The following guidelines are provided to assist in selecting a suitable valve for any application. These guidelines are intended to cover all physical features and capabilities or limitations of different types of valves and which may be suitable for one particular application. The user must fully evaluate the pros and cons of using a particular type of valve and arrive at the most suitable selection, taking into consideration the life span expected and costs involved.
Identify Application Characteristics
Identify the system and various modes of system operation: startup, normal operation, accident condition, standby, shutdown, et cetera.
Identify the flow medium and its properties, flow rate during all modes of system operation, system design pressure, and design temperature.
Identify the pipe size, pipe wall thickness, piping material, piping joint(s) to be used, and any other information which relates to valves in the system.
Establish the code of jurisdiction, which governs the construction of the system, component, or equipment. The codes related to piping systems are discussed in Chap. A4. The applicable code (ASME B31.1, B31.3, B31.5, B31.8, B31.9, B31.11, ASME Sec. I, Sec. III, Sec. VIII, etc.) does contain requirements for valves. Become familiar with the code requirements for valves in general and for the system under consideration in particular.
Identify the valve standards referenced in the code of jurisdiction. Each code lists the valve standards that are acceptable for construction of valves to be used in piping within the jurisdiction of the code. Use valves complying with the valve standards listed in the code of jurisdiction. The most commonly used valve standards are listed in the beginning of this chapter.
Establish the pressure drop through the valve that can be critical on overall system performance. For example, the pressure drop through the stop valve(s) in the main steam system of a power facility is critical to achieve the guaranteed performance of the plant.
Select Type of Valve Required
In reference to the various modes of system operation, determine the func- tion(s) a valve has to perform.
Based upon the valve functions, the valve type(s) can be selected. The valve needed may be an isolation valve or a stop valve. What are the choices available?
Similarly, the valve required may be a check valve or the valve required to stop the flow in reverse direction. There are many different types of check valve. Which is the most suitable?
Does flow need to be throttled? Based upon the amount of throttling required, one may select a globe valve, butterfly valve, or a needle valve. Size limitations also play a role in the availability of these valves.
If flow has to be regulated and controlled based on variation in pressure, temperature, fluid level, or the design limitations of a component or equipment, the valve required would be a control valve.
The first step is to determine the valve type: isolation, check, or a control valve. If the component or the piping system is required to be protected against overpressure built up, then one of the pressure-relief devices ought to be selected.
To begin the selection process, Table A10.11 provides a good starting point.
The next step is to narrow down the choices of valves to be used. As discussed earlier, there are several different designs available in different categories of valves. For example, large-size butterfly valves are preferred to large-size gate valves as stop valves in low-pressure and low-temperature cooling water systems, due to space, weight, actuator, and cost considerations
Select Valve Size
Pipe size will indicate the valve size unless there are other requirements that may make it necessary to install a smaller- or larger-than-pipe-size valve.
The valve availability is one of those factors. In addition, some valves are not manufactured in certain small or large sizes. Refer to valve vendor catalogs and evaluate application requirements and valve features in addition to space, cost and, operational concerns.
Select Valve-End Connection
Types of piping joints to be used depend upon several considerations, such as ease of removal and replacement of components in the piping system, frequency of repairs and replacements, life span of valves, and trim items. For example, the power plants utilize welded joints, whereas the petroleum refineries and chemical plants use flanged joints predominantly. Use of threaded, soldering, and brazing joints is common in plumbing piping systems.
Check code requirements concerning piping joints. At times codes prohibit or restrict the use of different types of joints based upon size, pressure, temperature, materials of construction, flow medium, and other criteria. When leakage through joints is a concern, use of a threaded joint may be prohibited or limited by the code or by prudent engineering.
When valve-body material is different from the pipe material, transition pieces may be needed to attach the valve to piping.
To prevent galvanic corrosion between valve and pipe flanges, insulating flanges may be needed.
Select Valve-Body, Bonnet, and Trim Materials
Flow medium and its characteristics will help in selecting the valve-body and valve-trim materials. The flow-medium characteristics include: liquid, gaseous, vapor or two-phase flow, viscosity, clean fluid, dirty fluid, suspended impurities, pH- value (0 to 14), and pressure, and temperatures during different modes of system operation. Nonmetallic, stainless steel, or high-alloy piping may be utilized. Accord- ingly, valve materials should be selected keeping in mind the manufacturing limita- tions and availability of valve types and sizes required.
Flow rate will dictate requirements for the valve-flow coefficient. Should the valve be full port, standard port, reduced port, et cetera?
Valve materials for pressure-retaining parts must be in accordance with the applicable code and acceptable valve standard.
Materials for valve parts other than pressure retaining parts must be suitable for withstanding all conditions of loading and assist the valve in performing its design functions.
Identify Seat-Leakage Criteria
Determine the minimum and maximum acceptable seat leakage across the valve seat when the valve is in closed position during various modes of system operation. In some cases the seat leakage may not be of concern, while in other applications, such as in piping handling cryogenic fluids, radioactive materials (liq- uids, gases, and mixtures) or toxic and hazardous waste materials may be a serious concern and must be limited to an acceptable level.
The applicable valve standard may specify the acceptable seat leakage when the valve is tested in the shop. Alternatively, more stringent criteria may be specified. Refer to the valve standard, such as MSS SP-61 and API 598.
Identify Requirements for Valve-Stem Packing Arrangement
If the valve is to be connected to a piping system or to equipment which is continuously maintained at vacuum, the stem packing must be suitable to prevent inward air leakage. Inverted–V Teflon packing is used to temperatures up to 400°F. Graphite packing may be specified at temperatures above 400°F. Contact the valve manufacturer to ensure effectiveness of the packing.
When fugitive emission is a concern, specify the requirements for a suitable packing arrangement. In cryogenic applications the leakage of fluid across the valve seat must not come in contact with the stem packing.
Take into Consideration Maintenance Requirements
Maintenance considerations are important for the selection of valves. The plant designer must provide for access, assembly, and disassembly of valves.
Space limitations may impose restrictions on the use of a particular type of valve even though it may be the most suitable valve for the application. Select an alternative valve.
When plants are designed for long life, the valves selected must not require frequent maintenance, with the exception of items such as the replacement of packing or lubrication.
A low-initial-cost valve may necessitate frequent repairs or replacements of the valve or valve parts. Be aware of future costs involved.
High initial costs may be prohibitive. Therefore, a compromise may be made to choose the right valve for the application.
Does the valve require an actuator? If needed, select the proper actuator, keeping in mind the facilities and utilities available at the location.
Failure mode desired will dictate the type of power actuator.
Gear-actuated valves require special attention with regard to the size of the actuator. The actuator size would depend on the maximum rim pull that can be applied without use of crowbars, rods, or hammers. The rim pull may vary from as low as 50 lb (22 kgf) to 250 lb (113 kgf). Consideration must be given to the plant-operating individuals and their safety and health. A high rim pull may result in injury to an operator.
The above-stated guidelines are provided to assist the user in arriving at a reasonable solution for selecting and applying valves. They are not to be considered the only guidelines. They are for initiating the thought process and offering users critical information for making a final decision. #Little_PEng