Is the problem of parallel operation of multi-stage pumps under outlet throttling the cause of one pump breaking down every 15 days?
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I would like to consult the experts: the P1407 high-pressure ash water pump in our company’s water-coal slurry gasification system is a 6-stage centrifugal pump with symmetrically arranged impellers, manufactured by Chongqing Pump Factory. Model ZDF250-135×6, head 800m, flow rate 250m3/h; the drive motor is an explosion-proof motor of 10kv and 800kw. On-site, 2 in use and 1 as backup. Under normal operating conditions, two pumps operate in parallel; the water flow required by the system is around 410 m3/h, with a system pressure of 6.5 MPa. The rated current of the motor is 54.1 A; under conditions that satisfy the system’s water demand, both pumps are adjusted by controlling the opening degree of their outlet valves and monitoring the current values. The pump outlet valve is a gate valve; when the opening degree of this valve is 3 thread strokes, the current reaches approximately 49 A. The flow rate of both pumps then reaches 410 m3/h, so the valves operate at this opening degree throughout. Since March of this year, the P1407 pump has been experiencing frequent failures; on average, it needs to be taken out of service for maintenance every 15 days due to excessive vibration. Subsequently, the impeller of one of them was cut, reducing its diameter by 9 mm. The pump with the smaller impeller and the pump with the larger impeller are currently operating in parallel; the pump with the smaller impeller has its outlet valve fully open, while the outlet gate valve of the pump with the larger impeller remains at a opening degree of less than 20% (out of concern that increasing this opening degree might reduce the flow rate of the pump with the smaller impeller). It is currently running. I would like to ask everyone: 1. Is it permissible to adjust the outlet valve in this way for pumps with different efficiencies when they are connected in parallel? And how should it be adjusted more appropriately? 2. In a pump with a large impeller, increasing the outlet valve opening raises the head pressure; shouldn’t this further lead to a decrease in flow rate for a pump with a small impeller?
The pump does not perform well at low flow rates; the outlet valve of this large impeller pump is only open to 20%, so the flow rate is well below the rated value. This results in low efficiency and an increased likelihood of failures. Regarding the issue with the pressure head, when the valve is closed to maintain pressure, the pressure in front of the valve is high; whether this affects the other pump depends on the pressure behind the valve.
This post was last edited by blade_wzy on 2019-7-30 at 16:56. Are the design conditions consistent with the actual operating conditions? Using the method of adjusting the valve opening isn’t very good; why wasn’t a model with lower flow rate chosen during the initial selection process?
Hello, has this issue been resolved yet?
Using an outlet gate valve to regulate flow is indeed not a good approach, as the operating requirement for such valves is to be fully open or fully closed. The system pressure is only 6.5 MPa, and the pump’s own head is 800 m; the system backpressure is insufficient, which results in the outlet valves of the two pumps being opened to a small degree. If these valves are opened further, the motor will experience overcurrent. The issue still requires discussion with the designers regarding the matching of the pumps and the system; cutting the impeller is also an option, but it’s best if the two pumps have similar performance characteristics.
Excessive vibration does not necessarily require cutting the impeller; have you checked the offset of the balance disk? Is there wear on the balance seat?