Pump Short-Cycling and a Pressure Tank That Won't Hold Air
A small booster station with two 25 hp vertical turbine pumps (lead/lag) maintains 65 psi in a 6,000-gallon hydropneumatic pressure tank. The lead pump has begun short-cycling: roughly 40 starts per hour instead of the normal 4–6. The tank's air charge was last checked 8 months ago. The operator hears water sloshing when tapping the tank and the air gauge reads 38 psi while the system pressure swings between 55 and 68 psi. The pump motor draws normal current and the discharge check valve was replaced last year. The station serves a hospital with a 24/7 demand floor, so you cannot simply shut the station down without coordination.
1. What is the most likely cause of the short-cycling, and how do you confirm it before touching anything?
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The symptoms point to a lost air charge / waterlogged pressure tank, not a pump or check-valve failure.
Evidence:
- 40 starts/hour vs 4–6 means the tank has almost no usable air volume to buffer between pump cycles. A properly charged tank lets the pump run long enough to refill the air cushion between cut-in and cut-out.
- Sloshing when tapped is the classic sign of a waterlogged tank - water where air should be.
- Air gauge 38 psi vs system 55–68 psi: the precharge should typically be set a few psi below the cut-in pressure (here, roughly 50–52 psi if cut-in is 55). A 38 psi reading is far below that, consistent with a bladder/air loss.
- Normal motor current and a new check valve argue against a mechanical pump problem.
Confirm before acting:
- Compare the tank air pressure to the pump cut-in/cut-out settings (read the pressure switch setpoints).
- Isolate the tank and check whether it holds air (a bladder tank that won't hold precharge has a failed bladder; a captive-air tank may just need recharging).
- Check for water at the air valve - that confirms a ruptured bladder.
- Log the start count with a counter or by observation to establish the baseline.
Do not just add air to a tank with a ruptured bladder; it will waterlog again within days.
2. How do you restore the station without interrupting the hospital, and what do you change to prevent recurrence?
Read the worked answer
Restore service without a shutdown:
- The station has two pumps and a hospital demand floor. Coordinate with the hospital, then run on the lag pump alone (or manually hold one pump running) to carry the load while you isolate the pressure tank. A single 25 hp pump sized for the station can usually hold the hospital load; verify discharge pressure stays in the acceptable band before isolating.
- Isolate the tank with its isolation valve, drain it, and inspect: if water comes out the air valve or the bladder is ruptured, replace the bladder/tank. If it is a captive-air tank, drain and re-establish the correct air precharge (a few psi below cut-in, e.g., ~50–52 psi for a 55 psi cut-in).
- Return the tank to service, then restore automatic lead/lag and confirm the start count drops back toward 4–6 per hour.
Prevent recurrence:
- Add air charge verification to the PM schedule (e.g., quarterly, or monthly for a critical station), with a logged precharge value.
- Record pump starts and runtime; a rising start count is an early warning.
- Check the bladder/tank age and plan replacement before failure.
- For a hospital, consider a redundant tank or VFD control so a single tank failure cannot short-cycle the pumps; a VFD also reduces start frequency.
- Document the hospital coordination and the isolation sequence in the station log.