(a) Starting with Closed or Throttled Discharge Valve:
It’s common practice to start a centrifugal pump with the discharge valve closed or throttled for several reasons:
- Reduced Starting Load: When the discharge valve is closed, the pump initially operates against minimal resistance. This reduces the starting torque required by the motor, minimizing strain on the motor and electrical system.
- Reduced Risk of Cavitation: A closed discharge valve reduces the flow rate through the pump during startup. This can help prevent cavitation, a phenomenon where bubbles form and collapse due to low pressure at the pump inlet, potentially damaging the impeller.
- Safer System Startup: A closed discharge valve allows for a controlled system startup. The pump can be brought up to speed gradually, allowing for any trapped air to purge from the system before full pressure is established.
(b) Not Leaving Discharge Valve Closed After Starting:
Leaving the discharge valve closed after starting the pump can have detrimental effects:
- Overheating: With no flow through the pump, the liquid inside the casing cannot circulate and absorb heat generated by the motor and bearings. This can lead to overheating and potential damage to the pump components.
- Excessive Pressure Build-Up: If the discharge valve remains closed for an extended period, the pump continues to deliver fluid, leading to a rapid pressure build-up within the system. This can exceed the pressure rating of the pipes or equipment, potentially causing leaks or ruptures.
- Pump Damage: In extreme cases, excessive pressure due to a closed discharge valve can lead to mechanical damage to the pump housing or internal components.
(c) Why Relief Valve Might Not Be Necessary:
While relief valves are crucial for positive displacement pumps, they might not always be necessary for centrifugal pumps. Here’s why:
- Self-Limiting Flow: Centrifugal pumps are not positive displacement pumps. Their flow rate is not constant but increases with increasing discharge pressure. As the discharge pressure rises due to a closed valve, the flow rate through the pump naturally decreases.
- System Relief Measures: Some systems might already have pressure relief valves or safety devices installed elsewhere in the piping system. These can act as a safeguard against excessive pressure build-up in case of a closed discharge valve.
- Application Considerations: For applications with low-pressure requirements and limited risk of system overpressure, a relief valve might be deemed unnecessary.
However, there are situations where a relief valve is still recommended for centrifugal pumps:
- High-Pressure Systems: In systems with high operating pressures, a relief valve provides an extra layer of safety by protecting the pump and piping from exceeding their pressure limitations.
- Dead-End Systems: If the discharge line terminates in a dead-end with no pressure relief path, a relief valve on the pump itself is essential to prevent excessive pressure build-up.
- Variable Speed Pumps: For pumps with variable speed drives, a relief valve is important to safeguard against unexpected pressure surges in case of control system malfunctions.
Ultimately, the decision to include a relief valve in a centrifugal pump system depends on a risk assessment considering factors like system pressure, potential for closed discharge scenarios, and the presence of other safety measures.