Factors to consider when selecting a diverter valve

Factors to consider when selecting a diverter valve Selecting a diverter valve for your pneumatic conveying system can be a tough job, especially when you consider how many diverter valves are on the market. Yet a properly selected diverter valve can keep your operation running smoothly, cut maintenance costs, and improve your conveying system s efficiency. After outlining factors you should consider to select a diverter valve for your system, this article provides descriptions of commonly used diverter valves, how they work, how they re applied, and their pros and cons. Before you can select a diverter valve for your pneumatic conveying system, you need to consider several factors: your application type, your conveying system type, your material characteristics, valve cost, and valve options. APPLICATION TYPE - Most diverter valves, also called two way diverter valves, convey material from one source to two destinations. In some applications, a two-way diverter valve is installed backward in the line to convey material from two sources to one destination. When used this way, it s called a two-way converger valve. However, be aware that turning a conventional two-way diverter valve into a converger valve may require the manufacturer to make costly design modifications that will increase the valve s purchase price. It may be more practical to select a valve specifically designed for converging. If your system has more than two material sources or destinations, you need to consider how each diverter valve can be installed to meet your system s design criteria, as well as, your budget. One way to meet these constraints is to install a diverter valve with more than two ports, called a multiport diverter valve. CONVEYING SYSTEM TYPE - Your diverter valve choice will depend on whether your pneumatic conveying system is dilute- or dense-phase and operates under pressure or vacuum. A dilute-phase system can have a conveying line pressure up to 15 psig or vacuum pressure down to 30 inches mercury, while a dense-phase system can have a conveying line pressure up to 90 psig. Make sure the valve you select will operate well in your conveying phase and at your system pressure or vacuum. You also need to consider your pneumatic conveying system s pressure or vacuum capabilities and line diameter, as well as the expected pressure drop across the valve, to help determine the valve s minimum and maximum size. This ensures that the valve will function properly and efficiently after it s installed in your system. To help you narrow the field, many manufacturers

pressure-rate each of their valves and provide a valve-sizing chart listing each valve s specifications and limitations. MATERIAL CHARACTERISTICS - Consider your material s characteristics, such as particle size and abrasiveness, to choose a diverter valve that can handle them. Particles that are too large for the valve you select can become jammed between the diverter s internal components, preventing the valve from fully closing off the specified downstream conveying line or lines (the closed line or closed lines) to air and material flow. Particles that are too small for your valve can become packed between the internal components, binding the diverter, or the material can pass into the closed line. Material that s too abrasive for your valve s design can erode the valve components, creating a gap for air and material to pass through into the closed line. Air passing through this gap will create a pressure drop across the system that affects the system s capacity and material passing through to the closed line may contaminate your finished product. To fix these problems, you must shut down the conveying system and remove the valve to either clean or replace its internal parts. This creates lengthy production downtime, increases maintenance and production costs, and reduces your system s efficiency. To prevent these problems, identify your material characteristics and choose a valve that can handle them. COST - A diverter valve s purchase price is only one of many costs you need to consider. Others include shipping, installation, and maintenance costs, material cross-contamination costs associated with internal valve leaks, and lost-production costs caused by maintenance downtime. You can discover these and other performance-related costs by asking for information from users who have installed a particular valve in an application similar to yours. OPTIONS - Depending on your material characteristics, you can specify that a diverter valve and its components be constructed of cast aluminum, cast iron, stainless steel, or a specialty alloy. And depending on your pneumatic conveying system s specifications and power availability, you can often specify that a valve be actuated manually, by air, or by an electrical motor. You can also specify a valve with an air-controlled solenoid and position indicating switches. Now, keep these selection factors in mind as you explore the types of available diverter valves. The following information covers five common diverter valves: rotary-plug, rotary-blade, flapper, sliding-blade, and flexible-tube. Multiple-source, multiple destination diverter valve configurations are also discussed. ROTARY-PLUG DIVERTER VALVE - The housing of a rotary-plug diverter valve (also called a tunnel diverter valve), as shown in Figure 1A, contains a solid rotating plug with a smooth tunnel-like hole bored through it. The material flows through the tunnel from an upstream conveying line to one of two downstream lines. To divert flow from one downstream line to the other, the plug rotates on a central axis, like a dial, about 150 degrees so that the tunnel s previous outlet becomes the tunnel s new inlet and the tunnel s previous inlet becomes the tunnel s new outlet to the other downstream line. In another version of this valve, called a parallel-tunnel diverter valve, shown in Figure 1B, two parallel tunnels are bored through the rotating plug. This reduces valve wear, because the plug has to rotate only about 45 degrees to shift the material flow from one downstream line to the other. The rotary-plug diverter valve is typically used for handling pellets rather than powders, because powder can pack between the rotating plug and housing. Shifting the valve on the fly (that is, diverting material flow while the conveying system is operating) isn t recommended, because it can cause the valve s upstream conveying line to become completely blocked. The valve can be used in dilute-and dense-phase applications and be pressure-rated based on your application needs, typically up to 15 psig for dilute-phase pressure conveying and down to 30 inches mercury for dilute-phase vacuum conveying. Manufacturers can also modify the valve to achieve higher pressure ratings, allowing it to handle abrasive materials in dense-phase high-pressure applications. The rotary-plug diverter valve can have a precision-machined cast housing and heavy pipe flanges for connecting the valve to the conveying line. The primary advantages of the rotary-plug diverter valve are its shallow material deflection angle, which creates a low pressure drop in the pneumatic conveying system, and its ability to handle abrasive materials in high-pressure dense-phase applications. As long as the conveying line is purged before conveying a different material, the rotary-plug diverter valve s smooth-bore tunnel eliminates material cross-contamination when handling non-dusty granules or pellets. The primary disadvantages of the rotary-plug diverter valve concern it s cost, susceptibility to seal abrasion, and inability to shift on the fly. 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. Valve replacement parts are also costly. The rotary-plug diverter valve requires clearance between the housing and sealing surfaces in order to actuate. Because the valve s sealing surfaces wear with each plug rotation, material can become packed in the clearance gap and bind the plug, preventing the valve from operating properly. To fix it, you must remove the valve, clean it out, and replace the seal or housing.

ROTARY-BLADE DIVERTER VALVE - The housing of a rotary-blade diverter valve, as shown in Figure 1C, contains a flat Figure 1 solid-metal disc that rotates on a central axis to divert material from one downstream conveying line to another. A metal shaft runs through the housing s center and lengthwise through the disc, supporting the disc and creating its central axis. An externally mounted actuator connected to the metal shaft triggers the disc s rotation, and material flowing from the upstream conveying line hits the disc s flat surface and is deflected into the appropriate downstream line. The rotary-blade diverter valve is typically used for granules and powders, especially in dilute-phase systems, because it s less subject to powder packing problems than the rotary-plug diverter valve. However, shifting on the fly isn t recommended because the valve s rotating disc can trap material in the sealing areas during the shift. Like the rotary-plug diverter valve, this valve can be used in diluteand dense-phase systems and be pressure-rated based on your needs, typically up to 15 psig for dilute-phase pressure conveying and down to 30 inches mercury for dilute-phase vacuum conveying. The valve can be modified by the manufacturer to achieve higher pressure ratings so it can handle abrasive materials in dense-phase high-pressure applications. The valve can have a precision- machined cast housing and heavy pipe flanges for connecting it to the conveying line. The primary advantages of the rotary-blade diverter valve are the same as those of the rotary-plug diverter valve: its shallow material deflection angle, which creates a low pressure drop in the pneumatic conveying system, and its ability to handle 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 fly. 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 flapper diverter valve (also called a swing diverter valve), as shown in Figure 2, uses a swinging metal flapper (or gate) to divert material flow 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 flapper s bottom, creating a hinge point that allows the flapper to swing back and forth. In some models,

the flapper seals against a replaceable polyurethane liner. The flapper diverter valve can be made with pipe flanges 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 flow direction has been established, it can t be reversed without costly valve modifications. Most flapper diverter valves in dilute-phase vacuum conveying systems are limited to low-vacuum applications. In high-vacuum applications, the flapper can lose its seal, because the vacuum tends to pull the flapper from its internal sealing surface. With some powders, the flapper diverter valve can shift on the fly. However, even with these powders, some material can become trapped between the flapper and sealing surface, creating a gap for air and material leakage into the closed line. Shifting this valve on the fly is even less suitable for materials with large particles. 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 difficult, expensive, and time-consuming to replace, select a flapper 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 flow via its flat, rectangular, sliding metal blade with a hole near its center. The sliding blade is installed so that it intersects and extends beyond both downstream conveying lines. To divert the flow, an actuator slides the hole over a downstream line and the airflow carries the material through the hole into the line. The sliding blade s solid section stops material flow to the other downstream line. The blade can be carbon steel, aluminum, 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 flapper 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 flapper 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 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 Figure 3 Figure 2

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 fly with most materials. The sliding-blade diverter valve has a precision-fabricated structural frame, as well as, fabricated plastic-tube or metalpipe 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 flanges. The valve is available in two-, three-, and four-way configurations 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. systems. However, the valve isn t suited to handle severely abrasive materials because they will wear the hose. The flexible hose is typically constructed of abrasion-resistant rubber, polymer, or flexible steel and is often housed in an open hose support frame. Some manufacturers offer an enclosed hose support frame to prevent material from spilling onto the plant floor if the hose fails. The flexible-tube diverter valve can use one of two seal arrangements. One is a wear-compensating seal, which ensures that a positive air-and-material-seal is maintained when shifting. This seal allows the valve to shift on the fly and function in both pressure and vacuum dilute-phase Figure 4 One disadvantage of the sliding-blade diverter valve is that material flowing 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, creating the potential for material cross-contamination. However, installing the valve vertically, so that the material flows upward, eliminates this problem because gravity eventually pulls any material from the segment. A naturally occurring high-pressure airfoil, As shown in Figure 3, prevents most of the material from entering the closed-line segment, deflecting it back into the material stream. However, the airfoil can t completely prevent cross-contamination because a small amount of material can still find its way into the segment if the valve isn t installed vertically. FLEXIBLE-TUBE DIVERTER VALVE - The flexible-tube diverter valve, as shown in Figure 4, operates similarly to the sliding-blade diverter valve to divert flow in a conveying line. A tube stub is welded directly above a sliding blade s hole, which is installed just like a sliding-blade diverter valve, and a flexible hose is attached to the tube stub. The flexible hose is then attached to the upstream conveying line. To switch the flow from one downstream conveying line to the other, an actuator shifts the sliding blade and flexible hose together. The flexible-tube diverter valve can convey powders and pellets from one source to two destinations (diverter) or from two sources to a single destination (converger) in pressure or vacuum dilute-phase and pressure densephase conveying

systems. The other is a pneumatic seal, which uses an inflatable seal to handle the high pressures in dense-phase systems. With this seal, the valve can t shift on the fly because the seal must be deflated before shifting and reinflated after shifting before the Figure 5 material flow can be restarted. One advantage of the flexible-tube diverter valve is that very little pressure drop is created across the valve because the sliding-blade design provides smooth valve actuation and a positive air and material shutoff to the closed line. The valve virtually eliminates material cross-contamination because the upstream conveying line can be completely purged before the valve is shifted. However, to work effectively, the valve s hose must be long enough to endure the torsional stress caused by constant shifting. If the hose is too short, the constant shifting will fatigue the hose and cause it to break. Hose length depends on both the conveying line s diameter and the shifting distance the larger the line diameter or the greater the shifting distance, the longer the hose. MULTIPLE-SOURCE, MULTIPLE-DESTINATION DIVERTER VALVE CONFIGURATIONS - In many applications, material must be conveyed from multiple sources to multiple destinations. Traditionally, this has been done with a manually operated hose-and-manifold station, which requires a worker to uncouple an upstream line from one source and recouple it to a downstream line that carries the material to the required destination. But this manual method is less than ideal. Toxic or hazardous materials can spill when the lines are uncoupled. Material can be contaminated if the upstream line is coupled with the wrong downstream line. Workers can injure themselves when uncoupling and coupling lines. Downtime for uncoupling and coupling the lines can decrease production rates. One way to successfully automate this process is to combine and stack various two-, three-, and four-way diverter valves, as shown in Figure 5. Because the operating principles of standard rotary-plug, rotary-blade, and flapper diverter valves don t typically allow themselves to work in these configurations, sliding-blade or flexible-tube diverter valves, or a combination of them, are typically used to convey materials from multiple sources to multiple destinations. Typically, a three- or four-way diverter valve assembly is custom-manufactured for the application. The assembly tends to be compact, making it easier to install than a hose and manifold station. The assembly is independently mounted on a stand alone frame so it can be transported and installed as a single unit. The assembly can include connections for compressed air, electrical power, and operator controls. Be aware, however, that the multiport diverter valve assembly has a high onsite installation cost.