By Jeff Welle
Every pump operator needs to know how to draft, but few do so confidently. Drafting failures are usually the result of operator error, because of a lack of proficiency, but confidence is easily attained with training. With a little practice, an average crew (two to three firefighters) can set up a three-tube draft in less than three minutes.
Drafting is not totally reserved for rural areas. An existing fireground supply from a hydrant can be augmented by adding another LDH line and engine drafting from a nearby static source. This allows the draft pump's potential to be exploited. A pump will be more efficient from a static source since there is no hydrant restriction.
Drafting is relatively simple when a few rules and limitations are considered. Correlating lift, vacuum, atmospheric pressure, and the area's elevation can be confusing. These drafting considerations are important; however, you may be confident that a reasonably placed pump at a static water source, located anywhere on this earth, will produce water. The pump may have limitations related to lift, but it should produce water.
When a pump doesn't produce water, it usually isn't the pump; it's the setup. The pump is a predictable component. The real question is: Will there be enough atmospheric pressure to overcome what the pump operator believes is a "reasonable hard tube setup?"
The pump's dependable lift is the height that water can be expected to rise upward from a static water source and provide a continuous reliable fire flow. Learn the area's dependable lift expectations through practice. Realistically, the highest lift an operator would want to overcome is about 10 feet, and the lift limitation becomes greater as the area's elevation increases. More importantly, higher lift requirements will reduce the quantity of water available to the pump; therefore, high lift attempts are counterproductive. Get the pump as close to the source as possible, preferably requiring less than 10 feet of lift.
Don't confuse a 10-foot lift with the tube distance. The water source could potentially be blocked by 30 feet of brush, requiring a multiple tube operation. A multiple-tube operation does create some additional resistance for the pump; however, the lift requirement is the critical consideration. High spots and dips in the tube can trap air and inhibit operational efficiency. Avoid bottom feeding and silt suction by using a floating strainer or rope to hold the barrel strainer off the bottom of the static water source. Conversely, ensure that the strainer makes it to the bottom deep end of a swimming pool.
The primer evacuates air from the pump and hard tubes as atmospheric pressure forces water towards the impellers. From primer activation until the pump creates pressure should take no longer than 30-45 seconds. If the pump is unable to produce water in this time, the setup should be suspected and evaluated. If it is an initial engine placement error related to the lift requirement, the engine must be repositioned. The hard tube/pump setup with a vacuum leak is easily isolated and corrected. The problem will be in the hard tube connections, open drains/discharge, or even the tank-to-pump valve.
Centrifugal pumps like back pressure. Avoid back pressure fluctuations by operating valves slowly and smoothly. Back pressure fluctuations are arguably the most common reason for loss of draft. Opening a discharge valve too quickly will allow air from the supply hose into the pump, pushing the water away from the pump and back down the tube. The key to successful drafting is opening the discharge valve slowly until a solid water flow/back pressure has been established in the line being supplied. Attempting to rush this aspect of the operation will ultimately lead to loss of prime.
An engine can be equipped with a second separate primer unit specifically for the front suction. This option gives the pump operator the ability to accomplish the initial draft and water flow. When time permits, the front intake can be placed in service with another set of tubes to increase flow without disturbing the initial water distribution. Providing the pump with another pathway to introduce water will reduce the operation's restriction and increase flow potential. Drafting with an engine using two large intakes will produce more water than the pump manufacturer's rating. This resistance reduction can also be applied to portable pumps using an intake adaptor and larger tube; this enhances the portable pump's flow.
If the pump cavitates at the static source, the pump operator is simply demanding more water than the pump can deliver with that particular setup. The pump and hard tube setup has restrictions such as pump intake piping, tube size/length, and lift. Cavitation is simply an imbalance between supply and demand and is one of the best indicators of the drafting operation's limit. Briefly taking the pump somewhere near cavitation will determine the operation's capability. A supply pump's prolonged cavitation situation will have a global fireground impact.
Required equipment on a drafting vehicle should include 2,500 to 3,500 feet of LDH, a variety of water pick-up appliances, and the necessary adaptors needed to work with surrounding engines and hose. A draft engine should carry four to five tubes. A three-tube operation is about the most restriction a pump will want to encounter from a single large intake. However, the extra tubes allow the pump operator to place two large intakes in service simultaneously to enhance the pump's flow rate ability. The extra tubes can also be used for open relays, portable pumps, or for another engine's operation.
Ladders, tarps, and pike poles are used for water retention. Retaining the flow of even a small stream and using a low-level strainer can produce a surprisingly acceptable supply rate. An LDH drafting-specific vehicle will also have a chainsaw for cutting ice or small trees inhibiting access and a portable pump capable of respectable short distance LDH flows. The portable pump flow can supply a Class A Pump from a remote static source. An engine with a large hosebed can easily house 3,000 feet of five-inch hose. That's what makes drafting vehicles unique; they're built with a specific purpose. The draft-specific vehicle will have a large hosebed, a short wheelbase, and piping that reduces resistance.
Creating a water preplan book with distances from static sources is critical. This invaluable information will determine the draft operation's potential before the engine leaves the station. Drafting practice and observation of a few guidelines will assist the firefighters in becoming confident in their drafting operation skills. Don't pull higher than 10 feet of lift and don't use more than three tubes together. These considerations dictate the initial engine placement. When water is pulled to the pump, open the discharge gate valve slowly until water flow/back pressure has been established and pump pressure fluctuation has subsided.
Any firefighter who practices drafting operations will become more confident in his skills, and will develop a greater tactical awareness for supply operations on the fireground. Good officers come from strong pump operators.
Jeff Welle is a career paramedic, firefighter, and registered nurse. For more information on drafting, LDH relays, and easy fire ground hydraulics, please visit the Web site: hydraulics4jakes.com