Valves
abrasive materials in high-pressure dense- phase applications. The primary disadvantages of the rotary- blade diverter valve, like those of the rotary-plug diverter valve, concern its cost, susceptibility to seal abrasion, and inability to shift “on the fl y”. This valve usually costs much more than other valves, because its precision- machined cast housing is expensive to fabricate, hard to install, and costly to repair. Replacement parts are also costly. The rotary-blade diverter valve requires clearance between the housing and sealing surfaces in order to actuate. The valve’s sealing surfaces tend to wear quickly when exposed to abrasive materials at high velocities. This enlarges the clearance gap and allows air and material to migrate into the closed line, which decreases the conveying system’s air volume and can cause material cross- contamination.
FLAPPER DIVERTER VALVE
The fl apper diverter valve (also called a swing diverter valve), as shown in Figure 2, uses a swinging metal fl apper (or gate) to divert material fl ow from the upstream conveying line to one of two downstream lines. A metal shaft runs though the housing between the downstream lines and lengthwise through the fl apper’s bottom, creating a hinge point that allows the fl apper to swing back and forth. In some models, the fl apper seals against a replaceable polyurethane liner. The fl apper diverter valve can be made with pipe fl anges or stub ends (used with compression couplings) for connection to a conveying line. The valve can be used in both dilute- and dense-phase pressure conveying systems to convey powders, granules, or pellets from one source to two destinations (diverter) or from two sources to one destination (converger). However, once the valve’s material fl ow direction has been established, it can’t be reversed without costly valve modifi cations. Most fl apper diverter valves in dilute-phase vacuum conveying systems are limited to low-vacuum applications. In high- vacuum applications, the fl apper can lose its
40 March 2014 Solids and Bulk Handling Figure 2. Flapper Diverter Valve Illustration
seal, because the vacuum tends to pull the fl apper from its internal sealing surface. With some powders, the fl apper diverter valve can shift “on the fl y”. However, even with these powders, some material can become trapped between the fl apper and sealing surface, creating a gap for air and material leakage into the closed line. Shifting this valve “on the fl y” is even less suitable for materials with large particles. The fl apper diverter valve’s primary advantages are that it typically weighs less and costs less to purchase, install, and maintain than the rotary-plug and rotary- blade diverter valves. One of the fl apper diverter valve’s disadvantages is that its seals are directly in the material stream, they can wear rapidly when the valve handles even mildly abrasive materials. And as the seals wear, air and material can leak past the seal into the closed line, potentially causing conveying air loss, material cross-contamination, and line blockages. When the valve handles an abrasive material, the seals must be replaced more frequently, which can lead to extended downtime and increased production losses.
Because worn seals are diffi cult, expensive, and time-consuming to replace, select a fl apper diverter valve that can be serviced without removing it from your conveying line.
SLIDING-BLADE DIVERTER VALVE
The sliding-blade diverter valve, as shown in Figure 3, diverts material fl ow via its fl at, rectangular, sliding metal blade with a hole near its centre. The sliding blade is installed so that it intersects and extends beyond both downstream conveying lines. To divert the fl ow, an actuator slides the hole over a downstream line and the airfl ow carries the
can be carbon steel, aluminium, or stainless steel, and the hole is the same size as the downstream conveying lines’ inside diameter. The valve’s wear-compensating seals consist of a polymer pressure plate with a rubber or silicone backing. The backing pushes the pressure plate against the sliding blade, eliminating gaps that would allow air and material to leak into the closed line. The sliding-blade diverter valve can convey both powders and pellets from a single source to multiple destinations (diverter) or from multiple sources to a single destination (converger) in either pressure or vacuum dilute-phase conveying systems. Because the valve’s seals create a positive seal across the closed line, the valve can shift “on the fl y” with most materials. The sliding-blade diverter valve has a precision-fabricated structural frame, as well as, fabricated plastic-tube or metal- pipe attachment points, called weldments. The weldments match the conveying line construction material and serve as attachment points to connect the valve to the lines using compression couplings or fabricated fl anges. The valve is available in two-, three-, and four-way confi gurations for more cost-effective conveying system design. The sliding-blade diverter valve is lightweight and easy to install, and its simple design allows you to make maintenance and seal adjustments without removing the valve. Additionally, the valve’s weldments are easily replaced if abrasive material wears through them, eliminating the expense of replacing the valve’s entire precision- cast housing in abrasive applications.
Figure 3. Single Blade Diverter Illustration
material through the hole into the line. The sliding blade’s solid section stops material fl ow to the other downstream line. The blade
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One disadvantage of the sliding-blade diverter valve is that material fl owing through the blade’s hole creates a slightly greater pressure drop across the valve than is typically produced in other valves. Another disadvantage is that the valve doesn’t function well if installed horizontally because material may not
be completely purged from the closed-line segment between the upstream conveying line and the blade’s upstream surface,
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