TYPE OF VALVES
Based on the movement of the disc, there are three main types of valves:
- LINEAR MOTION: gate, globe, diaphragm, pinch and check
- ROTARY MOTION: butterfly valves, ball, plug, eccentric- and swing check
- QUARTER TURN: valves that require approximately a quarter turn motion, from 0 to 90° of the stem to move from fully close to fully open position or vice versa.
More in detail, the main type of valves are:
Gate valves open and close the flow of the fluid through the pipeline (and belong therefore to the family of the on-off valves). The wedge of the valve is positioned between two parallel (or oblique) seats positioned perpendicular to the flow. The fluid flows horizontally in gate valves and is not subject to pressure drops. Gate valves cannot be used to modulate and regulate the flow of the fluid, as the high speed of the fluid would damage the wedge and the seats of a partly open / partly closed gate valve. The main type of gate valves in the oil & gas industry are: API 600 (cast body carbon / alloy steel), API 603 (cast body stainless), API 6D (pipeline gate valve, expanding gast / slab gate type), API 602 (forged body gate valves), pressure seal gate valves (for high pressure applications). Knife-gate valves are a last type of gate valve with a very sharp sized and machined disc. Knife gate valves are manufactured according to the TAPPI standards. In terms of construction, gate valves may have rising / non rising stems and different types of wedges (solid wedge, flexible wedge and split wedge).
Globe valves are used to regulate the fluid flowing through the pipeline (modulating valve) or in applications that require a frequent and quick opening / closing of the flow (as the movement required to close / open the valve is very short for this type of valve). Globe valves are available in various models (straight and y pattern), sizes and material grades (from cast steel to forged steel bodies) to cover the various requirements in terms of fluids, working temperatures and pressures. A frequently asked question is what is the difference between gate and globe valves: gate valves are used to close / open the flow, whereas globe valves are used to modulate the flow. The most common types of globe valves in the petrochemical industry are: BS 1873 (carbon / alloy cast body globe), API 603 (stainless steel cast body), API 602 (forged body). Pressure seal globes are used for high pressure applications.
Check valves (also named "non-return") are used to protect equipment as pumps, as they prevent potentially damaging backflows. These valves provide a tight seal as well as a fast closing in case of unwanted back-flows. The swing check type is the most common and is designed to produce a low-pressure drop in the pipeline. Compared to the API 600 type, widely used for petrochemical applications, API 603 valves are used with the corrosive media and abrasive conditions. High-pressure check valves are also available on the market, and they have pressure seal cover with different configurations. API 602 check valves (piston check, ball check) are made of forged steel. Dual plate check valves are special types that are used to protect pumps and compressors from potentially damaging backflows (they conform to the API 594 norm).
Ball valves have a spherical disc that is able to modulate the flow of a fluid in the pipeline. The ball inside the valve has a hole through which the flow passes as the hole is aligned with the two ends of the device. Otherwise, when the hole is in a perpendicular position to the ends, the flow is interrupted. The lever of the valve is aligned to the position of the ball, so the open / close status is clearly visible. The two main types are the floating ball and the trunnion mounted type. Both types can be side entry or top entry. Three-way ball valves and wafer ball valves are also available on the market. The key norm for ball valves is API 6D and BS 5351 (forged type). Ball valves, as well as butterfly valves and plug valves, belong to the quarter turn family. Stainless steel ball valves are used for corrosive pipeline applications. A specific type are the so called "double block and bleed ball valves " (DBB): they feature a double seating surface that seals the pressure on both sides of the device.
Butterfly valves are used to cut off or modulate the flow of a fluid. Manual butterfly are often preferred to other valves as they are cheaper, lighter and easier to maintain (but for some application, actuated types are also available). The key part of this type of valve, the disc, is positioned at the center of the pipe and the disc is connected via a rod to an external actuator. Differently from a ball valve, the disc is always present within the flow and creates a specific pressure drop. Resilient butterfly valves (otherwise defined “concentric”) have seats in soft material (EDPM, VITON, NBR, BUNA-N seats or similar) and have lowest pressure ratings than high-performance type (also called double eccentric). A third type of that has a metal to metal seal is the triple eccentric type (a type that competes with ball valves for some applications). Butterfly valves used in petrochemical are manufactured in accordance with the API 609 Norm.
Y-strainers and basket strainers are used to filter solids from a the fluid flowing in a pipeline by means of a perforated or wire-mesh filtering element (i.e. the strainer). These devices are relatively cheap equipment to protect downstream mechanical equipment from damages due to debris like rust, pipe scale, sediment, and other solids that flow together with the conveyed fluid. Strainers are available with both cast and forged body materials.
There are three different designs of plug valves. The short pattern design type has compact face to face dimensions and port areas accounting for 40% to 60% of a full bore valve. Regular pattern have longer face to face dimensions and a port area of 50-70% the full bore plug valves and are designed to provides minimal loss of flow. Full bore design have long face to face dimensions and a round port. This last configuration provides unrestricted flow and allows pigging operations of the pipeline. Modern plug valves are an evolution of a basic design that was developed back in the Roman empire. API 6A design is used for upstream operations.
PRESSURE RELIEF VALVE
The goal of a pressure relief valves (PRV or safety) is to release possible over pressures of specific equipment (example boilers, pressure vessels) or pipelines; they open automatically as a set pressure (or temperature) is exceeded and the overpressure is released. Pressure relief valves are available in most material grades from carbon steel to stainless steel and can manage over pressure of any fluid (hydrocarbons, gas, steam or water). One of the main components of this type of valves is the spring. PRV valves can be used as a safety valve or a relief valve depending on the application. What is the difference between a pressure relief valve and a safety valve? The term pressure relief to valves used to control overpressures in vessels that contain liquids (the opening of the valve is not sudden, but proportional to the increase of the pressure in the vessel). The term safety valve is instead used for mechanical devices that have compressed fluids or for vessels filled with gas (safety valves open all of a sudden, as the set pressure of the valve is reached). It is anyway possible that these terms are interchangeable. Safety valves are available either in the API 526 and ASME construction.
A control valve is used to control parameters such as process pressure, temperature, level, flow, pressure. These valves open or close the flow (partly or totally) on the basis of signals captured by specific sensors that compare actual to target process values. Control functions can be implemented by globe, butterfly and ball valves. Solenoid valves generally provide control functions in a small size piping or instrumentation system.
Most of the above valves, may have different types of connections with tubes and process equipment:
- flanged: connected to other piping equipment by means of single or double flanges.
- butt weld: connected to piping by welded joints
- screwed (NPT connections): the connection is in this case screwed (typical for forged valves)
- socket: connected by fillet welds