adjusting return pump impeller clearance

In this issue, you will learn how to identify and repair the other common cause of low boiler pressure: excessive clearance between the impeller and raceway in the Burks 5 and 7 CT family of turbine pumps used on many of the return systems found in the dry cleaning and laundry industry.

Unlike problems caused by excessively high water temperature in the return tank (see TROUBLESHOOTING BOILERS GONE BAD II), the water temperature in the return tank and outlet of the return pump will not be excessive. (The pipe at the inlet side of the check valve nearest the boiler will be hot but hardly evaporate spit). The culprit here is pump wear; a simple pump impeller adjustment will have you running in no time. Here’s the reason why this happens. As the pump is used, the impeller and raceway slowly erode, increasing the physical clearance between them. This lowers the maximum pressure the pump can push against (sometimes called deadhead pressure). Eventually, you’ll notice that the return pump is always running, and the burner is now turned on and off by the low water level cut-off. By this time, you will have wished that the low water level alarm would “just shut the $#&% up” and will be making frequent trips to the boiler room to press the water level reset switch. Before going further, we need to be careful to ensure there is a sufficient water level in the return tank and that the strainer between the return tank and condensate return pump is clear and allows water to get to the pump. These conditions will mimic the symptoms of a pump impeller clearance problem.

 I know I repeat myself, but YOU MUST SHUT OFF THE BOILER AND BLOW DOWN UNTIL THERE IS NO BOILER PRESSURE! DO NOT DO ANY REPAIRS ON A BOILER UNDER PRESSURE UNDER ANY CIRCUMSTANCE!

Here’s a list of tools you’ll need for this job:

Large flat blade screwdriver; small hammer; spanner wrench for impeller adjustment nut (it is well worth investing in this tool instead of trying to use a punch); vise grip pliers (needle nose type if possible); and a 5/16” combination open end/box wrench; Spray can of WD-40 or a good penetrating oil.

BE SURE THE ELECTRICAL POWER TO THE BOILER AND PUMP IS OFF! Once the boiler is no longer under pressure, get some decent lighting on the return pump area and wipe it down, removing dust and debris. While you’re down there, ensure the air vents (the cutouts at the end/ends of the motor) are free of obstructing lint. Adjust the vise grips to grab the PUMP SHAFT near the motor gently. Rotate the shaft. If it swings very easily up and down, you must adjust the pump impeller. On the older Burks pumps, a plastic cap or metal plug covers the rear shaft bearing at the center of the pump. On the pump end of the motor is a round IMPELLER ADJUSTMENT NUT (SEE FIGURE 1) about 2 inches in diameter with 6 blind holes drilled around its circumference. The PUMP SHAFT goes through the center of this IMPELLER ADJUSTMENT NUT, through the inboard wall of the pump housing, then through a rotating seal and into the pump impeller (not visible because it is enclosed in the pump housing). The pump impeller is pressed on the end of this shaft. Spray some WD-40 over the shaft and on IMPELLER ADJUSTMENT NUT, alsowhere the impeller shaft enters the outboard wall of the pump housing. If you have time, allow the WD-40 to penetrate the parts for a few minutes.

Near the IMPELLER ADJUSTMENT NUT is an IMPELLER ADJUSTMENT RETAINER (a small flat slotted piece of metal with a right-angled tab at the end which goes into a hole in the IMPELLER ADJUSTMENT NUT) held in place by a small 5/16 headed bolt called the IMPELLER ADJUSTMENT RETAINER BOLT. Using a 5/16 wrench, unscrew the small IMPELLER ADJUSTMENT RETAINER BOLT from the pump body and remove the IMPELLER ADJUSTMENT RETAINER. Now you’re ready to actually to make the adjustment.

To better understand where we are heading, let’s take a moment and visualize what we’re doing. Remember that the cause of our problem is too much distance between the impeller (the rotating part on the end of the PUMP SHAFT) and the raceway (the stationary part that also is the outboard end of the pump housing where the inlet and outlet pipes are screwed into). We will adjust the pump by moving the impeller closer to the raceway; here’s how.

Adjust your spanner so the protrusions at the ends fit into holes in the impeller adjustment nut. Rotate the IMPELLER ADJUSTMENT NUT clockwise (I use the mnemonic “clockwise is closer” to remember the effect of rotating the IMPELLER ADJUSTMENT NUT) direction facing the nut. After about every 45 degrees of rotation on the impeller adjustment nut, try rotating the PUMP SHAFT using the vise grips. Continue the adjusting until you feel a lot of resistance when attempting to rotate the PUMP SHAFT. That resistance indicates the impellor is resting against the raceway with no clearance, and you are ready to make the clearance adjustment.

Turn the IMPELLER ADJUSTMENT NUT in a counterclockwise direction about 10 degrees at a time and try to rotate the impeller shaft after each adjustment. When the impeller shaft drags slightly, adjust back about 3 degrees, and you’ll have the proper adjustment.

Place the tab end of the IMPELLER ADJUSTMENT RETAINER into a hole in the impeller adjustment nut and bolt it in place with the IMPELLER ADJUSTMENT RETAINER BOLT. You’re now finished with the adjustment; remove tools from the work area. Turn on the power and switch the pump on while listening to be sure the motor sounds smooth without straining or grinding noises. IF THERE IS AN INDICATION OF A PROBLEM, IMMEDIATELY TURN OFF THE POWER and check to make sure the valves between the return tank and pump, and the pump and boiler are opened. If all valves were correct, re-adjust the impeller for slightly more clearance.

Well, that’s it for this month. Look for a downloadable PDF file of this article on my FIXIT-ACADEMY website www.fixit-academy.net  , where even an amateur can learn to make their own equipment repairs.