By Chad Edmondson (JMP) and Norman Hall (RLD)
As we conclude this series on how to select a centrifugal pump for your hydronic applications we want to leave you with the following “best practices” for overall safe, efficient, and long lasting performance.
Specify Pumps with an OSHA Approved Coupler Guard. This really should be a non-negotiable feature of any flexible coupled pump. Coupler guards prevent any dangling items (neckties, chains, etc.) from getting caught up in the coupler as the shaft is spinning. Such incidents can cause serious injury and have been known to result in lawsuits.
State and Federal codes state that when a motor is flexibly-coupled to a pump shaft, there should be an OSHA-approved guard. Bell & Gossett provides OSHA-approved coupler guards, like the one show below, as a standard feature on any pump it sells. That is not the case with every manufacturer so it is essential that you include this requirement in the specs. Otherwise you get the old-style, U-shaped strip of sheet metal which is not OSHA approved.
Look for Pumps with a True Back Pull-0ut Design. A pump with a true back-pullout design as shown below allows servicing the pump without moving the motor.
Select pumps with a Solid-foot Mounted Volute. Pumps with an integrally cast volute foot help you maintain pump alignment during maintenance. Also, since the piping can be quite heavy, this support keeps distortion from occurring which can lead to premature failure. This is especially important on hot water systems because of the continual pipe expansion.
Choose a Drop-out Spacer Coupling. This allows for removal of the bearing frame and rotational element without disturbing the pump alignment or electrical connections. Make sure the coupler type specified will operate without issue when used with a variable speed drive.
Consider Specifying a Trolley and Gantry for Inline Pumps. Remember, the motors for vertical inline pumps must be removed for servicing the pump bearings. Since these motors tend to be heavy, do your client a favor and include a “hoisting” mechanism so that the motor can be carefully lifted away from the pump for servicing. If this is not an option, make sure there is plenty of space around the pumps so a contractor can locate equipment to service the vertical inline pump.
Double Suction Pump Pipe Orientation. Avoid using an elbow that has a plane that is parallel to the pump shaft. Otherwise the flow will be severely non-uniform, creating fluctuating high pressures on one side of the impeller inlet, while starving the other side. This can also cause noisy operation and even damage due to cavitation. This needs to be a consideration when choosing between a base-mounted or vertical inline double suction pump. If the pump shaft is parallel to the elbow then a suction diffuser should be used
For double suction base-mounted pumps, depending on the piping arrangement, you can better orient the suction and discharge flanges in one of the following ways to eliminate an elbow:
- Horizontal discharge and horizontal suction (VSH)
- Vertical discharge with horizontal suction (VSCS)
- Vertical discharge and vertical suction if you have a Vertical split case pump (VSX)
Finally, the chart below is an excellent general reference for when to specify which type of pump. As you will notice, all the pump types have flow ranges that are beyond what is actually optimal in terms of operating efficiency – sometimes well beyond. Ideally you want to stick with a pump type that will operate primarily within its optimal flow range (shown in red). This is not only improves efficiency, but also service life.