| Nominal Valve Size | Flanged valves commonly use nominal pipe sizes from NPS 1/2 through NPS 24 when designed to ASME B16.5 flange dimensions. Larger sizes generally require ASME B16.47 or a project-specific standard. | The valve should suit the required flow capacity without creating excessive pressure loss. Selecting a valve only by matching the pipeline diameter may result in poor throttling control or an oversized valve. | Confirm line size, required flow range, minimum and maximum flow, available pressure drop, and the manufacturer’s flow coefficient data. |
| Pressure Class | Common ASME pressure classes are Class 150, 300, 600, 900, 1500, and 2500. | A higher class generally provides greater pressure capability, but the allowable pressure depends on the valve material and operating temperature. Class number must not be treated as a direct pressure value in bar or psi. | Select the class from the applicable pressure-temperature rating tables in ASME B16.34, the flange standard, and the project design code. |
| Design Pressure | The design pressure includes the maximum expected operating pressure plus the specified design margin and any transient pressure effects. | The selected valve rating must exceed the design pressure at the actual design temperature. Pump shutoff pressure, thermal expansion, water hammer, and relief-system conditions may govern the selection. | Use the highest credible pressure, not only the normal operating pressure, and check startup, shutdown, blocked-in, and upset conditions. |
| Operating Temperature | Pressure-temperature ratings vary with body, bonnet, trim, gasket, and bolting materials. Elevated temperature normally reduces allowable pressure. | The required pressure class may increase as temperature rises. Low-temperature service may also require impact-tested materials to prevent brittle fracture. | Check minimum, normal, maximum, and upset temperatures together with the complete material specification and pressure-temperature table. |
| Flow Capacity and Cv | For a liquid, a commonly used preliminary relationship is Cv = Q × √(SG / ΔP), where Q is flow in US gallons per minute, SG is specific gravity, and ΔP is pressure drop in psi. | The valve should operate within a controllable portion of its travel. Excessive pressure drop can cause noise, vibration, flashing, or cavitation; insufficient pressure drop can reduce control authority. | Obtain rated Cv values at several valve openings and perform detailed sizing for compressible, flashing, or cavitating fluids. |
| Fluid Type and Phase | Typical services include water, steam, thermal oil, hydrocarbons, gases, corrosive liquids, and fluids containing suspended solids. | Globe valves are suitable for isolation and throttling, but the body, trim, packing, gasket, and seat design must match the fluid’s chemical and physical properties. | Review density, viscosity, vapor pressure, corrosivity, toxicity, solids content, flashing tendency, and compatibility with all wetted materials. |
| Body and Trim Material | Common material groups include carbon steel for general industrial service, stainless steel for improved corrosion resistance, and alloy materials for demanding temperature or chemical conditions. | Material selection affects corrosion resistance, erosion resistance, temperature capability, maintenance life, and pressure-temperature rating. | Specify body, bonnet, stem, plug, seat, packing, gasket, and bolting materials separately where required by the process specification. |
| Flange Connection | Flange dimensions, bolt-hole patterns, facing, and nominal size should comply with the applicable flange standard, commonly ASME B16.5 for NPS 1/2 through NPS 24. | A matching pressure class alone does not guarantee compatibility. Facing type, outside diameter, bolt circle, bolt-hole diameter, and gasket arrangement must also match. | Confirm raised-face or ring-type-joint requirements, flange material, gasket type, bolting grade, and pipe-wall clearance before purchase. |
| Face-to-Face Dimension | Face-to-face dimensions are commonly specified using ASME B16.10 or the applicable project standard. | Incorrect face-to-face dimensions can prevent installation or require field modifications, especially when replacing an existing valve. | Measure the existing valve or verify the required dimension from the piping layout and the approved dimensional drawing. |
| Shutoff and Leakage Requirement | Seat leakage testing may be specified under API 598, ISO 5208, or another contractual testing standard. | A valve intended mainly for throttling may have different seat requirements from a valve used for critical isolation. The required leakage class should be defined before selection. | State the test standard, test pressure, allowable leakage, seat direction, and whether a metal-seated or resilient-seated design is required. |
| Cavitation, Flashing, and Noise | Cavitation may occur when liquid pressure falls below vapor pressure and subsequently recovers. Flashing occurs when vapor remains downstream. | These conditions can damage seats and trim, increase noise, and cause vibration. A standard single-stage trim may be unsuitable for high pressure-drop liquid service. | Compare inlet and outlet pressure with vapor pressure and request a noise, cavitation, and velocity assessment for severe-service applications. |
| Actuation and Operating Force | Manual handwheel operation is common for smaller or infrequently operated valves; larger or automated valves may require pneumatic, electric, or hydraulic actuation. | Differential pressure, packing friction, temperature, and valve size determine required stem thrust or torque and actuator capacity. | Check maximum unbalanced force, breakaway load, actuator safety factor, fail position, travel switches, and available utilities. |
| Installation Orientation | Globe valves are commonly installed with the flow direction indicated by the body arrow; some designs have preferred orientations for steam or special services. | Correct orientation supports proper shutoff performance, reduces erosion, and helps prevent damage to the plug and seat. | Verify the body flow arrow, stem orientation, drainability, accessibility, support requirements, and clearance for bonnet removal. |
| Testing and Documentation | Typical documentation may include material certificates, pressure-test records, dimensional drawings, and valve performance data. | Complete documentation confirms that the valve meets the specified size, rating, materials, inspection, and testing requirements. | Request certified dimensional drawings, pressure-temperature ratings, hydrostatic and seat-test results, material certificates, and maintenance instructions. |
