suction question

aidanki · 18-03-2014 12:50am #1

Hi all,

farming backround here and have an MEng in Engineering.......

the machine in the pic is a slurry tanker, its not mine but something has been bothering my engineering mind, lets assume there are no reinforcement rings within the tank

slurry tanker operates as follows,
A Vaccum is created to the tune of 1bar in th tank via the pump on the front and slurry is sucked up into the tank.

farmer then drives to the field and Pressurises the tanker again to the tune of 1bar and the slurry is spread on the land

now the bit that is bothering me........
if i look on the net I will see several similar pics of old tanks, ever single one collapsed inward not a single one has exploded, its not no pics its they all collapse inwards

the bit Im struggling with is why do these tankers only collapse inward ?

the whole gauge pressure absolute pressure atmosphere isn't stacking up in my head, pressure on the tanker walls is -1bar during sucking and when spreading its +1bar. what am I missing ?

18-03-2014 9:52am

aidanki wrote: »

A Vaccum is created to the tune of 1bar in th tank via the pump on the front and slurry is sucked up into the tank.

So you are saying that the tank now has -1 Bar Gauge in it?
At a glance I would not expect that tank to be able to withstand full vacuum. If you add slurry to a tank why would this create a vacuum?
Where are you getting this pressure from, a gauge?

farmer then drives to the field and Pressurises the tanker again to the tune of 1bar and the slurry is spread on the land

I assume that you mean that some sort of relief valve (or conservation vent) was opened to permit the tank pressure to equalise with atmospheric pressure (so back to 0 BarG) ?

now the bit that is bothering me........
if i look on the net I will see several similar pics of old tanks, ever single one collapsed inward not a single one has exploded, its not no pics its they all collapse inwards

If atmospheric pressure and the pressure within the tank are equal why would the tank explode?

the bit Im struggling with is why do these tankers only collapse inward ?

Because a negative pressure has been created within the tank.
I would guess that the pump is removing the slurry from the tank and the pressure relief device on the tank has failed. Therefore the action of removing the slurry and not replacing it with air is causing a negative pressure within the tank. This is causing the tank walls to collapse inwards.

enda1 · 18-03-2014 10:29am

The buckling strength of the tank is a lot lower than the tensile strength of the steel walls. So it's easier to collapse than explode. (That's apart from everything said above)

spideog7 · 18-03-2014 2:56pm

2011 wrote: »

So you are saying that the tank now has -1 Bar Gauge in it?

At a glance I would not expect that tank to be able to withstand full vacuum. If you add slurry to a tank why would this create a vacuum?

You're drawing a negative pressure on the tank to suck up the slurry, it's unlikely it would ever become a full vacuum though. Generally it's operated manually (at least one as old as the one in that picture). You connect up the pipe, open the inlet, start the pump, watch the little window at the top of the tank and once you see the liquid get up to it you close the inlet and turn off the pump. I'm assuming there is some sort of check valve on the input to prevent slurry getting sucked into the pump, which presumably also provides an alternate input air path for the pump. You can also hear the load on the pump change once this happens.

2011 wrote: »

I assume that you mean that some sort of relief valve (or conservation vent) was opened to permit the tank pressure to equalise with atmospheric pressure (so back to 0 BarG) ?

If atmospheric pressure and the pressure within the tank are equal why would the tank explode?

No, once the tank is full and you want to spread the slurry you change the direction of the pump and create a positive pressure in the air pocket above the slurry (same as a regular pressure tank on your water supply). Once you open the nozzle at the back this then blows the slurry out the nozzle onto the splash plate. The pump continues pressurising the whole time, so presumably you could cause it to fail if you closed the valve with the tank empty (of slurry) and didn't disable the pump (and no other protection was in place).

2011 wrote: »

Because a negative pressure has been created within the tank. I would guess that the pump is removing the slurry from the tank and the pressure relief device on the tank has failed. Therefore the action of removing the slurry and not replacing it with air is causing a negative pressure within the tank. This is causing the tank walls to collapse inwards.

Judging from the picture this happened when filling, so the pump is creating a negative pressure by evacuating the air, which should be replaced by slurry. The only way I could see it collapse like that is if the pipe got clogged or (more likely) someone forgot to open the inlet to the tank.

I would expect there to be some sort of pressure valve on the pump to prevent it over pressurising (or de-pressurising) the tank. An exploding pressure vessel would strike me as being a lot more dangerous than an imploding one so I'd expect significant more protection for that failure.

18-03-2014 3:45pm

I think you are correct spideog7, slurry is not my forte

However, I think I am correct about a pressure relief device failing.

aidanki · 18-03-2014 7:57pm

spideog7 wrote: »

You're drawing a negative pressure on the tank to suck up the slurry, it's unlikely it would ever become a full vacuum though. Generally it's operated manually (at least one as old as the one in that picture). You connect up the pipe, open the inlet, start the pump, watch the little window at the top of the tank and once you see the liquid get up to it you close the inlet and turn off the pump. I'm assuming there is some sort of check valve on the input to prevent slurry getting sucked into the pump, which presumably also provides an alternate input air path for the pump. You can also hear the load on the pump change once this happens.

No, once the tank is full and you want to spread the slurry you change the direction of the pump and create a positive pressure in the air pocket above the slurry (same as a regular pressure tank on your water supply). Once you open the nozzle at the back this then blows the slurry out the nozzle onto the splash plate. The pump continues pressurising the whole time, so presumably you could cause it to fail if you closed the valve with the tank empty (of slurry) and didn't disable the pump (and no other protection was in place).

Judging from the picture this happened when filling, so the pump is creating a negative pressure by evacuating the air, which should be replaced by slurry. The only way I could see it collapse like that is if the pipe got clogged or (more likely) someone forgot to open the inlet to the tank.

I would expect there to be some sort of pressure valve on the pump to prevent it over pressurising (or de-pressurising) the tank. An exploding pressure vessel would strike me as being a lot more dangerous than an imploding one so I'd expect significant more protection for that failure.

spideog got there before me

slurry is sucked in under a vacuum and slurry is spread under pressure, there is a gauge on the top of tank indicating pressure be it positive or negative, no slurry passes through the pump, safety valves take care of that

this would be a link to the pump manual

http://www.natvac.com/pdf/OM-Model-MECII-5-8000.pdf

to save people reading it

max vaccum is 27 inch mercury or 13.25 psi
max spreading pressure is 30inch mercury or 15 psi

so why do all these tanks always fail inwards rather than outwards ?

timmy4u2 · 18-03-2014 8:53pm

aidanki wrote: »

spideog got there before me

slurry is sucked in under a vacuum and slurry is spread under pressure, there is a gauge on the top of tank indicating pressure be it positive or negative, no slurry passes through the pump, safety valves take care of that

this would be a link to the pump manual

http://www.natvac.com/pdf/OM-Model-MECII-5-8000.pdf

to save people reading it
.
max vaccum is 27 inch mercury or 13.25 psi
max spreading pressure is 30inch mercury or 15 psi

so why do all these tanks always fail inwards rather than outwards ?

Boyles law. When volume decreases pressure increases.
I think the pressure on the surface is 14.1 lbs per sq inch which is 1 bar atmospheric pressure or thereabouts so takes very little for an implosion to occur.

I knew that knowledge of that law would come in handy sometime

timmy4u2 · 18-03-2014 8:57pm

Boyle's Law, a principle that describes the relationship between the pressure and volume of a gas. According to this law, the pressure exerted by a gas held at a constant temperature varies inversely with the volume of the gas. For example, if the volume is halved, the pressure is doubled; and if the volume is doubled, the pressure is halved. The reason for this effect is that a gas is made up of loosely spaced molecules moving at random. If a gas is compressed in a container, these molecules are pushed together; thus, the gas occupies less volume. The molecules, having less space in which to move, hit the walls of the container more frequently and thus exert an increased pressure.
Boyle's Law actually applies only to an ideal, theoretical gas. When real gases are compressed at a constant temperature, changes in the relationship between pressure and volume occur. However, the law is acccurate enough to be useful in a number of practical applications. It is used, for example, in calculating the volume and pressure of internal-combustion engines and steam engines.
The law was first stated in 1662 by Robert Boyle. In 1676, Edme Mariotte of France independently stated the same law, and it is sometimes called Mariotte's Law.
Stated as a formula, Boyle's Law reads:
V1/V2=P2/P1 (at constant temperature)
where V1 equals the original volume, V2 equals the new volume, P1 the original pressure, and P2 the new pressure.

There, you have it now.

aidanki · 18-03-2014 10:57pm

I know the boyles law bit,

but why did implode rather than explode

timmy4u2 · 18-03-2014 11:28pm

aidanki wrote: »

I know the boyles law bit,

but why did implode rather than explode

Surely boyles law has demonstrated that

.

For example, if the volume is halved, the pressure is doubled; and if the volume is doubled, the pressure is halved.

aidanki · 19-03-2014 12:18am

aidanki wrote: »

spideog got there before me

slurry is sucked in under a vacuum and slurry is spread under pressure, there is a gauge on the top of tank indicating pressure be it positive or negative, no slurry passes through the pump, safety valves take care of that

this would be a link to the pump manual

http://www.natvac.com/pdf/OM-Model-MECII-5-8000.pdf

to save people reading it

max vaccum is 27 inch mercury or 13.25 psi
max spreading pressure is 30inch mercury or 15 psi

so why do all these tanks always fail inwards rather than outwards ?

timmy4u2 wrote: »

Surely boyles law has demonstrated that

.

for my post above and the pump manual more pressure is generated during emptying than suction

so the question still remains why did the tanker not explode rather than implode

time to fill via suction would be less ~2mins compared to ~4mins to empty

mickdw · 19-03-2014 12:27am

Regardless of the pressure control systems involved, the shape/design of the tank lends itself to collapse via simple distortion whereas to explode, the Material would have to fail/fracture.
You can suck the air out of an empty coke bottle with your mouth but good luck trying to explode the bottle by blowing into it.

timmy4u2 · 19-03-2014 12:38am

http://glacialtill.wordpress.com/2011/01/14/imploding-tankers-or-when-chemistry-finally-gotinteresting/

aidanki · 19-03-2014 12:48am

mickdw wrote: »

Regardless of the pressure control systems involved, the shape/design of the tank lends itself to collapse via simple distortion whereas to explode, the Material would have to fail/fracture.
You can suck the air out of an empty coke bottle with your mouth but good luck trying to explode the bottle by blowing into it.

I agree, what is the mathematical for this buckling though ?

I dont think its the classic membrane

timmy4u2 · 19-03-2014 1:18am

aidanki wrote: »

I agree, what is the mathematical for this buckling though ?

I dont think its the classic membrane

V1/V2=P2/P1

spideog7 · 19-03-2014 2:02am

timmy4u2 wrote: »

V1/V2=P2/P1

You're over simplifying it, I think the OP gets the basic premise of Boyle's law. The question is not why did implode but rather why didn't it explode, the chemistry is the same and Boyle's law applies equally in both circumstances.

My guess is the responses suggesting it's a structural thing are closer to the truth, but would you not expect a valve fitting or something to fail before the whole tank would collapse?

timmy4u2 · 19-03-2014 9:32am

spideog7 wrote: »

You're over simplifying it, I think the OP gets the basic premise of Boyle's law. The question is not why did implode but rather why didn't it explode, the chemistry is the same and Boyle's law applies equally in both circumstances.

My guess is the responses suggesting it's a structural thing are closer to the truth, but would you not expect a valve fitting or something to fail before the whole tank would collapse?

When you are creating a vacuum the implosion is assisted by atmospheric pressure, when you are pressurising you have to work against atmospheric pressure

CatFromHue · 19-03-2014 1:52pm

enda1 wrote: »

The buckling strength of the tank is a lot lower than the tensile strength of the steel walls. So it's easier to collapse than explode. (That's apart from everything said above)

Is this not it?

I used to work in the vacuum industry (hey yo

:D ) and there would be plenty of say hoses for example which could take 10-16bar or sometimes even a lot more but put them under a rough vacuum and they'd collapse.

The principle would be the same for pressure/vacuum vessels. From just looking at that slurry tank it doesn't look to have any strength features for vacuum.

Here's another example

https://www.youtube.com/watch?v=Zz95_VvTxZM

A railroad tank car demonstration imploding after placing a vacuum on the tank with the vacuum safety valves disabled or removed

aidanki · 19-03-2014 2:18pm

CatFromHue wrote: »

Is this not it?

I used to work in the vacuum industry (hey yo :D ) and there would be plenty of say hoses for example which could take 10-16bar or sometimes even a lot more but put them under a rough vacuum and they'd collapse.

The principle would be the same for pressure/vacuum vessels. From just looking at that slurry tank it doesn't look to have any strength features for vacuum.

Here's another example

https://www.youtube.com/watch?v=Zz95_VvTxZM

A railroad tank car demonstration imploding after placing a vacuum on the tank with the vacuum safety valves disabled or removed

Im with you on this, what equation describes that though ?

interesting paper here http://rennes.ucc.ie/~bill/repository/2012-TWS-55_1-10.pdf I must read and understand it

suction question

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