Our first basis for selection is determined by the required purpose of the valve within the installation; therefore, our first selection is drawn within the first step by valve category:
Isolation:
We wish to block the flow across the valve when required.
Non-Return:
We wish to avoid the flow to return to the pressurized side when pressure decreases or fully extinguishes itself.
Throttling:
We wish to change the flow in terms of rate, diverting it, mixing it or operate the valve remotely.
Safety:
We wish to protect people and properties from overpressure in pressure vessels.
Air Valves and anti-surge tanks:
Specifically refereed to water, we wish to eliminate the air off the water ducts and avoid hammer strikes, which would affect the pipe systems.
Steam Traps:
We wish to eliminate condensate from the steam system thus keeping the live steam within the heating system.
The next factor of our selection would lead us to determine the correct valve design pressure, which results after plotting the working pressure and temperature. The value has to be plotted into the available common tables of pressure class standards being the following the most common ones:
| DIN |
ANSI |
| PN 10 |
Class 125 |
| PN 16 |
Class 150 |
| PN 25 |
Class 300 |
| PN 40 |
Class 600 |
| PN 64 |
Class 900 |
| PN 100 |
Class 1500 |
| PN 250 |
Clase2500 |
These typical pressure classes should not be regarded further than a helpful assistance to define as design pressure class after having plotted the effective working pressure and temperature. It should be bear in mind that maximal working Pressure decreases along with the temperature Increase and vice versa. (consult our Pressure / Temperature charts available in our technical data documentation).
Upon having defined the Valve Design Pressure, we should go for the proper valve diverse components material selection in consideration of the chemical compatibility, corrosion resistance and abrasion properties. The key valve components to be born in mind for the material selection are the following:
- Pressure retaining components, usually body and bonnet shells.
- Soft goods, usually body gaskets and stem packing units.
- Fasteners and external paint / coatings.
As for the material choice of pressure retaining parts, we should observe the process fluid compatibility against the metal parts. (our Compatibility Material selector would be helpful for this step). Other factors than chemical compatibility such as abrasion, fluid velocity, sediment media, temperature will also lead us to the proper choice of material, for example alloyed steels for temperatures higher than 400ºC, internal rubber lining for slurries and so on.
As for the packing material and soft goods, the fluid temperature would be a determining factor. Limits of temperature advised by the manufacturer should be observed in what the soft goods are concerned. (For example, do not exceed 90ºC when NBR sealing is present). There are two sort of sealing in a valve: the internal seat sealing and the external atmospheric sealing.
Last but not least, the fasteners material may be an important factor in line with environmental corrosion at the plant. The use of coatings and marine grade paints might be required for the endurance of the valve.
Valves may be operated one of the following ways:
- Manually by hand wheel or lever
- Self-acting by the fluid
- Externally powered
The operating mode selection comes determined by the actual plant operator needs for example non accessible valves, operation intervals, external power availability, economy, accuracy of control and so on being these factors dependable on each situation.
Valves are standarised to a defined Nominal Diametre in compliance with International Standards. Sizing a valve is usually carried out coming from flow rate and similar factors (refer to our sizing formulas indicated in our data sheets). When sizing a valve some aspects such as cavitation and pressure drop are necessary (see Glossary and Definitions).
The application engineer is the ultimate responsible to combine the before mentioned factors with a rational one leading to the economic criteria as well as market trends and availability. Though using the top range of materials and design pressure will ensure a trouble free valve for the application, this way would not follow rational criteria of economics.
This is precisely why the valve selection is not a simple job since a suitable combination of technical parametres with the rationalization of economics need to be set in place for the effectiveness of the task. The wide spectrum of potential valve selectors, being application engineers, plant owners, contractors, valve manufacturers and professional distributors have an arduous task in selecting the most adequate valve out of the wide choice available.
This document is devised by the Comeval Valve Systems training and school section throughout a well-structured valve selection process. It is highly recommended to support yourselves on other bibliography as well as other sections of our web site where more informative materials are available for example:
Our staff is available to answer all enquiries and Technical questions with regard to Valve Selection Through our mail box formacion@comeval.es
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