How to Use the Friction Loss Calculator?

MeyerFire University | TL104.01

By Joe Meyer, PE

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TRANSCRIPT

​The Friction Loss Calculator 

Today we've got an overview of the Friction Loss Calculator. This has a free version online. It also is a part of our MeyerFire Toolkit package.  

I'm going to show an example and kind of walk you through some of the uses for the friction loss calculator. It's one of my favorite tools that we have available.  

If you're online, you go to MeyerFire.com. You'll find this under the Toolkit > Friction Loss Calculator. That'll take you to our online version right here. We also have this as part of our toolkit package.  

And if you're on here, you would just go down to friction loss and this is our Friction Loss Calculator.  

This video is part of our MeyerFire University package, but this tool specifically is also part of our Toolkit package, so it's available to more users than just those that are in the University program.  

The Friction Loss Calculator  

So this is a Friction Loss Calculator. It's based on the Hazen Williams formula for friction loss within a pipe network. 

Basically, in this tool we're working from left to right, starting with the flow, different diameters of pipe, different thicknesses of pipe, the C-factor associated with the roughness of the pipe. 

The pipe length and then we get all of our calculated values and losses over here.  

If we want to override any of these individual columns, for example, the thickness, we can do that right here. 

If we want to override the C-factor globally instead of doing it individually, we can do that down here. 

We want to add any fittings in this quick calc. This is where we would do it. You could add Tees, Firelock Tees. 

Should add elbows and if you wanted to do that you could also change the quantities that are associated with those here.

So what is this used for? Well, in any system, if we've got water flowing through a pipe, we know that the water is going to rub on the inner wall of that pipe, and it's going to slow down water as it nears the edge of that pipe.

That's where we get that friction water is not going to move at the very edge of the pipe, and it's effectively going to slow down.

Or cause pressure loss along the outer edge of that water flow profile. So what is that friction loss that becomes really important? Is it very, very minor or is it debilitating for a system?

If we are looking at sizing and individual pipe, pressure loss becomes really important. One of the ways that I use this calculator often is when I size underground pipe. I don't always know before I do a full set of hydraulic calculations for a system in the amount of flow I’m going to have in the system demand. 

Overall I can estimate that using some other tools, but I don't really need to know that level of detail early in a job to figure out an underground size.

If I know we're going to have storage or warehouse, I don't know, I'm probably going to be in the ballpark of a 6 and 8-inch or maybe in extreme cases, even a 10-inch underground. If I know I've got a light commercial NFPA 13 system, well, it might be a 4-inch or 6-inch. 

And if I get into residential or very low density calculations, it might be less than a 4-inch.

But approximately, you know, where does that fall? Well, what I commonly do here with the flow is I want to estimate the system demand.

So if I know for instance, that I have a light hazard density, let's say a 0.10 / 1500 square feet, I can take a 0.10 x 1500 square feet.

And that's going to be our straight density in a perfectly efficient system where our calculated area is exactly 1500 square feet.

That very rarely is ever the case. Our remote area could be smaller and 1,500 square feet. It could be more, but we're generally going to have some overage, so I like to add in 30% for that overage, so I'm going to multiply this by 1.3 to account for 30% overage. Now our straight system demand without any hoses added in is 195 gallons per minute. 

That's an estimate for this system flow.

So if I want to add in a hose demand when I'm figuring the underground size, that's a little bit more conservative and some consultants like to do that. 

I can also add say 100 GPM for a Light Hazard system. Now I know a demand is going to be about 295 gallons per minute. The other big important factor here is length, how long is our underground pipe?

You can make this an equivalent length if you want to include fittings and tees or down here you can include an assumption.

So let's say I'm going to have a tee and an elbow. Let's just go standard elbow. One of those and a tee. When I tap the system standard tee.

What is the pipe length from the point where I stub into the building to the street tap? In this case, let's just say it's 150.

And then for underground pipe if I'm using ductile iron, it's wet. It could have a C-factor of 140.

In some systems I've done copper underground lines that are very small, which has this different C-factor of 150, plastic in some cases can also have a C-factor at 1:50, but let's just assume ductile iron and I'll put 140 in this global override for C-factor.

For ductile iron you could do some specifics. Here Schedule 10 is going to be a close enough approximation for what I want to do today.

But if I've got I say, a light hazard flow and I've got a pipe length of 150 feet.

Now I can see in pounds per square inch what the friction loss is on each of these incoming services.

If our project is extremely tight, for instance, I do work in one city where our static and residual pressures would generally be about 40 to 45 psi static and the residual is 35 to 40 psi at about 1,000 to 1,500 gpm. Very low available water supply where every pound counts. In that case losing 10 psi on an underground. It just doesn't work. It's way too much to lose in general. Personally, I like to make sure that we're not burdening the system on the inside.

By squeezing down the size of the underground on the outside basically, I generally don't want to go as small as humanly possible on the underground, because that makes the calculation tighter on the inside, and if there's any changes or future expansion of the building, any future changes of the building, then we’re kind of pinning ourselves in a corner. We really never want to go back and re-dig an underground line because it's undersized.  

So generally speaking, I like to have the friction loss on the underground be less than 5 to 10% of the overall friction loss for the job. Now in this Light Hazard system, let's say we have 50 or 60 pounds to work with on our water supply. In that case, a 2.8 psi loss, which is right here.  

Not very much. I think I'd be comfortable with that on a 4-inch underground and NFPA 13 says that you can only use on an NFPA 13 system.

You can only use it for an underground if it's hydraulically calculated and there is not a hydrant on that 4-inch line.

So we would have to check that out to even use a 4-inch. Otherwise, it's going to point us and say your 6-inch is the minimum size.  

But you can see the difference here. 2.8 psi versus a 0.4 psi.

If we're really tight, maybe we need to go with a .4 PS I, but in most cases I could live with that 2.8 psi. 

Going down to a 3-inch while it might be tempting to help save on that material.

We're losing an additional what 7 pounds there? That's a lot. That's a lot, because we might have to upsize a main inside the building.

Maybe our riser gets bigger, maybe our backflow gets bigger. Those things start to add up, so it's tempting I guess if you have a lot of pressure.   

You could potentially go down to a 3-inch, but you can also very quickly see that 2-1/2 inch completely off the table here.

It's not good even if we have 100 pounds to work with, we don't want to lose 26 of that just in the underground and obviously a 2-inch and greater. That's just way too much pressure loss. We don't have that much to work with on the system, and that's for a light hazard.

We could do that same exercise, let's say ordinary Hazard Group two in a shell space 0.20 / 1500 square feet, 30% overage and for kicks let's add in that hose to be included with the underground and NFPA 13 says we can add that outside hose outside at the tap if we don't have any hose valves in the building, but if we want to be conservative anyways, Add all of the hose flow to that underground. Now we've got a flow of 640 GPM.

On that same pipe length, C-factor all of that.

You'll see we've got 41 pounds of loss on a 3-inch. That's really just not going to work. A 4-inch might be possible, depending on what our water supply is.

But if we got flows that high, losing a 1.8 pounds less than 2 pounds on an underground at a 6-inch, that's probably going to be our safest bet.

It's a little bit different when we're doing shop drawing work and run the contractor side of things because we generally should have all of that information available to us. Flow test information, we know the building and how it's shaping out.

We have a good idea of what those hazards are going to be. We're trying to be as economical and efficient as possible, but if you're on the consulting side and you're looking out for the future use of the building.  

The changes to the building. 

You can choose to be slightly more conservative, and in this case you know going with a 6-inch on an Ordinary Hazard Group 2 probably going to make sense.

So that's the gist of how the friction loss calculator works.

I personally like to use it for underground sizing or if I've got a main, and I'm trying to estimate a size I can hop in here real quick, get an idea of the friction loss that's associated with that.

There's also velocities on here. This isn't used as much. Sometimes specifications will limit a velocity of water flow in a pipe to a certain amount.

I believe there used to be like a 32 feet per second limitation in a insurer specification. Most of those have all gone away with the advent of hydraulic calculations.

Velocity isn't really a concern anymore because friction losses is essentially self-limiting. The slower that the water moves in the pipe, the less frictionless there is more beneficial that it is to us as a designer to have less friction, loss. Velocity in other systems are limited because of noise and vibration on the system. A fire event is so rare.

We don't have those same issues and then also obviously when there's a fire or tend to be less concerned about the sound that's coming through that pipe due to its own vibration.

Any questions post in the comments below, that's an overview of the Friction Loss Calculator. 

I'm Joe Meyer, this is MeyerFire University.