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.
Link to SUPCON
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:
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.
Equal PercentageThe 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:
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
Contact us to discuss your requirements of flow control valve do. Our experienced sales team can help you identify the options that best suit your needs.
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:
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.
References:
How to Select the Right Control Valve for Your Process?
Control valves play an important role in various process industries. They are designed to control the flow of fluids in a system. They are designed to start and stop as well as throttle the fluid flow to ensure efficient process operation. Owing to their increasing use in process industries, today, it is easy to find these valves in different specifications. This large selection may be confusing to users who are purchasing it for the first time. There are several factors to be considered when making a selection. This post aims to ease the control valve selection by providing a few easy-to-follow tips.
Control valves are differentiated based on their designs and other characteristics. The following are a few important types of control valves in use today:
Linear globe valves
: These valves are made of cylindrical or disk plug element, and have a spherical body and a stationary ring seat. Linear globe valves allow easy access to internal elements, and they are used to regulate the flow of a variety of fluids.Linear angle valves
: The control valves are designed for severe applications, which may have large noise, flashing, and flow rates.Rotary valves
: In this type of valve, the liquid flow is regulated through the rotation of passages present in a transverse plug. These valves are ideal for applications, where fluids being transferred may contain solids. In many applications, rotary valves serve as a replacement for globe and angle valves. These valves are designed for severe applications, and they are suited for applications where cleanliness is not mandatory.Linear three-way control valves
: These control valves feature two outlets and one inlet or vice-versa. Linear three-way control valves are used for applications, where liquids may be involved. They enable easy mixing of liquids from two different pipes into a single pipe. These valves can be found in water chillers, fan coils, air handling units, and so on.The purpose of this section is to provide you points that will ease the control valve selection process for your next project.
Fluid to be Transferred
: What type of fluid will be transferred at your processing plant? The control valves are designed for controlling the flow of media such as natural gas, compressed air, steam, liquefied petroleum, gasoline, hydraulic fluid, slurry, and water. So, it needs to be ensured that the valve works perfectly for all these fluids.Material Grades
: As said earlier, today, it is easy to find the industrial valves in various stainless steel material grades such as ASTM, ASTM A217 grade, A216 grade WCC, ASTM A351 CF8M, and so on.Flow Control Characteristics
: Address the maximum and minimum or acceptable temperature range of the fluid to be transferred. Also, address the minimum and maximum inlet and outlet pressures that a valve is expected to handle. This will help you understand the flow control characteristics of the valve. Ensure that the valve chosen offers consistent control across various temperatures. Also, they meet equal or linear percentage or several other characteristics. Understand that these flow characteristics will vary across different processes, and the valve must be able to assure stability across all conditions.Compatibility with Remote Systems
: Owing to the criticality of the process environment, many processes require remote control and monitoring. It is important to ensure that the valve chosen complies with the remote process management systems.Rugged Construction
: Most industrial processes are conducted in ambient, harsh environments; so, if the chosen valve is unable to meet these diverse requirements, then it may fail to serve the purpose. You need to check if the valve can withstand the extremes and minimums as well as upset conditions, and can operate virtually.In addition to the above-discussed tips, you can discuss your requirements with the manufacturer. Whenever you are considering these valves for your project, ensure that they are sourced from a reliable supplier like The Transmitter Shop. The company provides a new surplus, reconditioned, and factory new control valves.
Want more information on pressure control valve pneumatic? Feel free to contact us.