Differentiating between PRV, Multi Stage Multi Outlet and Series Pumping
The urban migration rate is on the rise every year. This is what’s driving the growth of cities; large cities are even larger today and will continue to grow.
The urban migration rate is on the rise every year. This is what’s driving the growth of cities; large cities are even larger today and will continue to grow. The city skylines are also getting higher and denser every year. This means newer challenges for protecting the lives of people occupying these skyscrapers, especially from a potential fire hazard. Designing a fire suppression system for a high-rise building with due attention to the real estate it occupies is challenging and at times very complex to install. Selecting the heart of the fire suppression system, a fire pump is not easy. Often times the zone split of a high-rise building will be dictated by the fire pump selection.
Fire suppression systems for high-rise or super high-rise buildings utilize multiple fire pumps arranged in a series to deliver water to the farthest sprinkler (hydraulically) in that very building zone. Selection and design intention typically is not to protect the entire high-rise at once, but to protect individual building zones at one time. This conventional solution is a pretty expensive way of meeting the purpose of fire protection as it typically involves multiple pumps, pump drivers, and controllers. It also needs considerable space for it to be housed in the building, which is very dear to any skyscraper. Design and installation of such systems is a complex affair as the system designer has a limited set of options from listed and approved products to pick from.
If one does not wish to use multiple pumps due to space limitations, pre-existing pipework, or any other reason, a single high-pressure pump, duly sized to meet pressure requirements for hydraulically farthest sprinkler can be employed. In this type of fire suppression system, use of pressure reducing valves (PRV) is often inevitable to avoid overpressurization of branches and sprinklers. However, NFPA 20 discourages the use of pressure-reducing devices on the discharge side of a fire pump due to the known history of installation and maintenance issues associated with PRVs. Without use of PRVs, fire system components rated for high-pressure availability is rare and that means these components are relatively expensive. In such a system, the fire pump will always run to meet peak pressure requirement at the demanded flow, which may result in paying that month’s electricity bill at higher rates than normal due to spike rate.
How about a third solution, which offers the best of the above two solutions – a single fire pump that does deliver exact pressure at each zone of the building without using pressure reducing valves. We are talking about a multi-stage multi-outlet (MSMO) pump, which offers interstage outlets to serve pressure requirements of individual zones of a high-rise building. Each inter-stage outlet is hydraulically sized to meet specific pressures as needed by the building zone it serves.
This product is novel to the United States commercial fire protection market, but the working principle and its application is an old and commonly followed practice in critical pumping applications where pressurized water from a multistage pump is utilized for forced cooling of bearings or for shaft sealing. In essence, its working principle can be easily compared with a diesel-engine-driven fire pump which is also providing cooling water for engine cooling.
To understand the basic working and sizing principle of an MSMO, let’s consider a three-zone high-rise building needing rated 1,000 gpm flowrate and zone split as given below:
Zone 1 – 130 psi
Zone 2 – 220 psi
Zone 3 – 360 psi
To aid simplicity, let’s assume city mains average water pressure of 50 psi and each pumping stage of an MSMO pump rated for 1,000 gpm produces 50 psi at its maximum impeller diameter.
For Zone 1 sizing pump needs to deliver a discharge pressure of 130 psi with 50 psi suction pressure, the pump needs to create 130 – 50 = 80 psi of differential pressure, which it can do in two stages. That means the first pump outlet needs to be at the second stage of pump, with each stage creating 50 psi pressure.
For Zone 2 sizing which needs 220 psi from the pump, our first outlet is already sized for 130 psi and is situated at the second stage of the pump. We need to find the pressure boost that pump would need to produce, i.e., 220 – 130 = 90 psi, with each stage creating 50 psi. We would need two more stages added to the pump to produce 220 psi discharge pressure, meaning our second pump outlet will be at the fourth stage.
Zone 3 will need 360 – 220 psi = 140 psi boost, which can be obtained by adding three more stages to the pump, making the seventh stage our final pump discharge outlet.
This means we would need a seven-stage pump with hydraulically sized discharge outlets at the second, fourth, and seventh stages, serving Zone 1, Zone 2, and Zone 3, respectively. Now let’s make another assumption of a fire initiated at Zone 1. Sprinklers serving the area of fire are now fully open and the main fire pump will be started as soon as the pressure in the piping drops below the set point. Though all of the impellers within the pump are rotating, only the first outlet, which serves Zone 1, will be active and outlets 2 and 3 serving Zones 2 and 3 will be at the state of churn as there is no flow possible through Zones 2 and 3. Another critical point to remember is that fluids follow the path of least resistance, meaning the pump will discharge its entire capacity through the active outlets and remaining pump stages. Though the impellers are rotating within it, they will be inactive and will not create any further pressure boost. The power consumed by the pump will correspond to a two-stage pump power consumption and not to a seven-stage pump power consumption.
This means for the above-stated application, a correctly sized MSMO pump can replace three individual fire pumps arranged in a series and work more efficiently without needing a PRV on the downstream side of the pump to prevent over pressurization. An MSMO pump offers considerable savings of space, overall equipment costs, and minimum start lag.
As stated before, this product is novel to the United States but is already employed in Europe, Asian, and the Middle East for the purposes of providing fire protection to high-rise buildings per BS EN 12845:2015 Fixed Firefighting Systems. Automatic Sprinkler Systems. Design, Installation, and Maintenance.
I have purposefully avoided to make any sprinkler system design-related comments in this article as the intention is to provide a brief overview of working principle, sizing, and selection of an MSMO pump. The overall sprinkler system design for a conventional pump in a series system can be applied to designing a system with MSMO pumps with little state- or county-specific code variations. All the discharge outlets will need individual pressure sensing lines, driver sizing will be based on final outlet peak power, three separate controllers or one controller with three pressure lines with hi-low settings are the elements similar to a conventional pump in series type system design. NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, 2016 and 2019 editions have advised the basis of system design with MSMO pumps, the verbiage being used in NFPA 20 for MSMO pumps is “Multi-Stage Multi-Port” pump. Fire pump approval standards such as UL 448, Standard for Centrifugal Stationary Pumps for Fire-Protection Service, is amended to include MSMO pumps and FM has introduced approval standard FM 1310, Centrifugal Fire Pumps (Multi-Stage Outlet Type), specifically to address MSMO pumps. Apart from high-rise buildings, MSMO fire pumps can also be used in applications such as large warehouses, remote train stations, metros, etc.
To sum it up, an MSMO pump offers benefits of a PRV-based single high-pressure pump with flexibility and code compliance levels of a conventional pump in a series fire system. It is a reliable, efficient, and cost-effective solution for providing fire protection to occupants of high-rise buildings.Hhopeful that this new technology will help to modernize fire protection in many old buildings where it really matters and many new buildings to use this revolutionary product in near future.