The primary purpose of a Pressure Boosting System is to maintain constant pressure in the building (surprising, right?!). In Part II of this Tech Talk, we explain the factors to consider when determining the pressure requirements for a building.
Pressure Boosting Systems – Pressure – Part 2 Tech Talk Transcript:
Hi, I’m Tim Zacharias with Cougar USA. On this Tech talk, we’re going to be talking about calculating the system pressure required for a pressure boosting application of a commercial building. This is part two of a three-part series, so if you haven’t seen part 1 on the flow calculation, make sure to check that one out as well.
When we are looking at the pressure required for a pressure boosting system in a commercial building, the first thing we wanted to look at is our static pressure. Now, this is going to be literally what the weight of the water is to fill up the riser in the building and get it to the top of the building. So our very nice and friendly building over here is 20 stories at 231 feet so if we were to convert that to pressure, that would be 100 PSI. So if we filled this riser in this building with water, but it wasn’t under pressure we would see 100 PSI on our gauge, that would be our static pressure. Now, a little trick that we use a lot of times on existing buildings is a lot of times we don’t always know what the actual height of the building is, so we can kind of back into it by looking at the number of floors of boost. So if we assume that every floor is about 10 to 12 ft that’s going to be about 5 PSI per floor, and you can see our math works out pretty well in this one to be similar to 100 PSI not always going to be the case and all buildings, but it will get you close.
So for this example, we have 100 PSI as our static pressure. Now, the residual pressure is anything else that we need on top of our static pressure to make the water usable at the top. Our cooling tower is going to be one of the main reasons that we need this additional residual pressure up at the top. It could also be for our flush valves, it could be to go to water heaters, whatever it is, make sure that you have that residual pressure. We’d like to see a minimum of 45 PSI, that’s going to be able to get you through backflow preventer, and then out to the cooling tower, you know, if you got to go also through a softener and things like that and you might want to have a higher residual pressure than that.
Another thing to consider is going to be friction loss. Now, to calculate the actual friction loss in the building you’re going to want to have the length of pipe and any set fittings, things like that, to be able to convert those two equivalent lengths of pipe and then look at your flow rates to see what your actual friction loss is. When we’re dealing with pressure boosting applications, typically, we are going to be looking at lower flow rates most of the time, you know, 2200 gallons per minute, and if we have decent pipe sizes (2 inches and larger) the friction loss and can be pretty minimal. So we do have some, but for this example, we’re going to go pretty low and just have the 5 PSI there for friction loss, so that would give us a system pressure of 150 PSI. Now what that means is that we need that pressure here, at the bottom of the building, we need to have 150 PSI on the discharge of our pumps to be able to see this 45 PSI up here at the top for it to be usable.
So that’s what we’re going to want to see in the discharge of our pump system. And the other thing that we need to consider once we have our system pressure is, what are we getting from the city supply? Now, this gets a little bit complicated when we working in the city of Houston, because we have the code requirement to go through the atmosphere storage tank. So inside the city limits of Houston, our City supply is going to be zero effectively. We do get whatever the head is in the tank, but for these purposes, we typically use a zero as our suction pressure.
If you were in a municipality that did not have that code requirement, you could take whatever the city supply pressure is from the city and subtract it out from this here. So I will show you what that looks like. Let’s say that we have a 50 psi coming in from the city, we would have 50 PSI that we would subtract from our system pressure there, meaning our pump boost is only 100 PSI. So out of the 150 PSI required for the building, the pumps are only responsible for 100, the city pressure is taken care of 50 of that. But what we are dealing with most of the time here in Houston is where we have zero pressure coming in from the city because we were starting from that storage tank.
So we’re going to show that example here, and the effect that has on the pumps elections if we assume, Zero here for incoming pressure, now our pump boost is going to be the same as our system pressure. So the pumps are going to be responsible for all of the Boost that is in that system pressure. It is very important to know if you have to go to the storage tank or not so you can see how much that affects the pump boost that is required.
Make sure you check out part 1 and part 3 of this series. Part 3 we’re going to look at how to take your flow and pressure calculations and turn them into a system selection.
If you have any other questions, please feel free to reach out or check out our other videos on our site. Thanks