Power Basics of Industrial Valve selection – Exclusive 10 Points ...

Introduction:

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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.

Kindly take a special attention to the "TIPS" sections. I offer here a real added-value information based on my own experience.

Enjoy reading and don't hesitate to give any feedback, additional information or clarification.

With humble regards

Michel F. Bolle - 17.7.

THE 10 POINTS VALVE SELECTION CHECK-LIST (by Michel F. Bolle - July )

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

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 :

A very good source for information about different types of valves and their applications are the catalogues of the valve manufacturers. Usually, they state the main applications for each type of valve. Example: “Gate / Globe Valves excellent for Steam”

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//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//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.

For more information, please visit DIN Standard Globe Valve.

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 aluminum bronze valves would be supplied unpainted, there are some installations that use piping color 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 - 17.7.

A valve is a mechanical device used in fluid systems to control, direct, isolate, mix, or regulate the flow or pressure of a fluid, by modifying the passageway through the pipe. When a valve is in the open position, the passageway is unobstructed and the fluid flows in a direction from higher pressure to lower pressure. By partially or fully obstructing the passageway through the valve, the flow can be reduced or completely stopped.

Technically, valves are pipe fittings but due to their critical function, wide variety of types, and operation options they are generally considered separately from other fittings. Valves are often the costliest components of the piping system in a plant; the cost of valves can be as much as 30% of the overall piping cost.

At the design stage of a manufacturing process or a piping system, engineers generally specify the type of valve suitable for the desired function on the Process and Instrumentation Diagram (P&ID). A P&ID is a schematic illustration of the functional relationship of piping, instrumentation and system components. Different types of valves commonly found on P&IDs are introduced and discusses in the following section.

Gate valves are inexpensive and easy to disassemble for maintenance. They offer low fluid resistance due to a straight path that allows the fluid to flow straight through without change in direction.

During use, gate valves are normally kept in either a fully open or a fully closed position as they are not recommended for throttling applications. The valve design makes it difficult to control the flow.

Furthermore, the fluid slapping against a partially open gate can damage the valve. The multi-turn design of these valves makes them unsuitable for applications where rapid operation is desired.

A globe valve generally has a spherical body that contains a movable plug element and a stationary ring seat. The two halves of the body are separated by an internal baffle. The plug element is connected to an operating hand wheel through a stem. As the wheel is turned, the screw action moves the plug towards or away from the ring seat. Globe valves can be used for stop/start function as well as to regulate fluid flow. They are useful in applications where good flow control is desired and leak tightness is important such as cooling water systems, feedwater and chemical feed systems, and fuel/lubricating oil systems.

Globe valves cost more than gate valves but provide better shut off and throttling control. The design of these valves forces the fluid to change direction as it flows through; this creates high pressure loss and turbulence. Globe valves are generally heavier than other valves. They are not suitable for fluids containing particles, high viscosity fluids and other mediums that are prone to choking.

Ball valves are smaller, lighter and cheaper when compared to gate valves of the same size and rating. They allow visual detection of the open or closed state of the valve. In an open state, the handle is aligned with the direction of flow; in a closed state the handle is perpendicular to the flow direction. In addition, ball valves offer ease of operation, high volume flow, high pressure, long service life and possibility of repair of seats and seals without removing the valve body from the line. On the other hand, ball valves are difficult to clean which can lead to contamination. They are not suitable for applications that require continuous throttling.

A check valve is used to prevent backflow in piping systems. The valve opens due to the pressure of the fluid passing through the pipeline. Reverse flow closes the valve and prevents the fluid from moving in the opposite direction. Check valves do not require external control. They work automatically and hence do not have an operating handle or stem. The simplest check valves are designed with a one-way flap mechanism.

Plug valves use a cylindrical or conically tapered plug to allow or restrict fluid flow. The plug has one or more hollow passageways that allow the fluid to pass through when the passage is aligned with the flow direction. A quarter-turn rotary motion is required to turn the plug and block the fluid flow. Plug valves perform well in slurry applications. The wiping action of the plug does not allow suspended particles to accumulate and form an obstruction. Typical applications of plug valves include gaseous, and vapor services, natural gas piping systems, oil piping systems, coal, mineral and sewage applications. They are also suitable for vacuum and high-pressure applications.

Plug valves offer a long service life and high reliability which makes them suitable for corrosive, abrasive, and toxic materials. They are usually used as on-off stop valves to provide quick bubble-tight shutoff. The valves can be repaired and cleaned without removal of the body from the piping system. Plug valves are not recommended for throttling applications and applications that require high frequency switching. Compared to ball valves, they cost more and require greater force to actuate, due to higher friction.

The “butterfly” refers to a rotatable metal disc mounted on a rod. A quarter turn of the disc is required to open or shut-off the valve. In the closed position, the disk completely blocks off the passageway. In the open position, the face of the disc is parallel to the flow direction and allows nearly unrestricted fluid flow. Butterfly valves find applications in cooling water systems, compressed air or gas applications, fire protection, slurry services, vacuum service, and high-pressure / high-temperature water and steam services.

Butterfly valves have a short circular body which reduces space requirement and makes them light weight and easier to install. The compact design also makes them suitable for large valve applications. Butterfly valves have a high coefficient of flow. They offer good sealing even at low pressure. On the other hand, butterfly valves can be difficult to clean, and the throttling function is limited to low differential pressure and unguided disc movement can be affected by flow turbulence.

Needle valves are similar in design to globe valves. The plugging element in these valves is a long, tapered, needle-like plunger which fits into a matching seat to stop the flow. A stem is turned to insert or retracted the plunger, opening or restricting the flow path. The finely-threaded stem requires many turns to fully retract the plunger; this feature allows precise regulation of the flow rate. Needle valves are typically used in small diameter piping systems where very fine control of flow is desired. Needle valves are generally suitable for systems with low viscosity fluids flowing at low flow rates. The most common application is to regulate flow to fragile gauges in order to protect them from damage caused by an abrupt pressure surge. They are also used as bleed valves to relieve pressure or to drain off the fluid for analysis or for maintenance.

Pinch valves can be used to start, stop, and regulate fluid flow. Since these valves have fewer components, they are relatively cheaper and more leak-proof as compared to other valves. These valves are ideal for pneumatic conveying of solid materials and for handling of slurries. Pinch valves minimize turbulence and friction during flow. They are, however, not suitable for gases and for high-temperature or high-pressure applications. A high pressure-differential or vacuum in the system can cause the tubing to collapse or deform resulting in incomplete opening of the valve.

A diaphragm valve, also known as a membrane valve, consists of an elastomeric diaphragm, and a saddle/seat upon which the diaphragm closes. A linear compressor is used to push the thin, flexible diaphragm into contact with the seat/saddle to close the valve. Diaphragm valves also allow partial closure of the passageway making them suitable for throttling applications. They are suitable for viscous fluids and fluids containing solid materials as the solids are not trapped in the valve mechanism. Diaphragm valves are commonly used in water treatment facilities, pharmaceutical, food and chemical plants, vacuum services and corrosive applications.

There are several different ways in which valves can be classified. They can be grouped based on their function, type of operating mechanism, end connections, mechanical motion, construction material, pressure and temperature ratings and port size. When choosing a valve, it is important to consider each of these classifications to ensure that the valve is suitable for the desired application.

Regulation valves are used for regulation of pressure and fluid flow with suitable precision. These valves can be safely and efficiently used in processes that require partial open or closed state.

Some valves are suitable to perform both functions while others are recommended only for one. The table below classifies the valves discussed above according to their function.

Valves are available in a large variety of designs and configurations. The large number of available options make valve selection a difficult task. Choosing the best valve for an application requires consideration of the required function, fluid service conditions and characteristics, frequency of operation, isolation or regulation requirements, maintenance needs, and the desired level of safety and reliability. Valve manufacturers can help you select the best valve for your application.

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