Identify Application Characteristics
2. Identify the flow medium and its properties, flow rate during all modes of system operation, system design pressure, and design temperature.
3. 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.
4. Establish the code of jurisdiction, which governs the construction of the system, component, or equipment. 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.
5. 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.
6. 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
2. 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?
3. 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?
4. 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.
5. 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.
6. 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.
7. To begin the selection process, Table A provides a good starting point.
|Table A: Valve Types and Typical Applications|
1. Only angle-globe valves can be used for a 90-degree change in direction of flow.
2. Check valves (other than the stop-check valves) stop flow only in one (reverse) direction. Stopcheck
valves can be and are used as stop, block, or isolation valves, in addition to being used as a
3. Some designs of ball-and-plug valves (contact the valve manufacturer) are suitable for throttling
4. Multiport ball-and-plug valves are used for changing the direction of flow and mixing flows.
Select Valve Size
2. 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
2. 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.
3. When valve-body material is different from the pipe material, transition pieces may be needed to attach the valve to piping.
4. To prevent galvanic corrosion between valve and pipe flanges, insulating flanges may be needed.
Select Valve-Body, Bonnet, and Trim Materials
2. Flow rate will dictate requirements for the valve-flow coefficient. Should the valve be full port, standard port, reduced port, et cetera?
3. Valve materials for pressure-retaining parts must be in accordance with the applicable code and acceptable valve standard.
4. 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
2. 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
2. When fugitive emission is a concern, specify the requirements for a suitable packing arrangement.
3. In cryogenic applications the leakage of fluid across the valve seat must not come in contact with the stem packing.
Be Aware of Piping Layout and Valve Orientation
2. If a gate valve is installed with its stem in a horizontal plane, the bonnet cavity may not fully drain after hydrostatic testing, be filled with condensed vapor when the system is shut down, or contain chemicals if the system was cleaned using chemicals. Such situations may result in damage to the valve due to pressure binding or chemical reaction. A drain or vent in the bonnet may be needed to alleviate potential problems.
Take into Consideration Maintenance Requirements
3. 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.
2. High initial costs may be prohibitive. Therefore, a compromise may be made to choose the right valve for the application.
2. Failure mode desired will dictate the type of power actuator.
3. 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.