RPZ Backflows in Pump Suction

[bmlxd40]

I have a building with a pump and a RPZ in the vault outside. During the pump rated flow of 750 gpm, the RPZ has considerable loss (About 9.5 psi. My concern is at pump churn how to accurately calculate the available suction pressure so I can calculate the maximum churn pressure. If I understand RPZ's correctly, during a true static condition, there would be no pressure loss since there is zero flow. However, pump churn is not a true static condition since we have a casing relief valve that is flowing a considerable volume of water.

My question is this: Does the casing relief valve discharge enough to lower the pressure at the pump suction flange by an amount that should be considered? What is worth noting is that the RPZ has its greatest losses at very low flows.

With the gap on my 24 hour test, and the gap between the friction losses at churn vs pump rated flow, I am toeing the line so to speak on getting the pump pressures right

I am hoping someone here can recall a first hand test experience with an RPZ and a pump at churn.

 

[SprinklerDesigner2]

I assume the reason behind all this is to keep the maximum churn pressure below 175 psi?

What I would do is calculate the maximum churn pressure without any friction loss through the backflow. If the city static is 70 psi 105 psi would be the maximum pressure I would have at churn while in real life I would keep it around 95 psi for safety.

I have noticed the curves provided with both RPZ's and double checks can sometimes be way off when it comes to real world measuring and most often the actual head loss is more than what the literature claims it to be.

Also, the vault you mention would be above ground, correct?

 

[MatthewJWillis]

Does this help?

From 2013 NFPA 20

4.7.7*MaximumPressureforCentrifugalPumps.
4.7.7.1 The net pump shutoff (churn) pressure plus the maxi-mum static suction pressure, adjusted for elevation, shall not ex-ceed the pressure for which the system components are rated.
4.7.7.2* Pressure relief valves and pressure regulating devices in the fire pump installation shall not be used as a means to meet the requirements of 4.7.7.1.

R/
Matt

 

[bmlxd40]

SD2, that is exactly what my goal is. Try to keep my churn less than 175 while still getting my 100 at the top.

My pressures are as follows and all are pump room pressures that have been adjusted for elevation.

Maximum 30 minute high point of the 24 hour test is a static pressure of 65 psi.


Lowest 30 minute period of the 24 hour test:
Static of 57psi
Residual of 43 psi while flowing 1040 gpm.

I did as you suggested and based my churn pressures on zero loss through the RPZ since that is a worst case scenario. If, in fact, there is a pressure loss through the RPZ because of the discharge of the casing relief valve, then that will only help lower my churn pressure. I have a HSC pump with an extremely flat curve, and that helps me too. The pump has a rating of 750 gpm @ 110 psi and a churn pressure of 112.5 psi.

Hydracalc comes up with a maximum churn of 176.65 when all the numbers are calculated exactly. I did a little rounding in my numbers above. That gives me 102 psi and some change at my roof manifold while flowing 500 GPM, plus the 250 at my other standpipe and that is including almost 10 psi of loss through the RPZ. The roof manifold is at an elevation of 85'. I am using a wall hydrant at the roof rather than standard hose valves to get rid of the loss through the 2½" angle valves. I wish I could find published data for loss through a wall hydrant, but I haven't had much success so far.

I HATE cutting things this close, but the only other option I see is to increase pump pressure and add PRV's on my first floor, and I am doing everything I can to avoid that.

 

[TravisMack]

Do you have this with all 4" pipe? How tall are you going up? You may have to use some 6" in the horizontal bulk, if you aren't already.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[bmlxd40]

All 6" bulk and standpipe. My roof manifold is at 85'. I only have a little over 5 PSI of friction loss between the RM and discharge flange. Unfortunately, the gap in the 24 hour test coupled with the loss through the RPZ is what is hurting me here, I believe.

 

[TravisMack]

So you have about 49 psi at 750 gpm. You are using a pump to give 110 at 750. So, you have 159 total PSI to work with. 37 psi in elevation. 7 psi for angle valve, nipple and outlet. So basically 144 + about 12 psi for the RPZ. I would think that 3 psi for friction loss is going to be quite tight.

Are you sure you need an automatic standpipe. Roof manifold at 85' would likely put your floor level below 75' (assuming 9' ceiling and at least 2' deep roof truss). As such, you have a mid-rise and should be able to do a manual wet standpipe, unless your jurisdiction mandates automatic. If you follow NFPA 14, you will get about 200 psi or so at 750 gpm. You should confirm with the local FD what kind of pressure they can give you at 750 gpm.

Hopefully you can get this to a manual wet standpipe and save yourself the issue with the pump churn pressures. You may also be able to run the pressures up high on the lower levels and use steel pipe and higher pressure sprinklers if you must do automatic standpipe. Again, no idea what type of project this is, so this may not be acceptable.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[SprinklerDesigner2]

I might be wrong but it looks to me like the building might not be a high rise?

The roof manifold is at an elevation of 85'.

Correct me if I am wrong but isn't a high-rise building defined in Chapter 2 of the IBC as a “building with an occupied floor located more than 75 feet (22 860 mm) above the lowest level of fire department vehicle access."?

For the record, being the coward that I am, I would never make the decision whether or not a building is a high rise as that would be practicing architecture without a license which, especially you a layout technician is wrong, could get the company into a whole lot of trouble.

One thing people need to recognize is Georgia has some rather strange rules that don't exist anywhere else in the country and they can be found here:

120-3-3 State Minimum Fire Safety Standards - effective 01-01-2015

That is the last published rules.

We are interested in the changes to NFPA #14 which begins at the bottom of page 43:

(13) NFPA 14, 2013 Edition, Standard for the Installation of Standpipe, and Hose Systems
Modifications:
(a) Modifications to Chapter 1:
1. Delete Section 1-1 in its entirety and substitute in its place the following:
“1-1 Scope. The State's minimum requirements for standpipes shall be established by the IBC (Refer to Table 102.13, CODES REFERENCE GUIDE) of the International Fire Code, as adopted by Chapter 120-3-3 of the Rules and Regulations of the Safety Fire Commissioner). In addition, the requirements for occupant hoses are eliminated for new and existing buildings subject to the approval of the authority having jurisdiction. Where the installation of standpipes and /or hose systems is required, this standard covers the minimum requirements for the installation of standpipes and hose systems for buildings and structures. This standard does not cover requirements for periodic inspection, testing, and maintenance of standpipe systems. (See NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems.)”
(b) Modification to Chapter 7:
1. Delete 7.8.1 in its entirety and substitute in its place the following:
“7.8.1 Hydraulically designed standpipe systems shall be designed to provide the waterflow rate required by Section 7.10 at a minimum residual pressure of 100 psi (6.9 bar) at the outlet of the hydraulically most remote 2-1/2 inch (65 mm) hose connection and 65 psi (4.5 bar) at the outlet of the hydraulically most remote 1-1/2 (38 mm) hose station.
7.8.1.1 Where the local Fire Chief or local Fire Code Official having fire suppression jurisdiction permits lower than 100 psi (6.9 bar) for 2-1/2 inch (65 mm) hose connections, based upon local suppression tactics, the pressure shall be permitted to be reduced to not less than 65 psi (4.5 bar).
7.8.1.2 Where the building is protected throughout by a supervised automatic sprinkler system and the building is not a high-rise, as defined in 3.3.5, the minimum residual pressure provisions shall not be mandatory when the standpipe system piping is a minimum of eight inches (8”) nominal diameter.
7.8.1.3 Existing high-rise buildings, as defined in 3.3.5, that are protected throughout by a supervised automatic sprinkler system shall be permitted a reduction of the minimum residual pressure requirement of 100 psi (6.9 bar) at the hydraulically most remote 2-1/2 inch (63.5 mm) hose connection to 65 psi (4.5 bar).
7.8.1.4* Manual standpipe systems shall be designed to provide 100 psi (6.9 bar) at the topmost outlet with the calculations terminating at the fire department connection. ”
2. Delete 7.8.2.1 in its entirety and substitute in its place the following:
“7.8.2.1 Pipe schedule designed standpipe systems shall have piping sized in accordance with the pipe schedule in Table 7.8.2.1 to provide the required waterflow rate at a minimum residual pressure of 100 psi (6.9 bar) at the topmost 2-1/2 inch (65 mm) hose connection and 65 psi (4.5 bar) at the topmost 1-1/2 inch (38 mm) hose connection.
7.8.2.1.1 Where the local Fire Chief or local Fire Code Official having fire suppression jurisdiction permits lower than 100 psi (6.9 bar) for 2-1/2 inch (65 mm) hose connections, based upon local suppression tactics, the pressure shall be permitted to be reduced to not less than 65 psi (4.5 bar).
7.8.2.1.2 Where the building is protected throughout by a supervised automatic sprinkler system and the building is not a high-rise, as defined in 3.3.9, the minimum residual pressure provisions shall not be mandatory when the standpipe system piping is a minimum of eight inches (8”) nominal diameter.
7.8.2.1.3 Existing high-rise buildings, as defined in 3.3.9, that are protected throughout by a supervised automatic sprinkler system shall be permitted a reduction of the minimum residual pressure requirement of 100 psi (6.9 bar) at the hydraulically most remote 2-1/2 inch (63.5 mm) hose connection to 65 psi (4.5 bar).”
3. Insert a new subsection 7.12.3.4 to read as follows:
“7.12.4 Location. The location of fire department connections shall be approved by the Fire Chief as set forth in subsection 501.5 of the International Fire Code, as adopted by Chapter 120-3-3 of the Rules and Regulations of the Safety Fire Commissioner.”
(d) Modification to Chapter 10:
1. Add a new subsection 10.1.3.1 to read as follows:
“10.1.3.1 A letter certifying that all pressure restricting and pressure reducing equipment is installed and set per NFPA requirements and manufacturer’s instructions shall be presented to the inspector along with test certificates at the time of final inspection.”
(e) Modification to Annex A:
1. Renumber A.7.8.1.1 to A.7.8.1.4 to read as follows:
“A.7.8.1.4 It is not the intent of this standard to provide an automatic water supply for manual standpipe systems. Manual standpipe systems are designed (sized) to provide 100 psi (6.9 bar) at the topmost outlet using a fire department pumper as the source of flow and pressure.”

With the exception of an open parking garage I have never been allowed to install a manual standpipe system in the State of Georgia. I haven't seen it lately, haven't bothered to look as long as the state gives me my 8" out, but I do believe there is something in the Georgia version of the IBC that only allows manual standpipes in open parking garages. I know I saw it once but to find it again I would really have to do some serious looking.

And the reason I asked if this building was a high rise there's this little gem that, as far as I know, is unique to Georgia:

7.8.2.1.2 Where the building is protected throughout by a supervised automatic sprinkler system and the building is not a high-rise, as defined in 3.3.9, the minimum residual pressure provisions shall not be mandatory when the standpipe system piping is a minimum of eight inches (8”) nominal diameter.

I've done this a number of times where in a four or five story motel I can do away with the fire pump completely as long as I run the entire standpipe in 8" pipe and this is from beginning to end. The way I read it if all your pipe is 8" you could have one psi of residual pressure at the roof manifold and you are good to go.

Trying to remember but over the past 10 years I know I have done at least half a dozen four and five story motels that have standpipes, that have typical city static pressures of 50 psi and and as the building is not a high rise all these have been approved and half of them were reviewed and approved by the state fire marshals office in Atlanta.

The 8" rule really comes in handy around Savannah where the local fire officials deem all electric is not reliable citing hurricanes. Yep, you do a fire pump in Savannah it will be diesel or have an electric backup generator with automatic transfer switch. Given this running the standpipe all in 8" is always the less expensive way to go.

I got one coming up right now a four story motel, a standpipe is required (fourth floor over 30') and due to some architectural trade offs we have a full NFPA #13 system. To compound issues the entire building is constructed of TJI's so running any large pipe through the building is a no go. How does one hang 8" from any wood structure mess less composite wood joists?

I have a backflow in the front yard where I place the fire department connection and then run 8" underground to each stairwell so the only place I have 8" is the stairwell and it is 8" all the way up to and including the roof manifold.

And I have heard of people in Georgia going with manual standipes in other than open parking garages and getting away with it. I would really, really like to know how they do this? Anyone give me some guidance on this?

Back to the original building where the ceiling of the top floor certainly is no higher than say 82' given the roof manifold is at 85'. At 82' given city water pressure of say 50 psi at sprinkler demand we would still have 35 psi at the top floor. Yeah, the sprinkler pipe might have to be increased a bit considering Georgia will be demanding a 10 psi "safety factor" in any calculations but top floor of an office or hotel certainly wouldn't be impossible.

Are RPZ's required on all fire sprinkler systems or only ones having a fire pump? Rules around Georgia can change a lot depending on jurisdiction.

If the building is not a high rise another option might be to run the standpipe in 8", we're already running 6" so it isn't that much of an increase, having a separate riser in a stairwell for sprinkler only. For the sprinkler riser use a 250 gpm @ say 60 psi pump? Sometimes this works out even better if the local authority requires hose valves on intermediate landings anyway. So give them their hose valves on the intermediate landings and run 3" up on the main floors with a floor control valve.

 

[TravisMack]

SD2:

I used to do work for a multi-family sprinkler contractor in GA. We did all of our standpipes as manual wet/dry standpipes. They were always approved by the local AHJ. We just made sure to contact the FD to have them tell us what pressure they wanted us to use at the FDC. Your info above states it is permitted.

7.8.1.4* Manual standpipe systems shall be designed to provide 100 psi (6.9 bar) at the topmost outlet with the calculations terminating at the fire department connection. ”

I don't see anything in that writing that says that Manual is not permitted. NFPA 14 is pretty clear on when Manual is an option. The fire code generally says go to NFPA 14 for standpipe criteria. There is some that is conflicting like main or intermediate landings.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

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[bmlxd40]

It is a high rise for sure. The roof level has a kitchen, among other things, so they are saying that is the highest occupied floor. Plus, the architect's code compliance sheet list it as a highrise. We have done several non high-rise buildings as manual standpipes with no issues whatsoever. I have checked 120-3-3 and cant find where it s prohibited any longer, though it may have been at one time.

 

[TSDavis]

Although not part of the actual standard, A.4.7.2 says......
"It is not the intent of this subsection to restrict the use of pressure reducing valves downstream of the discharge isolation valve for the purpose of meeting the requirements of 4.7.7."

So in essence, once you've passed the pump discharge iso. you can add a PRV. You could produce a flat curve at that point. Still not the best option as these are prone to failure.

If you are going to use this method to reduce pressure it's important that any hydraulic demand points fall below maximum pressure available at the PRV. Easy to get carried away with the sprinkler layout when you have a ton of pressure available.

Terry S. Davis
Triton Fire Group
h2o-racing@comcast.net