Sam Turbo- Case Study- Failure of shaft of Dump Tower Pump

Customer Complaint:

"Today at 5.00 am machine was shut for 4 hours due to the shearing of the shaft dump tower pump. The pump was supplied by you and the photograph of the failed shaft is attached herewith for your reference.

So requested you to send your technical person to analyze the immediate failure of the shaft"

Pump Specifications:

Liquid: Pulp, Consistency: 3.5%, Capacity: 413 m3/hr, Head: 59m, Motor: 110KW, Speed: 1400 rpm VFD

Site Observations:

Reasons for Shaft Shearing:

The pump is supplied for 59m. So, it has to show 5.9 kg/cms if the total system head of 59 m ( Static + Friction) exists in the system. A pressure gauge will show the total actual available head in the system above the gauge(Static +all friction loss in the pipe and other equipment fitted in the delivery line).

The pump is selected at the BEP-Best Efficiency Point for the given 413 m3/hr and 59m. Since the actual head is only 35m, the impeller dia/Speed is fixed, and the pump duty point is shifted to the end of the curve and runs there continuously. At this point, the radial load on the rotating parts will be very high, and NPSH R will increase very high. So, the smallest dia of the shaft will get sheared.

Further, since the suction head is varying, the total head will also vary and the pump operates in a range instead of at one point. When the suction level is at the lowest, the total head will be more, and hence flow will be less. As the suction level increases, the total head will reduce, leading to more flow. When the pump restarts, the suction tank is full, the total head is less. Hence, more flow. As the level comes down, the Total Head increases and the flow reduces.

The suction valve should always be kept open. It should not be throttled. For any flow control, only the delivery valve is to be controlled. If the suction valve is throttled, the severe loss will take place and the pump will not perform as per curve and no one can predict how the pump will perform. The flow will drastically be reduced, the radial load will increase, etc. Slow opening of the suction valve while restarting is one of the reasons for shaft shearing.

Solution:

Since the system head is less, we have to

1. Trim the impeller to the required size for 35m

2. Reduce the speed if VFD is available.

3. Go for the new small-sized pump.

The customer is running the pump with a full dia impeller at 1450 rpm even though they have VFD control for this pump. We requested the customer to run the pump around 1100 rpm with VFD. Since the total head is varying, they have to operate at 1200 rpm when the liquid level is low and slowing reduce the speed to 1000 rpm when the tank level increases so that the pump always runs in the BEP area, having very less radial load acting on the rotating assembly.

In the following 1 st picture, the RED Line shows how load increases when the Duty point shifts to –RHS-towards end of cure or LHS-towards shut off.

Requested customer to always keep the suction value fully open. Only, when pump has to be removed from location, suction valve has to be shut.