8 replies to this topic

[Kent8541]

Hi guys, I am seeking advice on the correct method in sizing nitrogen storage tank with an output of 525 m³/hr @ 7kg/cm² after the vaporizer.
Intend to have a 24 hours 7 days storage tank.


The following are some calculations I did & wonder if this is correct.

Volume required for 7 days (24 hours operation) = 525 x 24 x 7 = 88,200 m^{3
Pressure = 7 x 9.81 x 100 x 100 = 686,700 Pa}
Assume temperature of 25 ^oC
By ideal gas law (PV = nRT), n = (686,700 Pa x 88,200 m³) / (298K x 8.314) = 24,446,086 mol

Density of liquid nitrogen = 0.808g/ml,
Amount of liquid nitrogen = (24,446,086 mol x 28.0067 g/mol) / 0.808 = 847,344,328.4 ml = 847,344.3284 litres

[kkala]

Hi guys, I am seeking advice on the correct method in sizing nitrogen storage tank with an output of 525 m³/hr @ 7kg/cm² after the vaporizer.Intend to have a 24 hours 7 days storage tank.
The following are some calculations I did & wonder if this is correct.

Volume required for 7 days (24 hours operation) = 525 x 24 x 7 = 88,200 m³
OK. Be sure that demand of 525 m3/h refers to actual (not normal or standard) conditions.

^{Pressure = 7 x 9.81 x 100 x 100 = 686,700 Pa}
OK, seeing that 7 kg/cm2 = 7*9.81 N/cm2 = 7*9.81*100^2 N/m2. Check that pressure P is 7 kg/cm2 abs, not 7 kg/cm2 g = 8.033 kg/cm2 abs.

Assume temperature of 25 ^oC
Note: Average minimum daily temperature would give more conservative results (difference up to ~ 10%) in this specific case. Vaporizer output may be at temperature lower than 25 oC, and nitrogen distribution lines are not insulated. .

By ideal gas law (PV = nRT), n = (686,700 Pa x 88,200 m³) / (298K x 8.314) = 24,446,086 mol
Considering 1 kmol of ideal gas at normal conditions (molar volume=22.412 m3/kmol): R=(1.01325E+5)*22.412/273.15=8313.7~8314 Pam3/kmol/oK=8.314 Pam3/mol/oK, which is OK.
Consequently resulting n is OK. Compressibility factor ~ 0.995 at said conditions (Perry, 7th ed, Table 2-179).

Density of liquid nitrogen = 0.808g/ml
0.808 g/ml = 808 g/l, spec volume= 1.238 l/kg. Note: This represents liquid nitrogen at atmospheric pressure. See Perry, 7th edition, Table 2-288 (saturated nitrogen)

Amount of liquid nitrogen = (24,446,086 mol x 28.0067 g/mol) / 0.808 = 847,344,328.4 ml = 847,344.3284 litres
OK. So net tank volume is about 850 m3. Compressibility consideration would give a slightly higher volume (by~4 m3), which is not significant. Atmospheric cryogenic storage is economically feasible at much larger capacities. Said tank is assumed to be an insulated pressure vessel with a PSV covering the case of maximum liquid evaporation (e.g. in fire case). In normal conditions evaporated nitrogen (through tank natural heating from ambient) partially covers plant demand, while vaporizers cover the rest nitrogen demand at any time.

Above assumes that nitrogen plant production has stopped. A less conservative approach is to assume that nitrogen plant is operating during this week, while nitrogen storage supplies the balance through vaporizers.

Edited by kkala, 14 May 2011 - 04:41 PM.

[Zauberberg]

First, you have to specify whether the flow of 525 m3/h3 relates to 7 kg/cm2 (abs or gauge?), or it refers to standard conditions (specify at what temperature and pressure). There is a huge difference in calculation results if you need 525 standard m3/hr, or 525 m3/hr at actual process conditions (7 kg/cm2 abs or gauge). From that point onwards, the calculation is very much straightforward.

You should also include some design margin for the volume flow required, and also take into consideration the fact that you need some vapor space in the 100% liquid full tank in order to have fully operational and trouble-free system.

[Kent8541]

Hi kkala, thanks for the advices & info. It's very helpful of u.

Hi Zauberberg, the required flow & pressure were not specify clearly in the documents/ specifications, so I suppose it is to be of 525 m3/hr at actual process conditions (7kg/cm2 abs). As a Building Engineering background, I have no experience in designing the nitrogen system for industrial plant, any advice of where can i seek more information & advices? Am my calculation in order/correct?

[Zauberberg]

Whoever expects from you to furnish design calculations with incomplete and confusing input data, must be either an ignorant person, or insane one. It comes down to the same in any way.

The main reason why I am saying this, is because the difference in liquid N2 volumes for these two cases (525 m3/hr at standard or actual conditions) is approximately 7 times. Now, if you design a tank for 7 times more volume than it is really required, you are wasting loads of money and probably looking like a goof in the eyes of Operations personnel who will be running that unit. On the other hand if you design it for a smaller volume and it shows like you actually need 525 m3/hr at actual conditions then you have wasted money even more, because you have designed, constructed, and commissioned a system that will need to be replaced in future because it is undersized. I hope you realize the importance of having complete and accurate input data for any kind of design calculations.

My advice to you is to revert to the original source of input data, and clarify the issues being discussed in this thread. With the arguments mentioned above, I am very much sure that you are on the right track.

[kkala]

I agree that clarification of design requirements is the first priority, needing following actions.
Do the checks suggested in previous posts.
Clarify any query that you may have in advance. This will save time later.
Ask those having written the documents / specifications to give clear values of nitrogen demand (that 525 m3/h) and pressure (that 7 kg/cm2).
This data is usually issued in written form to avoid any confusion.

[Art Montemayor]

I have been following this thread, spotted some errors, and developed a spreadsheet where I generated the detailed calculations that define the required size of cryogenic vessel being discussed. My calculations show the OP's calculations are erroneous and should be rejected. I held back when Zauberberg posted his very important and key response.

No engineer should endeavor to get involved in a process specification without thorough and expert knowledge of the process itself. As Zauberberg has clearly pointed out:

• This is a CRYOGENIC VESSEL. And this fact hasn't even been mentioned! Liquid Nitrogen (LIN) is normally delivered and stored IN THE SATURATED STATE at approximately 100 to 150 psig - perhaps higher. Nowhere is this mentioned in the original post. These basic facts - and others - are the reasons for the major flaws and errors in the calculation - and resulting specifications.
• It is vital and important to ensure a SAFE vapor expansion volume in any CRYOGENIC storage tank. Otherwise, a dangerous safety hazard is created. Everyone knowledgeable in the industry - such as Zauberberg - knows that this expansion volume must be AT LEAST 10% of the total, and perhaps more. This also is not even mentioned.
• A temperature of "25 ^oC" is mentioned as the resulting vaporizer output temperature. This simply can't be obtained with a simply vaporizer. You are dealing with superheated vapor at this point and you have to focus on your vaporizer heating medium - which, for cryogenic fluids, is usually atmospheric air. The type of vaporizer is usually a natural convection, atmospheric vaporizer and the resulting temperature will be very cold unless "boosted" by another heating element. Superheating will cause higher pressures and no mention is made on how this is to be controlled.
• Vaporizing a cryogen is a very special unit operation and should be left to the supplier of the cryogen and the storage tank - they are the experts who can best analyze, evaluate, and specify the required equipment if given the scope of work.

I commend both Kkala and Zauber for jumping on this thread and correcting what could have turned into a costly and unsafe mistaken specification.

[DB Shah]

Dear Kent,

I had similar assingment couple of years back. We needed N2 during shut down/start up when our captive N2 plant was under shutdown. We explored for tank installation which being cryogenic was a costly affair. We had Liq N2 supplier company (BOC) @ 60 kms from our plant area. We decided to import liq N2 through road tankers (10 M3 capacity). The system is working nicely since installation 2008.

Some points which you should consider while designing-

1 As stated by Art/Zauberberg & Kkala, make sure of your volumes - std M3 & acutal M3 will make a huge difference; and also confirm pressure units - abs/gauge.

2 Cryogenic tanks' concerns;

3 Evaporator - We get liq N2 at 10~15 Barg pressure in tanker. We are able to evaporate & heat the N2 with an apporach of 5~10°C. Our ambient temp during May/June is 40°C. We are able to achieve N2 outlet of 35°C. Kindly check your ambient conditions.
You will need to install parellal evaporater. After certain hours of operation you will have icing on the fins of evaporater & will require de-icing.

4 TRVs & Safety valves

5 Low temperature switch to isolate the system if evaporter outlet temp goes too low.

6 Check for low temperatures of safety valve discharge lines installed on evaporter outlet due to expansion.

I am attaching schematic of our system, maybe this will be useful.
DB Shah

Attached Files

•  Liq N2.xlsx   26.13KB   465 downloads

[kkala]

The idea of kkala's post (14th May 11) is that liquid nitrogen is produced and stored in an insulated vessel. Design pressure of this vessel is higher than atmospheric (e.g. 150 psig or higher, mentioned by Art Montemayor), since atmospheric storage is not expected to be economical with a net capacity of 850 m3. The vessel has no refrigeration, but naturally evaporated nitrogen plus the ouput of vaporizers cover the plant requirements at any moment. I assume the scheme is met at plants locally producing nitrogen.
A LNG storage without refrigeration (sending excess evaporation to flare) would be pretty similar.

In case of no plant demand, PSV on the vessel will release nitrogen vapor to atmosphere. Vessel operating pressure is normally substantially lower than its design pressure. But vessel capacity should cover the case when PSV is open and vessel is full. Density of liquid nitrogen is about 658 kg/m3 under 150 psig saturation pressure, so 684654 kg of liquid nitrogen will occupy 1040 m3, not 850 m3 as net volume. This "liquid expansion" volume is different to "vapor expansion" volume, clarifications on the latter would be welcomed (vessels are usually not more than 85% full).

Temperature of 25 oC (reservation expressed in kkala's post) can be obtained only through use of cooling water as heating medium in vaporizer.Superheating of evaporated nitrogen looks necessary, to avoid such low operating temperatures as -170 oC (corresponding to 8 Barg saturation pressure). It has been assumed that gaseous nitrogen soon acquires ambient temperature in the distribution line.

Edited by kkala, 17 May 2011 - 03:35 PM.