Module 2 : Valves

Question 1

What is the purpose of a valve?

Answer 1

To vary the quantity and direction of flow of a fluid or gas in a piping system

Question 2

What are the common valve materials?

Answer 2

• Carbon Steel
• Stainless steel
• Brass
• Plastic

Question 3

How are valves connected to pipes?

Answer 3

• Commonly by a bolted flange or threaded connection
• Less common is soldered or welded in place

Question 4

What are the 7 types of valves?

Answer 4

1. Globe valve
2. Gate valve
3. Butterfly valve
4. Ball valve
5. Plug valve
6. Check valve
7. Safety/ relief valve

Question 5

Purpose of Globe Valves

Answer 5

To control flow within a system

Question 6

Construction of a globe valve: what are the parts of a globe valve

Answer 6

1. Body
2. Bonnet
3. Disc
4. Stem aka spindle

Question 6

Material of a Globe valve

Answer 6

• Depends on application
• Factors to consider in selection of material: size of valve, pressure in system, type of system, medium in the system
• Most commonly carbon steel, stainless steel, brass or brass

Question 7

The body of the globe valve

Answer 7

Purpose : main structural component of the valve, it is the strongest part of the valve
Material : that of the valve itself
Construction : normally cast and then machined to the required tolerances.
- The body houses the seat, which has a tapered machined face that received the disc to seal off flow of medium. The seat is one part of the sealing system. The seat is often made of a different material than the body, such as fancy steel, but must be compatible with the base metal; it can be removed to be replaced, instead of needing to replace the whole valve. The inner edge of the seat is tapered b/c it is easier to use and will last longer
- The top of the body has a machined face to receive the bonnet. There is a passage drilled to allow for the passage of the spindle. Holes are drilled circumferentially in this face and are threaded for studs that will secure the bonnet. A groove is machined in way of the center hole to allow for a gasket to keep the bonnet and body concentric.
Operation : connects to the inlet and outlet sides of the pipe through threaded or flanged connections (most common), OR welded (high pressure systems).

Question 8

The bonnet of the globe valve

Answer 8

Purpose: the other half of the main structure
Material: that of the valve itself
Construction: Machined to sit on top of the body. Secured in place by studs AND a gasket for sealing purposes. A raised piece is cast into the bottom of the bonnet called a spigot to match the groove in the body and deform the gasket
- The top of the bonnet has a threaded arch (AKA yoke) to receive the spindle.
- Additionally, a hole is machined vertically through the bonnet such that the spindle can pass through and into the body of the valve. A recess is machined where the spindle will pass and is filled with packing.
Operation:
Nice to know: in some valves, the packing gland is threaded and that eliminates the need for a yoke

Question 9

What is packing?

Answer 9

Purpose: Used to seal shafts and spindles
Material: fibrous material
Construction: square cross section
Operation: Placed circumferentially around the stem and pushed down into the recess in the bonnet. Held in place by a packing gland which is secured by two threaded studs and nuts. When the gland is tightened down, it compresses the packing and forms a seal against the stem.
Nice to know: Must take care not to overtighten the gland as it could cause the spindle to bind (not turn) and/or wear/erode the surfaces of the packing.

Question 10

The stem of the globe valve

Answer 10

AKA spindle
Purpose:
Material: generally high carbon steel for steel and brass valves OR stainless steel for SS valves
Construction: The top is machined square to receive the valve wheel (handle). the stem is threaded through the yoke. The section that passes by the packing is machined smooth to ensure proper sealing. The bottom of the stem is fabricated to match the disc valve
Operation:

Question 11

The valve disc of a globe valve

Answer 11

AKA plug
Purpose: a circular plug that is attached to the bottom of the valve spindle and sits against the seat to create a seal
Material: Same material as the valve seat
Construction: Tapered to match the valve seat
Operation: The disc must remain concentric to the seat to seal properly - this can be done by fitting guides (wings) to the disc OR to the seat

Question 12

Operation of the globe valve

Answer 12

• When the spindle is rotated counterclockwise, it will withdraw from the valve body and lift the disc off the seat, thus OPENING the valve.
• When the spindle is rotated clockwise, it will lower the disc onto the seat and CLOSE flow.
• This type of valve is called a Screw Down Screw Lift (SDSL) valve.
• Globe valves allow for passage of fluid in both directions.
• If the disc is mechanically secured to the spindle, we call it a Screw Lift Valve.
• Some valves do NOT have the disc secured to the spindle; they rely on the pressure of the medium to lift the disc after the spindle is lifted. Once the pressure subsides, the disc will drop down even if the spindle remains raised. This is called a Screw Down Non-Return Valve (SDNR)
• Always installed so that the inlet (high pressure side) hits the bottom of the valve disc so that pressure is on the disc instead of the packing gland.
• When liquid passes through the valve, there is considerable resistance to flow which will cause a drop in pressure BUT the piping system is designed to account for this loss.
• Prone to damage due to the kinematic energy of the media (undergoing an abrupt directional change), causing turbulence that could lead to erosion. Additionally, if debris in fluid, the valve can become clogged.

Question 13

Nice to know: Globe valves

Answer 13

• Come in various configurations such as straight, right angle or Y shapes.
• Choice depends on application

Question 14

2. Gate Valves

Answer 14

Purpose: control flow within a piping system. Acts in such a way as to eliminate the resistance in flow and drop in pressure found in globe valves.
Material:
Construction: the interior of the valve is cast with a wedge shaped seat to receive a wedge shaped disc.
Operation:
- When the spindle is rotated counterclockwise, it will withdraw from the valve body and lift the disc off the seat, thus OPENING the valve.
- When the spindle is rotated clockwise, it will lower into the valve and CLOSE flow.
- The disc moves in a set of guides to ensure the correct orientation with respect to the seat.
- This valve is considered “fully open” or “fully closed”
- This valve is good for fluids with debris however it cannot regulate flow.
- The valve is not as easy to repair.

Question 15

3. Butterfly valves

Answer 15

Purpose: control flow within a piping system. Acts in such a way to greatly reduce resistance to flow. Used when there isn’t the space for a globe valve
Material:
- The body is circular and normally cast from carbon steel
- Round stainless steel or brass disc held in place by the spindle
- A ring (rubber or Teflon) is fitted inside the valve and forms a seal with the disc. It serves as the gasket to seal the valve to the flanges
Construction:
- The body could be flanged at inlet and outlet OR not flanged (Lugged - with threaded protrusions cast into the periphery OR Lugless/ Wafer - designed to sit between the sealing faces of the pipe flanges)
- The spindle extends out the top of the valve, held in place with brass bushings and sealed with an o-ring or packed gland. The bottom of the spindle sits in a recess in the lower part of the body
- The edges of the disc extend outwards over the inlet and outlet connections of the valve and are used to seal the valve to the pipes
- A handle is attached to the top of the spindle, often fitted with a locking arrangement to lock the valve fully open / fully closed / somewhere in between. Large valves will be turned by a gearbox.
Operation:
- When the spindle is rotated, the disc turns and the valve is opened for the passage of fluid. The orientation of the handle is the same as that of the disc.
- Commonly used on systems containing water, such as main engine cooling systems

Question 16

4. Ball Valve

Answer 16

Summary: contains a ball shaped plug within the valve body
Purpose:
Material:
- The ball is normally constructed of brass, stainless steel, or plastic
- The plug is made of chromed carbon steel, plastic, or stainless steel
Construction:
- The body of the valve is composed of two sections that screw together.
- The spherical plug rests on a Teflon seat inside the body and has a centrally drilled passageway.
- The top of the plug has a slot to receive the spindle, sealed with an o-ring or fibre washer
- The handle is secured to the top of the spindle with a small screw
- The inlet and outlet connections may be threaded or flanges, depending on the application
Operation: when the handle is rotated, the drilled passageway lines up with the inlet and outlet passages and permits the flow of liquid. The ball is stopped from over-rotating by means of tabs cast into the body such that the handle can only rotate 90 deg.
Nice to know: cannot be used on fuel or lube oil lines; only for water systems. Cannot be used for high pressure systems.

Question 17

5. Plug valve

Answer 17

AKA cock valve
Summary: tapered or parallel sided plug that can be rotated a quarter turn within the valve body. A hole is drilled through the plug and opens the inlet/outlet passage when rotated
Purpose:
Material: normally brass or carbon steel - choice depends on application
Construction:
- The plug may be tapered or straight and will have a horizontally drilled passageway. The plug sits down in the body of the valve and is held in place by a bolt secured into the bottom.
- The inlet and outlet are normally threaded but may be flanged or fitted with a compression fitting.
- The handle is normally in line with the drilled passageway and can turn 90deg.
Operation: when the handle is rotated, the drilled passageway lines up with the inlet and outlet passages and permits the flow of liquid
- Only low pressure applications
Nice to know: when the valve is mounted vertically, the valve handle will be perpendicular to the drilled passageway. This is done to ensure the valve will not accidentally open due to vibrations in the system.

Question 18

6. Check valve

Answer 18

AKA Non-Return Valve
Purpose: prevents the flow of liquid in the wrong direction.
Material:
Construction: Similar to the globe valve BUT no spindle and cannot be manually closed. A guide will be fitted to the disc to ensure it sits properly.
Operation: Normally mounted vertically, but if needed horizontally, will use a swing check valve (valve disc hinged to the upper part of the valve body that closes against an axial seat).
Typically used in bilge and ballast systems to prevent accidental flooding of compartments.
Nice to know: the direction of flow is indicated by an arrow cast into the outside of the body.
- Should not be used if there’s debris in the fluid

Question 19

6a. Scupper valves

Answer 19

AKA Storm Valve
Purpose: suited for fluids that may contain debris as less tendency for debris to get trapped between the disc and the seat
Material:
Construction: similar to a swing-check valve, has a yoke cast into the bonnet and a spindle that when lowered, forces the disc onto the seat
Operation: used on sewage and water drain lines that connect to the ship’s hull.

Question 20

7a. Safety valve

Answer 20

Purpose: fitted to a pressure vessel, the valve will open fully when a specified pressure is reached. Often fitted in pairs depending on application or relevant regulation.
Material:
Construction:
Operation:
- Pressure drops dramatically when opened
- Boilers, air tanks

Question 21

7b. Relief valve

Answer 21

Purpose: To relieve pressure in a system to prevent overpressure conditions; when pressure is above a set point, the valve will gradually open. The opening is proportional to the pressure in the system
Material: Normally cast steel to withstand the high pressure that may be present
- The seat must be soft enough not to damage the valve disc yet durable for repeated operation. Most often, seat is made of stainless steel, with or without a coating for improved corrosion resistance. The valve disc is made of the same material as the seat
- The spindle is made of high carbon steel and is highly polished in way of the packing gland.
- The spring is fabricated from stainless steel and has sufficient strength so that the seat can open and close at the required pressure.
Construction:
- May be connected by a threaded fitting (low pressure) or flange (high pressure).
- The valve disc sits on the seat and will have wings or a pin cast into the bottom to ensure it sits concentrically on the seat
- The spindle is attached to the seat and is fitted with a plate that acts on the spring
- A helical spring is fitted between the plate and the yoke, secured by two threaded studs, and exerts pressure on the disc. The pressure can be adjusted by tightening the nuts on the studs.
Operation:
- Operated by pressure acting on the valve seat -> when sufficient pressure is reached to overcome the downward pressure of the spring, the disc will lift off the seat. When pressure drops, the spring will force the disc back down and flow is stopped.
- Often set to lift at 8%-15% over the working system pressure. Specifics vary by system and type of medium. See Marine Machinery Regulations for specifics.
- The outlet is often connected to a pipe that will direct the medium either to the bilge or out of the machinery space
- May be fitted with manual means of activation called “easing gear”. Could be as simple as a ring fitted into a hole drilled in the end of the spindle (for small valves) or a lever (for larger valves). For boilers or pressure vessels, a cable arrangement would allow for the valve to be operated at a location away from the top of the boiler.

Question 22

Valve chests

Answer 22

• Allows for several vales to take advantage of the same suction and discharge lines.
• Takes up less space than a full series of valves
• Possible to have screw lift and screw down non-return valves in the same chest.
• If used for oil AND water, will be fitted with an arrangement to ensure the mediums are isolated from each other -> called a changeover chest: use of an interchangeable blank that is swapped from one side of the chest to the other; not common on ships for fear of contamination
• Commonly constructed of bronze or cast steel with a bronze seat; discs are brass or steel depending on the application

Question 23

Critical systems

Answer 23

Systems where failure directly effects the seaworthiness of the vessel.
Eg. fire main - critical
Domestic fresh water or sewage - non-critical
Main engine fuel - non-critical
Hydraulics: cargo cranes - non-critical
Hydraulics: steering gear - very critical