Jun. 10, 2024
Mechanical Parts & Fabrication Services
Courtesy: CFE Media and Technology
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Manufacturing and processing facilities often work with systems that manage fluids in their operations. These facilities must control the supply and flow of different service fluids crucial for routine processes. The service fluids can be gaseous, liquid or semi-solid (slurries), bearing unique physical and chemical properties.
Any system with a working fluid will need valves to control fluid flow, while also being able to handle operating characteristics like temperature and pressure. Valves come into play in these situations, by performing the functions needed to manage the working fluid.
With the many different types of valves and even more choices for customization, choosing a valve can seem like a daunting task. Regardless of the nature of the application, impacts on safety and effectiveness are always a top priority. Here are some practical factors to start with when choosing an appropriate valve.
Not all fluid systems are pressurized to the same level. Different types of service will require higher pressure levels than others. However, the entire system should be able to deal with the demands of the conditions, and within a reasonable factor of safety. Line pressure refers specifically to the force exerted throughout the area of the valve body. It provides a figure for the full upstream pressure of the fluid as it enters the valve.
Going beyond design pressure limits means running the risk of ineffective valve operations. Leaking fluids is a common consequence of exceeding pressure limits, causing line losses and safety concerns. Any additional stress beyond the design parameters of a valve can also compromise sealing components and can lead to the valve degrading.
When selecting valves based on the line pressure of the piping system, process engineers should evaluate the desired pressure drop across the valve and the entire piping system. The valve trim and sizing should offer the least frictional resistance to keep the pressure drop across the valve low and reduce line pressure losses &#; which may affect subsequent manufacturing processes. This explains why gate valves, which have low-pressure drops at fully open positions, are preferable to globe valves in industrial piping systems where line pressure should remain constant.
Certain valves require additional considerations such as the set pressure for safety relief valves (SRV). Aside from evaluating the maximum pressure expected from the line, the design needs to account for the pressure level at which the valve opens a path to relieve system pressure. A similar concept applies to check valves, where cracking pressure is the minimum pressure where it starts to allow flow in one direction.
Different valve types and their varying mechanisms manage fluid flow in distinctive ways. For instance, a ball valve offers superb sealing for shut-off applications. However, a needle valve might provide better precision for controlling specific flow rates. The valve&#;s inner workings tell a lot about how they steer the working fluid and how they perform the required function.
Valve selection requires understanding what the flow needs to be. Broad classifications of valve functions include whether the valve needs to switch the flow on or off, regulate the amount of flow or change the flow&#;s direction.
On-off applications require the valve to either allow or restrict flow. The way a ball valve works is an excellent example of a rapid response, where a hollowed-out spherical ball either aligns with or blocks fluid flow. A gate valve provides the same function &#; shutting off flow &#; using a plate or obstructive tool that acts as a gate. Butterfly valves can also provide on-off fluid service using a metal disc (butterfly) that completes quarter-turn rotations around a fixed axis to permit or stop the flow of service fluids.
Flow control functions need more precise increments in adjusting the flow rate. Rather than shutting and opening the flow passage, the valve should be able to handle more meticulous control. Controlling flow rates is only possible if the action of opening the valve has a predictable relationship with the variation of flow allowed through the valve. Needle valves achieve this level of precision by employing a needle-shaped plunger &#; often coupled with a screw-type controller &#; that restricts flow. One can achieve linear flow control using V-ball valves whose flow rates increase with shaft rotations. V-ball valves have low pressure drops, provide bubble-tight shut-off, and are designed for high pressure and flow service.
Process engineers can achieve precise flow control using sliding gate valves that feature a compact design and require low actuation forces. Sliding gate valves are lightweight and compact, which make them more responsive to line pressure changes. Plug valves also are suitable alternatives when considering precise flow control in different industrial piping systems.
Think of controlling directional flow as an expansion of the concept of on/off applications. It can refer to either the limitation of flow direction or the management of multiple possible inlets and outlets. Check valves are an example of limiting fluid flow to go one way.
A minimum set pressure will allow flow rates to go in one direction but not the other.
Another example of directional control is a situation in which the flow is not shut in one path but redirected onto another. Multi-port valves that allow on-off functionality, such as a 3-way ball valve, are available in configurations that permit multiple exit ports or inlets.
As with pressure, temperature affects the characteristics of both the medium flowing through the line and the valve itself. Working fluids can have varying energy levels dependent on temperature levels. Extreme temperature conditions can also exacerbate the corrosive effects of fluids on certain materials. Several materials also make up individual valve components and these materials withstand high and low temperatures differently.
Fluids, especially gases, often take up more space as the temperature rises. Gases expand and contract with varying temperatures. Air and other gases that experience high temperatures tend to be less dense &#; resulting in increased pressure rating requirements if the fluid is enclosed, or higher flow rates if allowed to pass through. Valve considerations need to account for higher pressure ratings or more precise increments for flow control in these situations.
Aside from affecting the fluid that goes through the system, temperature ratings also affect the valve and its components. Materials often contract at low temperatures and expand with higher temperatures. Because many different materials make up a valve, temperature differences can result in non-uniform changes in the components of a valve.
Metals and metal alloys are the most common materials used for the valve body. Various stainless steel variants are a sensible start for non-corrosive gases up to around 400 °F. Austenitic steels and nickel alloys are typical alternatives for higher temperatures that are also viable for corrosive service. On the other hand, Teflon is considered a versatile seat material that works for both ends of the temperature spectrum &#; as opposed to EPDM rubber, which offers a narrower range of temperature allowances. Always consult a valve expert when designing or sizing valves for critical fluid service to avoid valve problems or failures during use.
Typical temperature ranges for valve types also apply. All industrial valves are assigned temperature classes depending on the manufacturing materials and testing standards. Categorizing valves based on allowable temperature range makes it easier to select the correct process valves and ensure durable service. High-temperature process valves use strong manufacturing materials (including alloys) to prevent chemical reactions with different service fluids, or deformations when controlling fluid flow at high process temperatures. Exercise caution when selecting valves for low-temperature (cryogenic) applications.
Selecting a valve comes with budget constraints. Different types of valves have varying levels of associated construction costs. The type of valve material and the anticipated medium affect any special requirements for the valve construction material and the budget. Customizations regarding valve operation and automation also incur additional costs. Utilizing actuators requires more specialized components, which translates to added expenses.
Looking specifically at valve construction, simpler valves with fewer moving parts often cost less for the same rating. For instance, a gate valve typically costs less than a ball valve for similar specifications due to its design. However, the very design that makes gate valves cheaper in construction also makes them less effective when it comes to sealing.
Choosing to operate a valve manually or automatically is another consideration that affects total cost. Selecting an actuator that allows remote and automated control can sustain additional expenses. Actuators typically employ hydraulic, pneumatic or electrical means of operation. Choose a valve within the company&#;s budget and can provide dependable and durable fluid control. The goal is to minimize the total cost of valve ownership &#; purchasing and maintaining valves throughout their useful lives &#; irrespective of their valve cycles and sizes.
Hydraulic actuators use compressed oil, which allows quick-response operations for large-scale valves due high-force capacities. Additional safety precautions also should be in place when handling hydraulic fluid because maintenance practices can be complex.
Pneumatic actuators use air instead of oil, making them more suitable for hazardous applications. However, there will be drawbacks in reduced precision due to the compressibility of air. It is crucial to maintain accessibility to instrument-quality air. The presence of impurities in compressed air can accelerate the wear and tear of actuators and adjacent valve components. With smaller-scale operations, electrical actuators are relatively inexpensive alternatives. They are also generally more compact and lighter if slower actuation speeds are acceptable.
Budget and effectiveness become a balancing act depending on the requirements of an application. While there are opportunities to get cheaper alternatives, the impact on achieving business objectives and workplace safety remains the priority.
Valve selection is not a choice guided by preference. Instead, most considerations arise from utility and the necessity of meeting the specifications of the operating conditions.
Ball valves are designed for industrial fluid applications and a good starting point when selecting a valve. They offer reliable sealing that lends itself to on-off applications. Because the hollowed-out portion of the ball allows unobstructed flow when switched on, it achieves minimal pressure reduction across the valve. While most ball valve types are suitable for moderate to high pressures, large ball valve applications can use a trunnion-mounted ball valve to offer additional mechanical support for stability.
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Gate valves are a suitable alternative for the same on-off applications if the user prefers a more gradual flow release. This preference is more applicable to water systems where the effect of water hammer requires additional consideration. For any other application, gate valves also have the advantage of generally costing less than other alternatives.
For functions requiring flow control, needle valves offer high levels of precision. Typical applications include gas calibration and lines carrying clear fluids such as propane. A more economical option for industrial, high-flow functions are globe valves.
Globe valves utilize a disc element that linearly adjusts its position to obstruct or permit flow. This element is usually coupled with a screw-type control that allows for gradual flow rate increments. Increasing or decreasing the amount of the opening proportionally affects the amount of flow, which then allows control. Different configurations of globe valves also enable varying flow patterns such as crossflow and Y-flow arrangements.
Butterfly valves are another option for industrial piping systems. They have a compact and lightweight design, rendering them more responsive to pressure changes in the pipeline with excellent sealing characteristics. Several butterfly valve designs are available to meet industrial fluid system pressure and temperature demands.
Regardless of the type of valve used for various applications, design considerations should also include assessing suitable valve materials. The toxicity of fluids and any corrosive properties mandate using materials that can chemically handle the wear. Exceedingly hot or cold conditions also require proper material type.
While there&#;s a broad range of valve options, the good news is facilities can count on having a valve that will perform the duties they require. Regardless of what type of valve design one ends up with, it is always a top priority to keep operations safe and effective. Always work with a valve expert when selecting valves for different fluid applications to ensure proper sizing and maximize the productivity and efficiency of industrial processes.
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The wedge is the sealing part of a gate valve and is therefore crucial. Consider the following:
The wedge nut connects the wedge to the stem. There are two basic wedge nut designs; A loose wedge nut design where the brass nut slides in a slot in the wedge core, and a fixed wedge nut design where the nut is expanded in the wedge core. With a fixed wedge nut design the number of movable parts is reduced, thus eliminating the risk of corrosion as a result of moving parts damaging the rubber surface of the wedge core. A fixed wedge nut design is therefore recommended.
The wedge is exposed to friction and stress forces when the valve is opened and closed during operation of the pipeline. Guides in the wedge fitting to corresponding grooves in the body help stabilizing the wedge position during operation and ensure that the stem does not bend downstream due to the flow velocity. Wedge shoes help ensuring that the rubber on the wedge surface is not worn through as a result of the friction between the wedge and the guiderail in the body. Make sure that the wedge shoes are fixed to the wedge and that the rubber layer underneath is sufficient to prevent corrosion of the wedge core.
It is vital for the tightness of the valve that the wedge is fully vulcanized with rubber and that the rubber volume on the sealing area of the wedge is sufficient to absorb impurities in the seat. A strong bonding between the rubber and the wedge core is important to ensure a correct seal even when the rubber is compressed, and to prevent creeping corrosion even if a sharp object penetrates the rubber during closing of the valve.
The rubber quality is critical for the durability as well as for the valve function. The rubber must be able to withstand continuous impact from impurities and chemicals without being damaged and it must be able to absorb small impurities in the seat to close tight. Consider the following:
The compression set means the rubber&#;s ability to regain its original shape after having been compressed. The EN 681-1 standard states the minimum requirements for the compression set value, but the better the compression set, the better is the rubber&#;s ability to regain its shape and close 100% tight year after year.
Organic substances migrate from the rubber compound and act as nutrients for microorganisms, which will then start forming biofilm causing contamination of the drinking water. Select valves with a wedge rubber that ensures minimum formation of biofilm.
Chlorine and other chemicals are commonly used to clean new pipelines or disinfect old ones. Ozone and chlorine may also be added in low concentrations to make the water drinkable. The rubber compound must not degrade or crack as a result of chemical treatment of the drinking water, as it would cause corrosion of the wedge core.
All rubber components in contact with the drinking water should carry a drinking water approval. If no local approvals are required, the rubber in direct contact with the drinking water should hold one of the major approvals like DVGW/KTW, KIWA or NF.
The external corrosion protection is critical for the service life of the valve. A uniform and even epoxy coating in compliance with DIN part 1, EN and GSK* requirements is recommended and involves the following:
According to ISO -4.
Min. 250 μm on all areas.
The curing of the epoxy coating is to be checked in a cross linkage test (MIBK test). One drop of methyl isobutyl ketone is put on a test piece. After 30 seconds the test area is wiped with a clean white cloth. The test surface may not become matt or smeared, and the cloth must remain clean.
A stainless steel cylinder is dropped on the coated surface through a one meter long tube. After each impact the component is to be electrically tested, and no electrical breakthrough shall occur.
A 3kV detector with a brush electrode is used to reveal and locate any pinholes in the coating.
There are two important design issues:
The sealing placed in the bonnet around the stem retaining the pressure inside the valve/pipeline. Stem sealings should always be designed to be maintenance-free and should last the service life of the valve or at least fulfil the service life demands according to EN -2. The main seal retaining the inside pressure should preferably be designed as a hydraulic seal giving tighter seal with increased internal pressure. Backup seals should be placed around the stem. To protect the sealings against contamination from outside, a sealing should be placed around the stem on the top. For safety and health reasons a drinking water approved high quality EPDM rubber compound must be used where direct contact to drinking water occurs.
Tightness between the bonnet and the body can be obtained by using a gasket embedded in a recess in the valve. This design ensures that the gasket will remain correctly positioned and not be blown out as a result of pressure surges. To protect the bonnet bolts against corrosion the bonnet gasket should encircle the bolts, and the bolts should be embedded in the valve in such a way that no threads are exposed to the surroundings.
When operating a gate valve either by handwheel or by means of an electric actuator it is important to pay attention to the operating and closing torque.
The torque needed to operate the valve from the open position to the closed position, should be between 5 Nm and 30 Nm depending on the valve size. It is important to consider that valves having an operating torque less than 5 Nm encourages the operator of the valve to close the valve to fast thus risking water hammer and pressure surges in the pipeline.
The torque needed to close the valve to a drop tight position. This torque should for handwheel operated valves be balanced against the handwheel diameter in such a way that it does not present the operator with a rim-force in excess of 30-40 kg. When operating the valve with an electric actuator or manual gearbox the torque should be within the limits of a standard range actuator. It is important to notice that the actuators normally have a torque range that is quite wide, and often it is the ISO flange connection between valve and actuator that determines the actuator choice. As a main rule valves with ISO flange connection should have max. closing torques as stated below:
To enable the use of pipe cleaning devices the inside diameter of the valves should correspond to the nominal size of the valve.
* GSK stands for Gütegemeinshaft Schwerer Korrosionsschutz, and is an independent quality association with about 30 members, all leading European valve and fittings manufacturers. GSK outlines requirements for the coating itself and for the control procedures of the finished coating.
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