Drafting Failure at Elevation: Reading the Vacuum Gauge
You are drafting from a pond at 5,000 feet elevation. Vertical lift from the water surface to the pump intake is 18 feet. You are using 20 feet of 6-inch hard suction with a barrel strainer. The primer runs and the pump will not take water. Two tests have been run: with the strainer capped off and out of the water, the primer pulls and holds 10 inHg. With the strainer submerged in about 14 inches of water over a silt bottom, the vacuum gauge climbs to 7 inHg and stops there, and no water reaches the pump. The engine has drafted without trouble at lower elevations and has run off hydrants all shift. Ambient temperature is 88°F.
1. Compute the theoretical maximum lift at this elevation and judge whether 18 feet is a workable lift here.
Read the worked answer
Step 1 - atmospheric pressure at 5,000 ft. Pressure falls roughly 1 psi per 2,000 ft of elevation: 14.7 − 2.5 ≈ 12.2 psi (about 24.9 inHg).
Step 2 - theoretical water column. 12.2 psi × 2.31 ft per psi ≈ 28 feet. (Same answer via 24.9 inHg × 1.13 ft per inHg.)
Step 3 - practical limit. A real draft cannot use the whole theoretical column, because suction-hose friction, the strainer, and the need to keep velocity in the hose consume the rest. A common planning figure is about two-thirds of theoretical, ≈ 19 feet here.
Judgment: 18 feet leaves roughly 1 foot of margin. That is marginal, not comfortable. It can work, but only if everything else is perfect - no air leak, clean strainer, short suction line, pump intake valve fully open. It is not a lift to troubleshoot casually, and any correction that adds lift will fail. Reducing the lift by repositioning is a legitimate first-choice fix, not a last resort.
2. Interpret the two vacuum readings. What do they tell you, and what do they rule out?
Read the worked answer
Capped-suction test - 10 inHg held: the primer, pump intake, suction hose, and couplings hold vacuum. There is no gross air leak in the assembly, and the primer is producing vacuum. This is a pass, and it is why 'the primer is broken' is the wrong diagnosis.
Submerged test - stalls at 7 inHg: 7 inHg is only about 8 feet of lift equivalent, far short of the 18 feet this installation needs. The pump is not developing the vacuum required to pull water up the pipe.
Critical reasoning step: a clogged or buried strainer would drive the vacuum HIGHER, not lower - a restricted intake makes the pump pull harder against nothing. A low, stalled vacuum means air is entering the suction stream or the vacuum is being broken under load.
Therefore, the likely causes are:
- Air entrainment at the strainer - only 14 inches of water over a silt bottom is not enough submergence; the pump creates a vortex and pulls air.
- A coupling or gasket that seals when capped but leaks under flow - a hardened or missing gasket, a coupling not fully seated, or the pump intake valve/cap not fully closed.
- A partially open valve on the suction side - including a tank-to-pump or drain valve left cracked.
Ruled out or deprioritized: primer failure (test passed), a completely plugged strainer (would show high vacuum), and elevation alone (18 ft is within the marginal but achievable range computed above).
3. Give your corrective actions in order, and state the safety limit that governs when you stop trying to make this draft work.
Read the worked answer
In order - cheapest and most likely first:
- Fix submergence. Move the strainer to deeper water or float it so at least 2 feet of water stands above it and no vortex forms. This matches the low-vacuum evidence and costs nothing.
- Check every suction gasket and coupling by hand and by eye, plus the pump intake cap, drain, and tank-to-pump valve. Re-seat or replace gaskets.
- Confirm the suction valve is fully open and no other valve is bleeding air into the intake.
- Re-prime and watch the gauge. Water should reach the pump with the gauge climbing past the lift requirement. Judge by the gauge, not by the sound.
- If it still fails, reduce the lift - reposition the engine closer to the water. With roughly 1 foot of margin, this is the reliable fix.
Safety limit: never let the crew keep working a marginal draft that is producing no water while a fire is waiting. Set a hard time limit, maintain a backup water source or a defensive posture, and tell the IC immediately that the water supply is not established. Also do not run the pump dry or overheat the primer chasing a draft; and never send anyone down a bank or into the water without the proper harness and a second rescuer in place.