7 replies to this topic

[chem_buddy]

Hi all

 

We have Nitrogen system which produces gaseous N2 as well as Liquid N2.

Liquid N2 is stored in Liquid Nitrogen Storage vessels at 135 psig and -169 deg C (saturated condition)

This liquid N2 needs to be transferred to Truck tanker of 5 barg pressure from holder without pump facility.

Pipe size limitation is 2 inch. Please suggest me how to calculate the loading time for liquid N2 truck tanker of 30 m3 capacity.

 

How to calculate the flow rate? Is it based on velocity criteria or differential pressur criteria?

How to control the flow because it will on the basis of differential pressure. Your experince and suggestion on this matter would be really appreciated.

Edited by chem_buddy, 21 June 2015 - 10:28 PM.

[Bodhisatya]

Dear

 

The governing factor in your case is velocity,for any cryogenic liquid (be it subcooled or saturated fluid) the max considered velocity is 1 m/s .While designing the unloading line ,surely that factor was taken into consideration.Use continuity equation to calculate the max volumetric flow through this line.

 

The tanker capacity can be divided by this flow-rate to obtain the time required,simple arithmetic.

 

Do you have any control valve at tank unloading line or manual control ?

 

If manual control,then the valve should be slowly operated ,as initially high dP between tank and Tanker will facilitate high flow and hence two phase flow ,if uncontrolled it will lead to huge pressure rise in the tank.Be careful in handling the same.

 

First cool down the tanker and the tank unloading line properly ,once the system is sufficiently cooled,gradually keep opening the tank unloading valve.

 

Also make sure the pressure building coil of your tank is fit to operate,as Liquid level of Tank starts depleting pressure will fall,which will lead to low dP between tank and tanker and in turn reduced flow.

 

Upload a sketch or P&ID of your tank area for better understanding.

 

Cheers

 

Bodhisatya 

[chem_buddy]

Hi Bodhisatya

 

Thank you for your reply & explanation. It is really appreciated. There is no control valve. Only manual globe valve will be provided. Presently there is no system available for transfer. I have to develop one. I will try to provide a conceptual skectch ASAP. 

 

Regarding calculations, I considered different line size like 1.5" & 2" and kept the max velocity limitation as 1.2 m/s. As per the calculation, I got the max flow rate for 1.5" line size and 2" line size considering the velocity criteria. Accordingly, I calculated the loading time for truck tanker.

However I am not sure how to control the flow rate or velocity using manual globe valve. Because as per my understanding, Liquid N2 from Tank to truck will flow on the basis of differential pressure. In addition, since there will be no flowmeter then how to ensure the flow rate through pipe. 

If there is more pressure drop, then liquid N2 will convert to Gaseous N2 because it is almost at saturated conditions. 

Please advise if you understand my confusion. If you need more explanation then please let me know.

 

Thanks

Edited by chem_buddy, 22 June 2015 - 08:50 PM.

[Bodhisatya]

I have explained you the procedure of charging the tanker from tank,initially it should very slow else you will incur huge loss of liquid due to flashes of vapor and liquid.

 

Once the system is cool enough the dP should drive the liquid into tanker.Back calculate flow rate based on the time required to fill the tanker in absence of flowmeter ,as it is in your case.

 

The entire control is in ur hand.Manipulate the valve based on the tanker condition.

 

Considering all the losses from tank to tanker via globe valve and line,u need to set Tank Pressure ,as always for transfer to happen a min dP of 1.5 bar(g) is kept.

 

U r yet to confirm the condition of the Pressure building coil attached with the tank.

 

Additional Query : What is the process cycle for ur N2 plant ,is it waste gas expansion cycle?What is the product mix of ur plant (Nm3/hr of Gas and Liquid).What is the air flow to Cold box in ur plant?What is the product purity?

 

Regards

Bodhisatya

[Pilesar]

   The tanker maximum pressure is lower than the storage tank pressure and there is a safety risk of overpressuring the tanker if the two vessels are directly connected. It may be prudent to transfer from your main storage to another vessel controlled at a lower pressure before final transfer to the tanker to mitigate the overpressure risk.

   Assuming you have some other way to protect against tanker overpressure and you transfer directly to the tanker from the main storage tank, the tanker vent will be completely open to the atmosphere and you will flash about 30% of the transferred liquid. The fill valves should be manually adjusted to control the pressure. After the tanker is filled, the tanker will be disconnected from storage, the tanker vent will be closed and the tanker will slowly pressurize during its journey as the contents heat.

[chem_buddy]

Hi Bodhisatya and Pilesar

 

Thank you for your reply. I understand the transfer process. 

 

If I assume that liquid N2 truck with capacity of 30m3 to be filled in 1 hrs without pump then trasnfer flow rate will be 30m3/hr.

 

Now If I consider 2" line size for 30m3/hr then the velocity at this flow rate is very high (6m/s) which is not acceptable. Higher line sizes are also difficult due to handling issue. Therefore the limitation is up to 2". In addition, I have time limitation of 1 to 2 hr. 

Therefore I asked whether we have to consider the velocity criteria or not. Altough I understand that I can reduce the flow rate by manual valve.

 

By the way what is the normal pressure inside the truck before starting the transfer process ? Does any body have any idea.

N2 purity is 99.7% 

[Bodhisatya]

ChemBuddy

 

Requesting you to answer my query.

[chem_buddy]

Dear Bodhisatya

I have already informed you about the N2 purity. I am really sorry but I can not share the flow rate details for Gas and Liquid N2 and Air due to comapny security matters. In addition, I think these details are not required for answering to my queries. My question is just related to transfer of Liquid N2 from storage to truck without pump facility. Storage tank can be pressurized up to 12.5 barg pressure and PSV is installed for safety during high pressure. 

I would like to confirm again the process during transfer without pump. As a first step I think valve to transfer liquid N2 from Tank to truck will be slowly open to avoid the N2 losses.Truck need to be pressurized up to saturated conditions and then we will be able to transfer on the basis of available head. Is it correct method. Do we need to provide truck vent connection to storage?

Since the liquid from Tank will start building the level in truck. How to calculate the loading time because pressure diff. will change with time.