Good new year,

We have a fixed roof crude oil tank. This tank receives high RVP crude oil in case of a plant upset. Something seems to be strange to me regarding the pressure controls on the tank. The set point of the blanketing valve is 800 mm WC (31.5” WC) whereas the set point of the relief valve to flare (a conservation vent) is 200 mm WC (7.9” WC).
The relieving pressure of the conservation valve is 912 mm WC(36” WC)..
This leads to the situation where the two valves are opened at same time specially during normal venting (to flare), and consequently unjustified blanketing gas (FG) consumption.

During the plant upset condition the pressure inside the tank goes beyond 1,200 mm WC (47.24” WC) which is the maximum range of the pressure transmitter on the top of the tank even though the plant is not at its maximum capacity. As a result, we cannot monitor the pressure inside the tank in order to avoid the emergency relief pressure set at 1,500 mm WC (59.05’’ WC)to atmosphere.
The same pressure settings is used for all the fixed roof storage tanks in the plant.
Your comments will be helpful to me.

Best regards,
Benabed

A Happy New Year to you.

Your description doesn't make any sense to me. You say, but then you say, Aren't these two the same thing; you call one a conservation vent and the other a conservation valve? If they are not, then please use proper terminology for us to be able to see what is really going on here.

I would go out to the field and verify all set conditions and put all the instruments, label them and show their set points on a sketch and post this sketch for us to see.

Strictly from what you write, the reason the tank pressure is so high during releif is that it can't handle the inflow of blanketing gas, which at this point is really a sweep and not a blanket.

Dear pleckner.

The set pressure of the conservation vent and not valve is the pressure at which the vent begins to open, on the other hand the relieving pressure is the pressure at which the vent is fully opened.

I am awaiting your reply.

Benabed

Benabed:

You ask for comments, so I’ll give you mine – which coincide with Phil’s – and also attach the workbook you see in this post to aid you in giving us additional information and also helping you identify the type of relief scenario that you may need.

Look over the sketch of your application that I’ve included in the Tab “Background” and correct it - if necessary - and re-submit it to this thread as Phil has requested. This will enable all of us to fully understand the installation as it is presently functioning.

Study the various logic and decision making steps that I’ve outlined in the workbook for your application. It appears, as Phil indicates, that you have no Dead Band in your pressure settings and you are merely sweeping the tank vapor space with expensive nitrogen.

The setting on the Conservation Vent (CV) is the important item to consider in arranging your set points. The actual pressure that the CV “stagnates” at is normally of little concern – except that it should not exceed the MAWP (Maximum Allowable Working Pressure) on the tank. You should furnish the MAWP and the MAVP (Maximum Allowable Vacuum Pressure) of your tank as well.

Your tank pressure relief does not seem to be working. Your CV seems to be undersized, but we need more information. This could be a very hazardous situation. We don’t really know how hazardous because we don’t have knowledge of the MAWP, MAVP, and the mechanical conditions of your tank.

When you resubmit your sketch with the appropriate data, don't submit the entire workbook. Just submit the sketch in one worksheet - as Phil has requested.

Await your reply.

Click to view attachment

Art and Phil:

First of all, the tank is blanketed with fuel gas and not with nitrogen. The available information that I have right now are: MAWP = 1,700 mmWC and MAVP = -50 mmWC. Also included is a sketch representing the tank with the diferent connections.

Thank you for your help.

The sketch was not uploaded to this website.

Please try again.

Phil

Here is the sketch of the tank as requested.

Click to view attachment

Benabed:

I had to edit your submitted copy of the spreadsheet.

For some reason, there was a copy of a Google Website picture imbedded in the spreadsheet and I deleted it because the file was very big. I also zipped the file and it is now of a manageable size.

The fuel gas supply regulator will keep opening until the tank pressure reaches about 800 mmWC but at the same time, the breather will open at 200 mmWC to allow relief. The fuel gas supply valve will continue to open to try to maintain that 800 mmWC setting but the breather, if designed big enough, will not allow this as it is trying to maintain 200 mmWC. Thus the fuel gas supply regulator will probably go to its fully opened position, never being able to achieve the 800 mmWC it wants to, and all control is lost. This is not a blanket system but a horribly designed sweep system where there will be a constant flow of fuel gas into the tank and out to flare.

If the breather is not sized to handle the maximum pump-in rate possible plus the fuel gas rate (at a fully opened regulator position with a pressure differential of whatever the upstream pressue is minus 200 mmWC) plus an associated thermal expansion rate (see API 2000), then the emegency relief will open. Adding to this you are blind after 1,200 mmWC! This is not a place I would want to be near anytime in my life.

So my comment is, you obviously suspect that this is a poor design (and a very dangerous design at that) because you wrote about it, so...change this design immediately!! Drop the regulator pressure to around 150 mmWC and change the transmitter to allow a maximum reading of at least 1,750 mmWC.

By the way, you haven't given us the desgin pressure of this tank so I would check this against your pressure relief settings immediately. I do not have a warm and cozy feeling that you are even protected properly.

And a question to you, how long has this design been in place and has there ever been a recorded failure of any of your tanks with the settings like this?

Benabed:

Thanks to your supplied sketch and, as Phil has indicated, we now have a very good idea of the state of your installation. I concur with Phil's concern for the safety surrounding the operation of your tank.

I would also point out something that also stands out in your sketch: There is a Locked Open (LO) block valve in the overhead line of the feed "boot" vessel that feeds the tank with both flash liquid and flash vapor. This means that if this valve is totally left open, all flash vapors generated by the feed entering the "boot" vessel will immediately exit out to the flare - together with all natural gas being fed to the storage tank in order to try to keep it under a blanket. The blanket gas is all being vented to flare 100% of the time.

I have incorporated this type of "boot" vessel (actually a flash tank) when handling hot crudes from a pressurized source. The boot handles the flash vapors and allows a bleed-off of excess vapors to a Vapor Recovery Unit (VRU). However, you don't show this and the way you have sketched it, all vapors - including the blanket vapors - are all being routed to the flare. In effect, there is no blanketing taking place - just a lot of wasteful, sweep gas, as Phil has indicated.

I would immediately undertake a major revamp of this badly designed control scheme. It just doesn't work.

Thanks Art, I missed that little bit of (very important) information about the vent line from this boot. No matter what he sets the pressure regulator at, the regulator is going to be a fully opened, pouring fuel gas to the flare. I'm not sure with this configuration it will even act as an effective sweep.

Art & Phil:

This design is in place since mid 2006. Actually the emergency vent is leaking, this means that we have already reached the setting which is 1500 mmWC so we are aproaching the relieving value which is about 1575 mm WC. For the other tanks used for water storage they have not been in operation yet.
You ask me to change or to revamp the existing design, but you don't tell me what to do. What concrete action i should take from the design point of view.First to avoid the the exessive rise in the pressure during the filling opération, is it due to the wrong setting of the pressue or to the breather which is undersized. Seconcdly to recover the fuel gas which is continusly sent to the flare. What about the gas line connecting the tank to the boot. Do you think it is necessary for the relief system.
What change i should undertake to the original design to be effectif. Please be specific
I need your assistance in this matter. Thank you.

Benabed:

Are you asking us to design the system for you?

Our Forum is not structured, organized, nor paid to furnish design packages. We can assist you in your design by pointing out hazards, errors, shortcuts, safety tips, and even check your calculations sometimes. This is done under a voluntary, "best effort" basis. But designing your system for you means we undertake the liability of the design - depending on who implements it, who operates it, and who writes up the operating instructions.

I don't know about Phil, but I normally get paid for organizing and putting together an engineering design. I even critique and evaluate process designs in my present position. And I am paid for it as a professional engineer.

I have assumed you are a professional engineer and are capable of understanding and analyzing what we have recommended and taking that and doing the design and calculations for the changes and modifications we have recommended. If you are not capable of taking our input and designing and implementing it, then you can hire a local engineering firm - who I can assure you will do the same thing Phil and I have done, charge you an engineering fee for the consulting service and then propose to do the engineering work on the changes for a fee or a "cost-plus" amount.

As I said, I've already designed this type of hot, high vapor pressure crude oil flash drum with adjoining storage tank under natural gas blanketing for an international oil company. And it worked like a charm. I even have the P&IDs I developed for that project and the calculations. However, I can't use them on another application. What I could do is do your project under contract through the company I work for - but that is not my function on this Forum. I don't try to sell myself, my services, or my company's services on this Forum. I believe that to be unethical since we don't pay any advertising money to the Forum. I am sure Phil is in a similar position and can't compromise his present employer either. We just try to help and aid other engineers by sharing our knowldege and experience - for free.

I hope that we have been helpful in confirming what you already suspected: you have a hazardous and potentially dangerous tank operation and somebody should alert the owner and operator so that remedial engineering can start immediately in order to avoid any accident and fuel waste.

I'm a little confused because you do show what looks like a vent line from the boot going directly to flare via a locked-opened block valve and you show an equalization line going into the tank. If the vent line from the boot is indeed wide open to the flare then all the pad gas should be by-passing the breather and emergency vent! However, from what you say the gas can't be doing this because the emergency relief is simmering?!? I have to conclude:

(1) The block valve is not opened as you indicate on the sketch but closed

or

(2) the vent line from the "boot" to flare is very, very small and is actually acting as a restriction

or

(3) some detail is missing on this sketch

To get more specific also means we need more information. For instance, what are the sizes of the lines from the boot? What type of breather are you using? You do know that these come in basically two flavors, one where there is a spring on the vacuum side and weights (pallets) on the pressure side or the other-way-around. Perhaps the spring is on the pressure side and is binding or broke and not allowing the pressure side to open. When these devices are delivered, they typically have a plate put in to protect the weights. This protection has to be removed before installation so that the weighs are allowed to move. If the weights are on the pressure side and this protection is still in place then that would not allow the brether to vent.

So, you need to check these things out.

If you are getting excessive pressure during filling then the breather is undersized; it isn't a matter of set pressure. The sizing basis for the pressure side of the breather is the maximum pump-in rate possible plus an associated thermal expansion rate. You must follow the procedure given in API 2000 or at least check API 2000 against what you currently have. By the way, you should have the calculations for this venting. If you don't, go back to the designer and ask for them!! If you can't go back to the designer, then you must generate them yourself now and use API 2000. On top of all this, you need to check the sizing of your vent system. Why? Because this whole design is suspect and at this point in time, you should be thinking the entire system is undersized. You need to prove it to yourself it is OK.

I also gave you a suggested setting for your pad gas regulator and suggested a pressure range for your pressure transmitter in my last post.



The boot must be protected as well as the tank but if the vent line from the boot is indeed wide open to the flare as shown in the sketch (but I question this) and it is of proper size to keep the "boot" at or below design pressure (during operation and for a fire) then it should not require an additional relief and you could then take out the equalization line to the tank. But if it needs protection say from a fire, then it can still be used to tie into the tank and allow the tank's emergency relief to handle the boot as well. This also means you need to consider the volume of liquid in the boot as an adder to the tank volume when determing the emergency relief requirements and you need to make sure the boot equalization line is large enough to handle the load from the boot to the tank during fire.

Art has experience in designing these systems whereas I do not so I'll let Art chime in with any specific design improvements he feels the system may need.

Again, check on the things I brought up and get back to us.

To Art:
I think that the purpose of the forum is to share experience as a professional engineer with young engineer like me.I just want you to answer the specific questions i asked in the last post as Phil did.
Thanks you for the insight given to the problem we are facing.
To Phil:
The block valve on the line from the boot is actually 50% opened, but according to the P&ID it should be fully opened.The vent lines from the tank and the boot as well as the eqalizing line are 6'' diameter.
I am also concerned by the back pressure of the CV which should not excced 700 mm WC and may be affected by the vent line of the boot.

A conservation and/or breather vent is a pressure differential devices. This means that if you have a back pressure of 700 mmWC on the device and the set point is 200 mmWC, then the pressure in the tank must be a minimum of 900 mmWC before the pressure side of the breather will open. This means that the Fuel Gas regulator, set for 800 mmWC will go closed when you reach enough pressure in the tank to start it to vent.

If you told the vendor when sizing the conservation vent that the back pressure could be 700 mmWC and the actual back pressure is less, then the conservation vent will open at lower pressure than 200 mmWC.



If this is indeed true then it may be in violation of your safety protocols.

Look, the entire design/set points of this system appears to be way out of whack. Take a deep breath, sit down and start this design from scratch and determine where you really need to be. This will then help you define what design changes will be necessary to make it work correctly.