# Thread: Effect of Undersized Hard Suction?

1. ## Effect of Undersized Hard Suction?

Hi All,

I'm looking for a formula or rule of thumb to quantify the impact of using undersized hard suction when drafting. Our 1250 gpm pumper is right now outfitted with 4 1/2" suction instead of 6" because that's what our old truck had and they wanted one size of hard suction on both trucks. The intakes are taken down from 6" to 4 1/2" with adapters. I'd like to show how this is hurting us, and why we should be using 6", but I haven't been able to find much in the fire hydraulics book I have. Most of the formulas in there are for discharge, and this is a little bit different. I'm assuming that pump rating, hose diameter, hose length, and drafting height all come into play. There is one table in the book that almost has the info I want, but it's not quite what I need (wrong format). Any help would be appreciated.

Andy

2. You've effectively cut your pump from 1250 GPM to 750. There are tables for calculating what you can get, I'll try to dig one up in the next few days.

Stay safe out there, everyone goes home!

3. I don't have my hydraulics books handy right now. I'm sure one of the engineers here will know the answer right off the top of their head.

Have you considered doing a flow test and comparing the 4 1/2" and 6" suctions?

4. Originally Posted by Pelican631
Hi All,

I'm looking for a formula or rule of thumb to quantify the impact of using undersized hard suction when drafting. Our 1250 gpm pumper is right now outfitted with 4 1/2" suction instead of 6" because that's what our old truck had and they wanted one size of hard suction on both trucks. The intakes are taken down from 6" to 4 1/2" with adapters. I'd like to show how this is hurting us, and why we should be using 6", but I haven't been able to find much in the fire hydraulics book I have. Most of the formulas in there are for discharge, and this is a little bit different. I'm assuming that pump rating, hose diameter, hose length, and drafting height all come into play. There is one table in the book that almost has the info I want, but it's not quite what I need (wrong format). Any help would be appreciated.

Andy

This is crazy, do you have LDH or did they pull that off and go back to 2 .5 ? I agree with KenNFD1219 get out there and prove it. hopefully it will open some eyes up.

5. I currently own and operate a 1958 FWD 750GPM pumper. When it was delivered new to my and chiefengineer11's company (CE11 was 17 when it was brand new and delivered) it was officially rated for 750GPM. In 1968, the truck was sent to Hahn Fire Apparatus (relatively close to our company) and received a minor facelift- a third discharge was added onto the driver's side, theoretically turning it into a 1000GPM truck.

One problem though- it will do 1000+ GPM on a hydrant with good pressure, I have done it. The problem is the 4.5" steamers.

At a muster one time, not too long after I purchased the truck, CE11 and myself decided to "see what it could do." Thought I dont remember specifically what we were pumping, I do remember dual 3.5" lines going into a deluge, and another 3" line going into another deluge by itself. If I remember right, we were moving somewhere in the neighborhood of 875GPM + or - a few. One thing that helped was the use of a floating strainer.

The lift was only 3-4' tops. Would I have moved 1000+ if I had 6" steamers? Absolutely no doubt about it. I had 2 more discharges available, the only thing that could have stopped me was the lack of more available RPM's (if I remember right I was running a 3200 rpm engine at 28-2900 for the 875GPM....and was that before or after CE11 smacked me and made me put it in "volume" mode??? I cant remember......) I am sure he will chime in here with the technical analysis and the proper charts.

6. Originally Posted by chiefengineer11
You've effectively cut your pump from 1250 GPM to 750. There are tables for calculating what you can get, I'll try to dig one up in the next few days.

Stay safe out there, everyone goes home!
Wouldn't you be able to bring this back to capacity by adding a second suction similar to what you have to do with pumps over 1500gpm?

Dual 4.5"s or the 4.5" and an auxilary 2.5" or 3"?

I know for our 1750 we pump test at 1-6" and 3" of the front suction. Or We have also used a jumbo siamese 2-6" suctions into the siamese and then on to the 6" intake. We haven't cranked up the new 2000gpm Q-max yet but once we get open water that will be on the list of things to do....

Obviously, getting the 6" that the truck should have is the "better" solution but you could work around that with other options.....

The other side of the coin is how often do you really need more than what the 4.5" suction will supply?

7. Originally Posted by ChiefDog
Wouldn't you be able to bring this back to capacity by adding a second suction similar to what you have to do with pumps over 1500gpm?
Certainly Chief, what you suggest could be done and it would work as you describe. But my read of what Pelican is looking for is good reason to equip the engine properly so that it will function as it was designed to.

I recall a derating chart in a book I had, "Fire Service Hydraulics" by Fred Sheppard. Unfortunately, I no longer have that book. I do have NFPA's "Operating Fire Department Pumpers" dated 1967. In Chapter XVI it has several tables that can be used to give an idea of what the effects of undersized hard sleeves would be.

One such table is "Allowances for Friction Loss in the Suction Hose" (hard sleeves, as we prefer to call them today). But the only sleeves shown in the table are the acceptable ones for the size of the pump. Even that can give an idea of what would occur. For example, it indicates that at 10' lift through 20' of 4-1/2" the friction loss would be 8-1/2 psi. 1250 gpm through 6" would have a loss of 7 psi. Just from that you can infer that at 1250 gpm, the loss through 4-1/2 would be pretty high.

Another table "Atmospheric Pressure Available to Overcome Suction Losses" shows that it takes 4.3 psi to "lift" the water 10 feet (same as elevation loss). Since atmospheric pressure at sea level is 14.7 psia, there's 10.3 psi left to overcome friction loss. You're already at 8-1/2 psi loss at 750 gpm. How much more water will you be able to move through that 4-1/2 before you get to 10.3 psi loss?

Taking note of FWDbuff's numbers from what he and I did a few years ago as validation, I would submit that you could count yourself lucky to get 850 through 20' of 4-1/2 at 10' lift.

Note: All of this assumes old time smooth bore hard sleeves. I believe that the newer lightweight sleeves and even Cordflex will flow even less because of additional losses due to turbulence (eddy currents) from the spiral rings.

Anyone else out there have some real world expreriments to report on?

Stay safe out there, everyone goes home!

8. Originally Posted by KenNFD1219
Have you considered doing a flow test and comparing the 4 1/2" and 6" suctions?
We don't have any 6" to do a comparison with...we'd have to borrow some from a neighboring department

Originally Posted by dday05
This is crazy, do you have LDH or did they pull that off and go back to 2 .5 ? I agree with KenNFD1219 get out there and prove it. hopefully it will open some eyes up.
Funny you should mention that. Imagine my surprise when I joined and looked around and noticed that there's not a lick of LDH anywhere to be found. 2.5 on everything. When I suggest moving towards LDH I get everything from "it's too heavy to load" to "those Storz connectors are too hard to use" to (this one is great) "those Storz connectors are too confusing"!

Change is slow and hard...I'm hoping to gather enough facts to start to chisel through the arguments, but facts and data aren't always enough to overcome apathy.

Originally Posted by chiefengineer11
Originally Posted by ChiefDog
Wouldn't you be able to bring this back to capacity by adding a second suction similar to what you have to do with pumps over 1500gpm?
Certainly Chief, what you suggest could be done and it would work as you describe. But my read of what Pelican is looking for is good reason to equip the engine properly so that it will function as it was designed to.
Exactly. And, at a rural fire, it's a heck of a lot easier to just have to keep one portable tank on one side of the truck full than using one on each side just to overcome the undersized suction.

Originally Posted by chiefengineer11
I recall a derating chart in a book I had, "Fire Service Hydraulics" by Fred Sheppard. Unfortunately, I no longer have that book. I do have NFPA's "Operating Fire Department Pumpers" dated 1967. In Chapter XVI it has several tables that can be used to give an idea of what the effects of undersized hard sleeves would be.

One such table is "Allowances for Friction Loss in the Suction Hose" (hard sleeves, as we prefer to call them today). But the only sleeves shown in the table are the acceptable ones for the size of the pump. Even that can give an idea of what would occur. For example, it indicates that at 10' lift through 20' of 4-1/2" the friction loss would be 8-1/2 psi. 1250 gpm through 6" would have a loss of 7 psi. Just from that you can infer that at 1250 gpm, the loss through 4-1/2 would be pretty high.

Another table "Atmospheric Pressure Available to Overcome Suction Losses" shows that it takes 4.3 psi to "lift" the water 10 feet (same as elevation loss). Since atmospheric pressure at sea level is 14.7 psia, there's 10.3 psi left to overcome friction loss. You're already at 8-1/2 psi loss at 750 gpm. How much more water will you be able to move through that 4-1/2 before you get to 10.3 psi loss?

Taking note of FWDbuff's numbers from what he and I did a few years ago as validation, I would submit that you could count yourself lucky to get 850 through 20' of 4-1/2 at 10' lift.

Note: All of this assumes old time smooth bore hard sleeves. I believe that the newer lightweight sleeves and even Cordflex will flow even less because of additional losses due to turbulence (eddy currents) from the spiral rings.
Good info. The book I have is the second edition of Fire Service Hydraulics, edited by James Casey. The book is older than I am, but water hasn't changed much over the years. It has the friction loss table you mention, but like you said, it assumes that the hose is sized properly for the pump. The derate table you no longer have would have been ideal.

Originally Posted by ChiefDog
The other side of the coin is how often do you really need more than what the 4.5" suction will supply?
This is a somewhat valid argument, and if anyone is opposed to getting 6", this is the argument they'd probably use. However, I think it's silly to choke off the capacity of the pump for no good reason. If using the same piece of suction on more than one truck is really a concern, adapters are cheap. They paid a lot of money for that 1250 pump...why derate it to a 750?

Thanks to everyone for all of the replies...lots of good info being posted. If anyone does happen to have a pump derating table or formula that would be ideal, but I think I can glean enough to start making a good case for 6".

Andy

9. Originally Posted by Pelican631

Funny you should mention that. Imagine my surprise when I joined and looked around and noticed that there's not a lick of LDH anywhere to be found. 2.5 on everything. When I suggest moving towards LDH I get everything from "it's too heavy to load" to "those Storz connectors are too hard to use" to (this one is great) "those Storz connectors are too confusing"!

Change is slow and hard...I'm hoping to gather enough facts to start to chisel through the arguments, but facts and data aren't always enough to overcome apathy.

I guess Good luck!! I think angus has an old tape it's about LDH and it's not to bad to view. It might work to get people thinking.

10. Originally Posted by chiefengineer11
Another table "Atmospheric Pressure Available to Overcome Suction Losses" shows that it takes 4.3 psi to "lift" the water 10 feet (same as elevation loss).
Peeking around the corner. . .But I thought the pump sucked the water in.

Ducking for cover

11. Originally Posted by KenNFD1219
Peeking around the corner. . .But I thought the pump sucked the water in.

Ducking for cover
It's frequenly expressed that way, but what's really happening is that when you prime the pump, you're removing air from it thereby reducing the pressure inside the sleeves and the pump to something less than atmospheric pressure. When that occurs, the atmospheric pressure outside the pump pushes the static water up the sleeves and into the pump. Expressing it this way make it easier to understand the effect of the atmosphere on drafting. It's also why we like to use the term "hard sleeves" instead of "suction hoses."

Stay safe out there, everyone goes home!

12. Originally Posted by Pelican631
We don't have any 6" to do a comparison with...we'd have to borrow some from a neighboring department

Funny you should mention that. Imagine my surprise when I joined and looked around and noticed that there's not a lick of LDH anywhere to be found. 2.5 on everything. When I suggest moving towards LDH I get everything from "it's too heavy to load" to "those Storz connectors are too hard to use" to (this one is great) "those Storz connectors are too confusing"!

Change is slow and hard...I'm hoping to gather enough facts to start to chisel through the arguments, but facts and data aren't always enough to overcome apathy.

Exactly. And, at a rural fire, it's a heck of a lot easier to just have to keep one portable tank on one side of the truck full than using one on each side just to overcome the undersized suction.

Good info. The book I have is the second edition of Fire Service Hydraulics, edited by James Casey. The book is older than I am, but water hasn't changed much over the years. It has the friction loss table you mention, but like you said, it assumes that the hose is sized properly for the pump. The derate table you no longer have would have been ideal.

This is a somewhat valid argument, and if anyone is opposed to getting 6", this is the argument they'd probably use. However, I think it's silly to choke off the capacity of the pump for no good reason. If using the same piece of suction on more than one truck is really a concern, adapters are cheap. They paid a lot of money for that 1250 pump...why derate it to a 750?

Thanks to everyone for all of the replies...lots of good info being posted. If anyone does happen to have a pump derating table or formula that would be ideal, but I think I can glean enough to start making a good case for 6".

Andy
Andy,

You don't know me so you can't see or realize what I was doing.

We just put a 2000gpm Q-max in service, it is the second big water pump we have purchased new, third 1500 or better we have bought since I became Chief. I influenced the purchase of our first LDH and spent a few thousand on adaptors and stuff to equipt the new 2000 pump.

The difference between the Q-flo gpm ratings is not as much as if you jump to a big case Q-MAX. So you have spent some more but not as much as you could have to get into the 1250 range.

Most of what I was getting at is the real world work arounds so that in the mean time you can get your 1250 up to 1250 if you really did need a flow of that amount. Including using a auxilary suction off the same side as your 4.5" if the truck is so equipted. I would guess it is since you said you have all 2.5" hose and no LDH. Ask for adaptors that will at least allow you to use mutual aid department's ldh if called upon.

It takes time to change "old ways". Most people do not know even simple hydraulic theory let alone have the understanding to figure it out. Myself included on the real sit down and calculate it equations. So, volunteer to help get an outside service provider to do your annual pump service and testing at draft. They probably have 6", deck gun and piotot gauges. Make sure a few key people are there and the calculations do not lie!! Plant the seed and they will see that the truck does better and you will get your wish.

13. CE:

My post was in reference to an infamous thread a couple of years ago.

I have carried both the friction loss pocket calculator and the slide rule friction loss calculator from Akron Brass for many years. Only used them a couple of times on the fire ground but they have been very helpful in pre-incident planning for water supply requirements.

14. Originally Posted by KenNFD1219
CE:

My post was in reference to an infamous thread a couple of years ago.

I have carried both the friction loss pocket calculator and the slide rule friction loss calculator from Akron Brass for many years. Only used them a couple of times on the fire ground but they have been very helpful in pre-incident planning for water supply requirements.
Agreed on all points. I've studied this stuff since way back. I first learned friction loss calculations in the mid sixties at what's now MFRI. That 1967 edition of the NFPA book, I've had it since it came out. I have the same Akron calculator (actually, I've worn two of them out and going to buy the third. I've got a circular slide rule calculator that I bought way back from a chief in Longview, Wash. who was making and selling them. I think his name was Floyd Grant.

You know what, though? Everybody thinks I sit there with these things trying to figure out what to set the pump at. Ha! Little do they know, the reality is, on the fireground I make water come out of the nozzles or get to the next engine in the relay. If the streams look good, that's great. Nothing else needs to be done.

But if it's not going well, I've got the training, the knowledge and the tools to help me understand what's wrong and what, if anything, I can do to fix it. Although I have to admit, there's been cases where even that's not enough.

Stay safe out there, everyone goes home!

15. Originally Posted by chiefengineer11
on the fireground I make water come out of the nozzles
Yes he does. After we scream at him about 10 times to charge the line!

16. An e-mail to the pump manufacturer will probably get you an answer from their engineers.

You paid for a pump that meets the requirements of UL & NFPA to deliver 1250 GPM. My guess is that 4.5" derates it to 750 GPM although it will probably deliver closer to 1000 GPM at draft or a good hydrant.

I haven't worked with Hale pumps in over thirty years but Waterous pumps use one size impeller for 500 to 1250 GPM and another size for 1250 to 2000 GPM (approximate break point). The difference is the size of the intakes bolted to the pump case. The number of discharges is normally speced by the FD because of special features (CAFS, A and B Foam systems, # and size of crosslays, etc).

Good luck on trying to convert to the 6" and LDH.

Stay Safe
IACOJ

17. ## Suction Hose

Years ago with a Ford/Great Eastern we went the other way and put 6" suction on our CM750 Pump. That made it into a pump capable of moving 1100-1250 gpm. I guess if all they have is 2 1/2 hose it doesnt really matter if the suction is 4 1/2 or 6" You can only ram so much water down a 2 1/2

18. Ok so after poring over the formulas a little more and badgering my wastewater engineer wife, here's what I've come up with. Someone tell me if this is totally whacked.

Friction Loss = KLQ^2/d^5

Where:

K = a constant
L = length of hose
Q = flow
d = hose diameter

Now, if we assume (this is where this might all go to heck...someone tell me if this is an ok assumption to make just for illustration purposes) that the friction loss of a 6" sleeve when the pump is flowing 1250 gpm at some pressure (value doesn't matter, as long as we keep pressure constant for the 6" and 4.5" cases) is the maximum that the pump can overcome, then we can set that friction loss equal to the friction loss when using 4.5" and solve for the Q (flow) in the 4.5" case. The constant K and length L reduce out as long as we assume they are the same for both cases. In other words:

Friction loss = K*L*12.5^2/6^5 = K*L*Q^2/4.5^5

Which reduces to:

12.5^2/6^5 = Q^2/4.5^5

Solving for Q (4.5" flow) gives 610 gpm.

Is this valid? We're on vacation right now, but my wife promised to look up her textbooks when we get home and see if she can figure it out. I'm an electrical engineer, so this isn't quite as intuitive to me as my electrons, even though the concepts are very similar.

Thanks again for all of the replies.

Andy

19. Pelican, I think you're going off on a slight tangent. If I understand your question, friction loss isn't really the main issue. Set it aside for a minute.

You want to know the difference between drafting with an unrestricted 6 inch suction hose and an unrestricted 4.5 inch suction hose. Right?

Friction loss in suction hose where you are usually dealing with 1 or 2 lengths is negligible. The factor to consider is the volume of water that can flow through 4.5 vs 6 inch hose. If my rust old math is correct the inner area of a 4.5 inch suction hose is 63 inches. The inner area of a 6 inch suction hose is 113 inches. That's about an 80% increase in the waterway to your pump. Your best comparison for finding written documentation is probably the flow rate charts for LDH as this is the real comparison you want to make. "Do you want your pump drafting through a suction hose with an inner area of 63 inches or do you want to use one that is 80% larger"? It's really a no brainer.

From personal experience, with a 1,250 pump in good condition, while drafting with 4.5 inch hose we achieved flow of about 800 GPM, when we used the 6 ich hose we achieve flow of about 1,350 GPM or about a 68% improvement. We then threw away the 4.5 inch hose to eliminate the temptation.

20. Originally Posted by Pelican631
Ok so after poring over the formulas a little more and badgering my wastewater engineer wife, here's what I've come up with. Someone tell me if this is totally whacked.

Friction Loss = KLQ^2/d^5

Where:

K = a constant
L = length of hose
Q = flow
d = hose diameter

Now, if we assume (this is where this might all go to heck...someone tell me if this is an ok assumption to make just for illustration purposes) that the friction loss of a 6" sleeve when the pump is flowing 1250 gpm at some pressure (value doesn't matter, as long as we keep pressure constant for the 6" and 4.5" cases) is the maximum that the pump can overcome, then we can set that friction loss equal to the friction loss when using 4.5" and solve for the Q (flow) in the 4.5" case. The constant K and length L reduce out as long as we assume they are the same for both cases. In other words:

Friction loss = K*L*12.5^2/6^5 = K*L*Q^2/4.5^5

Which reduces to:

12.5^2/6^5 = Q^2/4.5^5

Solving for Q (4.5" flow) gives 610 gpm.

Is this valid? We're on vacation right now, but my wife promised to look up her textbooks when we get home and see if she can figure it out. I'm an electrical engineer, so this isn't quite as intuitive to me as my electrons, even though the concepts are very similar.

Thanks again for all of the replies.

Andy
Andy,

I am not much of a formula guy, more of a guy who likes to go get it done.

I do know that the standard for 1000 gpm pump is a 5" suction hose. Once you get to 1250 gpm you need to move to 6" suction hose. 1500 gpm can be done with a single suction hose, however when you move up to 2000 gpm you will need duel 6" suctions.

Having done real world testing on all my departments pumps here are the results:
Our 1250 gpm pump which uses 6" suction hose, it is very easy to flow the rated capacity of the pump and then some.

On our 2000 GPM pump you must use 2 - 6" suction hoses to flow the rated capacity. With a single 6" we are able to flow around 1850 - 1900 GPM.

On our 1750 GPM pump depending on the operator you can some times get 1750 GPM on a single 6" suction. However for our yearly pump testing we always use 2 - 6" suction hose's.

On our 1000 GPM pump we use a single 5" suction and flow way in excess of 1000 GPM.

I find sometimes you need to take the "leadership" by the hand and show them how things work before they will get it!!!!!!!!

Hope that helps.

21. Originally Posted by Chief1FF
Andy,

I do know that the standard for 1000 gpm and 1250 gpm pumps is a 5" suction hose. Once you get above 1250 gpm you need to move to 6" suction hose. 1500 gpm can be done with a single suction hose, however when you move up to 200o gpm you will need duel 6" suctions.
Hate to differ with you, Chief, but the standard for 1250 GPM pumps is 6" sleeves. To quote Casey Stengel, "You can look it up."

Stay safe out there, everyone goes home!

22. Originally Posted by chiefengineer11
Hate to differ with you, Chief, but the standard for 1250 GPM pumps is 6" sleeves. To quote Casey Stengel, "You can look it up."

Stay safe out there, everyone goes home!
CE11,

Sorry, you are correct. Don't know where my brian was this morning, lack of enough coffee before posting.

Lesson learned - Drink enough coffee and be awake before posting.

23. Originally Posted by Chief1FF
CE11,

Lesson learned - Drink enough coffee and be awake before posting.
Heartily agreed, - learned from years of trucking - Truck must have fuel, driver must have coffee!

24. If it's relevant, our 1986 pumper rated at 1000gpm. Supply 5" hard sleeve. Recent pump test with 5 intake, 2x 10' sleeves (20') floating strainer from a lake source (approx 1' from water surface to ground level) was 1260gpm max. Discharge was thru 2x 2.5" lines 100ft ea line).

25. Well I think I found a table in the book I have that gives me what I'm looking for, more or less. The table is titled "Relative Carrying Capacity of Pipes". The description says:

"This table shows the relative carrying capacity of pipes 4 to 60 inches in diameter for the same loss of head. Under such conditions of flow the velocity in the larger pipe is greater than in the smaller so that the larger pipe will discharge at a rate greater than that which would be expected by a comparison of their respective cross-sectional areas. The values in the table are based on the Hazen-Williams equation in which the discharges vary as the 2.63 power of the respective diameters."

The table doesn't have 4.5" pipe, but it does have 4" and 6". The table shows that 6" has 2.9 times the carrying capacity of 4" for the same loss of head. We can calculate the 4.5" case if we want:

(6^2.63)/(4.5^2.63) = 2.13

So the 6" pipe will carry 2.13 times more water than the 4.5" pipe. So if my max flow rate with 6" is 1250 gpm, then my max flow rate with 4.5" will be 1250/2.13 = 587 gpm. This is pretty close to the 610 gpm I had calculated using a different method.

Of course the 2.63 power number was derived experimentally, so it's probably not quite correct. From the numbers Jim gave, it's probably a little too severe.

I think I can safely (based on several formulas and the real-world numbers people have given here) that we'll only get somewhere between 600 and 800 gpm out of the pump as-is. I can't wait until pump test time !

Thanks again for all of the replies...much appreciated.

Andy

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