Introduction of ball valves, globe valve
The appearance of a ball valve was after the Second World War. Although the invention of a ball valve dates back to the early 20th century, this structural patent failed to complete its commercialization steps due to limitations in the materials industry and the mechanical processing industry. DuPont in the United States invented a high polymer material polytetrafluoroethylene (PTFE) plastic until 1943. This kind of material has advantages of sufficient tensile and compressive strength, certain elastoplasticity, good self-lubricating properties and excellent corrosion resistance, which is very suitable as a sealing material and has very reliable sealing effect. In addition, a ball with high roundness and good surface finish can be manufactured as a closure member of a ball valve due to the development of ball grinding machines. A new type of valve with full bore and 90° rotatory angular travel enters the valve market, drawing much attention. Traditional valve products such as stop valves, gate valves, plug valves and butterfly valves are gradually replaced by ball valves, and ball valves are greatly used, ranging from small diameters to large diameters, low pressure to high pressure, normal temperature to high temperature, high temperature to low temperature. At present, the maximum diameter of the ball valve has reached 60 Inch, and the lowest temperature can reach liquid hydrogen temperature -254℃.The highest temperature can reach from 850 to 900℃. All of these make ball valves suitable for all kinds of media, which become the most promising type of valve.
Ball valves can be divided into floating ball valves and trunnion ball valves based on structure.
Ball valves can be classified into top entry ball valves and side entry ball valves. Side entry ball valves can also be divided into one piece ball valves, two-piece ball valves and three-piece ball valves according to the structure of the valve body. One piece ball valves’ valve bodies are integral; two-piece ball valves consist of main valve bodies and auxiliary valve bodies and three-piece ball valves are composed of one main valve body and two auxiliary valve bodies.
Ball valves can be classified into soft sealing ball valves and hard sealing ball valves according to the valve sealing material. Sealing materials of soft sealing ball valves are high polymer materials such as polytetrafluoroethylene (PTFE), reinforced polytetrafluoroethylene and nylon as well as rubber. Sealing materials of hard sealing ball valves are metals.
A Globe valves is a linear motion valve and are primarily designed to stop, start and regulate flow. The disk of a Globe valve can be totally removed from the flowpath or it can completely close the flowpath.
Conventional Globe valves may be used for isolation and throttling services. Although these valves exhibit slightly higher pressure drops than straight=through valves (e.g., gate, plug, ball, etc.), they may be used where the pressure drop through the valve is not a controlling factor.
Because the entire system pressure exerted on the disc is transferred to the valve stem, the practical size limit for these valves is NPS 12 (DN 300). Globe valves larger than NPS 12 (DN 300) are an exception rather than the rule. Larger valves would require that enormous forces be exerted on the stem to open or close the valve under pressure. Globe valves in sizes up to NPS 48 (DN 1200) have been manufactured and used.
Globe valves are extensively employed to control flow. The range of flow control, pressure drop, and duty must be considered in the design of the valve to avert premature failure and to assure satisfactory service. Valves subjected to high-differential pressure-throttling service require specially designed valve trim.
Generally the maximum differential pressure across the valve disc should not exceed 20 percent of the maximum upstream pressure or 200 psi (1380 kPa), whichever is less. Valves with special trim may be designed for applications exceeding these differential pressure limits.
Applications of the globe valve
Globe valves used for high temperature and high voltage power stations
The valve body and the valve bonnet of globe valves used for high temperature and high voltage power stations adopt self sealed pressure or clamp connections. The connection between the valve body and the pipeline is butt welding connection. The valve body mostly is made of chrome molybdenum steel or chrome molybdenum vanadium steel, and the sealing surface is welded by hard alloys. Therefore, the globe valves are resistant to high temperature, high pressure and heat; the sealing surface of the globe valve is wear, scratch and corrosion resistance, and the globe valves have good sealing performance and long service life. All of these make the globe valves very suitable for high temperature and high pressure water, steam, oil and superheated steam pipelines in thermal power industrial systems, petrochemical systems and metallurgical industries.
Globe valves used for oxygen pipelines
Globe valves used for oxygen pipelines are strictly design according to the requirements of the oxygen pipeline. The outer part of the stuffing box is strictly sealed so as to prevent external dirt from entering the stuffing box. There are grounding devices on both ends of flanges of valve bodies. After the pipe is installed, ground wire should be connected in order to prevent static ignition. Austenitic stainless steel or copper alloy is often selected for the housing material of the globe valve, which makes the globe valve have good electrical conductivity and not easily generate static ignition. The sealing pair’s material is PTFE which is good for the valve body’s material. The sealing performance is good, and the leakage rate is zero after inspection. The globe valves are mainly applied to oxygen pipelines in metallurgical systems as well as other industries.
Globe valves used for liquefied petroleum gas
Globe valves used for liquefied petroleum gas are designed for pipelines or devices of liquefied petroleum gas. The structure of globe valves used for liquefied petroleum gas should confirm to fireproof requirements. The packing box is made of polytetrafluoroethylene, which is reliable in sealing and has no external leakage. The sealing pair’s material is made of Teflon or nylon which is good for the valve body’s material, and the sealing is reliable. The globe valves are suitable for pipeline systems of liquefied petroleum gas, as an opening and closing device, and they can also be used as shut off valves for other pipelines whose media’s temperature is lower than 80℃.
Globe valves with upper threaded stems
Globe valves with upper threaded stems are not in direct contact with media, and they are used for different working conditions based on different materials of housing and inner parts. If the housing is made of carbon steel, the sealing pair should be made of alloy steel, the packing box flexible graphite and the valve stem Cr13 stainless steel. This kind of globe valve is used for water, steam and oil pipelines. The housing adopts 12Cr18Ni9 or 06Cr19Ni10, and the sealing pair’s material is the same as the valve body’s material or hard alloys; the packing box is made of PTFE, and the valve stem is made of 1Cr18Ni12Mo2Ti. This kind of globe valve is applied to pipelines or devices which carry corrosive media, mainly acetoxy. The maximum nominal size of the upper threaded stem globe valve is generally DN200. It is best to install internal or external bypass valves for the upper threaded stem globe valve whose nominal size is above DN200.
Connection of valves
Usually valves have 7 types of connection, they are as bellow:
1. Flange connection: Flange connections are the most common form of connection between valves and piping or equipment. It refers to the detachable connection of the flange, the gasket and the bolt as a set of combined sealing structures. Flange connection can be applied to valves of various nominal sizes and nominal pressures, but there are certain restrictions on the use temperature. In high temperature conditions, the flanges will be prone to creep and cause leakage. Under normal circumstances flange connection is recommended for use at temperatures below 350 °C.
According to the shape of the joint surface, it can be divided into the following types: FF, RF, RTJ.
2.Wafer connection: A connection form in which the valve and the two pipes are directly clamped together by bolts.
3.The welded connection refers to a connection form in which the valve body has a welding groove and is connected to the piping system by welding. API600, ASME B 16.34 and other standards have specified welding bevels. The welded connection between the valve and the pipe is divided into a butt weld connection (BW) and a socket weld connection (SW). Socket weld end shall comply with the provisions of JB/T 1751. Butt weld connection (BW) can be applied to various sizes, various pressures and high temperature conditions. Socket weld connection (SW) is generally suitable for valves of ≤ DN50.
4.NPT connection: This is a simple connection method often used for small valves.
7.Internal self-tightening connection