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Introduction to valves
What is a valve ?
A valve is a mechanical device that regulates the fluid in a piping system. Its function can be stopping the flow, controlling the flow rate or pressure, diverting or mixing, or preventing backflow. Basically, a valve is an assembly of a body with some kind of control element which can be moved to open or close the valve. The body is the pressure boundary which prevents leakage to the environment, while the control element avoids internal leakage when closed. The valve is connected to a piping system and we’ll discuss later the different types of connections.
There are many different types of valves, each with its specific characteristics, which have to be considered when selecting a valve type for an application. Some valve types can be more suitable for a specific application. Many valve related problems in plants are caused by wrongly selected valves.
Isolating valves and control valves
Isolation valves are also called on/off valves or block valves and normally are either fully open or fully closed. Control valves are used to control the flow and can have any position in between fully open and fully closed. Some valve types are typically used for isolation (gate valve) or regulation (globe valve), while other type can be used for both (butterfly valve, plug valve).
Valve components
               Body : this is the pressure-containing component and main part of a valve that contains the internal parts or trim, and can have many different forms. Sometimes also referred to as valve casing or shell. Metal valve bodies can be cast or forged and are available in a wide range of materials. Since the body is in direct contact with the fluid, selecting the right body material for the application is very important.
               Bonnet : The valve bonnet acts as a cover on the valve body, both can be screwed, bolted or welded together. It contains the stem , valve packing and at the top it supports the actuator. To access the valve internals for maintenance, the bonnet has to be removed. When the bonnet is welded, there will be zero leakage (no fugitive emissions), but the easy removal of the bonnet as with the screwed or bolted version is of course not possible. Between the body and bolted bonnet is a gasket to get a tight seal. Not all valves have a bonnet (like ball valves), because the construction design is different.
               Stem : The stem connects the valve closing element with the operator, being a lever, handwheel or actuator. When sizing an actuator, it must be checked that the maximum torque given by the actuator does not exceed the MAST (Maximum Allowable Stem Torque), otherwise the valve stem can by subjected to torsion and damaged. For linear motion valves, two types of stem exist : rising and non-rising. The first one will rise above the handwheel when the valve is opened, giving a clear indication of the position of the valve. When space above the valve is limited, a non-rising stem would be the preferred option.
               Packing : The packing ensures there is no leakage from the inside of the valve to the environment. It seals the space between the valve body or bonnet and the stem. It is usually a material like graphite or PTFE. The valve packing must be properly compressed to get a tight seal, but if the packing is too tight, the valve operating torque will increase and the stem can be damaged. The valve packing is one of the main sources of valve fugitive emissions.
               Trim : The internal parts of a valve that are in direct contact with the fluid are collectively referred to as valve trim. It typically includes disc, seats and stem and the term is mainly used with linear motion valves like gate and globe (control) valves. The seats can be from metal (integral to the body) , called hard seats, or of softer materials like PTFE, FKM (Viton), EPDM and are called soft seats. The hard seats usually have a small amount of leakage, while soft seats give 100% tight shutoff, but are limited in temperature and can be damaged more easily.
               Gaskets : To get a seal between two static valve parts (like two body parts bolted together), a gasket is installed. This can be in a fibrous material, graphite, PTFE or a metallic gasket like a spiral wound gasket.
The ports of a valve are the valve ends that are connected to the piping. The ports can have different forms (see valve end connections).
Most valves have two connections or “ports”, and are therefore referred to as “two-way” valves (1 inlet and 1 outlet); when the valves has multiple ports, for instance to mix of divert a flow, they are referred to as “3-way” of “4-way” valves, depending on the number of ports. Typically a three-way ball valve comes with a T- or L-shaped drilling of the ball. The L-bore allows to guide the flow from one specific port to another and block off a third. The T-port can do the same, but is more commonly used to mix (2 inlets, 1 outlet port) or divert (1 inlet, 2 outlet ports) flow.
In instrumentation, 5 ports or more are not uncommon.
Valve end connections
Valves can have different types of ends to mate with the piping in the installation.
Flanged : the most common type of valve connection, it needs a flange gasket between the valve flange and the pipe flange to ensure tight sealing. The selection of the right gasket is an important but often neglected issue , but will not be discussed here.
Threaded (also called screwed) : only used for smaller valves up to 4”, often even limited to 2”. The thread can be on the inside of the end (female thread) or on the outside (male). Threads can be either to BSP or NPT standard. Valves with threaded ends are less expensive and easy to install.
Socket weld : also used for smaller valves up to 2”(DN50); used where high pressure, high temperature or dangerous fluids are encountered. In a socket weld, the pipe is inserted into the socket of the valve, backed off slightly to leave a space between the end of the pipe and the bottom of the socket, and the weld is made around the outside diameter of the socket to the outside diameter of the pipe. The gap at the bottom of the pipe prevents thermal expansion from stressing the joint during or after welding. Care should be taken to see that the valve body material is compatible with the adjoining pipe material.
Butt weld : The ends of a valve with butt weld ends are beveled to match a similar bevel on the connecting pipe of corresponding size and schedule (wall thickness). The angle machined to both ends of valve and pipe are defined, so when they are placed together face-face, there is a circumferential V shaped gap serving as groove to put the weld material in.
Both socket weld and butt weld connections are 100% leak-tight , but valves cannot be replaced easily.
Flangeless : Flangeless valves have no flange connection as part of the body but are simply bolted or clamped between the adjoining pipeline flanges. They are typically used with butterfly valves, but also with check valves, ball valves and rupture discs. Two main types exist : “wafer” or “lug” style. The latter one can also be used when connected to only one pipe flange, referred to as an end of line duty.
Sanitary(Clamp) ends : Sanitary, or “clamp” valve ends have a flat face (a bit similar to a flange) with an o-ring groove that is suitable for mating with another clamp (or “ferrule”) end. This type of end connection is very commonly used with sanitary/food applications because it is easy to remove from the line for cleaning, repair or replacement.
Reduced bore and full bore
A full bore valve has an orifice diameter similar to the end connections, giving the valve maximum flow capacity, low pressure drop and is used when “pigging” is required.
A reduced bore valve has an orifice diameter which is smaller than the end connection which leads to lower capacity, higher pressure drop, higher flow velocity (causing wear) and pigging is no longer possible. Reduced bore valves have a lower price.
These terms are mostly used in association with ball valves.
Valve actuation/Operators
Based on their way of operation, valves can be divided roughly in two categories : multi-turn valves (also called linear motion valves) and quarter-turn valves (also called rotational motion valves).
With multi-turn valves, a handwheel is turned a certain number of turns to move the closure element up and down in a linear motion to open or close the valve.
The quarter turn valves are operated by a lever and require a quarter turn (90°) of the stem to operate the valve from fully open to fully closed, as in ball valves, butterfly valves and plug valves. Some (three or four-way) ball valves can be operated to half a turn (180°), but for ease, they are also classified as quarter-turn valves.
There are also valves that are self-operated and perform their function totally independent , like check valves, safety valves, float valves and steam traps.
Valves can be automated for different reasons with electric, pneumatic or hydraulic actuators. The first two are the most common in the industry.
Pressure ratings/ pressure class (PN)
The valve rating is determined by the type of service, material used for the valve, maximum line pressure and temperature.
The maximum allowable pressure decreases with rising temperature and this curve depends on the material used. However, it should be noted that these maximum pressure/temperature limits are only valid for the body material and operating conditions of the valve can be limited by other materials used in the valve, like PTFE seats or EPDM O-rings.
Two classification systems are used in the valve world : ASME and ISO/DIN
ASME Class 300 is defined as a “dimensionless number indirectly related to the pressure-retaining ability as the function of temperature of the component.” At the beginning of the development of this rating system, the number referred to the maximum allowable pressure at 750°F (except class 150). Typical class ratings are a fixed set of numbers : 150, 300, 600, 900,1500, 2500 and 4500 (and class 800 for small forged valves to API 602)
In the ISO(DIN) classification system , pressure class is referred to as PN followed by a number of a fixed set of numbers : 6, 10, 16, 20, 25, 40, 50, (64), 100, 150(160), 250, and up.
PN means “Pressure Nominale” and the number indicates the maximum cold working pressure ratings in bar.
Valve size -  NPS / DN
The Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes and valves. Valve size is specified with a non-dimensional number: the Nominal Pipe Size (NPS) for diameter based on inches (i.e. NPS 2”).
The European designation equivalent to NPS is DN (Diamètre Nominal/nominal diameter). The nominal diameter is specified according to EN ISO 6708 using the abbreviation DN (Diameter Nominal) followed by a dimensionless number corresponding to the approximate internal diameter in millimeters (i.e. DN50). The term NB (nominal bore) is also sometimes used interchangeably with NPS.
Valve body materials
Valve bodies can be made of different metals like brass, bronze, cast iron, steel, alloy steels and stainless steels and other exotic alloys. Valves can also be made from thermoplastic materials like PVC, PP, PE and PVDF and are used for acids at low pressure/temperature application because of their excellent corrosion resistance.
The final choice of valve body material is a critical one and must be based on application conditions.
Valve selection
The first three things to know are the operating conditions : fluid, pressure and temperature
Further questions should be asked to eliminate options and find the right valve type:
- required function : on/off (àisolation or block valve)or throttling (àcontrol or regulating valve)
- frequency of operation
- weight (support requirements) and size (when limited space is available, like on a ship)
- availability
- cost
- special conditions : pigging requirements, cryogenic (NACE), fire-safe, offshore protection, special testing requirements, …
- automation : some valve types are easier/cheaper to automate than others
- field serviceability
- environmental requirements (fugitive emissions)
- operating time : a quarter turn ball valve closes much quicker than for instance a gate valve
- …
This is just a summary of one chapter of a soon to be released book about valves where each subject is discussed in much more details. Interested? Contact us for more information.
© 2013 - Gope Media Group

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