Water Delivery System Design

Editor’s Note: This is the last of a three-part series on fire apparatus water delivery system design. The first part covered the water’s route from the fire pump volute outlet through the pump discharge manifold, apparatus plumbing, discharge valve and fire hose to the nozzle or other fire appliance. The second part covered the suction inlet side of the system, and the third looks at the pump drive system.

The fire pump drive system must be matched to the flow and pressure required and the water supply available. The process guarantees the performance capability of the apparatus at the end of the hose, where the wet stuff meets the red stuff.

There are several different pump drive systems. Each has several major components to consider, and two have drive system limits, beyond available engine power, that need to be taken into consideration.

The most common setup is the split driveline pump drive. This design cuts the main drive shaft between the chassis transmission and the rear axle and adds inline a shiftable gearbox, which allows either driving down the road or operating the pump. This system requires a chassis transmission that can be held in the correct gear for pumping and that doesn’t have a first gear ratio high enough to multiply the engine’s torque to the point where it exceeds the pump’s lower drive torque rating. The truck transmission should also not have any oil temperature problems while pumping.

Full Power Reliably

The good news is this setup is very good at delivering full engine power reliably for more hours than most departments could ever use. The engine selected for this setup, as well as the others, needs the power to meet the National Fire Protection Association 1901 apparatus standard rating, but it also needs the power to meet the designed pumping discharge and intake requirements.

 The second most common setup is the transmission power takeoff pump drive. This system requires a chassis transmission that has a PTO opening and driving gear that can be used to drive the fire pump. The PTO, which is bolted to the chassis transmission, must clear the cab, chassis frame and front suspension. Additionally the PTO must have a continuous duty torque rating suitable for both the NFPA 1901 ratings and the designed pumping requirements. The PTO also needs a design life that matches the department’s yearly average usage at the designed pumping performance. This drive setup has the limitation of the PTO’s design torque rating and designed life. The transmission should also not have any oil temperature problems while pumping.

Front Engine Pump Drive

The third and least common setup is front engine pump drive. This design drives the pump off the front of the engine crankshaft. This requires a cooling system designed for a front PTO drive. The other big issue is the crankshaft’s ability to transmit power. This rating is usually lower than the available engine power and could be a limiting factor. Front PTO drives are not available on many chassis. This system requires only the chassis transmission to be in neutral.

No matter which drive is used, the design pumping requirements could be very different than the basic NFPA 1901 ratings. The goal is to meet NFPA while performing the designed performance at the lowest practical engine rpm. For most applications where the designed operating pressure is 120 to 140 psi, the simultaneous total flows are less than 80 percent of the pump rating and the engine is at least 320 hp and over 860 foot-pounds of torque, it is common to be pumping close to the rpm that coincides with the maximum torque rating speed of the engine. The drive should have the component life suitable for the department’s usage history and projected future usage. With a split driveline setup, this is normally not an issue, but it can be on some PTO drive applications with big pumps and high usage.

What does all of this really mean? Spec the performance you want at each discharge, match the discharge requirements to the suction intake layout, design the pump drive to match the performance requirements and maximize the performance of your next pumper.

Editor’s Note: Gary Handwerk is global pump product manager for Hale Products. He has been involved with the fire service industry for more than 36 years, working for various fire apparatus and pump manufacturers. He has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for more than 15 years.

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Water Delivery System Design

Editor’s Note: This is the second of a three-part series on fire apparatus water delivery system design. The first part covered the water’s route from the fire pump volute outlet through the pump discharge manifold, apparatus plumbing, discharge valve and fire hose to the nozzle or other fire appliance. The second part covers the suction inlet side of the system, and the third looks at the pump drive system.

The suction inlet side of the pump system is critical to maximizing overall pump performance. It has to be matched to the flow and pressure required to deliver the desired discharge.

When evaluating inlet conditions and how they affect pump performance, it is best to look separately at pumping from the tank, pumping from a pressurized water source and pumping from a draft. For each water supply condition, the discharge flows required for your operations should be compared to the available water supply capabilities.

A Good Base Line

Most departments operate at intake and discharge conditions that are different than the ratings in the National Fire Protection Association’s 1901 apparatus standard. So asking for a specific NFPA pump rating is just a good base line. The goal is to make sure that what you really need to do the job can be achieved.

Tank-to-pump performance guidelines include a range of considerations.

The performance required when operating from the tank is calculated by adding the individual discharge line flows that will be used simultaneously during tank pumping operations. The performance that can be achieved is based on the pump size, the pump-to-tank plumbing design and valve and tank design.

The Tank-To-Pump Line

The design of the booster tank is critical to the tank-to-pump flow performance. An improperly designed tank sump, anti-swirl plate, tank vent system or baffle system can reduce the achievable pumping performance. Be very specific in your specifications as to the performance required from the tank-to-pump line.

As with other piping, the plumbing between the booster tank and the pump suction should be the correct size, as straight as possible and full flow. In general, the higher the pump NFPA rating, the greater the flow that can be achieved through a specific plumbing line design. And the larger the tank-to-pump line, the greater the flow capability. (See accompanying chart for performance capabilities based on straight direct full flow plumbing, proper tank design and a full opening tank-to-pump line check valve.)

Pumping From Draft

Guidelines for pumping from draft include similar considerations.

The performance required while drafting from an open body of water is calculated by adding the individual discharge line flows that will be used simultaneously during drafting operations. Performance is based on the pump size; inlet plumbing design; suction hose type, size and length; vertical lift; intake hose strainer; elevation of site; and any intake valving.

The suction hose couplings should not have blunt straight edges where the coupling meets the inside of the hose. Keep suction hose as short as possible and avoid a hump in the hose because it can create an air pocket.

Use 6-inch suction hose on any apparatus with a pump 500 gpm or larger. The bigger hose will aid pump performance and will allow you to interchange hose on the fire ground. The pump NFPA rating certification, which comes from the pump manufacturer, does not change if you select a larger size suction hose. So you gain performance that can be achievable.

As with other piping, the inlet plumbing should be the correct size, be straight as possible and full flow.

In general the higher the pump NFPA rating, the greater the flow that can be achieved through a specific suction hose set up, given lift or given length. As a pump flows more water, it loses vacuum at the eye of the impeller, thereby reducing the amount of suction lift achievable.

As a good rule, a pump that is rated at a 10-foot lift is capable of approximately 26 to 27 feet of lift at 50 percent of its rated capacity. When a specific performance is required at a specific lift that is greater than the normal NFPA rating, it is best to look at the rating data from the pump manufacturer or contact the pump manufacturer.

Not all suction strainers are created equal, and many are so restrictive they exceed the losses allotted by NFPA 1901. This is especially true at ratings of 1,250 gpm and larger. The popular short barrel strainers are particularly bad. The lowest loss style is a basket strainer that has a radiused entrance into the hose coupling end.

Effect Of Elevation

When operating at any elevation above sea level, performance is reduced. The NFPA rating is based on a 2,000-foot elevation, which is equal to an additional 2.38 feet of lift. An elevation over this will drop the performance below the pump’s NFPA rating. If you have a need to have suction hose lay over 20 feet, the performance will be below the pump’s NFPA rating.

When pumping from a pressurized source, the performance required is also calculated by adding the individual discharge line flows that will be used simultaneously during operations to determine the flow required.

The performance that can be achieved is based on the pump size; inlet plumbing design; supply hose type, size and length; elevation change between the water source and the pump intake; flow and pressure at the pressurized water source; and any intake valving.

The supply hose lay will be the biggest factor that limits the required performance. The friction loss in the hose lay for the required discharge flow must not be so high that, at the required flow, there is less than 40 psi entering the pump intake. To achieve the required discharge performance, it may be necessary to increase the supply hose diameter or add a second hose line.

As with other piping, the inlet plumbing should be the correct size, be as straight as possible and full flow.

Additional Capacity

In general the higher the pump NFPA rating, the greater the flow that can be put through the pump. In some cases as much as 50 percent additional capacity can be achieved. A 2,000-gpm pump could flow 3,000 gpm if the water intake supply is adequate and the engine power is available.

Every 2.31 feet of elevation change requires an additional 1psi to compensate.

Remember to specify the performance you expect at each discharge and match the discharge requirements to the suction intake layout and design to maximize the performance of your next pumper.

Editor’s Note: Gary Handwerk is global pump product manager for Hale Products. He has been involved with the fire service industry for more than 36 years, working for various fire apparatus and pump manufacturers. He has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for more than 15 years.

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