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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).
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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 • 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 • 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
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 • 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 • 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
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.
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 • 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. 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. 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. 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. 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. 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. 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.
