Why Suction Diffusers Are Important
Suction diffusers pack a lot of functionality inside their deceptively simple castings. Not only do they reduce turbulence in the water before it enters the pump, but they also play a critical role in the performance and efficiency of the entire system. In this article, we’ll discuss the functionality and advantages of suction diffusers and explain why careful selection is critical.
How Suction Diffusers Reduce Turbulence
Turbulent flow at the pump suction is a noisy precursor to pump damage and premature failure. Installing an elbow on the suction side of a centrifugal pump can cause significant turbulence due to the sharp 90-degree directional change. Instead of using an elbow, it is recommended to use a suction diffuser to reduce the water turbulence at the pump suction. If a suction diffuser not going to be used, the piping design must include at least five pipe diameters of pipe length between the last piping elbow and the pump suction to reduce turbulence. This extra bit of piping allows the water to settle; however, it also creates additional equipment and installation costs and takes up precious floor space.
It is recommended to use a suction diffuser that combines the roles of an elbow, strainer, and flow straightener all in one package to eliminate the need for that extra piping (Figure 1). An internal baffle “slices” the flow into four separate veins, which serves to “straighten” the flow before it enters the pump. This x-shaped piece fits snuggly inside a perforated carbon or stainless-steel cylinder. The perforations prevent large sediment or debris from entering the pump. In addition, the cylinder sits inside a fine mesh strainer which is only used for start-ups and should be discarded within 24-48 hours of operation. Do not forget to discard this strainer!
B&G’s Suction Diffuser Plus includes all the above, plus a shallow, internal flow cone device that helps eliminate the recirculation that occurs inside the diffuser body due to the 90-degree directional change it imposes (Figure 2). These design features help to reduce the stress on the pump, while also allowing it to retain its full efficiency.
Why Careful Selection (and Specification) Is Important
Suction diffusers vary in their internal design. These design variances create a notable difference in pressure drop—even between valves with the same size connections. Tests have shown that some suction diffuser designs provide as much as 5% more efficiency than same-sized models from other manufacturers, simply due to the lower pressure drop.
This underscores why knowing the specific pressure drop through a particular suction diffuser is so critical to pump performance and system efficiency. Selecting a suction diffuser isn’t just about matching inlet and outlet pipe sizes. Per ASHRAE 90.1, engineers must include the pressure drop of the specific brand, model, and size of the suction diffuser in the detailed pump head calculation. As an extra measure, we also recommend that the pressure drop (not just the size and brand) be specified right along with the suction diffuser itself. That way, any substitutions will be required to have the same pressure drop as the original specification. Otherwise, the owner could end up with a pump that can’t deliver maximum flow.
To illustrate this point, let’s say the engineer has selected a pump that is sized for 1000 GPM at 60 feet of head. The engineer has completed a detailed pump head calculation based on a piping system with a B&G Model HG-3 suction diffuser with an 8-inch system connection and a 6-inch pump connection. The Cv of this suction diffuser is 1050, which translates into a pressure drop of 2.1 feet at 1000 GPM.
Now let’s say the installer wants to substitute this suction diffuser with another brand with the same connection sizes. He or she chooses the same size valve from a different manufacturer, but the Cv of this valve is only 554 GPM. That means that at design flow (1000 GPM) the pressure drop through this substitution valve would be nearly 7.5 feet! (Note that these values are based on real-world valve data.)
The pressure drop of the substitution valve is more than triple that of the originally specified valve. Not only will this result in increased energy costs for the life of the system, but the pump head calculation is no longer correct. The system now needs a pump that is sized for 1000 GPM at 65 FT head!
Don’t underestimate the lifetime role that a suction diffuser plays in a hydronic HVAC system. And never assume that any two suction diffusers are created equally in terms of pressure drop. If you need any advice on parts or would like to order a Bell & Gossett or Armstrong Pump suction guide, you can contact Inline Sales & Service.