All About Relief Valves

Question 1

Vessels connected to one another can be protected by a common relief device, provided there is no isolation valve in the piping that connects the vessels which may accidentally render one or more vessels unprotected. True or false?

Answer 1

True

Question 2

All pressure vessels require overpressure protection. True or false?

Answer 2

True

Question 3

All low-pressure storage tanks require pressure and vacuum relief for normal operation. True or false?

Answer 3

True

Question 4

Lines that are in continuous service and kept indoors require relief protection

Answer 4

False. Only lines not used in continuous service, and kept under the hot sun, require relief protection.

Question 5

A vessel jacket of the reactor does not normally require relief protection

Answer 5

False. A vessel jacket requires its own overpressure protection.

Question 6

How does a leak in cooling water coils cause an overpressurisation scenario?

Answer 6

If the water vaporises upon coming in contact with the hot reactor mixture, it can expand in volume rapidly.

If the water reacts with the reactor mixture and gives off vapour in the process.

Question 7

Compressors and turbines require relief devices on the discharge side for deadhead protection. T or F?

Answer 7

True.

Question 8

Positive displacement pumps require relief devices on both the discharge and suction side. T or F?

Answer 8

False. Discharge side only.

Question 9

There should be no isolation valves on the relief line for single relief valves. T or F?

Answer 9

True

Question 10

A isolation valve can be installed on the relief line in a CSO or CSC arrangement, if there is more than one relief line. T or F?

Answer 10

True

Question 11

Why are flare lines usually marked with ‘No Pockets’?

Answer 11

To permit free draining and ensure no blockage from trapped liquid.

Question 12

The required flow area for two-phase flow is often smaller than single-phase flow. T or F?

Answer 12

False. 2-phase flows require a larger relief area. The discharge piping also needs to be sized larger.

Question 13

What is the purpose of a KO Drum?

Answer 13

Remove liquid from relieved gases

Question 14

What is the purpose of a Seal Pot?

Answer 14

Prevent flash-back of flame from Flare Stack.

Question 15

Explain how a Seal Pot works

Answer 15

In the event of negative pressure, a backfire from the flare may occur.

Water in seal pot helps to extinguish the potential back fire.

This prevents the fire from being carried to upstream process lines.

Question 16

What is the purpose of a Flare Stack?

Answer 16

To safely burn off relieved gases

Question 17

What are the 3 considerations when designing a flare stack?

Answer 17

The pilot flame must remain lit at all times.

Air should be injected to promote smokeless burning.

Steam should be injected to atomise the hydrocarbon into smaller sizes for more efficient burning.

The height of the flare must be determined based on amount of heat generated.

The diameter of the flare must be suitable to maintain a stable flame.

Question 18

What is the purpose of a Cyclone Separator?

Answer 18

Remove entrained liquid before KO Drum

Question 19

What is the purpose of a Condenser

Answer 19

Recover valuable components and reduce volume of relief material that require treatment

Question 20

What is a Scrubber

Answer 20

Remove toxic components from relieved gases by contacting it with a lean solvent

Question 21

In a scrubber, the lean solvent is sent for further treatment, while the rich vapour may be sent to a flare. True or false?

Answer 21

False. The vapour would be lean by the time it exists the scrubber, not rich.

Question 22

The pressure at which the relief valve begins to activate is known as the __________

Answer 22

Set pressure

Question 23

The pressure increase in the vessel over the set pressure is known as the ______

Answer 23

Overpressure

Question 24

Relieving Pressure = ______ + _______

Answer 24

Set pressure + overpressure

Question 25

The maximum operating pressure in a reactor is known as the _______

Answer 25

Set pressure

Question 26

The maximum GAUGE pressure allowed at the top of the vessel at a specified temperature is known as the _________

Answer 26

Maximum Allowable Working Pressure (MAWP)

Question 27

The design pressure is the maximum GAUGE pressure allowed at the top of the vessel at a specified temperature. T or F?

Answer 27

True. MAWP and design pressure are same thing.

Question 28

The relieving pressure is the total pressure in the tank when the relief valve begins to activate. True or false?

Answer 28

True