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Truck Mounted Water Tank
9

Truck Mounted Water Tank

Truck Mounted Water Tank

(OP)
Our company equips trucks with required equipment and structure for different applications.

We have a truck mounted with a 10,000 lit (2700 US gal) Poly Propelene tank (External Dimension height: 1600mm X Width: 2500mm X Length: 2800mm). The tank is mounted on a frame under the tank using bolts which are tightened directly on to the Polypropylene tank. This frame is mounted to the chassis through rubber flexible joints. There are no other supports or fixtures holding the tank except the above.

Our Engineering department claims that there are no issues and the type of mounting is sufficient. Unfortunately there is no study or calculation supporting conditions like when the truck applies a sudden brake, takes a sharp curve or drives off-road. We did a validation by conducting a road test, however we (as quality control) are looking for some supporting calculations which can ensure safety of the mounting system in the above conditions. Would appreciate if someone can throw some light on this. Thanks

RE: Truck Mounted Water Tank

That reminds me a bit on calculations which are done by car manufacturers who calculate whether the seats stay where they are when there is a sudden brake. My doubt would be more the flexible joints between frame and truck chassis than the bolts which hold the tank.

RE: Truck Mounted Water Tank

You are right to be worried.

Tear out of bolts through the plastic.

Cold flow of the plastic over time causing the tank to become loose.

RE: Truck Mounted Water Tank

(OP)
Can someone suggest a way to calculate the force that the mounting bolts may be subject to, in the event that the truck be driven in circles at a particular velocity etc? Thanks for any help in advance!!

RE: Truck Mounted Water Tank

Hi alexM71
Those fixing bolts could be subjected to various loads due:- accelerating, cornering, braking, swishing of fluid in the tank and we have very little information from your post.
I would suggest you ask engineering department why they consider there are no issue's, do the trucks only run very slowly or are they roadway trucks?
You could also ask if they have considered the loads imposed on the bolts due to the things I listed above.

desertfox

RE: Truck Mounted Water Tank

(OP)
Thanks for your time desertfox,

In fact I discussed with our Engineering dept but it looks like things are running on assumptions and they are looking to have a design validation done by conducting a road test to simulate actual use of the truck. But I'm not convinced and believe that even if everything goes fine in a road test, there is still a risk factor.

I can provide any required details if we can have some reliable results.

The truck will be used for fire fighting and hence it becomes more critical to make sure there is no risk the way the tank is mounted. Thanks

RE: Truck Mounted Water Tank

Who's going to perform the high-G turn validation after 5 years of weather, with the tank 3/4 full after sitting in the hot sun all day?

RE: Truck Mounted Water Tank

AlexM71:
We really need some cross sections through the tank and frame, lengthwise and across the truck/tank frame, and details of the connections currently contemplated; including dimensions, etc. We can’t see what you are imagining from here. I would think you would want some baffling in the tank to control sloshing, maybe some soft stiffeners as part of the tank internals, and I would want two structural rails inside the tank and in line with the side sills of the tank’s support frame. What I would want to do is pinch the plastic tank shell btwn. the inside rails and the support frame members at many smaller bolting points, so as to distribute this loading to the tank shell. Then the real structural connections to the truck frame are made at the bottom flange of the tank support frame. These are concentrated load which the plastic tank shell would never be able to withstand.

RE: Truck Mounted Water Tank

Here in my locale it is very common to see these poly tanks in use by the ag industry. They are almost always secured by steel banding around the tank which fastens them into a cradle support. There is no way I would trust a bolted foot mount or other such arrangement, especially for highway use.

My 2 cents.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: Truck Mounted Water Tank

On the one hand, I am in the engineering department of my company and everyone else knows better than we do - production, quality, sourcing, even the truck drivers. Boy, they could do some really good designs if we would just get out of the way.

On the other hand, we in engineering do make some real boners from time to time, so I've been around long enough to know to listen to dissenting voices. If I can't justify my designs, I know to go back to the drawing boards (euphamistically speaking, that is.)

Are your engineers real engineers, guys who went to school, paid their dues, etc. and got degrees, or are they guys who are promoted draftsmen, ex-salesmen etc.? That could make a big difference in what opinion I form of their design from your meager descriptions. If the former, then they should be able to make a reasonable defense of their design approach and show you the calculations that justify their design decisions. There may be mitigating factors involved that aren't appearant to you. On the other hand, if they aren't really qualified for their responsibilities, then you may have a disaster on your hands and you are right to raise the alarm.

I'd like to hear from them if there is some way for you to post their comments.

rmw

RE: Truck Mounted Water Tank

Can you reveal who designed and manufactured this tank?

Mike Halloran
Pembroke Pines, FL, USA

RE: Truck Mounted Water Tank

Designed the tank?

Or, "designed" the bolts holding the tank on tot he bottom of the truck frame with only bolts holding it up?

I'd expect failure of the bolts pulling through the plastic around the holes. Begin an inspection program and look for trouble before you get sued for a hazmat cargo drop onto the freeway at rush hour. You (as part of the QA dept) can doi that much immediately.

Not good publicity there.

RE: Truck Mounted Water Tank

I would take as a first approximation the maximum force on the vehicle to be due instantaneous braking, leaving the maximum coefficient of friction ( I found = 1.0, dry asphalt against new tire)as the limiting force= 1.0 *W
So the lateral force would be in the vicinity of the loaded weight of the tank. This would also be the maximum centripetal force no matter what the speed/arc radius.

Putting a safety factor of 2, I would calculate the design force to be

2*W

which would suffice for normal driving.

Also, fatigue life would have to be added to the force equation of the structural components as well as the flexible mounts life over weather and time.

However,collision forces would no doubt dominate all forces and it would behoove the designer to guarantee the integrity of the support structure under those extreme conditions; this would require an additional structure( bands for example) to prevent the tank from separation after shear failure of the main support.

RE: Truck Mounted Water Tank

(OP)
Thanks a lot for all the responses and for your time..
Very much appreciate the links posted.. some of them really gave some good information. What I noticed is, the vehicles I saw in one of the link does have PP tanks but those are too small and doesnt seem to pose any risk of shearing.
To have a better idea of what I am tryin to convey, I have attached some drawings which shows the actual vehicle and how the tank is mounted on to the vehicle. Hope this will help to reach some kind of calculations to understand how reliable the vehicle is, as far as the tank mounting is concerned. Thanks once again!!

RE: Truck Mounted Water Tank

Looks like you just changed the material spec on a steel tank drawing.

Mike Halloran
Pembroke Pines, FL, USA

RE: Truck Mounted Water Tank

(OP)
Mike, I really don't know what made you say that and why do you think I would do such a thing when I am seriously looking for a solution.
This is the drawing I got from our Engineering dept. I don't know if it was copied from a different project with ss tank as we do both PP and steel tanks.

RE: Truck Mounted Water Tank

This is not a tough question. Look at how NASA design the shuttle, you will know why. It is the "g" force you have to establish, forward, backward ( brake, accelerate), and sideway (cornering), and up and down if the driver hit a bump or pothole. Once you establish the value of "g" at each direction, you will know the force and any combination of the forces since it can happen simultaneously. All the forces will be then transferred to the bolts, structures, bracing. Do not consider friction to counter the force in case you have loose bolt or bracing.. Then check fatigue on all parts due to how many times it will occur.
We ship pressure vessels by all means: truck, railroad, barge, ocean ship, and the "g" factors are well established in order to secure the vessels.

RE: Truck Mounted Water Tank

AlexM71:
That plastic tank makes a great corrosion resistant liner, but it is not intended to really be a self supporting structural tank or container. As your lower support frame details show, the gravity load of the tank and water are supported by a grid work of beams and then transmitted to the truck frame. This grid work of beams must be strong enough to do this; with the plastic basically acting as a tension membrane to span over this grid work of beams to contain the water. This type of tank must also be supported at least on its four side by some steel structure. This side structure reacts water pressure loads and lateral loads due to transit. The top may not have to be fully supported.

I would liken your tank and its support structure to your putting a trash bag inside a plastic tote box and filling it with water. These tote boxes are about 14x14x14" with a plastic grillage for the sides and bottom. Your trash bag contains the water and spans, in tension, across the side and bottom openings, and the tote box acts as the primary structure. Do you and the engineering dept. work for the same company, or compete and fight with each other? You should be communicating with them on these matters on a regular basis, and they should be willing to explain their design thinking to you.

RE: Truck Mounted Water Tank

3
A tank of that size to be made in quantity other than one would normally be rotationally molded, for which purpose it would have generously rounded corners, and if not intended just to line a steel tank, the flat faces would be at least partially corrugated for stiffness, and the tank would have a minimal number of penetrations and be supported on a cradle retained by straps, not by bolted pads fused to a flat bottom.

The part of the tank drawing we can see suggests that the tank is to be fusion welded from PP plate. That's a very unusual way to fabricate a plastic tank, because fusion welding of plastic is skill-intensive and slow, hence expensive.

I'm not saying it's impossible; there are outfits that specialize in making plastic tanks that way. I was not aware of them until I drifted into the margins of the oil industry, where cost and weight are not a consideration.

If you intend to buy the tank from one of those oil-industry tank makers, and they are aware of your intended use and support method and have agreed to it, then I apologize. ... but you can see how the drawing also resembles the work product of a drafter unfamiliar with plastic tankage, given a drawing for a steel tank and asked to draw a plastic one.

Mike Halloran
Pembroke Pines, FL, USA

RE: Truck Mounted Water Tank

Does the tank manufacturer intend for its tanks to be subjected to dynamic loading or was the tank designed as a static vessel? Makes a big difference. What is the strain compatibility of the system between the tank and supports, between the supports and the truck frame? What is the damping capability of the support system?

For your road test, what type of instrumentation do you anticipate using? Will you have both accelerometers and strain gages? Will your sampling rate on both be sufficient to check the dynamic response?

So many questions come to mind. I reiterate rmw's questions about your engineering department. They should be the ones providing calculations to prove their assumptions...not depending on an ass-backwards approach to let QC find all their issues...some of which could be easily missed in a limited road test.

RE: Truck Mounted Water Tank

(OP)
Thanks for all the replies,

Can someone give more details on calculating the g-force in different conditions like braking, left / right cornering etc? Would appreciate if we can have some info on finding the force on the mounting bolts as well based on the current design.
I am insisting our Engineering for more details on their calculations as well. I will post it here if I get something really worth. Thanks

RE: Truck Mounted Water Tank

Am I the only one who sees a problem when seeing the words "firetruck" and "polypropylene"?
I mean, if you're fighting fires, I assume it can happen that you're standing pretty close to them. With a melting temp of 130°C I wouldn't really trust that.

Also:

Quote (wikipedia)

Polypropylene is liable to chain degradation from exposure to heat

and when someone before said:

Quote:

Looks like you just changed the material spec on a steel tank drawing.
I think he meant:
Looks like your engineers just changed the material spec on the steel tank drawing.

And I agree with him.
No offense.

NX 7.5.5.4 with Teamcenter 8 on win7 64
Intel Xeon @3.2GHz
8GB RAM
Nvidia Quadro 2000

RE: Truck Mounted Water Tank

While we're at it, the grillage under the tank doesn't look stiff enough to isolate the tank from chassis flex.

Do I understand that the tank mounting bolts enter threaded holes tapped into PP pads?

Never mind driving the truck in circles. Drive it diagonally over a curb a few times, and some of those bolts will drop right out.

Mike Halloran
Pembroke Pines, FL, USA

RE: Truck Mounted Water Tank

This whole thing is scary.

You do NOT design things like this for "normal use" loads.

You need to design for abuse and collision loads.

What happens when the truck needs to jump a curb at speed?

What happens when the truck collides with another vehicle?

What happens when another vehicle collides with the truck (side impact)?

RE: Truck Mounted Water Tank

this isn't the first fireengine you guys have designed, is it ?

presumably the others worked out ok ?? if they didn't, you've got your answer !

if the engineers say it's ok, put one of them (the manager, the one calling the shots) in the cab when you test drive it !

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