Valves, Types, Parts, Function, Operation

VALVES

INTRODUCTION

The most common mechanical device used in process operations is the valve.  Valves are used to control the flow of liquids and gases in piping systems.

A wide range of products, pressures, and temperatures are used in processing systems, so valves are made in a wide range of types, shapes and materials.

 

HAZARDS ASSOCIATED WITH VALVES

Valves are used to minimise the hazards associated with the transfer of fluids by providing methods of controlling or isolating the pressure and flow of fluids, It is mainly when valves fail that hazards arise, depending on the chemical and physical nature of the fluid being handled.

Often the failure of a valve will go unnoticed as they are situated within the pipe where their operation cannot be seen. Extra precautions should be taken when performing tasks that rely on the correct operation of a valve.

 

For example, when pipelines are being dismantled for maintenance, any isolating valves in the system must be closed and if possible, the pipeline drained. Adequate safety equipment should be worn and pipelines opened slowly to prevent large amounts of fluid leaking out in the event of an isolation valve failure.

 

Any valve that is stuck should not be forced open as this may cause fluid to leak out past the stem and onto operators. Any valves that are known to be leaking, either internally or externally, should be reported and changed as soon as possible.

Any unusual trends in valve operation should also be reported but no adjustments should be made without prior consultation with senior personnel and investigations carried out to discover the reasons for the unusual trend.

 

2        IDENTIFY VALVES & DESCRIBE THEIR FUNCTIONS

Different types of valves perform different functions.  Some are used to start or stop flow in pipelines.  Other valves are used to regulate flow in pipelines.

Figure 1 Valve open and closed position

The element changes position to open and close the valve.

 

Figure 2 valve major parts

 

3        MAJOR VALVE PARTS

3.1       Body

 

The body is the part of a valve that is attached to pipesand hold all the parts together.  The gases or liquids flow through the body when the valve is open.

 

3.2       Opening Element

This is the part that opens and closes the valve.  All valves have an opening element in the valve body.  Opening elements are made in various shapes and sizes.  They can move up and down on a stem, or rotate around a central pivot point.

The opening element stops or allows flow through a valve.  We will discuss types of opening elements further when we look at different types of common valves. When the valve is closed, the opening element fits against the valve seat.

 

3.3       Stem

The stem raises or lowers the opening element.  In manually operated valves, the steam is usually threaded.  In ball valves and butterfly valves, the stem does not go up or down.  It turns inside the bonnet.  In pneumatic and hydraulic valves, the threaded stem is replaced by a smooth rod that moves up and down in the valve.

 

3.4       Handwheel / Handle

On a manual valve, a hand wheel or handle is attached to the stem.  The hand wheel or handle allows the operator to turn the stem.  The hand wheel is held on the stem by the hand wheel nut.

 

3.5       Bonnet

The bonnet is a separate housing that is bolted tightly to the top of the valve body.  The stem passes through the bonnet.  The bonnet holds the stem in position.

 

3.6       PACKING GLAND

The packing gland is held in place by bolts, or sometimes it is screwed into place.  This gland keeps the packing tight.  Packing is a soft material inside the bonnet, placed around the stem to make a tight seal.  The packing keeps pressurized gas or liquid in the valve from escaping to the atmosphere.

 

4        TYPES OF VALVES AND THEIR FUNCTIONS

We will discuss types of valves that are either manually or automatically operated.  They are:

  • Butterfly Valve
  • Globe Valve
  • Ball Valve
  • Gate Valve
  • Plug

4.1       Butterfly Valve

The opening element in a butterfly valve is a flat disc.  The disc rotates around a central pivot point and fits against the valve seat when fully closed. The flat disc fills the entire area of the valve body.  The butterfly valve offers very little resistance to flow when it is fully open.

 

Figure 3 butterfly valve

The opening element of a butterfly valve is a flat disc.  When the disc is perpendicular to the pipeline, it stops the flow.

Butterfly valves need only one-quarter of a turn to move from a fully closed to a fully open position.  A handle is often used instead of a hand wheel to turn the valve stem.

Butterfly valves are used as control valves in low-pressure gas pipelines. They can be set at any position between fully open and fully closed.

Butterfly valves are also sometimes used in low-pressure liquid pipelines.

Butterfly valves are not used in high-pressure pipelines because they do not provide complete shutoff.

 

A butterfly valve and its engineering symbol are shown in Figure 4 butterfly valve and it’s engineering symbol

 

Figure 4 butterfly valve and it’s engineering symbol

The vertical lines at either end of the symbol mean that the valve has flanges on the valve body.  You may see these lines on any type of valve

 

4.2       Globe Valve

The opening element in a globe valve is a plug shaped disc operated by a hand wheel.  The disc moves away from the valve seat to open the valve. The disc fits tightly into the seat to close the valve.  When closed, the valve completely shuts off the flow of liquid or gas.

 

Figure 5 globe valve

The opening element is a disc, which moves down to shut off flow.  The element can be positioned to allow different rates of flow.

 

 

The main function of the globe valve is to regulate the flow of liquids and gases.  The valve may be placed in any position between fully open and fully closed.

Globe valves are used mainly as control valves, when the rate, volume, and pressure of flow must be frequently adjusted.

A globe valve and its engineering symbol are shown in Figure 6 globe valve and it’s engineering symbol

Figure 6 globe valve and it’s engineering symbol

Globe valves are primarily used as control valves to regulate fluid flow.

 

 

 

4.3       Ball Valve

In a ball valve, the opening element is a ball with a hole through the centre. When the valve is fully open, gas or liquid flows through the valve easily.  The hole in the ball is the same size as the pipe, so there is no obstruction to flow.

Figure 7 ball valve

The ball element rotates within the valve body to open and close the valve. Large ball valves are operated by hand wheels.  Small ball valves have a handle, connected to the stem that turns the ball.

 

A one-quarter turn of the valve stem will rotate the ball 90°.  This is enough to move the valve from a full open to a full closed position.

 

Ball valves have the advantage that they can be opened and closed more quickly.  They are used in high-pressure liquid and gas pipelines as quick shutoff valves.

Ball valves are usually set in a fully open or fully closed position.  Because of their construction, ball valves can be opened slowly without damage to the ball.  For this reason, they can be used as bypass valves.  Bypass valves are opened slowly to allow pressure to build up gradually in a PIpeline. Both ball valves and butterfly valves open and close with a 90° turn.

 

A ball valve and its engineering symbol are shown in Figure 8 ball valve and it’s engineering symbol below

Figure 8 ball valve and it’s engineering symbol

4.4       Gate Valve

The opening element is a gate valve is a wedge shaped disc or gate attached to the end of a threaded stem.  The gate fits into a wedged shaped seat in the valve body to stop flow through the valve.

Turning the hand wheel raises and lowers the valve gate.  When the valve is fully opened, the gate is positioned above the passage in the valve body. This allows full flow in either direction, with little or no restriction.

 

When the gate valve is fully closed, the gate fills the passage and it stops flow through the valve completely.

Figure 9 rising and non rising stem valves (open and closed)

The gate element is raised and lowered by the hand wheel.   When the valve is closed, the gate fits tightly against the valve seat, stopping flow completely.

 

4.4.1        Rising Stem

Large gate valves normally have rising stems.  When the hand wheels turned, the stem rises out of the valve bonnet through the hand wheel.  The stem shows the position of the disc, and shows if the valve is opened or closed.  In small gate valves, the wheel and stem may rise together.

 

4.4.2       Non Rising Stem

Some gate valves have a nonrising stem.  The stem does not come out of the valve body.  The gate moves up on the stem.  This type of valve is used where there is not enough space for the stem to rise out of the valve body. Most gate valves are manually operated.

 

Gate valves provide a good seal against flow.  For this reason, they are normally used as isolation valves in high-pressure hydrocarbon pipelines. Gate valves are the most commonly used valves in process operations plants.

A gate valve and its engineering symbol are shown in Figure 10 gate valve and it’s engineering symbol

Figure 10 gate valve and it’s engineering symbol

Gate valves can stop flow completely even on high-pressure lines.   For this reason, they are the most common valves in process operations.

 

4.5       Plug Valve

This multi-turn valve derives its name from the rotating plug, which forms the closing element; the plug may be cylindrical or truncated.

In the open position the fluid flows through the hole in the plug.

Lubricated plug valves rely on a sealing compound injected between the plug and the valve body

Whilst sleeved plug valves are fitted with a soft insert between the plug and the body.

Figure 11 plug valve and it’s engineering symbol

 

 

4.6       Needle Valves

Needle valves are a form of the globe valve and are intended for low flow, fine adjustment conditions. A needle valve and its engineering symbol are shown in Figure 12 needle valve

Figure 12 needle valve

The spindle ends in a tapered needle, which moves into the valve seat to restrict or shut off the flow of fluid through it. A hand wheel operating the spindle controls the ‘in and out’ motion of the needle to and from the valve seat.

 

Advantages

  • Very precise flow control at low rates

Disadvantages

  • Needle valves impose a high restriction to the fluid flow, so there is a very high pressure-loss,
  • The small flow passage between the needle and seat restricts the use of this valve to low flow rate
  • The seat and needle are prone to damage if the fluid being handled contains suspended

 

 

1          AUTOMATIC VALVES & THEIR FUNCTION

Automatic valves, once installed, operate by themselves.  They are not opened and closed by an operator or actuator.

1.1       Check Valves (Non Return Valves N.R.V )

Check valves are used to permit flow in only one direction in a pipeline. There are two types of check valves:

  • Swing Check Valves
  • Ball Check Valves

 

1.1.1           Swing Check Valves

In a swing check valve, the opening element is a disc attached to a hinged arm. Figure 13 swing check valve

Figure 13 swing check valve

Upstream pressure causes the hinged disc to rise, opening the valve.   Higher downstream pressure will cause the valve to close.

The pressure of liquid or gas flowing through the valve holds the disc open, allowing full flow.  If flow stops, or the pressure downstream of the valve becomes higher than the pressure upstream, the hinged arm will swing down. The disc will cover the passage and seal the valve.  The higher downstream pressure will press against the disc to keep it tightly closed.  None of the liquid or gas that has already passed through the valve will be allowed to flow back through the valve. Swing check valves can be installed vertically (the disc must point up in the flow direction) or horizontally.  Gravity will close the valve if flow stops.

 

 

 

1.1.2          Ball Check Valve

The opening element in a ball check valve is a ball.

The ball check valve works on the same principle as the swing check valve. The pressure of liquid or gas in the pipeline opens the valve by raising the ball from the valve seat.  If flow stops or reverses direction, the ball drops back into the seat.  The higher pressure of the liquid or gas downstream holds the valve closed.

The ball check valve shown in Figure 14 ball check valve open and closed, must be installed in a horizontal position.  Some ball check valves are made so they can be installed vertically. That is, flow from below raises the ball to allow liquid or gas to pass through the valve.

There will usually be an arrow on the outside of the check valve body to indicate the direction of flow.

Figure 14 ball check valve open and closed

Higher downstream pressure will cause the element to seat, closing the valve.

 

1.2       Function Of Check Valves

Check valves are used to control the direction of flow in a pipeline.  Swing and ball check valves can be used in both liquid and gas pipelines.  They are often installed downstream of reciprocating pumps and compressors.

A check valve and its engineering symbol are shown in Figure 15 check valve and its engineering symbol

Figure 15 check valve and its engineering symbol

The circle on the symbol indicates the upstream side of the check valve.   The arrow on the valve casing indicates the direction of flow for installation.

 

1.3       Safety Relief Valve

Safety relief valves prevent excess pressure, which may cause serious accidents.

The opening element in the safety relief valve shown in Figure 16 safety valve is a disc held closed by a spring.  The amount of tension on the spring is adjustable. Maintenance personnel set the tension on the spring when the valve is installed.

The safety relief valve pressure setting is always higher than the normal operating pressure of the system it is protecting.  It is always lower than the pressure, which could damage the system.

Figure 16 safety valve

Safety relief valves prevent damage to equipment from overpressure.   When fluid pressure at the inlet exceeds the spring tension, the valve disc will rise, opening the valve.

 

For example, a safety relief valve is often used to protect a positive displacement pump or compressor.  If the normal discharge pressure is 150psi, the safety relief valve might be set at 180psi.   If the pressure in the discharge pipeline goes up to 180psi, the pressure will overcome the tension of the valve spring.  The valve disc will lift up off its seat.  The liquid or gas causing the high pressure will then escape through the valve as shown in Figure 17 safety relief valve operation

Figure 17 safety relief valve operation

 

 

 

The valve opens when the fluid pressure at the inlet is higher than the setting on the relief valve.

Once the pressure in the pipeline falls below 180psi, the spring will force the valve disc back down in its seat.

Safety relief valves are used in both liquid and gas pipelines.  These valves are also installed on process vessels and tanks to protect them from excess pressure build-up.

 

Some safety relief valves will allow the liquid or gas to escape to the atmosphere.  Hydrocarbon liquids and gasses that escape through safety relief valves are piped to a burn pit or flare.

Flow through the relief valve will only occur under high-pressure conditions.

A safety relief valve and its engineering symbol are shown Figure 18 safety valve and its engineering symbol

Figure 18 safety valve and its engineering symbol

 

2        THE OPERATION VALVES

In this part of the module, you will learn how valves are operated.

  • Valves can be operated manually or automatically.
  • Valves can be operated locally or by remote cont

2.1       Manual Operation

Manual operation occurs when an operator opens, closes, or adjusts a valve. Figure 19 manual operation shows one method of manual operation.  The operator turns a hand wheel or moves a handle that changes the position of the valve disc or ball.

 

2.1.1            Handwheel Operation

Figure 19 manual operation

The operator turns the hand wheel clockwise to close the valve.   He opens the valve by turning the hand wheel counter clockwise. (Usually can be opposite)

In process operations, all valves with hand wheels operate the same way; you turn the hand wheel in a counter clockwise direction to open the valve and in a clockwise direction to close the valve.   Generally the hand wheels have arrows marked on the surface to show the direction for opening and closing the valve.

 

There are three important points to remember for hand wheel operation of gate valves:

  • Grease the stem after operati This helps to lubricate the stem, prevent corrosion, and keep sand out of the threads.  In some plants you will put a plastic hose over the threads after greasing them.
  • Always turn the hand wheel ¼ turn closed after fully opening the va This prevents the valve from seizing or sticking in the open position.
  • Always turn the hand wheel ¼ turn open after fully closing the valv This prevents the valve from seizing or sticking in the closed position.

 

2.1.2         Handle Operation

In general, to open a handle-operated valve you put the handle in line with the pipe work.   To close the valve you turn the handle 90 to the pipe work.   The movement from open to close is 90.

2.2       Local Operation

Figure 20 shows local operation of a motor-operated valve (MOV).

Local operation occurs at the valve.   If the valve is operated by a motor, the controls for the motor are located near the valve.

Figure 20 local operation of an MOV

The local controls are located close to the valve, in the field. The operator in Figure 20 local operation of an MOV is pressing a button that operates an electric motor. The electric motor does the work of opening or closing the valve.   This is local operation of the valve.

 

 

2.3       Remote Operation

Remote means at a distance, or away from.   Remote valve operation is usually performed by control room personnel.   A remotely operated valve is equipped with a valve actuator.   This is a mechanical device that provides force to open, close, or adjust a valve.

Valve actuators may be operated electrically, pneumatically, or hydraulically. In figure 20, the operator is using an electrically operated actuator to operate a valve.

 

InFigure 21 remote operation, electric wires connect the valve actuator mechanism to a pushbutton switch in the control room. A valve position indicator will be located near the pushbutton switch that operates the valve. The indicator shows whether the valve is open or closed.   A control room operator can operate the valve by pressing the pushbutton switch while he watches the position indicator.

Figure 21 remote operation

Remote valve operation is normally performed in the plant control room.

 

2.4       Automatic Operation

Automatic operation of a valve is when an instrument control system is used to open, close, or adjust a valve Figure 22 controlling water level compares automatic control with manual control.

Figure 22 controlling water level

Figure 22 controlling water level shows an example of automatic control.   The valve position changes automatically to adjust the water level in the tank.

The objective in each system is to control the water level.   The system below is a manual system.  An operator opens and closes the valve in the inlet line as necessary to keep the water level at the set point.  In this case, the set point is 50%.   The operator observes the level indicator and adds water when the level drops below 50%.

 

The system in figure 22 is an automatic system.  A sensing instrument continuously monitors the water level in the tank.  Another instrument, called a transmitter, sends an electronic or pneumatic signal corresponding to the water level from the sensor to a controller.  A controller is an instrument that takes the place of the operator in adjusting the valve.

The controller is preset to a specific set point.  The controller compares its set point with the information it receives from the transmitter about the actual water level.  If the water level is not exactly at the set point, the controller automatically adjusts the control value to bring the level back to the set point.

 

2.5        Local Automatic Operation

Local automatic operation means that a process instrument control system is located near the process control valve.  A plant operator sets the instrument controller to a required set point.  The automatic controller operates the valve as necessary to control the process.

2.6        Remote Automatic Operation

In remote automatic operation, the controller that operates the valve is located in a control room.   The control room may be quite far from the valve. A control room operator sets a required set point on the controller.  The controller opens and closes the valve to maintain the process at set point.

 

 

 

valves detail and classification 👌

Introduction

A Valve is a device that regulates the flow of gases, liquids or loose materials through an aperture, such as a pipe, by opening, closing or obstructing a port or passageway. A valve controls system or process fluid flow and pressure by performing any of the following functions:

 

* Stopping and starting fluid flow
* Varying (throttling) the amount of fluid flow
* Controlling the direction of fluid flow
* Regulating downstream system or process pressure
* Relieving component or piping over pressure

 

There are many valve designs and types that satisfy one or more of the functions identified above. A multitude of valve types and designs safely accommodate a wide variety of industrial applications.

 

Because of the diversity of the types of systems, fluids, and environments in which valves must operate, a vast array of valve types have been developed. Examples of the common types are the ball valve, butterfly valve, globe valve, gate valve, plug valve, diaphragm valve, reducing valve, needle vave, check valve and safety/relief valve. Each type of valve has been designed to meet specific needs. Some valves are capable of throttling flow, other valve types can only stop flow, others work well in corrosive systems, and others handle high pressure fluids. Each valve type has certain inherent advantages and disadvantages. Understanding these differences and how they affect the valve’s application or operation is necessary for the successful operation of a facility.

 

 

Although all valves have the same basic components and function to control flow in some fashion, the method of controlling the flow can vary dramatically. In general, there are four methods of controlling flow through a valve.

1. Move a disc, or plug into or against an orifice (for example, globe or needle or some types of check valves).

2. Slide a flat, cylindrical, or spherical surface across an orifice (for example, gate and plug valves).

3. Rotate a disc or ellipse about a shaft extending across the diameter of an orifice(for example, a butterfly or ball or some types of check valves).

4. Move a flexible material into the flow passage (for example, diaphragm valves).

Each method of controlling flow has characteristics that makes it the best choice for a given application of function.

 

Types of Valves

In piping following types of valves are used depending on the requirements. The cost of Valve in the piping system is up to 20 to 30% of the overall piping cost. And the cost of a given type and size of the valve can vary 100%. It means that if you choose ball valve over butterfly valve for the same function. It can cost you more. So, the selection of valves is essential to the economics, as well as operation, of the process plants.

 

1. Gate valve
Gate valve is the most common type of valve in any process plant. It is a linear motion valve used to start or stop fluid flow. In service, these valves are either in fully open or fully closed position. Gate valves are used in almost all fluid services such as air, fuel gas, feedwater, steam, lube oil, hydrocarbon, and all most any services. Gate valve provides good shutoff.

 

Types of gate valve

a. Wedge Gate Valves: – Commonly used in industrial piping for stop or isolating – to turn on and shut off the flow as opposed to regulating flow. Gate valves are named from the gate-like disc which operates at a right angle to the path of the flow. Gate valves are general service valves that can be made in a broad spectrum of sizes using a variety of different materials. Wedge gate valves are metal seated but are also available with resilient seat insert if drip tight shut off is required. They can meet the demands of a wide range of pressure and temperature conditions and is available in full port. Advantages – low pressure drop, straight through flow either direction. Disadvantages – slow acting, bulky. Not drip tight shut off (over 150NB). Do not partially open as this will cause damage to seat/disc.

 

b. Knife Gate Valves: – Useful for many applications in larger sized pipework (50mm up). Unlike traditional gate valves, they are able to throttle (at lower pressures) depending on line media and degree of opening. Metal seated knife gate valves are not leak tight shut off. Some knife gate valves have a resilient seat in order to ensure they close drip-tight. Available in v-port, o-port and lined they are ideally suited for the control of effluent, slurries, waste products, semi solids, pulp, bulk powders. Most knife gate valves are designed for single flow direction.

 

c. Parallel Slide Gate Valves: – Popular in steam applications as the energised disc design handles thermal expansion without sticking like wedge gate valves. Another advantage is lower torque then wedge gate valves especially in venturi (Ferranti) reduced bore configuration. Parallel slide valves consist of two parallel gates that are energised against the seat at all times by springs or a wedging spreader bar between the two gates. No mechanical stress is exerted between the discs, and the valve is not subjected to dangerous strains in opening and closing. This design of valve maintains fluid tightness without the aid of wedging action. These valves are used for saturated and super heated steam.

 

d. Pipeline Slab Gate Valves: – Available in parallel solid slab and expanding 2 piece wedging slab. Both styles protect the seat area from the flow in all operating positions. These valves have a full through conduit configuration with a hole in the slab. Slab style gate valves have seats that are spring energised. The expanding slab features two opposed sliding v-shape segments that maintain pressure against the seats. These valves are for API6D pipeline applications but are also used for API6A wellhead valves. All these valves are made in metal to metal and soft seat configuration.

 

2. Globe Valve
Globe valve is used to stop, start, and regulate the fluid flow. Globe Valves are used in the systems where flow control is required and leak tightness is also necessary. Globe valve provides better shut off as compared to gate valve and it is costlier than gate valve.

 

3. Check Valve
The check valve prevents backflow in the piping system. The pressure of the fluid passing through a pipeline opens the valve, while any reversal of flow will close the valve.

 

Types of check valve

a. Swing Check Valves: – Swing & Wafer checks cease reverse flow with a flap that swings onto a seat. Use swing checks only for forward flow that is horizontal or vertical upward.

b. Piston Check Valves: – Piston & Spring checks cease reverse flow with a spring loaded plunger.

c. Ball Check Valves:- Ball checks have a ball that slides into a hole as forward flow slows. Consider a ball check for semi-solids such as pulp or effluent.

d. Titlting Disc Check Valves: – Tilting Disc check valves are similar to Swing check valves but in most installations, slamming is minimised upon reversal of fl ow so noise and vibration are reduced.

 

4. Plug valve
Plug valve is Quarter-turn rotary motion Valve that uses a tapered or cylindrical plug to stop or start the flow. The disk is in plug shape, which has a passage to pass the flow. Plug valve used as on-off stop valves and capable of providing bubble tight shutoff. Plug valve can be used in vacuum to high-pressure & temperature applications

 

5. Ball Valve
A Ball valve is a quarter-turn rotary motion valve that uses a ball-shaped disk to stop or start the flow. Most ball valves are of the quick-acting type, which requires a 90° turn of the valve handle to operate the valve. The ball valve is Smaller and lighter than a gate valve of same size and rating.

 

6. Butterfly Valve
A Butterfly valve is a quarter-turn rotary motion valve, that is used to stop, regulate, and start the flow. Butterfly valve has a short circular body. Butterfly Valve is suitable for large valve applications due to Compact, lightweight design that requires considerably less space, as compared to other valves.

 

7. Needle Valve
Needle valves are similar to a globe valve in design with the biggest difference is the sharp needle like a disk. Needle valves are designed to give very accurate control of flow in small diameter piping systems. They get their name from their sharp-pointed conical disc and matching seat.

 

8. Pinch Valve
The pinch valve is also known as clamp valve. It is a linear motion valve. Used to start, regulate, and stop fluid flow. It uses a rubber tube, also known as a pinch tube and a pinch mechanism to control the fluid. Pinch Valve is ideally suited for the handling of slurries, liquids with large amounts of suspended solids, and systems that convey solid material pneumatically.

 

9. Pressure Relief Valve
A pressure Relief valve or pressure safety valve are used to protect equipment or piping system during an overpressure event or in the event of vacuum. This valve releases the pressure or vacuum at pre-defined set pressure.

 

Valves Functions

Valve serve a various function within the piping system. Such as

* Stopping and starting a fluid flow. Depending on whether a valve is open or closed, it let pass the process fluid or halt the fluid.
* Throttling the fluid flow. Some of the valves let you throttle the fluid depending open % of total opening. Lesser the opening higher the throttling and otherwise.
* Controlling the direction of a fluid flow. Multiport valve lets you decide the way fluid will go.
* Regulating a flow or pressure within the piping system. Some of the automatic control valves maintain the flow and pressure within the system by adjusting opening and closing.
* Relieve pressure or vacuum from the piping system and equipment. Pressure and vacuum relief valve safeguard the process system from overpressure and during vacuum condition.

 

Common Metal Types used in Valve Manufacture

The following is a general review of common valve materials used in general industrial, commercial and process valve construction.

* Aluminum – A non-ferrous metal, very lightweight, approximately one-third the weight of steel. Aluminum exhibits excellent atmospheric corrosion resistance, but can be very reactive with other metals. In valves, aluminum is mainly used as for exterior components such as a hand wheels or identification tags.

 

* Copper – Among the most important properties of wrought copper materials is their thermal and electrical conductivity, corrosion resistance, wear resistance, and ductility. Wrought copper performs well in high temperature applications and is easily joined by soldering or brazing. Wrought copper is generally only used for fittings.

 

* Bronze – One of the first alloys developed in the Bronze Age is generally accepted as the industry standard for pressure rated bronze valves and fittings. Bronze has a higher strength than pure copper, is easily cast, has improved machinability, and is very easily joined by soldering or brazing. Bronze is very resistant to pitting corrosion, with general resistance to a wide range of chemicals.

 

* Silicone Bronze – Has the ductility of copper but much more strength. Silicon bronze has equal or greater corrosion resistance to that of copper. Commonly used as a stem material in pressure-rated valves, silicon bronze has greater resistance to stress corrosion cracking than common brasses.

 

* Aluminum Bronze – The most widely accepted disc material used in butterfly valves, aluminum bronze is heat treatable and has the strength of steel. Formation of an aluminum oxide layer on exposed surfaces makes this metal very corrosion resistant. Not recommended for high pH wet systems.

 

* Brass – Generally good corrosion resistance. Susceptible to de-zincification in specific applications; excellent machinability. Primary uses for wrought brass are for ball valve stems and balls, and iron valve stems. A forging grade of brass is used in commercial ball valve bodies and end pieces.

 

* Grey Iron – An alloy of iron, carbon and silicon; easily cast; good pressure tightness in the as-cast condition. Grey iron has excellent dampening properties and is easily machined. It is the standard material for bodies and bonnets of Class 125 iron body valves. Grey iron has corrosion resistance that is improved over steel in certain environments.

 

* Ductile Iron – Has composition similar to gray iron. Special treatment modifies metallurgical structure, which yields higher mechanical properties; some grades are heat treated to improve ductility. Ductile iron has the strength properties of steel using similar casting techniques to that of grey iron and is used for class 250 (as well as class 125 in larger sizes).

 

* Carbon Steel – Very good mechanical properties; good resistance to stress corrosion and sulfides. Carbon steel has high and low temperature strength, is very tough and has excellent fatigue strength. Mainly used in gate, globe, and check valves for applications up to 454ºC, and in one-, two-, and three-piece ball valves. Can be forged or cast, with forgings being superior especially for larges sizes in very high classes.

 

* 3% Nickel Iron – Improved corrosion resistance over gray and ductile iron. Higher temperature as well as corrosion resistance and mechanical properties. Very resistant to oxidising atmospheres.

* Nickel-Plated Ductile Iron – Nickel coatings have received wide acceptance for use in chemical processing. These coatings have very high tensile strength, 50 to 225 ksi. To some extent, the hardness of a material is indicative of its resistance to abrasion and wear characteristics. Nickel plating is widely specified as a disc coating for butterfly valves. For industrial and petroleum ball valves, superior electroless nickel plating (ENP) is used in carbon steel valve components and is in fact superior to stainless steel in hardness but with similar corrosion properties.

 

* 400 Series Stainless Steel – An alloy of iron, carbon, and chromium. This stainless is normally magnetic due to its martensitic structure and iron-content. 400 series stainless steel is resistant to high temperature oxidation and has improved physical and mechanical properties over carbon steel. Most 400 series stainless steels are heat-treatable. The most common applications in valves are, for stem material in butterfly valves, and trim components such as seat, backseat bushings, discs, wedges etc. in cast steel gate, globe and check valves.

 

 

* 316 Stainless Steel – An alloy of iron, carbon, nickel, and chromium. A non-magnetic stainless steel with more ductility than 400 series SS. Austenitic in structure, 316 stainless steel has very good corrosion resistance to a wide range of environments, is not susceptible to stress corrosion cracking (however it is not suitable for higher levels of H2S typically found in wellhead applications) and is not affected by heat treatment. Very commonly used in valve body and/or trim material.

 

* 17-4 PH Stainless Steel – Is a martensitic precipitation/age hardened stainless steel offering high strength and hardness. 17.4 PH withstands corrosive attack better than any of the 400 series stainless steels and in most conditions its corrosion resistance closely approaches that of 300 series stainless steel. 17.4 PH is primarily used as a stem material for butterfly and ball valves as well as any valve application requiring a superior strength stem.

 

* Alloy 20Cb-3 – This alloy has higher amounts of nickel and chromium than 300 series stainless steel and with the addition of columbium, this alloy retards stress corrosion cracking and has improved resistance to sulfuric acid. Alloy 20 is widely used in all phases of chemical processing.

 

* Monel – Is a nickel-copper alloy used primarily as interior trim on all types of valves. One of the most specified materials for corrosion resistance to sea and salt water. Monel is also very resistant to strong caustic solutions.

 

* Stellite – Cobalt base alloy, one of the best all-purpose hard facing alloys. Very resistant to heat, abrasion, corrosion, impact, galling, oxidation, thermal shock and erosion. Stellite takes a high polish and is used in steel valve seat rings. Normally applied with transfer plasma-arc; Stellite hardness is not affected by heat treatment.

 

* Hastelloy C – A high nickel-chromium molybdenum alloy, which has outstanding resistance to a wide variety of chemical process environments including strong oxidisers such as wet chlorine, chlorine gas, and ferric chloride. Hastelloy C is also resistant to nitric, hydrochloric, and sulfuric acids at moderate temperatures

 

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