Your application is a classic example of “looking at the entire picture” of what is involved in the handling of your ground-processed tobacco.

This is a Class 5 material, which has some unique handling properties, one of which is its ability to physically interlock or mat. The net result is that it will not be easily ‘poured’ out of the ‘cardboard carton’ that it is received in. So in order to get the tobacco out of the carton, you will most likely have to lift and invert the 12.5 - 13 cubic feet of tobacco carton, dumping the entire contents into a holding or supply hopper. This can be accomplished via ‘skip hoists’, drum or carton dumper [which are commercially available].

Class 5 materials will also resist flow due to consolidation head pressures, moisture absorption, and other externally applied factors so you want to empty the material as quickly or as soon as possible. If extended storage times are part of the system, a recycle system may be necessary in order to reduce potential flow problems due to head load consolidation.

We would recommend a specifically designed live bottom bin or PosiBin, for holding this material. The PosiBin will provide controlled discharge, on demand, to the rate-controlling device located below it.

For cursory rate control a ‘screw type’ of feeder is preferred although others concepts may also work with somewhat less efficiency. The screw feeder must be carefully designed and integrated with the PosiBin unit above to ensure a well-performing package.

Based on question and clarification requests, we note that this gentlemen has a packaging application / installation, handling a 1 – 2 mm ‘rubber crumb’, which is being stored in a 59 inch diameter x 55 inch straight wall height x 78 degree hopper angle. He states that the discharge outlet or opening is 31-½ inches diameter. The bulk density of this material is stated to be 28 PCF, no required flow rate.

Any material handling application is made up of at least two components: Storage/discharge and metering or rate regulation.

Rubber crumb is a Class 5 material, meaning it will physically interlock, absorb vibration, and will not flow easily due to gravitational effects.

What is puzzling is the stated discharge problem. The bin-hopper is not considered to be very ‘large’ with significant head loading. Considering a repose angle in excess of 50 degrees, a working hopper capacity would be less than 1400 lbs. With this configuration [31 ½ inch discharge opening!!] the rubber crumb should flow, unless it is being restricted in some manner not identified.

The gentlemen should locate and identify this bottleneck. Due to the Class 5 properties, and his existing bin-hopper, which he does not want to replace, we do not believe brute force / vibration applied to the hopper or straight wall is a solution. We would suspect the rubber crumb is forming a stable arch or bridge between the hopper outlet and downstream device, stated to be a bag filler.

Once the bottleneck has been located corrective mechanical measures may be required, such as an active flow promotion device like a mechanical stirrer to break up the bridge at the bottleneck. We would also look at contact materials, which will lower the wall friction in the hopper and straight wall. [Teflon perhaps?]. In some cases the lower portion of the cylindrical hopper is replaced with a series of large diameter, slowly rotating screws, or a mechanical Bin Activator [60 inch diameter in this case] for flow promotion.

He should also review the design and operation of the bag filler to see how the filler components relate to a Class 5 material operation. REMEMBER: Storage/Discharge and Regulation must be correctly integrated!

I am are pleased to offer some comments in answering your topic question regarding problems associated with a knife gate valve installed on the discharge of a process ribbon blender. [Assume most materials are very fine particulates?]

Knife gate valves [KGV’s] are not ‘new’. If you research the early recorded use of such cut off gates, you will find a link to the sale and fabrication of KGV’s to a Swedish company dating back to 1650! Since then, different valve types, products and designs were made available, starting in the 1800’s through present day.

Unfortunately, not a lot of technical improvements were made, particularly for KGV’s during these early years, simply due to the fact that these valves [and other types as well] were made for use in liquid and gas control, whose properties are known and fairly constant [I.E. consistent and uniform]. When faced with a solids control problem, engineers were forced to select and make fit, KGV’s designed for liquids or gases.

A quick review of US Patents issued or pending from 1960 through the present, indicate most deal with valve operation improvements rather than design improvements. Valve operating improvements to seals, packing, seats, and valve liners are most often cited. The design remains the same with improvements being directed to the internal components.

There are a few valve companies who have come along over the last twenty years or so, who have specialized in dry bulk solid valves due to the very problems referenced. One of these is Salina-Vortex, which you are familiar with.

Your problem looks like it is due to a seal problem associated with the knife gate sealing against the body. Powder material can build up in the open chest area and remain ‘trapped’ eventually causing the problem you describe. Several solutions include: installing an inlet shield or shear protector, which is designed to minimize direct powder contact with expose seals, and other moving parts. This generally works best when the KGV is slightly ‘oversized’; providing purged air to the enclosure for seal / seat protection; or the most obvious is to replace the valve with an “Orifice Gate” design which has slide gate blade/seal modifications referenced above and consider air purging as well.

I hope this has been helpful?

There are various methods and techniques used for sampling in order to obtain a representative sample for evaluation, testing, quality control, etc. Guidelines for sampling solids [liquids and gases as well] have been established and can be sourced through ASME, MIL STD’s, ANSI/ASQC, ISO 9000 Quality Control Procedures, even the National Institute of Oilseed Products have established sampling procedures!

Books have been written on the statistical relevance of sampling and how results affect certification of final product quality or in the evaluation of production efficiency. Physical sampling is one method of statistical quality control.

Sample size and sample frequency will be dictated by process and individual plant QC standards.

Generally, sampling standards are divided into three types of inspection: normal; tightened, and reduced inspection. Normal cycles are based on initial production runs and process, similar to start up phases in a typical plant; tightened sampling is incorporated if a history of plant problems or quality control issues have occurred, basically more frequent and detailed sampling; reduced sampling is used when sampling is required for QC issues but do not occur frequently because historical data has proven the plant production to meet specification.

We assume you have a Sampling Plan in place, including the frequency of sampling, the methodology of how the sample will be extracted, and any specific guidelines you intend to use.

Obtaining a representative Sample can be accomplished through various means, either a portion of product flow or a complete cross section of product flow by means of a suitable sample device, often call a sample thief.

As you have noted, you are already considering installation of a sample thief. A question a supplier will ask is if you are going to do a point of entry type sample or an entire cross sectional sample, which would require a full diverter installed in the transport line.

As far as sample sources, you need to look no further than Powder/Bulk Solids for a complete list of companies that manufacture ALL types of samplers and have engineering expertise specific to your needs.

There are many factors that must be considered when selecting a feeder, such as whether the feeder is vibratory, screw, belt, rotary, tray, en masse, etc. Design factors, which should be considered but not be limited to, include: functions to be accomplished, properties of the material, conveying speeds and distances, and economics.
If, after reviewing of all details such as those above, it appears that a vibratory pan (or tube) is the best selection, then some basic guidelines would apply. Remember all feeding and conveying applications require a well engineered and designed storage/discharge supply capability. Poor flow to the feeder will result in poor feeder performance. You did not indicate how you intend to supply the aggregate to the feeder. What sort of feed accuracy is required, and is control feedback information is required?
A pan (or tubular) feeder/conveyor is simply a trough (or tube) with vibration (force) applied in a manner so as to convey or feed bulk materials. The vibration or force is applied to the feeder at a very specific angle, so as to lift and move the individual particle along the feeder’s surface.
A particle sitting on the surface is moved upward and forward with the feeder. The feeder retracts on its downward stroke, with the particle continuing forward, landing further along the feeder’s surface. The more dense the particle, the more efficiently it is conveyed. The cycle continues. This ‘throw and catch’ motion conveys material along the feeder’s surface at velocities variable from 0 – 100 ft / min (or more!), depending upon the combination of drive frequency, amplitude, drive angle, and material properties. A major difference between a feeder and a conveyor is use of high amplitude and low frequency for a conveyor to high frequency and low amplitude for a feeder. For feeder calculations, a pan (or tube) velocity of 25 ft/min is generally used. (Remember this may vary based on material properties, etc.)
For vibratory feeder and conveyors, many drive configurations can be used including a.c. unbalanced ‘vibrators’, electromagnetic, and others. The resultant displacement must be linear, at an angle to the trough (or tube). The feeder/conveyor will be supported/mounted on suitable isolator mountings. (Some conveyors have other means of support from ‘leaf springs’, air mounts/springs, cables or wire rope, etc., depending upon the type of unit selected.)
Vibratory feeders generally operate in the range from 900 cpm with displacements (stroke) of ¼ in., to a high frequency of 7,200 cpm with displacements of 0.035 in., with the norm from 3,600 to 5,700 cpm.
Vibratory conveyors seldom exceed 1000 cpm (more like 300 – 600 cpm with displacements from ¼  to 4 in.
Pan Feeder capacity (tn/hr) = [W] [D] [R] [d] /33.3, where:

W = width of pan, in ft
D = material depth, in ft
R = linear velocity, in ft/min
d= bulk density, in lb/ cu ft.

Your application, 70 tn/hr with a bulk density of 120 lb/cu ft., results in a volumetric ‘rate  of 1,167 cu ft/hr, or 19.44 cu ft/min.
For sizing pan width and an appropriate nozzle we are going to use a ‘typical’ horizontal velocity of 25 ft/min, with the equation: 19.44 cfm/25fpm = 0.7776 sq ft. x 144 = 111.9 sq in., which represents bed width and height. (Here we are using inches since most pans are sized in inches). Therefore, pan width and product depth must be equal to some combination of 111.9 sq in.
When considering nozzle or gate openings, chute diameters, etc. with granular, aggregate particulates, a rule of thumb is to use six times the largest particle size, in this case ¾ in. Using this ratio we have: (6)(0.75) = 4 ½ in. Dividing 111.9 by 4.5 results in a nozzle width of 24.8 in. We can be creative here to minimize pan configuration, etc., by shortening the width and increasing bed depth of the infeed nozzle. Therefore a 20 in. wide pan, with a bed depth of +6 in., using a convey velocity of 25 ft/min will safely obtain your desired convey rate.
A quick check: WDRd/33.3 = (1.66)(0.5)(25)(120) /33.3 = 74.77 tn/hr!
For this configuration electromechanical, inverter-controlled drives or electromagnetic drive versions are available. I do not know of any pneumatic drive which would be suitable for such an application.

In the industry we see more and more applications for eductor (venturi) systems. In some cases, the eductor may be used in dry bulk/gas conveying, or as in your case, dry bulk/liquid conveying. Eductor uses are not new, concept and application go back to the ‘originators’ - Bernoulli, and Venturi, et al (circa: 1700- 1800’s). Eductors have been in practical use in liquid and gas handling applications as ‘injectors’ or ‘ejectors’. These are typically low pressure devices, deriving the name from the Latin, Educere, meaning “one that leads out”. Used very often with liquids and gas motive fluids, primarily due to the fact that both gas and liquids have very predictive properties that are necessary for correct and efficient eductor design features. Venturi’s have been used in automotive carburetors for over a hundred years with good (expected) results. Using an eductor for moving a liquid (or gas) and solid from point “A” to point “B” becomes a bit more of a challenge, since the ‘motive fluid’ and the ‘conveyed material’ must be reviewed for compatibility.

Next,the convey loop is considered, such as the horizontal distance, the vertical distance, and the number of elbows, all of which will impact the pressure drop the system sees. System design for a dry bulk/liquid system thus becomes more complex. You did not indicate what your two dry bulk solids are, or their properties, so we can offer generalized comments here. Ideally, the conveyed material and motive fluid should be compatible, that is, easily mixed or soluble. An insoluble solid or liquid application may require a more aggressive approach to conveying. Assuming that the eductor system has been correctly designed to the above conditions, then the most important feature of the system becomes the dry bulk feeding equipment.

Remember: gas and liquid systems are easily designed due to the documented consistent properties of these states, and a vast amount of design knowledge is available. The motive fluid parameters are therefore known and considered. The addition of the dry bulk solids to the eductor system is very important to the overall operation of the system. Dry solids feeders should be designed so they provide an accurate, reliable, uniform, and consistent flow of solid into the eductor, in a ’starve feed’ flow pattern.

It is also preferred that when handling a dry bulk solid that the solid be easily soluble in the motive fluid. For this, ‘wetting cones’ are recommended to facilitate the introduction of solid to liquid flow stream. Eductor systems work best when the number of variations (or variables) such as dry bulk solid feed is eliminated or reduced by using state-of-the-art solids feeders. Such feed systems should be selected based on manufacturers’ experience in business, years and numbers of working applications with the dry bulk solids in question, and reliability and durability as would be applied to maintenance (how many moving parts does the feeder have to theoretically break?).

I assume you have your reasons for using propylene glycol as the motive media? This should not present any major concerns since basic properties are similar to water, and there are many eductor systems in use handling water. You may also want to contact eductor (venturi) manufacturers for their comments. There are a few good companies selling eductors for use with solids, which also have experience as well. Good luck with your application.

The concept of moving material via screws, or helix, is not new or recent. Historically, the concept of the helix goes back to the ancient Greeks, such as Archimedes and Vitruvius. Industrial uses for lifting and conveying (via screws) in this country is generally credited to Oliver Evans, who wrote and published ‘”The Young Millwright and Miller’s Guide” in 1795. This is considered the first reference source for flour milling and production processes, which was later used in other industries. Ever since the early days of industrial manufacturing, basic equipment, such as screw conveyors and screw feeders, has been improved upon by others. With improvements comes variety. Today there are probably a dozen or more reputable companies manufacturing screw feeders here in the United States alone. It is also fair to state that each will submit for consideration their design that falls into the category of ‘A better Mouse Trap’!

In my opinion, there is little or no difference between a single screw and a so called ‘dual screw’. As I recall, the ‘dual screw’ concept was publicized/advertised as being designed to handle adhesive or cohesive powders, and could do so based on the two intermeshing, self-cleaning screws.

In fact, Class 4 materials (powder sluggish, adhesive-cohesive) are being handled with single screws probably more often than a dual-screw configuration due primarily to the sheer number of single-screw feeders working in industry today!

What is more important in selecting a screw feeder, other than a single-helix or dual-helix design, are the variables that must be considered in basic selection. Some of these are:
1. Material: Properties and Classification. Standards that apply to the material.
2. Performance requirements: feed rates, accuracy, and life cycle.
3. Environment/Ambient conditions: indoors or outdoors, food plant, pharma plant, foundry, etc.
4. Maintenance: spare part availability and COST, the number of moving parts to wear and break.
5. Manufacturer’s Integrity: ability to provide convenient field service, long term solvency,
and business philosophy.
6. Historical Experience: actual field installations and experience with your material, in your
industry (i.e., reference list), test lab services.
7. Cost: initial cost, cost to operate, manufacturer’s commercial terms, and warranty.

The material you want to store - discharge - meter into process belongs to a class of very difficult to handle materials, Class 5. Class 5 materials are fibrous, flaky, flocculent, or chips that have long, irregular particle shapes, substantial diameter to length ratios, will physically and mechanically interlock, and often are light and fluffy with bulk densities less than 30-40 pcf. Most Class 5 materials will not be handled with typical, “standard catalog” equipment. Most often, very special equipment is required for these materials, as is in your case.

There are feeders being used successfully with similar materials in various industries such as textiles, tobacco, road building - resurfacing, process operations using long strand (>1/2 in.) fiberglass, and plastic film and regrinds, as examples. These feeders are also referred to as ‘Pin Feeders’. As the name implies, material is discharged by means of a series of drums with multiple, intermeshing pins located on the outer surfaces of the drums. Such ‘Pin Feeders’ have long been used in the handling of cotton and flax fibers in cotton ginning, for example.

Your problem must be broken down into two parts: storage and discharge, and then rate control or metering. A suitably designed storage vessel would be required to hold the 10-15 tn [1660 cft], which in itself would present a sizeable challenge for discharge into the feeder or conveyor. I would expect substantial bridging of material if any converging slopes were incorporated in the vessel design. Once the vessel design problem is solved, then the type of control device can be configured to mate to the vessel.

Our suggestion would be to look in the textile industry first if you have not already done so.