VALVE Body-Bonnet Design

Gate valves are normally available in five different body/bonnet joint designs. They are: screwed, union, bolted-bonnet, welded-bonnet & pressure-seal.

* The screwed joint is the simplest design. However it is only used for inexpensive bronze valves that rarely if ever require disassembled.
* The union joint is also primarily used on bronze valves, but the union design allows for easier disassembly for repair and maintenance.
* The bolted-bonnet joint is the most popular joint and it is used on the vast majority of gate valves in industrial use today. Unlike threaded and union bonnet valves, the bolted-bonnet connection requires a gasket to seal the joint between the body and bonnet. On lower pressure valves, sheet gasket materials are used. ANSI Class 150 steel valves usually employ a corrugated soft iron or graphite/corrugated soft iron gasket. Valves of class 300 and higher employ either a spiral-wound or ring joint type gasket.
* The pressure-seal joint is energized by the fluid pressure in the valve body acting upon a wedge shaped, soft iron or graphite gasket wedged between the body and bonnet. On a pressure-seal valve, the higher the body cavity pressure, the greater the force on the gasket. Pressure-seal bonnets are used extensively for high-pressure high-temperature applications, such as the power industry. Pressure-seal valves are much lighter than their corresponding bolted bonnet designs. Due to the pressure energization of the seal ring, they are normally not used in pressure classes below ANSI class 600.
* Welded bonnets are a very popular body-bonnet joint for compact steel valves in sizes ½” through 2” and pressure classes 800 through 2500, where disassembly is not required. The higher pressure welded-bonnet type valves rely on threads to handle the force generated by the body cavity pressure, while a small peripheral weld bevel actually contains the pressure. Like pressure-seal valves, welded-bonnet valves are much lighter than their bolted-bonnet counterparts.

Disc Design VALVE disc or gate

Gate valves can have one of two different disc designs: parallel or tapered type. Both operate on the principle of a closure element (disc or gate) sliding into a slot in the pipeline and closing off the fluid path. The tapered disc of the “wedge gate” valve is machined to match a pair of body seats set at the same angle, usually about 10o. If machined correctly, as the tapered disc engages the seats, it locks firmly into place, stopping the flow.

Three types of wedge gates are available: solid disc, one piece flexible type, and two piece split design.

* The solid wedge has been around the longest and at one time virtually all wedge gates were the solid type. The drawback to a solid design is that it does not have any flexibility and if there is any valve body/seat distortion due to extreme temperature fluctuations or pipe stresses, the solid disc can become jammed in the seats. The solid disc is still standard on bronze, cast iron, water service and compact carbon steel valves (API 602 type). Today, solid discs are usually only available as special order items on large diameter gate valves.
* The flexible wedge type is just that- flexible. By the addition of a groove or slot around its periphery, the flexible disc can adapt to temperature changes and adverse piping stresses without binding. The flexible design also is a little easier to manufacture, in that minor imperfections in the seating surface angles can be compensated for by the disc’s flexibility. The “flex-wedge” design is by far the most common type seen on commodity gate valves used in industrial applications.
* The split wedge type consists of a two-piece design with mating surfaces on the back side of each disc half. These mating surfaces allow the downward stem thrust to be uniformly transferred to the disc faces and onto the seats. This flexible design also provides protection against jamming due to thermal expansion. A disadvantage to the split design is that in “dirty” services, residue or debris can cake in between the disc halves, causing the valve to improperly seat or even jam. Split wedge designs are commonly found on stainless steel and high alloy valves, as well as many small bronze valves.

Wedge gates are guided by grooves or ribs cast or welded into the body of the valve. These wedge guides keep the disc in alignment as it opens or closes and also keeps the disc from sliding against the downstream seat during opening and closing.

The second disc design is the parallel type. Unlike the wedge type gate valve, which relies on the stem thrust to “wedge” the disc into the seats to seal, the parallel seat valve needs some assistance to seal properly. The sealing assistance is usually in the form of a spring loaded or mechanically activated spreading action between the two disc halves as the valve closes fully. On most parallel seat designs the friction and sealing force is relieved as the gate disengages from the seats.

The most common use for parallel disc valves today is in the pipeline industry, where elastomer seat seals and ambient operating temperatures make valve virtually leak proof. Parallel gates are also used in some high pressure, high temperature steam applications, to help reduce the possibility of locking the disc in the closed position due to a radical change in temperature.

Regardless of disc design or type, the gate valve closure element must come in perfect contact with seats in the valve body. The body seats may be welded, screwed, pressed or swaged in, or be integral with the valve body. Most industrial steel gate valves utilize seat rings that are welded into the valve body. For most of the 20th century the norm was screwed in seat rings in steel valves. However, advances in welding and valve repair techniques made the screwed-in rings obsolete. Seat rings and valve discs are also often overlaid with corrosion or abrasion resistant alloys to increase their service life.

VALVE Body-Bonnet Design TRIM

The word “trim” is often overheard when valve professionals are talking about industrial gate valves. Trim has nothing to do with how slim and fit a valve is, rather it refers to the internal components of a valve that are exposed to great stress or subject to a harsh combination of erosion and corrosion. In a gate valve the trim components are the stem, disc seating area, body seats and backseat, if applicable. Common utility bronze or brass valves usually have trim parts of the same material as the body and bonnet. Cast and ductile iron valves have either all iron trim components or occasionally bronze trim. The term for an iron valve with bronze trim is “iron body, bronze mounted” or IBBM for short.
Because of their weldability, steel valves can be furnished with a number of different trims. Stellite, Hastelloy, 316ss, 347ss, Monel, and Alloy 20 are some of the materials regularly used for gate valve trim.

During most of the 19th century, valves were predominantly supplied with screwed end connections, even in sizes as large as 12” NPT. Since that time the flanged end connection has become the most popular. Other end connection types in use today include screwed, ring-type-joint (RTJ), Victaulic, Greyloc and water works “mechanical joint”.

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2 Responses to “VALVE Body-Bonnet Design TRIM”

1. aliaswn Says:
September 10th, 2007 at 11:01 pm

Valve assembly comprising a valve base, a valve bonnet attached thereto, a rotatable valve stem extending coaxially through the valve bonnet, a rotatable valve handle attached to the valve stem and having a plane of rotation which is generally perpendicular to the axis of the valve stem, and an indicator tab attached to the valve base or the valve bonnet and extending outward from the valve body or valve bonnet in a direction generally parallel to the plane of rotation of the valve handle. The rotatable valve handle can be disposed in at least a first position and a second position, wherein the rotatable handle when disposed in the first position renders the indicator tab invisible when viewed in a direction generally perpendicular to the plane of rotation of the valve handle, and wherein the rotatable handle when disposed in the second position renders the indicator tab visible when viewed in a direction generally perpendicular to the plane of rotation of the valve handle.
2. aliaswn Says:
September 10th, 2007 at 11:02 pm

There is a wide range of safety valves available to meet the many different applications and performance criteria demanded by different industries. Furthermore, national standards define many varying types of safety valve.

The ASME standard I and ASME standard VIII for boiler and pressure vessel applications and the ASME / ANSI PTC 25.3 standard for safety valves and relief valves provide the following definition. These standards set performance characteristics as well as defining the different types of safety valves that are used:

* ASME I valve - A safety relief valve conforming to the requirements of Section I of the ASME pressure vessel code for boiler applications which will open within 3% overpressure and close within 4%. It will usually feature two blowdown rings, and is identified by a National Board ‘V’ stamp.
* ASME VIII valve - A safety relief valve conforming to the requirements of Section VIII of the ASME pressure vessel code for pressure vessel applications which will open within 10% overpressure and close within 7%. Identified by a National Board ‘UV’ stamp.
* Low lift safety valve - The actual position of the disc determines the discharge area of the valve.
* Full lift safety valve - The discharge area is not determined by the position of the disc.
* Full bore safety valve - A safety valve having no protrusions in the bore, and wherein the valve lifts to an extent sufficient for the minimum area at any section, at or below the seat, to become the controlling orifice.
* Conventional safety relief valve - The spring housing is vented to the discharge side, hence operational characteristics are directly affected by changes in the backpressure to the valve.
* Balanced safety relief valve - A balanced valve incorporates a means of minimising the effect of backpressure on the operational characteristics of the valve.
* Pilot operated pressure relief valve - The major relieving device is combined with, and is controlled by, a self-actuated auxiliary pressure relief device.
* Power-actuated safety relief valve - A pressure relief valve in which the major pressure relieving device is combined with, and controlled by, a device requiring an external source of energy.

The following types of safety valve are defined in the DIN 3320 standard, which relates to safety valves sold in Germany and other parts of Europe:

* Standard safety valve - A valve which, following opening, reaches the degree of lift necessary for the mass flowrate to be discharged within a pressure rise of not more than 10%. (The valve is characterised by a pop type action and is sometimes known as high lift).
* Full lift (Vollhub) safety valve - A safety valve which, after commencement of lift, opens rapidly within a 5% pressure rise up to the full lift as limited by the design. The amount of lift up to the rapid opening (proportional range) shall not be more than 20%.
* Direct loaded safety valve - A safety valve in which the opening force underneath the valve disc is opposed by a closing force such as a spring or a weight.
* Proportional safety valve - A safety valve which opens more or less steadily in relation to the increase in pressure. Sudden opening within a 10% lift range will not occur without pressure increase. Following opening within a pressure of not more than 10%, these safety valves achieve the lift necessary for the mass flow to be discharged.
* Diaphragm safety valve - A direct loaded safety valve wherein linear moving and rotating elements and springs are protected against the effects of the fluid by a diaphragm.
* Bellows safety valve - A direct loaded safety valve wherein sliding and (partially or fully) rotating elements and springs are protected against the effects of the fluids by a bellows. The bellows may be of such a design that it compensates for influences of backpressure.
* Controlled safety valve - Consists of a main valve and a control device. It also includes direct acting safety valves with supplementary loading in which, until the set pressure is reached, an additional force increases the closing force.

The British Standard BS 6759 lists the following types of safety valve:

* Direct loaded - A safety valve in which the loading due to the fluid pressure underneath the valve disc is opposed only by direct mechanical loading such as a weight, a lever and weight, or a spring.
* Conventional safety valve - A safety valve of the direct loaded type, the set pressure of which will be affected by changes in the superimposed backpressure.
* Assisted safety valve - A direct loaded safety valve which, by means of a powered assistance mechanism, is lifted at a pressure below the unassisted set pressure and will, even in the event of failure of the assistance mechanism, comply with all the relevant requirements for safety valves.
* Pilot operated (indirect loaded) safety valve - The operation is initiated and controlled by the fluid discharged from a pilot valve, which is itself a direct loaded safety valve.
* Balanced bellows safety valve - A valve incorporating a bellows which has an effective area equal to that of the valve seat, to eliminate the effect of backpressure on the set pressure of the valve, and which effectively prevents the discharging fluid entering the bonnet space.
* Balanced bellows safety valve with auxiliary piston - A balanced bellows valve incorporating an auxiliary piston, having an effective area equal to the valve seat, which becomes effective in the event of bellows failure.
* Balanced piston safety valve - A valve incorporating a piston which has an area equal to that of the valve seat, to eliminate the effect of backpressure on the set pressure of the valve.
* Bellows seal safety valve - A valve incorporating a bellows, which prevents discharging fluid from entering the bonnet space.

In addition, the BS 759 standard pertaining to safety fittings for application to boilers, defines full lift, high lift and lift safety valves:

* Lift safety valve (ordinary class) - The valve member lifts automatically a distance of at least 1/24th of the bore of the seating member, with an overpressure not exceeding 10% of the set pressure.
* High lift safety valve - Valve member lifts automatically a distance of at least 1/12th of the bore of the seating member, with an overpressure not exceeding 10% of the set pressure.
* Full lift safety valve - Valve member lifts automatically to give a discharge area between 100% and 80% of the minimum area, at an overpressure not exceeding 5% of the set pressure.

The following table summarises the performance of different types of safety valve set out by the various standards.

Trim TYPES VALVE

High Flow Plug and Cage Trim

The Plug and Cage trim design gives the maximum flow capacity for a cage trim choke valve. This trim type is proven for effectiveness in high flow liquid and dual phase flow.

In the closed position, the plug makes contact with a prepared shoulder in the cage to facilitate positive shut off.

This robust trim is most often furnished with linear characteristic in surface hardened stainless steel or tungsten carbide for erosive service.


External Sleeve Trim

The External Sleeve type trim uses a flow sleeve moving over the outside of a ported cage to control flow. A metal-to-metal seat design on the outside of the flow sleeve and out of the high velocity flow assures positive shut off and an extended seat life.

High erosion resistance of this trim design leads to its use in severe service that may include high pressure drops and fluids with entrained solids, such as formations sands. This trim is furnished in tungsten carbide with an equal percentage characteristic.

ED Labyrinth Trim

The ED “Energy Dissipating” disk stack is a severe service trim solution to reduce noise, prevent cavitation and with the correct material selection, resist erosion.

The disk stack consists of a multiple set of tortuous paths in parallel to each other. Several mechanisms are utilized in the design to assist with the conversion of energy into heat without problems of cavitation in liquids and the problems of high noise levels with gasses caused by high velocities.

The ED trim is typically used in high pressure drop valves in water injection and gas production/blowdown.

Lubricating Control Valve Packing

A lubricator or lubricator/isolating valve is required for semi-metallic packing and is recommended for graphite asbestos and TFE-impregnated asbestos packing. The lubricator or lubricator/isolating valve combination should be installed on the side of the valve bonnet, replacing the pipe plug used with packing types not requiring lubrication. Use Dow Corning X-2 lubricant or equivalent for standard service up to 450°F(232°C) and Hooker chemical Corporation Fluorolube Lubricant or equivalent for chemical service up to 300°F (149°C). With lubricator, isolating valve, and pipe nipple (if used) completely filled with lubricant and installed on bonnet, open isolating valve (if used) and rotate lubricator bolt a full turn clockwise to force lubricant into the packing box. Close the isolating valve after each lubrication.

Replacing Threaded Seat Rings CONTROL VALVE

Replacing Threaded Seat Rings CONTROL VALVE

Many conventional sliding-stem control valves use threaded-in seat rings. Severe service conditions can cause damage to the seating surface of the seat ring(s) so that the valve does not shut off satisfactorily. In that event, replacement of the seat ring(s) will be necessary.
Before trying to remove the seat ring(s), check to see if the ring has been tack-welded to the valve body. If so, cut away the weld and apply penetrating oil to the seat ring threads before trying to remove the ring. The following procedure for seat ring removal assumes that a seat ring puller, such as that shown in figure 5-4, is being used . If a puller is not available, a lathe or boring mill may be used to remove the ring(s).

1. Place the proper size seat lug bar across the seat rings so that the bar contacts the seat lugs as shown.

2. Insert drive wrench and place enough spacer rings over the wrench so that the hold-down clamp will rest about 1/4-inch above the body flange.

Slip hold-down clamp onto drive wrench and secure the clamp to the body with two cap screws (or hex nuts for steel bodies) from the bonnet. Do not tighten cap screws or nuts.

3. Use turning bar to unscrew the seat ring. Struck seat rings may require additional force on the turning bar. Slip a 3- to 5-foot length of pipe over one end of the turning bar with a heavy hammer to break the ring loose. In addition, a large pipe wrench can be used on the drive wrench near the hold-down clamp.

4. After the seat ring is loose, alternately unscrew the flange bolts (or nuts) on the hold-down clamp and continue to unscrew seat ring.

5. Before installing new ring(s), thoroughly clean threads in the body port(s). Apply pipe compound to the threads of the new seat ring(s).

Note

On double-port bodies, one of the seat rings is smaller than the other. On direct-acting valves (push-down-to-close action), install the smaller ring in the body port farther from the bonnet before installing the larger ring. On reverse-acting valves (push-down-to-open action), install the smaller ring in the body port closer to the bonnet before installing the larger ring.

Screw the ring(s) in to the body. Use the seat ring puller, lathe, or boring mill to tighten seat rings in the body. Remove all excess pipe compound after tightening. The seat ring can be spot welded in place to ensure that it does not loosen.

6. Reassemble the valve.

Replacing Stem Packing CONTROL VALVE

Replacing Stem Packing CONTROL VALVE

Bonnet packing, which provides the pressure seal around the steam of a globe-style valve body, may need to be replaced if leakage develops around the stem, or inspection. Before starting to remove packing nuts, make sure there is no pressure in the valve body.
If the packing is of the split ring variety, it can be removed (with considerable difficulty) without removing the actuator by digging it out of the packing box with a narrow, sharp tool. This is not recommended, because the wall of the packing box or the stem could easily be scratched, thereby causing leakage when the new packing was installed.
Don’t try to blow out the old packing rings by applying pressure to the lubricator hole in the bonnet. This can be dangerous and frequently doesn’t work very well anyway. (Many packing arrangements have about half of the rings below the lubricator opening.)

The approved method is to :

1. Separate the valve stem and actuator stem connection.

2. Remove the actuator from the valve body.

3. Remove the bonnet and pull out the valve plug and stem.

4. Insert a rod (preferably slightly larger than the stem) through the bottom of the packing box and push or drive the old packing box and push or drive the old packing out the top of the bonnet. ( Don’t use the valve plug stem because the threads could be damaged in the process.)

5. Clean the packing box. Inspect the stem for scratches or imperfections that could damage new packing.

6. Check the valve plug, seat ring, and trim parts as appropriate.

7. Re-assemble the valve body and put the bonnet in position.

8. Tighten body/bonnet bolting in sequence similar to that described for flanges on page 100.

9. Slide new packing parts over the stem in proper sequence, being careful that the stem threads do not damaged the packing rings

10. Install the packing follower, flange, and packing nuts.

11. For spring-loaded TFE V-ring packing, tighten the packing nuts as far as they will go. For other varieties, tighten in services only enough to prevent leakage.

12. Replace and tighten the actuator on to the body. Position and tighten the stem connector to provide desired valve plug travel.

CONTROL VALVE Maintenance INTRODUCTION

CONTROL VALVE Maintenance INTRODUCTION

In order to perform even routine maintenance procedures on a control valve, it is important that the maintenance man have a thorough understanding of the fundamental construction and operation of the valve. Without this knowledge, the equipment could be damaged inadvertently, or could cause injury to the maintenance man and others in the area. Most valve manufacturers in their detailed instruction and operation manuals. Usually, a sectional drawing of the equipment is also furnished to help in understanding the operation of the equipment as well as to provide identification of component parts.

In all major types of control valves, the actuator provides force to position a movable valve plug, disc, or ball in relation to a stationary seat ring or sealing surface. The moveable member should respond freely to changes in actuator loading pressure. If proper operation is not being received, service is indicated. Before any maintenance procedures are started, be sure that all line pressure is shut off and released from the valve body and also that would damage the equipment or injure personnel.

Often corporate maintenance policy or existing codes require preventive maintenance on a regular schedule. Usually such programs include inspection for damaged of all major valve components and replacement of all gaskets, O-ring seals, diaphragms, and other elastomer parts. Following is a series of commonly performed maintenance procedures and some general instructions for performing each procedure instructions are normally furnished with control valve equipment and should be carefully followed.

* Replacing Actuator Diaphragm
* Replacing Stem Packing
* Replacing Threaded Seat Rings
* Grinding Metal Seats
* Lubricating Control Valve Packing

Replacing Actuator Diaphragm ^
After isolating the valve from all pressure, relieve all spring, if possible. (On some spring and diaphragm actuators for use on rotary-shaft valve bodies. spring compression is not externally adjustable. Initial spring compression is set at the factory and does not need to be released in order to change the diaphragm.) Remove the upper diaphragm case. On direct-acting actuators, the diaphragm head assembly must be dismantled to change the diaphragm.
Most pneumatic spring-and-diaphragm actuators utilize a molded diaphragm for control valve service. The molded diaphragm facilitates installation, provides a relatively uniform effective area throughout the valve’s travel range, and permits greater travel than could be possible if a flat-sheet diaphragm were used. If a flat-sheet diaphragm is used in an emergency repair situation, it should be replaced with a molded diaphragm as soon as possible.
When re-assembling the diaphragm case, tighten the cap screws around the perimeter of the case firmly and evenly to prevent leakage.