OIL & GAS VALVES
This article gives some basic information about oil & gas valves (used for pipelines, process piping and, other petrochemical applications). We also indicate how valves should be specified and ordered to valves manufacturers.
WHAT ARE VALVES?
As a general definition, 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 determining factors for oil & gas pipelines reliability, safety, and performance.
Based on the movement of the disc, there are three main families of valves:
- LINEAR MOTION: gate, globe, diaphragm, pinch, and check valves
- ROTARY MOTION: butterfly valves, ball, plug, eccentric- and swing check valves
- 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.
Based on their body construction, valves are divided into:
- CAST (the body is obtained by casting steel)
- FORGED (the body is manufactured by forging steel)
Valves may be manually operated (via levers, wheels, gears) or actuated (via electromechanical devices, called actuators, that may be electric, pneumatic, hydraulic and gas over oil).
TYPES OF OIL & GAS VALVES
The valves listed below account for approx 80% of the total volumes of industrial valves sales in the world (a market of approximately 50 billions USD per year). Given their importance, we have dedicated a specific page of our knowledge base to each of them, which you can ready by following the links.
- GATE VALVES: read here
- GLOBE VALVES: read here
- CHECK VALVES: read here
- BALL VALVES: read here
- BUTTERFLY VALVES: read here
- STRAINERS: read here
- CONTROL VALVES: read here
Less common valves are:
There are three different designs of plug valves. The short pattern design type has a compact face to face dimensions and port areas accounting for 40% to 60% of a full-bore valve. The regular pattern has a longer face to face dimensions and a port area of 50-70% the full-bore plug valves and is designed to provide minimal loss of flow. Full bore design has 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 AND SAFETY VALVES
The goal of a pressure relief valves (PRV or safety) is to release possible overpressures 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 overpressure 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 any way possible that these terms are interchangeable. Safety valves are available either in the API 526 and ASME construction.
For each type of valve, hundreds of possible configurations are actually available (based on the combinations of parameters as valve type, sub-type, bore size, pressure rating, manufacturing norm, body and trim materials, valve operation, etc).
Valves are used to:
- 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 a 3-ways 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)
VALVE SELECTION (PETROCHEMICAL)
The downloadable presentation is a useful guideline on how to select the correct valve type based on the application.
PARTS OF OIL & GAS VALVES
Valves are rather complex mechanical devices, consisting of multiple parts and components:
BODY OF THE VALVE
The bonnet is another important part of most valves. When dismantled from the body, it allows the access to the internal components of the valve to execute maintenance activities or replacement of parts (such as the disc, seat, stem, etc). Valve bonnets are available in many designs and models (the most typical are: bolted, round bolted, welded, pressure seal) and are manufactured in cast steel or forged steel (generally using the same material grade of the body). The bonnet is connected with the body by a threaded, bolted, or welded joint connections – and gaskets are used in between the body and the bonnet. Manufacturers strive to optimize the design of this part (to reduce the overall dimension of the device and to ensure that the connection is tight and leakage-proof).
The valve trim is a collective name for all the internal parts of the valve that can be removed and replaced and that have a direct contact with the fluid flowing through the pipeline. Typically, the trim includes components as the disc, the seat, the stem, the glands, the bushings and the sleeves needed to guide the stem (but the specific elements that constitute the trim of a valve depends on the valve type). The trim is a fundamental component of the performance and the operation of the valve and the trim materials combination shall be chosen with care. The most used trim combinations for gate, globe and check valves are defined by the API trim chart.
The disc is the part of the valve that opens, closes or modulate the flow of the fluid within the pipeline, depending on its relative position to the seat. In the case of a gate valve, the gate (wedge) is the disc of the valve (whereas the disc is called “ball” for ball valves).
After the body and the bonnet, the disc is the third most important element that determines the performance of the valve and its tight seal. Discs are generally manufactured in forged steel and are often reinforced (hard faced) to enhance the mechanical properties of the base material.
Seats accommodate the movements of the disc, and valves may have one or multiple seats. Globe or a swing-check valve are fitted with one seat that, in connection with the disc, creates the seal that interrupts the flow of the fluid. Gate valves have instead two seats, one on the upper side and the other on the lower side of the valve. Seats are often hard-faced. A proper finish of the seats is required to ensure a good seal when the valve is in the closed position.
The stem of a valve is used to maneuver the valve (open/close) because it moves the disc inside the valve. The stem is linked to the valve actuator or to the manual handwheel (or lever) at one end and is connected to the valve disc on the other end. In the case of gate and globe valves, the stem exercises a linear motion on the disc, whereas for a ball, butterfly and plug valves the disc rotates to open or close the valve (therefore such valves are called “quarter turn valves”). Stems are made of forged steel and are connected to the disc by threading or other means. To prevent the leakages of the valve, a proper finish of the stem surface is necessary.
There are five types of valve stems:
- Rotary Stem: This is a standard type of the ball, plug, and butterfly valves. A quarter-turn motion of the stem opens or closes the valve.
- Sliding Stem: In this case, the stem does not execute any rotation. The stem slides in and out the valve to open or close it. This design is common in hand-operated lever rapid opening valves. It is also used in control valves are operated by hydraulic or pneumatic cylinders.
- Rising type with outside screw and yoke (“OS&Y”): the external side of the stem is threaded while the part of the stem which is inside the valve is plain. The threads of the stem are isolated from the medium by the packing. Two alternative designs are available. The “OS&Y” design is common for valves above 2″.
- Rising type with an inside screw (“IS&Y”): The threaded part of the stem is positioned inside the valve body, whereas the stem packing lays outside. With this design, the stem threads are in touch with the medium flowing through the pipeline. Once rotated, the stem and the hand wheel rise together and open the valve.
- Non-rising stem type with inside screw: The threaded part of the stem is inside the valve and does not rise. The valve disc floats on the stem, like a nut if the stem is rotated. Stem threads are in contact with the media of the pipeline, and as such, may be exposed to its corrosive impact. This is the reason why such design is used when the available space to position the valve is too narrow to permit linear movement, and the media does not cause erosion, corrosion or abrasion of the stem material.
The gasket that seals the stem with the bonnet is called packing, and comprises the following components:
- Gland follower, which is a sleeve that compresses the packing, by a gland into the stuffing box.
- Gland, a type of bushing, which compresses the packing into the stuffing box.
- Stuffing box, a chamber in which the packing gets compressed.
- Packing, available in different materials, like PTFE, elastomers, fibrous material, etc.
- A backseat is a seating inside the bonnet. The back seat provides a seal between the stem and bonnet and prevents system pressure from building against the valve packing once the valve is fully open. Back seats are often used in gate and globe valves.
- The valve packaging shall be properly designed and manufactured to minimize the possible damages to the stem and minimize the risk of leakages of fluids. On the other hand, it is necessary to observe that a too tight packing may affect the stem.
A hand operated or manual valve is generally equipped with a hand wheel that can be rotated clockwise or counter-clockwise to open and/or close the valve (typical for gate and globe valves). Ball, plug or butterfly are actuated using a lever (manual quarter turn valves).
In the following cases, it is not either possible nor advisable to use manual valves:
- Large dimension valves that operate at high pressures
- Valves that need to be controlled from a remote location
- Valves that require, for the nature of the process, a very fast open or close operation
In all these cases, a valve actuator is needed. The actuator produces linear and rotary motion able to open or close a valve (the actual movement depends of course on the type of the valve, linear or quarter turn). The main types of valves actuators are:
- Gear Actuators
- Electric actuators
- Pneumatic actuators (pneumatic control valves use this type frequently)
- Hydraulic actuators
- Gas over oil actuators
- Solenoid Actuators
Rotork actuators and Auma actuators have the largest market shares within the petrochemical industry. Actuators are costly parts of valves.
HOW TO ORDER OIL & GAS VALVES
To specify a valve to suppliers correctly, 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)
EXAMPLE: HOW TO ORDER GATE, GLOBE, CHECK VALVES
Each manufacturer has own valves ordering sheets that map the valve configuration parameters that user has to consider:
GS – F – 6″ / 150 – 316 – B
1 2 3 4 5
|1. Valve type||2. End type||3. Size / Class||4. Body Material||5. Options|
|C: Check Valve
CL: Lift Check Valve
CS: Check pressure Sealed Valve
CW: Swing Check Valve
G: Gate Valve
GG: Forged Gate Valve
GL: Light Type Gate Valve (API 603)
GS: Gate Pressure Sealed Valve
O: Globe Valve
OB: Globe Bellowed Sealed Valve
OS: Globe Pressure Sealed Valve
|F: Flanged End
T: Threaded End
W: Butt Weld End
S: Socket Weld End
|Size: NPS 1/2 – 80″
10K: JIS 10K
|GG: Forged Gate Valve
316: Casting S.S CF8M
304: Casting S.S CF8
F316: Forgings S.S F316
F304: Forgings S.S F304
WCB: Steel WCB
LCB: Steel LCB
HB: Hastelloy B
G: Gear Operator