Though NFPA70 (National Electrical Code) 50th Edition has passed the NFPA Membership Vote, an Appeal has been filed with the Standards Council by several NFPA20 Technical Committee Members. The Appeal pertains to a new electrical connection arrangement for electric fire pumps accepted by NFPA70 Panel 13. The Appeal will be reviewed by the Standards Council at their July meeting.
On Monday, June 13th, the lives of two Bronx native brothers, aged five and twelve, were lost to a fire caused by an overloaded extension cord. Though the passive fire alarm system was functional, it did not activate in time to allow the boys to exit their second floor apartment. The apartment was not equipped with automatic fire sprinklers. Seven fire fighters were injured extinguishing the blaze.
HIGH-RISE FIRE PROTECTION SYSTEM DESIGN: ZONING & SERIES PUMPING
This article is the third of a series of articles on high-rise fire protection system design. The last issue looked at how zoning is accomplished in a high-rise building. This issue will look at the controls necessary to make the system work reliably and effectively.
Now that we understand how the zoned fire protection system works, let's look at the controls necessary to make the system work. To do so, we will need to refer to Figure 1.
In Figure 1, each successive pump supplies water to a specific zone in the building. All pumps except the High Zone Pump also boost the pressure for the next pump feeding the next zone in the system. For example, Pump 1 supplies the Low Zone and in turn provides water supply to Pump 2. Pump 2 supplies the Mid Zone and in turn provides water supply for Pump 3 via the break tank.
Considering the water flow and supply, we can see that pumps must be started in sequence to be able to supply the adequate water flow and pressure to the appropriate zone in the system.
To ensure the system operates to design specifications, we need to look at the system both during emergency conditions and during normal conditions (when the system is waiting to respond to an emergency).
Emergency Conditions – Fire Pump Controller Sequence of Operation
The standard combination automatic / non-automatic fire pump controller can be applied to operate high-rise fire pumps. In the automatic mode, each fire pump controller would start as required when a pressure drop is sensed in its zone. The only required modification is the use of sequence start timers to ensure each pump has adequate water flow and pressure available on the supply side before starting.
Referring to Figure 1 once again, let’s look at each zone in turn.
- LOW ZONE: It is clear that Pump 1 can be started without delay when a pressure drop is sensed. Water is supplied from the city main (or reservoir) and as such, when a pressure drop is sensed in the Low Zone, Pump 1 can be started exactly as it would in a single zone system.
- MID ZONE: If a pressure drop is sensed in the Mid Zone and Pump 2 was started immediately, Pump 2 would not have adequate water supply to meet the pressure and flow requirements of the Mid Zone. As such, when a pressure drop is sensed in the Mid Zone, Fire Pump Controller 2 must send a signal to Fire Pump Controller 1 to start. Once Pump 1 is started (after a time delay), Fire Pump Controller 2 must then call Pump 2 to start.
- HIGH ZONE: The High Zone control is less intuitive. What should happen when a pressure drop is sensed in the High Zone? Do we need to start Pump 1 and Pump 2 before starting Pump 3? This is where the break tank becomes important. Though there are no guidelines for break tank sizing in either NFPA13 or NFPA14, it is generally recommended to size the break tank for 3 to 5 minutes of full flow (150% the rating of the fire pump, or the hydraulically calculated residual flow conditions). This allows us to start Pump 3 immediately upon sensing a pressure drop in the High Zone.
To complete the control design, we must now consider how Pump 1 and Pump 2 are going to start. Clearly, Pump 3 cannot indefinitely feed the high zone without Pump 1 and Pump 2 starting to provide makeup water to the break tank. There are two ways of doing this:
- SEQUENCE STARTING VIA FIRE PUMP CONTROLLER 3 - The obvious solution to the problem of starting Pumps 1 and 2 is to provide a start signal to Pump 1 and 2 in sequence from Fire Pump Controller 3. Doing so, however requires additional control logic and wiring (running wire from Fire Pump Controller 3 the length of the building to Fire Pump Controller 2 in the basement mechanical room).
- PRESSURE SWITCH STARTING - Provided the fill valve for the break tank is capable of supplying the full residual flow of the High Zone (Pump 3), the act of opening the fill valve causes a pressure drop in the Mid Zone. Once the pressure drop occurs in the Mid Zone, Pumps 1 and 2 will start in sequence as designed. No additional control logic is required.
You will note that the sequence starting of the fire pumps ensures adequate water flow and pressure are available in as little time as possible. The sequence starting also reduces starting loads on the utility or generator power sources. If a single pump is used to boost pressure for the whole building, we would need to start a 200 to 400hp motor (depending on the building flow requirements). With zoning of the system and sequence starting applied, the power source need only start one 75 to 125hp motor at a time. When looking at the combined horsepower of the pumps, generally the horsepower is equal to that of a single pump applied for the whole building.
As in any application where motors above 30hp are applied, it is recommended to use a reduced voltage starting to limit the starting current in high-rise applications. As a further note on the design, it is also recommended to use a mechanical float or altitude valve to fill the break tank. Two valves in parallel are recommended to allow service of each valve without taking the High Zone fire protection system off-line. The valves should be sized such that they can supply the full residual flow requirement for the High Zone.
Normal Conditions – Jockey Pump Sequence of Operation
Because we have three differing pressure zones in the building, we will also require three jockey pumps. Each jockey pump should be installed as in a single fire pump - single jockey pump application.
Is there a need to sequence start the jockey pumps? Surprisingly, the answer to this question is "no". The jockey pumps can be applied without modification to the jockey pump controllers if the jockey pumps are not oversized. High-rise buildings require a minimum 6" standpipe. If the jockey pumps are not oversized, each 6" standpipe holds a large volume of water relative to the jockey pump design flow. The large volume of water held in the standpipes themselves provides adequate water supply for pressurization of the next zone in the system. As such, there is no need to start Jockey Pump 1 before starting Jockey Pump 2. If Jockey Pump 2 is called to start, Jockey Pump 1 will start shortly after to re-pressurize the Low Zone. What in effect happens is that Jockey Pump 2 takes water from the Low Zone Standpipe inducing a pressure drop in the Low Zone and starting Jockey Pump 1. This can be done without concern for simultaneous starting of the jockey pumps. Further, starting the jockey pumps in short succession, or in the worst case, simultaneously, should not cause an excessive burden on the power source.
As a closing note, Jockey Pump 2 will serve to provide makeup water to the break tank during non-emergency conditions. If a large water demand occurs in the High Zone, Jockey Pump 2 will very shortly be unable to maintain pressure in the Mid Zone resulting in the sequence starting of Fire Pump 1 and 2 as designed.
FIRE PUMP TIPS
QUESTION: If I install a listed flow meter with test loop as permitted by NFPA20, is an outside fire pump test header still required?
ANSWER: Flow meters should be installed on fire pump systems where it is either impractical or impossible to perform the fire pump acceptance test (as required by NFPA20) and/or the annual flow test (as required by NFPA25). Regardless of flow meter installation, a fire pump acceptance test requires full flow discharging water from an outlet to test the fire pump suction water supply. In addition, though NFPA25 allows annual tests on the pump to be performed using a listed flow meter, NFPA25 still requires that full flow be established once every three years to ensure the water supply is still adequate for the fire protection system. A hose valve header should be installed on all fire pumps for these purposes.
There is an exception to the above. Some jurisdictions require full flow tests to be performed at the furthest outlet(s) of a standpipe system. Where acceptable to the AHJ, the full flow test on the standpipe system can serve the purpose of testing the water supply. When testing a pump using the standpipe hose connections, provisions must be made to ensure the pump and flow test are conducted safely.
NEW FROM ARMSTRONG
Ask your local Armstrong Sales and Service Representative about application of variable speed pressure limiting control on fire pumps. The product is available in electric and diesel configurations to solve over-pressure problems in a variety of automatic sprinkler system applications. As always, we welcome all fire protection professionals to visit us at http://www.armstrongpumps.com/fire_protection.asp.