Valves can be found just about anywhere today: in our homes, under the street, in commercial buildings and in thousands of places within power and water plants, paper mills, refineries, chemical plants and other industrial and infrastructure facilities.
The valve industry is truly broad-shouldered, with segments varying from water distribution to nuclear power to upstream and downstream oil and gas. Each of these end-user industries use some basic types of valves; however, the details of construction and materials are often very different. Here’s a sampling:
In the world of water distribution, the pressures are almost always relatively low and the temperatures ambient. Those two application facts allow a number of valve design elements that would not be found on more challenged equipment such as high- temperature steam valves. The ambient temperature of water service allows use of elastomers and rubber seals not suitable elsewhere. These soft materials allow water valves to be equipped to tightly seal off drips.
Another consideration in water service valves is choice in materials of construction. Cast and ductile irons are used extensively in water systems, especially large outside diameter lines. Very small lines can be handled quite well with bronze valve materials.
The pressures that most waterworks valves see are usually well below 200 psi. This means thicker- walled higher-pressure designs are not needed. That having been said, there are cases where water valves are built to handle higher pressures, up to around 300 psi. These applications are usually on long aqueducts close to the pressure source. Sometimes higher-pressure water valves also are found at the highest-pressure points in a tall dam.
The American Water Works Association (AWWA) has issued specifications covering many different types of valves and actuators used in waterworks applications.
The flip side of fresh potable water going into a facility or structure is the wastewater or sewer output. These lines collect all the waste fluid and solids and direct them to a sewage treatment plant. These treatment plants feature a lot of low pressure piping and valves to perform their “dirty work.” The requirements for wastewater valves in many cases is much more lenient than the requirements for clean water service. Iron gate and check valves are the most popular choices for this type of service. Standard valves in this service are built in accordance with AWWA specifications.
Most of the electric power generated in the United States is generated in steam plants using fossil-fuel and high-speed turbines. Peeling back the cover of a modern power plant would yield a view of high-pressure, high-temperature piping systems. These main lines are the most critical in the steam power generation process.
Gate valves remain a main choice for power plant on/off applications, although special purpose, Y-pattern globe valves are also found. High- performance, critical-service ball valves are gaining popularity with some power plant designers and are making inroads in this once linear-valve-dominated world.
Metallurgy is critical for valves in power applications, especially those operating in the supercritical or ultra-supercritical operating ranges of pressure and temperature. F91, F92, C12A, along with several Inconel and stainless-steel alloys are commonly used in today’s power plants. Pressure classes include 1500, 2500 and in some cases 4500. The modulating nature of peak power plants (those that operate only as needed) also puts a huge strain on valves and piping, requiring robust designs to handle the extreme combination of cycling, temperature and pressure.
In addition to the main steam valving, power plants are loaded with ancillary pipelines, populated by a myriad of gate, globe, check, butterfly and ball valves.
Nuclear power plants operate on the same steam/high-speed turbine principle. The primary difference is that in a nuclear power plant, the steam is created by heat from the fission process. Nuclear power plant valves are similar to their fossil-fueled cousins, except for their pedigree and the added requirement of absolute reliability. Nuclear valves are manufactured to extremely high standards, with the qualifying and inspection documentation filling hundreds of pages.
OIL AND GAS PRODUCTION
Oil and gas wells and production facilities are heavy users of valves, including many heavy-duty valves. Although gushers of oil spewing hundreds of feet in the air are no longer likely to occur, the image illustrates the potential pressure of underground oil and gas. This is why well heads or Christmas trees are placed at the top of a well’s long string of pipe. These assemblies, with their combination of valves and special fittings, are designed to handle pressures upwards of 10,000 psi. While rarely found on wells dug on land these days, the extreme high pressures are often found on deep offshore wells.
Wellhead equipment design is covered by API specifications such as 6A, Specification for Wellhead and Christmas Tree Equipment. The valves covered in 6A are designed for extremely high pressures but modest temperatures. Most Christmas trees contain gate valves and special globe valves called chokes. The chokes are used to regulate the flow from the well.
In addition to the wellheads themselves, many ancillary facilities populate an oil or gas field. Process equipment to pre-treat the oil or gas requires a number of valves. These valves are usually carbon steel rated for lower classes.
Occasionally, a highly corrosive fluid—hydrogen sulfide—is present in the raw petroleum stream. This material, also called sour gas, can be lethal. To beat the challenges of sour gas, special materials or material processing techniques in accordance with NACE International specification MR0175 must be followed.
The piping systems for offshore oil rigs and production facilities contain a multitude of valves built to many different specifications to handle the wide variety of flow control challenges. These facilities also contain various control system loops and pressure relief devices.
For oil production facilities, the arterial heart is the actual oil or gas recovery piping system. Although not always on the platform itself, many production systems use Christmas trees and piping systems that operate in the inhospitable depths of 10,000 feet or more. This production equipment is built to many exacting American Petroleum Institute (API) standards and referenced in several API Recommended Practices (RPs).
On most large oil platforms, additional processes are applied to the raw fluid coming from the wellhead. These include separating water from the hydrocarbons and separating gas and natural gas liquids from the fluid stream. These post-Christmas tree piping systems are generally built to American Society of Mechanical Engineers B31.3 piping codes with the valves designed in accordance with API valve specifications such as API 594, API 600, API 602, API 608 and API 609.
Some of these systems may also contain API 6D gate, ball and check valves. Since any pipelines on the platform or drill ship are internal to the facility, the strict requirements to use API 6D valves for pipelines do not apply. Although multiple valve types are used in these piping systems, the valve type of choice is the ball valve.
Although most pipelines are hidden from view, their presence is usually evident. Small signs stating “petroleum pipeline” are one obvious indicator of the presence of underground transportation piping. These pipelines are equipped with many important valves all along their length. Emergency pipeline shutoff valves are found at intervals as specified by standards, codes and laws. These valves serve the vital service of isolating a section of a pipeline in case of a leak or when maintenance is required.
Also scattered along a pipeline route are facilities where the line emerges from the ground and line access is available. These stations are the home for “pig” launching equipment, which consists of devices inserted into the pipelines either to inspect or clean the line. These pig launching stations usually contain several valves, either gate or ball types. All of the valves on a pipeline system must be full-port (full-opening) to allow for the passage of pigs.
Pipelines also need energy to combat the friction of the pipeline and maintain the pressure and flow of the line. Compressor or pumping stations that look like small versions of a process plant without the tall cracking towers are used. These stations are home to dozens of gate, ball and check pipeline valves.
The pipelines themselves are designed in accordance with various standards and codes, while pipeline valves follow API 6D Pipeline Valves.
There are also smaller pipelines that feed into houses and commercial structures. These lines provide water and gas and are guarded by shutoff valves.
Large municipalities, particularly in the northern part of the United States, provide steam for heating requirements of commercial customers. These steam supply lines are equipped with a variety of valves to control and regulate the steam supply. Although the fluid is steam, the pressures and temperatures are lower than those found in power plant steam generation. A variety of valve types are used in this service, although the venerable plug valve is still a popular choice.
REFINERY AND PETROCHEMICAL
Refinery valves account for more industrial valve usage than any other valve segment. Refineries are home to both corrosive fluids and in some cases, high temperatures.
These factors dictate how valves are built in accordance with API valve design specifications such as API 600 (gate valves), API 608 (ball valves) and API 594 (check valves). Because of the harsh service encountered by many of these valves, extra corrosion allowance is often needed. This allowance is manifested through greater wall thicknesses that are specified in the API design documents.
Virtually every major valve type can be found in abundance in a typical large refinery. The ubiquitous gate valve is still the king of the hill with the largest population, but quarter-turn valves are taking an increasingly large amount of their market share. The quarter-turn products making successful inroads in this industry (which was also once dominated by linear products) include high performance triple offset butterfly valves and metal-seated ball valves.
Standard gate, globe and check valves are still found en-masse, and because of the heartiness of their design and economy of manufacturing, will not disappear any time soon.
Pressure ratings for refinery valves run the gamut from Class 150 to Class 1500, with Class 300 the most popular.
Plain carbon steels, such as grade WCB (cast) and A-105 (forged) are the most popular materials specified and used in valves for refinery service. Many refining process applications push the upper temperature limits of plain carbon steels, and higher-temperature alloys are specified for these applications. The most popular of these are the chrome/moly steels such as 1-1/4% Cr, 2-1/4% Cr, 5% Cr and 9% Cr. Stainless steels and high-nickel alloys are also used in some particularly harsh refining processes.
Post time: Jul-10-2020