34 Questions
What is the main purpose of a pressure relief valve?
To relieve excess pressure
What is the difference between a Pressure Relief Valve and a Temperature and Pressure (T&P) Relief Valve?
One is actuated by pressure, the other by temperature
What is characteristic of a Relief Valve?
Gradual opening proportional to pressure increase
What is the primary difference between a Safety Relief Valve and a Safety Valve?
One is for compressible fluids, the other for incompressible fluids
According to ASME PTC-25, how many types of pressure relief devices are defined?
11
What is the purpose of the ASME BPVC I, Part PG-73.1.4?
To adopt ASME PTC-25 definitions for pressure relief devices
Why are terms like 'safety valve', 'relief valve', and 'safety relief valve' often used indiscriminately?
Because they have great similarity in construction, operation, and purpose
What is the significance of Figure 1 in the context of pressure relief valves?
It summarizes the main types of pressure relief valves
What is the purpose of the guides at the bottom and top of the valve disc?
To allow the valve disc to move only in a vertical direction
What is the function of the huddling chamber in a safety valve?
To increase the effective area of the disc
What is blowdown defined as in ASME PTC-25?
The difference between actual popping pressure and actual reseating pressure
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 change 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 with a larger drop in boiler pressure
What is the maximum percentage by which the PRV set pressure can be increased or decreased?
5%
How is the locking screw secured in place?
With a wire seal installed by the jurisdictional Boiler Inspector
What is the purpose of the lip (skirt) on the safety valve disc?
To increase the effective area of the disc
What happens when the valve begins to lift?
Steam rushes into the huddling chamber
What must be furnished and installed on the valve when a PRV set pressure is changed?
A new valve data tag
To decrease the blowoff pressure, what should be done to the adjusting nut?
Screw the adjusting nut upwards
What is the maximum steam temperature for which the cast-iron safety valve shown in Figure 7 is suitable?
230°C
According to ASME BPVC I, Part PG-71, how should safety valves be placed on the drum, superheater, reheater inlet and outlet headers?
In an upright position with the spindle vertical
What should be done to the discharge pipe, if used, according to ASME BPVC I, Part PG-71?
Support it separately, leaving ample room for expansion
What is the standard that covers the design, material selection, capacity, testing, adjustments, and sealing of safety valves?
ASME BPVC I.F
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 percentage above the highest pressure at which any valve is set that the pressure relief valve can allow the pressure to rise?
6%
What is the primary purpose of a safety valve in steam service?
To relieve compressible fluids
What material is prohibited for superheater pressure relief valves operating over 230°C?
Bronze
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 typically adjusted?
By screwing the nut up or down
What is the minimum pressure required to operate the try lever on a safety valve?
75% of boiler operating pressure
What is the purpose of the pressure relief valve in a superheater?
To relieve excess steam pressure
What is the consequence of setting the set pressure of a superheater safety valve to the steam drum pressure?
The safety valve will never open
What is the purpose of the authorized inspector's seal on the safety valve?
To prevent access to the adjusting nut
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
This quiz covers the procedures for adjusting the pressure of a safety valve and installing a new valve data tag. It also explains how to increase or decrease the popping pressure.
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