How to Choose the Best Valve for Your Industrial Application

It is important to consider the factors that affect complex fluid system design, including:

• Do you need to stop and start flow?
• Do you need to control the direction of the flow?
• Do you need to regulate flow rate?
• Do you need to protect your system from overpressure?
• How often will you cycle the valve?
  

You should ask all these questions about your fluid system before finalizing your valve selection. In this article, we will provide examples of different industrial valves to help you make the right choice.

Safety First

Fluid systems sometimes operate at high pressures and temperatures, and occasionally they carry hazardous materials that could pose a threat to the operators if leaks occur. To ensure fluid systems do not pose unnecessary hazards, best practices for installation and operation should be followed.

Valves play critical roles in enabling safely functioning fluid systems. For example, a safety shut-off valve or pressure relief valve can prevent your system from reaching overpressure, a potentially dangerous scenario that may lead to a blowout. That is why it is so critical to select the right valve for the function you are trying to achieve, which starts with understanding how flow operates within your system.

Understanding Flow

Ultimately, valves are designed to control flow, which is defined as a substance’s movement in a steady and continuous way through your system from higher pressures to lower pressures. Using a flowmeter, the pace of flow is recorded as a ratio of distance or volume per unit of time. For example, flow might be measured as meters per second, liters per minute, gallons per day, or other similar measurement criteria.

The diameter of a valve’s end connection and its flow path determine how well valves allow flow to occur. Manufacturers often include a flow coefficient, or Cv, with their valves, which gives operators a better understanding of how much flow a valve will control. The higher a Cv, the higher the flow rate – although, higher is not always better. The Cv you choose is highly dependent on the valve type and application. In some situations, that may mean the Cv will be close to zero.

While this may seem complicated, manufacturers can often help you determine the right valve selection for your application. To determine Cv or flow based on pressures, flow rates, temperatures, and media within your system, ask your manufacturer if they have a Cv calculator to offer the guidance you need.

Valve Functions and Types

Choosing the right valve for your application may initially seem overwhelming. After all, valves come in many sizes, configurations, materials of construction, and actuation modes. To make the best choice, it is always good practice to ask the first question in valve selection: What do I want the valve to do? Once you have answered that question, you can more easily decide what specific valves you need, which generally fall into one of five categories.

On/Off is the most basic type of valve function.start or stop the flow of fluid, and there are many different types of valves to choose from, including:

• Ball valves. Ball valves can be used in a wide range of applications, from general-purpose to critical-service applications. They are useful for reliable, leak-tight shut-off and have a low overall cost of ownership. (

  Figure 1)

• Bellows valves. Bellows valves are packless, making them a good choice when the seal to atmosphere is critical and access for maintenance is limited. A welded seal divides the lower half of the valve, where the system media resides, from the upper parts of the valve, where actuation is initiated. The stem, which is entirely encased in a metal bellows, moves up and down without rotating, sealing over the inlet.

  
  

• Diaphragm valves. Diaphragm valves have a long cycle life, provide effective shut-off, and can be found in a wide variety of sizes, materials, and configurations. Actuator options include manual, pneumatic, and locking. Consider a diaphragm valve in high-purity and ultrahigh-purity applications. (

  Figure 2

  )

• Gate valves.

  Gate valves are designed primarily for blocking flow rather than regulating it. They contain a plate-like barrier (gate) that can be inserted into the stream of a fluid to block its flow.

• Rising plug valves. Similar to a gate valve but in instrumentation sizing, a rising plug valve lifts a plug out of the flow path to achieve a full flow. They are often used in applications requiring a straight-through flow path and rough-flow control. (

  Figure 3

  )

2. Flow control valves are designed to regulate the flow of a fluid through the system. The amount of regulation depends on what type of valve is selected and can range from simple regulation to fine metering. The most common flow control valves are needle valves (Figure 4), which can provide on/off functionality if necessary. Orifice size, stem type, and stem position, which are controlled by the turning of the valve handle, will determine the pace of flow.

3. Directional flow valves guide the flow in the proper manner and are used to change the direction of the flow, if desired. The most common valves used for directional control are check valves (Figure 5) or multiport ball valves. It is important to note that directional flow valves do not throttle flow; instead, they operate in either the on or off position.

4. Much like the overpressure valves, excess control valves are designed to prevent problems if the flow levels at the valve rise to unsustainable levels. If excess flow happens downstream, the valve’s poppet is activated to the fully forward position, which prevents an uncontrolled release of system media.

5. In the case of overpressure protection valves, the goal is to prevent pressure buildup beyond a preset limit. For this application, systems typically rely on relief valves (Figure 6) or rupture discs. Relief valves are essential to any system that operates under pressure to avoid blowouts and can be calibrated to open after the pressure reaches predetermined levels. They are generally considered the last line of defense when pressures rise too high and can protect plants by allowing production to continue by relieving overpressure when it occurs.

Consider Cost of Ownership

The true cost of a valve is not its purchase price – it is the purchase price plus the cost of owning and maintaining or replacing that valve over time. To calculate the cost of ownership, you must know how long a valve will operate in your system between maintenance checks.

Maintenance costs must not only be calculated on the cost of replacement parts, but also in labor and downtime. Note that some valves are much easier to service than others. Some can be serviced in place; others must be removed from the process line.

Making the Right Selection

As you design your fluid system to be as optimized as possible, selecting the right valves to meet your application needs is important. The better your understanding of each valve and its proper function, the better able you will be to make the decision that most fully meets your specific needs.

Work with a supplier who offers training on valve selection, identification, and troubleshooting so your team can remain up to speed and knowledgeable about the latest in valve innovations.

An original version of this article appeared on the Swagelok Reference Point blog here: swagelok.com.

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Joe Bush is a Senior Product Manager for Swagelok Company.

I’m fully aware that there are already many check-lists for valve selection available on the market. Some of them quite simple and unfortunately also incomplete, others are so complicated that a “normal brained” person like me has trouble understanding the whole picture.

Through the years, and working in different valve-application fields such as Oil & Gas, Power Generation and Waterworks, I have built up my own check-list. It covers all the most important points to be considered when selecting on-off valves. I tried to include some personal experiences, tips and external links that might help to reduce mistakes while selecting valves.

Nothing really new for the experienced valve specialists out there, but maybe a real help for any “Newbie” getting involved in the valves selection process.

THE 10 POINTS VALVE SELECTiION CHECK-LIST (by Michel F. Bolle - 2018)

1 ) Applicable main standard – DIN (EN)/ASME and or other?

While often forgotten, this is one of the most important points to clarify in the valve selection process. There are two main standards on the valves market EN (European) and ASME (American).

Depending on the location where a plant is built or located one or the other standard might be used. Usually, this information can be found in the technical specification of a project.

Other standards might be applicable such as GHOST (Russian) or AWWA (Amercian Water Works Association) among many others. In the EU, for example, most valves fall under PED standard requirements. So it would be impossible to install I the EU, a big size high-pressure gate valve made in the USA without CE marking and the related PED approval.

TIPS:

According to EN standards pressure class indication is given in PN and size in DN, while according to American standards we speak about #(lbs) for pressure class and “ (Inches) for the size. When working on a new project, or a replacement project to not take this as a rule. A Valve specified as DN 400 and PN100 does not necessarily mean it is according to EN standards. There is a lot of confusion on the market, people mix up EN & ASME and there also many “hybrid”(mix of standards) valves on the market. Better check twice!

2) Type of Valve?

The type of valve needed is usually given by the process and the main function of the valve. Do we want to use the valve only for isolation (on-off) or do we need to control a certain flow? Might the valve be used to prevent back-flow or as a safety valve? Do we need a valve for steam (most likely a gate valve for isolation) or for water (most likely a Butterfly Valve).

Another important point de define from the beginning is to decide if you need a valve that seals in both directions (Ex. soft seated centric butterfly valve), or a valve that needs to seal only in one direction (Ex. double eccentric butterfly valve)

If working on replacement projects the choice is quite easy as usually the existing valves are replaced by the same type.

TIPS :

3) Size of Valve?

Usually, a valve is mounted in a piping system where the size of the pipe is already given. In this case, it is quite easy as the size of the valve will be the same as the piping size. Nevertheless, there are some factors which can influence the choice of the size of the valve such as:

- Max Allowable pressure drop (if any)

- Max Flow rate (if any)

While defining the size of the valve, these two factors have to be considered. There are some exceptional cases where the size of the valve might be bigger than the pipe size or smaller (which can be an important costs saving factor).

TIPS :

Consider that for many valve types such as globe valves or Ball Valves there are two options available on the market; Full Bore or Regular Bore (Reduced bore). In order to choose the right one, it is important to know if there is any requirement for max allowable pressure drop?

As an example: If you have a valve at a bottom of a pipe just to drain water, the pressure drop might not be so important and you can use a reduced bore valve. On the other hand having a valve in a steam system that brings steam to a turbine, pressure drop requirements might be crucial for the performance of the turbine.

4) Pressure Class?

In many cases, the pressure class for a valve is already given by the pressure class of the defined piping system.

Otherwise, the pressure class of a valve is the result of the combination of Pressure, Temperature and main body/bonnet materials used.

Here is a great example of a table for pressure classes definition according to ASME B16.34 (Source: Globalsupplyline):

http://globalsupplyline.com.au/wp-content/uploads/2014/10/Valve_Material_Temperature.pdf 

TIPS:

In order to define the right pressure class it is very important to know the “design” pressure & temperature for the piping system. If you define a pressure class according to “operating” conditions your selected valve might not withstand the “worst case” scenario.

5) Body/Bonnet Material?

The choice of the material of construction of the main parts (body/bonnet) of your valves depends on the medium. It is part of the job of the valves specifying engineer to check first material compatibility (resistance) with the medium (gas, steam, fluide, etc). In addition to this the design conditions of the piping system have to be considered (for example castings in WCB has a limit of temperature at 425C)

If the material of the piping system has already been defined, then you are lucky, as for the valve you can choose the same grade material.

TIPS :

Do not forget that the ambient conditions, can also be an important factor for the choice of the valve material. While A105 carbon steel might be a great material to be used in central Europe for a globe valve , in Russia (due to the low winter temperatures) you might have to use LF2 material instead of A105. Very salty ambient conditions might let you choose Aluminium Bronze as Valve Body Material instead of Stainless Steel.

6) TRIM Materials?

The choice of trim (stem/seat/disc or ball) materials are most likely to be in line with the chosen body/bonnet materials and will depend also on the medium, design conditions, as well on the leakage rate we want to reach.

While speaking about the trim, here should also be included the choice of all sealing material directly connected to the trim (Ex : Gaskets, Packings, etc)

Other factors to consider are who often a valve is used and for how long it should last. Depending on this we might choose a higher grade for trim materials

TIPS:

If there is the possibility of having a vacuum condition while the valve is in service trim materials & design might have to be different (Example : Lantern ring on gatevalve packings)

Widely used on the market is the API Trim chart. Here you find a useful link:

https://blog.projectmaterials.com/valves/api-trim-chart/

7) Leakage Rate (valve testing)?

Directly related to trim, but often forgotten is the “maximum allowable leakage rate”. There might be some applications where this is not important, but when we start talking about this requirement, it gets quite tricky and we need to be sure which standard is applicable. Checking the questions of the leakage rate will also end-up by defining the applicable valve testing standard.

Here is an excellent overview and explanation of the different standards:

http://globalsupplyline.com.au/wp-content/uploads/2014/10/Valve_Leakage_Rates_Test_Std.pdf

TIPS :

The definition of “0” leakage doesn’t mean anything by itself and is often confused with “Bubble tight”… Always check which is the applicable standard for the maximum allowable leakage rate.

8) Connections?

The most commonly used connections for valves are ; screwed (Ex : NPT or Gaz), welded (SW or BW) or Flanged.

Usually, the choice of the of the connection is given by the philosophy of the whole piping system. Safety and emission aspects might also be considered, as well as how the maintenance should be done.

Consider also that in any case the valve connection must match 100% the pipe connection.

TIPS :

Out of experience there are some general market tendencies of the use of end-connections:

- Power Plant Steam Systems                     : Welded (un to 2” SW, larger BW)

- Raffineries                                                 : Flanged

- Gas Systems                                              : Welded

- House heating systems                           : Screwed

- Water distribution / wastewater          : Flanged

9) Actuation

Once we have defined our valve, we need to figure out how we want to operate it. The most common ways to actuate a valve are:

- Hand actuated (lever / Hand wheel)

- Electric Actuator

- Pneumatic Actuator

- Hydraulic Actuator

- Self Actuated (by the medium)

TIPS:

Consider that for electric actuators the voltages are different depending of the location of the installation. This has to be checked absolutely when defining the actuator. Sizes of hydraulic and pneumatic actuators depend on available min air or oil pressure.

10) Painting

One out of two NCR’s for valves I have seen in the past 20 years have been related to painting.

For many projects, there are available painting specifications and/or colour codes. The choice of painting can depend on many things such as temperature resistance (medium & ambience), location, colour codes, required painting thickness et….

While often forgotten in the selection process of valves, it is a key element.

TIPS:

While usually stainless steel or aluminium bronze valves would be supplied unpainted, there are some installations that use piping colour codes these valves might also be painted. Better check twice.

Other points to consider for the selection of valves:

- Accessories (limit switches, solenoid valves, lantern rings)

- Noise

- Emission

- Weight

- Dimensions

- Installation (for example horizontal or vertical)

Valve selection can be quite tricky, but it must not be. Following a simple check-list ensuring that during the selection process all the important key-points are considered is a good step in the right direction.

If you are missing some knowledge somewhere, let’s say “material resistance” speak to your trusted valve partner. According to my experience valve manufacturers are always willing to advise.

Last but not least, consider that the time used to make the right valve selection is a very smart investment! A lot of valve problems on production sites are due to poor and wrong valve selection.

Feel free to comment and share this article. Improvements are very welcome.

Michel F. Bolle - Industrial Valve Expert - 20.6.2018