Valves

Question 1

Explain the effect of valve position on back pressure and flow rate in piping systems.

Answer 1

When a valve is opened, flow through the valve increases and D/P, or headloss, across it decreases. Closing a valve causes D/P, backpressure, and headloss across the valve to increase and flow and downstream pressure to decrease.

Question 2

What are the five basic parts of a valve?

Answer 2

The body, bonnet, disk, seat rings, and stem

Question 3

Describe the function of the valve bonnet.

Answer 3

The bonnet mates with the body of the valve andsupports the stem, disk, and the device used toposition the valve (actuator).

Question 4

Describe the function of the valve disk.

Answer 4

The disk provides the means to control the flowthrough the valve. Throttling increases D/P across thevalve, which reduces downstream pressure andflow.

Question 5

Describe the function of the valve body.

Answer 5

The body is the main element of the valve assembly. Itdirects the flow through the valve and past the disk and its mating surface, the seat.

Question 6

Describe the function of the valve stem.

Answer 6

The stem attaches to the disk and extends out of the valve to allow an outside force to position the disk within the valve. Packing provides a seal between the bonnet and stem to prevent the fluid from escaping. The stem has a smooth surface finish that allows it to be sealed.

Question 7

Describe the function of the seat rings.

Answer 7

Seat rings provide the seating surface for the disk. In some designs, the body is machined to serve as the seating surface, and seat rings are not used. In other designs, forged seat rings are threaded or welded to the body to provide the seating surface.

Question 8

State the purpose and usage of valve backseats.

Answer 8

The stem may have an area with a wider diameter that isbeveled to mate with a similarly beveled surface in the bonnet of the valve. This area is known as the backseat of the valve. Not all valves have backseats. The primary purpose of the valve backseat is to remove system pressure from the packing in the event that the packing leakage is excessive and adjustment cannot correct theproblem.

Question 9

Describe the function and design characteristics of a Globe valve.

Answer 9

Globe valves are used to stop, start, and regulate fluid flow. The seat of the globe valve is either perpendicular to the flow of the liquid or at a sharp angle to normal flow. This makes the fluid going through the valve follow a torturous flow path. This flow path results in a high (DP) pressure loss across the valve.

Question 10

Describe the function and design characteristics of a Needle valve

Answer 10

Needle valves are used to make fine adjustments to the amount of the fluid allowed to flow through them. a needle valve has a long narrow point on the end of thevalve stem that serves as the disk. This permits a gradual change in the size of the opening when operated, which allows more precise throttling than is possible with a standard globe valve.

Question 11

Describe the function and design characteristics of a Gate valve

Answer 11

Gate valves are used to start or stop flow, but they are not intended to regulate or throttle flow. In a gate valve, the disk is completely removed from the flow stream when it is fully open. This characteristic offers virtually no resistance to flow when the valve is open, and, therefore, little pressure drop occurs across an open gate valve.

Question 12

Describe the function of a Butterfly valve

Answer 12

Butterfly valves are used to stop, start, and regulate fluid flow. They are used in high flow, large diameter, and low pressure systems, such as circulating water. Throttling is not as good as a globe valve.

Question 13

Describe the function of plug and ball valves.

Answer 13

Plug and ball valves are used to stop or start fluid flow. The names are derived from the shape of the disk, which resembles a plug or a ball. The primary disadvantages of plug valves and ball valves are (1) cost, (2) relatively rapid wear and corrosion of plug/ball-to-body seats, and (3) inability to regulate flow.

Question 14

Describe the function of a Diaphragm valve

Answer 14

Diaphragm valves are used to start, regulate, and stop fluid flow. Diaphragm valves have long lasting reliability,zero stem leakage, provide unrestricted streamlined flow with minimum loss of pressure, and precise throttling when required. Diaphragm valves are not used for high pressure or high temperature fluids; they are used withradioactive or corrosive liquids and gases.

Question 15

Describe the function of a Check valve

Answer 15

Check valves are designed to permit flow in only one direction. They are closed when downstream pressure is greater than upstream pressure, blocking flow in the reverse direction (known as backflow). There are three basic types of check valves: (1) swing check, (2) liftcheck, and (3) stop check.

Question 16

List the common types of check valves.

Answer 16

Swing check, lift check, and stop check.

Question 17

State the purpose of valve packing.

Answer 17

On all valves where the stem penetrates the pressure boundary, a seal is required to keep the fluid that is flowing through the valve from leaking out around the stem. To properly seal, both the stuffing box and the stem must have a smooth, clean finish.

Question 18

State the purpose of an overpressure protection valve.

Answer 18

To provide component or system overpressure protection.

Question 19

Describe the function and operation of a relief valve.

Answer 19

It begins to open when system pressure reaches the preset pressure setpoint of the valve. It will continue to open as the pressure increases, until the valve is fully open. At that point, the valve is relieving at its maximum rated capacity. It closes when system pressure decreases below the preset setpoint pressure.

Question 20

Describe the function and operation of a safety valve.

Answer 20

Safety valves provide a large volume release path at low overpressure conditions. They are designed to fully open instantly at the preset pressure setpoint. They do not have any accumulation like relief valves.

Question 21

What is relief valve accumulation?

Answer 21

That pressure above the preset pressure, at which the valve is fully open. Accumulation is normally expressed in percent of the preset pressure.

Question 22

What is safety valve blowdown?

Answer 22

Safety valves stay fully open until the pressure falls to below the preset pressure; then they close sharply. Blowdown refers to the difference between the preset pressure setpoint and the actual pressure at which the a safety valve reseats (shuts). Blowdown is also expressed in percent of the pressure setpoint.

Question 23

When comparing steam overpressure protectionvalves with water overpressure protection valves, what two thumb rules apply?

Answer 23

First, mass flowrate, or mass lost by the system, is lower for a valve that relieves steam. Second, the pressure drop is faster when a valve lifts to relieve a vessel filledsolid with water than when the valve passes steam. This is why relief valves are mainly used on liquid systems, while safety valves are mainly used on steam or gas systems.

Question 24

Describe how a motor-operated valve would react to a loss of electrical power.

Answer 24

It fails in the “as is” position due to the removal of the operating torque. Now requires manual operation.

Question 25

Describe the operation of the manual declutch lever on a typical motor-operated valve.

Answer 25

It disengages the motor. Operate the declutch lever and the hand wheel to engage manual operator. Will automatically return to motor when motor operates.

Question 26

Describe the precautions used while manually operating a motor-operated valve.

Answer 26

Care should be taken during manual operation to avoid using excessive valve seating or backseating torque (normally prevented by the torque switches and limit switches). The torque required to break the valve free may exceed the available motor torque. It is good practice to tag open the power supply to the motor prior to manual operation, when feasible.

Question 27

Describe the four basic types of pneumatic actuators.

Answer 27

Gas cylinder, gas piston, gas diaphragm, and gas vane.

Question 28

Describe how thermal binding and pressure locking of valves occurs.

Answer 28

Thermal expansion and contraction can cause thevalve seat to bind against the valve disk. Pressure binding occurs when liquid trapped in the bonnet isheated and begins to pressurize the valve bonnet.If there are no means of pressure relief via valvepacking leakage or disk/seat leakage, thepressure build-up could prevent valve operation.

Question 29

List three methods to prevent thermal binding and pressure locking of valves.

Answer 29

One, valves can be cracked opened and reshut periodically during the cooldown. Two, using the proper closing torque can prevent the binding of the seat and disk. Three, pressure binding can be prevented by using a relief valve or vent valve on the valve bonnet to equalize or reduce the pressure buildup.

Question 30

Identify potential causes of valve operator difficulties following maintenance or valve operation.

Answer 30

Valve sticking may also occur due to improper packing adjustment. If the packing gland of a valve is overtightened, stem binding can occur, which makes valve operation difficult.

Question 31

Describe the precautions associated with transferring a valve controller between manual and automatic modes.

Answer 31

Ensure the manual and automatic control signals are matched to prevent rapid repositioning of the valve. This technique, known as a “bumpless transfer”, minimizes system disturbances and possible valve damage

Question 32

Describe the method for verifying valve stroke time using a stopwatch.

Answer 32

Stroke time is measured from the time the valve is given the open or close signal, until it reaches the desired position.