A Double Dump Valve or a Rotary Valve Airlock?
The age-old question as to which of these two material handling components is the best usually comes down to maintenance and performance.
A Double Dump Valve is a material handling component that has been around for a very long time and is commonly used as an airlock! In theory, a Double Dump Valve functions by moving material into one compartment, the gate closes sealing off the atmosphere, and then the second gate opens releasing the material into the next phase of the material handling process. The gate then closes, and the cycle starts all over again. Double Dump Valves can be powered by many sources, whether pneumatic, electric, or gravity. With the exception of the Double Dump Valve’s middle door, there are not many impact zones, reducing the number of wear issues caused by friction and material impact. This is certainly a benefit when considering a Double Dump Valve.
Although in theory, Double Dump Valves should provide excellent airlock functionality. Experience has shown that theory and “real-world” operations tend to be very different. Double Dump Valves are complex pieces of machinery with a lot of moving parts and pieces, this increases the need for maintenance and adjustment. The constant opening and closing of the various doors and the intricacies of fittings within the Double Dump Valve create a number of high-maintenance areas, with air leakage being the most common problem. One of the main advantages of a Double Dump Valve is the direct separation of atmospheres, which should result in significantly less air leakage. Air leakage is due to material getting packed into the seals, lodged between the doors and door channels, or building up in the door channel itself causing the doors to be misaligned when closed. These issues are magnified if moisture is present or if the material has sticky characteristics. In this case, a heavy-duty Self-Cleaning Rotary Valve Airlock would be the optimum choice, but we will save this discussion for another day.
In most material handling applications where Double Dump Valves are being utilized, the material gets packed into the doors and channels of the valve and/or the seals. If this is not consistently cleaned and/or replaced the foreign material compromising the structural integrity of the seal will most certainly result in air leakage and less throughput. Unless maintenance personnel are extremely attentive and available to provide ongoing daily maintenance, air leakage will be common, resulting in less throughput and an increase in costly downtime.
Many will boast that Double Dump Valves are not subject to the same wear and leakage as other material handling devices, but this could not be further from the truth. Like any mechanical piece of equipment, Double Dump Valves are subject to wear and leakage, and if they are not maintained properly and at regular intervals, performance will decrease dramatically. Since coming out of COVID, maintenance teams continue to be severely short-staffed. As a result, maintenance personnel’s attention is generally focused on the higher-priority areas within the industrial manufacturing plant versus the daily cleanings and adjustments that a Double Dump Valve requires. The cost of downtime, especially in a production line moving massive amounts of material can add up quickly, so putting a component in place with a very low maintenance requirement is an important consideration.
Rotary Valve Airlocks: The reality is that not all Rotary Valve Airlocks are created equal, so when the statement is made that a “rotary is a rotary,” this is simply not true! Due to the number of Rotary Airlock manufacturers and the varying quality standards of each, there are large differences between some Rotary Airlock manufacturers and others with the most “common” being the material that most Rotary Airlocks are manufactured from. For instance, common Rotary Valve Airlocks are made out of 20-30 Rockwell cast-iron material, which is subject to increased wear rates. In comparison, Heavy-Duty Rotary Valve Airlocks such as those manufactured by Precision Machine & Manufacturing (PMM) provide a number of operational benefits.
All PMM Rotary Valve Airlocks are 100% constructed from a U.S sourced, 50 Rockwell, abrasion-resistant steel alloy with a high nickel content. The high nickel content has a low coefficient of friction, which further reduces wear. PMM surface treats all Rotary Valve Airlocks with ion-nitride, which is far better for the environment than hard chrome, pushing the overall material hardness to almost 60 Rockwell.
Another big difference between Rotary Airlock manufacturers is in relation to airlock capability, most common Rotary Airlocks have 6-rotor pockets, which only allows for one rotor vane to be in contact with each side of the housing. Only having one rotor vane in contact with each side of the housing leaves a greater chance of air leakage when compared to a Rotary Valve Airlock with two rotor vanes in contact with each side of the housing. PMM’s Rotary Valve Airlocks, which all have a standard 8-vane closed-ended rotor configuration, and a maximum rotor to housing clearance of .005” provide a near perfect airlock functionality. There are number of important features built into all PMM Rotary Valve Airlocks that make them NFPA-Compliant and can be further investigated here: https://nfparotaryvalve.com/.
As far as maintenance is concerned, PMM’s Rotary Valve Airlocks do not have many moving parts. The rotor, which is turned by an external gear motor, turns within the housing. External, heavy-duty, piloted flanged bearings are used, maintaining perfect rotor alignment and limiting air leakage. However, because of the impact zones at the top of the valve, or the abrasive nature of the material being moved, wear can and does occur, which will require a bit of maintenance during regular “shut-down” intervals, which is significantly less than the daily/weekly maintenance a Double Dump Valve requires.
Overall, 95% of PMM Rotary Airlock Valve’s maintenance consists of a simple visual inspection, greasing the external, heavy-duty, piloted outboard bearings, and an occasional adjustment of the packing, which is extremely simple in comparison to a Double Dump Valve! Should a PMM Rotary Valve Airlock require the replacement of a wear item such as a rotor, wear plate, or barrel side (housing) for instance, these items can all be pre-ordered and onsite in anticipation of the upcoming shut-down and quickly replaced in just a few hours!
In Conclusion: Getting back to the original question, are Rotary Valve Airlocks better than Double Dump Valves? The answer really depends upon the manufacturer of the Rotary Valve Airlock and the materials used to construct it. Manufacturing techniques and the materials to construct Rotary Valve Airlocks vary so greatly among manufacturers, comparing two types of Rotary Valves Airlocks would be an apples to oranges comparison, and comparing the different types of Rotary Valve Airlocks to a quality Double Dump Valve will result in very different results as well.
For instance, could a light-duty cast-iron Rotary Valve Airlock outperform a quality Double Dump Valve? Not likely! However, could a Heavy-Duty Rotary Valve Airlock, which is constructed the way Precision Machine & Manufacturing builds their Rotary Valve Airlocks, outperform a quality Double Dump Valve? Absolutely, and we do so quite regularly!
There is a place and time for all equipment, which is why all material handling applications are carefully evaluated and the appropriate component specified based on the parameters of the material, the throughput, and the temperature! In our many years of experience solving the most challenging material handling problems, we have found that “real” maintenance only occurs during shutdowns twice a year. Therefore, the component achieving the required throughput, with minimal maintenance and maximum longevity is generally the option preferred and is a big reason why PMM’s Rotary Valve Airlocks are regularly called upon to replace failed Double Dump Valves.
To learn more about Precision Machine & Manufacturing’s Rotary Valve Airlocks, navigate to this page: PMM Rotary Valve Airlocks
