Pump Myth #1: Backward Rotation

Over the next few blogs, I will attempt to address some myths that exist regarding centrifugal pumps. Today we will talk about reverse rotation. Reverse rotation may exist for either of two common reasons:

1) The most common reason is that when a three-phase motor is wired up and connected to voltage, there exists a 50% chance that it will turn the wrong way. So, when the installer first starts the motor, the proper procedure is to “bump” it. That is, connect it for a very short time (usually accomplished with the “Hand” or “On” position of the starter). The pump will begin to rotate. By observing the rotation of the shaft, the installer can tell if it matches the rotation arrow on the pump casing. Note that it is extremely important to fill the system first, as the pump must be full of fluid to avoid damage to the seal! If it does not, the solution is to switch any two of the three power leads. This can be done at the motor, but it is often more convenient to do it at the motor controller (starter of VFD). Very occasionally, this is not done, or somehow phase reversal later happens UPSTREAM of the motor in question. A common myth is that in reverse rotation, the pump causes backward flow, that is, IN the discharge and OUT the suction. In reality, a pump operating in reverse rotation because of wiring or phase change will pump in the normal direction. Now, it won’t pump very well. Its flow will be reduced, as will its head. The NPSH characteristic will not match the cataloged value. The pump may be noisier than normal. Operation in this mode may cause several problems. Flow switches or differential pressure switches at boilers or chillers may fail to close. Hot water boilers may cut out on high limit (and chillers on freeze stat), as the flows are so small that the temperature change thru the boiler or chiller is excessive. I was once on a project where the boilers had cycled on high limit millions of times because of extended reverse rotation! In any case, the reduced flow and head will probably lead to a building or process that does not heat or cool properly. So, checking rotation on startup really is important. The seal may leak and even be damaged after a period of operation (though I have not personally witnessed this). The biggest problem might occur in a parallel pumping application where the other pump operates normally. The higher head developed by the properly-operating pump would cause the “backward” pump to operate at shutoff condition, likely eventually damaging the backward pump (see our Course HYD-150, section on parallel pumping with dissimilar pumps). Go ahead–you can read it without buying the course.

2) The other reason for reverse rotation can sometimes happen in parallel pump installations where one pump is off. Short circuiting of flow thru the off pump should be prevented by a check valve. If the check valve leaks, reverse flow from the discharge to the suction connection WILL occur in the “off” pump and this WILL cause the pump to rotate in reverse. The rotational forces and the speed of rotation depend upon the amount of the check valve leak, the head differential, and the impeller design. Again, a seal leak may eventually occur but the BIG problem is when the reverse-operating pump is called upon to start. If the reverse-rotation speed is at all significant, significant stresses will be placed upon the shaft, the coupling and the motor. Think about throwing the car into drive while backing up rapidly! Failure of the coupling and/or motor is possible. So, while I can’t think of a specific myth regarding THIS type of reverse rotation, I can say that it’s important to take care of it when it occurs. So, it is a good idea for a maintenance person to take a quick look at “off” pumps in parallel installations to be sure that reverse rotation is not occurring.

Any comments relating to YOUR experiences with reverse rotation are certainly welcome.