May I ask if you have vibrators or air cannons fixed to the hopper? ■

Originally Posted by fgaona

We have a bolted silo, 1000 ton chip plastic storage capacity, mass flow design, discharge flow 120 ton/hr.

We are finding the bolts in the middle section of the cone (vertical bolts position) are loosing and breaking, always in the same position, same plate etc.

Any experience with similar problem?

I apreciate your comments.

The bolts installed are apparently loosening and shearing from the stesses due to ambient heat, mass flow, and funnel flow.

My questions in regard to this issue are:

is it a glass fused to steel tank or stainless steel?

Why have you not stated the type of bolt and mounting? Is it a carriage bolt using a square punched hole for the bolt head?

What was the SAE grade of metric bolt used and were heavy flat washers used as well as lock washers for the nuts?

Was Loctite used as a thread locker when the silo was assembled?

Were the nuts and bolts properly torqued when installed?

if you could post some pictures that would help but I think you are deailng with improper fasteners and fasteners that are not properly torqued and lack of thread locker and flat washers.

lzaharis ■

Dear fgaona,

The failed bolt at the picture indicates a fatigue failure.

If you can post a close up from the failed cross section it might be possible to determine whether the failure was caused by bending, shearing or torque.

If the type of fatigue loading is decided upon, then a matching force mechanism can be investigated and from there a solution found.

The force transmission between overlapping plates can be effected by friction (bolt tensioning necessary) or by shear (exact fitting bolts required).

Riveting instead of bolting might be a solution.

Fatigue is mostly caused by too big deflections of the material.

Is there a seal between the two plates that allows this flexibility?

Consult a strength calculation engineer.

Success

Teus ■

What does the silo supplier say about your problem?? ■

Ruling out mechanical load cause. Does the silo honk when you are discharging? i.e. cause noticeable vibrations or makes noise?

Plastics are prime candidates for slipstick behaviour in silos especially mass flow ones. The material moves on the silo wall then stops and then starts again. This causes vibration and noise in silos and ultimately lead to fatigue failure. ■

The bolts you are using are are failing for a number of reasons.

1. The steel sheets are not stamped with square holes to use large carriage head bolts

as is normally done.

2. The bolts appear to have a flat flange instead of a bolt head of some type

whch will not add in the bolts holding power and allows more slippage and movement.

3 The coarse threaded bolts head design and the torqueing by hand are not enough

to eliminate the problems.

I would do it this way to stop the problem and cure it for good:

I would purchase an air impact wrench or (nut runner with the

proper torque settings) for dry installation of the fine threaded

bolts and to aid in swift installation of the new bolts.

Purchase 2 allen head socket wrenches rated for impact wrench

service having one as a spare.

Purchase the proper sized box spanner for the nylock nut or sign

one out from your tool crib provided it is in good condition to avoid slippage orinjury

while tightening the socket head capscrews with the nut runner/impact wrench

purchase the following in fine thread grade eight strength fasteners:

a. grade 8 full length thread Allen head/socket head capscrews with fine thread 32 T.P.I. to replace the all the bolts used in the silo entirely

b. 2-grade 8 socket head split lock washers FOR EVERY BOLT USED TO REPLACE THE OLD ONES

c. enough grade 8 nylock nuts for the number of bolts required

Installation:

a.. remove each old bolt and clean the area of any sealant, and install

the socket head capscrew making sure that- two of the hex head cap screw lock washers

are used for each bolt-one one the interior and one on the exterior of the silo.

b. start each nylock nut by hand to avoid cross threading and then

tighten with air powered nut runner or impact wrench.

c. use a torque wrench to verify the proper SAE dry torque for each fastener,

the nylock nuts will hold and not loosen and the fine thread bolts will have

the holding power to eliminate the problem for good.

lzaharis ■

Originally Posted by fgaona

Teus

There is a silicone gasket between the plates but the idea of this gasket is only to seal between plate.

Designer

The supplier has been checked the all the calculations and they say all is ok, at the beginning they thought the problem was related the torque, material bolts etc but I think they do not is happen and they are looking a support to resolve this issue.

Mantoo

Your are right, tyhe silo has some vibrations based on the behavior you describe, we measured this vibration and compare with others small silo and the vibration level is higher, I think the behavior or type of flow we have in this silo is producing higher forces.

I forget to say this silo is a blender silo internally have pipes fixed in the walls of the silo to do the mix the pellets.

Izaharis

Are you sure if I change the botls, nuts etc and follow the procedure you description this problem will be result?

======================================================

If the sheet interior is clean and the bolt holes are true and the allen bolts fit with close tolerance-no wiggle, no slop, bolt shoulder fully supported, I see no problem with the 24 thread per inch grade 8 allen bolts, 2 grade eight allen bolt split lock washers, 24 thread per inch nylock nuts.

IF the there is slop in each drilled hole you may end up using the next larger diameter bolt and the larger corrresponding washers and 24 T.P.I. grade 8 nylock nylock nuts.

MY thoughts:

I would do this first-replace the suspect ring of bolts with the proper size

grade 8 24-T.P.I. full thread allen bolts, grade 8 allen washers-2 per bolt, 24 T.P.I. grade 8 nylock nuts properly torque the bolts and nuts-some one has to be inside holding the spanner or the impact wrench and torque check the bolts/nuts to the proper foot pounds/newton meters.

BE SURE TO TORQUE THEM DRY as the torque values go down hill if lubrication is used- you also do not want any lubricant on the nylock nut as it will affect the holding ability of the nylon in the nut threads.

Dip each bolt in a can of rubbing alcohol in a safe work area witrh lots of ventilation and then wipe them dry with a clean cloth; it will help if you run a 24 TPI die over the threads immediately prior to installing the bolts as well.

If you find no issues with this ring of replacement allen bolts split lock washers and nylock nuts after a week of operating you have solved the problem and I see no reason why the rest of the bolts cannot be changed as they are holding each sheet together with every other sheet in unison.

If flexing is still evident the problem is in either the steel thickness being too thin or strength of materials being too low.

lzaharis ■

The silo is said to be of mass flow design, but what is the surface like on the inside where the bolts are fitted? Any deviation from a smooth surface could give rise to an arch stress field and high radial loads that increases the hoop stress on the verticla joints. Particularly high values would be generated by suddenly applied loads caused by silo 'quaking'. brought on by slip-stick on the walls. ■

Nothing short of removing the inverted cone and installing an open hopper wide than the silos diameter will cancel the issues you have as it will allow total drawdown at all times where the inverted cone and the smaller diameter tube hinder its flow. ■

Thicker plates will make no basic difference to the flow situation. Attention should be directed to the internal finish to secure a smooth surface by way of countersunk bolts and ensuring no projections to eliminate slip retraints.

The other hazard mentioned, of silo quaking, is awkward to counter because in one mode stick-slip will prevent wall slip on a small cross section of mass flow channel, so that progressive collapse take place until the arch size is unstable and the total contents move to fill the reduced voidage of the region that has emptied, when the cycle then repeats.

I consider it necessary to develop continuous flow over a large enough cross section that the flow forces exceed the static friction value and movement is continuous in the form of rapid waves of dilation moving up the contents. The frequency of collapses should indicate the volume 're-filled' in a quake, and hence the critical diameter needed to motive total mass movement. It may be practical to achieve an equivalant size of flow channel with an insert or use a screw feeder to develop the necessary live cross section. The approach will no doubt depend how serious the problem is.

Lyn ■

It would be imprudent to advise without a proper investigation of the situation. If you care to send copy of the silo drawing and details of the wall friction value of the silo contents to lyn@ajax.co.uk, I would have a better understanding of the construction and may be able to point a way forward. ■