At a 70 mm bore, a 1 m/s water velocity requires about 3.85 L/s (231 L/min); that is a useful starting point for flushing air from the coil, not a complete pump specification. Select the purge pump from the required flow at the coil’s actual pressure drop, then use a suitable hydraulic test pump to reach the specified test pressure after removing as much air as practical.
Separate air-purge flow from hydrotest pressure
The job has two different requirements. Air removal is a flow problem: water must carry bubbles through the coil and out of a vent or discharge. Pressure testing is a pressure-control problem: a pump must raise the water-filled system to the project’s required test pressure without exceeding the rating of any component. One pump can perform both jobs only if its flow and pressure characteristics suit both operating modes.
The coil’s 1 m inlet and outlet elevation does not determine the purge flow. Because the ports are at the same height, their net elevation difference is zero, but the loop’s high point can still trap air. The pump must overcome pipe, bend, valve, hose, and fitting losses to sustain the selected flow. For an open discharge, also account for the elevation difference between the source water level and discharge point.
| Quantity | Known value or calculation | Where to read or verify |
|---|---|---|
| Pipe inside diameter | 70 mm (0.070 m) | Confirm the actual bore, including any different hose or fittings. |
| Flow at 1 m/s | 3.85 L/s, approximately 231 L/min | Measure or read at the coil operating pressure, not merely the pump’s free-discharge rating. |
| Flow at 2 m/s | 7.70 L/s, approximately 462 L/min | Use only if the system and components can accommodate the resulting flow and pressure loss. |
| Hydrotest pressure | Not specified | Obtain the required value and limits from the test procedure and component documentation. |
| Total flow-path length | Not established by the dimensions given | Measure the actual pipe length and account for fittings, valves, and hoses when estimating pressure drop. |
Compare continuous flushing with manual pressure pumping
The evidence supports two practical approaches, but they solve different parts of the task. A higher-flow pump can move bubbles through the coil. A hand hydraulic test pump can pressurize a water-filled system, but it is slow when it must first compress a large trapped air volume. The preferred sequence is to fill and purge with a suitable flow path, then use a hydraulic test pump for controlled pressure rise.
| Approach | Best use | Main limitation |
|---|---|---|
| Continuous water flushing | Carry air bubbles through the coil toward an open, high-point vent or discharge. | Required pump duty depends on actual system resistance; a velocity target alone does not specify pump pressure. |
| Hand hydraulic test pump | Raise pressure after the coil is substantially full of water and vented. | Slow if used to displace or compress a substantial volume of trapped air. |
| One pump for both tasks | Possible when its flow-at-pressure curve covers purge duty and it can also safely control test pressure. | High-flow capability does not automatically provide safe pressure control; verify pump limits and system ratings. |
Choose the sequential approach unless pump documentation and the test plan demonstrate that one unit can safely meet both duties. The comments give greater than 1 m/s as a rule of thumb for moving air, including downward flow, and mention a 1–2 m/s range. Treat that as an initial purge target, not a guaranteed air-removal criterion for every coil geometry.
Calculate the coil flow target from bore and velocity
For a circular pipe, calculate flow from the cross-sectional area and mean velocity:
Q = v × AA = π × D² / 4
With D = 0.070 m, the area is approximately 0.00385 m². At v = 1 m/s, Q = 0.00385 m³/s, or about 3.85 L/s and 231 L/min. At 2 m/s, flow doubles to about 7.70 L/s or 462 L/min.
These results assume the quoted 70 mm inside diameter is the bore throughout the flow path and the stated velocity is the average water velocity in that bore. Any smaller port, hose, valve, or fitting can become the controlling restriction. Check actual flow with a meter or a verified pump curve at the operating condition; a pump’s nominal or maximum flow at zero head does not establish delivered flow through the coil.
Use the dimensions without mistaking geometry for flow length
The stated 75 mm outside diameter and 70 mm inside diameter yield an approximate wall thickness of 2.5 mm if the pipe is concentric and dimensions are nominal: (75 − 70) / 2 = 2.5 mm. That wall calculation does not provide the pipe’s pressure rating; obtain the rating from the coil documentation.
The 1 m coil diameter describes a geometric dimension, not a complete hydraulic length. The stated 3 m “length” is ambiguous: it may be the pipe length, or a coil dimension. Do not use it as total wetted pipe length until the drawing or physical measurement resolves that meaning. If 3 m is confirmed as actual 70 mm-bore pipe length, its straight-pipe water volume is approximately 11.5 L, calculated as area multiplied by length. That estimate excludes hoses, fittings, and any additional coil turns.
A complete pump selection needs more than the bore and nominal dimensions. Determine total flow-path length, fitting and valve configuration, the pump’s suction arrangement, and required discharge pressure. Use manufacturer pump curves and a pressure-drop calculation or measurement for the actual circuit. A larger target velocity raises flow and usually raises system pressure loss; do not infer the required pressure from the velocity alone.
Fill and vent the high point before raising test pressure
Air collects at high points because buoyancy drives it upward against the water flow. A coil with ports at equal elevations can therefore retain air at the top even when water enters and leaves at the bottom or sides. A purge outlet must give that air a route out; circulating water through a closed, unvented loop can move a pocket without removing it.
- Confirm the required test pressure, coil rating, and the ratings of every connected hose, valve, fitting, gauge, and pump component. Resolve whether the 3 m figure is pipe length and identify the coil’s highest point.
- Arrange a fill connection at a low point and provide a vent or discharge at the high point. Keep the water supply available and avoid drawing air into the suction side.
- Fill slowly until water reaches the high-point outlet. Open the vent and continue filling or flushing until the discharge is continuous and free of visible air. If the test arrangement permits, use the calculated velocity target as a starting point for flushing through the coil.
- Close the vent only after the trapped air has been released. Inspect for leaks and confirm the coil remains water-filled before connecting or operating the pressure pump.
- Use a hydraulic test pump to raise pressure gradually to the test procedure’s specified value. Monitor a suitable gauge and inspect the circuit while pressure rises; stay within the lowest applicable component rating.
The amount of water in the circuit may help estimate fill and purge time, but it does not decide the required pump flow by itself. If the 3 m figure is actual straight pipe length, the coil’s estimated volume is about 11.5 L; the true system volume is higher when connecting pipe and equipment are included.
Distinguish trapped air, leaks, and inadequate pump duty
A slow pressure rise can indicate compressible trapped air, but it is not a standalone pass/fail test. Gauge response, pump delivery, leakage, hose expansion, and system volume also affect pressure buildup. Judge air removal from the fill and vent process, not an assumed “nearly instantaneous” pressure response.
| Observed condition | Likely mechanism | Diagnostic action |
|---|---|---|
| Air continues to emerge from the high-point outlet | The loop is still purging, or flow is not carrying the pocket to the outlet. | Keep the vent open during filling; check flow path and actual flow through restrictions. |
| Pressure rises slowly during hand pumping | Trapped air may be compressing; pump displacement, leakage, or system compliance may also contribute. | Continue venting before pressure testing, inspect for leaks, and check pump and gauge operation. |
| Target purge velocity cannot be reached | Pump flow at actual pressure is below the target, or a restriction is limiting flow. | Measure flow and check the pump curve, bore changes, valves, hoses, and fittings. |
| Pressure changes after the vent is closed | Residual gas, leakage, temperature change, or equipment compliance may be affecting pressure. | Check for visible leaks, confirm the venting sequence, and follow the test procedure’s stabilization and acceptance criteria. |
A pressure build test can reveal a problem, but the source’s “instantaneous” benchmark has no stated time or instrumentation criterion. Use the specified pressure-test procedure and its acceptance limits rather than treating pressure-rise speed as proof that all air has been removed.
Verify delivered flow and controlled pressure
Before relying on the selected pump, verify the two duties separately. For purge, measure flow through the installed coil and calculate velocity using the actual bore: v = Q / A. At a 70 mm bore, about 231 L/min corresponds to 1 m/s. Confirm that the high-point vent discharges air and then a steady water stream.
For pressure testing, verify that the pump can reach the specified test pressure with adequate control and that the gauge is appropriate for the required range. Inspect connections and the coil for leakage during the prescribed test period. Record the pressure and observations required by the test plan. If pressure cannot be reached or held, isolate the cause before increasing pump output; a higher flow rating is not a remedy for leakage or an inadequate component pressure rating.
Stop if the required test pressure, coil rating, or a connected component’s pressure limit is unknown or conflicting, or if leakage or damage appears. Resolve the test basis with the responsible engineer and the coil or pump manufacturer’s official support channel before pressurizing further.
Frequently asked questions
Can I size the pump from the coil diameter alone?
No. The coil’s 1 m geometric diameter does not set flow. Use the 70 mm bore and target velocity to calculate flow, then check pump delivery against the actual circuit pressure drop and flow-path restrictions.
Can I use 1 m/s as the required air-removal velocity?
Use it as an initial rule-of-thumb target, not a guarantee. For a 70 mm bore it equals about 3.85 L/s (231 L/min); confirm that air exits the high-point vent under the installed flow conditions.
Does a hand pump work for the hydrotest?
Yes, for controlled hydraulic pressure after filling and venting. It will take longer if it must first compress a substantial trapped-air volume, so remove as much air as practical before using it.
Can a fast pressure rise prove the coil contains no air?
No. Pressure-rise speed also depends on pump delivery, leakage, gauge response, and system compliance. Follow the test procedure’s acceptance criteria and verify air removal at the vent; stop and consult the responsible engineer or official manufacturer support if pressure limits or abnormal behavior remain unresolved.