CS10:
I am going to make some comments here because I have been confronted with this same type of “issue” on many past occasions, all from young inexperienced engineers working under me. I refer to your statement: “if my pump transfer the fluid into the tank at its maximum design condition (highest discharge flow)”.
First - and foremost - a pump (and I don’t care what type of pump is involved) will NOT GENERATE PRESSURE ON ITS OWN VOLITION. A pump’s discharge pressure is solely the result of any resistance it confronts on its discharge side. This discharge pressure can be caused by the discharge piping, fittings, and valving - especially if you have a discharge control valve installed. A centrifugal pump will generate a discharge pressure sufficient to overcome any discharge resistance it confronts UP TO ITS DESIGN AND MECHANICAL CAPABILITIES (as seen on its customary Data Sheet and Performance Curve). Once it meets those limits, it will “dead head” or begin to cavitate with ZERO flow rate through its discharge piping - unless you install a return flow line with controls - usually a flow back to its suction source. All centrifugal pumps are subject to the same characteristics. Positive displacement pumps also follow the same basic rule: they will only generate a discharge pressure related to the resistance imposed on their discharge side. However, unlike centrifugals, they cannot vary their pumped flow rate in accordance with their discharge pressure - as centrifugals do. Therefore, you must design a failsafe discharge system for them to prevent an exorbitant pressure discharge. Most PD pumps will continue to pump regardless of the discharge resistance (including a total discharge blockage) until something happens to relieve that situation - such as a total shutdown or a relief system - or a mechanical breakup or rupture in the worst case.
By the way you have written your post, you are insinuating that the storage tank (undefined as to type) is receiving the full discharge pressure of your pump. If your tank is an API storage tank, this can’t be the case. Therefore, Breizh’s usually useful comments and advice don’t apply. I believe ProinWV also assumes that you have an API tank and he is applying the same basic pump characteristic information I mention above. Fallah also assumes an API tank, I believe. All our expert engineers are correct and right on with their comments - but only by applying the assumption of an API tank and that you are regulating the inlet liquid fluid flow rate and that you have a maintained vapor pressure in the tank’s head space.
Your stated tank design pressure is 0.4 psig. This puts it well within the design limits of an API tank and I will assume that is what it is also. I will also assume you are pumping and storing a fluid with a vapor pressure below atmospheric and that you are applying a nitrogen or other inert blanking system. Under this conditions the 20” emergency vent size is probably sufficient - depending on the emergency scenario it was designed for. Bear in mind that it is an EMERGENCY relief device - not an operational relief device. It should be designed for emergency scenarios - such as an external fire case or a sudden influx of excess inlet fluids. A relief of blank gas does not apply here because that is an operational scenario subject to the operation of the pressure relief vent valve described by Breizh's literature. You should read and study the excellent information Breizh supplies on Protectoseal vent relief devices - both operational and emergency.
My response(s) to your questions/statements are:
My question is, if my pump transfer the fluid into the tank at its maximum design condition (highest discharge flow),
As explained above, your tank does not receive your pump’s maximun design pressure although it can - and should receive its maximum flow rate - as long as it is instrumented to handle any excess flow rate or fill.
Is it possible to over-pressure the tank if flow from 2" pipe?
As explained above, you are wrongly worrying about the wrong condition. You cannot pressurize your tank with the fill pump(s) - unless you a fluid supply source that is more than the tank’s capacity and you have no controls on your fill pump (such as a max fill limit on your tank and your emergency relief device fails to work (a double jeopardy). This is an improbability - but I assume you have a properly safe design installation.
If it does not over-pressure the tank, how can be best explain the correlation between 2" pipe vs 20" emergency vent (to confirm that this vent is adequate to handle the max. condition from the pump)? My question is, if my pump transfer the fluid into the tank at its maximum design condition (highest discharge flow), Is it possible to over-pressure the tank if flow from 2" pipe?
As explained above, NO!
If it does not over-pressure the tank, how can be best explain the correlation between 2" pipe vs 20" emergency vent (to confirm that this vent is adequate to handle the max. condition from the pump)?
Again, I’ve explained how the filling of an API storage tank is usually and generally designed to operate. The size of the tank’s fill line has absolutely NOTHING to do with the size of the emergency device on the tank. YOU (assuming that you are responsible for this tankage) should be intimately familiar and know exactly how your tank is designed to operate - including all its safety and controlling devices - and all credible safety scenarios. That means that you must have had a direct participation in any HAZOP meeting and design results regarding this particular tank and also have played a significant role in deciding the safety and operational features of the entire tankage area. If this has not happened, you are in more potential serious trouble than what you imagine. That is one reason that I am making this rather long and detailed response to your queries.
If you are, indeed, the responsible operating engineer for this storage area, you should by all engineering standards and accepted good practices, be totally in command and understanding of the equipment, instrumentation, and infrastructure of your tankage area. That is the advice and - many times, instructions and orders - I have given young or inexperienced engineers in the past. I offer this advice to you now with the hopes that you will accept it as a voluntary attempt to help you in your efforts to operate a safe and efficient operation while maintaining your operators and career in good, safe standing.