Engine Operations›Relay Pumping
The water is too far from the fire for one pump to reach it.
That is the whole trigger. Not a bigger fire — a longer distance.
“During some incidents, the water source may be found remote from the fire scene. In these instances, relay pumping may have to be employed.”
Engine Company Operations, Water Supply, p.5-14.
Two or more pumpers in series. Discharge of one feeds the intake of the next.
Each pump takes what the pump behind it hands over and puts the pressure back on top. The line never sees one pump doing all the work.
“Using two or more pumpers to move water over a long distance by operating them in series; water discharged from one pumper flows through hoses to the inlet of the next pumper, and so on.”
The manual then describes the chain end to end: a pumper at the water source pumps under pressure through one or more supply lines to a pumper further down the line; that pumper boosts the pressure to supply the next, and so on until water reaches the attack pumper.
Engine Company Operations, Water Supply, p.5-14.
A relay is rehearsed, not improvised.
The manual puts pre-planning, training and coordination in the definition itself. The night you need one is not the night to work out the spacing.
“In order to be effective, relay operations require pre-planning, training, and coordinating of all participants.”
Engine Company Operations, Water Supply, p.5-14.
Orem runs one method: the Constant Pressure Relay.
Everybody in the middle holds the same pressure. You are not solving a fresh hydraulics problem at every rig.
“Orem Fire Department uses the Constant Pressure Relay method for establishing hydraulic pressures and distances during relay operations. For more specific information see the current APPARATUS DRIVER / OPERATOR HYDRAULIC CALCULATION STANDARDS in Target Solutions.”
Engine Company Operations, Water Supply, p.5-15.
Source pumper — biggest pump goes on the water.
Takes water from the hydrant or the static source and pumps it under pressure to the next apparatus. It sits on the “key” hydrant. The manual also calls this the water supply pumper.
“Water supply pumper — Pumper that takes water from a hydrant or static source and pumps it under pressure to the next apparatus in the relay pumping operation. Water supply pumpers should be the apparatus with the largest pumping capacity. Some jurisdictions refer to water supply pumpers as source pumpers.”
Engine Company Operations, Water Supply, p.5-14. The “key hydrant” placement is from the Constant Pressure Relay card, p.5-15.
Relay pumper — catches pressure, raises it, hands it on.
It can be a smaller pump than the source, because it is not starting from zero. It inherits the energy the pumpers behind it already put into the line.
“Relay pumper — Pumper or pumpers connected within the relay that receive water from the source pumper or another relay pumper, raises the pressure, and then supplies water to the next apparatus. This pumper may be of smaller capacity due to its ability to use the acquired energy of previous pumpers in the relay.”
Engine Company Operations, Water Supply, p.5-14.
Attack pumper — the only rig doing fireground math.
It receives from the relay and supplies the attack lines and appliances. Everyone behind it holds a fixed number; this one sets pump discharge pressure for the lines it is feeding.
“Fire attack pumper — Pumping apparatus located at the fire scene that receives water from the relay and is responsible for supplying the attack lines and appliances required for fire suppression.”
The Constant Pressure Relay card places it at a forward “key” attack position, and gives it the one variable job: “Attack Pumper — adjust PDP as needed making sure to dump excess pressure.”
Engine Company Operations, Water Supply, p.5-14 (roles) and p.5-15 (positions and pressure); Apparatus Driver/Operator Hydraulic Calculation Standards, Constant Pressure Relay.
Put the attack pumper in first.
The relay is built backwards from the fire. Where the attack pumper parks decides where every other rig lands.
“Step 1, Position Attack Pumper”
Apparatus Driver/Operator Hydraulic Calculation Standards, Implementing a constant pressure relay operation; the same card is printed in Engine Company Operations at p.5-15.
Put the source pumper on the key hydrant.
Now you have both ends. Everything else fills the gap between them.
“Step 2, Position Source Pumper at ‘Key’ hydrant”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
Lay the hose. Drop a relay pumper every 750 feet.
750 is the spacing you count out, and it is the same 750 the max-volume table is built on. Change the spacing and the flow numbers below stop applying.
750 ft — even intervals“Step 3, Lay out hose and place Relay Pumpers at 750 foot intervals”
And from the same card, under key positions: “Relay Pumper/Pumpers — Spaced evenly throughout the relay at intervals of 750 feet.”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
Every pumper except the source opens a discharge. Air has to leave before water arrives.
The muscle memory: if you are not on the hydrant, you have an open discharge before the source ever throttles up.
“Step 4, All pumpers except source pumper open a discharge to exhaust air from the lines”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
Source pumper throttles up to 175 psi.
That is the number the whole method is named for. It does not change with the fire, the hose, or the tip.
“Step 5, Source pumper throttles up to 175 psi”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
Wait for a steady stream out of your open discharge. Close it. Throttle to 175.
Steady stream, not a spit. That is the whole cue — it tells you the air is out and the water behind you has arrived. Then the next pumper down does exactly the same thing, and the next, all the way to the attack pumper.
The move, repeated at every relay pumper
“Step 6, 1st Relay pumper closes unused discharge once a steady stream of water flows through it, then throttles up to 175 psi. – All successive Relay pumpers follow the same procedure”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
Every driver/operator sets their intake relief valve.
Every one. Not just the rig at the end of the line.
“Step 7, All Driver/Operators set their intake relief valves”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
Attack pumper adjusts PDP to supply the attack lines.
The relay behind it is now a fixed 175 psi supply. The attack engineer works the normal problem from the nozzle back.
“Step 8, Attack pumper adjusts PDP to supply attack lines. – Maintain water flow during temporary shutdowns by using one or more discharges as waste or dump lines”
Apparatus Driver/Operator Hydraulic Calculation Standards; Engine Company Operations p.5-15.
175psi
Source & relay PDP
Held by the source pumper and by every relay pumper in the chain. The attack pumper is the exception — it sets its own.
20psi
Residual intake, minimum
The 175 figure is built to leave 20 psi residual at the next pumper in the relay. That is the floor you protect.
750ft
Interval between pumpers
Evenly spaced the length of the relay. The flow ceilings below are all quoted at this distance.
175 is a ceiling on distance, not a licence to stretch.
The pressure is fixed, so the only thing keeping water at the next intake is the spacing. Stretch a leg past 750 feet and the residual at the far end is what pays for it.
Under Maximum volume relay pump pressure:
“Source and Relay pumpers — Maintain 175 psi PDP”
“Attack Pumper — adjust PDP as needed making sure to dump excess pressure”
“* PDP accounts for 20 psi residual pressure for the next pumper in the relay”
Apparatus Driver/Operator Hydraulic Calculation Standards, Constant Pressure Relay (Limited Max Volume); printed in Engine Company Operations at p.5-15.
20 psi is the same floor you already hold on Evolutions 1.7 and 1.8.
A two-engine pumped hydrant is a relay with one leg. The residual you are told to protect there is the residual you protect at every intake in a long relay.
Evolution 1.7 (supply pumper) setup: “Residual on the attack pumper should remain above 20psi.” Out-of-cab step 6: “Adjust pump pressure, if needed, to maintain 20 psi minimum residual pressure at the attack pumper.”
Evolution 1.8 (attack pumper) out-of-cab step 4: “Maintain Communications with supply pumper to ensure 20psi residual pressure.” Step 7: “Request additional pressure, if needed, to maintain 20 psi minimum residual pressure.”
ADO—Pumper Task Book, Evolutions 1.7 and 1.8.
Swipe the table sideways.
| Hose layout | One 2½ | One 3 | One 4 | One 5 | Two 2½’s | One 2½ & one 3 | Two 3’s |
|---|---|---|---|---|---|---|---|
| Max flow | 321gpm | 508gpm | 1017gpm | 1607gpm | 643gpm | 830gpm | 1017gpm |
One 5” carries roughly three times what one 3” carries.
1,607 gpm against 508 gpm over the same 750 feet. That single comparison is the argument for laying LDH on a long supply, and it is the number to have in your head before someone asks whether 3” will do.
Read the layout against the tip you are feeding, not against the fire.
The relay does not care how big the fire is. It caps at whatever the hose will pass. Pick the tip first, then check the layout can feed it.
Swipe the table sideways.
| Hand-line tip | gpm @ 50 psi | Master stream tip | gpm @ 80 psi |
|---|---|---|---|
| 7/8” | 160 | 1 3/8” | 500 |
| 15/16” | 185 | 1 1/2” | 600 |
| 1” | 200 | 1 3/4” | 800 |
| 1 1/8” | 265 | 2” | 1,000 |
| 1 3/16” | 300 | ||
| 1 1/4” | 325 |
Flow ceilings: Apparatus Driver/Operator Hydraulic Calculation Standards, Maximum volume at 750 feet by hose layout (printed in Engine Company Operations at p.5-15). Tip flows: same standard, Solid stream / smooth bore tips. The comparison between the two tables is arithmetic on printed figures, not a printed rule.
These ceilings belong to 750-foot spacing.
The card is titled Limited Max Volume for a reason — it is the flow you can hold at the fixed pressure and the fixed interval. Move the pumpers and you are outside the table.
One pumper buys you 750 feet. Distance is a pumper count.
Because the pressure never changes, reach is not something you calculate — it is something you count. Every rig you can put in the line adds another 750 feet.
A mile is 5,280 feet — seven 750-foot legs and change.
5,280 ÷ 750 = 7.04. So a mile of relay is seven legs: the source on the hydrant, six relay pumpers spaced out behind it, and the attack pumper on the end. Ask for the engines before you commit to the lay, not after.
The Constant Pressure Relay card prints both numbers it takes to do this: the 750-foot interval under Key positions in a relay operation, and “1 mile = 5280 Feet” as the last line on the card.
The division is mine, not the manual’s — the card gives you the two figures and leaves you to count. There is no printed maximum total relay length.
Apparatus Driver/Operator Hydraulic Calculation Standards, Constant Pressure Relay (Limited Max Volume); printed in Engine Company Operations at p.5-15.
Distance and volume trade against each other.
The pressure is fixed at 175 psi, so a longer relay is not paid for with more pressure — it is paid for with more rigs. And the flow you get at the end is still whatever the hose layout in the table will pass.
Pumping the hydrant is a two-rig relay by another name.
On the high-flow hydrant list it sits second of four — better than stacking lines off one hydrant, not as good as pulling from separate hydrants.
Least preferred to best preferred:
Note on the page: the lead sentence says “three basic tactics” and then prints four bullets. The four-item ranked list is what is printed.
Engine Company Operations, Water Supply, p.5-13.
At a standpipe building, the 3rd due relays into the 2nd due.
2nd due takes the FDC with two 3” lines off the nearest hydrant. 3rd due comes off the next nearest hydrant and relay pumps into it. That is where the redundancy comes from — two engines in series on one Siamese, not two engines both hooked to it.
“The second Engine Company arriving on the first alarm should take a position at the appropriate FDC standpipe connection and supply two 3” lines into the FDC Siamese and connect to the nearest hydrant. The third arriving Engine Company should relay pump to the second engine company from the next nearest hydrant. This procedure should be continued by the next arriving engine companies if additional FDC connections to the building are available.”
The reason given on the same page: an independent, redundant supply so that advancing crews keep water if a pumper suffers a mechanical failure or the FDC feeds only a single stairwell riser. On a dry system those pumpers are the only water there is.
Engine Company Operations, Standpipe Operations, p.6-16. More on that job: Standpipe Operations.
The bigger the building, the more FDCs — and the more relays.
Large buildings often carry multiple FDC connections on different street fronts. Each one gets the same pairing repeated by the next engines in.
“The larger the building, the more likely multiple FDC connections will be present, often on different street fronts.”
Engine Company Operations, Standpipe Operations, p.6-16.
Keep water moving during a temporary shutdown — dump it, don’t stop it.
Step 8 tells you how: open one or more discharges as waste or dump lines. The relay stays alive while the lines are off.
“Maintain water flow during temporary shutdowns by using one or more discharges as waste or dump lines”
And on the attack pumper’s standing job: “adjust PDP as needed making sure to dump excess pressure.”
Apparatus Driver/Operator Hydraulic Calculation Standards, Constant Pressure Relay; Engine Company Operations p.5-15.
Communication between engineers is constant, not occasional.
The task book puts it in the setup, before a single step: talk to each other or you end up with too little pressure or too much. Say it out loud every time a line goes into or out of service.
“Communications must be constant between the engineers to avoid insufficient pressure or excessive pressure.”
And on the supply side, out-of-cab step 5: “Maintain communication between engineers when additional lines are being placed into service.”
ADO—Pumper Task Book, Evolutions 1.7 and 1.8, setup 2; Evolution 1.7 out-of-cab 5.
SHUTDOWN ORDER
The order is fixed and it runs from the fire back toward the water — the opposite direction from the way you built the relay. It is written the same way in both evolutions.
“To end operations shut down all lines at attack pumper first then supply pumper last.” — Evolution 1.8, out-of-cab 9. Evolution 1.7, out-of-cab 7: “Shut down lines at attack pumper first when ending operations then the supply pumper.”
Evolution 1.7 — Pumping a hydrant as the supply pumper
Two pumpers, at least 100 feet apart, the far one flowing at least one hand line. You are on the hydrant. Fill the supply line to the attack pumper, set to RPM mode, set pressure at an idle and let the attack engineer call for more. Note pump pressure and intake pressure. Adjust to hold 20 psi minimum residual at the attack pumper.
This is the source-pumper half of a relay with the middle taken out.
Evolution 1.8 — Attack pumper on a pumped hydrant
Same two rigs, minimum 100 feet of supply hose, and you are on the fire end flowing at least two attack lines. Begin flowing from one line, establish the connections from the supply pumper, call for water, then hold communications to keep 20 psi residual. Shut a hand line down and watch what the residual does — that reading is the whole reason the relay is built around a residual floor.
Both evolutions are on the Engine Operations home. The generic pumping sequence they both open with is on Engine Evolution.
Related reading from the manual
Water Supply for the lays and the hydrant tactics this sits inside · Standpipe Operations for the 2nd-and-3rd-due FDC pairing · Master Stream Operations for what you are feeding at the end of a big relay · Quick Reference Guide for the arriving-company order.
NOT IN THE MANUAL
The card is titled Constant Pressure Relay (Limited Max Volume) — it bounds flow, not distance. It gives the 750-foot interval, the 175 psi, the 20 psi residual and the flow ceilings, and it gives you “1 mile = 5280 Feet” to count with. It does not state a maximum total length, a maximum number of pumpers in the chain, or what to do when you run out of rigs before you run out of distance.
Engine Company Operations sends the specifics to the same place: “For more specific information see the current APPARATUS DRIVER / OPERATOR HYDRAULIC CALCULATION STANDARDS in Target Solutions” (p.5-15). If you need a hard limit for a pre-plan, that is the document and a training officer — not a number to work out on the fireground.
Sources. OFD Engine Company Operations 2023 V-1 — Water Supply, pp.5-13–5-15; Standpipe Operations, p.6-16. Apparatus Driver/Operator Hydraulic Calculation Standards — Constant Pressure Relay (Limited Max Volume) and Know the Flow smooth bore tip table. ADO—Pumper Task Book, Evolutions 1.7 and 1.8.
Engine Company Operations and the task book paginate per chapter. Page numbers above are chapter pages, the way they are printed in the footer of the manual.