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Truck Mounted Water Tank 9

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alexM71

Automotive
Jul 20, 2012
6
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
 
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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.
 
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
 
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.
 
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
 
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:
wikipedia said:
Polypropylene is liable to chain degradation from exposure to heat

and when someone before said:
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
 
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
 
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)?

 
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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