Control Valve Selection
A previous blog-post discussed the importance of control valve sizing and energy optimization opportunities. This blog-post will focus more on the topic of control valve selection although, both topics shouldn’t be considered to be mutually exclusive.
When selecting a control valve for process plant, there are many things to be considered. These can include the valve flow characteristic, size, valve body and trim materials, noise, potential for damage from cavitation or flashing, actuator type and size, dynamic response to changes in control signal etc. This summarises the typical considerations when making a control valve selection.
Selecting an improperly sized control valve can have serious consequences on safety, operation and productivity. The following list outlines some of the things to consider when making a control valve selection:
- Give careful consideration to selecting the correct materials of construction. Take into consideration the components of the valve that come in to contact with the process fluid such as the valve body, the valve seat or any other valve components exposed to the process fluid.
- Consider the operating temperature and pressure the control valve will be exposed to. Consider the local ambient atmosphere and any corrosives that can occur which may affect the exterior of the valve.
- Consider the degree of control you require and ensure the selected valve is mechanically capable of achieving the desired operating conditions.
- Consider the inherent flow characteristic of the control valve you are selecting. Different valve types have different flow characteristics. The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position. This item is discussed in a little more detail later.
- Aim for optimal valve travel. When a valve is sized correctly, the range of operation will correspond well to the control range of the valve. Some industry literature recommends travel at normal flow should fall within 50 to 70 percent opening angle. Travel at maximum flow should fall below 90 percent whereas travel for minimum flows should be above 20 percent open to avoid erosion of the trim. When modelling a control valve in FluidFlow, the software enunciates a warning message if the valve position falls outside of an optimal operating range. Users can also adjust the desired settings for minimum and maximum valve positions. This helps prompt the designer into considering valve position for the given design operating conditions. Ultimately, it also helps the designer make a better and more appropriate valve selection for the application in hand.
- Avoid oversizing a control valve. If the control valve is too large for the required application, only a small percentage of travel is required. This is due to only a small change in valve position having a large effect on flow which in turn makes the valve hunt. This can cause excessive wear. Some published literature sources recommend sizing a control valve at about 70% to 90% of travel.
This list represents just some of the criteria to be considered when selecting a control valve. It is generally recommended that the final valve selection is discussed with an appropriate and experienced supplier or manufacturer before making your final selection.
Control Valve Flow Characteristic
The flow characteristic of a valve represents the inherent relationship between the valve opening and flow rate. As a valve gradually opens, the flow characteristic allows a certain amount of flow though the valve at a particular opening percentage. This permits predictable flow regulation through the valve. The most common flow characteristics are linear, quick opening and equal percentage.
Linear Flow Characteristic
This flow characteristic exhibits a linear relationship between valve position and flow rate. The flow through the valve varies directly with valve stem position.
Linear Flow Characteristic – FluidFlow.
Quick Opening Flow Characteristic
The flow characteristic of a quick opening valve is such that for a relatively small initial change in valve stem travel, a large increase in flow occurs. The noticeable characteristic of this valve type is that maximum flow is achieved at a relatively low percentage of the valve stem range.
Quick Opening Flow Characteristic – FluidFlow.
Equal Percentage Flow Characteristic
The flow characteristic of an equal percentage valve produces equal percentage changes in the existing flow for equal increments of valve travel. The change in flow rate is proportional to the flow rate just before the change in position is made.
The above summarises the most common valve flow characteristics.
Control valves actually have two characteristics, inherent and installed characteristic. The inherent characteristic is that published by a valve manufacturer based on tests conducted on a system where care is taken to ensure the pressure drop across the valve is held constant at all valve opening positions and flow rates. The inherent characteristic therefore represents the valve flow capacity and valve opening position when there are no system effects involved.
The installed characteristic is the relationship between the valve position and flow in the system taking into account any changes in the pressure differential available to the control valve due to the flow squared relationship between flow and piping pressure losses and/or the behaviour of a centrifugal pump’s head curve.
The performance of control valves in a process system can have a dramatic effect on the plant efficiency, asset life cycle costs and overall profitability. It therefore goes without saying that the cost-effective operation of any plant, industrial or otherwise, requires considered design and careful control valve sizing and selection. A correctly sized control valve can provide significant savings as well as increase process availability, reduce process variability and reduce maintenance costs. Correctly sized control valves also last longer than unmatched or incorrectly sized valves.
Oversized valves have a higher capital cost and tend to cause instability in the operation of the system whereas undersized valves simply won’t pass the required flow of fluid in the line.
As designers, it is therefore worth giving careful consideration to both the sizing and selection of the control valve to affect efficient and effective operation of a process plant whilst optimising operating costs.
Selecting the Right Device
The essential steps required to optimize control valve performance as well as prevent erosion problems include proper valve sizing and selection of valve body and trim materials. They could mean the difference between continued operation and unplanned shutdowns. There are of course other decisions involved in selecting the right valve solution. Many companies choose globe type valves for their proven performance and life cycle advantages. When compared to other available valve designs, this valve offers:
- Better control performance.
- Better low or partial load performance.
- High differential pressure across the valve.
- Smaller physical profile than a comparable ball valve.
- Use for steam, water or water/glycol fluids.
In general, a globe valve modulates flow through movement of a valve plug in relation to the ports located within the valve body. The plug is connected to the valve stem which in turn (no pun intended !!!) is connected to the actuator.
Importance of Trim Material
Proper control valve selection can result in a high level of performance but how can this be maintained? Like other piping components, control valves can wear over time which can produce continued deterioration of the initial control valve performance. Left unchecked, this progressive deterioration can eventually lead to failure, shutdowns we well as the associated repair costs and financial impact of equipment shutdowns.
Trim refers to the internal elements of a control valve and these elements are a crucial consideration in the process of valve selection. Trim typically includes the valve seat, disc and stem as well as the sleeves within the valve which are required to guide the stem. The interface between the disc and seat along with the relation of the disc position to the seat normally determines the performance of the control valve.
A control valve’s trim may be selected to create a variety of passage shapes that control the flow in specific ways. The gap within the valve opens by moving the plug, disc or valve away from the seat. The length of the valve stroke determines the opening size and how much fluid passes the seat. Changing the size of the internal gap can increase, decrease or maintain the flow though the valve. Whenever the process parameter or variable being controlled does not equal the design requirement, the control valve operates and alters the opening to achieve the setpoint conditions.
Manufacturing plants can encounter significant problems from erosion or weakening of valve bodies or trim components from severe process conditions. Typical damage can include seal rings and gasket loss, stem, body and trim retainer wear on the seat ledge, plug, seat ring and cage wear and packing leakage.
There are several common causes for premature trim wear in control valves. One example would be where flashing occurs, i.e., when the pressure of the flowing fluid falls below its vapor pressure and changes the fluid phase-state from a liquid to a vapor. Small vapor cavities are formed under these conditions which cause wear at the outlet of the valve and its trim components.
Cavitation is similar to flashing except the fluid pressure recovers to a level above its vapor pressure at flowing conditions. This causes the vapor cavities to implode producing impinging jets with the potential cause severe erosive damage. Outgassing occurs when the pressure of a fluid drops below the saturation pressure of a dissolved gas. When this point is reached, the gas separates from the fluid or solution and produces a high velocity erosive vapor droplets. The simplest way of appreciating this occurrence is to think of an unopened can of soda/soft drink/fizzy pop. Once we open the can, which of course is under pressure, the sudden pressure drop causes some of the carbon dioxide to escape from the solution as a gas. When the outgassing condition arises in a flow system, in addition to the wear from vapor droplets, it can lead to vibration and eventually the trim can no longer shutoff the flow or maintain the desired flow stability.
Benefits to plant operators
Demanding business or manufacturing environments require the most accurate and reliable control of production processes possible. The failure to meet and achieve specific operating standards can produce an inherently inefficient plant, can lead to serious consequences for quality and safety and can significantly affect the financial margins for the final product. Optimum control valve performance is therefore vital in preventing such scenarios.
Industrial organisations can benefit greatly from working closely with their manufacturer representatives or instrumentation suppliers to initially specify appropriate measurement and control devices. This collaboration can achieve important performance criteria including:
- Precise flow and pressure control. This produces stable and consistent production results.
- Efficient energy usage.
- Reduced operating costs.
- Fewer unplanned and undesirable plant shutdowns.
- Increased plant availability.
- Lower maintenance and repair costs resulting in longer valve trim life.
Control valves are required to withstand the erosive effects of the flowing fluid while maintaining an accurate position to control the process. In order to successfully perform these tasks, control valves need to be sized accurately and correctly for the application as well as being designed, built and selected such that it is appropriate for the process operating conditions.
- Processing Magazine.