After the hotel fire pump starts, the pipes vibrate severely and the pressure in the fire protection pipe network is unstable.
Okay, as a hotel MEP specialist, in response to your typical fault of "severe pipeline vibration after hotel fire pumps start, unstable pressure in the fire protection pipe network," I will provide a systematic, point-by-point detailed analysis and solution based on national standards (GB) and actual engineering cases.
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I. Problem Characterization and Standard Reference
Problem characterization: This phenomenon is a typical coupled fault of hydraulic transients (water hammer) and mechanical resonance in the fire water supply system. It not only affects system reliability but may also cause pipe rupture and equipment damage, seriously threatening hotel fire safety.
Core referenced standards:
1. GB 50974-2014 Technical Code for Fire Water Supply and Hydrant Systems
- Article 5.1.13: The startup, operation and switching of fire pumps shall ensure that their flow and head meet design requirements. Facilities to prevent water hammer damage shall be installed on the pump discharge header.
- Article 5.1.16: Pressure gauges shall be installed on the suction and discharge pipes of fire pumps and shall comply with the provisions.
- Article 8.3.3: A pressure relief valve shall be installed in the pipe network of the fire water supply system, and the set pressure of the pressure relief valve shall not exceed 1.2 times the design working pressure of the system.
- Article 13.2.6: During system commissioning, a simulated fire-extinguishing function test shall be conducted to check whether pump startup, pressure, flow, etc. are normal.
- Article 3.5.2: Pipes shall be securely fixed, and the installation of pipe supports and hangers shall meet design requirements, with spacing not greater than the specified values in the code.
- Article 5.2.1: The installation of fire pumps shall meet design requirements and vibration isolation treatment shall be carried out.
II. Cause Analysis (with Case Study)
Based on field experience, severe vibration and unstable pressure are usually caused by a combination of the following core factors. We analyze this with a typical case of a five-star hotel basement fire pump room:
Case background: A hotel fire pump room uses 2 main pumps (Q=40L/s, H=120m) and 1 jockey pump. After the main pump starts, the pump body and discharge piping vibrate severely, the pressure gauge pointer swings greatly (0.8MPa~1.6MPa), and the jockey pump starts and stops frequently.
Cause 1: Water hammer effect (the main culprit for unstable pressure)
- Standard point: Article 5.1.13 of GB 50974-2014 requires facilities to prevent water hammer damage.
- Mechanism: The fire pump instantly reaches rated speed from zero, and the water flow generates enormous kinetic energy impact in the pipe. When the check valve closes quickly during pump shutdown, the water flow is instantly cut off, and kinetic energy is converted into a pressure wave that oscillates back and forth along the pipe, forming a water hammer.
- Specific case manifestation: The hotel pump room was not equipped with a pressure-sustaining/pressure-relief valve, and the ordinary swing check valve used on the discharge pipe closed too quickly (about 0.2 seconds), causing the pressure peak to reach as high as 1.8MPa (system design working pressure 1.4MPa) each time the main pump started or stopped, far exceeding the pressure-bearing capacity of the pipes and equipment, with the pressure gauge swinging severely.
Cause 2: Pipeline system resonance (the main culprit for severe vibration)
- Standard point: Article 3.5.2 of GB 50261-2017 has strict requirements for pipe supports and hangers.
- Mechanism: When the rotational frequency of the pump (e.g., 2950rpm corresponding to 49Hz) is close to the natural frequency of the pipes and supports, mechanical resonance occurs, amplifying slight vibration dozens of times.
- Specific case manifestation: The hotel pump room had excessively large spacing of pipe supports and hangers (more than 3 meters, while the code requires that DN150 pipes should not exceed 2.5 meters), and vibration-damping supports were not installed. The 49Hz vibration frequency during pump operation coincided exactly with the natural frequency (about 48Hz) of an approximately 8-meter-long DN150 horizontal pipe section, causing the vibration amplitude of that pipe section to exceed 5mm and produce a loud roaring noise.
Cause 3: Problems with the pump and piping accessories
- Standard point: Article 5.1.16 of GB 50974-2014 and Article 5.2.1 of GB 50261-2017.
- Mechanism:
- Cavitation: The suction pipe is not tight or the water level is too low, drawing in air; bubbles collapse in the high-pressure area and produce impact.
- Coupling misalignment: Deviation between the motor and pump axes generates periodic radial force.
- Insufficient foundation stiffness: The pump foundation is not vibration-isolated or the anchor bolts are loose.
- Specific case manifestation: Inspection found that the sealing gasket at the connection between the suction pipe and the pump body of the hotel pump had aged and was leaking air, causing a slight cavitation noise (similar to the sound of rolling stones) during pump operation, which aggravated pressure fluctuation.
Cause 4: Mismatch between jockey system and main pump system parameters
- Standard point: Article 5.3.3 of GB 50974-2014 (design flow of jockey pump matching the main pump).
- Mechanism: The set values of jockey pump start/stop pressure are unreasonable, or the air pressure in the jockey tank is insufficient, so the jockey pump cannot effectively compensate for minor leakage in the pipe network, causing the pressure to be already close to the lower limit before the main pump starts, and the pressure surge at the instant of main pump startup is greater.
- Specific case manifestation: The hotel jockey pump start pressure was set at 1.2MPa and stop pressure at 1.4MPa, while the main pump start pressure was 1.0MPa. Due to normal leakage in the pipe network, the pressure often dropped to 1.0MPa; at this time the main pump started, and because the jockey pump had stopped working, the pipe network was in an "idle" state, making the water hammer effect more severe.
III. Systematic Solution (Step-by-Step Implementation)
Step 1: Emergency handling and testing
1. Shutdown inspection: Immediately stop all fire pumps and check the pump body, valves, pipes and supports for obvious deformation or rupture.
2. Pressure recording: Record the jockey pump start/stop pressure values and the pressure peaks during main pump start/stop.
3. Vibration measurement: Use a vibration analyzer to measure the vibration frequency and amplitude of the pump body, motor and key pipe nodes.
Step 2: Eliminate Water Hammer (Core Measure)
1. Install water hammer arresters (recommended as priority): Install bladder-type water hammer arresters on the discharge header of each main pump. According to GB 50974-2014, their volume should be capable of absorbing the maximum water hammer energy. In this case, we selected bladder-type arresters with DN200 and rated pressure of 2.5MPa, with pre-charged nitrogen pressure at 90% of the system working pressure (approximately 1.26MPa).
2. Replace check valves: Replace ordinary swing check valves with slow-closing check valves (such as micro-resistance slow-closing butterfly check valves). Their closing time is adjustable (recommended setting: 5–10 seconds), which can effectively suppress pump-stop water hammer. After replacement in this case, the peak pressure dropped from 1.8MPa to 1.5MPa.
3. Add pressure relief valves: Install pressure-holding/pressure relief valves on the discharge header, with the relief pressure set at 1.1 times the system design working pressure (approximately 1.54MPa). Once pressure exceeds the limit, it automatically relieves to the fire water tank, protecting the piping network.
Step 3: Eliminate Mechanical Resonance
1. Reinforce pipe supports: In accordance with GB 50261-2017 requirements, increase the density of pipe supports and hangers, with spacing controlled within the code-permitted range (e.g., DN150 pipe ≤2.5 meters). Add anti-sway braces at elbows, tees, and valves.
2. Add vibration isolation measures: Install rubber vibration isolation pads or spring vibration isolators between pipes and supports to change the natural frequency of the piping system, keeping it away from the pump operating frequency.
3. Adjust pump foundation: Check anchor bolt torque and re-grout if necessary. Install vibration isolators between the pump base and foundation.
Step 4: Optimize Pressure Stabilization and Control System
1. Adjust jockey pump parameters: According to GB 50974-2014, adjust the jockey pump start/stop pressures. Recommended: start pressure = main pump start pressure + 0.05–0.1MPa (adjusted to 1.05MPa in this case), stop pressure = start pressure + 0.1–0.15MPa (adjusted to 1.2MPa). Ensure that when the main pump starts, the jockey pump is still running and the piping network is at high pressure, reducing water hammer impact.
2. Inspect and charge: Check the pre-charge pressure of the pressure stabilization tank (pneumatic tank), ensuring it is 0.6–0.8 times the jockey pump start pressure (0.7��1.05=0.735MPa in this case), and check whether the bladder is ruptured.
3. Optimize control logic: Change the main pump starting method from "hard start (direct-on-line start)" to "soft start" or "variable frequency start," which can significantly reduce the flow and pressure surge at the moment of startup.
Step 5: Investigate Cavitation and Leakage
1. Inspect suction pipe: Check the sealing integrity of the suction pipe and eliminate air leakage points. Ensure the horizontal section of the suction pipe has a slope of not less than 0.005, sloping toward the pump, to prevent gas accumulation.
2. Adjust water level: Ensure the minimum water level of the fire water tank is at least 0.5–1 meter above the pump suction inlet to prevent vortex air entrainment.
3. Coupling alignment: Use a laser alignment instrument to re-align the coaxiality of the motor and pump coupling, with error controlled within 0.05mm.
IV. Implementation Results and Verification
After completing the above modifications (using the case hotel as an example):
1. Vibration values: Pump body and pipe vibration velocity decreased from 12mm/s to 1.5mm/s (below the 4.5mm/s limit required by GB 50261).
2. Pressure stability: Pressure fluctuation range during main pump startup reduced from ±0.4MPa to ±0.05MPa; jockey pump start/stop frequency reduced from 30 times per hour to 2 times per hour.
3. Safety: No abnormal pipe shaking, no leakage, and stable system operation.
V. Summary and Recommendations
The hotel fire protection system is a life-safety engineering system. When "severe vibration and unstable pressure" occur, do not blindly test-run pumps or allow prolonged operation under faulty conditions. The principle of "diagnose first, then remediate" must be followed, with focus on investigating the three core issues: water hammer protection, pipe resonance, and pressure stabilization matching. Recommendations:
1. Regular maintenance: Conduct functional testing and maintenance of water hammer arresters, pressure relief valves, check valves, and pressure stabilization tanks quarterly.
2. Establish records: Record pressure, vibration, current, and other data from each test to build a health file for trend analysis.
3. Professional inspection: Every two years, engage a qualified testing agency to perform hydraulic transient analysis on the fire pump units and piping network to predict risks in advance.
The above analysis and solutions strictly follow GB codes and incorporate practical engineering experience, with the hope of helping you thoroughly resolve this hazard. If you have further detailed questions, please feel free to continue the discussion.