Domestic Water Pressure Booster Sizing Part 5: Selecting The Pressure Booster
/By Chad Edmondson
In our last blog, we determined that we required a 56 PSI pressure boost in our 100 Room Apartment example. Previous to that we determined (using the Hunters Curve) that we had a maximum total demand of 180 gpm. With these two values we are now prepared to select our pressure booster. But before we go there, let’s go over a few things about pressure boosters in general.
Domestic water pressure boosters will typically have one to four pumps. The number of pumps specified for a given system will depend on the demand (required gpm) and whether or not the application requires any redundancy. A “simplex” system with just a single pump is sufficient to handle most small demands assuming redundancy isn’t required. However, if the application is for a hospital or other critical operation, a duplex is often specified for back-up.
Multiple pump systems (duplex, triplex or quadplex) are used for larger flow systems with duty split evenly between the pumps (i.e. 50%/50% for a duplex, 33%/33%/33% for a triplex, etc.) In these cases a single standby pump can be added if there is a need for redundancy.
In most cases it is more efficient to operate a booster with multiple smaller pumps than one or two large pumps. Demand profiles only spike once or twice a day so the ability to stage smaller pumps on and off can be very advantageous. More often than not you will find that only one pump will operate at a time in a properly sized duplex system.
Finally, most pressure boosters have centrifugal pumps that are either single stage (low pressure applications) or multi-stage (high pressure applications). Single stage pumps are generally better suited for applications with high demand (flow) and lower pressure boost requirement (100 PSI or less). Multistage pumps are ideal for higher pressure boost applications or if you are pumping out of a tank due to the low suction inlet. For high pressure applications it is important to make sure the pump casing can withstand the highest possible discharge pressure. It is also a good idea to specify that a redundant high pressure safety cut-off switch be included with the booster if over-pressuring the system is a concern. This should be set for the maximum pressure that the system can handle.
Now, back to our example application and that 180 GPM at 56 PSI requirement…
Selecting the Pump(s)
The first thing we need to do is convert our 56 PSI to feet of head. To do that we simply multiply by 2.31:
2.31 FT/PSI x 56 PSI = 129 FT HD
So we need to select the pressure booster based on 180 GPM at 129 FT HD booster. We have determined that a duplex pumping arrangement with a 50%/50% duty split is good for this application. In this particular example we are going to consult a booster curve to make our model selection. HYFAB makes this easy with the following Model Identification Curve:
180 GPM at 129 FT HD puts us in the yellow zone, the operating range for the MVP-850 model. The last three digits of the model number (noted as “XXX” in the image above) refer to the voltage in our system. In this example, we have 460/3/60 voltage, therefore we will choose an MVP-850-460 variable speed booster system.
Since booster systems almost always operate at part load, this duplex selection with two identical 5HP pumps, each operating at 50% of the total flow demand, will save operating cost and gives us some redundancy if one pump goes down or needs servicing.
