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Q: For many applications, valve sizing and selection will determine the ultimate success or failure of the larger process. In your opinion, what are some key steps end-users can take to ensure the proper sizing and selection of valves?
A: For the purpose of the following line of questions, let’s define the type of valves we are discussing. Years ago, control valves were generally thought of as throttling control valves. Today, however, many consider actuated on-off valves as control valves as well. For the sake of this discussion, I am going to assume we are considering both. Throttling control valves include the rising-stem type of valves, both soft-seated and metal-seated, and quarter-turn valves, both soft-seated and metal-seated with standard trim or characterized trim, coaxial, angle-seated, and diaphragm valves.
Once the appropriate type, or types, of valves for the application have been identified, the user should then attempt to determine the “approximate” size of the valve that is required. A user can effectively determine a rough valve size either by using an appropriate formula or sizing program.
Regarding valve selection, users should first analyze the material of construction of the valve based upon compatibility with the media being handled. If there are a number of choices, select a few and rate them regarding their relative compatibility. The end-user is generally the best source for this determination, because in the majority of cases, it is the user that has prior experience with the media being handled, and they know more about it than anyone else. For example, the users may know the piping material that has been successful in handling their particular media. Quite often, the media is a unique mixture of chemicals. Standard chemical compatibility charts may be inappropriate for materials of construction in such cases.
Know the characteristics of the media you are handling – is it a relatively easy product to handle (air, water, inert gas) or is it toxic, corrosive, abrasive, carcinogenic; does it polymerize, solidify, crystallize; does it contain fine particulates; is it a slurry, does it have a high coefficient of expansion? All of these characteristics should be considered.
With the size, the materials of construction, the characteristics and the pressure/temperature of the media, as well as the pressure class of the piping system, the end-user can now look for the appropriate type (or types) of valve to handle the application. Armed with this information, the user can approach valve vendors for guidance in choosing the appropriate valves for the application. And once the appropriate valve has been chosen, the user can proceed to the task of sizing.
Valve sizing is the procedure of determining the appropriate flow coefficient (Cv) for the valve and the application. End-users may want to calculate the Cv required or use a particular manufacturer’s sizing program (if one is available). When sizing a valve, it is very important to use accurate information and not to make assumptions.
Some manufacturers tailor their sizing programs to the type of valves they offer. Most sizing programs utilize a simplified sizing technique. These programs have certain parameter limitations. Applications that are beyond the limitations of such programs must rely on more intricate mathematical sizing calculations. Sizing programs will likely present different limitations depending on whether the media is liquid or gas/steam.* (Please see the application data provided below for accurate valve sizing.)
In the case of throttling control valves, the customer must determine how accurately the selected valve will have to produce the determined CV. This is one of the areas where valve sizing and valve selection are interdependent.
Although other factors may affect the accuracy of the valve assembly, throttling valve types generally fall in the order provided below from most-accurate to least-accurate:
1. Rising-stem valves (i.e., globe valves) with custom-machined all metal trim (plug and seat).
2. Rising-stem valves (i.e., globe valves) with custom-machined soft trim (metal plug and soft seat, metal seat with soft insert).
3. Rising-stem valves (i.e., globe valves) with standard all-metal trim (plug and seat).
4. Rising-stem valves (i.e., globe valves) with soft trim (metal plug and soft seat, metal seat with soft insert).
5. Quarter-turn valves with metal seats and characterized trim.
6. Quarter-turn valves with soft seats and characterized trim.
7. Quarter-turn valves with metal seats and standard trim.
8. Quarter-turn valves with soft seats and standard trim.
After selection and sizing, the valve assembly can be configured. Configuration includes selection of the appropriate actuator (pneumatic or electric) and/or positioner (if required and for throttling applications). The user must determine if the actuator is to be fail-open, closed, or double-acting. If there is a positioner, what is the control signal and are other accessories required (i.e. limit switches, solenoid valves, etc)? What size actuator is required to properly actuate the valve? If pneumatic, what is the air-supply pressure? If the actuator is electric, what is the voltage available? For electrical components, what is the NEMA rating required? These are some of the questions that generally should be addressed during the configuration process.
Q: In your experience, what are some of the common errors end-users make in the area of valve sizing and selection? In those applications where improperly sized valves result in performance problems, what are some of the common reasons the valve was improperly sized in the first place?
A: In many cases, users today are not familiar with the wide variety of valves available, and, as a result, inappropriate valves are commonly specified. Often, to be “safe,” a valve larger than required is chosen. This factor, along with the popularity of ball valves, has resulted in the over-specification of full-port ball valves. Ball valves are wonderful devices; they are extremely versatile and quite often very cost effective, but they are not a panacea. If a standard-port valve or a valve with a reduced Cv is acceptable, there are very cost-effective, compact, fast-acting, pneumatically operated and electrically operated valves available – some of which are available for only clean services, while others support more general services. There are also other small, fast-acting valves that can handle very high cycle rates and even support steam applications better than ball valves. If the application is corrosive, toxic, abrasive, or slurry and it polymerizes, crystallizes, and/or solidifies, a ball valve may not be the best option.
Another prevalent problem is that users often “assume” various data points in their calculations or sizing program inputs, thus producing erroneous results.
Q: Prior to engaging in the valve sizing and selection process, what application data should an end-user be sure to gather to ensure success and streamline the sizing and selection process?
The following information is generally required:
I. Incompressible Flow
a. Data required for liquids:
- Inlet Pressure
- Pressure Drop
- Specific Gravity (In the event of an application with mixed chemicals, this
is a value that depends significantly on the knowledge of the end-user)
- Vapor Pressure
- Recovery Factor
II. Compressible Flow
a. Data required for gases or vapors:
- Inlet pressure
- Pressure drop
- Specific Gravity
b. Data required for steam:
- Inlet pressure
- Pressure drop
- Saturated temperature
Q: While valve manufacturers offer a variety of tools to assist users with valve sizing and selection, it can be a little unnerving for end-users to rely on a piece of software for the specification of technology. In your opinion, how effective are today’s valve sizing and selection tools in identifying the appropriate solution for a given application?
|The latest generation of online valve configuration tools provide a simplified approach for end-users to perform such tasks as actuator selection and evaluating and selecting various valve accessories.|
An example of a simple application would be potable water flowing at 150 F. A difficult application would be a corrosive, toxic, mixed chemical media with entrained solids flowing at high pressure and temperature. The valve selection process for the latter application would be far more complicated that the selection process for the former application.
To the best of my knowledge, there are no formalized valve “selection” tools. I believe this is primarily because of the complexity of the valve selection process for a significant number of applications, as well as the number of possible valve solutions available for a broad range of application scenarios.
That said, there is, however, a relatively new class of tools for valve users and specifiers called valve configurators. Configurators are generally online tools that aim to simplify actuator selection and the addition of various accessories to allow the user to build a virtual valve assembly, create the assembly part number, and either order the assembly online or use the accumulated information for specification and/or requisition purposes.
Q: Some end-users I’ve spoken to in recent years have expressed frustration at the complexity and cumbersome nature of valve sizing and selection. How has the latest generation of specification tools evolved to provide a more intuitive and streamlined approach on valve sizing & selection?
A: I can understand the frustration of some end-users. But as I mentioned before, some applications are fairly simple, while others can be extremely complex. This is the reason I went into so much detail above. I was trying to convey the possible complexities of some applications. You have the valve selection exercise, the valve sizing exercise, and the valve assembly configuration exercise, all of which are interrelated.
I feel the valve assembly configurator offers a more intuitive and streamlined approach for end-users, but it is neither a valve selection nor sizing tool. This tool would be better described as a valve actuator sizing tool.
Nevertheless, I believe online valve configurators are the most exciting recent advance in modern valve application tools. These tools can save the user a significant amount of time by enabling quick-and-easy configuration, identification and pricing of valve assemblies. Further, configurators can be used, in many instances, to explore and compare various options in a minimum amount of time.
Q: When looking back and into the future, what are some of the key improvements you’ve seen in the way valves are sized and selected today as compared to the past, and how do you see the sizing and selection process evolving to provide added improvement going forward? How is valve sizing and selection today better than it was yesterday, and how will it be better tomorrow than it is today?
A: The only real improvement I‘ve seen in this arena is the implementation of the online valve configuration tool. I believe there is significant potential for improvement in this technology by increasing the breadth of the types of valves included.
Automated valve sizing programs were the last significant improvement in sizing, and they have been around for quite a while. I really don’t see any improvement coming except for the continued proliferation of essentially the same programs.
Quite frankly, I believe valve selection is worse today than it was yesterday. I believe that, due to the complexity of valve technologies and applications today, selection must be performed by a knowledgeable, experienced and objective specialist who is familiar with the various types of valves on the market and the media that is being handled. Such individuals can be assets to the sizing and selection process because they bring with them a solid understanding of present-day material compatibility issues. However, due to the downsizing and/or demise of our pharmaceutical, chemical and architect engineering firms, many mechanical engineers with valve expertise have been lost.
As such, I don’t see an improvement in valve selection unless or until corporations recognize the importance of valve expertise within their organizations and do something about it. Even then, I believe it will take years to develop the experience and knowledge needed.