4BE3 U3 Obj 1 Easy

64 Questions

What is the primary function of pressure relief valves?

To relieve pressure from pressurized equipment

According to ASME PTC-25, how many types of pressure relief devices are defined?

What is the main difference between a Relief Valve and a Safety Relief Valve?

Opening characteristics

Which type of pressure relief valve is normally used for compressible fluids?

Safety Valve

What is the purpose of ASME BPVC I, Part PG-73.1.4?

To establish definitions for pressure relief devices

What is the main difference between a Safety Valve and a Safety Relief Valve?

Opening characteristics

Why are the terms 'safety valve', 'relief valve', and 'safety relief valve' often used interchangeably?

Due to similar construction and purpose of pressure relief valves

What is the name of the standard that defines pressure relief valves?

ASME PTC-25

What is the minimum number of pressure relief valves required for a boiler with more than 500 ft2 of bare tube water-heating surface?

Two

What is the maximum allowed pressure rise above the highest pressure at which any valve is set?

What should be considered when determining the set pressure and relieving capacity of a superheater PRV?

The pressure drop upstream of the valve

What is the primary characteristic of a safety valve?

Rapid opening

What is the typical material used for constructing safety valves for superheaters operating over 230°C?

Alloy steel

What holds a safety valve disc tightly against its seat?

A spring

What is the purpose of the exposed spring in the safety valve shown in Figure 2?

To prevent the high temperature of superheated steam from weakening the spring

How is the set point pressure of a safety valve adjusted?

By screwing the nut up or down

Why is the try lever used on a safety valve?

To manually lift the valve

What is the purpose of the seal attached to the adjusting nut by an authorized inspector?

To prevent tampering with the adjusting nut

What must be furnished and installed on the valve when a PRV set pressure is changed?

A new valve data tag

What is the purpose of loosening the jam nut on the adjusting nut before adjusting the pressure at which the safety valve opens?

To allow adjustment of the set pressure

What is the recommended installation position for safety valves mounted on the drum, superheater, reheater inlet and outlet headers?

Upright position with the spindle vertical

According to ASME BPVC I, Part PG-73.1.4, which definition of pressure relief devices applies?

ASME PTC-25 definitions

What is the name of the standard that defines 11 different types of pressure relief valves?

ASME PTC-25

What is the characteristic of a Relief Valve, according to ASME PTC-25?

Gradual opening proportional to the increase in pressure

What is the minimum number of pressure relief valves required for a boiler with more than 500 ft2 of bare tube water-heating surface?

Two

What is the purpose of a discharge pipe in a safety valve arrangement?

To allow for expansion and prevent force on the valve

What is the maximum steam temperature that a cast-iron safety valve can handle, according to Figure 7?

230°C

What is the term that refers to all forms of reclosing pressure relief devices?

Pressure Relief Valve

What is the maximum allowed rise in pressure above the highest pressure at which any valve is set?

What is the purpose of considering the pressure drop upstream of each pressure relief valve?

To ensure the superheater PRV opens before the drum PRVs

What is the characteristic of a safety valve that distinguishes it from other pressure relief valves?

Rapid opening

What material is prohibited for use in superheater pressure relief valves operating over 230°C?

Bronze

What is the purpose of the exposed spring in Figure 2?

To prevent the high temperature of superheated steam from weakening the spring

What is the function of the nut 'L' in Figure 3?

To adjust the point at which the valve will lift and relieve the pressure

Why is the try lever used to manually lift the valve?

To test the valve operation

What is the purpose of the guides on the valve disc?

To ensure consistent operation and tightness of the disc

What happens if the safety valve is opened without pressure in the boiler?

The try lever or valve spindle can be damaged

What is the purpose of the seal attached to the nut 'L'?

To prevent access to the adjusting nut

What is the purpose of the guides at the bottom and top of the valve disc?

To ensure the valve disc moves only in a vertical direction

What is the function of the huddling chamber in the safety valve?

To increase the effective area of the disc

What happens when the boiler pressure drops to below the popping pressure?

The valve closes quickly

What is the minimum blowdown required by ASME BPVC I, Part PG-72.1?

2 psi or 2% of the set pressure

What is the purpose of the blowdown adjusting ring?

To vary the size of the huddling chamber escape port opening

What happens when the blowdown adjusting ring blocks the huddling chamber escape ports?

The valve recloses at a higher pressure

How is the blowdown adjusting ring secured in place?

With a locking screw

What is the maximum percentage change allowed for the PRV set pressure?

What is the purpose of the compression spring in the PRV?

To hold the valve closed

What is the definition of blowdown according to ASME PTC-25?

The difference between the actual popping pressure and the actual reseating pressure

What is the purpose of screwing the adjusting nut upwards on a safety valve?

To decrease the spring compression

What is the maximum steam temperature that the cast-iron safety valve in Figure 7 can handle?

230°C

What does ASME BPVC I, Part PG-71 cover?

Installation of PRVs

How should safety valves be mounted on the boiler, according to ASME BPVC I, Part PG-71?

In an upright position with the spindle vertical

What should be done with the discharge pipe, if used, according to ASME BPVC I, Part PG-71?

It should be separately supported, leaving ample room for expansion

Which ASME BPVC section is similar to the discharge piping arrangement shown in Figure 8?

Section VII

What is the purpose of the huddling chamber in a safety valve?

To increase the effective area of the disc

What happens when the valve begins to lift and steam rushes into the huddling chamber?

The valve suddenly lifts and 'pops' to its full opening

What determines the lifting force exerted on the disc by the boiler pressure?

The area of the disc exposed to the pressure

What is blowdown, according to ASME PTC-25?

The difference between the actual popping pressure of a pressure relief valve and the actual reseating pressure, expressed as a percentage of set pressure or in pressure units

What is the minimum blowdown required by ASME BPVC I, Part PG-72.1?

2 psi (15 kPa) or 2% of the set pressure

What is the purpose of the blowdown adjusting ring?

To vary the size of the huddling chamber escape port opening

What happens when the blowdown adjusting ring blocks the huddling chamber escape ports?

The steam under the lip cannot exit from the chamber

How can the PRV set pressure be changed?

By changing the compression of the spring

What must be done when the PRV set pressure is changed?

A new valve data tag must be furnished and installed on the valve, and the valve must be resealed

What must be loosened before adjusting the pressure at which the safety valve opens?

The jam nut on the adjusting nut

Study Notes

Pressure Relief Valves

Automatically open to relieve pressure and reclose after normal conditions have been restored
Exist in many different varieties, with 11 types defined by ASME PTC-25 Pressure Relief Devices

Types of Pressure Relief Valves

Pressure Relief Valve: designed to actuate on inlet static pressure and reclose after normal conditions have been restored
Temperature and Pressure (T&P) Relief Valve: may be actuated by pressure at the valve inlet or by temperature at the valve inlet
Relief Valve: characterized by gradual opening, generally proportional to the increase in pressure, used for incompressible fluids
Safety Relief Valve: characterized by rapid opening or gradual opening, proportional to the increase in pressure, used for compressible or incompressible fluids
Safety Valve: characterized by rapid opening, normally used to relieve compressible fluids

Safety Valve Construction

Made of cast iron, steel, alloy steel, or bronze
Cast iron and bronze safety valves are used for lower pressures and lower temperatures
ASME BPVC Section I, Part PG-73 prohibits the use of valves with bronze parts for superheaters operating over 230°C
ASME BPVC Section I, Part PG-68 requires all superheater and reheater pressure relief valves to be constructed of steel or alloy steel

Safety Valve Operation

Held shut with a spring that holds a valve disc tightly against a seat
When steam pressure reaches the set point pressure (popping pressure), the disc rises slightly from the seat, and steam begins to escape
The valve disc has a lip (skirt) that forms a small compartment called a huddling chamber, which increases the effective area of the disc
The valve lifts suddenly and "pops" to its full opening, and remains open until the pressure drops to below the popping pressure

Blowdown

Defined as the difference between actual popping pressure and actual reseating pressure, expressed as a percentage of set pressure or in pressure units
ASME BPVC I, Part PG-72.1 states that pressure relief valves shall be designed and constructed to operate with a minimum blowdown of 2 psi (15 kPa) or 2% of the set pressure

Installation and Adjustment

Blowdown adjusting rings can be moved up or down to vary the size of the huddling chamber escape port opening
Once adjusted to a particular position, the rings can be locked into place with a set screw, and then sealed in position
The adjustment ring can completely block off, partially obstruct, or fully open the huddling chamber escape ports### Boiler Pressure and Safety Valve Reclosure
The boiler pressure must drop significantly for the valve to reclose.
When the huddling chamber escape ports are fully open, the steam under the lip can exit, creating a lower pressure under the lip, allowing the valve to reclose with less drop in boiler pressure.

Adjusting the Blowdown Ring

The adjustment ring can be set to an intermediate position, partially obstructing the escape ports, allowing the valve to reclose at a pressure in-between.
To change the position of the blowdown adjusting ring, remove the locking screw and turn the adjusting ring in the desired direction with a screwdriver.
After adjusting, lock the ring into place with the locking screw.

Preventing Unauthorized Tampering

The locking screw is secured in place by a wire seal, installed by the jurisdictional Boiler Inspector, to prevent unauthorized tampering with the blowdown adjustment.

PRV Spring and Set Pressure

The PRV spring is designed for a certain range of opening (or popping) pressure.
The PRV set pressure can be increased or decreased by a maximum of five percent (5%) from the setting marked on the valve.
Changing the set pressure requires a new valve data tag, and the valve must be resealed.

Adjusting the PRV Set Pressure

Before adjusting the pressure, loosen the jam nut on the adjusting nut.
To increase the popping pressure, screw the adjusting nut down to further compress the spring.
To decrease the blowoff pressure, screw the adjusting nut upwards to decrease the spring compression.

Safety Valve Design and Installation

ASME BPVC I covers safety valve design, material selection, capacity, testing, adjustments, and sealing.
ASME BPVC I, Part PG-71 covers the installation of PRVs.
Safety valves must be installed in an upright position with the spindle vertical, connected to the boiler independently, and located as close as possible to the boiler without unnecessary intervening pipe or fitting.

Discharge Piping Arrangement

The discharge pipe should be separately supported, leaving ample room for expansion so that no force is exerted on the valve.
The arrangement is similar to that shown in ASME BPVC, Section VII.

Learn about pressure relief valves, their functions, and different types, including pressure relief valves and temperature and pressure relief valves.

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