When a newly serviced horizontal split case pump performs poorly, a good troubleshooting procedure can help eliminate a number of possibilities, including problems with the pump, the fluid being pumped (pumping fluid), or the pipes, fittings, and containers (system) connected to the pump. An experienced technician with a basic understanding of pump curves and performance parameters can quickly narrow down the possibilities, especially those related to  pumps.

Horizontal Split Case Pumps

To determine if the problem is with the pump, measure the pump's total dynamic head (TDH), flow, and efficiency and compare them to the pump's curve. TDH is the difference between the pump's discharge and suction pressures, converted to feet or meters of head (Note: If there is little or no head or flow at startup, shut down the pump immediately and verify that there is adequate fluid in the pump, i.e., the pump chamber is full of liquid. Running the pump dry may damage the seals). If the operating point is on the pump curve, the pump is operating properly. Therefore, the problem is with the system or pumping media characteristics. If the operating point is below the pump curve, the problem may be with the pump, the system, or the pumping (including media characteristics). For any specific flow, there is a corresponding head. The design of the impeller determines the specific flow at which the pump is most efficient - the best efficiency point (BEP). Many pump problems and some system problems cause the pump to operate at a point below its normal pump curve. A technician who understands this relationship can measure the pump's operating parameters and isolate the problem to the pump, the pumping, or the system.

Pumped Media Properties

Environmental conditions such as temperature change the viscosity of the pumped media, which can change the pump's head, flow, and efficiency. Mineral oil is a good example of a fluid that changes viscosity with temperature fluctuations. When the pumped media is a strong acid or base, dilution changes its specific gravity, which affects the power curve. To determine if the problem is with the pumped media, its properties need to be verified. Testing the pumped media for viscosity, specific gravity, and temperature is convenient and inexpensive. Standard conversion tables and formulas provided by the Hydraulic Society and other organizations can then be used to determine if the pumped media is adversely affecting the pump's performance.

System

Once the fluid properties have been ruled out as an influence, the problem is with the horizontal split case pump or system. Again, if the pump is operating on the pump curve, it is working properly. In this case, the problem must be with the system the pump is connected to. There are three possibilities:

1. Either the flow is too low, so the head is too high

2. Either the head is too low, indicating that the flow is too high

When considering head and flow, remember that the pump is operating correctly on its curve. Therefore, if one is too low, the other must be too high.

3. Another possibility is that the wrong pump is being used in the application. Either by poor design or by incorrect installation of components, including designing/installing the wrong impeller.

Too Low Flow (Too High Head) - Too low flow usually indicates a restriction in the line. If the restriction (resistance) is in the suction line, cavitation may be occurring. Otherwise, the restriction may be in the discharge line. Other possibilities are that the suction static head is too low or the discharge static head is too high. For example, the suction tank/tank may have a float switch that fails to shut off the pump when the level drops below the set point. Similarly, the high level switch on the discharge tank/tank may be faulty.

Low head (too much flow) - Low head means too much flow, and most likely not going where it should. Leaks in the system can be internal or external. A diverter valve that allows too much flow to bypass, or a failed check valve that causes flow to circulate back through a parallel pump, can cause too much flow and too little head. In a buried municipal water system, a major leak or line rupture can cause too much flow, which can cause low head (low line pressure).

What could be wrong?

When an open pump fails to run on curve, and other causes have been ruled out, the most likely causes are:

- Damaged impeller

- Clogged impeller 

- Clogged volute

- Excessive wear ring or impeller clearance

Other causes may be related to the speed of the horizontal split case pump – the shaft spinning in the impeller or the incorrect driver speed. While the driver speed can be verified externally, investigating other causes requires opening the pump.