9 replies to this topic

[CS10]

Hi Everyone,

 

I have some questions regarding the emergency vent for LP storage tank. 

 

If I have a feed stream to the my tank coming from high-pressure source i.e. pump, and flow through a pipe with a size of 2" pipe.

The installed emergency vent on my tank with a size of 20", and tank design pressure is 2.8 kPag (Tank Size: 4(D) x 5(L).

 

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?
• 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)?

 

 

Thanks in advance,

 

 

[fallah]

 

• Is it possible to over-pressure the tank if flow from 2" pipe?
• 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)?

 

Hi,

 

1- At a glance, no...

2- In general, a 20" EV could be adequate for a 20" incoming flow line from a pump discharge

[proinwv]

You need to determine the inlet flow rate and relate that to the vent capacity and resulting pressure to maintain that flow. While the sizing is likely adequate, I would still confirm and record the condition that will result should the vent be required to open. Remember that the tank pressure will reach the vent opening pressure then it may drop depending upon the vent capacity.

[breizh]

Hi,

May be not the last update but it will give you direction for your work.

good luck

Breizh

Edited by breizh, 19 July 2019 - 08:12 PM.

[Art Montemayor]

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.

[PhilippM]

What pieces of equipment are upstream of the pump and what does their overpressure protection look like?

Maybe there is a possibility of vapor breakthrough resulting in a much larger volumetric flow than the maximum pump-in rate

 

What pieces of equipment are downstream of the tank? Maybe there is a backflow scenario, again with much larger flows than the maximum pump-in rate.

[CS10]

Hi Experts,
 
Thank you for all your feedback and comments, much appreciated. This give me more insight to this topic and specially with regards with the task I am doing.
 
Just for information, I'm doing the engineering review to confirm whether the existing over-pressure protection system which installed is adequate to relief under any relieving scenario.
 
Based on the API Standard 2000, I have done a calculation and found that the relief valve is adequately sized (assuming stored fluid have the same characteristics as Hexane).

While doing that, I also identified the high-pressure sources (from pump) as I mentioned on my early post, which I was not too sure if this can lead to over-pressure my tank.
 
I have attached a simplified sketch to show the scenario I identified.

Attached Files

•  Sketch.JPG   36.55KB   4 downloads

Edited by CS10, 22 July 2019 - 02:50 AM.

[PhilippM]

 
Just for information, I'm doing the engineering review to confirm whether the existing over-pressure protection system which installed is adequate to relief under any relieving scenario.
 

 

Just make sure you also check the "Other Circumstances" (API 2000, section 3.2.5), as it not really evident from your sketch what other pieces of equipment are connected to the system you've shown in the sketch. You mentioned the stored fluid had similar properties as Hexane, while calling it "Waster Water" in the sketch, so I'm guessing it's not just a pump that transfers waste water from one atmospheric tank to another one. When I check the overpressure protection of any system, I usually make sure to look at the entire flow path up to the next piece of equipment with a defined pressure "boundary condition" (i.e. a process vessel with a relief valve or an atmospheric tank)

[Art Montemayor]

CS10:

 

You cannot make a credible hazard analysis based on the sketch you have supplied.  You only show:

• a check valve and a ball block valve in the discharge line;
• a "goose neck" vent nozzle on the tank roof.  Is this tank at atmospheric pressure?
• waste water is the stored fluid; why are you considering hexane?

You state the relief valve is adequately sized.  Which relief valve - the emergency relief?

How do you start up the centrifugal pump shown - with the discharge fully open to maximum flow?  That is definitely not the way to start up a centrifugal pump.

 

The bottom line to your situation is that you can't analyze your storage facility's potential hazards without having an AS BUILT, updated, P&ID.  You can't do it with the simplified sketch you have furnished.  You've also only furnished us "simplified" basic data - and that also won't fly in a serious hazop.

[Pilesar]

So you have a wastewater storage tank open to the atmosphere with a 2" vent. Normally this vent is to handle the vapor in and out during normal liquid transfer operation. The 2" vent is likely large enough for the normal scenario. But you also have a 'tank liquid overfill' scenario which cannot be handled by the 2" vent. That is where the large emergency vent would be needed if you do not have an overflow nozzle on the tank. You will need to calculate each scenario and the required vent sizes. I think you assumed hexane in your calcs because you do not know the actual composition of the wastewater and there are unknown hydrocarbons in it and you wish to be conservative. That may be common for PSV calcs but brings up another concern. If you really do have the chance of hydrocarbon vapor in the tank, you probably do not want this 2" vent open to the atmosphere to allow air to breath in and out! This would allow a flammable atmosphere in the tank lacking only a spark. This scenario would also exercise the large emergency vent and maybe that would not be large enough. A continuously monitored nitrogen purge would be the safest alternative. Consider whether a flame arrestor is also desired. You don't show a dip pipe for your feed, but that would also help with grounding for a light hydrocarbon feed case. An alternative for a wastewater tank is to permanently remove the cover of the emergency vent. Is there a reason to keep it closed? You could even add large holes in the side near the top for the liquid overflow scenario and to make sure there would be no pressure buildup.