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What is a Check Valve?

What is a Check Valve?

The valve that is used to prevent backflow in a piping system is known as a check valve. It is also known as a non-return valve or NRV. The pressure of the fluid passing through a pipeline opens the valve, while any reversal of flow will close the valve.

It allows full unobstructed flow and automatically shuts as pressure decreases. The exact operation will vary depending on the mechanism of the valve.

Parts of Non-Return Valve

It consists of the body, cover, disk, hinge pin, and seat ring. In the image below, you can see the parts of the valve.

check valve parts

Types of Valves

The type of disk will decide the type of valve. The most common types of Check valves are

  • Swing Type.
    • Top Hinged
    • Tilting disk
  • Lift Type
    • Piston Type
    • Ball type
  • Dual Plate Type
  • Stop Check Valve

Let’s learn about each of them.

Swing Check Valve

swing check valve with parts

Image – DOE Handbook

The disc in a swing-type valve is unguided as it fully opens or closes. This Valve operates when there is flow in the line and gets fully closed when there is no flow. Turbulence and pressure drop in the valve is very low. Disk and seat designs can be metal to metal or metal to composite.

The angle between the seat and the vertical plane is known as the seating angle and varies from 0 to 45 degrees.  Usually, the seat angles are in the range of 5 to 7 degrees. Larger seat angles reduce the disc travel, resulting in quick closing, thus minimizing the possibility of a water hammer. A vertical seat has a 0-degree angle.

The swing-type valve allows full, unobstructed flow and automatically closes as pressure decreases. Usually installed in combination with gate valves because they provide relatively free flow combinations.

A basic swing-type valve consists of a valve body, a bonnet, and a disk that is connected to a hinge.

Tilting disk Check Valve

Image – DOE Handbook

The tilting disc-type valve is designed to overcome conventional swing-type valves’ weaknesses. The design of the tilting disk enables the valve to open fully and remain steady at lower flow rates and close quickly when the forwarding flow stops.

The dome-shaped disc floats in the flow and fluid flow on both the bottom and top of the disk surfaces. As the disk is spring-loaded, the spring force helps the valve close fast when forward flow pressure reduces.  In the image above, you can see the flow from the valve.

Tilting Disc type Valve is available in wafer type and lug type design.

Ball Type and Plug Type Lift NRV

The seat design of a lift Check valve is similar to a Globe valve. A piston or a ball is usually used as a disk.

Lift Check valves are particularly suitable for high-pressure service where the velocity of flow is high. The disk is perfectly set on the seat with full contact. They are suitable for installation in horizontal or vertical pipelines with upward flow.

When the flow enters below the seat, a disk is raised from the seat by the pressure of the upward flow. When the flow stops or reverses, the backflow and gravity force the disk downward to set on the seat. Commonly used in piping systems that used globe valves as a flow control valve.

ball and plug type lift check valve with cross section

You can see the plug or piston type and ball type check valve here. These valves provide superior leak-tight characteristics to those of swing check valves.

Some design in plug type uses spring to retain the disk in a closed position. This will ensure that the valve allows fluid flow only when there is enough pressure in the flow direction.

A ball-type valve is very simple as it works on the gravity principle. When there is enough pressure in the flow, it lifts the ball upward, but when pressure is reduced, the ball rolls down and closes the opening.

 

Dual Plate / Dual Disc Check Valve

dual plate check valve

A dual plate check valve is known as a butterfly check valve, Folding Disc Check valve, double-disc, or splits disc check valve. As the name suggests, two halves of the disk move towards the centreline with the forward flow, and with reverse flow, two halves open and rest on the seat to close the flow (Flapping action).

The use of the Dual Plate Check Valve is popular in low-pressure liquid and gaseous services. Its lightweight and compact construction make it a preferable choice when space and convenience are important.

It is 80 to 90% lighter than the conventional full-body check valve. Frequently used in systems that used butterfly valves. The cost of installation & maintenance is very low compared to other type

Stop Check Valve

stop check valve parts

Image- DOE Handbook

Stop Check Valve is a combination of a lift check valve and a globe valve. It can either be used as a check valve or as an isolation (stop) valve like a globe valve. These valves can be closed with the help of a stem that is not connected to the valve disc during normal operation, making it possible to use these valves as a regular NRV.

However, the stem is used when needed to hold the free-floating disc against the valve seat, just like a globe valve. These valves are available in tee, wye, and angle patterns. Swing and piston lift valves are commonly used as stop check valves.

Application of Check Valve (NRV)

Check valves (Non-return valves) are used in a piping system to prevent backflow. The rotary equipment’s discharge line, such as the pump and compressor, is always fitted with a check valve to prevent backflow.

Advantages & Disadvantages

Can you guess why I have not mentioned any advantages or disadvantages of the check valve?

The only function of a Non-return valve is to prevent backflow. There is no better alternative. Yes, you can choose the best NRV from the various available type that you have learned, but you cannot supplement the Non-return valve with another valve.

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What is Globe Valve?

A globe valve is a linear motion valve that stops, starts, and regulates fluid flow. The globe valve disk can be removed entirely from the flow path, or it will completely close the flow path. During the opening and closing of the valve, the disc moves perpendicularly to the seat.

This movement creates the annular space between the disk and seat ring that gradually closes as the valve closes. This characteristic provides the globe valve good throttling ability required for regulating the flow.

Leakage from the globe valve seat is less as compared to the gate valve, mainly due to right-angle contact between the disc and seat ring, which allows a tighter seal between the seat and the disc

Globe Valve Diagram

In the below globe valve diagram, you can see how the globe valve functions. The image also shows flow direction.

globe valve diagram

Image- By Petteri Aimonen

Globe valves can be arranged in such a way that the disk closes against the flow or in the same direction of flow.

When the disk closes in the flow direction, the fluid’s kinetic energy helps close but obstructs the opening. This characteristic is preferable when a quick-acting stop is required.

When the disk closes against the flow direction, the fluid’s kinetic energy obstructs closing but helps open the valve. This characteristic is preferable when a quick-acting start is required.

Globe Valve Parts

In the image below, you can see the globe valve parts such as Body, Bonnet, Stem, Seat, Disk, etc.

globe valve with parts drawing

Image- TROUVAY & CAUVIN

Globe Valve Disk Types

Globe valve is available in many different types of disc arrangement. The most used disk designs are listed below.

  1. Ball Type
  2. Needle Type
  3. Composite type
globe valve disc types - ball type and needle type

The ball disk design is used in low-pressure and low-temperature systems. It is capable of throttling flow, but in principle, it is used to stop and start the flow.

Needle disk design provides better throttling as compared to ball or composition disk design. A wide variety of long and tapered plug disks are available to suit different flow conditions.

A composition disk is used to achieve better shutoff. A hard, non-metallic insert ring is used in composition disk design.

Types of Globe Valve

Depending on the type of body, there are three types of globe valves;

  1. Z types
  2. Y types
  3. Angle Types

  4. Z types Globe Valve

    The simplest design and most common type is a Z-body. The Z-shaped partition inside the globular body contains the seat. The horizontal seating arrangement of the seat allows the stem and disk to travel perpendicular to the pipe axis resulting in a very high-pressure loss.

    The valve seat is easily accessible through the bonnet, which is attached to a large opening at the top of the valve body. The stem passes through the bonnet like a gate valve.

    This design simplifies manufacturing, installation, and repair. This type of valve is used where pressure drop is not a concern and throttling is required.

    Z types Globe Valve with cross section drawing

    Y types Globe Valve

    The Y-type design is a solution for the high-pressure drop problem in Z-type valves. The seat and stem are angled at approximately 45° to the pipe axis in this type. Y-body valves are used in high pressure and other critical services where pressure drop is concerned.

    Angle types Globe Valve

    The angle globe valve turns the flow direction by 90 degrees without using an elbow and one extra pipe weld. Disk open against the flow. This type of globe valve can also be used in the fluctuating flow condition, as they can handle the slugging effect.

    Globe Valve Types based on Body Bonnet Connection

    Screwed bonnet: This is the simplest design available, and it is used for inexpensive valves.

    Bolted-bonnet: This is the most popular design and is used in a large number of globe valves. This requires a gasket to seal the joint between the body and the bonnet.

    Welded-Bonnet: This is a popular design where disassembly is not required. They are lighter in weight than their bolted-bonnet counterparts.

    Pressure-Seal Bonnet: This type is used extensively for high-pressure, high-temperature applications. The higher the body cavity pressure, the greater the force on the gasket in a pressure-seal valve.

    Application of Globe valve

    Globe Valves are used in systems where flow control is required, and leak tightness is also important.

    • It is used in high-point vents and low-point drains when leak tightness and safety are major concerns. Otherwise, you can use a gate valve for the drain and vent.
    • It can be used in Feed-water, chemical, air, lube oil, and almost all services where pressure drop is not an issue.
    • This valve is also used as an automatic control valve, but in that case, the stem of the valve is a smooth stem rather than threaded and is opened and closed by the lifting action of an actuator assembly.

    Advantages

    • Better shut off as compared to gate valve.
    • Good for frequent operation as no fear of wear of seat and disk
    • Easy to repair, as the seat and disk can be accessed from the valve top
    • Fast operation compares to gate valve due to shorter stroke length
    • Usually operated by an automatic actuator

    Disadvantages

    • High head loss from two or more right-angle turns of flowing fluid within the valve body.
    • Obstructions and discontinuities in the flow path lead to a high head loss.
    • In a large high-pressure line, pulsations and impacts can damage internal trim parts.
    • A large valve requires considerable power to open and create noise while in operation.
    • It is heavier than other valves of the same pressure rating.
    STV is one of the leading China globe valve suppliers focus on the design, manufacturing and supply of various globe valves. We are not only provide high-qualified products but also whole industrial valve solutions.
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Valve Trim and Parts Including API Trim Charts

A valve is an assembled product. Valve external parts and Valve trim parts such as Body, Bonnet, Disk/wedge, Seat, Stam, Gland Packing / Stud & Bolt / Gasket / Handwheel are all manufactured independently and assembled in a valve factory.

In the image below, you can see the main parts of a valve.

Parts of a valve

Image – Velan

You can see the cutout of the gate valve. The main components of the valve are listed below.

  • Body
  • Bonnet
  • Stem
  • Disk/Wedge
  • Seat, Packings
  • York
  • Bolt
  • Sleeves
  • Actuator
  • Backseat

Let’s learn about each of these parts of a valve.

YouTube Video

Valve Body or Shell

The body is the main pressure retaining part and accommodates valve trim. It provides the passage for fluid flow. The body may be cast, forged, or fabricated. Sometimes valve bodies are manufactured by a combination of cast, forged, or fabricated parts. Various metals, alloys, and non-metals are used to manufacture the valve body. The valve body is also known as a shell.

The ends of the valve are designed to connect the valve with pipe or equipment. Ends connections can be a butt, socket, threaded, or flanged type, and sometimes it simply sandwiches between two pipe flanges that are known as wafer ends.

Valve body parts

A valve body has different types of passages through which fluid passes. The design of these passages depends on the function of a valve.

  • The first body type is Reduced bore; in this type, the passage diameter of the valve is smaller than the connecting This is the most common design as it will reduce overall valve cost and, at the same time it, narrows the fluid flow.
  • The second type is a Full bore; in this type, the inside passage diameter of the valve is the same as connecting pipe. This type of body is used when pigging is required. Pigging is used for various purposes, such as cleaning and inspection of the pipeline.
  • The third type is Crossflow or Split section body, this kind of body is used mainly in the globe valve, piston, or plug type check valve.

You can see the images of all three types of bodies.

Full bore vs reduce bore valve

You can now easily work out the difference between the full-bore valve and reducing the bore valve from the above image.

Bonnet or Cover

The cover for the valve body is known as a bonnet. Like valve bodies, bonnets are also available in many designs.

Valve bonnet

Some bonnets function simply as a valve cover. For example, the swing check valve is shown in the photo. While others support valve internals and accessories such as the stem, disk, and actuator. In the case of the gate, globe, stop check, and diaphragm valves, the bonnet contains an opening for the valve stem to pass through. Usually, a stuffing box is also a part of the bonnet.

Some valves have a bonnetless design in which the valve body and bonnet are combined into one. You can see the bonnetless valve photo. In a split body ball valve, there is no bonnet because the body is split into two sections.

There are many ways to connect a bonnet with a body, such as bolting, threading, and welding. The body-bonnet joint is one of the primary sources of the leak; that is why it should be a pressure-tight. The bonnet is cast or forged of the same material as the body.

Valve bonnet types

What is Valve Trim?

The removable and replaceable internal parts of the valve that come in contact with the flow medium are collectively known as valve trim. Disc, valve seat, and stem are common for all the valves.

Valve Trim components will change with the types of valves. Valve-specific trim includes a back seat, glands, spacers, guides, bushings, retaining pins, and internal springs. Here in the image, you can see the gate valve trim parts. Because of the trim parts, disk movement and flow control are possible.

gate Valve trim parts

Disk

The first valve trim part is a disc. The disc is the part that allows, throttles, or stops fluid flow depending on its position. Types of disks define the name of the valve such as gate, ball, plug, and needle valve’s disk are also of the same shape as the name.

A valve disc could be cast, forged, or fabricated.  The valve disk is sometimes required hard facing to improve wear resistance. Disk needed smooth machine surface to reduce the friction with a seat. The valve disk is a pressure-retaining part.

That means the disk holds the pressure.  When the valve is open, the disc does not perform pressure-retaining or -containing functions. However, when the valve is closed, the disc performs pressure-retaining functions.

Valve Disk Types

A disc rests against the stationary valve seat when the valve is in the closed position. It can be moved away from the seat by the movement of the stem. However, in check and safety-relief valves, fluid flow and pressure move the disc away from the seat.

Seat

The seat provides the seating surface for the disk. Here, you can see the gate valve seat in the above image. A valve may have multiple seats. In the case of a globe valve and swing-check valve, there is one seat. A gate valve and ball valve have two seats; one on the upstream side and the other on the downstream side.

The valve leakage rate is directly proportional to the effectiveness of the seal between the valve disc and seat(s).  Valve seats may be integral or replaceable rings. Valves are generally provided with a screwed, welded, or integrally cast or forged seat and hardened by heat treatment or by the hard facing of Stellite weld overlay.

A fine surface finish of the seating area is necessary for proper sealing. Some ball valves & plug valves used the non-metallic seat for non-critical services.  Valve manufacturers have developed several designs of combination valve seats involving elastomer and metal seats that are effective in achieving the desired leak tightness, which cannot be achieved only by metal seats.

Back Seat

The back seat is comprised of a shoulder on the stem and a mating surface on the underside of the bonnet. You can see it in the image. It forms a seal when the stem is in a fully open position. It prevents leakage of flow medium to the packing chamber and consequently to the environment. The back seat enables the replacement of the gland packing when the valve is in service

Stem

The stem connects the actuator and disk. It moves and positions the valve disk. The valve stem transports the required motion to the disc, plug, or ball for opening, closing, or positioning the valve. The stem connects the actuator, handwheel, or the lever of the valve at one end and the disc on the other end.

In gate and globe valves, the linear motion of the disc opens or closes the valve, while in the plug, ball, and butterfly valves, the disc rotates to open or shut the valve. Stems are typically forged from stainless steel and connected to the disk by threaded or welded joints.

Valve non pressure parts

Bonnet Bolt & Gland Eyebolt

Bonnet bolt or stud, hold the bonnet and body to create a presser tight seal between them. Gland eyebolt serves two functions. First, it connects the gland flange and bonnet. Second, when you tighten the bolt, it pushes the gland bush to retain gland packing in the stuffing box.

York, Yoke Bushing, Yoke Nut

The yoke is also called arms. It connects the valve body or bonnet with the actuating mechanism. The yoke and bonnet are designed as a one-piece construction in many valve designs. A yoke must be sturdy enough to withstand forces, moments, and torque developed by the actuator.

The top of the yoke holds a yoke nut. The valve stem passes through the York. It converts the rotary motion of the actuator into linear motion and moves the valve stem.

Yoke Bushings, also known as stem nut, is an internally threaded nut held at the top of a yoke through which the valve stem pass. Usually, the yoke nut and bush are made of a softer material than the stem to reduce the effort of valve opening. Valves that require greater effort to open or close are provided with anti-freeze yoke-sleeve bearings that minimize the friction between the hardened stem and the yoke bushing.

Non-pressure Retaining Parts of a Valve

Gland Flange is used to provide support to the gland bush to keep the gland packing under tension in the stuffing box.

The Bush or Gland sleeve keeps the gland packing inside the stuffing box.

Stem packing or Gland packing or contained in the stuffing box. Gland packings are made from graphite or PTFE as required by services. Proper compression of gland packing is required to prevent the leak from the stem.

With the help of gland flange and sleeve, you can compress the gland packing. Gland packing is one of the primary sources of fugitive emission in a process plant. Regular maintenance is required to ensure the proper function of packing.

Valve Trim Chart

Trim materials such as disks, seats, stem, back sheet, and sleeves are grouped together and assigned one number called Trim No. or Combination number. This will element the requirement of defining material grade for each component.

  • API 600 & 602 gives the list of Trim material that can be used in the valve.
  • The most common trim grades are ASTM A410(13Cr), ASTM A316, Alloy 20 (19Cr-29Ni), and Monel (CuNi Alloy).

Here in the image, you can see the simplified chart of the trim material. Against trim number, the seat, disc, backseat, and stem material are specified. This makes it easier to order the valve as you just have to specify trim no based on the requirements and need not specify the material for each of the parts. This list is included in the resource section.

API trim chart for valve
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