Definition according to the Valve Manufacturers Association of America. :
A valve is a mechanical device that turns on and off, regulates, modulates or isolates the rate, volume, pressure or direction of liquids, gases, slurries or dry materials through a pipeline, chute or similar passageway.In General:
Valve is a very important piping component which plays a vital role in the safe ,economic design of a plant.
To understand the importance and the function of a valve one could very take the example of the valve present
in the heart of a human body.
Note: this discussion is only for the understanding the importance and functions of a valve in a system.This article will NOT present and CLAIM any authentic medical procedure and functions of a heart. The author is not a doctor and has no medical knowledge as such.
Our body gets energy from fresh oxygenated blood which should be supplied from lungs .Hence to do that our heart which acts as a pump to supply blood to all parts of the body with balanced pressure range works continuously without stopping from birth until death. To avoid any backflow of pure blood into impure blood while pumping the blood, a device to check the flow is present in the heart which are called heart valves. This acts like a check valve(non-return valve).It also regulates the flow of blood to have a uniform pumping pressure of the blood .
There are four valves in the heart which are all non-return valves (check valves). These valves are very thin like tissue paper and are constantly opening and closing .This sudden closing and opening of valve will produce heart beats.These are the famous LUB-DUB sounds of the heart or also called heart beats which are actually the sounds of the valve closing and opening.
Every day, heart beats around 100,000 times, sending 2,000 gallons of blood circulatiog through the body. It’s similar to the size of the fist Heart has the amazing job of keeping blood flowing through the 60,000 miles of blood vessels that feed the organs and tissues. Any damage to the heart or its valves can reduce that pumping power, forcing the heart to work harder just to keep up with the body’s demand for blood..If this valve malfunctions then the valve will block the supply of blood which is indeed fatal for the survival of human being.
So as we see that the primary function and importance of the valve is designed by the nature itself.
Similarly valves play a vital role in the working of any process system.The location of the valve,type,rating are very important in the safe design of a plant.
Four main functions of a valve
TYPICAL VALVE USAGE IN POWER INDUSTRY
The following are some applications where valves are used to accomplish the system design objective.
A. To start and stop fluid flow as required by the process, e.g., sluice water flow for bottom ash conveying.
B. To modulate fluid flow as required by the process, e.g., deaerator storage tank level control.
C. To balance the flow between a number of parallel flow paths, e.g., cooling water discharge from lube oil coolers.
D. To fill and/or vent the system during startup, e.g., boiler filling valves or high point vents.
E. To drain the system after shutdown or to carry out repairs, e.g., equipment drains.
F. To prevent over pressure and potential failure of equipment, e.g., safety valves
G. To isolate equipment or control valves for repairs or maintenance, e.g., block valves upstream and downstream of a heater and a by pass valve.
H. To reduce consequences of pipe breaks (i.e., insurance against an unexpected but possible event), e.g., isolation valve at a branch line or sectionalizing valves in a fire water loop.
I. Provisions for future additions, e.g., valves at connection points for future additions.
J. Instrument isolation, e.g., instrument root valves.
K. Connections for purging the equipment prior to repairs, e.g., such as steam in and steam out.
Valves in petrochemical and refining installations are subject to numerous standards and specifications issued by many supporting organizations. Today’s valve standards are dynamic documents that reflect sound engineering practice, changes in market demands and changes in technology and manufacturing procedures.But the history of Valve industry standards was driven by feedback from the costomers-be it years of effective service life, catastrophic failure or incompatibility with similar products from other manufacturers.There was no standerdisation of valves as such.
Standerdisation of valves begin during the same period when boilers were extensively used or during the industralization phase.
In 1839 ,The first and foremost valve to be patented was a Gate Valve to Charley W. Peckham a NewHaven resident.
From then onwards, as more and more sophisticated boilers were manufactured with increase in steam pressure ,leading to produce more enhanced design valves and other types of valves to be designed.The control of steam was the driving force behind virtually every new valve design to come to life from the draughtsman’s table during the period from 1850 through the turn of the 19th Century.
Other inventors were also striving to improve gate valve design, primarily in the area of disc and seat construction. In 1896, William Jennings, an engineer for the Pratt & Cady Company, patented the screwed-in seat ring design that would remain the standard for the next 75 years, until advancements in welding technology rendered the screwed-in arrangement obsolete.
As the steam powered industrial revolution churned across the United States during the first quarter of this century, concern over boiler and pressure vessel design increased as some catastrophic disasters involving pressure vessels resulted in great loss of life and property. This situation led to the creation of the “Boiler Code”, which forever altered the future of all pressure containing components, including valves.
Boilers and standardization of boilers change the future of valve industry.The standardization of boilers lead to the standardization of inline components used .
One group, The Manufacturers Standardization Society of the Valve & Fitting Industry (MSS), issued its first standard in 1924, and is still today at the forefront of valve standards activities. Over the years many MSS documents have been the basis for follow-up ASME and American Petroleum Institute (API) standards. The American Standards Association (ASA) published their first document covering standardized flanges and flanged fittings in 1927.
This embryonic period for the American valve industry saw many future flow control manufacturing icons get their start: William Teller Crane (Crane); Edmund H. Lunken (Lunkenheimer); William Powell (Powell); H. G. Ludlow (Ludlow); Rufus B. Chapman (Chapman); Charles Jenkins (Jenkins); Daniel Kennedy (Kennedy); and Rufus Pratt (Pratt & Cady). All of these men patented their valve designs and founded companies that would later become well known in the field of flow control.
As the steam power industry matured and boiler temperatures and pressures increased, the valves had to keep up. In some case boiler manufacturers couldn’t get the valves they needed, so they designed and manufactured their own. Not until the first decades of the 20th century did steam pressures begin to move past the 150-200 psi level. Until then, these low pressure applications were easily handled by the brass and iron valves of the day. Cast iron valves could also easily handle the modest 350-400 degree temperatures in these days prior to high temperature superheaters and steam turbines.
Innovations in the power industry resulted in the rise of operating temperatures and pressures around the 1915-1925 period. This situation fostered the rapid development of a new valve material- steel. Steel could take the pressures and the temperatures that piping in the new central power stations required. During this period of maturation, the steel gate valve adopted the design and appearance that it is still known for today. As operating temperatures and pressures continued to rise, new chrome/moly alloys began to be employed in valve construction.
Powerful new compounds developed by the chemical and petro-chemical industries created corrosion challenges for the valve manufacturers. These challenges were met by a host of new alloys such as Hastelloy, Alloy 20 and Inconel. The last big advance in gate valve design occurred during the early 1940’s with the invention of the pressure-seal bonnet. The pressure design reduced the mass and weight of large high pressure gate valves by as much as 40%. Pressure seal valves are now the default style of valve for use in power plants.
Since the flurry of pressure-seal patent activity of the 40’s and early 50’s, valve design work has been focused on other types of valves, particularly ball and butterfly valves.
All aspects of valve design, functionality, inspection and testing are covered in dozens of ASME, API and MSS documents. This dizzying amount of codes, standards and specifications can make the specification and procurement of valve products a job for only a seasoned valve engineering expert.
This situation puts more responsibility on both the manufacturer and valve purchaser/specifier. A good understanding of the primary standards affecting these products is paramount.
1-Disk Type(Closing Down Valves): Closed piston type member such as globe valve,piston valve
2-Wedge type(Sliding Down Valves): Sliding wedge type member such as gate valve
3-Ball type(Rotary Valve): Revolving ball type member such as ball valve,butterfly valve
4-Flex-Body Valve: Flexible member diaphragm type valve such as pinch valve, diaphragm valve
• Regulating flow
• Isolating flow
• Back flow prevention
• Pressure Relief
This is the most widely used classification and selection of valves in a system.
1)Linear Motion Valves
2)Lift type Check Valve
In-Line Ball Check Valve
Stop Check Valve
3)Rotary Motion Valves
Swing Check Valve
Tilting Disc Check Valve
Folding Dick Check Valve
Stop Check Valve
4)Quarter Turn Valves
Small Bore (usually 2-inch and below)
Large Bore Valves(usually 2-inch above)
Note: This classification is very important because almost all small bore valves used are forged steels and all the large bore valves used are castings.Hence for material specification ,this classification on the size of valves is necessary.
STANDARD CLASS 150, 300, 400, 600, 900, 1500, 2500 AND 4500
• STANDARD CLASS 150 THROUGH 4500 VALVES
– WELDING END VALVES: CLASS 150 THROUGH 4500
– THREADED END VALVES: CLASS 150 THROUGH 2500
– RATINGS FOR THREEADED END VALVES ARE LIMITED TO 1000 F
– FLANGED END VALVES: CLASS 150 THROUGH 2500
– THREADED END OR SOCKET WELDING END VALVES ARE LIMITED TO NPS 2-1/2 AND SMALLER
SPECIAL CLASS 150 THROUGH 4500 VALVES
– SPECIAL CLASS 150 THROUGH 4500 RATINGS APPLY TO WELDING END VALVES
– SPECIAL CLASS RATINGS DO NOT APPLY TO FLANGED END VALVES
– SPECIAL CLASS RATINGS DO NOT APPLY TO THREADED END OR SOCKET WELDING END VALVES GREATER THAN NPS 2-1/2
– RATINGS GREATER THAN CLASS 2500 OR RATINGS AT TEMPERATURE GREATER THAN 1000 F DO NOT APPLY TO THREADED END VALVES
– SPECIAL CLASS VALVES ARE MARKED “B16.34 SPL”
LIMITED CLASS VALVES
– LIMITED CLASS RATINGS APPLY TO:
• NPS 2-1/2 AND SMALLER VALVES
• WELDING END VALVES
• THREADED END VALVES
– LIMITED CLASS RATINGS ARE CALCULATED PER ANNEX G
– LIMITED CLASS VALVES MUST COMPLY WITH ANNEX G REQUIREMENTS
– LIMITED CLASS RATINGS DO NOT APPLY TO FLANGED END VALVES
– LIMITED CLASS VALVES ARE MARKED “B16.34 LTD”
INTERMEDIATE CLASS VALVES
– ONLY WELDING END OR THREADED END VALVES CAN BE ASSIGNED INTERMEDIATE CLASS RATING
– INTERMEDIATE CLASS RATINGS CAN BE APPLIED TO
• STANDARD CLASS VALVES
• SPECIAL CLASS VALVES
• LIMITED CLASS VALVES
Maximum allowable working pressure corresponding to the steam temperature is marked on the valves
Marked as “SWP” rating
Must not be used at pressure and temperature exceeding the “SWP” rated pressure at the listed temperature
NFPA rated valves are used in fire protection systems in USA
Usually rated for 175 or 250 psi for fire service
Underwriters laboratory (UL) listed
Factory mutual (FM) approved
NFPA rated valve for 175 or 250 psi may be rated much higher for general service.
General Service Rating
May be considered equivalent to “CWP”
NFPA rated valve for 175 psi may have a general service rating of 400 psi
ASME B16.34 class 600 or API class 800
NFPA 175 psi or CWP 400 psi