5 Reasons Why Your Business Needs Cage guided valve？

What Are Process Control Valves?

Control valves are critical to the establishment of an efficient process and smooth running over the lifetime of a system. The controller sends signals to the valves which can change the size and direction of fluid flow, and in turn affect other process parameters, such as liquid level, temperature and pressure.

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How To Choose The Right Process Control Valves

The many different types of process control require valves of all sizes, types, materials and modes of actuation. Knowing how to choose the right process control valves might seem a little daunting. You will have to consider technical specifications such as the level of control a particular valve will provide, its delivery time, and the amount of resolution it can achieve, not to mention its life expectancy and maintenance requirements. Your choice will also be affected by cost, and may demand application-specific expertise.

An additional factor to evaluate is how versatile the valves may be; whether they can cope with a wide range of different flows and still maintain stable flow control. They may be required to have connectivity that enables remote monitoring and technology that makes them easy to maintain. In our experience at Rowse Pneumatics, we have found that linear angle valves and rotary valves offer the most wide-ranging applicability.

Linear Valves

Linear valves are simply designed and easy to maintain. They are also extremely versatile, offering a wide range of pressure classes, sizes and design options. Linear valves are typically more expensive, usually using pneumatic or hydraulic actuators. However, they are also more resistant than rotary valves to cavitation, erosion and excessive noise. They tend to operate more slowly, but have greater accuracy and positional stability of the closure element. Linear valves can be cage-guided, stem-guided or top/bottom-guided.

Linear Angle Valves

Linear angle valves are a type of globe valve where the inlet and outlet ports are situated at right angles to each other, and are often used in tight spaces. In this type of valve; a closure element, a slat, a disc or something flexible like a diaphragm is moved in and out of the angled valve seat by means of a linear actuator. The valves can be cage-guided or stem-guided, and have a range of design options. These include restricted trim, expanded outlet connections for low-flow applications, and outlet liners that help to resist damage. They are best suited for more demanding applications, where noise, large flow rates and flashing can be more easily mitigated with appropriate technology.

Linear Globe Valves

Linear globe valves are a popular choice for their noise-mitigating properties, their ability to regulate a wide range of flow requirements, and their internal accessibility which allows easy maintenance. A spherical body typically contains a stationary ring seat with a cylindrical or disc-shaped plug element. These valves are available in all design styles, with top/bottom-guided and stem-guided valves being more suitable for general industrial purposes and less demanding applications. Cage-guided valves perform well in a wider range of applications, and can be fitted with mitigating features for noise and cavitation, but they tend to be more expensive. All types of globe valve can be controlled by all modes of actuation, making them a great deal more versatile.

Three-Way Control Valve

Linear three-way control valves are commonly used in industrial applications, and have a combination of one and two inlets and outlets, which enables one-into-two fluid separation or two-into-one mixing. This type of valve is usually cage or stem-guided, and is used to control the amount of pressure or flow in between two sections of the piping system. Typically, this valve is not used in very demanding applications, being more suited for things like boilers, water chillers, fan coils and air-handling units.

Rotary Valves

Rotary valves regulate flow by rotating one or more passages in a transverse plug. They can incorporate a wide variety of adaptive technology, which makes them very versatile. They typically offer a less tortuous inline path than linear valves, while still providing similar rangeability, flow control and severe-system resilience. Their design is inherently advantageous to applications with actuating fluid which may contain particulates and can't be kept scrupulously clean.

Other Factors To Consider

The prevailing standards of the country and industry must be taken into account when deciding how to choose the right process control valves, as well as the varying expectations of different customers. The priorities of the end-user are the most important. This usually means a versatile, robust valve, combining the necessary degree of control with a long service life and the greatest ease of use and maintenance.

Standardised performance is also desirable to make repair and replacement of parts more straightforward. Valves should be able to perform equally well in a wide range of fluid environments: clean, dirty, abrasive, corrosive or viscous, and at high temperatures or pressures. While incorporating a filtration unit in such environments is possible, it makes the system more complicated and requires an additional maintenance programme.

The valves must be capable of consistent, stable flow control in all conditions so as to prevent production downtime. They must match the specific control characteristics required for their particular process conditions, such as load, linear parameters and equation. Valves must also be able to handle the entire range of potential flow changes that occur in the process. They should have adequate and compatible connectivity to a management system, to enable remote control and monitoring of processing applications.

Process control valves should also be selected with ease of proper maintenance in mind, including:

• maintenance access without the valve body having to be removed from the system
• inspection and adjustment access to the valve packing
• easy disconnection and reconnection from the actuator, such as coupled or splined shaft connections
• shaft protection which should be blowout-proof
• self-purging, enclosed actuator linkage
• self-aligning seat ring to ensure tight shut-off

Taking all these factors into consideration is no easy task, but it's a necessary one to ensure you choose the right process control valves for your particular circumstances. If you have any queries or concerns about your selection, our expert team at Rowse Pneumatics will always be happy to help.

Many end users ask for a review of their technical specifications to identify areas adding unnecessary cost and lead time to their valve, actuator, positioner and related accessory purchases.

They need help evaluating if their requirements were truly needed or were actually more than required for their specific applications. A review of specifications typically reveals instances where cost-saving products and options are overlooked, along with inclusion of unnecessary items and procedures.

In many cases, these mistakes drive up cost and lead time because users specify “gold-plated” control valves in applications where less expensive options would work just as well. These over-specified valves not only cost more up front, but can also be harder to maintain and require more spare parts. The preferred alternative is to specify just the valve needed (see Image 1), saving time and money.

This article describes issues often seen when reviewing end user technical specifications and shows how process engineers can specify just what they need – and no more – when ordering valves, actuators, positioners and related accessories.

Reducing costs to green light projects

The biggest challenge facing most capital projects is cost justification. All too often, a valve upgrade or new valve project is based on what has been done traditionally. Unfortunately, this often includes the use of outdated technology, unnecessary options and overly stringent specifications and standards, which can add layers of avoidable costs.

Process engineers are increasingly recognizing the need to incorporate a design without these layers. Instead, they want a fit-for-purpose approach using industry standards and common designs versus customization that adds cost and development time.

Starting at this zero-base by using only what is necessary for the project to be safe, legal and reliable, valve vendors can help evaluate the application and make value-driven choices.

Optimizing specifications

When specifying valves, the end user, control valve vendor and perhaps a systems integrator or engineering, procurement and construction (EPC) firm should evaluate the company’s valve requirements early in the process. All parties involved should discuss how each party can help optimize the specifications and eliminate excessive requirements. These areas include optimizing the control valve, welding and nondestructive evaluation (NDE) specifications.

Some of the requirements leading to unnecessary added costs and excessive lead times include:

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• All assembly must occur in North America
• No castings from India or China
• Third-party witness of all welding

The first and second points are outdated remnants of a world gone by. Most valve manufacturers have global facilities with quality equal to American- made products.

For example, an Emerson customer was able to waive the assembly requirement, saving the project $1.2 million and reducing lead time by 20 weeks.

The “no castings” requirement is a repeat offender in specifications and one that regularly meets resistance to removal. Many valve manufacturers have global quality standards enforced on all the foundries they use. Understanding what these standards are and ensuring they are acceptable to an EPC or end user can save 25 percent on project cost and reduce lead times by up to 12 weeks.

The “third-party witness of all welding” requirement can also be dismissed in almost every case. A valve supplier can provide weld procedures for approval by the user and EPC and skip having someone actually come and witness the welding. The welding will still be in compliance with all applicable regulations and standards. This saved a customer $200,000 and reduced the lead time of the project by 12 weeks.

Unnecessary specifications

While reviewing multiple technical specifications from customers just this past year, areas were often discovered where products and technology would be excluded for reasons that may have been true years ago, but no longer apply. Using the word “shall” in a spec dictates no deviation from what is written, often forcing vendors to provide more expensive solutions.

Some unnecessary specifications include:

• Provide throttling ball valve for pulp mill applications.
• Provide globe valves for hot gas recycle applications.
• Cage-guided valves should not be used with high-viscosity fluids, fluids that contain solids or in slurries.
• Rotary valves shall have splined shafts to limit lost motion.
• Stellite is not acceptable in boiler feedwater due to attack of water-treating chemicals.

Such requirements often deny a plant the chance to use new or different proven technologies. For example, a high-performance butterfly valve works just as well as a ball valve in most pulp mill applications. Angle valves can handle hot gas recycle applications just as well as globe valves in many instances, saving weight and often cost and lead time.

Often multiple solutions for the same application, such as outgassing, exist. In these instances, service conditions, sizing methods particular to the application (such as bracket sizing), and end user experience should help determine what valve is the best fit for a particular application. In outgassing applications, there are some cases where a more cost-effective rotary solution can be used, and others where a highly engineered severe-service solution must be used to withstand harsh operating conditions.

The requirement that cage-guided valves should not be used with high-viscosity fluids, fluids that contain solids or in slurries does not allow a plant to use new trim designs even when they would be perfectly acceptable and even a lower cost option in many cases.

The requirement that control valves with special trim for noise reduction should have globe bodies and cage trims eliminates the use of angle bodies or an even more cost-effective solution – a rotary valve with a noise attenuator. Depending on the application, an angle valve might be a better option than a globe valve, or a rotary valve with an attenuator could reduce noise levels and save money (see Figure 1).

The requirement that rotary valves shall have splined shafts to limit lost motion excludes large sizes of rotary valves and scotch yoke pneumatic actuators, often to the detriment of project costs and lead times.

As for the stellite requirement, in Emerson concluded an investigation revealing that feedwater treatment technologies and methods have changed significantly over the past 25 years, allowing Alloy 6 stellite to be an acceptable solution. And in many cases, 440C stainless steel is more cost-efficient and provides similar erosion resistance.

Exclusion of options

Gold-plated specs tend to eliminate less expensive options, often requiring more expensive solutions with no corresponding operational improvements. Some of these include:

• Threaded seat rings are not acceptable.
• Bonnet bolts shall not be used to attach actuators or mounting brackets.
• Reverse-acting spring diaphragm actuators that incorporate seals or glands should be avoided.
• Valve yokes may be cast iron for fluid operating temperatures up to 800ºF, but shall be cast steel for temperatures exceeding 800ºF.

The threaded seat ring exclusion and the no-bonnet-bolts spec drive a user to more expensive valves by eliminating cost-saving alternatives. Stating that reverse-acting spring diaphragm actuators incorporating seals or glands should be avoided is incorrect. If a valve must fail open, it needs a reverse-acting actuator.

Specifying that valve yokes may be cast iron or cast steel for various temperatures is an incomplete spec because it does not state if extension bonnets can help with high-temperature applications, which is often a more cost-effective solution.

It is difficult to capture all the options offered by valve manufacturers, and it is hard to know if something put into a specification will eliminate an option that could bring the overall project cost down. This is why early review and optimization of specifications and requirements is critical. If valve vendor technical personnel can review specs before the project is bid, they can walk users through the specs and identify problematic areas.

Optimization of work processes

The project team should evaluate its documentation requirements and their impact on the overall schedule. In many cases, end users continue to rely on outmoded and outdated paperwork requirements and procedures. The goal should be to optimize the provision and use of:

• Data sheets and sizing calculations
• Drawings and procedures
• Data packs
• Electronic data exchange, FF2.0 to SPI
• Submittal and approval process

Optimizing these documentation requirements can result in substantial savings. For example, with outdated work procedures, the project team must manually enter data and create/modify 3D control valve shapes for the piping design. In a typical project, this can require:

• Editing 500 data sheets x 1 hour average per tag = 500 engineering hours
• Entering 500 data sheets into a computer system x 0.5 hour each = 250 work hours
• Building 500 valves in 3D models at 0.5 hour each = 250 design hours
• Checking, revisions and management of change = 250 work hours
• Total: 1,250 work hours

Many valve manufacturers now offer computer-based work procedures requiring considerably less time. For example, some procedures allow end users and EPCs to receive data electronically from the vendor to build pre-populated spec sheets, 3D shapes and dimensions – with only a few fields requiring editing – resulting in:

• Editing 500 data sheets x 0.2 hour average per tag = 100 engineering hours
• Importing 500 data sheets into valve sizing software x 0.002 hour each = 1 work hour
• Placing 500 DDP shapes in 3D model at 3 minutes each = 25 design hours
• No checking necessary. Revisions and management of change = 20 work hours
• Total: 145 work hours

Using manual data editing adds 2.5 man-hours per valve, while using automated procedures takes only about 0.3 man-hours per each valve. Automated work processes also reduce rework during construction and minimize schedule delays.

Summary

Working with outdated and unnecessary valve, actuator, positioner and accessory specifications results in increased project costs and lead time by denying access to the latest technology and less expensive solutions. Reviewing valve specifications to identify unnecessary specifications is a necessary first step to cut costs and lead times, and this must be done early in the project.

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