Table of Contents
A brush truck rolls toward a grass fire with a brush truck skid unit rated at 100 psi and 50 gpm. The crew lays 300 ft of 1 1/2 inch hose up a slope. Published fire-service friction loss coefficients put that lay near 6 psi per 100 ft at 50 gpm. At 100 gpm the same lay burns 72 psi before the tip sees water. The nozzle dribbles, the tank still drains, and the fire keeps moving. Every number in that failure was chosen months earlier, on a purchase order, for a skid nobody flow-tested.
A fire skid unit is a pump, tank, hose, and nozzle chain on a frame that one vehicle carries or tows. The chain holds only as well as its weakest sizing step. This guide walks the whole chain for wildland fire skid units. It covers drive selection, tank math, hose delivery, foam pickup, and the acceptance test that proves the system before the call. If your hazard is fixed rather than mobile, the fire protection nozzles guide covers deluge, monitor, and building patterns instead.
The Snapshot
- Flow needs an orifice: the relation gpm ≈ 29.83 × d² × √P puts 100 gpm at 100 psi through a 0.58 inch orifice. A 0.50 inch tip flows 74.6 gpm under the same head.
- Duration is arithmetic: a 300 gallon fire skid unit at 50 gpm delivers 6 minutes. At 100 gpm the same tank empties in 3 minutes, and then the crew is moving water by hand.
- Power is small but real: 100 gpm at 100 psi is about 4.3 kW hydraulic, so a pto driven water pump needs roughly 6.6 kW at the shaft once 65 percent efficiency is counted.
- The class is standardized: NFPA 1906 covers slip-on fire fighting packages with pumps from 10 to 500 gpm and tanks of 50 gal or more. NWCG engine typing then sets the minimum tank and pump figures for type-rated trucks.
Why Mobile Skid Systems Fail On The Hill
Fixed fire systems fail when demand exceeds design. Mobile systems fail the same way, one step earlier, because the operator picks the duty in the field and the vehicle sets the payload.
Three failure families repeat across the market. The pressure deficit: friction in a long, small hose lay eats the pump’s whole output before water reaches the tip. The volume deficit: the tank is sized for what fits, not for the minutes the nozzle needs under load. The duty deficit: the drive cannot hold pressure while the truck moves, or it overheats at the rpm the job demands.
A fourth risk shows up on wildfire skid units and never on fixed systems, and that is contamination. Drafting from a creek, a portable tank, or a mine sump pulls grit, sand, and organic debris into the pump and the orifices. A skid that passes its shop test can clog on the hillside within minutes. Fire fighting skid units that draft regularly therefore need bore sizes and passages that tolerate dirty water.
The Sizing Chain From Pump To Nozzle
Every mobile water system is one chain: source, pump, drive, hose, appliance, nozzle. Each link passes a number to the next, and the nozzle decides what the chain must deliver.
The orifice is the entry point for all of it. For water, gpm ≈ 29.83 × d² × √P, where d is orifice diameter in inches and P is pressure in psi. Substitute d = 0.50 in and P = 100 psi. The square root of 100 is 10, d² is 0.25, and Q = 29.83 × 0.25 × 10 = 74.6 gpm. Work the same equation backward for a target. To flow 100 gpm at 100 psi, d² = 100 ÷ (29.83 × 10) = 0.335, so d = 0.58 in.
Pressure changes move flow by the square root rule, GPM₂ = GPM₁ × √(P₂ ÷ P₁). A tip rated 30 gpm at 100 psi needs a real 100 psi at the nozzle. Run it on a skid that makes only 50 psi, and the flow becomes 30 × √(0.5) = 21.2 gpm. The orifice does not change and the pattern angle does not change. Only flow and effective reach fall.
The metric form of the same physics is Q (L/min) ≈ 0.65 × d(mm)² × √P(bar). Take a 14.7 mm orifice at 6.9 bar, which is 100 psi. Then 0.65 × 216.1 × 2.63 = about 369 L/min, and 369 ÷ 3.785 = 97.5 gpm. The two forms agree within rounding, so one rating table serves both unit systems. For the K-factor treatment behind nozzle ratings, see spray nozzle flow rate calculation.
Choosing The Drive: PTO, Engine, Or Hydraulic
The drive sets the flow ceiling more than the pump body does. Three arrangements dominate skid work, and each one fails in a different way.
A pto driven water pump takes power from the engine accessory drive or a transmission power take-off. It produces rated flow only near rated engine rpm, so pump-and-roll work needs a control arrangement that holds pressure at working road speed. NFPA 1906 devotes attention to pump-and-roll riding positions and speed limits for exactly this reason: the operating mode is riskier than stationary pumping.
A dedicated engine on the skid decouples the pump from the truck. That matters when the truck must reposition while the line stays charged. It also matters when the skid is a trailer drafting at a site with the tow vehicle parked. A skid unit for fire truck chassis duty cannot always free the engine, so the accessory drive and its clutch take the load instead.
Hydraulic drive is the third route. A hydraulic motor fed from a UTV, tractor, or truck auxiliary circuit needs no mechanical coupling. That arrangement simplifies mounting and lets the skid move between machines. Utility vehicles use this route most.
Sizing the drive follows one equation, kW = Q(L/min) × P(bar) ÷ 612. Take 250 gpm at 100 psi. The flow is 250 × 3.785 = 946 L/min, and the pressure is 100 ÷ 14.5 = 6.9 bar. Then 946 × 6.9 ÷ 612 = 10.7 kW at the water end. Pump efficiency takes a further cut: at 65 percent, the shaft demand is about 16.4 kW, or roughly 22 hp. A 100 gpm unit needs about 4.3 kW hydraulic and near 9 hp. On a utv fire skid unit, a circuit that spares only 1 to 2 kW limits the pump to about 25 gpm at 100 psi.
Tank Volume Is A Duration Setting
Water weighs 8.34 lb per gallon. A 300 gal tank therefore carries 300 × 8.34 = 2,502 lb of water, or about 1,136 kg. That is before the tank shell, pump, plumbing, and frame. The loaded skid mass sits on a chassis with a payload and center-of-gravity rating. That rating is what limits tank size on the smallest platforms.
Tank volume is the hard cap on fire skid units for utv duty. Published cargo ratings for utility side-by-sides commonly fall between 400 and 1,500 lb. With a pump and foam tank aboard, that caps realistic water weight at roughly 50 to 100 gal. The rest of the job has to come from drafting, relay, or a tender, and the nozzle flow has to respect that reality.
Duration is volume divided by flow, and the nozzle sets the divisor. A 300 gallon fire skid unit flowing 50 gpm runs 6 minutes. On a smooth-bore tip sized for 100 gpm, the same tank runs 3 minutes and is dry. NWCG typing bakes this tension into the classes. A Type 7 vehicle carries a 50 gal minimum tank and pumps 10 gpm at 100 psi, which is a 5 minute duration by design. A Type 6 carries a 150 gal minimum at 50 gpm, which lands at 3 minutes. Refill logistics, not tank size in isolation, set the working cycle.
Duty Classes At A Glance
Use this matrix to place a platform before selecting a pump. Rows marked as minimums come from NWCG engine typing; the other values are typical for the class in published equipment literature.
| Duty class | Tank volume | Pump flow at pressure | Drive | Constraint |
|---|---|---|---|---|
| UTV skid | 25 to 100 gal | 5 to 25 gpm at 50 to 100 psi | hydraulic or electric | payload and cargo rating |
| Brush truck, pickup chassis | 150 to 500 gal | 30 to 150 gpm at 100 psi | PTO or on-board engine | NWCG Type 6 minimum is 150 gal and 50 gpm |
| Slip-on module | 200 to 400 gal | 100 to 250 gpm at 100 to 150 psi | dedicated engine | NFPA 1906 pump range is 10 to 500 gpm |
| Trailer skid | 200 to 500 gal | 50 to 250 gpm | towed, engine driven | draft or relay duty site |
| Type rated engine | 500 to 1,000 gal | 150 gpm at 250 psi minimum | PTO | NWCG Type 3 minimum is 500 gal and 150 gpm |
Each row carries its own arithmetic. In the pickup class, brush truck skid units usually land at 30 to 150 gpm and 150 to 300 gal. That is a duration window of 2 to 10 minutes, depending on the tip. The right skid unit for brush truck work is sized on one question: how many minutes must the nozzle run between refills? Every other number follows from that.
Strainers, Pickup Lines, And Priming
The pickup side of the chain decides whether the pump ever sees clean water at the right pressure. A skid that drafts needs a strainer sized so its open area does not throttle the suction line. It also needs a suction hose stiff enough to resist collapse under vacuum. A hose that flattens under lift starves the pump and shows up as cavitation noise and lost pressure.
Priming is the next gate. A pump mounted above the water line will not lift on its own unless the body is self-priming or fitted with a primer circuit. Published practice for portable skid pumps covers wet-priming rotary designs and self-priming centrifugal pumps. Drafting from a static source favors a self-priming or rotary pump, while a tank-fed skid can use a standard centrifugal pump with a foot valve.
The strainer is also the cheapest defense for the orifices downstream. Debris that passes a coarse screen will lodge in a narrow tip, and a partly blocked orifice changes the whole pressure balance of the system.
Hose Lay Decides What The Nozzle Sees
The nozzle receives only what is left after the hose takes its share. Published fire-service friction loss coefficients put 1 1/2 inch hose near a coefficient of 24. Flow is expressed in hundreds of gpm and length in hundreds of feet. At 50 gpm the loss is 24 × 0.5² = 6 psi per 100 ft. At 100 gpm it is 24 × 1.0² = 24 psi per 100 ft. At 150 gpm it climbs to 24 × 1.5² = 54 psi per 100 ft.
Scale those numbers to a real lay. A 300 ft line at 50 gpm costs 18 psi, which a 100 psi skid pump absorbs easily. At 100 gpm the same line costs 72 psi, and the pump then needs that plus nozzle pressure to hold rated flow. A 1 1 2 fire hose nozzle, the 38 mm handline class, typically wants 50 to 100 psi at the tip. The pressure balance rarely closes on a long small-diameter lay, and the symptom is a soft stream that looks like a nozzle fault.
Two levers fix it without changing the pump. Move the nozzle closer, or move up in hose diameter and accept the weight. A 1 3/4 inch line at the same 100 gpm loses about 15.5 psi per 100 ft. That cuts a 300 ft loss from 72 psi to about 47 psi. For compact handlines on a skid, the pattern and fitting decisions are covered in the hose nozzle selection guide.
Foam Pickup And Mixing At The Eductor
Class A foam is proportioned at published rates around 0.1 to 1.0 percent by volume. For wildland wet lines and mop-up, 0.3 to 0.5 percent is common. A venturi style foam eductor draws concentrate in proportion to solution flow, and the ratio holds only inside the eductor’s rated flow window. Fit a unit rated for 100 gpm, then run 40 gpm through it. The pickup draws rich or lean depending on the nozzle and pressure at the time.
The mixing arithmetic is direct and worth doing before a purchase. At 0.5 percent and 100 gpm of solution, concentrate draw is 100 × 0.005 = 0.5 gpm, which is about 1.9 L/min. A 5 gal concentrate jug therefore lasts 10 minutes at that setting and 5 minutes at 1.0 percent. A 300 gallon tank empties in 3 minutes at the same flow, so the concentrate outlasts the water. The foam budget is not the limiting factor on a small skid. Tank capacity is.
NFPA 1906 requires proportioning systems on wildland apparatus to be type tested, and each installation to be individually tested after it is built. Copy that model on any skid: verify the pickup rate at the working flow, not just the rated flow. For small tanks and low-flow pumps, how an eductor works covers the same physics at a smaller scale.
Water Curtains And Cannon Duty
Some skid jobs are not fire attack at all, and sizing them like an attack line wastes water. Where a job calls for water curtain fire protection between a burning brush edge and an exposure, the goal is coverage, not penetration. Wide sheets at low pressure do that work.
Coverage follows the fan geometry, width ≈ 2 × distance × tan(θ/2). A 65 degree fog tip at 3 m covers 2 × 3 × tan(32.5°) = 3.8 m of curtain. A 30 degree cone at 25 m covers 2 × 25 × tan(15°) = 13.4 m, which is monitor work rather than handline work. Overlap of 10 to 15 percent between passes keeps the sheet continuous, and that overlap is what usually gets forgotten on a moving vehicle.
Dust suppression water cannons share the same hardware envelope but a different droplet target. Larger droplets resist wind and carry to the pile, so wide angle and full cone patterns at moderate pressure beat fine fog for knockdown. A roadside or mining version of a fire skid often wants 100 to 250 gpm at 50 to 100 psi. That sits inside the NFPA 1906 pump range but at the top of the duration curve. Tip strategy for that duty is covered in the dust suppression nozzles guide.
Where the same skid also protects a structure, purpose-built sprinkler nozzles on the vehicle or the roof run from the same pump. That is the bridge back to fixed fire protection nozzles, which cover deluge and monitor design for buildings rather than trucks.
Tanks, Plastics, And Materials Life
Tank material is a weight and chemistry decision as much as a corrosion decision. Plastic fire skid tanks, rotomolded in polyethylene, cut tare weight and resist corrosion. That matters on a skid that drafts from creeks, ash, and mine water. Steel and aluminum shells take impact better and hold shape in a rollover. They need coating inside and out, and they add mass on a chassis already paying for water.
Whatever the shell, the tank needs baffles to control surge. A 2,500 lb water mass that shifts on a curve changes how a light chassis handles. Unbaffled tanks are a known cause of loss of control on modified vehicles.
Nozzle materials follow the water quality. Brass bodies and stainless steel orifice inserts hold bore size against abrasive grit longer than plastic. A tip that keeps its bore keeps its flow. Flow is the number the entire system was sized around, so a worn tip is a system fault rather than a nozzle fault. An orifice that reads 8 percent oversize flows (1.08)² − 1 = 0.166, or about 16 percent more water. That shortens every tank interval in the plan.
Fittings deserve the same care. NFPA 1963 standardizes threadless coupling families and hose appliance threads, which is what lets a skid discharge mate with other departments at a multi-agency fire. Carry adapters for the thread families in your response area, and inspect them at every service.
Between seasons a skid tank is just another tank. Cleaning residue from the sump and the walls uses the same rotary heads as any process vessel. A tank cleaning nozzle rated for the tank volume keeps the pickup strainer clear and the water honest.
Common Sizing Mistakes
Most fire skids leave a shop carrying at least one of these faults, and each check is cheap on paper.
| Mistake | What happens | Check that prevents it |
|---|---|---|
| Nozzle chosen before the pump | Rated flow is never reached and the stream looks weak | Size the orifice with 29.83 × d² × √P against the real pump curve |
| Tank sized by what fits under the deck | Duration falls under 3 minutes under load | Divide tank volume by design flow and require 5 minutes for initial attack |
| Engine drive assumed to cover PTO duty | No pressure while the truck repositions | Confirm pump-and-roll capability and the NFPA 1906 position rules |
| One hose size for every lay | Friction loss eats the tip pressure | Compare loss = 24 × Q² per 100 ft in 1 1/2 inch hose against the lay length |
| Eductor run outside its rated window | Foam ratio drifts and the wet line fails to knock down | Test the pickup with a graduated cylinder at the working flow |
| Orifice left to wear until it fails | Over-delivery drains the tank early | Inspect tips seasonally and replace at the wear limit |
Structural firefighting skid units and wildland firefighting skid units share this table but weight the rows differently. A structure-focused build favors pressure and reach, while a brush unit favors duration. Writing the priority order down before buying keeps the arithmetic honest.
Verification Before Acceptance
A skid unit is accepted on measurements, not on the sum of its parts. The checks below cover the links that actually fail in service.
- Timed flow test: run the pump against a marked tank level and confirm the draw matches rated flow within 5 percent. At 50 gpm a 300 gallon tank should drop 50 gal in one minute.
- Tip pressure check: fit a gauge at the nozzle, not just at the pump, and record pressure with every pattern the skid carries. A 20 psi discrepancy between gauges points at the lay, not the pump.
- Orifice inspection: measure the bore of each tip against its rated diameter and set a wear limit. A 5 percent oversize hole already flows (1.05)² − 1 = 10 percent more water than it should.
- Foam pickup test: run the eductor at the working flow and measure concentrate draw with a graduated cylinder.
- Suction and priming check: lift from a static source at the deepest expected draft and verify the pump primes and holds without cavitation noise.
- Coupling and adapter audit: verify each discharge against NFPA 1963 threadless coupling families and the thread families used locally.
Record the numbers in the vehicle file. Flow drift between seasons is the earliest sign of a worn orifice or a tired pump.
FAQ
How do I match a pump to the nozzle on a fire skid unit?
Start from the nozzle the duty needs, then work backward. If the job calls for 67 gpm at 100 psi, solve 29.83 × d² × √P = 67 for the bore. Then check the pump curve at that flow and pressure. A pump rated 100 gpm at 100 psi cannot hold that flow at 250 psi, and the curve shows the shortfall before you buy.
Is a pto driven water pump better than a portable engine on a brush truck?
It depends on whether the truck must move while the line is charged. A PTO pump runs from the engine and supports pump-and-roll work. It also loses flow at idle and loads the truck engine. A portable engine keeps the skid independent, which suits trailer duty and static drafting.
How long will a 300 gallon fire skid unit actually spray?
Divide the tank by the flow. At 30 gpm it runs 10 minutes, at 50 gpm it runs 6 minutes, and at 100 gpm it runs 3 minutes. Choose the tip for the duration the tactic needs, then confirm the pump can hold that flow at the tip pressure the pattern requires. Duration is a nozzle decision as much as a tank decision.
Can one skid unit run foam, a water curtain, and dust suppression?
Yes, if the pump envelope covers every nozzle you hang on it. Class A pickup at 0.5 percent needs a stable solution flow. A water curtain needs wide fog at 50 to 100 psi, and dust suppression water cannons want larger droplets at moderate pressure. Change tips and appliances for each duty rather than changing pumps.
Do plastic fire skid tanks hold up in wildland service?
They hold up well against corrosion, which is why plastic fire skid tanks dominate the UTV and trailer classes. The limits are UV exposure, abrasion from sliding loads, and impact in cold weather. Keep them shaded or lined, baffle them against surge, and inspect the shell and fittings at the start of every season.
The Bottom Line
A skid that cannot make pressure is a tank of dead weight on a hillside. Orifice, drive, tank, lay, and pickup line must all close on paper before anyone closes a purchase order. Nobody redoes arithmetic at the fire line.
BoreJet supplies the nozzle end of that chain. That means flow-tested tips for skid pumps, wide angle models for curtains, and eductor hardware for Class A foam. Send the pump curve, the tank volume, and the duty you actually work, and the sizing comes back with every link closed. Start with the contact form and we will review the whole chain with you.
Next Step
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Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.