One of my previous Pumps & Systems columns expounded on an urban myth that all centrifugal pumps arrive on the jobsite ready to operate—“plug and play.” In almost every case, this myth is “busted.” Most every pump requires at least five areas of attention prior to startup, including setting the impeller clearance, which is the focus of this month’s column.
This example uses the most common type of pump—a B73.1 ANSI pump. Most manufacturers include an instruction manual and warning tag that advise the following information about the startup process:
Alignment: The alignment (driver to pump) as received is probably not correct, and it is surely not the final alignment that would satisfy acceptance criteria. Even if the factory completed an accurate alignment, it will change during the transit and installation phases. When the base is installed on the foundation and the piping is bolted on, the alignment will change.
Bearing housing: There is a 98 percent chance that no oil is in the bearing housing.
Mechanical seal: The mechanical seal is not set to the proper dimension. It may be in a safe (sleeping/transit) position with shipping clips installed. The mechanical seal may not even be installed. These are additional reasons why the coupling is not installed (fully) at the factory.
Phase rotation: Assume the driver is a 3-phase motor and the factory has no data in reference to your electrical system phase rotation. There is a 50-50 chance the factory will have the phase rotation correct. If it is wrong, you risk destroying your new pump.
This is another reason why the coupling is not installed.
Impeller clearance: Last but not least—the focus of this month’s column—is setting the impeller clearance. I continually witness this step ignored or completed improperly more than any of these mentioned issues.
Even if the purchase order specified for the factory to set the impeller clearance at a certain dimension, why would you not check the clearance in the field to verify the setting is correct? Unless there is an unbroken chain of custody with the pump, you cannot be positive and, besides, it is easy to check.
Why do you care if the impeller is set incorrectly? In addition to reduced efficiency, you are effectively reducing the impeller’s size. Just 0.015 to 0.020 inches off the correct clearance can reduce the performance of a 10-inch impeller to perform like 9.5 inches.
There are primarily two main styles of ANSI pump impellers. This article will not address the subsets of low flow/high head (constant velocity) and recessed impeller (vortex) designs.
There are two main styles of pump/impeller arrangements: open impeller and reverse-vane impeller.
With an open (semi-open) impeller (see Image 1), the clearance is indicated (set) off of the pump casing to the front side of the impeller (vane tips). With a reverse-vane impeller (see Image 2), the clearance is set off of the stuffing box to the back side of the impeller.
Both are good designs, but determining which one is better depends on the fluid properties you are pumping and perhaps the hydraulic conditions dictated by the system design. If the question of which design to use was up for debate, I would gladly accept a position on either side.
The concern is that many mechanics do not set the impeller to the proper clearance no matter which style of pump and impeller is used. On either style, the entire rotating assembly (shaft, impeller and bearings) can be moved axially utilizing design features at the coupling end of the pump bearing housing. In both cases, with the pump assembled you can move the impeller (the entire rotating assembly) toward the suction end of the pump until you touch the casing and in the other direction until the impeller touches the stuffing box (sometimes called the sealing chamber or back plate).
The difference is that with open-style impellers the correct running clearance is set off of the casing (see Figure 1), and with reverse-vane impellers the clearance is set off of the stuffing box or back plate (see Figure 2).
The actual impeller clearance dimension will vary by manufacturer, but more so by pump frame size and the temperature of the fluid being pumped. Refer to the manufacturer’s instruction book for the correct clearance based on frame size and temperature.