PROCEDURE NO.
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D. Ayon
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APPROVED APPROVED BY DEPARTMENT: SUBJECT:
PROCESS ENGINEERING
A. Bourji
SINGLE SINGLE PHASE PHASE LIQUID LIQUID FLOW FLOW - WATER WATER HAMME HAMMER R AND SURGE SURGE PRESSURE DESIGN DESIGN GUIDE
REVISION DATE
REV.
22-Jan-2004
Table of Contents
1.0 SCOPE.........................................................................................................................1 2.0 RESPONSIBILITIES ...................................................................................................1 3.0 DEFINITIONS.......... DEFINITIONS................... .................. .................. ................. ................. .................. .................. .................. ......................................1 .............................1 4.0 WATER HAMMER AND SURGE PRESSURE ............. ...................... .................. ....................................3 ...........................3 5.0 REFERENCES............ REFERENCES..................... .................. ................. ................. .................. .................. .................. .........................................10 ................................10
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PROCEDURE NO.
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PTD-DGS-119 PREPARED BY
PROCESS TECHNOLOGY PROCEDURES
D. Ayon
OF
1
12
DATE
22-Jan-2004
APPROVED BY DEPARTMENT: SUBJECT:
PROCESS ENGINEERING
SINGLE PHASE LIQUID FLOW - WATER HAMMER AND SURGE PRESSURE DESIGN GUIDE
A. Bourji REVISION DATE
REV.
22-Jan-2004
1.0 SCOPE
This design guide presents the basic physical principles involved in water hammer and pressure surge. In addition, this design guide provides a method for approximating surge pressure in simple cases (see Figure 1), which can be used for preliminary design or to check the results of commercially available programs. For detailed analysis of more complex systems, SURGE2000 software is available within the Process Engineering department. Figure 1.
Surge – Simple Case
2.0 RESPONSIBILITIES
The process engineer is responsible for calculating water hammer pressure and for determining the means of reducing it when it exceeds the pressure capabilities of pipe and/or components in a piping system. 3.0 Definitions a
β b D E L ∆ p
ρ
τ t c
∆V
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= = = =
= = = = =
pressure wave velocity liquid bulk modulus of elasticity pipe wall thickness internal diameter of pipe = modulus of elasticity of pipe wall = the distance from the valve to the pipe inlet maximum pressure rise liquid density [slugs/ft 3 (BG Units), or kg/m 3 (SI Units)] critical time valve closure time change in liquid velocity
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SINGLE PHASE LIQUID FLOW – WATER HAMMER AND PRESSURE SURGE DESIGN GUIDE
Note: Consistent units must be used for all formulas. It is recommended that the process engineer, when using the formulas supplied in this text, write out the units for each variable to ensure proper cancellation of units.
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SINGLE PHASE LIQUID FLOW – WATER HAMMER AND PRESSURE SURGE DESIGN GUIDE
4.0 WATER HAMMER AND SURGE PRESSURE
4.1
Potential Surge Pressure Hazards
A pressure surge is generated in a piping system whenever there is a change in flow rate of the liquid in the line. Surge pressures can cause extremely rapid changes in pressure, rapid enough to cause metallic percussions or a pounding of the line commonly known as water hammer. Potential causes of surge pressures are:
• • • • 4.2
Closure of an automatic valve Rapid closure or opening of a manual or power-operated valve Slamming shut of a non-return (check) valve Starting or stopping of a pump Pressure Wave Velocity
When a fast closing valve decelerates liquid flowing in a pipe, the kinetic energy of the flowing liquid is converted to surge pressure as the liquid compresses and the pipe wall stretches. The surge pressure propagates in a wave upstream to the pipe inlet at the speed of sound in the liquid, modified by the physical characteristics of the pipe. A return “negative” pressure wave is reflected back to the valve from the pipe inlet. The cycle of successive reflections of the pressure wave between the pipe inlet and valve result in alternating pressure increases and decreases which are gradually attenuated by fluid friction and imperfect elasticity of the pipe. Equation 4-1 will give the velocity of the pressure wave. a=
• • • •
β ρ 1 + ( β E )( D b)
(Eq. 4-1)
Notes See Figure 2 for average bulk modulus for crude oil, fuel oil, gas oil, and gasoline See Figure 3 for average bulk modulus for lubricating oils See Figure 4 for bulk modulus of water See Table 1 for modulus of elasticity for metals
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SINGLE PHASE LIQUID FLOW – WATER HAMMER AND PRESSURE SURGE DESIGN GUIDE Figure 2.
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SINGLE PHASE LIQUID FLOW – WATER HAMMER AND PRESSURE SURGE DESIGN GUIDE Figure 3.
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SINGLE PHASE LIQUID FLOW – WATER HAMMER AND PRESSURE SURGE DESIGN GUIDE Figure 4.
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4.3
Surge Pressure Rise
The time that it takes the pressure wave to travel from the valve to the pipe inlet and back is the critical time period τ . The critical time can be calculated using Equation 4-2.
τ =
2 L a
(Eq. 4-2)
4.3.1 Valve Closure Time < Critical Time When a valve closes faster than the critical time τ , the flow stoppage in the pipe is considered instantaneous. The maximum pressure rise for the instantaneous change in flow is calculated using Equation 4-3.
∆ p = ρ a∆V
(Eq. 4-3)
4.3.2 Valve Closure Time > Critical Time When the valve closure time exceeds the critical time, the pressure surge is reduced due to the cancellation between direct and reflected pressure waves. Equation 4-4 provides a rough approximation of the maximum pressure rise when the valve closure time exceeds the critical time.
τ ∆ p = ρ a∆V t c 4.4
(Eq. 4-4)
Pressure Capability
Pressure capability of the pipe is determined from the piping material class design pressure and design code allowance for pressure variation. For ASME B31.3-2002 design code: “Subject to the owner’s approval, it is permissible to exceed the pressure rating or allowable stress for pressure design at the temperature of the increased condition by not more than: (a) 33% for no more than 10 hr at any one time and no more than 100 hr/yr; or (b) 20% for not more than 50 hr at any one time and no more than 500 hr/yr.” For ASME B31.4 design code:
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“…Pressure rise due to surges and other variations from normal operations shall not exceed the internal design pressure at any point in the piping system and equipment by more than 10%. 4.5
Allowable Surge Pressure
Equation 4-5 determines allowable surge pressure: PS=PC -PO
(Eq. 4-5)
Where: PS PC PO 4.6
= = =
Allowable surge pressure Pressure capability of piping Piping operating pressure
Excess Surge Pressure
If the allowable surge pressure is greater than the maximum surge pressure, the piping is capable of withstanding the maximum surge pressure. If the pipe is not capable of withstanding the maximum surge pressure, the closing time of shutoff must be increased to reduce the surge pressure. Table 2 lists the minimum closing time for gate valves and permits comparison with the calculated critical time. If the valve cannot be closed slowly enough, or if a pump shutdown or startup is the reason for the water hammer, an air chamber or surge tank may be used to absorb all or most of the pressure rise. Commercial surge arrestors are available for most applications.
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Diameter of Valve (inches) 4 6 8 10 12 14 16 18 20 22 24 or larger
Number of Turns to Open (Typical) 9 13 27 32 38 45 52 58 64 76 76
Minimum Time of Closure (seconds) 9 18 42 58 69 91 205 117 158 158 158
5.0 REFERENCES
The following publications are used as references for this design guide: 1. American Society of Mechanical Engineers, Chemical Plant and Petroleum Refinery Piping, B31.3. 2. American Society of Mechanical Engineers, Pipeline Transportation Systems For Liquid Hydrocarbons And Other Liquids, B31.4. 3. Frick, T.C., Petroleum Handbook, p. 6.59. 4. Marks, L. S., Mechanical Engineers Handbook, McGraw-Hill Book Company, New York. 5. Moody, L. F., Water Hammer Surge - Analysis and Control, "Appendix F: Simplified Derivation of Water-Hammer Formula," Philadelphia. 6. Wood, D. J., and Funk, J.E., Computer Analysis of Transient Flow in Pipe Networks Including Surge Control Devices.
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