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Questions and Answers

What defines a Temperature and Pressure (T&P) Relief Valve?

A pressure relief valve that can only be actuated by pressure at the valve inlet

A pressure relief valve that can be actuated by pressure or temperature at the valve inlet (correct)

A pressure relief valve that is characterized by gradual opening

A pressure relief valve that can only be actuated by temperature at the valve inlet

What is a characteristic of a Relief Valve?

Rapid opening

Use for relieving compressible fluids only

Gradual opening that is generally proportional to the increase in pressure (correct)

Actuation only by temperature at the valve inlet

What does ASME BPVC I, Part PG-73.1.4 state about pressure relief devices?

Pressure relief devices are only used for compressible fluids

Unless otherwise defined, the definitions relating to pressure relief devices in ASME PTC-25 do not apply

Pressure relief devices are not defined in ASME PTC-25

Unless otherwise defined, the definitions relating to pressure relief devices in ASME PTC-25 shall apply (correct)

What is a characteristic of a Safety Relief Valve?

Rapid opening or gradual opening that is generally proportional to the increase in pressure and can be used for compressible or incompressible fluids Signup and view all the answers

What is the purpose of a Pressure Relief Valve?

To relieve pressure from pressurized equipment and then reclose Signup and view all the answers

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

Safety Valves have rapid opening, while Relief Valves have gradual opening Signup and view all the answers

What do all pressure relief valves have in common?

They are all actuated by pressure at the valve inlet and relieve pressure from pressurized equipment Signup and view all the answers

Why are the terms used to categorize pressure relief valves often confusing?

Because they are all similar in construction, operation, and purpose Signup and view all the answers

What is required when the set pressure of a PRV is changed?

A new valve data tag must be furnished and installed on the valve, and the valve must be resealed Signup and view all the answers

What is the purpose of loosening the jam nut on the adjusting nut?

To adjust the pressure at which the safety valve opens Signup and view all the answers

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

230°C Signup and view all the answers

What is the purpose of ASME BPVC I, Part PG-71?

To govern the installation of PRVs Signup and view all the answers

How should safety valves mounted on the drum, superheater, reheater inlet and outlet headers be installed?

In an upright position with the spindle vertical Signup and view all the answers

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

To leave ample room for expansion, so that no force is exerted on the valve Signup and view all the answers

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

To prevent the valve disc from moving horizontally Signup and view all the answers

What happens when the valve starts to open and steam fills the huddling chamber?

The effective area of the disc increases Signup and view all the answers

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

The area of the disc exposed to the pressure Signup and view all the answers

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

The difference between the actual popping pressure of a pressure relief valve and actual reseating pressure Signup and view all the answers

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

2 psi (15 kPa) or 2% of the set pressure Signup and view all the answers

What is the purpose of the blowdown adjusting ring?

To vary the size of the huddling chamber escape port opening Signup and view all the answers

What happens when the blowdown adjusting ring is moved to a position that blocks the huddling chamber escape ports?

The steam under the lip cannot exit from the chamber Signup and view all the answers

How can the PRV set pressure be adjusted?

By changing the compression of the spring Signup and view all the answers

What is the purpose of the wire seal on the locking screw?

To prevent unauthorized persons from tampering with the blowdown adjustment Signup and view all the answers

What happens when the valve is closing and the boiler pressure drops sufficiently?

The pressure under the lip decreases Signup and view all the answers

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 Signup and view all the answers

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

6% above the highest pressure at which any valve is set Signup and view all the answers

What is the main consideration for determining the set pressure of a superheater safety valve?

The pressure drop upstream of the valve Signup and view all the answers

What is the primary function of a safety valve in steam service?

To relieve compressible fluids Signup and view all the answers

What is the minimum temperature above which bronze safety valves are prohibited for use in superheaters?

230°C Signup and view all the answers

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 Signup and view all the answers

What is the purpose of the try lever on a safety valve?

To manually lift the valve Signup and view all the answers

What is the purpose of the seal attached to the adjusting nut on a safety valve?

To prevent tampering with the valve's set pressure Signup and view all the answers

What is the primary material used for the construction of superheater and reheater pressure relief valves?

Steel or alloy steel Signup and view all the answers

What is the purpose of the heavy coil spring in a safety valve?

To hold the valve disc tightly against the seat Signup and view all the answers

Study Notes

Pressure Relief Valves

Automatically open to relieve pressure and then automatically reclose
Exist in many different varieties, with 11 types defined by ASME PTC-25 Pressure Relief Devices
Each type has distinct operating characteristics and applications

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: 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, used for compressible or incompressible fluids
Safety Valve: characterized by rapid opening, used to relieve compressible fluids

Requirements for Boilers

Each boiler shall have at least one pressure relief valve
Boilers with more than 500 ft2 of bare tube water-heating surface or electric boilers with a power input more than 1100 kW shall have two or more pressure relief valves
Pressure relief valve capacity shall be such that the valve will discharge all the steam that can be generated by the boiler without allowing the pressure to rise more than 6% above the highest pressure at which any valve is set

Superheater and Reheater PRVs

Pressure drop upstream of each pressure relief valve shall be considered in the determination of set pressure and relieving capacity of that valve
Superheater PRV must open before the drum PRVs do and close after the drum PRVs

Safety Valve Construction

Made of cast iron, steel, alloy steel, or bronze
Used for relieving compressible fluids, commonly found on steam boilers, steam lines, compressed air vessels, and refrigerant receivers
Consists of a spring that holds a valve disc tightly against a seat, which opens when the steam pressure reaches the set point pressure

Safety Valve Operation

Valve disc has a lip (skirt) that forms a huddling chamber, which fills with steam when the valve starts to open, increasing the effective area of the disc
When the valve lifts, the pressure of the steam acts on this increased disc area, resulting in a greater force against the compression spring, causing the valve to pop wide open
The valve will remain open until the pressure drops to below the popping pressure

Blowdown

Defined as 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
Minimum blowdown of 2 psi (15 kPa) or 2% of the set pressure is required by ASME BPVC I, Part PG-72.1

Installation of PRVs

Safety valves mounted on the drum, superheater, reheater inlet and outlet headers must be placed in an upright position with the spindle vertical
Must be connected to the boiler independent of any other steam connection, and located as close as possible to the boiler without any unnecessary intervening pipe or fitting

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Description

Learn about pressure relief valves, their types, and operating characteristics. Understand the difference between pressure relief valves and temperature and pressure relief valves.