Monday, January 18, 2010


Codes usually set forth minimum requirements for design, materials, fabrication, erection, test, and inspection of piping systems, whereas standards contain de¬sign and construction rules and requirement for individual piping components such as elbows, tees, returns, flanges, valves, and other in-line items. Compli¬ance to Code is generally mandated by regulations imposed by regulatory and en¬forcement agencies. At times, the insurance carrier for the facility leaves hardly any choice for the owner but to comply with the requirements of a Code or Codes to ensure safety of the workers and the general public. Compliance to standards is normally required by the rules of the applicable Code or the purchaser's spec¬ification.

Each Code has limits on its jurisdiction, which are precisely defined in the Code. Similarly, the scope of application for each standard is defined in the stan¬dard. Therefore, users must become familiar with limits of application of a Code or standard before invoking their requirements in design and construction docu¬ments of a piping system.

The Codes and standards, which relate to piping systems and piping compo¬nents, are published by various organizations. These organizations have commit¬tees made up of representatives from industry associations, manufacturers, pro¬fessional groups, users, government agencies, insurance companies, and other interest groups. The committees are responsible for maintaining, updating, and re¬vising the Codes and standards in view of technological developments, research, ex¬perience feedback, problems, and changes in referenced Codes, standards, specifi¬cations, and regulations. The revisions to various Codes and standards are published periodically. Therefore, it is important that the engineers, designers, and other pro¬fessional and technical personnel stay informed with the latest editions, addenda, or revisions of the Codes and standards affecting their work.

While designing a piping system in accordance with a Code or a standard, the designer must comply with the most restrictive requirements which apply to any of the piping elements.

In regard to applicability of a particular edition, issue, addenda, or revision of a Code or standard, one must be aware of the national, state provincial, and local laws and regulations governing its applicability in addition to the commitments made by the owner and the limitations delineated in the Code or standard.

Tuesday, July 21, 2009

Plug Valve 2

1.Design Details
Design detail of plug valve are as follows :
a. There are two basic types of plug valves :
- Lubricated Type
- Nonlubricated Type
b. Plug may be :
- Tapered (More Common) or
- Cylindrical (Less Common)
c. Normally, plugs are tapered downward with bolted access cover to top. Inverted plug valves with bottom access are also available.
d. The term “lubricated” refers to sealant being injected under pressure into grooves within valve body and plug. The sealant helps to prevent internal leakage and also acts as a lubricant to reduce metal-to-metal friction. Nonlubricated valves depend on a resilient liner for sealing. Stem sealing is accomplished in a variety of ways.
e. Plug valve body configurations are :
- Shot Pattern
- Regular Pattern
- Venturi Pattern
- Multiport Patterns

2.Port Configuration

Port openings can be specified in a variety of configurations, such as round, rectangular, oval, trapezoidal, etc., resulting in restriction to flow characteristics that must be examined for specific applications; e.g., regular pattern valves provide a flow area of 40 to 100% of the connecting pipe area. Venturi patterns provide a flow area of 36 to 50% of the connecting pipe area. The area reductions vary with valve type and also with manufacturer. Generally, the greater the opening, the lower pressure drop, and the higher the cost.
Cylindrical plug valves may have a round opening identical to the pipe opening, thus introducing no additional pressure drop. They are not readily available and are seldom used, particularly in large sizes, because of the relatively high cost.

3. Actuators

Geared actuators are normally used for 4 inch and larger nonlubricated plug valves and for 6 inch and larger lubricated plug valves.

4. Limitations

Limitations of plug valves include the following :
a. They are subject to binding and galling.
b. They are not suited for steam service.
c. Lubricated plug valves require periodic lubrication.
d. The lubricant (sealant) may react or contaminate the fluid being carried.
e. Flowing fluid trapped inside the plug port may cause overpressure and failure (cracking) of the plug when the valve is closed.
f. It should be noted when handling water at freezing temperatures that there is a risk that water trapped inside a closed plug will freeze and crack the plug. This can be prevented by drilling a small hole in the plug on the downstream side.

5. Typical Applications

Typical applications of plug valves are in petroleum and natural gas for leakproof isolation, and in slurry service.
It should be noted when handling water at freezing temperatures that there is a risk that water trapped inside a closed plug will freeze and crack the plug. This can be prevented by drilling a small hole in the plug on the downstream side.

6. Eccentric Plug Valves

The eccentric plug valve is a quarter turn nonlubricated type. The valve consist of a body, bonnet, actuating mechanism, and a rotating vane-like plug disc that is eccentrically located off its trunnion shaft and passes flow by positioning the plug out of the flow path and stops flow by positioning the plug against as inlet or outlet port. These valves are manufactured in sizes from ½ inch to 54 inch NPS. Multiported eccentric valves are available in 3 inch to 16 inch pipe sizes.
Eccentric plug valves can be used in on-off operation and in throttling service for waste water, saltwater, corrosive drains, gas or air contaminated with suspended solids, and for liquid slurries. They have excellent flow characteristics and, when the valve is wide open the plug is out of the slurry stream, which reduces wear. In slurry service the flow direction is from under the seat, so that solids do not collect in the valve cavity. The preferred orientation is with the shaft horizontal with the plug in open position.
Special valves provided by bottom ash handling system vendors are often of the eccentric plug type.

Tuesday, June 16, 2009

Plug Valve 1

 Basic Design
A plug valves is essentially a ported, tapered or cylindrical plug in a housing. Rotation of the plug by 90 degrees (quarter turn) changes the position from open to closed or vice versa. Available sizes are from ½ inch to about 12 inch for nonlubricated valves and up to 30 inch nominal pipe size (NPS) for lubricated valves. The “eccentric plug” valves are a special case and are described separately at the end of this section.

Design Features
All two-way plug valves are bidirectional. However, three-way, four-way and five-way valves have a designated inlet. They require less headroom than most other valves and have a low center of gravity. They are of simple construction and easy to maintain.

Typical Applications

Plug valves are used as bubble-tight, on-off stop valves in a variety of fluid systems including air, gas, oil and liquid slurries, but are not used in steam service. In small sizes they are economically competitive with globe valves, while in large sizes they are more expensive than globe valves.
Because plug sealing methods do not promote “crud traps”, plug valves are frequently used in “slurry” service; however, if throttling accuracy is not important, nonlubricated plug valves can be used when the valves is equipped to hold the plug in position. Lubricated valves, if used in throttling service, will tend to lose sealant from sealant grooves exposed to fluid flow.

Monday, May 18, 2009

Ball Valves

1.Basic Design
The ball valve is basically a ported sphere in a housing. A quarter turn of the stem will cycle the valve from open to closed or vice-versa.

2.Size Range
Ball valves with are manufactured in sized ranging from ¼ inch to 48 inch and higher. Sizes up to 8 inch are used quite frequently. Ball valves in small sizes are relatively inexpensive.

3.Directional Seating Characteristics
Ball valves with two seats are normally bi-directional while those with a single seat are normally unidirectional. Multiport ball valves (2-Way, 3-Way, 4-Way) are unidirectional. The multiport ball valve is used frequently for diverting flow to several directions using a single valve.

4.Typical Applications
Ball valves are used as bubble-tight stop valves for relatively clean fluid systems, such as air, water, and gas. The popularity of this valve is enhanced by its very small space envelope for a given line size (although this advantage diminishes somewhat in larger sizes) and its quarter turn operation from the closed to open (or open to close) position. Typical applications include fuel oil and fuel gas service.

5.Design Features
Ball valves have a very low center of gravity and are simple designs. With few moving, yet accessible parts, this valve design rates very high with respect to achieving low maintenance costs.
These valves have low pressure drop and low leakage and they are rapid opening. In full port configuration they have the same flow restriction as a straight piece of pipe and generate little or no additional turbulence.

6.Construction Details
Construction details are as follows :
a. Ball valves are manufactured in three basic designs :
- Floating Ball
- Double Trunnion
b. Port sizes are as follows :
- Full Port
- Reduced Port
- Venturi Pattern
c. The full port ball valve has the lowest pressure drop (Or highest Cv) and the highest price of the three port configurations.

7. Limitations
Limitation of ball valves are as follows :
a. Standard ball valves should not be used for any sustained throttling service as severe erosion of the ball and seat ring my result if the ball and seats are not designed for throttling service. Some ball valves have special design features which makes them suitable for throttling service.
b. Soft-seated ball valves have a generic design problem that exists when suspended solid participles in the fluid system (resins, oxide particles, etc.) settle-out and become trapped in the cavities below the ball and in the vicinity of the stem or trunnion areas when the flow has stopped and this “crud” has had time to become encrusted. One solution to this problem is to position all valves mounted in a horizontal run of pipe so that the stems or trunnions are in the horizontal position. This valve position will promote the “self-scouring” motion when flow returns. Another solution is to drill and tap the body for a threaded plug under the ball or in the area of crud build-up; the tapped hole can be used for drawings and/or flushing.
c. Fluid trapped in the ball valve in the closed position may caused problems if the valve is not vented (e.g., water freezing and subsequent cracking the ball).
d. Because the valve can be quick opening and closing, rapid closure may cause water hammer or pressure surges.
e. Typical materials used for seating and seals on ball valves are listed with approximate (they are a function of mechanical design as well as material properties) recommended maximum service temperatures.

8.Access for Disassembly and maintenance
The internals of most ball valves may be accessed from one of the valve ends; this requires the pipe line to be loosened at the valve removed. More desirable are “top entry” designs. Also swing-away designs are available in small sites; this design allows in-service access, also which does not require dismantling the pipe line.

9.Basic Seat Seal Design
In order to maintain the bubble-tight closure expected of a ball valve, materials used for seating and sealing must be compatible for the system service temperatures. The majority of the ball valve design used a resilient material seat ring in contact with the polished surface of the ball disc. This valve seating design is described as “nonlubricated” and depends on the natural lubricating qualities of the material to provide low friction operation.

10.“Lubricated Seat” Designs
Some manufacturers produce valves with secondary seating as well as primary. This is accomplished by the use of sealant material forces under pressure into machined passages along the periphery of the ball affecting a seal that can be maintained until the primary seal can be replaced. This design is known as a lubricated seat design.

11.Stem Seals
Stem seal design varies with the valve sizes and service temperatures. Simplified stuffing boxes are used with flat packing rings compressed by screwed glands and stuffing boxes that us chevron type packing rings that are pressurized with a sealant. Other designs use a series of O-ring seals backed up by a secondary sealant seal.

12.Metal-To-Metal Seated Ball Valves
Heavy duty, meal-to-metal seated ball valves are available for high temperature, high pressure service.
This type of valve has been seen in increasing usage due to the following advantages :
a. High resistance to erosion, making this design an excellent selection in high solids service;
b. Temperature capability equal to metal seated gate or glove valves in high temperature service;
c. Options for throttling.
The drawbacks of these design are :
a. High initial costs;
b. A high stem torque requirement which means that a gearbox is required in smaller sizes than for a conventional ball valve.

Wednesday, April 15, 2009

Check Valve

1 Typical Usage
Check valves are used to block the pipe flow in one direction while permitting flow in the other direction. A typical application of a check valve is at the discharge of a pump. If the pump is idle on stand-by the check valve prevents reverse flow though this pump; the moment the pump start the flow can lift the check valve and go in the downstream direction. 

2 Primary Function

The primary function of a check valve is to prevent flow reversal. Check valves pass fluid freely in one direction and, if pressure reverses, close to stop flow in the other direction. Therefore, the principal force on the check valve is a water or steam hammer transient load.

3 Application Objectives

The major engineering concern in design and selection of check valves is to achieve, at normal flow condition, a low pressure drop across the valve, a reasonable wear rate on the valve element, and a stable operating mode. The design and selection of the check valve should also include the attainment of an acceptably low transient load and a tolerable leakage rate during and after valve closure.

Tuesday, April 7, 2009

Globe Valves

1 General Characteristics

The globe valve controls flow by raising or lowering a circular disc on a seat. Globe valve are normally manufactured in sizes from 3/8 inch to 16 inch. Globe valves larger than 16 inch can be obtained at a premium price. Normally flow direction is from under the seat, therefore, when the valve is closed the full line pressure is exerted on the disc and stem. On large valves the force needed to keep the valve closed and leaktight are great, hence, the size limitation. For the same reason, valve actuators tend to be larger than those for gate valves. Also a, self-locking gear is needed to hold the valve closed. The globe valve should be used primarily for throttling service at disc positions greater than 30% from the fully closed portion as a block valve. In vacuum service, globe valves are frequently installed opposite to the direction specified for pressure service.

2 Body Configurations

Globe valves are manufactures in three basic body configurations  :
a. Tee pattern
b. Wye pattern
c. Angle pattern (Angle Valve)

3 Disc Flow Characteristics

Discs may be specified with several contours depending on desired flow characteristics, such as :
a. Quick opening
b. Semi-throttle
c. Linear
d. Equal percentage

 Most globe valves are furnished with quick-opening or semi-throttling discs and are satisfactory for on-off service or when precise flow control is not required over the whole travel range of the valve. If control of flow is required over the complete travel range, other discs such as “linear” or “equal percentage” or, for small valves, needle plug are available.

4 Stem/Yoke Configurations

Globe valve stem/yokes are typically manufactured in two different combinations :
a. Outside screw and yoke with rising stem
b. Inside screw rising stem

5 “Packless” Designs

Most globe valves have a stuffing box as the stem seal, however, “packless” valves are also available. These design use a diaphragm or bellows to provide the seal and also have backup packing in case of diaphragm or bellows failure. They are used to prevent leakage to the environment and in vacuum service to prevent atmospheric leakage.

6 Backseats

Back seats may or may not be specified for globe valves; if specified, they are energized only when the valve is fully open. It is normal practice to specify a backseat; however, extreme caution should be exercised in using backseat.

7 Disc Guiding

Stem and disc to body alignment can be accomplished by “top guiding” or “body guiding”, generally in lower pressure valves with tee pattern. “Bottom guiding” is not recommended as these design can have a relatively short service life and may be prone to breakage. Good alignment of the disc to the seat is necessary for effective sealing.

8 Seats

Globe valve seats can be integral, replaceable, hard faced, soft faced, resilient, etc., as required. The seat material can be selected to suit the application.

9 Limitations

Small (2 inch or smaller) globe valves may be used as stop valves because the force under the disc is within the capability of the stem threads to keep the valve closed. For valves that usually remain partially opened, deep stuffing boxes are required, and the disc should be guided along the full stem travel.
In general, the maximum differential pressure across globe valves should not exceed 20% of the upsteam line pressure and never exceed 200 psi, unless a special trim and disc design has been provided by the valves manufacturer. Prior to using a given design, the valve manufacturer should be consulted regarding the valve design capability for these parameters.
Small globe valves can be used as piping or equipment high point vent or low point drain connections as well as root valves for instrument pressure connections. Globe valves with special trim are used for blowoff or blowdown system applications.

10 Body Configurations

The various body configurations of globe valves should be used and applied as follows :
a. Tee Pattern : To be used when severe throttling is required as in a by pass line around a control valve where pressure drop is not a concern. This pattern has the lowest flow coefficient (Cv).
b. Wye Pattern : To be used when valve is wide open or throttling during seasonal or startup operations. This valve type has very little resistance to flow and can be “cracked open” for long periods without severe erosion.
c. Angle Pattern : Similar in application to the wye pattern globe with a slightly lower Cv. A particularly good selection for systems that have periods of pulsating flow because this valve configuration adequately handles the “slugging” effect inherent with this type of flow. The “angle” valve, if appropriately located, can eliminate a 90 degree elbow.

Monday, March 30, 2009

Gate Valve

1 General

Gate valves are the most commonly used valves used valves used in on-off services. They are used where stop valves are required and will normally stop flow in either direction, i.e., they are “bidirectional”. They are often used as isolation valves.

2 Throttling Characteristics

Gate valve should be used in either the fully closed fully open position; never in a throttling mode position, except for :
a. Throttling Gate valves which use a “V-shaped port” in the seat to allow throttling;
b. Throttling Knife Gate valves with a “V-shaped gate” to allow throttling;

3 General Flow Caracteristics

They offer low resistance to flow and, therefore, cause low pressure drop.

4 Stem/Yoke Configurations

Gate valves are manufactured with the following stem/yoke combinations :
a. Outside Screw and Yoke, Rising Stem (OS&Y RS)
b. Inside Screw Nonrising Stem (ISNRS)
c. Inside Screw Rising Stem (ISRS)
The OS&Y RS is most adaptable to power actuators. Also its open-close position is visually indicated by the projection of stem above the handwheel; this is an important operating advantage and is mandatory in services such as fire protection.

5 Disc Variations

There are basically four disc design variations :
a. The double disc, Parallel seat type
b. The through conduit type
c. The knife gate
d. The wedge type. This type valve is manufactured comes in four disc designs :
• Solid wedge
• Hollow wedge
• Flexible wedge
• Split wedge
The solid wedge gate valve is the simplest design. The hollow wedge design is used only for small valves in low temperature applications. The flexible wedge gate valve is less susceptible to thermal binding and seat leakage than the solid wedge; however, it is more susceptible to thermal binding and seat leakage than the split wedge or double disc. The split wedge disc valve performs well in services where thermal binding may occur as with a flexible wedge gate valve, however, this design may be susceptible to “chattering”. The multiple disc, parallel seat type gate valve has a double seated disc. The conduit gate valve is a form of slide gate. Most commonly, flexible wedge gate valves are used for higher temperature services requiring larger size, and solid wedge gate valves of smaller sizes are used in lower temperature applications. Knife gate valves are characterized by a gate-like disc in which the leading edge is tapered to form a knife edge for cutting through suspended solids or scale accumulated against the seat. This design usually uni-directional with a solid seat ring on one side of the gate and widely spaced lugs on the order side to guide the gate; fluid pressure presses the gate against the seat. Most knife gates designs are either wafer style or have flanged ends, and are of the OS&Y design, usually bonnetless. Knife gate valves are used in “dirty” applications for fluids with suspended solids and scalling potential, such as return water service in bottom ash conveying systems.

6 Limitations

Limitations of gate valves are as follows :
a. Metal to metal seated gate valves, typically, do not close buble-tight, therefore, they should not be used where high leak tightness is required, e.g., in hazardous fluid service.
b. Solids or impurities could settle in the seat and make tight closing of the valve difficult or impossible in “dirty” or slurry service.
c. If the fluid is a scaling fluid, the seat groove may scale and make tight closing of the valve difficult.
d. Gate valves in large sizes are relatively expensive, compared to butterfly valves, for example.
e. In high temperature service some gate valves have the potential for developing a problem known as “thermal binding”.
- Wedge type gate valves may have a this problem. In simplified terms, this condition can occur when a valve is closed tightly while the high temperature side is still in operation. Subsequently, when the system is shutdown, and cooldown takes place, seats move inward more than the wedge shrinkage occurring during cooldown.
- Therefore, the differences in thermal contraction can bind the wedge to the seats tightly enough so that the valve cannot be reopened until the reactivated system temperature reheats the valve .
- To perform a quantitative analysis of this effect is quite complicates, therefore, when it appears likely that this phenomenon can occur, the preferred way in which the problem is dealt with is to specify a valve which has a low susceptibility to thermal binding.
f. Seat distortion caused by piping loads can lead to continuos leakage. In addition to thermal binding, wedge gate valves, in systems where severe temperature changes occur, are also subject to excessive seat leakage due to changes in angular relationship between the wedge and the seat faces caused by loads exerted on the valve ends.
g. “Pressure locking”(also known as “hydraulic” or “pressure binding”) is a problem which can occur when trapped liquid in the body-bonnet cavity of a closed double disc or split wedge gate valve is heated and the resulting high pressure loads the two disc pieces to exert a force which is too high for the operator to overcome. This pressure, in some cases, may exceed the rated capacity of the valve and result in “bonnet overpressurization”. The solutions to these problems may include the following :
- Drilling a hole in the upstream disc half;
- Piping the body-bonnet cavity to a lower pressure area, usually the valve downstream end.
h. Some disadvantages of gate valves include :
- The full stem travel requires many turns of a handwheel and many more turns when fitted with gearing.
- Due to the relatively long stem stroke and the outside appurtenances necessary to accommodate this stroke, a large space envelope is required.
- Body seat surfaces for a wedge type date valve may be difficult to machine or refinish due to the wedge angle, which is usually 3-5 degrees.
- Very little or slow movement of the disc near the fully closed position (which caused high velocity flow) may result in scoring of the sliding parts and seating surfaces (this phenomenon is often called “wire drawing”).
- Gate valves subjected to high pressure in the closed position must be equipped with small by-pass valves. These by-pass valves (normally globes) are opened first to equalized the pressure; then the gate valve may be opened.