firewater header- sediment flush?

[davefitz]

I have a question for the group, regarding the recommended method to remove 20 yrs of accumulated sediment in a large power plant's firewater header system.

For the type of plant under consideration, consider a coal fired electric power station with 4 coal fired boilers and a 20 yr history of operation. The most common water source to maintain firewater header pressure had been sourced from a lake , thru the ash sluice water supply pumps, to the firewater header system. This source of water , from a lake, did not have inlet screens nor were there added otehr filters in this system.

The problem is that the plant personnel had added hose bibs for miscelaneous uses ( ie , wash down trucks) sourced from the fire water headers ( typically 12" NPS headers ). This practice of using firewater had led to 2 issues: it caused ingress of oxygenc from the fresh water which over 20 yrs corroded carbon steel piping, and also the source fromt helake allowed sediment to ingress to the firewater headers.

Now , if a fire were to occur and require the true use of the firewater system, the large 1500-3000 gpm flows may entrain all the sediment and plug the sprinkler nozzles.

The questions are:
a) how common is this type of problem?
b) what is the preferred method to purge teh headers of sediment?
c) does this means of operation meet NFPA code?

 

[stookeyfpe]

Dave:

You have a number of issues. I'll preface by stating that I only worked once with a fire protection water supply system supplied by a lake or ocean (mine was supplied from the Artic Ocean).

At the facility I worked the system was designed with redundant screens. Monthly we would isolate one screen system, backflush it and inspect the screens for any large debris.

First, based on the presented statements, it appears that your system was never correctly designed. NFPA 24 section 5.8 requires the following:

Water supply connections from penstocks, flumes, rivers, lakes, or reservoirs shall be arranged to avoid mud and sediment and shall be provided with approved, double, removable screens or approved strainers installed in an approved manner.

I recgonize that this little statement does not offer a great deal of design. I did review the Factory Mutual Global requirements for water supplies. To avoid all of this again, what is the feasibility of connecting the pump to a static tank supplied by a potable water supply? I ask this because FM offered no guidance for your condition. I'm assuming they do not want the risk of the conditions you currently have and essentially will not insure such an installation. Again this an assumption on my part.

FM does offer some good guidance on methods for cleaning and possibly re-lining pipe.

Now on to suggestions for solving the problem. Does your system have isolation valves between the various headers? You could attempt to backflush the system from the various headers using clean water. This should remove debris. For a proper flush you should be looking for flow velocities on the order of 10 feet/second. You may want to seek assistance from the local FD because they may be able to provide an engine and tanker to provide the water and pump.

Regarding sprinkler pipe obstructions, this is more difficult. One suggested method is to isolate the sprinkler system at the control valve and modify the most remote piping by providing a flush connection. You may be able to flush water through the main drain connection.

My biggest concern is the reliability of your pipe. I would suggest a functional flow test from the hydraulically remote piping or fire protection system. You may lose some pipe and fittings in the process but it would better to have failure now (and take the appropriate fire watch and prevention procedures) rather than have the piping fail under an actual emergency condition.

Finally, non-fire protection uses should be disconnected. Your probably in violation of the local or state plumbing code by using non-potable water without backflow preventers from the fire protection system.

 

[davefitz]

stookeyfpe,

Thanks for the reply.

We are not interconnected with potable water at this plant , so the backflow preventor issue may be the only part of the firecode we have not yet violated.