Originally Posted by jasonliew18

Hi,

I am dealing with a project with closed loop pneumatic conveying system, however, i faced a problem on weighing part. please refer to picture attached.

When the hopper is pressurised, the weight of the hopper will increase. I am wondering why would this happen since I already put flexible pipe at top and bottom of the hopper to isolate the hopper...

Hello jason,

your problem is simply one of mass as the actual air you are pumping into the vessel has

weight all by itself. ■

Dear Jason,

You are determining a mass by measuring the force.

That means also that any additional force influences the measurement.

In your case it is not only the mass of the pressurizing nitrogen, but also the pressure itself.

In your installation configuration, the pressure acts also on the 2 top openings of 10”.

The action force is approx. :

0.2 * 2 * pi/4 * (0.25 * 100)^2 = 196 kgf

This force is not kept inside the weighing system as the flexible connections between the screw and the hopper isolate the action and the reaction.

By replacing (or extra mounting) the 2 pneumatically operated valves below the flexible connection, the pressurizing force will not be visible anymore.

The weighing process should not be a problem in your configuration, because that has already ended before you start pressurizing.

An engineer should always search for equilibrium of all acting forces. (gravity, pressure, inertia, spring, etc)

Have a nice day

Teus ■

TT is correct as always. However there is no indication of pressurisation on your PFD.

If you indicate the blow tank pressure line(s) it makes the situation clear.

Since you now know the story it should be possible to control and monitor the tank contents by accounting for the pressure download and applying Boyles Law during the emptying process. That will give you a calibration correction: but, as mentioned you shouldn't need it when you weigh the unpressurised vessel. There is also a chance that venting from the rotary discharge valve could maintain some pressure in the 'empty' tank and that would slow down your top up cycle.

Perhaps the approximate difference between the 200kg_f measured and the 196kg_f calculated by Teus is due to the entrained nitrogen weight suggested by Izaharis. ■

Dear louispanjang, Jason,

Indeed, the pdf does not show any pressurizing line.

There is also no vent line.

That leaves the question what happens during filling. The replaced nitrogen cannot escape from the tank and therefore, when the material is forced into the tank by the screws, the pressure must rise.

A measurement error is induced.

As Louis said, the venting from the rotary valve can cause pressurizing of the tank too.

As a matter of fact, this will happen when the pneumatic conveying starts.

The conveying pressure leaks from the rotary valve and the rotary vent back into the tank.

And because there is no vent, the pressure in the tank reaches the conveying pressure.

This theory could explain that the mass reading of the system increases with the conveying pressure.

It is up to you, Jason, to evaluate the system and modify it until you are satisfied with the performance.

Engineering at a distance of 15000 km (or more) is not really reliable.

Take care

Teus ■

Goes something like this "For a given mass of ideal gas the pressure is inversely proportional to the volume." Isothermally speaking.

So when the tank is powdered and pressured up the ullage is minimum and the 196kg_f superload is present.

When the powder is gone the ullage is about 3m³ and the pressure superload is reduced in inverse proportion.

You will have to calibrate you weighing system to account for the decreasing pressure or maintain constant pressure throughout the discharge.

This is down to the hidden tank pressure control.

You must appreciate vessel internal workings. Pressure drops acrosss packing in reactors impose tremendous thrusts on the trays and ledges and are critical for a safe tower design. Is your vessel coded? ■

The pressure source is most definitely the venting of the airlock. The flexible connectors do not isolate the pressure; conversely they are the creation of the force. The connectors act like a lifting airbag, you can multiply the surface area by the pressure to calculate the force.

You have to vent the hopper to minimize the pressure. I assume the conveying system is closed-loop nitrogen. Vent the hopper back into the low pressure side of the line. You may have to add a cyclone or filter receiver to separate the dust from the conveying gas. ■