Pressure Loss on a 300-ft Run
A three-plunger positive-displacement pump rated 20,000 psi and 10 gpm at 400 rpm is set up to clean a 12-in. process line. The crew assembled 300 ft of 3/8-in. ID high-pressure hose, an inlet filter, and a rotating nozzle rated 12 gpm at 10,000 psi. With the lance open, the pump gauge holds 6,500 psi, the nozzle does not turn, and cleaning is ineffective. The operator wants to close the bypass and bring pressure up to 12,000 psi. Supply is a 500-gal tote through a 1-in. line; the pump is running at 320 rpm. A light knock comes from the fluid end, and the filter differential gauge reads 18 psi against a clean-filter baseline of 2 psi. The nozzle has 60 hours on it, and its carbide inserts look washed out. The crew has a replacement nozzle of the same rated size but no smaller one on the truck.
1. Using flow and pressure relationships, explain why the pump cannot reach the nozzle's rated pressure.
Read the worked answer
Start with pump output, not with the pressure gauge. At 320 rpm, the pump's approximate output is 10 gpm x (320/400) = 8 gpm, assuming 100% volumetric efficiency. The nozzle is rated 12 gpm at 10,000 psi. Nozzle flow varies roughly with the square root of pressure, so at 6,500 psi the same nozzle would pass about 12 x sqrt(0.65) = 9.7 gpm, and a washed-out orifice would pass even more.
The pump cannot supply 9.7 gpm, so the nozzle demand exceeds pump output and pressure cannot build. The washed-out carbide inserts enlarge the effective orifice, which makes the mismatch worse. Separately, the 18-psi filter differential and the knock in the fluid end point to inlet starvation and cavitation, which further reduce effective pump output. The 6,500 psi reading is a symptom of insufficient flow, not a pressure setting to overcome.
2. Why is closing the bypass and raising pressure to 12,000 psi both ineffective and unsafe?
Read the worked answer
Closing the bypass does not create flow; it restricts or deadheads the pump. A positive-displacement pump will attempt to push against the restriction, so pressure can spike rapidly, the relief valve may lift, and the fluid end can be damaged by cavitation or overpressure. The nozzle still will not match pump output, so cleaning remains ineffective.
It is also unsafe because the hose, fittings, and nozzle have rated limits, and a pressure spike can exceed them. A partially blocked or stalled nozzle can produce water hammer, and a worn nozzle can suddenly pass more flow and drop pressure, causing the operator to lose control of the lance. The correct response is to restore inlet flow and match the nozzle to pump output, not to raise pressure.
3. List the checks and corrections you would make, in order, before returning the unit to service.
Read the worked answer
- Stop and relieve pressure: Shut down, relieve stored pressure, and lock out the pump drive.
- Check inlet supply: Verify the tote has adequate water, the 1-in. supply line is not collapsed or kinked, and the inlet filter is clean. An 18-psi differential is far above the 2-psi baseline and must be corrected.
- Verify pump speed: Confirm the pump is running at the correct rpm; at 320 rpm it is only delivering about 8 gpm.
- Measure nozzle condition: Inspect the carbide inserts and compare the actual orifice to the rated size. Replace the washed-out nozzle with a correctly sized one, not merely one of the same nominal rating if the old one is worn.
- Calculate the match: Select a nozzle whose flow at the target pressure does not exceed pump output at the actual operating speed, and account for hose friction loss over 300 ft.
- Check the gauge: Verify the pump gauge with a calibrated gauge before trusting the 6,500-psi reading.
- Restart gradually: Bring pressure up slowly, confirm the nozzle rotates, and check for knock, leaks, and stable pressure.