A: David Leonard, Rotolok, says:
A rotary valve serves two functions in any pneumatic conveying system, including that it works as a material feed control device to meter the material entering the conveying line and as an airlock to maintain suitable conveying air volume in the system pipeline for reliable operation. Because the rotary valve is a critical operational component, watching for valve wear and knowing when to implement maintenance measures to ensure the system’s continued reliability is important for plant operators.
The key indicator of increased rotary valve wear is typically increased air leakage through the valve from the high-pressure to the low-pressure side of the valve. In a positive-pressure pneumatic conveying system, the indicators will be from the valve outlet up through the valve into the material feed hopper above. In a vacuum conveying system, the wear indicators will be the reverse, appearing down through the valve into the conveying pipeline from the feed hopper. Due to the different operation of these two systems, the indicators of increased air leakage will differ in each case.
In a positive-pressure system, increased air leakage up in the material feed hopper will produce increased stored material fluidization, resulting in a reduction of the material’s bulk density. This, in turn, will lead to a decrease of the material mass flowrate through the valve, and therefore a similar decrease in the system’s conveying rate. This slowdown could be indicated in a production environment by seeing a longer material batch transfer time or by a generally lower system operating pressure due to the negatively impacted conveying rate. If an air leakage becomes too large and if it’s combined with insufficient valve venting, the worst-case scenario is that it can lead to trapped air above the valve and produce a complete material bridge resulting in a total loss of material flow from the feed hopper.
In addition to material feed issues, the increased leakage of air up through a valve will prove detrimental to the conveying pipeline operation in that only a reduced air volume will be available to convey the material within the system. This leakage process takes place over a period of time, but the gradual decrease in available conveying airflow eventually becomes insufficient to successfully convey the material through the system. This problem will be indicated by material dropping out of the airstream and causing an increase and fluctuation in the system’s conveying pressure. With reduced airflow volume, a system will initially continue to function but less reliably and with evermore frequently occurring line blockages. The system will eventually reach a stage where the leakage is too great and conveying airflow too low to continue to operate successfully. A rotary valve should be inspected as soon as these warning signs appear. Prior to a final failure, the system must have the clearances checked and closed back to factory settings via adjustments of the rotor blades or by installing a new rotor or valve.
In vacuum conveying systems, the indicators of wear and excessive air leakage are different and potentially less detrimental to the system in the early stages. However, increased rotor-housing clearances on material feed valves will typically lead to excessive air being drawn from the feed hopper and eventually produce an increased material flow flooding through the uncontrolled opened valve clearances. This will lead to an overfeeding of the vacuum conveying system and excessive operating pressure, resulting in a blocked conveying pipeline with the exhauster motor — the main drive motor of the air pump that provides the conveying air for the system — tripping at a high vacuum condition. This type of system fault is also often characterized by the exhauster drawing air in through its safety vacuum relief valve and an audible rush of airflow to the system in an attempt to equalize the pressure.
Air leakage in a vacuum system can be most significant where there are multiple rotary valve-controlled feed points into a conveying line. This leakage will result in the feed points furthest upstream in the system being starved of sufficient conveying air volume and air velocity in the pipe for the system to reliably operate. The result will be an increased occurrence of pipeline blockages and fluctuating pressure conditions within the pipeline. It’s generally considered good system design to provide a means of a complete airtight shutoff of infeed points, where possible, when not operating within a system. This can be achieved by using an alternative valve type such as a knife-gate valve or a butterfly valve that will provide a higher sealing efficiency than what a rotary valve can. The use of such a valve is often an effective means of avoiding system air leakage problems, thus prolonging the operating life of the system.
The problems that indicate rotary valve wear in a conveying system will vary with each system type and individual case. With close observation of the system’s operation and its associated operating parameters, the plant personnel should be able to recognize and predict potential issues at the early stages and take suitable action to reduce downtime and preserve the reliable operation of the conveying system.
Rotolok, Devon, UK, supplies rotary valves and airlocks, diverters, and slide-gate and control valves.