How to Use the Fire Pump Analyzer?

MeyerFire University | TL104.05

By Joe Meyer, PE

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TRANSCRIPT

So today let's introduce the Fire pump Analyzer.  

This is the downloadable Toolkit package. 

We also have an online version. But for the Fire Pump Analyzer is right here. And what we're ending up with is a combined city supply and fire pump pressure scenario. 

The goal here is to look at big picture.

When you have a fire pump and a city water supply information, plus you have an idea of the demand that your system is going to have, this is really to help what are your rated flow needs for your fire pump, to satisfy your system.
 
So let's just look at an example. Let's say that we've got flow tests that came in, and it's relatively low and flat of 45 PSI, 40 at 1,800. That's a relatively poor to average city pressure, but is relatively flat. 

Meaning that it's got a good handful of flow. 

You see, this is relatively speaking our curve is a very flat curve.

And let's say that our system demand, we’ve got an ordinary Hazard Group 2. 

That's maybe a three story or something like that.  

We can do as our estimator tool.

We can run a hydraulic calc, there's a number of ways that we can get estimates here. 

I'm just going to plug in some rough numbers 0.20 / 1,500 30% overage. And let's include, uh, hose allowance, just to be a little bit conservative.

So we can go into more detail about that separately, but just as an estimate and then our system demand tells us, hey, we're actually going to need, we're going to want 90 PSI.  

Oh, and here I've got hose allowance broken out separately, so let's just run this as a 1,500 with 1.3 overage.

OK, so.

This is where our demand falls.

And it's significantly higher than what the pressure from the city can support. So what size fire pump do we need?

This is an opportunity to say, you know the blue corresponds system demand. That orange is the city pressure. This allows us to play at the fire pump size and say you know, what realistically do we need? If we've got a combined flow of 640 GPM?

That's, let's see, and we take 500 GPM pump and add 20%. That's only going to get us up to 600 GPM.

So let's say our fire pump is 750.

Now the curve that's being drawn in here is a curve that meets the requirements of NFPA 20, meaning that the 150% rated pressure is exactly 65% of the rated pressure and the churn matches that curve.

So when we know actual pump curve values we want to plug that in but just from an estimation standpoint, this is this is a way to match a typical curve that NFPA 20 is outlining for us. So do we need 75 psi pump? Well, we're getting quite a bit of safety here. 

If our point of interest which we plug in here, this is going to be a new green point. If our point of interest is actually 640 GPM, then right here, what we're getting kicked back is our city pressure at that point is 44 PSI.

That's what the city is supplying.

That's right down here.

The pump is providing 80 psi. This is based on a 75 psi at 750 GPM rated pump.  

So our combined is 124 psi.

We don't really need that. It looks like you know our demand is 90 psi. Let's drop 20 pounds off that. Let's go with a 55 psi rated pump at 750. 

All these numbers are going to recalculate, so we're still getting 44 pounds out of the city. We're now getting 59 psi out of the pump.

Now keep in mind, 59 is slightly higher, but our flow if we're running back up the curve, our flow is at 640 at a demand point, not 750, so that makes sense and we're getting combined 103 psi at that point. So right now we're playing with 13 pounds of safety.  

If this is a pretty dialed in scenario, then that might be about spot on with what we're looking for.  

So let's reset and run through a second scenario with this analyzer tool. Let's say that we've got a standpipe demand sprinkler building and it's 1000 GPM as our demand. So I'm going to put in here 1000 GPM demand no hose flow.

And when we run it all the way, when we're running the demand all the way back to the source, we are needing 125 psi.

Flow test information comes in. Let's say we have a similar poor flow test at 45, 40. It's relatively flat 1200 GPM.  

This is, this is what we're getting now. Our point of interest in this case is 1,000 GPM, because that's the demand at the standpipe.

So this is going to run all the numbers right here and if we are looking at our pump size, 1,000 gpm demand. It's we're going to want 1000 GPM pump here, not for the reason that you might think a 750 GPM pump by NFPA 20 can handle 40% more flow than its rated.

So if you took 750 * 1.4, that's going to get us to 1,050 GPM.

So while we're allowed to do that, that's really stretching the ability of the pump.

For our demand point, there's a number of engineering practice and insurance requirements that says we really shouldn't go beyond 20% over the rated capacity of a pump. Meaning if we have a 750 GPM rated pump, and we're allowed to then design with the demand up to 900 GPM. Since here our standpipe demand is 1,000 GPM. Good engineering practice would say go with 1,000 GPM pump.

But what is this rated capacity? 

What does this need to be?

Well, if we've got a demand point of 125 psi.

And combined our city and pump at 1,000 GPM is only 116. We really. Let's say you know with the local safety factor, we actually want that to be 135 or 140. Well, let's start bumping this up 80.

85 Plus I, you'll see that our pump curve is going higher and higher. Here we're getting a combined higher pressure, which is that that green curve.

What if we're at 90 psi. I well now we're at 131 psi our demand points 125, about 95 psi.

Now we've got 11 psi cushion.

Combined at that point, we're getting 136 between the pump and our supply.

Our demand point is right here and you'll see if it was just the pump with, say, an atmospheric tank, this would be our supply curve.

It would fit with just the city. 

It would be this orange curve, but combined we’re now supplying more than what the system needs, so this tool is not meant to replace hydraulic calculations, and it's not meant to fine tune your pump size to get exactly what you're looking for, but it does give an idea on NFPA 20 curve for a pump and an approximate size and rated capacity that you'd be looking at if you're looking at preliminary numbers.  

Or you're specifying a project you could say, well, we're looking at a minimum pump size of 95 psi at 1,000 GPM. That could be a fair specification if that's where you know you’re going to be.

You could take those numbers, go to a pump manufacturer, or look up pump table charts and pull a horsepower off of that.

And now give that horsepower over to the electrical engineer and say if we've got generator backup, here's the size pump that we're looking at and start to plan power requirements or plan space requirements.

There's a whole number of things with fire pumps that this quick analysis tool will help to get an approximate size of your pump and you can start coordinating and working from there, but that's the the Fire Pump Analyzer it basically as an overview.  

That's that's really how we use this around here.

Thanks for watching.