fire water monitor design

[Kottam]

Hi All,
Fire water system of the offshore platform designed with monitors. The monitors are located away from the deluge skid, the piping between the akid and monitor is air filled. It is observed water hammer pressure during air evacuation from the system. what kind of pressure mitigation measure is the best solution here?
I really appreciate your reply.

Thank you.
Regards,
RR

 

[zacharialamsyah]

You can put ARV (Air Release Valve)to help protect fire protection systems from air lock and collapse by eliminating excess air or admitting air before a vacuum condition can occur.It will prevent water hammer in your fire water system

You also need Jockey Pump to maintain pressure of Fire Water System

CMIIW

Regards,
Zachari Alamsyah

 

[Kottam]

Hi, Thank you for your reply.
The monitor itslef is orifice and act as ARV. DO you think desgining of ARV at monitor will suppress the water hammer pressure?

 

[zacharialamsyah]

Yes, it will prevent the water hammer pressure.

Other method to reduce water hammer you can refer to this link

http://empoweringpumps.com/preventing-water-hammer-from-damaging-pumps-and-pipes/

Regards,
ZA

 

[gerhardl]

Hello, Kottam!

You wrote:
...'The monitor itslef is orifice and act as ARV. DO you think desgining of ARV at monitor will suppress the water hammer pressure?'

Any ARV or monitor effect will depend mainly on two factors:
a)- The layout of the pipeline system and your orifice: no air pockets remaining and acting as elastic buffers for the water.
b)- The size of the orifice in relation to the amount of fluid: you can very roughly calculate the speed of the amount of air (liter pr second) necessary to be evacuated through your orifice (note: net real opening, not nominal!), by estimate it to be equal to the amount of water (liter pr second) going into the system. In such a rough approximation the speed of escaping air should be well below critical flow, say 40-90 m/s.[sup][/sup]

You will most likely have one or both of these points working against you with your existing orifice solution!

It could also be an idea to check if incorporating a non-slam checkvalve between the water pump and the system in addition could hinder the waterhammer.

 

[Kottam]

Hi Gerhardl,
Thank you for your reply. I completely agree with you.
But how I can evaluate the air velocity which is function of the pressure pushing the air and length of the air filled pipe.
Is there any rule of thumb to evaluate the air velocity ?

Regards,
RK

 

[gerhardl]

Hi Kottam:

Very roughly you can make a series of approximation calculations.

1. Disregard volume in regard to pressure, pressure loss, pipelength, temperature, weight etc. The point is that what is coming in from deluge system of volume has to replace an equal volume of air. (I said it was a rough calculation !).

2. You have to know the input volume per time unit from the deluge system. If not, make a guesstimate or several, for instance 'not less than', 'absolutely not more than' and 'estimated average'.

3. You have to know the area of your orifice or the planned new area. If not guesstimates as above.

4. Calculate speed of air out using air volume out of the orifice area equal to the input volume from the deluge system.

5. Then compare and select an orifice diameter with a reasonable speed resulting. You will then also have incorporated a large safety margin, as actual speed perhaps (depending on selection) could be allowed to have double or even more velocity before it reaches critical value (sound of speed 330 m/s). See more in my first post regarding speed values.

6. Compare selected value to actual practical solutions and available commercial devices, reselect, control by recalculation and values/information/experience from other sources.