ARV sizing for firewater system on Offshore platform

[akupikree]

Hi all,

Im currently doing a transient analysis pressure surge for firewater system. I have problem in sizing the right Air Release Valve at the highest point of the system.

Comparing 2 cases,

Case 1

when the deluge valve for the largest fire zone is opened, there are large amount of water (1800 m3/hr) will flow out from the system thus creating a vacuum at the highest point. Ive then put the ARV at some point and the surge (due to vacuum formation) is solved. The ARV size is 4"x4" with the orifice size of 6/64" (2.34mm)

Case 2

when two firewater monitors at the helideck are activated, the amount of water flowing out the system is much smaller (270 m3/hr) compare to case 1. By using the same ARV size and location (supposed to), there is high pressure surge happened at the ARV section.

After i change the ARV size from 4"x4" to 1"x1", the surge problem is gone.


So, it looks like Case 1 needs a large ARV and Case 2 needs a small ARV. and there is no size in between is suitable.


Can anybody in here know why is this happening and how to solve it? Really appreciate if you guys can help me. thank you in advance.


Regards,

Fikree

 

[hsbcn]

You say that the problem is solved when you go from 4"x4" to 1"x1". What is the issue you want to resolve?

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[akupikree]

I have 2 cases for the same system. The ARV location and size shall be the same for both cases (ofcourse because its the same system on the same platform). However, based on my calculation in pipenet, the first case need larger size and the 2nd case need smaller size. Its impossible to have the same size for both case.

Thanks.

 

[stanier]

Suggest you use Ventomat air valves. These have multiple orifices. The largest is nozzle size of the air valve. Smaller orifice limits air egress to cushion surge. This type of air valve can be modelled in AFt's Impulse or Prof Norman lawgun's Hytran.



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[Pumpsonly]

You may have to relook on your simulation. The scenario described by you is quite opposite of a normal pipe line water hammer scenario.

in normal pipe line application, the water is actually in a motion and the vacuum is form due to sudden stopping of the pump. In the case of fire water system, the water in the pipe is in standstill before the deluge valve and the monitors are open. You will not get such a high flow rate instantly.The initial stored pressure in the fire main will not be able to provide you the designed flow rate. Full flow will only be achieved with the main fire pump in full speed.

 

[gerhardl]

Hi akupikree, I agree with hsbcn and pumpsonly, and have actually difficulties grasping the 'real picture' of what you are trying to avoid and what happens to create the necessity for ARVs.

Either way the real physics is the simple art of balancing input and output of water and maintaining the pressure in the line sufficiently high (through the pumps) to give the correct amount out of the given (or calculated) openings (deluge valves) without creating water hammer when filling and/or vacuum collapse if sudden vacuum should occur.

Excessive air relief out of the pipeline is only necessary if the pipelines are empty (filled with air) when you start to fill the system at full firefighting capacity (eg more than normal air relief at normal filling speed).

In any case: this is the simple way of calculating water in equals air out (opposite if vacuum). Air out of orifice should have a reasonable speeed comparable to normal speed of for instance steam/gas in pipelines (say 30 o 50m/s) and in addition the air relief valve/system must be equipped with damping, not to create waterhammer when the system is filled with water and water pressure builds up.

One valve of this type should be sufficient for both capacities.

Your orifices seems small if the above is the case.

Or have I misunderstood your problem or the physics?

 

[stanier]

You may need to consider other forms of surge mitigation such as a gas accumulator.

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