No matter what pump you choose or what type of apparatus you choose to have built, the intake and discharge manifolds, piping, and valves will affect operational performance.
National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, and NFPA 1906, Standard for Wildland Fire Apparatus, only go so far and do not always provide the maximum performance possible from the pump and drive system selected.
|1 This is a 5,250-gpm NFPA 1901 rated pump installed on a 600-hp custom pumper being tested using four sets of eight-inch suction hose. (Photos by author.)|
Obtaining the maximum available performance is critical on high-flow applications such as industrial or other large property fires but is also important at the extreme opposite end of the market on slip-style wildland/grass apparatus where the engine driving the pump is very small, without the extra power to overcome the losses in the plumbing.
The current NFPA standards reflect historical conditions, situations, and technologies and at best reflect the commonly used state-of the-art technologies. This safely covers most apparatus being built but may not always provide the optimum performance opportunities available from the pump and engine selected. This is the first of two articles that look at intake and discharge systems on most types of fire apparatus. They will look at the current standards, current practices, and what would work better for many fire departments to optimize performance.
Intake performance when drafting is limited to the atmospheric pressure available to push the water into the pump. Losses in the suction hose and intake manifold system consume some of that atmospheric pressure. The lower the losses, the more performance can be derived from the pump. When you consider that atmospheric pressure can be no more than 14.7 pounds per square inch (psi), even small losses make a big difference. Even when pumping from a pressurized source, losses in the intake hose and manifolds are robbing potential performance.
The suction intake systems, as we know them, are based on using the smallest possible suction hose to get the minimum performance. Why? Because we used hard black rubber, very heavy suction hose for decades, and 4½-inch hose was much lighter than a five-inch let alone six-inch suction hose. Today we have lightweight hose and couplings that are easy to handle and deploy. The hose also bends better for easier setup. So, the size hoses we are used to for a given size pump could be looked at in a different light. Even eight-inch modern hose is easier to handle than the old six-inch, black, hard suction we used for decades.
As a pump designer, I look at the velocity of the water traveling in the pipe, waterway, or hose as a reference point in any evaluation of this type. The water speed is commonly measured in feet per second. To calculate this, I use the following formula: Velocity in feet/second = [0.32 x gallons per minute (gpm)]/the area of the waterway in square inches.
Further study of pump and system design books and various industrial standards reveals that 12 feet per second is hydraulically the ideal maximum design speed in an intake waterway/hose system. So, let’s look at what is commonly purchased in the fire service and what the minimum NFPA 1901 requirements are. The lower the velocity number, the fewer restrictions exist in the setup (chart 1).
Why Larger Hose?
What NFPA 1901 stipulates as far as suction hose size for a given pump rating is the minimum suction hose size and quantity. This leaves the apparatus builder and the fire department free to use a larger size suction hose or more suction hose sets to achieve the performance needed. The following is a list of reasons larger or more hose sets could be specified:
- Pumping at an altitude of greater than 2,000 feet.
- Compensating for restrictive manifolding systems, usually caused by the pump’s location in relationship to the main intake connection location. A good example is a typical 500-gpm PTO-driven pump installation where the pump is below the truck frame, which needs a drop tee to get the side main suction intake down to the pump inlet.
- Compensating for intake gate valves.
- Achieving maximum drafting performance beyond the normal pump rating.
- Compensating for higher than normal suction lift.
- Standardizing on a common suction hose size. Typically this lets you use six-inch hose on all your apparatus with 750- to 2,000-gpm pumps. This maximizes the six-inch hose available on the fireground for an extended horizontal lay, sometimes as long as 1,000 feet.
If you study chart 1, it is easy to see that some of the industry’s most popular ratings have a very marginal suction hose situation; 1,500 gpm in particular has a very high velocity in the hose. From a pump engineer’s viewpoint, this is far from ideal. This also explains why many 1,500-gpm pumps, which meet the full NFPA suction conditions with a single hose set, are identical to some 2,000-gpm pumps. It also explains why some 1,500-gpm pumps need twin suction hose sets to achieve the full NFPA suction conditions.
|2 A very advanced design for a low-lose, low-turbulence midship suction manifold designed for large NFPA 1901 pumps rated from 5,000 to 6,000 gpm. Many of the design features of this manifold would apply to pumps in the 1,500- to 2,500-gpm rated sizes.|
Another issue in the current NFPA 1901 is the omission of fire pumps of greater than 4,000 gpm and 12-inch suction hose. The standard doesn’t stipulate a maximum size on either the pump or suction hose, but there is a lack of guidance on suction hose size and quantity for these larger pumps along with the related hose loss information. Until recently, there were no practical options for a fire pump of greater than 4,000 gpm. Today there are NFPA 1901-compliant fire pumps up to 6,000 gpm.
An updated, more inclusive, hose chart would look like chart 2.
In addition to matching the optimum suction hose package for your application, selecting the pump mounting and suction inlet locations will also play a part in reducing or optimizing the potential drafting performance available for a given pump.
The most common installation is the traditional midship location. This has the disadvantage of requiring the water to turn 90 degrees to enter the impeller eye. In actuality, there is more going on than just turning the water-there is actually a rotational process going on that needs to be controlled to maximize its effects. Manifold design that keeps the water velocity as low as practical is very important.
The pump being rear-mounted is the optimum configuration, especially if there is only one main direct suction inlet. If there are multiple main suction intake connections, care must be taken to combine the multiple water inlet flows in a manner that doesn’t create water swirl that enters the impeller eye. Additionally, the combined flow of all the inlets must be taken into account when sizing the main suction manifold.
Small, PTO-driven midship-mounted pumps are the hardest to optimize. The pump is usually mounted below or in between the chassis frame, which winds up having two 90-degree water turns that add to the system losses. Oversizing the manifolds, controlling the water as it transitions from the inlet connection to the eye of the pump impeller, and controlling the prerotation of the water as it enters the impeller eye are key to maximizing the pump’s performance.
Chart 3 indicates manifold sizing guidelines to optimize performance. Additional manifold sizing design rules for optimizing performance include the following:
- When making a transverse log manifold to accommodate multiple suction intake connections, the cross tube inside diameter (ID) must be at least one size larger.
- Having multiple large intake connections entering the main pump suction manifold just forward of the impeller eye requires the main manifold ID to increase one size.
Design rules for transitions, turbulence, and prerotation control in intake manifolds include the following:
- The transition from one ID to another must be made with a concentric-type adapter for either increasing or decreasing changes. Never create an abrupt change in size.
- Never use a square cross-section manifold. The corners will never see the correct velocities and can cause added flow losses or turbulence.
- Weld seams and pipe joints must be aligned and smooth. Ridges and edges will cause turbulence and disrupt the laminar flow of the water.
- Elbows are your enemy. Use as few as possible; increase pipe ID when practical.
- Baffling at the areas where water must turn or be directed may be desirable but may also cause more loss than they cure.
- The water entering the eye of the impeller should be accelerating. This is commonly done by reducing the main intake manifold ID gradually as the water gets closer to the impeller eye.
- The water entering the eye of the impeller will start to rotate along with the impeller. This is called prerotation and will cause a reduction in performance and in some cases cause a form of cavitation. The best way to control this phenomenon is to add baffles just forward of the impeller eye.
Design rules for proper priming of an intake manifold are as follows:
- Always have the primary ¾-inch primer tap as close as practical to the eye of the impeller on top of the manifold or pump outboard head.
- Have auxiliary primer lines running to any high points on the intake manifold and piping if the point is higher than the main priming port. The auxiliary lines should reduce in size as the points become closer to the main water intake point, starting with ½ inch and going down to 3⁄8 inch.
- On pumps more than 2,500 gpm, consider using multiple primer pumps to speed up the priming time.
What does all of this mean for the fire department ordering a new apparatus? Consider using the largest practical suction hose-it doesn’t matter if you are drafting or operating from a pressurized source. Look at your overall operation and other apparatus in the fleet and evaluate what you deal with in mutual-aid situations. Is there an opportunity to standardize on hose and connection sizes, over time, as new apparatus are put into service to optimize overall performance and flexibility? Do not be afraid to use eight-inch suction hose if you plan on doing high-flow drafting applications. Specify dual primers on any pump larger than 2,250 gpm.
Evaluate the potential bidders for your next apparatus on their intake manifolding designs. Also talk to potential pump manufacturers for their recommendations. Finally, adjust your department’s standard operating procedures to take advantage of the new apparatus’s potential.
GARY HANDWERK has been in the fire equipment industry for more than 43 years and has worked for various fire apparatus or fire pump manufacturers, holding positions in engineering or product management. He is president of US Fire Pump Co. He has been active with NFPA 1901, 1906, 1911, 1912, and 1925 for more than 25 years.