19 replies to this topic

[Attiq Ahmad]

Dear all,

We want to provide blanketing nitrogen to hydrocarbon storage tank but unfotunately we don't know storage tank design pressure. Only data which we have is tank thickness report.

Can any body tell how to calculate maximum pressure of blanketing Nitrogen while tank design pressure is unknown but thickness profile through out the storage tank is known?

regards,

Attiq

[Art Montemayor]

Attiq:

The method of calculating and rating an API storage tank are documented in text books and in API literature (which you should already have in hand and studied thoroughly). However, if you have never rated, don't try to do it. Employ an experienced, qualified, and registered Mechanical Engineer to do the formal calculations and rating. Make sure you obtain complete and detailed calculations from him, outlining the precise method and algorithms used by him/her.

What you are asking is very, very important and many owners of uncalculated storage tanks have failed or have refused to have their unknown tanks rated in the manner above. The result can be very tragic and sometimes disastrous. Many tank owners – including professional engineers – are unaware of the critical and weak design points in a cone-roof API storage tank and fail to take the elementary and safe steps that you are proposing and because of this, they have suffered tragic accidents with loss of human life and economic capital.

It is very, very important to have the accurate and documented identity of your storage tanks Maximum Allowable Working Pressure (MAWP) established formally and legally. This is a basic necessity for safe and reliable operation of your storage tank. Without this information you cannot set the operating pressure point of your tank's safety devices with certainty that you and your personnel will be safe.

I hope this helps you out.

[proinwv]

Let me add that not to forget to evaluate the MAWV or at least set pressures and vents that the tank is protected fully from entering any vacuum.

I would add to what Art implies in that all the construction details of the tank must be discovered to "reverse engineer" this to establish the tank capabilities.

Art, is it possible that a hydrocarbon tank could come under ASME, if above 15 psi?

[Attiq Ahmad]

Art & proinwv,

Thanx for quick help.

I also strongly agree with Art's suggestion to evaluate it from experienced mechanical engr.
Provinwv also highlighted an important point that we should evaluate this tank against vacuum.

Art,
Can you please tell (for information), do we follow API-650 while rating an existing API-650 storage tank or guidlines are given by some other API standard?

BACKGROUND:
we are taking a tank on rent basis (for one year) and owner of tank don't have tank data sheet nor tank drawing.. All he has is tank thickness report & radiography report. Existing system has no nitrogen blanketing and we want to install it for our storage chemical.

regards,

Attiq

[Qalander (Chem)]

Let me add that not to forget to evaluate the MAWV or at least set pressures and vents that the tank is protected fully from entering any vacuum.

I would add to what Art implies in that all the construction details of the tank must be discovered to "reverse engineer" this to establish the tank capabilities.

Art, is it possible that a hydrocarbon tank could come under ASME, if above 15 psi?

Dear Attiq, further to what is indicated above it is important to know your storage tank design basis;

That Should presumably be API 650 (usually considered as the basic design code& it is for atmospheric or near atmospheric)

thus the vapour spaces pressures are contolled/kept within few inches of water.
you may refer the recent post response to Afshin the other day; that gives the serious consequences possibilities as well.Storage Tank
Hoe this helps
Qalander

[Attiq Ahmad]

Qalander,
You are absolutely right that blanketing pressure is kept at few inches of water column (in W.C). But question is how many inches of water column press which this tank may bear?

Another problem related to pressure is that this tank is located near jetty where product will be unloaded from ship. During unloading, we have to have vapor balance line which will go to ship. Tank working pressure should be sufficient to bear this back pressure.

Thus, knowledge pressure is very important in this case.

For your info, I am also uploading PFD for this system.

regards,

Attiq

Attached Files

•  PFD.xls   68.5KB   376 downloads

[proinwv]

Have you calculated the pressure drop in the 600m hose and fittings returning vapors to the ship?

This back pressure will control the tank pressure, and may result in constant flow from your PVRV to the atmosphere.

Without seeing the numbers, I am very suspicious of this. It concerns me.

[Art Montemayor]

Attiq:

I believe everyone who reads this and studies the excellent, detailed schematic diagram you have furnished will realize that you know exactly what you propose to do and are being very professional and prepared in your planning and detailed engineering. This is the way ALL tank applications should be handled. Unfortunately, as I'm sure Paul Ostand will confirm, this is not always the case.

I consider this a very important subject and I take this time to apologize for omitting the important detail that Paul was kind enough to insert into our comments. The Maximum Allowable Working Vacuum (MAWV) is just as important as the MAWP. I regret omitting this important detail. The main purpose of my insisting on mentioning as much detail as possible on this subject is that I know – from experience – that this information is being read and (perhaps) also being applied. I take the opportunity to stress the study and reference to API 2000, API 650, and perhaps API 620. Regardless of whether you plan to do some of the calculations yourself or delegate them to a professional, it is important to be aware of the design guidelines and instructions found in the API documents. I realize that approximately 500 pages of technical information is a huge challenge; but it is all in your interest and safety.

API 650 is the instrument whereby you can design and rate your tank – up to a MAWP of 2.5 psig. For working pressures above that and below 15 psig, you are referred to API 620. Please see Appendix F, "Design of Tanks for Small Internal Pressures" in API 650. For vacuum operation (external pressure), see Appendix V in API 650. A maximum of 1 psi external normal pressure is allowed in API 650 for tank design. As Paul indicates, once you surpass the 15 psig ceiling of API 620, you are left with only ASME Section VIII to design under.

Using the confirmed and certified tank thickness report & radiography report on the tank condition you should be able to work with a professional mechanical engineer who will do the official and detailed rating of the tank. The rating engineer will need direct and personal access to the tank in order to do an accurate rating. He has to be able to physically inspect every key type of weld, joint, and plate conditions. He also has to check the required anchoring system and roof integrity. I have done many times what you are proposing. And working intimately with my rating mechanical engineer, I was able to identify (and employ) a safe MAWP and MAWV. I have identified MAWP values in excess of 2 psig and MAWVs of approximately 16 to 20 inches Water Column. All of this was valuable information when applied to chemical storage tanks that required blanketing. By maximizing the MAWP and MAWV PVSV settings, I was able to save valuable Nitrogen venting. In other words, I was able to amplify the safe pressure range under which the tank normally could operate without venting nitrogen or importing in make-up nitrogen. A storage tank can be reinforced and upgraded to achieve a higher MAWP – as well as a better MAWV. However, you have to know what you are doing. You state that the tank is located near a jetty where product will be unloaded from a ship. I don't define 500 meters length as "near". I believe the vapor balance pressure drop has to be carefully calculated and estimated in order to avoid too large a pressure drop.

I am attaching a copy of some storage tank workbooks that hopefully will be helpful in describing some of the aspects confronted in applying tank blanketing systems and safety relief devices. Be sure to note and study all 4 of Paul Ostand's articles included within these workbooks. I hope this helps you out.

Good luck.
 Art__s_Tank_Relief___N2_BlanketRev1.zip   1MB   939 downloads

[proinwv]

As usual Art writes an excellent reply, and more detailed than mine usually are.

I would amplify Art's comments about detailed information for anyone reading this thread or any forum. Your providing the schematic enabled us to immediately pinpoint a point that needs your attention, that we might not even have thought of without that detailed explanation.

Also as Art says, there are many tanks in service where the capabilities are not known to the user.

To me, the absence of a rating nameplate is a big warning sign. Red letters saying "User Beware!"

[Qalander (Chem)]

Dear Attiq,

You have proved to be a person with a good professional approach; that's really heartening.

Much said already that a number on whims can not and should not be advised at this should typically be dependent on the prevalent conditions of the ship and/or tank truck hooked up through the vapor balance pipeline.

However, even if we talk about very low figure of below 8.0 mm of water column the likelihood of what is pointed out by our 'proinwv' exists strongly.
I believe you are now in the best position to initiate an analytical thinking and decision-making process .

Best Regards
Qalander

[Attiq Ahmad]

Dear all,

Adding details to system being discussed, I am also sharing some detail calculations...

Flow of Liquid to be unloaded from ship = 500 tons/hr (equivalent to 400 m3/hr)

Thus, equivalent volume of vapors have to be vented from tank.

Hence,
Flow of vapors to be vented from tank = 1200 tons/hr (vapor desity is about 4.0 Kg/m3 near 0.5 ~ 1.0 psi)

Pressure drop through 6" vapor balance line (to jetty) = about 0.6 psi

Here the main question arises...

"Whether the tank can with stand back pressure of about 0.6 psi??"
OR
as an alternative, we should reduce the unloading flow rate to bring back pressure below tank design pressure?
OR
another alternative (but expansive one) is tha We should increase vapor balance line size to reduce pressure drop below tank design pressure.

Thus, knowing tank MAWP we can easily sort out this puzzle.

Regards,

Attiq

[Qalander (Chem)]

Dear Attiq,

Just a wild thought

Insertion of a Restriction Orifice & Control valve in such a way to ensuring that the pressure vapor receiving back line portion connected towards the tank in question;
remains regulated within limits identified/estimated MAWP as per the tank's mechanical strength survey NDT thickness scanning etc.

Regards
Qalander

[proinwv]

Attiq, when you say, "Thus, knowing tank MAWP we can easily sort out this puzzle." you have answered the question, as I believe you realize. The mentioned solutions depend upon the costs involved as well as the requirements of your operation. Of course, in considering the costs don't forget the cost of a damaged tank, personal injury, and downtime.

Qalander, I don't believe I understand what you meant when you said,"Insertion of a Restriction Orifice & Control valve in such a way to ensuring that the pressure vapor receiving back line portion connected towards the tank in question; remains regulated within limits identified/estimated MAWP as per the tank's mechanical strength survey NDT thickness scanning etc."

[Art Montemayor]

Excellent comment Paul.

Additionally, the math should be done carefully. Attiq says:

"Flow of Liquid to be unloaded from ship = 500 tons/hr (equivalent to 400 m3/hr)

Thus, equivalent volume of vapors have to be vented from tank.

Hence,
Flow of vapors to be vented from tank = 1200 tons/hr (vapor desity is about 4.0 Kg/m3 near 0.5 ~ 1.0 psi)"

And I note:

1) the liquid volumetric flow rate entering the tank is the return vapor flow rate going back to the ship; this flow rate is then = (400 m3/hr) (4.0 Kg/m3) = 1,600 kg/hr --- NOT 1200 tons/hr!

2) This is very heavy fluid! The 500 tons/hr divided by 400 m3/hr yields a 1.25 specific gravity = 78 lb/ft3. I thought water was heavy, at 62.4 lb/ft3!

I think Attiq has to furnish ALL the basic data and also make sure of his math.

[Qalander (Chem)]

Dear Paul hello/Good Night

I must admit my in-accurate & somewhat vague snetences. Definitely I probable remained in somewhat haste in sending off. I express now that these were two different things.

A flanged conection inserted with orifice mechanically restricting the flow (somewhat illogical and absurd indeed) I apologise. Howevever I really meant to insert a pressure control valve maintaining downstream side (towards tank) pressure within the limits.

The other statement merged was to indicate various possible means to obtain the mechanical integrity/strength level to calculate/estimate possible MAWP.

Sorry for being unclear as I was leaving office at that particular time and sent this poor show on my part as I am definitely not Infallable.

Thanks/Best regrds
qalander

[proinwv]

Qalander

Don't apologize, I've been known to do similar things.

Well I guess my basic theory is to limit the backpressure completely well below the tank MAWP and by a conservative calculation of backpressure using appropriate friction values and the maximum vapor transfer (of the heaviest vapor) rate. As it is we would be calculating this and this does involve estimating friction factors.

As previously stated, we need to establish a MAWP/MAWV.

I think Attiq knows just what to do.

Attiq, I have not checked your math and actually I won't do that.

[Attiq Ahmad]

Dear Art,

A very sharp pick up...!!! Some times I really introduce error in basic calculations when I am thinking in deep to find some engineering solutions.

Fortunately, Pressure drop was calculated on 1600 Kg/hr vapor flow.

Regards,

Attiq

[Qalander (Chem)]

Dear Attiq,

May I earnestly suggest to review/recheck you referenced 'Hydrocarbon's data' thoroughly as either the numerical values contain some error or otherwise the Defined H-C of such a heavy Sp.gravity seems very difficult even if we consider the Bitumen/Asphalt Grades.

Hope this should help anlytically in problem recognition & possible resolution as well.
Best Regards
Qalander

[Attiq Ahmad]

Dear Qalander,

Specific gravity of chemical is 1.25 and I am 100% sure about it. Actually it is chlorinated hydrocarbon.

There may be error in my simple calculations but not in estimation of chemical properties... (Just joking)

regards,

Attiq

[Qalander (Chem)]

Dear Qalander,

Specific gravity of chemical is 1.25 and I am 100% sure about it. Actually it is chlorinated hydrocarbon.

There may be error in my simple calculations but not in estimation of chemical properties... (Just joking)

regards,

Attiq

Dear Attiq
Thanks and Best regards for this belated but interesting info; now you seem relieved/ Puzzle resolved! I assume.
Best of Luck
Qalander