API576 Flashcards
Types of RV? (5)
safety valve relief valve conventional safety-relief valve balanced safety-relief valve pilot-operated pressure-relief valve.
safety valve
- normally used with compressible fluids
2. Rapid Open or pop action
The term pressure-relief valve is generic in nature,can be classified as a (3)
safety valve, relief valve, or a safety relief valvE
relief valve is a pressure-relief valve actuated by the difference between
static pressure upstream of the valve and superimposed backpressure downstream
Relief Valve
- opens in proportion to the pressure increase over the opening pressure (not pop action)
- Relief valves usually reach full lift at either 10 % or 25 % overpressure
- have closed bonnets to prevent the release of corrosive, toxic, flammable,
or expensive fluids. - used for incompressible fluids (liquids)
- should not be used:
in steam, air, gas, or other vapor services;
installations that impose any backpressure
as pressure control or bypass valves.
Safety Relief Valve
- May be used as either a safety or relief valve
- fully open at 10 % overpressure when in gas or vapor service.
- in liquid service, full lift will be achieved at approximately 10 % or 25 %
overpressure - Two types of Safety Relief Valves
Conventional Safety-relief Valve
Balanced Safety-relief Valve
cold differential test pressure
pressure at which a pressure-relief valve is adjusted to open on the test stand. The cold differential test pressure includes corrections for the service conditions of superimposed backpressure or temperature or both.
conventional valve can utilize a closed spring bonnet that will allow any backpressure to be contained within all areas of the valve downstream of the seat. This backpressure can affect the set pressure, stability, and available capacity that can be provided.
Conventional Safety Relief Valve par 4.4
- directly affected by changes in the backpressure
2.has a bonnet that encloses the spring and forms a pressure-tight cavity. - Used in flammable, hot, or toxic material
effect of temperature and backpressure on the set pressure
Balanced Safety-relief Valve par 4.7
- incorporates a bellows or other means for minimizing the effect of backpressure on the operational characteristics of the valve
- Used for;
flammable, hot, or toxic material, where high backpressures are present in gas, vapor, steam, air, or liquid services; in corrosive service
when the discharge from the valves is piped to remote 3. The bonnet of a balanced pressure-relief valve shall be vented to the atmosphere at all times for the bellows to perform properly.
blowdown
The difference between the
set pressure and the closing pressure of a pressure-relief valve,
Pilot Operated Valves used for
Used for;
1. where a large relief area and/or high set pressures
2. low differential exists between the normal vessel operating pressure and the set pressure
3. large low-pressure storage tanks
4. very short blowdown is required
5. where backpressure is very high and balance design is required, since pilot-operated
valves with the pilots either vented to the atmosphere or internally balanced are
6. inherently balanced by design;
sensing of pressure at one location and relief of fluid at another location;
Pilot Operated Valves NOT used for (3)
Not used; In dirty or fouling service Liquid service with vapors that will polymerize High temperature service (due to o-rings in pilot valve
There are three major rupture disk types:
forward acting, reverse acting, and graphite.
Forward-acting (3) types
convention / scored / composite
Forward-acting Conventional Rupture Disk (2)
- formed (domed), solid metal disk designed to burst at a rated pressure applied to the concave side
- These disks have a random opening pattern and are considered fragmenting designs that are NOT suitable for installation upstream of a pressure-relief valve.
Forward-acting Scored Rupture Disks
1 . burst along score lines at a rated pressure applied to the concave side
2. are considered fragmenting designs that are suitable for installation upstream of a pressure-relief valve.
Forward-acting Composite Rupture Disks
1 . composite rupture disk is a flat or domed multi-piece construction disk.
2. SOME are considered fragmenting designs that are suitable for installation upstream of a pressure-relief valve.
Reverse-acting Knife Blade Rupture Disks
Knife blades installed in holders should be constructed of corrosion-resistant material and should be in- spected periodically to ensure sufficient sharpness to open the disk. Dull or damaged knife blades will likely prevent proper opening of the disk. See Figure 11.
Reverse-acting Scored Rupture Disks
generally are considered fragmenting designs that are suitable for installation upstream of a pressure-relief valve.
Graphite Rupture Disks
2
Graphite, shear-loaded rupture disks are flat and are designed to open by bending and/or shearing of the membrane
Graphite rupture disks WILL fragment upon rupture.
Causes of improper performance (6)
- Corrosion is a basic cause of many of the difficulties encountered with pressure-relief devices a rupture disk device installed on the inlet or outlet of a pressure-relief valve can provide added corrosion protection
- lengthy piping to the valve inlet. These can cause a valve to chatter.
- Careless handling during maintenance
piping strains resulting - Improper blowdown ring settings. This can cause a valve to chatter.
- Severe oversizing of the pressure-relief valve
Failed Springs
Two forms of failure;
1.Weakened by corrosion
Failed springs are almost always due to corrosion
Weakened spring causes a reduction in set pressure and the possibility of premature opening.
- Spring breaks
Broken spring causes uncontrolled valve opening.
Pressure-relief valves that do not pop at inlet pressures of ____of CDTP should be considered as stuck shut
150 %
The primary reasons for applying rupture disks upstream of pressure-relief valves include the following. (4)
a) Prevent plugging of pressure-relief valve—
b) Prevent corrosion of pressure-relief valve—
c) Avoid cost of high alloy pressure-relief valve—High alloys or exotic alloys for rupture disk construction cost much less than a corresponding relief valve with the same alloy trim.
d) Prevent leakage through pressure-relief valve—
PRD set pressure tolerances allowed according to ASME for up to 70 PSI AND >70PSI
The set pressure tolerances, plus or minus, of pressure relief valves shall not exceed 2 psi (15 kPa) for pressures up to and including 70 psi (500 kPa)
and
3% for pressures above 70 psi (500 kPa).
the rupture disk shall be the (2) ________as the pressure-relief valve inlet
same nominal pipe size / Or larger.
accumulation
The pressure increase over the maximum allowable working pressure (MAWP) of the vessel or piping system,
superimposed backpressure
The static pressure that exists at the outlet of a pressure-relief device at the time the device is required to operate. (NOT YET OPEN)
backpressure
The pressure that exists at the outlet of a pressure-relief device as a result of the pressure in the discharge system. Backpressure is the sum of the superimposed and built-up backpressures.
built-up backpressure
The increase in pressure at the outlet of a pressure-relief device that develops as a result of flow after the pressure-relief device opens. (OPENS)
blowdown
The difference between the set pressure and the closing pressure of a pressure-relief valve, expressed as a percentage of the set pressure or in pressure units.
simmer
The audible or visible escape of compressible fluid between the seat and disc, which may occur at an inlet static pressure below the set pressure prior to opening.
type of valve that has a huddling chamber?
safety
Reverse-acting Scored Rupture Disk (2)
pressure applied on the convex side, thicker disk materials may be used, thereby lessening the effects of corrosion, eliminating the need for vacuum support, and providing LONGER SERVICE LIFE
AND
highest operating pressure
To prevent leakage of the loading fluid, an optical precision on the order of ____ light beads/bands according to manufacturer’s specifications should be maintained in the flatness of seating surfaces on metal-seated pressure-relief valves
three
what can cause chatter? (2)
- long piping between vessel and RV
- improper blowdown ring setting
- oversizing the RV
oversizing the RV
cause the valve to cycle (open/close repeatedly)
To ensure that the valve is opening, produce a ____ at the set pressure
distinct pop
as-received pop testing should be conducted prior to cleaning in order to yield accurate that will help establish
appropriate inspection and servicing interval.
Once the valve is set to pop at its CDTP, it should be checked for leakage. On the test block, it can be tested for seat tightness by increasing the pressure on the valve up to the manufacturer’s specified simmer pressure (oftentimes this is ________ of the CDTP)
90 %
There are three applications for Rupture Disk Devices:
- 5.3.2 Single Relieving Device
a) When complete discharge of the process contents can be tolerated when the disk ruptures.
b) When a fast opening response is required. - 5.3.3 Supplemental Relieving Device
a) To provide larger relief area for remote likelihood overpressure scenarios.
