It is important to consider many factors when selecting a control valve for a process plant. Among them are valve flow characteristics, size, valve body and trim materials, noise. The potential damage from cavitation or flashing, actuator type and size, and dynamic response to changes in control signals. The following lists the typical factors to consider when choosing a control valve.
Overview
An improperly sized control valve can have serious consequences on safety, performance, and productivity. Here is a list of things to consider when selecting a control valve:
- Make sure you choose the right materials for construction. Consider the components of the valve that come in contact with the process fluid. Such as the valve body, the valve seat, or any other components exposed to the process fluid.
- It is important to consider the operating temperature and pressure of the control valve. Be aware of the local ambient atmosphere and any corrosives. That may affect the exterior of the valve.
- Make sure the selected valve can achieve the operating conditions. You need mechanically and consider the degree of control you require.
- Consider the inherent flow characteristics of the control valve you are considering. Different types of valves have different flow characteristics. In general, a flow characteristic can be thought. As a change in the rate of flow in relation to a change in valve position. We will discuss this in more detail later.
- Control valves should not be oversized. When the valve size is too large for the application, only a small percentage of travel is required. Due to a small change in valve position having a large impact on flow. the valve hunts. This can lead to excessive wear. Various published literature sources recommend sizing a control valve at about 70% to 90% of travel.
The following list represents some of the criteria to consider when selecting a control valve. Prior to making a final valve selection. It is generally recommended that you consult an appropriate and experienced supplier or manufacturer.
Control Valve Flow Characteristic
The flow characteristic of a valve is the relationship between the valve opening and the flow rate. As a valve gradually opens. The flow characteristic allows a specific amount of flow to pass through it at a particular opening percentage. This permits predictable flow regulation. The three most common flow characteristics are linear, quick opening, and equal percentage.
Linear Flow Characteristic
There is a linear relationship between valve position and flow rate for this flow characteristic. Valve stem position directly affects the flow through the valve.
Quick Opening Flow Characteristic
Flow characteristics of quick opening valves are such that a relatively small change in valve stem travel leads to a large increase in inflow. A noticeable characteristic of this valve type is that maximum flow is achieved at a relatively low percentage of the valve stem range.
Equal Percentage Flow Characteristic
An equal percentage valve produces equal percentage changes in the existing flow for equal increments of valve travel. Changes in flow rate are proportional to the flow rate just before the change in position is made.
Below is a summary of the most common valve flow characteristics.
In reality, control valves have two characteristics, inherent and installed. Valve manufacturers publish the inherent characteristic of a valve based on testing conducted on a system where care is taken to ensure that the pressure drop across the valve remains constant at all valve opening positions and flow rates. As a result, the inherent characteristic represents the valve flow capacity and valve opening position when there are no system effects.
The installed characteristic is the relationship between the valve position and flow in a system taking into account changes in the pressure differential available to the control valve due to changes in the flow squared relationship between flow and piping pressure and/or the behaviour of centrifugal pump head curves.
Control valve performance in a process system can have a dramatic effect on the plant efficiency, asset life cycle costs, and overall profitability. Thus, it goes without saying that a plant’s efficient operation requires a considered design and careful selection and sizing of the control valves. When a control valve is size correctly, it can provide significant savings, as well as increase process availability, reduce process variability, and reduce maintenance costs. Similarly, correctly sized control valves last longer than unmatched or incorrectly sized valves.
As a result, oversized valves have a higher capital cost and are more likely to cause instability in the operation of the system, whereas undersized valves cannot pass the required flow of fluid in the line.
Therefore, as designers, it is crucial to carefully consider both the sizing and selection of control valves in order to achieve both efficient and effective operation of a process plant while minimizing operating costs.
Selecting the Right Device
Optimizing control valve performance as well as preventing erosion problems requires appropriate valve sizing and selection of valve body and trim materials. This could mean the difference between continued operation and unplanned shutdowns. Choosing the right valve solution involves other decisions as well. Globe-type valves are preferred by many companies due to their proven performance and long life cycle. Compared to other valve designs, this valve offers:
- Improved control performance.
- Improved performance at low or partial loads.
- The valve has a high differential pressure.
- Smaller physical profile than comparable ball valves.
- Can use with steam, water, or water/glycol fluids.
Globe valves modulate flow by moving a valve plug in relation to ports located within the valve body. Plugs connect to valve stems, which are in turn connected to actuators (no pun intended !!!).
Importance of Trim Material
The selection of the right control valve can result in high performance, but how can this be maintained? Control valves, like other piping components, can wear over time, resulting in continued degradation of their performance. If left unchecked, this progressive deterioration can lead to failures, shutdowns, as 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 crucial to valve selection. Valve trim includes the seat, disc, and stem of the valve as well as the sleeves that guide the stem. The performance of the control valve is determined by the interface between the disc and seat, as well as the relation between the disc position and the seat position.
It is possible to design a control valve’s trim to create a variety of passage shapes that control the flow in specific ways. By moving the plug, disc, or valve away from the seat, the gap within the valve opens. The length of the valve stroke determines how much fluid passes through the seat and the size of the opening. Changing the internal gap can increase, decrease or maintain the flow through the Valve Manufacturer in Qatar. When the process parameter or variable being control does not meet the setpoint, the control valve operates and alters the opening to achieve the setpoint.
Manufacturing
In manufacturing plants, erosion or weakening of valve bodies or trim components can cause significant problems. Damages can include seal rings and gasket loss, stem, body, and trim retainer wear on the seat ledge, plug, seat ring, cage wear, and packing leakage.
Premature trim wear in control valves is cause by a variety of factors. Flashing, for instance, occurs when the pressure of a flowing fluid drops below its vapor pressure, and the fluid transitions from a liquid state to a vapor phase. Small vapor cavities are form under these conditions, causing wear at the valve outlet and its trim components.
Cavitation is similar to flashing except that the fluid pressure recovers to a level above its vapor pressure when flowing. As a result of this, the vapor cavities implode, producing impinging jets capable of causing severe erosive damage. When a fluid’s pressure falls below the saturation pressure of dissolved gas, outgassing occurs. At this point, the gas separates from the liquid or solution and forms erosive vapor droplets at high velocity. It is easiest to comprehend this phenomenon if you think of an unopened can of soda/soft drink/fizzy pop. When we open the can, which is under pressure, some of the carbon dioxides will escape as gas. The outgassing condition in a flow system, in addition to the wear from vapor droplets, can cause vibration, and eventually. The trim will no longer be able to shut off the flow or maintain the desired flow stability.
Benefits to plant operators
Business and manufacturing environments require the most accurate and reliable control of production processes. Failure to comply with and achieve specific operating standards can result in inherently inefficient plants can result in serious consequences for quality and safety, and can significantly affect the profitability of the final product. To prevent such scenarios, the performance of control valves must be optimal.
Companies can gain considerable benefits from working closely with their manufacturers’ representatives or instrumentation suppliers to specify appropriate measurement and control devices. Through this collaboration, companies can achieve important performance criteria such as:
- Control of flow and pressure. Consistent and reliable results produce.
- Energy is efficiently use.
- Operating costs are reduce.
- There are fewer unplanned and undesirable plant shutdowns.
- The plant is more available.
- Longer valve trim life due to lower maintenance and repair costs.
Fluid flowing through control valves must be able to resist erosive forces while maintaining an accurate position to control the process. Control valves need to be size accurately and correctly for the application. As well as designed, constructed, and selected such that they are appropriate for the operating conditions.
