VALVES FOR PIPELINES AND PIPING SYSTEMS
WHAT IS A VALVE?
As a general defintion, valves are devices used to control, regulate and open / close the flow and / or the pressure of a pipeline and / or a piping system. Valves are highly critical and expensive components and one of the key determining factors of pipelines reliability, safety and performance.
There are many different type of valves, most often used in combination, which perform specific functions within the piping system or the pipeline.
The main type of valves used in the petrochemical and oil & gas industry are the following:
- gate (API 600 cast gate, API 602 forged, API 603 cast stainless steel gate, API 6D expanding gate, API 6D slab gate, knife gate, pressure seal gate, etc.)
- globe (BS 1873 cast globe valve, API 603 stainless steel globe, API 602 forged globe valve, y-globe, pressure seal globe...)
- check (swing check, piston check, ball check, spring check, pressure seal check, etc.)
- ball (floating, trunnion)
- butterfly (concentric, double eccentric, triple eccentric)
- strainers (y-shape, t-shape or basket strainers)
- plug (API 6A plug, API 6D plug)
- pressure relief/safety valve
- control valves (ball, butterfly and globe type)
- cryogenic valves ">(used when the operating temperature falls below 100° F - 73° C).
- others less
Thousands of possible valve specifications are possible, originating from the combinations of mutliple parameters such as valve type, subtype, size (bore size and class), manufacturing norm (main ones are ANSI/ASME and EN/DIN), body and trim materials, type of valve operation (manual or actuated), etc. ">In terms of valve body construction, valves are classified in cast and forged type (forged bodies are used for valves below 2 inch. or for larger valves that require specific mechanical performance). Finally, valves may be operated manually (levers, wheels, gears) or via an automated actuator (the main types of actuators are: electric, pneumatic, hydraulic and gas over oil).
Standard valves are generally available on the market via distributors that stock production from global manufacturers. Custom, big-size and exotic materials valves follow instead the "make to order" manufacturing process and are sold directly from producers to end users and EPC contractors. A last distribution channel are the online valves websites, that are gaining traction since a couple of years.
FUNCTION OF VALVES IN A PIPING SYSTEM
The most common functions that can be excuted by valves are:
- Start/stop the flow of the fluid (hydrocarbons, oil & gas, steam, water, acids) through the pipeline (example: gate type)
- Modulate the flow of the fluid through the pipeline (example: globe type)
- Control the flow of the fluid (control type)
- Change the direction of the flow (example 3-way ball)
- Regulate the pressure of a process (pressure reducing)
- Protect a piping system or a device (pump, motor, tank) from overpressures (safety or pressure relief) or back-pressures (check)
HOW TO SPECIFY A VALVE
To correctly specify a valve to suppliers, the following details have to be provided:
- Valve family (example ball valve), subfamily (example ball/trunnion) and exact type (ball/trunnion/top entry)
- Bore size, generally expressed in NPS or DN (main bore and reducing bore, if applicable)
- Valve rating / class (class range from 150# to 4500#)
- Manufacturing norm (example API 6D)
- Body material / trim material / packing etc
- Testing requirements
- Type of connection (flanged, threaded, butt weld, lug and others)
- Fluid in the pipeline (>oil, gas, water, steam, solids)
- Working temperature
- Working pressure
- Delivery time
- Origin restrictions (chinese valve manufacturers / indian valve manufacturers allowed or not)