10. PIPINGS/VALVES and PRESSURE VESSELS
Piping Standard
Schedule number
• Pipes were originally classified on the basis of wall thickness
as standard (extra strong, and double extra strong).
• Because of modern industrial demands for more exact spec,
pipes are now specified according to wall thickness by a
standard formula for schedule number designated by the
American Standards Association.
• Schedule number is defined by ASS as: = 1000 Ps/Ss
where Ps = safe working pressure
Ss = safe working fiber stress
Piping Standard
Schedule number
• Ten schedule numbers are in use at present.
• These are 10, 20, 40, 60, 80, 100, 120, 140, 160
• For pipe diameters up to 10 in, schedule 40 corresponds to the
former “standard” pipe and schedule 80 corresponds to the
former “extra strong” pipe.
Piping Standard
Schedule number
How they came up with the formula?
Bursting pressure of a thin walled cylinder may be estimated from
the following equation:
Pb = 2STtm/Dm
where: Pb = bursting pressure
ST = tensile strength
tm = minimum wall thickeness
Dm = mean diameter
Piping Standard
Schedule number
A safe working pressure Ps can be evaluated from equation if the
tensile strength is replaced by a safe working fiber stress Ss
Ps = 2Sstm/Dm
Piping Standard
Nominal pipe diameter
Pipe sizes are based on the approximate diameter and are reported as
nominal pipe sizes. Although the wall thickness varies depending on
the schedule number, the outside diameter of any pipe having a given
nominal size is constant and independent of the schedule number. This
permits the use of standard fittings and treading tools on pipes of
different schedule numbers
Piping Standard
Tubing
Copper tubing, brass tubing are used extensively in industrial
operations. Other metals, such as nicklel and stainless steel, are also
available in the form of tubing.
Although pipe specifications are based on standard nominal sizes,
tubing specs are based on the actual outside diameter with a
designated wall thickness.
Conventional system, such as the Birmingham wire gauge (BWG) are
used to indicate the wall thickness.
Piping Standard
Fitting and other piping auxiliaries
• Fittings, flanges, valves, flow meters, steam traps and many other
auxiliaries are often rated on the basis of the safe operating
pressure as:
25 psi – low pressure
125 psi - standard
250 psi – extra heavy
300 to 10,000 - hydraulic
Piping: Code and Standards
• The American National Standards Institute (ANSI) and the
American Petroleum Institute (API) have established
dimensional standards for most widely used piping components
• ANSI B31
- List of those standards can be found in the ANSI B31 code
section
- Section also lists specifications for pipe and fitting materials
and testing methods of the American Society for Testing and
Materials (ASTM)
- American Welding Society (AWS) specification
- Standard of the manufacturers Standardization Society of the
Valve and Fitting Industry (MSS)
Piping: Code and Standards
The design of piping system applied to this project is listed as
ASME B31.3
• ASME stands for American Society of Mechanical Engineer
• ASME 31.3 is actually a section of ANSI B31
• ASME (ANSI) 31.3 is a Standard Number and designation is
Chemical Plant and Petroleum Refinery Piping
• ASME (ANSI) 31.1 scope and application:
- For all piping within the property limits of the facilities engaged
in the processing or handling of chemical, petroleum or related
product unless specifically excluded by the code
• Information on latest issue can be obtain for ASME. 345 East
47th st. New York NY 10017
Valves
1.
Gate Valves
– used to minimize pressure drop in the open position and to
stop flow rather than to regulate it.
Valves
2. Globe Valves - offer ease in throttling
Valve
3. Diaphragm valves
Diaphragm valves
– limited to 50psi, excellent for fluid containing suspended solids
Valves
4.
Plug valve – for throttling service, little likelihood of leakage
when closed.
Valve
5. Ball valves
Valves
6. Butterfly valve
Valves
7. Needle Valves – offer flow adjustment on small lines
Valves
Valves
8. Check Valves
• Used to prevent reversal of flow
1. Swing check valve
- Normal design is for use only in horizontal lines
2. Lift check valves
- Vertical lift check valve – for installation in vertical line, where
the flow is normally upward
- Globe check valve – for use in horizontal lines
- Angle check valve – used for installation where a vertical line
with upward flow turn horizontal
3. Tilting disk check valve
- May be installed both horizontally and vertically. Less pressure
drop at low velocity but greater at high velocity. Arrests
slamming.
Pressure-Relieving Devices
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The possibilities for development of excess pressure exist in nearly
every process plant.
Excess pressure can develop from:
1. explosion
2. chemical reaction
3. reciprocating pumps or compressors
4. process upsets
5. external fire around equipment
In addition to the possible injury to personnel, the loss of equipment
can be serious and an economic setback
Most countries have laws specifying the minimum attention required
in the applications of pressure-relieving equipment in process and
steam power plants.
Types of Pressure Relieving Devices
1.
2.
3.
4.
5.
Relief valves
Safety valves
Safety-relief valve
Conventional and balance valves
Frangible disk (rupture disk)
Relief Valve
• A relief valve is an automatic pressure-relieving device actuated by
the static pressure upstream of the valve, and which opens further
with increase in pressure over the set pressure
• Used primarily for liquid services
• Rated capacity is usually attained at 25 percent over pressure
Relief Valve
Relief Valve
Relief Valve
Safety Valve
• A safety valve is an automatic pressure-relieving device actuated by
the static pressure upstream of the valve and characterized by full
opening or “pop” action upon opening
• Used for steam, gas or vapor service
• Rated capacity is reached at 3, 10 or 20 percent overpressure,
depending upon applicable code.
Safety relief valve
• Safety relief valve is an automatic pressure relieving device actuated
by the static pressure upstream of the valve and characterized by an
adjustment to allow either a “pop” or a “non pop” action and a nozzle
type entrance
• Used on steam, gas, vapor and liquid (with adjustments) and is
probably the most general type of valve in petrochemical and
chemical plants
• Rated capacity is reached at 3 or 10 percent overpressure,
depending upon code and/or process conditions.
Safety Relief Valve
Conventional and Balance Safety Valve
• Conventional safety valves operate satisfactorily only when there is
relatively constant back pressure. Changes in back pressure can
seriously affect its operational pressure and flow capacity
• Balance valves operate satisfactorily under varying back pressure
as this has little influence on performance
Frangible Disk (Rupture Disk)
• A rupture disk is a thin diaphragm (metal, plastic, non metallic) held
between flanges and designed to burst at a predetermined pressure.
• Each bursting requires the installation of a new disk
• Used in corrosive service, and for required bursting pressure not
easily accommodated by the conventional valve.
• Applicable to steam, gas vapor and liquid system
Definition of Pressure Relief Terms
1.
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Set pressure
Is the inlet pressure at which the safety or relief valve is adjusted
to open
This pressure is set regardless of any back pressure on the
discharge of the valve
Definition of Pressure Relief Terms
2.
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Overpressure
Pressure increase over the set pressure of the primary relieving
device is over pressure
It is the same as accumulation when the relieving device is set at
the maximum allowable working pressure of the vessel
Definition of Pressure Relief Terms
3.
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Accumulation
Pressure increase over the maximum allowable working pressure
of the vessel during discharge through the safety or relief valve,
expressed as a percent of that pressure.
Definition of Pressure Relief Terms
4.
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Maximum allowable working pressure
The maximum allowable working pressure of an unfired pressure
vessel is that pressure determined by code requirements, the
metal material of construction and its operating temperature,
above which the vessel may not be operated
For a given metal temperature, this pressure is the highest
pressure at which the safety device may be set to open.
Definition of Pressure Relief Terms
5.
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Operating pressure
Pressure (gage), to which the vessel is subjected in service
A processing vessel is usually designed for a maximum allowable
working pressure, which will provide a suitable margin above the
operating pressure to prevent undesirable operation of the relief
device
Margin is approximately 10 % higher, or 25 psi – which ever is
higher.
Definition of Pressure Relief Terms
6.
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Blowdown
The reduction in flowing pressure below the set point required for
a device to close
Is the difference between the set pressure and the reseating
pressure of a safety or relief valve
Expressed as percent of the set pressure or psi
Definition of Pressure Relief Terms
7.
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Back pressure
Pressure developed on the discharge side of safety valves is back
pressure
This pressure may be generated by the flowing fluid as it passes
through the relief discharge piping, or it may be an established
pressure as a part of a discharge vent system into which the valve
is discharging
It may be combination of these two.
Vacuum Breaker
Electric Solenoid
Emergency Relief Vent
Thermal Valve