10 replies to this topic

[vinod]

Friends

I have a water injection line with approximately 150 bar pressure and i am taking a tapping from it reducing the flow with an RO and then using a self actuated presure control valve to reduce the remaining pressure to 11 bar.This is then fed to a deluge valve and some fire water hose reels.

My questions are below:

1)How much pressure can i reduce using an RO from the water injection pressure of 150 bar.?
2)Is a self actuated pressure control valve and control valve same?
3)Do i need 3 flow parameters (minimum,normal and maximum) to define the valve?
4)How do i define the minimum and normal and maximum flow parameters for the valve,since my deluge valve downstream is an on/off valve and will open once fire is detected.Only difference is the number of fire hose reels working which i can define(2 working same time for maximum flow case and nil working duirng the minimum case)
5)What is the advantage of using a self actuated pressure control vale?
6)Is a set point pressure required while defining the control valve?

Regards

Vinod

[tarafdar]

I am not an expert on this subject,still I like to give the following informations from my experience.

1.Self actuated control valves are a class of control valve.There are many types of control valves.
2.Self acting pressure control valve acts by sensing downstream pressure.It can not control flow.A single contol valve can not control pressure & flow at the same time.
3.Set pressure of this type of valve can be changed by adjusting the sprig tension.
4.Flow depends on valve size & upstream pressure.
5.Advantage of this valve is it's simplicity.It does not require a secondery system(instrument air /electricity etc.).

But for fire water system I think your proposition is not suitable.For fire fighting you need enough flow with necessary head.How this can be served with water from injection line ,is not clear to me.

Thanks

[paulhorth]

Vinod,

This proposal strikes me as highly dangerous. YOU COULD EASILY KILL SOMEONE WITH THIS ARRANGEMENT.

It would never be accepted by the Fire and Safety engineers that I have worked with.

In the first place, you will need a PSV to protect all the hoses, deluge valves and fire system piping from overpressure, because they certainly will not be designed for 150 barg.

Next, 11 bar at a fire hose is sufficient to decapitate somebody. 7 or 8 barg is the maximum pressure recommended for fire hose service.

Thirdly, the pressure of 11 barg will be there only AFTER the flow has been established. The RO only reduces pressure when there is a flow. The pressure against closed valve ( the deluge or the hose valve) could easily be 150 barg, that is what will come out when the user opens the valve,probably knocking him on his back while the loose fire hose thrashes about out of control, or tears loose from its flange.

Fourthly, this system would not provide a secure source of fire water, because where would the fire water come from when the injection system is shut down? (which is what would happen if there was a fire...)

So it's not fit for purpose and is highly unsafe.

Paul

[vinod]

Paul

1)I have a restriction orifice which will bring my pressure form 150 bar to approximately 50 bar and then i have a SAPCV which will reduce the pressure to 15 bar.
2)Downstream of the SAPCV i have a PSV sized for regulator fauilure scenario and set at approximately 16 bar.
3)There are no fire water pumps or any other source of water and the client is happy with using the injection water for this purpose.
4)The hose reel system also has an eductor which reduces the pressure by 30 %,thereby taking the pressure the hose reel close to 8 to 9 bar.

Please clarify if this makes sense.

Paul

Could you please clarify the statement "The RO only reduces pressure when there is flow"?

Regards

Vinod

[paulhorth]

Vinod,
Thank you for the reply after three weeks. However, your reply chills my blood.
I will go through your responses.
First:

Could you please clarify the statement "The RO only reduces pressure when there is flow"?

This is simple. An RO is simply a hole in a piece of plate. If there is no flow in a horizontal piece of pipe, then the pressure in that pipe is the same all the way along its length. If you put a RO in this pipe, the pressure is still the same all the way along the pipe. Only when you open the valve at the outlet and flow starts, will you see a pressure difference across the RO.
So, if you have a closed line, you will have 150 bar upstream and also 150 bar downstream acting against your self-acting PCV.. Would you bet your life (or someone else's life) that your low cost PCV specified for 50 bar will be able to stay closed against 150 bar? at all times in the future?
When there is flow, the pressure drop is proportional to flow squared. So if you open the valve for only half the designed flow, the pressure drop across the RO will be only 25% of design, so instead of 150 - 50 = 100 bar DP, there will be 25 bar DP, leaving 150 - 25 = 125 bar downstream pressure.
If you don't understand how a RO works you have no business designing a safety critical system.

Second:

There are no fire water pumps or any other source of water and the client is happy with using the injection water for this purpose.

If you have no fire water pumps then you do not have a fire water system compliant with any code known to me. Regardless of the happiness of your client, you must comply with industry standards, the legal requirements at the location, professional standards, insurance company requirements etc. I ask again - what happens if there is a fire and the water injection (or the power supply) is shut down?

Third:
Your PSV will probably leak continuously because the PCV will be passing. So someone will isolate it.

Fourth:

The hose reel system also has an eductor which reduces the pressure by 30 %,thereby taking the pressure the hose reel close to 8 to 9 bar.

I don't exactly know what you mean by this, but would you be willing to stand in front of a hose connected to a 150 bar system, on the basis of a pressure drop through an eductor which is not controlled, only approximate and probably oversized "for safety", and specified by someone with no grasp of elementary fluid mechanics?
The 8 to 9 bar is still too high and will only be seen when full flow is established, too late for the victim.

Please tell us where this installation is, so I can warn everyone I care about never to enter it.

Paul

[breizh]

Vinod ,
As Paul explained to you you need to get support from experienced people on the matter. It's serious business , no improvisation .

Take a look at this link
http://www.cla-val.c...-protection.cfm.


Hope you understand .

Breizh

Edited by breizh, 28 October 2011 - 08:34 PM.

[S.AHMAD]

1. I agree that the arrangement is sub-standard and dangerous
2. The whole system piping (includes hose reel, deluge etc) must be designed to take 150 bar pressure (blocked discharge scenario)
3. I can guarantee that the PSV set at 16 bar will be poppig almost continuously.
4. Ability to apply HAZOPS mentally is important for process design engineer so that our proposal is safe and sound.

Edited by S.AHMAD, 28 October 2011 - 03:42 AM.

[kkala]

I believe, Vinod, you have to carefully consider previous posts and then propose a safer system to Client, probably after receiving additional advice from fellow engineers of your organization (who could also support the new proposal to the Client). The new proposal would be more expensive, Client may have objections, but explain that if the last (closest to atmosphere) fire water valve is closed, all network upstream of it would suffer a 150 bar pressure (as already pointed out). Two fire water networks (known to me) have deign pressure about 15 barg.
A way out could be an atmospheric fire water tank of adequate capacity, fed under level control from the 150 bar line (branch line piping design pressure=150 barg). This tank could feed new fire water pumps for the deluge system and hoses. Knowledge of specific local conditions could lead to a simpler, yet safe system; but "there are no fire water pumps or any other source of water", apparently in the area near the application.
Could you inform what the new fire fighting system (deluge+hoses) protects, as well as max flow rate? Just to have an idea of size and how critical the system is.

Edited by kkala, 30 October 2011 - 06:39 AM.

[paulhorth]

Vinod,
I agree with the proposal from Kkala for a firewater storage tank. This tank should have capacity for the likely duration of firefighting, this is typically considered as four hours minimum(I think specified in industry codes). I would add that the pumps need to have a secure power supply available when everything is shut down. This typically means diesel engine drives, with each engine fuelled from its own day tank. Ensuring these tanks are kept full is an operational responsibility, this is a weak point in the protection system.

Paul

[vinod]

Paul/Breizh/Ahmad/Kkala

Thanks a lot for the valuable comments

I am attaching a schematic of the system which i was talking to you.

Also when the deluge and foam hose reel is opened due to confirmed fire detection, it opens fully and this flow rate has to come downstream of the pressure control valve at approximately 11 bar.



Regards

Vinod

Attached Files

•  Query.xlsx   14.78KB   44 downloads

[paulhorth]

Vinod,
Thank you for this sketch. However it does not change the advice and conclusions given by me and others. I hope you will carefully consider what has been said in the various posts, try to understand them and act on this advice.
I have given several reasons why this system is unsafe and does not provide a secure source of firewater. I am not going to repeat them. I would ask you to think about the pressures in your system when it is static (not flowing) which will be the normal condition.

Paul