Valves for Piping Systems

Fluids and gassed do not just flow freely through piping systems. They must be regulated and at certain points stopped. There are a number of different types of valves used in piping systems, the most common types of stop valves being:

  • Ball valve
  • Butterfly valve
  • Globe valve
  • Check valves
  • Diaphragm valve
  • Process control valves
  • Safety Relief valves

Valves may be operated manually, either by a hand wheel, or a lever or operated automatically by a pneumatic actuator or electrical drive motor. Complex control systems will use feedback from an instrument to control these types of valves to regulate pressure, temperature or flow rate depending on the control parameters required.

Ball Valve

A ball valve is a valve with a spherical center which controls the flow through it. The sphere has a hole, or port, through the middle so that when the port is in line with both ends of the valve, flow will occur. When the valve is closed, the hole is perpendicular to the ends of the valve, and flow is blocked. The handle or lever is also in line with the port through the sphere which allows the operator to know whether the valve is opened or closed.

Ball valves do not offer the fine control that may be necessary in throttling applications; however they are durable and usually work to achieve perfect shutoff even after years of disuse and are suitable for high pressures and temperatures.

Butterfly Valve

The butterfly valve like the ball valve are part of the family of quarter turn valves, i.e. they only require a quarter turn to achieve their fully open position. The butterfly valve can be used for isolating or regulating flow. The closing mechanism takes the form of a disc whose position is again indicated by the position of the opening lever. Butterfly valves are generally favored because they are lower in cost to other valve designs as well as being lighter in weight, meaning less support is required. The disc is positioned in the center of the pipe, but unlike a ball valve, the disc is always present within the flow, therefore a pressure drop is always induced in the flow, regardless of valve position. (adsbygoogle = window.adsbygoogle || []).push({});

Globe Valve

 A globe valve is a type of valve used for regulating flow in a pipeline, consisting of a movable disk-type element (the plug) and a stationary ring seat in a generally spherical body. (See section through a globe valve below. While they can be used as a shut-off valve they are not generally selected for this function alone as the baffle inside the valve restricts flow even when the valve is fully open. In a globe valve, the plug is connected to a stem which is operated by screw action in manual valves. Typically, automated valves use sliding stems. Automated globe valves have a smooth stem rather than threaded and are opened and closed by an actuator assembly. When a globe valve is manually operated, the stem is turned by a hand wheel which requires 3 to 4 complete revolutions to open or close the valve.

Check Valve

A check valve, non-return valve or one-way valve is a mechanical valve, which normally allows fluid (liquid or gas) to flow through it in only one direction. An important concept in check valves is the cracking pressure (or opening pressure) which is the minimum upstream pressure at which the valve will operate. Typically the check valve is designed for and can therefore be specified for a specific cracking pressure.

Check valves are often used before and after pumps to ensure that they do not run dry once they have been primed or to prevent a system from draining if a pump was to fail. Diaphragm pumps use a ball check valve as part of the internal workings of the pump to ensure flow goes only in one direction. Check valves are also often used when multiple gases are mixed into one gas stream. A check valve is installed on each of the individual gas streams to prevent mixing of the gases in the original source. For example, if a fuel and an oxidizer are to be mixed, then check valves will normally be used on both the fuel and oxidizer sources to ensure that the original gas cylinders remain pure and therefore non-flammable.

Diaphragm Valve

Diaphragm valves (or membrane valves) consists of a valve body with two or more ports, a diaphragm, and a “saddle” or seat upon which the diaphragm closes the valve. Because the diaphragm valve has no seal cavities for contaminants or microbes to lodge and it can withstand sanitizing and sterilizing methods this type of valve is used extensively in biotechnology and pharmaceutical industries. There are two main categories of diaphragm valves: one type seals over a “weir” (saddle) and the other (sometimes called a “straight-way” valve) seals over a seat. The main difference is that a saddle-type valve has its two ports in line with each other on the opposite sides of the valve, whereas the seat-type has the in/out ports located at a 90 degree angle from one another. The saddle type is the most common in process applications and the seat-type is more commonly used as a tank bottom valve but exists also as a process valve. While diaphragm valves usually come in two-port or three-port forms, they can also come with much more. Diaphragm valve manufacturers are now offering bespoke valve manifolds to perform clean in place and sterilize in place functions while minimizing the dead space in the valve body.

When diaphragm valves are installed on the horizontal it is important to rotate them at an angle to ensure they are free draining around the weir. There is usually a mark on the body of the valve which indicates the correct angle of rotation.

In addition to the well-known, two way shut off diaphragm valve, other orientations of diaphragm valves include: two way divert valve, zero dead leg valve, sterile access port, block and bleed and tank bottom outlet valves just to name a few. (adsbygoogle = window.adsbygoogle || []).push({});

Process Control Valves

Process plants consist of many control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temperature, etc. within a required operating range to ensure the quality of the end product. A control loop consist of 3 main elements, an instrument or sensor to measure the process variable, a controller to process this information and to direct the control valve to respond and the control valve which manipulates the process fluid to regulate the process variable as close as possible to the desired set point. It is not accurate to say that the control valve is the most important part

Of the loop. It is useful to think of a control loop as an instrumentation chain. Like any other chain, the whole chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link.

There are numerous types of control valves some of which are self-regulating, like a pressure regulator on a gas bottle while others rely on feedback from instruments for their control. Control valves are normally installed as part of a

Set of valves including local isolation valves so that the control valve can be removed for maintenance and service at regular intervals. See illustration below which identifies the main components of a steam pressure reducing station which includes the necessary isolation valves, instruments, safety valves and steam traps which are required to provide effective steam pressure reduction.

Safety Relief Valve

A safety valve is a valve mechanism for the automatic release of a substance from a system when the pressure or temperature exceeds preset limits. It is a type of valve which is part of a larger family of valves known as pressure safety

Valves (PSV) or pressure relief valves (PRV). The ASME / ANSI PTC25.3 standards applicable to the USA define the following generic terms:

Pressure relief valve – A spring-loaded pressure relief valve which is designed to open to relieve excess pressure and to re close and prevent the further flow of fluid after normal conditions have been restored. It is characterized by a rapid-opening ‘pop’ action or by opening in a manner generally proportional to the increase in pressure over the

Opening pressure. It may be used for either compressible or in compressible fluids, depending on design, adjustment, or application. This is a general term, which includes safety valves, relief valves and safety relief valves.

Safety valve – A pressure relief valve actuated by inlet static pressure and characterized by rapid opening or pop action. Safety valves are primarily used with compressible gases and in particular for steam and air services. However, they can also be used for process type applications where they may be needed to protect the plant or to prevent spoilage of the product being processed.

Relief valve – A pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure. Relief valves are commonly used in liquid systems, especially for lower capacities and thermal expansion duty. They can also be used on pumped systems as pressure over spill devices.

Safety relief valve – A pressure relief valve characterized by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application, and which may be used either for liquid or compressible fluid. In general, the safety relief valve will perform as a safety valve when used in a compressible gas system, but it will open in proportion to the over pressure when used in liquid systems, as would a relief valve.

The European standards (BS 6759 and DIN 3320) provide the following definition:

Safety valve – A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a certified amount of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent the further flow of fluid after normal pressure conditions of service have been restored. Relief valves for liquid applications are generally characterized by the relatively small size of the valve necessary to provide protection from excess pressure caused by thermal expansion. In this case a small valve is adequate because most liquids are nearly in compressible, and so a relatively small amount of fluid discharged through the relief valve will produce a substantial reduction in pressure. Relief valves for gas applications have larger connections sizes due to a gas being compressible under pressure which will expand rapidly in volume when the pressure is lowered, i.e. when a relief valve lifts. (adsbygoogle = window.adsbygoogle || []).push({});

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