Pressure Boosting Systems – Pt3

By January 18, 2022Pressure

With few exceptions, the water demands of a Commercial Building are significantly overestimated, which has negative impacts on the long-term performance, reliability, and efficiency of a Booster System.  At Cougar USA, we design our systems to be most efficient at the partial load of the building, which improves performance & reliability and reduces operating costs.  In this Tech Talk, we cover the pump and system selection for Booster Systems.

Make sure to check out Parts I & II!

Pressure Boosting Systems – Selection – Part 3 Tech Talk Transcript:

Hi, I’m Tim Zacharias with Cougar USA, on this Tech talk we’re going to be going over system selection for pressure-boosting systems in commercial building applications. This is part three of a three-part series. So please check out Parts one and two if you haven’t already.

Now that we’ve determined our flow in our pressure for our building, we can take those and make an actual pump system selection. Based on our fixer unit counts for the worst-case scenario and specialty applications we have a potential load requirement thereof 300 gallons per minute, but we do want to take into consideration that partial load and that diversity factor that we talked about, the 15%. So we’re looking at about 50 GPM, there, is really where this pump is going to be operating most of the time. We want to pick our pump selections based on this 50 gallons per minute and not necessarily 300 gallons per minute.

In order to be covered, we want to make sure that we have enough pumps to cover that worst-case scenario if needed. So with the Grundfos Hydro MPC system boosterpaq, you can go up to five or six different pumps, so we can really get in, dial-in that pump selection to get it where we need it to be.

So what we don’t want to do is Select that pump, you know, let’s say we want two pumps, we want some redundancy. Set two pumps at 300 GPM each, so we’re going to be 100% redundant there. But, really, those pumps want to operate way out here and not back here at this partial load. A pump that is going to be efficient out here at this 300 GPM, is not going to want to operate back here at this 50 GPM where we’re going to see that load most of the time. So, instead, we’re going to pick based on this 50 gallons per minute and have multiple pumps. So we can do that, we can break this up into thirds, we can do 33-33-33 each at 100 gallons a minute. We could do 50-50-50 pumps at 150 gallons a minute each, or you can do four pumps at whatever, you know, 60 or 75 gallons a minute, start to add up your redundancy.

Now, when we’re looking at the redundancy, you know again this 300 GPM is going to be the worst-case scenario. Probably never going to hit that, so even if you were to go in this case, we’re going to do three pumps, each at 100 GPM, and really where that redundancy is going to come from is the diversity factor in the building. If you wanted to have an n + 1 you could go 4 at 100 or we could do a 3 at 150, but in any case, we really want to keep that pump selection to 100 to 150 GPM per pump because those are going to operate best at that partial load condition. They’re going to be able to run at this reduced speed and still be efficient, still be within their preferred operating range.

A couple of different options to give you some redundancy there, just depends on, you know, having a type of risk or how close are you going to be to the actual worst-case scenario there. But again, we would advise that you go with smaller pumps and had more of them to cover the redundancy rather than doing less larger pumps.

Okay, so that would be our flow right here, any of those selections would work well. When we’re talking about pressure now, remember we have to consider that pump versus the system boost. We want to make sure that our pump boost is correct, and in the city of Houston, that’s going to match the system boost because we’re starting from zero and that was 150 PSI or about 350 feet based on our example and again that’s represented here. But we want to look at when we’re selecting our pump, where we are going to be operating at these partial loads. It’s easy to select the pump out here where we are running it at 100% and we are satisfied that that Duty point, but ultimately again, we’re going to be operating back here somewhere at these partial speeds. So these could be in 95-90-85-80 even lower, potentially, depending on your flow rate. Where that pump is going to operate as the demand slows down so as that the man reduces the pump speed is going to slow down to maintain that constant pressure at the different flow rates

So we would like to see the pump be able to maintain Pressure at 80 to 85% speed at the partial loads. So if we can get to this 50 GPM and maintain 350 ft at about 80 -85% speed. That’s where we’re going to want to be, that’s going to give us the additional flow that we need as the demand increases. It also gives us some additional head in the system if we need it for whatever reason if we need to increase our set pressure.

So really two important takeaways here are going to be that our pumps are between we’re looking to try to select our pumps that do about 100 to 150 gallons a minute and add multiple to get redundancy and our worst-case scenario covered and then on the speed on the head side. We want our pumps to be able to maintain our set pressure at about 80 to 85% speed at that partial load condition.

If you have any questions on systems selections or anything that we’ve covered please reach out to us or check out our other videos. Thanks.