justification for class 300 valves 4

[smoothe]

Would appreciate a few comments.

Here's our situation:

We will be using seawater for our fire protection. Valves will be nickel aluminum bronze.

I believe class 150 NAB valves are rated at 19bar/276psi which is still higher than 245. Pump pressure at zero discharge is 215psi.

The water is polluted and rubbish are normally floating on the water. I'm worried if rubbish gets into the system, the pressure will shoot up above 215psi. Being a fire protection system, It seems "safer/smarter" to specify class 300. I'm also worried the EPC is just trying to save a few bucks by using class 150.

Q: How do we justify using class 300 valves instead of class 150?

 

[rmw]

Well now, according to your logic it would be much more 'smarter/safer' to specify class 1500 lb valves, and then again, much, much much more 'smarter/safer' to specify class 2500 lb valves.

Why not be as safe as you can be. Make the thing nuclear bomb proof.

Your valves as specified meet or exceed the service requirements. What is your beef. If you have the money, go for it, but don't expect the EPC to gold plate the job just to put suspenders on a belt (unless, of course, they are working for cost plus).

rmw

 

[pmover]

how would "rubbish getting into the system" result in higher pressures?

one would think that the rubbish+water is screened and filtered before entering the pump.

please rethink and revise your question if needed.

good luck!
-pmover

 

[stanier]

Has the EPC carried a waterhammer analysis of the various operating scenarios and determined the dynamic loading on the valves? Firewater systems are notorious for waterhammer when diesel pumps trip, hydrants open/close, secondary pumps kick in, lines break.

Steady state pressure is one thing but pressures can be very much greater without mitigation measures.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[smoothe]

Appreciate the comments guys. Thanks.
Our consultant is also claiming he won't guarantee the system if the EPC doesn't upgrade to class 300.

Rmw-I remembered when sizing motors for pumps, you normally move 1 step higher. So for valves, 1 pressure class higher. But I like the reasoning for nuclear proofing

pmmover - I would expect rubbish may clog the pipes. So similar to putting a thumb over a garden hose, the pressure will go up.

We do have basket strainers for our other pumps but rubbish (styrofoam, disposable utensils, plastic bags,even parts of flip flops) still gets in.

stanier - No waterhammer analysis was done. What are the effects of waterhammer? How high will the pressure kick? Will it exceed 275psi? We do have a PRV though.

 

[stanier]

Smoothe,

If there is column separation due to pump stop and the columns recombine you could end up with a 10 fold increase in pressure. Goodbye fire protection system just when you need it.

All standards and codes require a surge analysis to determine the design pressure rating. Without it you are just guessing.



“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[BigInch]

50% overpressures or more are possible. To be certain you must do a transient flow analysis.

Get a new EPC.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[BigInch]

Your EPC is derelict in his duty, if he refuses to run a transient anlaysis, or provide to your satisfaction sufficient justification for why one is not needed.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[stanier]

Perhaps you suggest to the consultant that they have failed in their duty of care in not having done the waterhammer analysis or specified that one should have been carried out by the EPC.

Is the design to NFPA standards? I am not familiar with these standards as I am in Australia. Others may be more familiar and be able to help if this is relevant here.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[smoothe]

Ok, I'll ask the EPC why this wasn't done. Wish we could fire them, the owner of the EPC is a significant shareholder in our company!

Pump pressure at zero discharge is 215psi. If I assume 50% overpressure (to be conservative), that's 322psi, which is above the rated 276psi. Wouldn't the PRV handle the kick? How long will this overpressure be? Is this enough to justify class 300?

 

[stanier]

“In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be."

Lord Kelvin [PLA, 1883-05-03]

Unless you do the analysis you DONT know what you have on your hands.

If the EPC is owned by a major shareholder then there is a conflict of interest. If the facility burns down then the insurance company will either not pay out or will chase the EPC company responsible and its directors. Insurance companies dont like giving money away. If someone dies in the fire you may go to jail.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[BigInch]

If you have a significant shareholder, perhaps he can bring some weight to bear on them. No matter what ANY code says, which are MINIMUM requirements NOT the end of the story, the engineer of record must produce a design that is functional. This day and age, any engineer would say in court that a transient analyis is "State of the Art" and thereby included in a "engineering design contract", which typically means "state of the art" as a basis. So, that would not be a design that ignores waterhammer.

If you buy into the reasoning that a 300# system will function for transient pressures, that could turn into your personal problem in that you may be severely critized for spending unwarranted money without proper justification. Your decision.

Many codes give an allowable, short term, transient pressure which is higher than design pressure, 10% or so, check YOUR code. Any pressure above that high transient allowable must be relieved, or the design pressure will need to be increased.

I would think that a reasonably sized system would justify the cost of the transient study, even if it was considered an extra payment although it is usually not in my world. It would be IMO a good idea to determine if you could use 150# or if you actually did need the 300#ers, or for that matter, 600#, but if it's only 100 feet of pipe and a couple of flanges, maybe it's not worth the argument. That's for you to decide.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[stanier]

Alternatively the cost of a surge mitigation measure may be less than providing Class 300 flanges for a whole system or burning down a refinery because the EPC contractor wants to save a couple of bucks. But you dont know until you engineer the system properly.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[trashcanman]

"
pmmover - I would expect rubbish may clog the pipes. So similar to putting a thumb over a garden hose, the pressure will go up."

I am calling this myth BUSTED. Putting your thumb over a garden hose does not increase pressure. It increases velocity due to the restricted opening. If the thumb trick did work, then why do we need pumps??? Why not just install a whole bunch of restrictions (valves) and create pressure out of nothing?

 

[BigInch]

It increases velocity while reducing cross-sectional area. Since the discharge coefficient is then greater, a greater head loss across the nozzle is required to drive the flow across the reduced area outlet. Since the outlet pressure at your thumb and atomospheric air interface is still 0 psig, more pressure than before is required at the entry to the nozzle to obtain enough differential pressure to accelerate the flow to that new velocity, because you presumedly did not reduce atomospheric pressure to obtain that additional head, or ...did you suck really, really hard.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[stanier]

The other aspect of getting marine rubbish into a firewater system is local corrosion. Firewater systems are generally static in nature. Corrosion mechanisms are vastly different under static conditions compared to flowing conditions. If there is any stainless steel in your system beware the corrosion is going to occur. Even super duplex stainless steel will corrode under static conditions , especially when the temperature is elevated.

http://www.nidi.org

provides some information on the mechanisms.

Sea water carries organisms such as the Zebra mussel. These will grow quite happily in stagnant sea water to the point where they will choke the system. Many desalination plant intakes have super chlorination and acid dosing facilities to remove the molluscs and dissolve their shells.

The build up of molluscs on the pipe walls will also create a very high surface roughness and affect the performance of the pumps. Any loose particles can indeed clog a system and take the pressure to the stalled head of the pump. The challenge isn't the increase in pressure but the reduction in flow when you are fighting a fire.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/