How to Select Valves for Powders, Granules, And Slurries

7/31/2019 How to Select Valves for Powders, Granules, And Slurries

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How to select valves forpowders, granules, and slurries.

by Paul Naberhaus

There are many troublesome valve installations causing tens of thousands ofdollars in product, maintenance labor and production time losses. In many cases,a higher quality valve can prevent costly shut downs or off spec product andpay for its extra cost in one prevented malfunction. A valve is not just a Valve!

Like any other processing system dry materials handling requires valving. Inmost cases basic liquid handling valves have been adapted. These all haveshortcomings when used on solids products. The most commonly used valvesfall into three categories as shown below:

Figure 1


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Figure 2


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Figure 3

Slide Gate Valves have problems with product getting pinched and/or cut, jamming the valve so it wont close completely and with product leaking andsifting out through the seals. (See Figure 1)

Butterfly Valves have a vane dead center in the flow stream that can causebridging and obstruct flow. Also, the seal is exposed to wear by the pinchingaction of the vane. (See Figure 2)

Ball valves are heavy and take a lot of headroom. More importantly they have adead space between the casting and ball that fills and packs with productcausing abrasive wear on the seals and cross contamination or spoilage betweenbatches. (See Figure 3)

One of the most successful valve designs for dry materials is the rotating disc ordome valve as manufactured by Roto-Disc Valve Company.


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Figure 4

The Roto-Disc valve operates on only a 1/4 turn action and the shafts are sealedby dual O rings on each side. There is no problem of dust escaping throughshaft leaks. The valve has 150 PSIG ANSI flange drilling so flange leakage is nota problem.

The Roto-Disc dome rotates out of the flow stream. Also, the valve cavityincreases in size in the direction of product flow. For example, the 6" valve hasan 8" outlet allowing free fall thru the valve. Both of these features allowunobstructed flow. The unique shape of the closure with the spherical machineddome pushes product aside rather than making crushing contact with it. Thesliding contact of two spherical surfaces, dome and seat tends to wipe the sealclean and promote a pressure tight seal. While it is not bubble tight, it is goodenough for most hi-vacuum applications.

Finally, The Roto-Disc valve can rotate through a static head of material withease, pushing material out of its way as it closes.

There are three basic designs of the Roto-Disc valve. First is the basic Roto-Discwith a standard seal of TFE. Seals are also available in Stellite, brass, PEEK andCelazole. These valves come standard in cast ductile, aluminum, 316 SS andHastelloy, in sizes from 3" to 24" diameter (See figures 5 and 6). Special seal ORings are used for vacuum or pressure seating if leaking must be minimized.Temperatures of applications range from -40 F to 750 F. Higher temperaturescan be accommodated by water-cooled jackets and domes. Roto-Disc standardvalves are rated to 100 PSIG but higher pressures can be accommodated.


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Figure 5


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Figure 6


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The second design is the Roto-Clean, a patented quick-assembly designnecessitating no tools. They are usually built-in stainless steel or aluminum foruse in the food, dairy and pharmaceutical industries. Figure 7a shows the valvecompletely apart. Not a piece is left together - yet assembly can be done in acouple of minutes. Clearances are maintained so no regaping is needed. Figure7b shows the valve at final assembly. In many cases the valve body is lifted to

the top plate for clamping where the top plate is welded to the outlet if a mixer orvessel. These are available in sizes 3" to 12" diameter.

Figure 7a


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Figure 7b

The third design is the Roto-Flate, an inflatable seal valve used where bubbletight or high-pressure sealing is important (See Figure 8). This valve is typicallyapplied in three cases: 1. High vacuums above 28.5 Hg, 2. Hazardousgases under pressure such as produced in chemical reactors, 3. Densephase pneumatic conveying systems . Seals can accommodate temperaturesto 400 F. These valves can likewise be water cooled for materials above 400 F.


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Figure 8

Other valves have been designed specifically for dry-material handling. A few ofthem are as follows: Pinch valves which use an elastomer sleeve encased in a

metal pipe to be externally pressured to pinch the elastomer flow channel closed.These are long and frequently suffer from elastomer wear problems. Iris valvesprovide an adjustable concentric hole like the lens of a camera. The opening isadjusted by tightening or loosening a stainless steel cable wrapped around afabric or elastomer sleeve. They too, suffer from abrasive wear and are limited tolower temperature and pressure applications.

Sliding disc valves that slide into a pocket within a casting can be effective forhigh temperatures and pressures. However, by their shape, they leave large

cavities that are hard to clean and can retain cross contaminating product.

There are many different valve types used in dry solids handling from simplehammer drive plates for maintenance cut off to sophisticated bubble tight hightemperature valves for reactors. Some valves are operated two or three timesper minute and some two or three times per month. Obviously, the selection ofthe type of valve should be made based on the application.

That is why Roto-Disc uses a data sheet showing a sketch of what is above the

valve and what is below. The sketch also indicates pressures, temperatures,cycle frequency as well as product characteristics. This is part of the orderacknowledgment procedure and is incorporated on the approval drawing.

Roto-Disc considers valves to be a capital item and takes care that the valve besuited to the application.

How to Select Valves for Powders, Granules, And Slurries

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