Centrifugal horizontal split case fire pumps - Suction 1

[FFP1]

I can only find the following references regarding the requirement that a centrifugal horizontal split case fire pump must have a positive suction supply.

6.1.2* Application. Centrifugal pumps shall not be used where a static suction lift is required.

A.6.1.2 The centrifugal pump is particularly suited to boost the pressure from a public or private supply or to pump from a storage tank where there is a positive static head.

I have a location with six 2,000 gpm @ 100 psi centrifugal horizontal split case fire pumps. I recently performed the annual flow test activities. This is a new customer for us, but the pumps have been in place for 15+ years. They area all arranged to take suction from an open top reservoir (approximately 100 ft. x 100 ft.) which is 15 ft. deep. The reservoir water level is 1 ft. above the top of the fire pump, so the STATIC suction pressure is 0.5 psi when the reservoir is completely full. Technically (based on the text listed above), this is acceptable based on NFPA. I do not think this arrangement should be considered acceptable. Centrifugal horizontal split case fire pumps are intended for applications which have positive suction pressures. What will happen after the deluge systems have operated for 30-60 minutes and the reservoir is 5-10 ft. below full?

Has anyone else seen a similar arrangement? Is this an acceptable arrangement based on NFPA?? Are there any NFPA requirements which I am missing which indicate this arrangement is unacceptable??

Thanks in advance!

 

[DavidCR]

I think that the code is not very explicit on this, but 5.14.3.1 and 5.14.3.2 of NFPA 20 (2003) contain the requirement to have positive pressure, and your case is not following the rule.

 

[chevy4x4trucks]

You're correct, you need to provide positive pressure at all times. I am assuming when the annual pump tests are run this is not enough to drop the water supply significantly or you are dumping the water back into reservoir.

If you run the water level down the pump will start taking in pockets of air and you could burn out the pump during a fire.

 

[FFP1]

Pump cavitation is exactly what I expect will occur if the water level drops more than 1-2 feet. I am just wondering why this issue is not clearly addressed in NFPA or if maybe I missed something more clearly stated in NFPA.

Thanks to David CR.....I was not aware of the text you referenced. These sections are very helpful and relevant.

 

[FFP1]

Obviously, if you combine 6.1.2, 5.14.3.1 and 5.14.3.2 all together the requirement is solid and clearly defined.

Thanks again to David CR

 

[KDLLC]

FFP1, there is commentary to section 6.1.2 in the 2007 handbook that reads (in part) "Once primed, a centrifugal pump can theoretically operate with a suction lift up to atmospheric pressure. Earlier editions of NFPA 20 allowed centrifugal pumps to operate under suction lift when they were provided with a tank of priming water. This arrangement proved problematic. It did not prove to be sufficiently reliable and was removed from the standard beginning with the 1974 edition."

The 1' of water above the pump suction would certainly act as the priming water. With the bottom of the resevoir 14' below the pump suction, the maximum lift required would be just over 6 psi, well under atmospheric pressure in most areas.

Do the pumps have suction pipes that extend down toward the bottom of the resevoir? Was the applicable version of NFPA 20 something prior to the 1974 edition? If both answers are 'yes', the installation may have met the original design standards.

Having said all that, I gotta agree with everyone else - not a good design.

 

[SprinklerDesigner2]

"FFP1, there is commentary to section 6.1.2 in the 2007 handbook that reads (in part) "Once primed, a centrifugal pump can theoretically operate with a suction lift up to atmospheric pressure. Earlier editions of NFPA 20 allowed centrifugal pumps to operate under suction lift when they were provided with a tank of priming water. This arrangement proved problematic. It did not prove to be sufficiently reliable and was removed from the standard beginning with the 1974 edition."'

Than God it wasn't all in my mind!

I started my sprinkler career in the mid 1970's and I remember reading the section on suction lift and priming water. I also remember seeing a couple large split case horizontal pumps, it was an oil refinery that was hurriedly built during the second world war, taking suction from Lake Erie just east of Toledo. Digging around old WW2 built sites makes one feel like an archaeologist because you never know what you will uncover.

These pumps were finally removed from service in the 1990's and I am thinking the water level had to be further down than just foot or two especially when there is a strong southwest wind which I've seen drop the level two or three feet piling it up on the Canadian side.

If the pump was installed prior to the mid to late 70's it's probably alright.

 

[FFP1]

I really appreciate the input from David CR, KDLLC, and SprinklerDesigner2. I was originally trained by FM Global and they told us these fire pumps require a positive suction supply, but I was unaware of the information each of you provided in your posts. I completed a search through NFPA 20, but did not find the information you three have posted.

This forum is very beneficial.

Thanks again to each of you for your input!