How to Use the Water Storage Calculator?

MeyerFire University | TL104.07

By Joe Meyer, PE

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RESOURCES

TL501 SERIES

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NOTES

• Notes Page [PDF]

TL501 SERIES

1. What is the Sprinkler Database?
2. How to Use the Obstruction Calculator for Beams?
3. How to Use the Obstruction Calculator for Soffits?
4. How to Use the NFPA 13 Translator?
5. What is Driving Design; the K-Factor or Density?
6. How to Estimate Clean Agent Quantities?
7. How to Calculate a Domestic Demand?
8. How to Select an Optimal K-Factor?
9. How to Quickly Calculate Friction Loss?
10. How to Analyze Fire Pump and Water Supplies from a "Big Picture" perspective?
11. How to Determine Fire Flow with the IFC Method?
12. How to Quickly Estimate Hydraulics for a Sprinkler System?
13. How to Estimate a Water Storage Tank Size?
14. How to Calculate Hanger Spacing by Weight?
15. How to Calculate the Size of a Trapeze Member?
16. How to Summarize Notes for Fire Alarm and Suppression?
17. How to Calculate Thrust Block Size?
18. How to Quickly Classify Combustible & Flammable Liquids?
19. Calculate the Volume & Air Compressor Size for a Dry System?

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TRANSCRIPT

So today we're talking about the water storage tank tool. 

Now this tool is used to help size water storage tanks, which in concept is very simple, but in real practice has some challenges or some logistical details that we kind of have to vet out in order to correctly size a water storage tank. Now, this is a quick tool, it's available online under the toolkit. If you go down to water storage, this is our cloud tool, and there's only a few parameters that you have to put in in order to size a water storage tank. We'll go over this in a little bit of detail, and then I'll go through what the calculations are considering as far as these inputs go so that when you're doing your estimates, you can also review the calculations, review the report and tweak your results as a proper engineering assessment, so that you get the information that you need, but also, you know, get the right answer. 

So, under this water storage tank tool, this is available again on our cloud platform. You can input your sprinkler demand in gallons per minute, your sprinkler duration in minutes. So if we have, let's say a light hazard building that the entire hazard classification for the whole building is light hazard to 0.1 over 1500 square feet. So, we could estimate that as 0.1 times 1500 square feet, and let's add 30% so 150 GPM times an additional 30%, sometimes 1.3’s for overflow. We got 195 GPM. 

Now, if we wanted to be conservative and add a hose allowance because we have hoses in the building, then we could add a hundred GPM. There's debate whether you even need to include a hose allowance for water storage tank, if you have no way to retrieve that. But let's say just as an example, we have 200 GPM. Most of light hazard building and our sprinkler duration. We have a monitored sprinkler system, NFPA 13 says that we can use 30 minutes for a monitored sprinkler system. That's monitored by fire alarms. So we could just pop in 30 minutes there. Then what we're doing is relating that demand and multiplying that demand by a time duration, to get a capacity for the water storage tank. Now that's the basic premise, is what is your flow rate and what is your duration, and that's, that's what NFPA 22 is telling us that we need to provide at a minimum, but there's also some other considerations. So, when you have a seismic design category of C, D, E, R, F, then you have to add space at the top of the tank so that when the building is moving laterally during an earthquake, that that water has some space to slosh around, and it doesn't immediately force water up to the top of the tank and somehow compromise the top of the tank. 

Essentially, we needed a larger air gap at the top, so that that water is allowed to slosh. And we say the term slosh, it it's a true term. The water's allowed to slosh around at the top of that tank. 

So, there's those considerations here again, the flow rate times your duration, that's the basic premise. The seismic design category is enforcing a larger air gap at the top of the tank, depending on whether the building or sorry, the tank is in a seismic design area or not. Then over here, we have fire flow settings. So, fire flow is provided to supply the fire department with the water that they need to manually extinguish a fire. There's several different considerations. And in some cases, if a water storage tank is providing fire flow, meaning it is storing the water that is for use for the fire department during a fire, then we would need to include fire flow as part of this effort in this calculation. In some cases, that water is brought in by the fire department or it's provided elsewhere, it's not a part of this consideration. Our water storage tank is not supplying the fire flow. 

In that case, we would leave the fire flow setting off. But as an example, if we wanted to add fire flow, we have two options. One is that we can do the fire flow setting and set a fire flow and GPM. So, let's say 1500 GPM and we have a flow duration of two hours. Well, a couple things just happen. One is that it's taking this flow rate, which is significantly higher than our sprinkler demand multiplying that by two hours. And you see that our storage tank capacity jumped a lot. So, when we go from zero, we have a very small water storage tank up to two hours. We have a large storage tank and our schematic drawing here is this is essentially a very rough section view of the dimensions of our storage tank. So right now, we have a very squat tank that is relatively short, only eight, essentially eight feet tall and 18 feet in diameter. Well, when we add time, we would essentially have an extremely tall water storage tank. And we can adjust that, you could take the diameter of the tank. Now this, this diameter of the tank, this is considering that we have a round cylindrical water storage tank, which in general is the most common. If you're looking to do some kind of an underground tank or a square tank or a cubic tank, then you'd have to make some other considerations, use the capacity more so than the height, but for our purposes, we usually have round cylindrical storage tanks. So that's why we use the diameter here. 

Now, just by clicking on this, you can then use a down arrow and start to adjust the diameter of the tank. And you'll notice as the diameter of the tank increases the required height, the minimum height decreases. So, the wider we go with the same capacity needs, the shorter that our tank is able to be. 

 If we have a tank that is supplying a lot of fire flow for a long period of time, our tank's gonna grow in size significantly. The other setting that we have for fire flow is a fixed volume. Let's say we need to provide a fixed volume of gallons that's a reserve or something like that. Well, we can add that reserve. Let's say 10,000 gallons is what we need to add. That's only incorporating this, this duration no longer applies, ‘cause we're not in the fire flow setting, we're only adding a fixed volume. 

So, we're adding 10,000 gallons. Well, we can put it in as a fixed volume for our purposes. I'm just gonna leave that at zero, just so we can do this calculation. And then the diameter of the tank, we went over this just briefly, but the diameter of the tank is, if you're looking down in plan view, the diameter of that cylinder and what that setting needs to be. 

So, in this case, we've got a very low demand overall. We go with a smaller diameter and it's relatively short. Our refill rate, this is the rate at which the tank is automatically refilled. This is an important consideration because NFPA 22 tells us that we have to be able to automatically refill the tank within eight hours, or there has to be some kind of approved alternative with the authority having jurisdiction. 

So, our default required time to refill is this eight hours, which is what NFPA 22 requires. But our refill rate, especially when we get into remote areas where we don't have a good water supply, this refill rate can be pretty tricky. If we have an existing well, we need to know that we reliably can get a certain flow rate from that well. Well, wells change over time in what they're able to supply. 60 gallons per minute is a lot of water out of a well for most rural areas. 

Let's say that we actually have a well that is 20 GPM. There's a few things that are happening here. One is that our storage capacity increases. As we go up, you'll see that the storage capacity decreases. The better our refill rate, the more we can use that in our calculation for water storage, but getting it back down to 20 GM, you'll notice that our storage capacity increases and we get this warning that the refill rate, which is nine hours is longer than eight hours and an alternative is required. 

This is saying that with 20 GPM, in order to supply the storage capacity that we would need for this tank, that our refill rate is longer than the eight hours that's mandated. Now, there's a few settings we can adjust here. One is that if we work with the authority having jurisdiction and they accept a different refill rate, they say it's okay to be 12 hours or we're okay to have an emergency standby water supplier that can refill within eight hours on contract or something to that effect. Then we may be able to adjust this or overwrite it. In general, our time to refill is eight hours. But let's say that we work with the HJ. We provide some alternative approaches for better fire protection in other areas. and they allow us to go to a 10-hour refill rate. Now our warning goes away and our storage capacity decreases because we're not carrying extra capacity just to allow for a little bit left over because we have a low refill rate. 

So last two items here, the air gap at the top of the tank. Now, if we have a seismic design category, we need an air gap of three feet. So, this is being overridden right now. And you'll notice that up until three feet, these settings don't change anything because our seismic design category is automatically overriding this air gap. Now, if we were to go to three and a half feet, our tank height is going to adjust. 

So now it's starting to adjust because it's greater than what the seismic design category requires. I'm just gonna leave that back at one foot, one foot's common. And if we go back to non-seismic, you'll notice now we lose two feet off the top of the tank. That's because we're in a non-seismic area and it's just using this setting for the air gap at the top of the tank. 

So now when I adjust that air gap at the top, it's adjusting the overall height of the tank. The other dimension that's important is the suction height above the base of the tank. So, this is our vortex plate and the height at which war are taking water, the suction from that tank. We have to have a little bit of extra space at the bottom so that the water's able to go up into that suction pipe. And that's essentially a wasted capacity at the bottom of that tank. 

The water that's below the suction line can never be sucked up. So, it's a volume of water that's essentially wasted. It has to be included when we're looking at the storage capacity of the tank overall, but it's wasted. So let's say our suction vortex is especially high, it's two feet off the base. So, we bump this all the way up to two feet, and now our storage capacity increased and the height of the tank increases in order to capture that. 

So all of that, there's a lot of calculation. There's a lot of background material that's going on with each of these considerations. And the way to tell what's going on is just to scroll down here and hit print. The other way is to go to the top and hit this calculation tab. 

And you'll see, this is a live calculation. That's breaking out each of these things individually. So why do we have the storage capacity that we do? What considerations are being made as it goes through? Well, this breaks it out. Does the calculation performs it and if we wanna print this, we should have a PDF friendly report here that's breaking out all the considerations that are being made and how we end up at the heights that we do. 

So, I'm hopeful that this tool will be helpful for when you're estimating water storage tanks. And I guess one important last detail is that we're providing minimums here. So just because we have 16,000 gallons, or we have 132,000 gallons or 300, and 47,000 gallons. These are the minimum storage capacities. So, the actual storage capacity of a tank is probably gonna be more like a nominal size. 

And what I mean by that is, you know, if we've got a eight-foot, eight-inch minimum height for our tank, we're probably not ordering it at eight feet, eight inch. It might come in heights of four feet or whatever our panel height is. So, if eight feet, eight inches is our minimum, this may actually end up being a 12-foot-tall tank, or you know, if we've got 31 feet, it may end up as a 32-foot tank, something like that. This is calculated our minimums that we need. So just be cognizant of that when we actually order, we're gonna go, we want to go up to the next highest size so that we meet the minimums. 

As a basic overview of our water storage tank. I hope this is helpful. I hope this is a helpful estimating tool. And then certainly use the report side of this, where you can PDF, save your calculations and know, just come back, and quickly enter the information. And again, if you need to revisit that. But I hope this tool is helpful for you as you're navigating estimating water storage tank size. 

I'm Joe Meyer, this is MeyerFire University.