Valves are a part of many daily functions, from ensuring your car does not overheat to successfully operating a chemical plant. They have been designed to perform a variety of functions such as stopping and starting, throttling, or acting as a no return for flow. There are a large variety of valves and valve configurations to suit all services and conditions; different uses (on/off, control), different fluids (liquid, gas etc.; combustible, toxic, corrosive etc.) different materials and different pressure and temperature conditions. They are primarily used for starting or stopping flow, regulating or throttling flow, preventing back flow or relieving and regulating pressure in fluid or gaseous handling applications.
- Isolation Valves – Used for systems that do not need the flow throttled and allow the flow and closes to stop flow.
- Non-Return Valves – Used to control the speed and capacity of flow through the system.
- Throttle Valves – Used to control the direction of flow. Flow in the desired direction opens the valve, while flow in the oppose direction forces the valve closed
Common valve types include Ball, Butterfly, Check, Diaphragm, Gate, Globe, Knife Gate, Parallel Slide, Pinch, Piston, Plug, Sluice, etc.
DO’S AND DON’TS:
As opined by the leading valve suppliers, Using an improperly applied sized or improperly sized valve can have serious consequences on operation, productivity and most important, safety. Most of the valves are not intended to be isolation valves and should not be used for isolating a process.
Factor in the degree of control you need and make sure your valve is mechanically capable. Too much dead-band leads to hunting and poor control. Dead-band is roughly defined as the amount of control signal required to affect a change in valve position. It is caused by worn, or loosely fitted mechanical linkages, or as a function of the controller setting. It can also be effected by the tolerances from mechanical sensors, friction inherent in the valve stems and seats, or from an undersized actuator.
Always carefully select the correct materials for the construction. Take into consideration the parts of the valve that comes in to contact with the process media such as the valve body, the seat and any other “wetted” parts. As pointed out by the experts, WRAS approved valves are always preferred in the global industrial valve usage.
Consider the operating pressure and operating temperature the control valve will see. Finally, also consider the ambient atmosphere and any corrosives that can occur and effect the exterior of the valve.
Put your flow sensor upstream of the control valve. Locating the flow sensor downstream of the control valve exposes it to an unstable flow stream which is caused by turbulent flow in the valve cavity.
The tendency for valves that have had very limited travel, or that haven’t moved at all, to “stick” is referred to as stiction. It typically is caused by the valves packing glands, seats or the pressure exerted against the disk. To overcome stiction, additional force needs to be applied by the actuator, which can lead to overshoot and poor control.
Don’t over-size your control valve as pointed out by expert valve control valve suppliers since these valves are frequently sized larger than needed for the flow loop they control. If the control valve is too large, only a small percentage of travel is used (because a small change in valve position has a large effect on flow), which in turn makes the valve hunt. This causes excessive wear. Try to always size a control valve at about 70%-90% of travel.
Tune your loop controller properly. A poorly tuned controller causes overshoot, undershoot and hunting. Make sure your proportional, integral, and derivative values are set). This is quite easy today using controllers with advanced, precise auto-tuning features seen in valves made in Europe, since this technology has replaced the old fashioned trial and error loop tuning method.
Think about the type of control valve you are using and its inherent flow characteristic. Different types of valve, and their disks, have very different flow characteristics (or profiles). The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position.
Globe control valves have linear characteristics which are preferred, while butterfly and gate valves have very non-linear flow characteristics, which can cause control problems. In order to create a linear flow characteristic through a non-linear control valve, manufacturers add specially designed disks or flow orifices which create a desired flow profile.
These are some of the more significant criteria to consider when selecting an industrial valve usage. Always discuss your application with an experienced valve manufacturers before making your final industrial valve selection. Contact Fevisa, leading valve stockist in Dubai for more details.