High elevation gravity tank and pressure relief

[DavidCR]

I have a hydro power plant fire protection system where the water is supplied by a gravity water tank.

The tank elevation is 133m (420´)above the PVC-C900 private service mains. So on static, and almost a plain curve on flow conditions, the pressure bellow will be aprox. 182psi. On the lower floors of the power house the static pressure gets to 214psi. There is no chance to relocate the tank.

AHJ here asks for pressure to be regulated to 65/85 psi for classII/classI hose stations. So I´m thinking of using hydraulic control type FM-UL type pressure reducing valves for flow conditions. Vendors (Claval,Watts,Bermad)of this valves recomend to use two parallel valves of different size in order to get a steady flow for a wide range of flows, one would work for low flows (i.e.a hose) and the other would work for the design full demmand scenario (as to NFPA 851).

Furthermore, AHJ here asks for a by-pass parallel line as an override of the pressure control devices, in order to guarantee the supply even it is not regulated for the case where the pressure reducing valves are in maintenance or if there is a failure. Here we had a fire in a alcohol plant where the pressure reducing valve failed and the water system was inoperative.

Inside the power house there is a sprinkler system and it is bellow ground level (479´bellow the tank) 208psi so class 175 sprinklers and devices will be in "danger".

My question...
I´m wondering that I will need also a main pressure relief valve (like the one required for a fire pump) to prevent overpressure on the accesories. And this will prevent overpressure when the bypass override is used, or when adjusting the pressure reducing valves, or if it fails t o adjust.

Initially I was thinking of small relief valves, but I suppose they work only for the case of closed systems that may suffer overpressure from cases like trapped air effects, day thermal cycles, etc. may develope overpressure. But in this case I suppose a small relief valve will not have the capacity to relief the pressure from the gravity tank.

Other option will be to go up in pressure class of devices. But I´m not sure if I will have money of availability problems.

I´d appreciate any comment on this case, if my thoughts about the main relief valves sounds fine, or if you could suggest any another idea on how to prevent presure problems, or if I´m mising something.

Thanks in advance.

 

[stookeyfpe]

Why do the main buildings require a standpipe Dave? Do you have a multi story building situtation requiring Class I fire service standpipes?

Are the buildings sprinklered? Being a power plant, I can already here the utility engineers proclaiming the impractical nature of sprinklers.

I'm just asking while being an old, cranky guy.

 

[DavidCR]

Hi "stook", and by the way, a thank you for your valuable hints in this forum.

As per NFPA 851 and AHJ, fire hose class II is used inside the building of the power house, it is one floor on ground level and 5 underground.

We use sprinklers (spray automatic nozzles sometimes instead of deluge systems) over the main lub-oil equipment in the power house. Also deluge system for transformers NFPA15, class III hose conections outside the buildings and detection system.

There is also a storage house that is sprinklered.

Even it is not the point of my question, please tell me more about the criteria of those utility enginneers, I know that the protection requirementsw on hydros are different from place to place, so for me it is important to compare different protection levels.

 

[stookeyfpe]

From a regulatory standpoint, power plants I've worked on are generally exempted by the State's corporation or utility commission. I have enforced IFC requirements for hazmats and the like but generally in the states I've worked in (Texas and AZ) the actual power generation equipment and buildings are regulated by the National Electrical Safety Code.

I have seen water spray and sprinklers for the lubrication units, as well as sprinklers in areas where hydrogen is stored as a lubricant to the turbines. Your standpipe arrangment is also consistent with what I've seen.

 

[rconner]

For whatever it is worth or for future reference, some manufacturers offer ductile iron pipe and restrained joint pipe systems for private fire main service, with even Factory Mutual listings of 350 psi for up to 16" size mains.