Abnormal pressures in fireline - out of box ideas?

[UtahWater]

I have been called to help troubleshoot some unusually high pressures in a fire line. The fire line experiences intermittent pressures (anectdotally from the client) on the order of 260 psi, which is about 70 psi higher than the storage tank at the top of the system. There is a PRV between the tank and fireline that should keep pressures at the fireline down around 125. If the PRV were to fail, we understand that pressures would rise to the HGL of the tank, which would be about 190 psi. How would pressures increase to a level higher than the tank though (sorry, I'm mixing my HGL and gauge pressure terms, hope it still makes sense)? The fire lines have a check valve in them so no water/pressure can back feed into the system. The fire protection company says the high pressure can be bled off of the fireline, which leads me to think that the high pressure is not (always) present in the distribution system. Has anyone seen a situation where a PRV intermittently works, and further that the surge pressure from such a failure gets locked into a closed system, like a fireline.

The residences also on the system supposedly all have regulators in their lines, and there isn't supposed to be any outside watering with culinary, so we haven't had any residential complaints of high pressures, ostensibly because all 300+ regulators are functioning properly and no one really is using culinary outside.

Any help? Pressures can't 'ratchet' up without outside help, so I'm stumped. I'll attach a little schematic for reference.

Thank you in advance for any ideas!

 

[RWF7437]

Are there air /vacuum valves at high points in the line(s) and are they working ?

 

[UtahWater]

There are no air/vacs that I'm aware of.

 

[RWF7437]

"There are no air/vacs that I'm aware of. "

Might there be some you're not aware of ?
A recording pressure gage on the fire line might yield some clues. Can you try that ?

By the way what does "using culinary outside" mean ? Never heard of this.

good luck

 

[UtahWater]

Another engineer in our firm designed the system before I came and I haven't been able to find reference to any air/vacs. I think the practice was to rely on service taps and thus kitchen sinks to act as air release points.

A recording guage on the system has been in place for two days and has been steady at the normal 125#. 'Course there haven't been any high pressures in the fireline since the recording guage has been put on, either. There is not a recorder on the fireline, only a pressure warning system that notifies the fire protection company when the pressure's off-kilter and what it is.

Using culinary outside means using potable water from the 'city' (company or district) to sprinkle lawns or wash cars or drives, etc. The other option is to use the secondary (non-potable) water system to take care of the outside water use.

Thank you,

 

[RWF7437]

Thank you for the answers Utahwater.
I had only heard of such a dual water system in Coalinga, CA before now. Still, there they call the non potable "blackwater" . Culinary water sounds like delicacy by comparison.

 

[MartinSr00]

Is there a pump somewhere in the system that could be supplying additional pressure?

Is the pressure transient? A surge perhaps?

Is the instrumentation telling you the truth?

Is there a place where water can be trapped?

If the guages are telling it right, there has to be some added energy somewhere to supply the extra head. That can be a higher tank somewhere in the system, decelerating water, a pump somewhere. Something like that.

If what you've said is correct, there's additional energy coming from somewhere.

 

[bimr]

One would think that you are experiencing some sort of surge or water hammer.

Several questions;

How long is the water main?

How long has the water main been in service?

Any grade changes in the water main?

Are there fire hydrants on the water main?

 

[UtahWater]

MartinSr00,
Is there a pump somewhere in the system that could be supplying additional pressure? [No]

Is the pressure transient? A surge perhaps? [This is my guess, I wanted to see if that reasoning made sense to others wiser and more experienced]

Is the instrumentation telling you the truth? [It has not been checked, but two blown expansion tanks on the fire loop system seem to verify the high pressures]

Is there a place where water can be trapped? [Yes, in the fireline...I'm guessing a surge occurs at times in the system as a whole, but it gets trapped in the fireline...does that sound like a valid explanation?...aside from a pump somewhere adding head, it's the only one I can think of]

bimr,
How long is the water main? [About 1 mile from the tank to the fireline, with the PRV at about the mid point]

How long has the water main been in service? [10 yrs]

Any grade changes in the water main? [Yes. One local low point and one local high point between the tank and PRV, one local low point between the PRV and fireline.]

Are there fire hydrants on the water main? [Yes, supposedly at 500' spacing ...at least that's how it was designed.]

Thank you all for your questions and responses.

 

[SteveWag]

If the fire line is sealed with a good check valve, and if the high pressure is only seen on the fire side of the check, then pressure in excess of line pressure is directly tied to temperature. Did/do these high pressure events seem to happen on hot days?
Steve

 

[UtahWater]

SteveWag, yes, the anectdotal story is that it happens when it's hot. The fire suppression company says that they have two types of systems, one type that has anti-freeze in it and one type that's just water. They say they experience the high pressures in both types of systems. I have never gone through sizing an expansion tank on a closed system...is there a chance that the tanks that blew weren't sized correctly to handle the thermal expansion, or is getting a surge 'locked' in the system still a possibility?

 

[cvg]

why can't you just install a pressure relief valve? Might be cheaper than an expansion tank.

http://www.tycovalves-na.com/ld/CROMC-0296-US.pdf

 

[SteveWag]

I suppose that a surge could become trapped but I have seen a surge trip a flow alarm. It was possible that the air tanks became waterlogged (full of water) and pressures became high enough to destroy the tanks. It takes very little change in temperature to cause a large change in pressure in a sealed and filled system.

Steve

 

[786392]

If the system gets somehow blocked-out and still fully primed and full state

then even slight temperature rise with heat absorption from any source(s) can play havoc.

Such a situation's usual remedy is very small size TRV(thermal reief valve i.e very small sized PRV)

Thus a small volume is bled out to control system pressure within limits from the blocked-out pipeline segment.
Hope this clarifies/proves helpful.

Best Regards
Qalander(Chem)

 

[gerhardl]

Is the problem solved and a cause found?

Another solution only indirectly mentionen above as PRV is to install an pilot operated overflow valve : eg constant pressure valve constructed to keep the pressure at least at a certain level within the system and relief/portion out water if pressure exceeds set pressure.

If you set this a selected distance above working pressure, below pressure limit it will relief repeatedly when set pressure is exceeded.

Could be a better and more longtime solution for repeated overpressures at uncertain conditions and interval if it is impossible to pinpoint exact cause and volume.

 

[UtahWater]

The cause has not yet been identified, but a possible cause has been eliminated, I think. With the turn to cooler weather, the temp outside has been as low as 30F, as high as 65F and avg about 55F. Both the supplying system (City water system) and the closed fire protection system have seen spikes of 150 psi. I think temp increase has been ruled out. The client seems more determined to place blame 1st, then implement a solution. Thanks for all your help. If and when the problem is found and a solution implemented, I'll post it for reference.

 

[bimr]

Have you considered inatalling a pressure relief valve on the pipeline? That pressure may cause the residential water lines to rupture.

http://www.cla-val.co.uk/documents/52-03.pdf?PHPSESSID=646a27c890eede8cb1937b00d5ba56ff