Boiler Feedwater Pump Cavitation During Load Swing
A boiler feedwater pump is a horizontal centrifugal pump, rated at 500 gpm at 400 ft head, driven by a 75-hp motor. It takes suction from a deaerator operating at 5 psig (227°F). The pump has been experiencing cavitation-like noise (gravelly sound) and vibration during periods of rapid load increase, especially when the boiler drum level drops and the feedwater control valve opens quickly. The pump is located 8 ft below the deaerator, and the suction piping is 6-inch schedule 40 with two elbows and a fully open gate valve. The pump has a suction strainer that was cleaned two months ago. The NPSH required by the pump is 12 ft at the design flow. The operator has noticed that the noise disappears when the flow is reduced. The plant is a cogeneration facility, and the boiler load swings frequently due to steam demand from a turbine. The pump has been in service for five years without overhaul.
1. Calculate the available NPSH (NPSHa) at the pump suction under design conditions and determine if cavitation is likely. Show your assumptions and calculations.
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NPSHa = pressure head at suction surface + static elevation head - vapor pressure head - friction losses. Assumptions: deaerator pressure 5 psig, water at saturation (227°F). Atmospheric pressure at sea level = 14.7 psia. Absolute pressure in deaerator = 14.7 + 5 = 19.7 psia. Vapor pressure of water at 227°F is approximately 19.3 psia (from steam tables). So pressure head = (19.7 - 19.3) psia × 2.31 ft/psi = 0.4 × 2.31 = 0.92 ft. Static elevation: pump is 8 ft below deaerator, so +8 ft. Friction losses: suction piping 6-inch schedule 40, flow 500 gpm. Use Darcy-Weisbach or Hazen-Williams. For 6-inch pipe, ID = 6.065 in, area = 0.2006 ft², velocity = 500 gpm / (448.8 gpm/ft³/s × 0.2006 ft²) = 500 / 90.0 = 5.56 ft/s. Equivalent length: 8 ft straight + two elbows (each ~10 ft for 6-inch) + gate valve (fully open ~2 ft) + strainer loss (assume 2 ft) = 8 + 20 + 2 + 2 = 32 ft. Friction factor f ~ 0.018 for clean steel. Head loss = f (L/D) (V²/2g) = 0.018 × (32/0.505) × (5.56²/64.4) = 0.018 × 63.4 × (30.9/64.4) = 0.018 × 63.4 × 0.48 = 0.55 ft. Add minor losses for entrance and exit: entrance 0.5, exit 1.0, so total K = 1.5, head = K V²/2g = 1.5 × 0.48 = 0.72 ft. Total friction ~1.3 ft. NPSHa = 0.92 + 8 - 1.3 = 7.62 ft. This is less than NPSHr of 12 ft, so cavitation is likely at design flow. The margin is negative, explaining the noise. When flow is reduced, friction losses decrease, so NPSHa increases, which is why noise disappears. The rapid load increase causes the control valve to open, increasing flow and friction losses, further reducing NPSHa. Also, the deaerator pressure may drop momentarily during load swings, reducing pressure head.
2. Recommend modifications to eliminate cavitation, considering both immediate operational changes and long-term fixes.
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Immediate operational changes: (1) Reduce the rate of load increase by slowing the feedwater control valve response; use a ramp or rate limit to prevent sudden flow surges. (2) Maintain a higher deaerator pressure (e.g., 10 psig) to increase NPSHa, if the deaerator and boiler can tolerate it; this raises the saturation temperature and pressure head. However, increasing deaerator pressure may require more steam and could affect boiler feedwater temperature. (3) Ensure the deaerator level is maintained at the normal setpoint to avoid low level and vortexing. (4) Open the suction gate valve fully (already assumed) and verify the strainer is clean. Long-term fixes: (1) Increase the suction pipe size from 6-inch to 8-inch to reduce velocity and friction loss; this would increase NPSHa by about 0.5 ft. (2) Raise the deaerator elevation or lower the pump to increase static head; the pump is already 8 ft below, but could be lowered further if space allows. (3) Install a larger pump with lower NPSHr, or select a pump with a suction specific speed that is more forgiving. (4) Add a booster pump or a low-speed pump. (5) Modify the suction piping to eliminate unnecessary elbows and use long-radius fittings. (6) Consider a variable frequency drive (VFD) to control flow more smoothly and avoid rapid valve movements. (7) Perform a proper NPSH study and possibly install a flow straightener. The most cost-effective immediate fix is to slow the control valve and raise deaerator pressure; the long-term fix is to increase suction pipe size and possibly replace the pump with one having lower NPSHr. Also, check the pump for wear; after five years, wear rings may be worn, increasing NPSHr. An overhaul may be needed.