Re: Silo Pressure Surge At The End Of A Pressurized Tank Discha…
Long long time ago trials were carried out on surge flows from standard road tankers into silos.
The surge flows were measured between 6000 - 13000 m3/h from a standard road tanker
surging from 2.0 -1.8 barg through a 4" pipe.
Obviously there are number of parameters which effect this flow. The measured flows were
much higher then expected and proved one thing for sure that fan assisted filter will never
catch the surge flow. ■
Re: Silo Pressure Surge At The End Of A Pressurized Tank Discha…
Dear Dr. Mantoo,
Each day, trials and test are carried out by operators, when a road tanker or cement tanker is unloaded.
The problem is that those field experiments are not documented.
By experimenting, the operators find the settings of the pneumatic unloading system for each pipeline and silo they deliver the material to.
Solutions like reducing the pressure before the pressure tank starts to empty the last amount of material is then the most common procedure.
Over the many years the road tankers were equipped with bigger compressors and pipe outlets operating on bigger pipelines.
Sometimes on bigger silos with bigger filters, but mostly on old existing silos with a small filter.
This is especially the case with cement carriers (inland and seagoing)
The inland cement barges, with over time increasing tank sizes, operate at pressures up to 3.2 – 3.5 barg on old, existing terminals making the situation worse.
That development caused a lot of dust clouds (mainly cement), forcing to change the filter size or the operating procedure.
The mentioned surge flow you mention in your reply are well possible, depending on the pipeline length, filter contamination and the tank discharge gradient at the end of the unloading cycle.
The numerical integration algorithm can do the calculations to predict the air flow surge.
Filter size, filter cleaning and maintenance have a significant influence.
The tank outlet, determining the discharge gradient at the end of the unloading cycle has also a significant influence.
Never forget to maintain the silo safety valve, so that it will open when necessary or the silo roof will come off. ■
Teus
Silo pressure surge at the end of a pressurized tank discharging.
When a pressurized tank is discharged into a silo, there is a moment that the tank is empty, while the pressure in the tank is still at maximum.
The air mass in the tank (tank volume * atm-gas density * (pressure+1)) is connected to the free volume in the silo at a pressure, close to atmospheric through an empty pipeline.
The, at that moment, existing high-pressure differential between the tank and the silo through an empty pipeline with a low resistance, causes an air mass surge to the silo, increasing the silo pressure.
At the same time, the silo air-evacuation is determined by the filter resistance and the caused silo pressure.
Initially, the silo pressure will rise and with the rising silo pressure the silo air-evacuation, the silo pressure rise is reduced.
During this event, the overall effect is that the silo pressure increases.
Until the air surge from the tank is decreased to a level that the air evacuation through the filter releases an equal amount of air from the silo as the air influx from the tank, the silo pressure reaches maximum.
From that point on the silo pressure decreases.
However, there is a real risk that the maximum silo pressure reaches the setting of the silo safety valve, which will open at that moment and a dust cloud is emitted into the air.
The air mass surge to the silo is determined by the tank pressure and the pipeline flow resistance.
Reducing the air mass surge to the silo is possible by reducing the tank pressure before the pressure tank is fully empty.
Reducing the tank pressure can be done by lowering the set discharging pressure before the tank is empty.
However, in many situations the material outflow is not suddenly changed from maximum capacity to zero but reduces gradually.
When the tank outflow reduces gradually, the tank pressure also reduces gradually and the air mass surge to the silo also reduces gradually, keeping the maximum silo pressure at a lower level.
Another solution to keep the maximum silo pressure below the silo safety valve setting is the application of a bigger silo filter, increasing the silo air-evacuation.
Whether that discharge tank pressure gradient is sufficient to prevent the silo safety valve to open is depending on the material outflow characteristics of the discharge tank, the pipe resistance, and the filter size (and filter contamination).
The application of a silo filter fan helps (a little) to prevent the silo safety valve from opening.
The calculation of such an event requires a pneumatic conveying calculation to determine the pipeline resistance factor as a function of the conveying rate.
The conveying rate gradient can be calculated from the discharge tank pressure gradient by measurement for a certain installation and material at the end of an unloading.
Once, the measurement data are available, a calculated estimate for other pipelines can be made, by assuming that the tank material outflow gradient is a discharge tank property for that material.
As the calculations are based on the discharge tank gradient, a numerical integration algorithm is required. ■
Teus