Originally Posted by Fahad

We are using close loop pneumatic conveying system (nitrogen) to transfer LLDPE from big silos into an extrusion hopper. The inlet line enters through cyclone where the LLDPE granules fill the hopper and the return line from the cyclone will back to filter cooler and blower suction again.

Previously, we had rotary feeder below the hopper to feed the downstream. After changing some systems in the downstream, the vendor required more flow of material to downstream and he replaced the rotary feeder by a slide valve. The problem here is how to release the extra pressure flowing to the downstream. Notice that a line was pull from the discharge channel to ATM but LLDPE granules escaped with each discharge making lot of losses.

thank you in advance,

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Fahad,

Closed loop conveying systems require an airlock at all discharge points to prevent loss of conveying gas. Replacing the rotary valve with a slide gate means that you have lost this airlock. You can reduce conveying gas leakage by maintaining a head of LLDPE granules above the slide gate. Pressure drop across this head should be greater than the gas pressure coming in. You can also reduce gas leakage by not opening the valve 100% but by reducing its opening to match the solids flow rate.

Regards,

Amrit Agarwal

Consulting Engineer

Pneumatic Conveying Consulting ■

Closed loop nitrogen systems are always tricky. With the air lock removed it is very likely that

huge quantities of air is being sucked into the system. If you need more volumetric throughput

then either increase the rpm of the rotary valve or just get a bigger rotary valve. Since you have

mentioned that it is a granular product and if it has no dust in it then in my opinion Nitrogen system

is not required in the first place. Granular material is not prone to dust explosions in the conveying

system. Unfortunately it is very common to see these system in petrochem industry when they are

not need. ■

Originally Posted by Mantoo

Closed loop nitrogen systems are always tricky. With the air lock removed it is very likely that

huge quantities of air is being sucked into the system. If you need more volumetric throughput

then either increase the rpm of the rotary valve or just get a bigger rotary valve. Since you have

mentioned that it is a granular product and if it has no dust in it then in my opinion Nitrogen system

is not required in the first place. Granular material is not prone to dust explosions in the conveying

system. Unfortunately it is very common to see these system in petrochem industry when they are

not need.

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FAHAD,

Closed loop systems using an inert gas such as N2 are a necessity because freshly produced granular LLDPE contains hydrocarbons. I think you are using

closed loop N2 conveying because of the dust/hydrocarbon mixture's explosion hazard.

You have probably already looked into speeding up the present rotary valve to increase its capacity. A larger rotary valve will also solve the problem but its cost will be high.

Are you using the Cyclone as a feed hopper for the pelletizing extruder or do have another hopper below the Cyclone?

Regards,

Amrit Agarwal

Pneumatic Conveying Consulting ■

Originally Posted by Fahad

Thank you gentlemen for your feedback, actually we are using N2 as conveying media to avoid any explosion hazards as you mentioned. The replacement of the rotary feeder was done because a weighing system required faster refill during normal operation. We have equalizing lines in the downstream to the slide valves and the weighing system to ATM as well. I wonder if the rotary feeder is not an option, what would be the alternatives.

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Fahad,

Looks like you are using a LIW (Loss in Weight) feeder to feed the extruder. This type of feeder requires batch filling of its refill hopper based on the sensed low and high weight. Refill time is less than a minute, generally about 15 to 30 seconds. With this, I now understand why the rotary feeder had to be replaced.

I need a layout of your present equipment, with dimensions, to fully understand your set-up. One possibility is a surge hopper above the LIW feeder and reuse the rotary valve.

Regards,

Amrit Agarwal

Pneumatic Conveying Consulting

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I rough sketch of the system will make it easier to give a suitable answer especially the details

of the weighing system you are feeding! If you want to replace RV then double dump valve is

an good option. ■

Originally Posted by Fahad

Dears,

thanks again for your kind feedback

i tried to upload the P&ID through the following link I hope it will work

http://s1.directupload.net/images/140312/6oo5rajq.jpg

thank you

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FAHAD,

Your P&ID shows that the granular resin flows from the Cyclone to a Weigh Hopper, Item 2614, and then to a Mixer, Item 2611 with a 12 inch slide gate between these two items. To solve your 2 problems with this arrangement, which are loss of N2 and excessive discharge rate, you may consider the following changes:

1. Maintain a high resin level in the weigh hopper and its discharge chute. Set the level so that the pressure drop across the resin bed is higher than the N2 pressure in the Cyclone.

2. Modify the 12 inch slide gate valve so that its opening can be controlled by the loss of weight of the weigh hopper. This can be done by using a flow rate controller.

You may contact me if you would like to discuss this scheme.

Amrit Agarwal

Pneumatic Conveying Consulting

Email: polypcc@aol.com ■

Originally Posted by Fahad

Dear Mr.Amrit Agarwal,

Thank you very much for your explanation I really appreciate your analysis. Regarding maintaining a high resin level in the weigh hopper, to what level we can relocate and what’s make the level solve the issue.

((Set the level so that the pressure drop across the resin bed is higher than the N2 pressure in the Cyclone.)) If I may ask you sir for more explanation.

2. Modify the 12 inch slide gate valve so that its opening can be controlled by the loss of weight of the weigh hopper. This can be done by using a flow rate controller. If I may ask you sir for more explanation.

I really appreciate your feedback sir

Fahad

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Fahad,

1. Required bed height can be calculated based on material's bulk density and the air pressure in the Cyclone.

2. Instead of using a fully open or fully closed slide gate valve, you should modify its operation so that its opening is adjustable and is controlled by the weight of material in the weigh hopper. For the controller, you should contact a vendor who makes loss in weight feeders, such as Acrison or KTron.

Regards,

Amrit Agarwal

Pneumatic Conveying Consulting

Email: polypcc@aol.com ■