanket gas supply line, I never consider velocity other than just as a curious check after I calculate the line size based on my desired (or available pressure drop). In other words, using the Darcy-Wiesbach relationship, I calculate the line ID by imposing a pressure drop that gives me a desired degree of control on the regulators, valves, and fittings in the system. My blanket gas is usually a process utility, available in a conveniently located header nearby, so it is easy to surmise what pressure drop I can tolerate and select.

By “20 to 60 fts” do you mean to state “20 to 60 feet per second”? If so, this is an extraordinarily very low velocity. I would normally expect a much higher (and acceptable) velocity. As long as the pressure drop is acceptable and the velocity is sub-sonic, the supply should function well. There is normally nothing to fear about any erosive or corrosive action on the part of blanket gas (Nitrogen?).

I hope this helps you out.


Art Montemayor
Spring, TX

 

[johngerhart]

The line size is not that important since I assume you will be supplying ~ 100 psi air or nitrogen . The more important sizing issue is the pressure regulator . It needs to have the DP range to add gas and not leak by or waste gas in the idle state .The pressure being regulated from 100 to < 5 inches/H2O may require 2 regulators to be accurate and reliable . The cfm going in will inherently be quicker than the GPM going out . Since the liquid outlet velocity is going to be ~5 ft/sec , the gas make up rate should not be an issue .

 

[sshep]

Be sure and consider all the ways that there can be liquid movement out of tank when sizing these vacuum protection systems- running 2 pumps, draining to sewer or grade, equalizing with other tanks, ect. For example, draining a tank with 40ft of head through a 4" nozzle can result in a flow larger than some pump systems are designed for. These requirements will be superimposed on the inbreathing that results from cooling tank vapors. Most of us have seen a few tank failures in our career, and the ways this can happen are virtually endless.

You give no details so we can assume that the standard vacuum breaking requirements per API2000 will be installed independent of the blanket system. If this is true there are generally no regulatory requirements for blanketing tanks- a notable exception is jet mixing of some products (static electricity concern). Blanketing is usually a matter of site or company practice done for product quality as often as for safety reasons. If you only have relatively small N2 branchlines or limited N2 supplies available, then it may not be economical or neccessary to provide a 100% blanket for all scenarios.

just a thought, sshep

 

[Latexman]

Don't forget about weather effects. Pumping out during a cold, hard rain storm could happen.


Good luck,
Latexman

 

[mohtogh]

Thanks guys.
your guidelines were so helpful.

 

[25362]

Some operators prefer to use close loop automatic controls on the oxygen content of the blanket gas at about 4% points below the MOC for the case in hand, with continuous monitoring of the O₂ content.

For example, if the MOC is 10%, the nitrogen (or CO2) supply valve would open at 8% and close at 6% oxygen set points, providing a reasonable margin of safety while saving inert gas.

Purging methods are used prior to charging the flammable material into the tank or vessel.