Key points
1: Introduction
Rotary Valves are compact devices widely used in bulk handling systems to control the discharge of powders and granular solids, normally under gravity, between two parts of a system; usually when there is a gas pressure differential between the two parts.
By definition, in their basic construction – ignoring any drive, they are the simplest of machines each incorporating just one moving element: a multi-vane rotor.
Yet until relatively recently rotary valves were often much maligned, due to customers often experiencing unreliable performance. As a result, many engineers actively sought alternative solutions or devices but with very limited success and only then in clearly defined applications.
Whereas the compact and economic simplicity of rotary valves provided for continuous development over time to become highly versatile devices, and are now established as important, reliable, and often vital components when correctly specified for the application.
That it can be a complex subject is reflected in the fact that manufacturers have a standard range of rotary valves supplemented by numerous standard options; as well as developing bespoke features or units when required.
One moving element!
Many thousands of options!
    2: Rotary Valve History
Bulk handling of particulate solids by automated control equipment is believed to have started at the beginning of the 20th century and was initially based on handling cereals and other basic, easy to handle bulk foodstuffs. Initially development progress was slow as there was little understanding or experience on how to manage other products / system demands.
It is assumed that Rotary Valves may have come into existence being an obvious and simple solution for the controlled powered discharge of storage vessels.
For many decades there were various terms adopted to identify such valves for this purpose including: Rotary Airlock, Airlock, Star Valve, Rotary Seal, and Vaned Valve amongst others. Some remain in use today; however, Rotary Valve is the most widely adopted generic term but not exclusively.
Wider development of systems engineering and valves was slow until around the 1950s when bulk handing started to develop into much more of a knowledge based, wide ranging, and accepted industry in its own right, and where operating needs were rapidly becoming much more widespread and demanding.
Despite the apparent simplicity, Rotary Valves have evolved to offer several functional uses:
- Control the product flow rate to the required value (fixed or variable).
- Maintain a gas pressure differential between the inlet & outlet (airlock function).
- Act as an explosion containment device.
- Act as an ‘Autonomous Safety Device’ for explosion and flame containment.
- Act as a process isolator / barrier.
As well as satisfying any one, or combination of these requirements, the valve supplied often has to do so while overcoming adverse or difficult product characteristics, often affected by the duty, environmental and legislative requirements.
Although now classed as a well-developed, mature industry; systems engineering continues to be faced with the challenges of new products, extended customer needs, and increasing legislation.
3: The Challenge
The generic description of a ‘valve’ is a device to control fluids i.e. gases, liquids, and slurries, including, when in a fluid condition, powders and particulate solids. However, unlike gases and liquids, bulk solids do not have stable physical handling characteristics, being vulnerable to different and sometimes variable environmental and production conditions, making predictable calculations for easy equipment design / selection difficult.
As is widely understood, handing characteristics of individual bulk solids at any given time, are heavily influenced by a range of factors and conditions. It has been suggested that bulk solids should be seen as being in the ‘fourth state of matter’ in that their physical values are defined by their immediate past history. With some applications operating conditions within the rotary valve can to a greater or lesser extent affect this state.
The challenge facing the applications engineer is to ensure that the valve supplied can operate effectively and reliably for an acceptable life span.
Notwithstanding the above, the greater majority of applications can be readily satisfied from a range of standard configurations and components and, where needed, a wide range of common developed features. The remainder tend to require bespoke engineering.
Cross-section of basic valve
4: Determining the Right Choice of Rotary Valve
The generic description of a ‘valve’ is a device to control fluids i.e. gases, liquids, and slurries; including, when in a fluid condition, powders and particulate solids. However, unlike gases and liquids, bulk solids do not have stable physical handling characteristics, being vulnerable to different and sometimes variable environmental and production conditions, making predictable calculations for easy equipment design / selection difficult.
As is widely understood, handing characteristics of individual bulk solids at any given time, are heavily influenced by a range of factors and conditions. It has been suggested that bulk solids should be seen as being in the ‘fourth state of matter’ in that their physical values are defined by their immediate past history. With some applications operating conditions within the rotary valve can to a greater or lesser extent affect this state.
The challenge facing the applications engineer is to ensure that the valve supplied can operate effectively and reliably for an acceptable life span.
Notwithstanding the above, the greater majority of applications can be readily satisfied from a range of standard configurations and components and, where needed, a wide range of common developed features. The remainder tend to require bespoke engineering.
The selection of final design and features is often not simple and evolves through six main areas of consideration which may need continuous reviewing until a final determination is arrived at.
4.1 Rotary Valve sizing / Speed
The selection of valve speed and size is not just a matter of mathematical calculation; it also takes into account the various factors that may negatively affect its performance from the theoretical maximum.
Almost invariably, customers specify capacity required by weight/time e.g. tonnes/hour.
A Rotary Valve is a volumetric discharger, so valve size is calculated using:
- Throughput weight required
- Product bulk density
- Filling efficiency
Where Filling efficiency is affected by several factors:
- Rotor speed
- Flow characteristics of the product – see Graph 1 curves 1 to 4.
- The bulk density at the point of entry into the rotor pockets
- Effect of leakage
- Size of the valve
- Feeding conditions (system design)
Graph 1 – Filling Efficiencies
Rotor Speed
It is important to select a size and rotor speed that ensures some of these factors are within manageable levels.
As a very general rule a rotor tip speed of 40 m/min is around the maximum after which a decrease in throughput results; at or anywhere near this speed, erratic performance is inevitable. (40 m/min equates to 85rpm for a 150mm diameter rotor and 17rpm for a 750mm diameter rotor).
To ensure reliable and stable throughput, most applications require selecting a valve size that gives rotor speeds below 25rpm for valves with rotor diameters up to 300mm; thereafter with larger valves it becomes progressively safer to run closer to the nominal 40 m/min tip speed due to the more accommodating larger throat sizes.
Final decision for suitable valve speed also takes into consideration the flow properties of the product being handled – Graph 1 offers general guidance.
4.2 Product Characteristics
These define the design and construction of the individual components. Knowledge of product handling characteristics and how they are affected by valve operation when under duty conditions is often essential for their final selection.
Caution is needed; for it is not uncommon for some products to act or be acted on adversely or be damaged when being handled within a Rotary Valve, and yet, not present a problem with other equipment.
Fine mildly abrasive product can become highly erosive when entrained in high velocity leakage that can occur within the valve operating clearances.
Some products are temperature sensitive and the inter-particle friction in the valve clearances can give rise to plasticising through particle fusion, undermining the product quality. Others, when under interface pressure or presence of moisture, can build up on the working surfaces and create unacceptable torque loads and / or distress noise.
Whenever any potential challenging issues arise, the means to alleviate such problems will need to be incorporated. Sometimes customer liaison is needed to mitigate any influential conditions within a system that would otherwise be difficult or expensive to control by means of the valve alone.
Food and pharmaceutical products require sterile easy clean conditions as does any product that requires purity.
Organic products can be explosive when airborne in dust form and requires special attention.
4.3 Pressure Differential
When it exists, it is a critical factor when assessing valve suitability, due to its creation of leakage.
The greater the pressure differential, the greater the leakage and the greater the effect it can have on the final choice of valve size and features. The particle size and handling characteristics of the product are also significant factors to be considered.
High leakage can significantly reduce material flow by reducing the bulk density through fluidisation and/or by physically opposing flow. Conversely, with some products and/or conditions it can aid product flow by creating agitation and/or fluidisation.
Whatever the net effect allowance must be made when sizing the rotary valve, something that becomes progressively less of a problem with increased valve sizes, as the larger throats make for better dispersion of leakage air.
The potentially damaging effects of leakage will not only determine features to be included, but also whether leakage needs to be utilised or suppressed; the latter may need the involvement of the customer at the design stage of the planned installation.
Valves should also be fitted directly to the outlet of any hopper so that leakage air can disperse easily. This is less of a problem when the valve is not operating in a flooded condition, i.e. when product enters the valve in a controlled cascade where leakage can naturally bypass the product flow.
4.4 Temperature Conditions
Extreme low and high operating temperatures: from Dry Ice at -800C to continuous discharge into a pneumatic conveying line of incinerator ash at 9600C, are two examples of successful applications that have challenged all aspects of engineering content and valve design.
It is not practical to go into detailed description and options in the context of this booklet, but it is important that there is understanding as to whether the valve temperature will increase slowly during production or may be subject to shock temperature loading i.e. sudden influx of hot product.
If the latter then the Rotor, being a lighter construction, will expand quicker than the Housing, which is also subject to external surface heat loss.
Clearances need to be set so that once operating temperature is reached, they are at the optimum clearance for the duty. This means that leakage before the optimum has been reached will be much greater.
4.5 Customer / Industry Requirements
These can be challenging aspects of supply, requiring the ability to offer flexibility in both detail valve design, manufacturing and commercial capability in order to satisfy specific customer needs and/or their individual industry standards.
Many users are not concerned about most aspects of valve supply providing it performs to specification, is durable and requires very basic documentation.
Other customers / Industries are more demanding to a varying degree with regard to detail design and required manufacturing practices. As well as specifying industry approved design features, there is often a requirement to be supported by extensive validation and acceptance tests, certification, documentation, post-delivery tests etc.
Between the two extremes is a wide range of differing requirements – Figure 1 shows examples of such valves.
Valves handling hazardous and/or toxic materials and/or operating in controlled environments will have further extended requirements.
Figure 1 – Some examples of valves designed for unusual applications.
Valve with Rotor Pocket Scraper Option. Principle of Operation: The ‘sweeping’ lower component ensures removal of sticky products that would otherwise not discharge by gravity.
Hypergienic’ Valve set as an ‘Autonomous Safety Device’ with extended features to satisfy requirements for handling highly toxic and sensitive pharmaceutical product.
Valve for handling light poor flowing products with extended inlet and inlet cone at precise angle required to meet product characteristics.
Stainless Steel valve designed to act as a process separator operating under Chlorine gas an internal pressure of 7 bar pressure. Pressure Differential 100mbar.
Tandem valve – available in two options:
- With two sealing Rotors to minimise leakage when handling very light products that are easily windswept.
As a Metering Sealing assembly with upper Rotor having reduced capacity with increased clearances feeding the lower sealing Rotor at a reduced rate to eliminate jamming.
Heat resisting valve operating at 5000C continuously discharging ash & gravel from a fluid bed dryer. Incorporates auto reversing to release any product jamming.
4.6 Legislation Requirements
There are several legislative guidelines that may be required to be complied with either singularly or jointly, such as:
European Union Machinery Directive
European Union Pressure Equipment Directive (PED)
European Union ATEX Directive for potentially explosive atmospheres
USDA (United States Department of Agriculture)
FDA – United States Food and Drug Administration
Whilst not a legislative guideline, EHEDG – (European Hygienic Engineering and Design Group) compliance only applies when requested by the customer.
Figure 2 – 600mm bore valve undergoing ATEX testing
(For explosion / flame containment validation by Notified Body)
Conclusion
Successful valve selection remains a combination of calculation, empirical data, and experience based keen judgement.
