Maximizing Flow With Midship Pumps

Dominic Colletti explains how making a few minor changes may provide a lot more water.


Your new apparatus committee has spent a lot of time sorting through details to come up with a good specification for a proposed new engine. This includes a requirement for a midship-mounted fire pump with the associated equipment to be able to move "big water." But how do you really know if you...


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Your new apparatus committee has spent a lot of time sorting through details to come up with a good specification for a proposed new engine. This includes a requirement for a midship-mounted fire pump with the associated equipment to be able to move "big water." But how do you really know if you have maximized the engine's flow-rate capability with the current equipment that's in the specification? Are a few large-diameter connections good enough?

First, let's quickly review why there is a need for big-water capability. When considering water-flow requirements and fire pump size, Pareto's Law, or the "80/20 principle," will come into play. As it relates to this subject, when planning a fire pump's rating on new apparatus, don't just set the apparatus up to handle 90% of working fires that cause 10% of the property damage. These are the run-of-the-mill fires typically extinguished with one or two 1¾-inch attack lines. Include real thought on how to handle the 10% of fire activity that causes 90% of property damage - this is where you will need to supply "big water." It makes good sense for most departments to size and equip a new midship fire pump to handle both situations. What follows are a few helpful tips and tricks to consider when reviewing an engine's midship pump specification to maximize water delivery rate.

  • Engine size. Tip number one is to maximize delivery rate by choosing a pump that will use the available power provided by your chassis' engine and transmission. The fire service pays good money for diesel motors with lots of power to move the fire truck from point A to point B. Why not put that power to work by choosing the right size fire pump to maximize gallon-per-minute delivery rates? That just makes good sense.

    When discussing how to size midship pumps, it is important to first talk about the two styles of midship that are available today from most pump manufacturers - the "small-body" and the "large-body" fire pump. Small-body midship pumps are usually rated up to a maximum of 1,250 gpm, while large body pumps can be rated up to and even over 2,000 gpm. The major physical difference between these two models of fire pump is the cross-sectional size of their waterway castings and impeller assemblies. While a large-body pump will cost more than a small-body pump, the price increase ends up being only a couple of percentage points of the engine's total purchase price. Is it worth it? Only you can decide - but remember that increased water-delivery rate is critical for putting a stop on a large volume of fire.

    Some may believe that it makes good sense for only large city or suburban departments to purchase large-body pumps, since they are more likely to have high-volume positive water supplies from pressurized hydrants. But rural fire departments should also carefully consider the large-body pump too. While it sounds counterintuitive to use a large-body fire pump in a rural fire department where water supplies may be scarce, large-body pumps have great benefit for rural departments that routinely draft under high-lift conditions and use long suction hoselays. The reason is that a large-body fire pump has a superior vacuum curve over a small-body pump. With a large-body pump, the dynamic which causes "lift" - vacuum at the impeller eye - occurs over a larger-gpm range to allow significantly higher volumes of water to be discharged versus a small-body pump. That's considering all other things to be equal, under high-challenge drafting conditions.

    When I see a new engine with a typical motor, say 330 horsepower or higher, equipped with a "small-body" 1,250-gpm fire pump, I generally ask the department why it did not have a higher-gpm fire pump installed. While the answers vary, few are fact based and some don't make sense. The fact is that most engines in this power class can provide enough power to drive a "large-body" pump rated at 1,500 gpm or higher. Why not equip the engine with a large-body pump to be able to move more water? Contact your fire pump manufacturer for specific listings.

  • Tank-to-pump flow rate. Tip number two is to consider installing a larger tank-to-pump valve and line, or even dual tank-to-pump valves, to achieve higher-discharge flow rates when using booster-tank water. Why configure your pump to achieve high tank-to-pump flow rates? The benefit is that you can knock down a large body of fire in 20 to 30 seconds with a blitz attack of 500 gpm from a portable monitor using tank water.

    When initially working from a booster-tank water supply for fire attack, one water-flow choke point is the tank-to-pump valve and line size. For example, a three-inch tank-to-pump line and valve will deliver approximately 500 gpm from the tank, while a four-inch tank-to-pump valve and line will deliver about 1,000 gpm. Dual four-inch tank valves will deliver even more. The exact flow rate you will achieve depends on a number of factors, including the engine's altitude above sea level and its booster tank's internal design, for example.

  • Water in and water out. Tip number three is to maximize water flow into and out of the fire pump through the use of large-diameter openings in the pump casting and the corresponding use of large-diameter inlet and discharge valves on those openings. The issue is simple: if we cannot satisfactorily get a high volume of water into the pump's impeller eye and then out of the pump discharge casting, it is a moot point to have a large impeller and a powerful diesel engine driving it. We must therefore plan to equip the midship pump with machined openings and valve accessories to maximize water flow. Read this as, "remove as many restrictions, turns and choke points as possible."

    As a preamble, there are various casting choices in fire pump design across several manufacturers. One does not need to be a registered professional engineer to see the differences between manufacturers' designs. If maximizing water flow is a priority, the fewer number of turns that water has to make, starting at the discharge of the impeller and exiting the casting at the discharge valve, the less turbulence and lower the friction loss there will be. It's pretty simple - one of the steps in making a good pump choice is to count the number of turns water must make to exit the pump casting.

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