PIPE WORK, VALVES AND FITTINGS

Transcription

1 CHAPTER - 6 PIPE WORK, VALVES AND FITTINGS 6 1 INTRODUCTION This chapter describes different types of pipes, sizing of pipes, different types of valves and fittings used in transferring water from the source to the desired location. Main pipe materials used in water industry are; * Cast Iron. * Ductile Iron. * Steel. * Asbestos Cement. * PVC. Each material has its own features, therefore a material suitable for the facility should be selected only after careful consideration of the application, chemical properties of the liquid being pumped, pressure exerted and cost. Cast iron had been the material used for pumping mains and distribution lines in the olden days. Today Ductile Iron pipes are commonly used for pumping mains because of their superior mechanical and physical properties. 6.2 COMPARISON OF DIFFERENT TYPES OF PIPES * PVC Pipes can withstand pressure upto 10 bar (Type 1000). They are easy to handle, cheap locally available and are commonly used in distribution systems. Mishandling of PVC pipes can lead to potential stress concentration points which could lead to premature failure particularly if surge is present. A good example is the Jayawadanapura hospital sewerage pumping main where more than 100 pipe bursts have taken place over a period of 10 years. It is better to avoid PVC pipes in pumping mains where pressure fluctuations due to surges are possible because fatigue arising from repeated surge effects can cause premature failure.

2 Asbestos cement pipes have been used in several places including for pumping mains and sewerage lines in Sri Lanka. Kalutara Integrated Water Supply Scheme, Soysapura Sewerage disposal line, Ratnapura and Balangoda Water Supply Schemes are a few examples. In all these places adverse results have been reported. Frequent pipe bursts, pipe leaks and collapsing of the pipe lines due to corrosive gaseous effects have been recorded. Now all these lines have been identified to be replaced with ductile and iron pipes. As these type of pipes are not properly coated with a cement or plastic coating, deterioration of the pipe line over a number of years is far more than ductile or cast iron pipes. Further in certain countries use of asbestos pipes in water industry have been stopped. Therefore it is the author's view that we should avoid the use of asbestos cement pipes in the future. Steel pipes are commonly used in pump houses especially when pipes run overhead due to its light weight and easy fabrication. Steel pipes eliminates flange joints and standard fittings. Steel pipes can be used upto 100 m head. The durability of steel pipes is less than that of ductile or cast iron and is about 25 years or less. For example suction pipes at Negombo newhigh lift pumps need replacement in about 12 years. Ductile cast iron are those in which the graphite is obtained in a Spheroidal or nodular form as a result of treatment of the molten metal with either Cerium or Magnesium. Ductile iron is also called Spheroidal graphite iron (S.G iron) or nodular cast iron. As the graphite is no longer in the form of flakes but gathered together in spherical globules, the fracture propagation lines are eliminated. As rich in graphite as cast iron, ductile cast iron conserves the traditional qualities together with three remarkable mechanical characteristics. * Resistance to traction and impact * High elongation. * High elastic limit

3 These characteristics of ductile cast iron are somewhat paradoxical with regard to those of conventional cast iron. Nevertheless ductile iron is really a cast iron: * The carbon content is the same as cast iron. * It is made in the same melting equipment. There are different classes of ductile iron pipes. The maximum working pressure of ductile iron pipes vary according to the class. 6.3 PIPE SIZING System losses depend on the size of the pipe in other words flow velocity is the governing factor which determines the losses in a system. Optimum flow velocities have been determined from previous experience. Recommended flow velocities differ for suction pipes and delivery pipes. Low velocities should be used on suction side to improve the NPSHA- Higher flow velocities can be used on the delivery side. Recommended flow velocities for water extracted from Hand Book of valves piping and pipe lines are given below. Pipe Bore Flow Velocity Inches mm ft/sec. m/sec Over 12" & 300 mm Table Recommended Suction Flow velocities 103

4 General formulae for suction pipes based on a flow velocity of 1.5 m/sec. or 5 ft/sec. are: Suction pipe diameter (mm) = V( 14 x liters/min) Suction pipe diameter (inches) = V[(Gal/min)/10] Pipe Bore Flow Velocity Inches mm ft/sec. m/sec Over 12" & 300 mm Table Recommended Delivery Flow Velocities General formulae for delivery pipes based on a flow velocity of 3 m/sec. or 10 ft/sec. are; Delivery Pipe diameter (mm) = V(7 x liters/min) Delivery Pipe diameter (inches) = V[(Gal/min)/20] A calculated pipe size from above formula will not necessarily be a standard pipe size. The choice would then be normally the next standard size up. If this is considerably larger than the calculated diameter, or inconvenient to use. next smaller size can be used provided that flow velocity with this size is not more than 10% of the recommended flow velocity. 104

5 However we should not forget that when conveying water over a long distance a larger pipe diameter will reduce the operating cost even if the capital cost is higher. Therefore, proper economic analysis should be carried out before finalizing the pipe size in such situations. For long distances flow velocities of 0.5 to 1.5 m/s is adopted. 6 4 INSTALLATION OF SUCTION AND DELIVERY PIPING Piping work has a serious influence on pump performance. If the piping work is not satisfactory the pump will not perform to the level expected to perform. Further the pump can fail prematurely. In piping work attention must be given to the following SUCTION PIPES * Pumps should have independent suction lines, otherwise air can get into the operating pump through the glands of the non operating pumps. * Suction pipe should be as short as possible and rise gradually towards the pump. Reducers on suction side must be of eccentric type. * Avoid placing of elbows adjacent to the pump. Provision of a straight pipe piece not less than 3 times the pump bore is recommended. * Sluice valve on suction side in a negative suction installation should be in the horizontal position to avoid air pockets. Suction side valves must not be throttled. * All joints in suction side should be perfect to avoid any possible loss of priming in a negative suction installation. * Suction bellmouths should be installed as indicated in Chapter 2. ' './.X DISCHARGE PIPES * The size of long discharge lines should be decided only after careful economic calculation as indicated in 6.3. * Air valves must be provided at high points. * Surge protection should be looked into.

6 6 4 3 COMMON TO BOTH SUCTION AND DISCHARGE PIPES * Weight of piping should not fall on the pump body. * Piping should not be connected to the pump by force, otherwise misalignment of the pump can happen. * Flexible couplings are preferable. * Pipe stands strands and supports should be strong enough to withstand all forces and stresses. 6 5 DIFFERENT TYPES OF VALVES There are many kinds of valves, most commonly used ones are discussed below. Valves used in pumping stations can be classified into four categories depending on their application namely; * Isolating valves. * Flow control valves. * Reverse flow protection valves. * Surge protection. The mode(s) of operation of the valve will depend on the type of the valve, and the different modes available for different types of valves are given below: * Manual ) ) Gate, Butterfly, and Rotary Valves * Electrical ) * Hydraulic - Rotary valves * Valve disc weight - Non return, Flap and foot valves 106

7 6.5.1 FOOT VALVES The purpose of the foot valve is to retain water in the suction pipe for priming pumps upto 250 mm bore. Foot valves are available in swing type and lift type. Swing type foot valves are widely used as the head loss in type is less than the lift type. The important properties of a foot valve are; * Water tightness. * Large effective area. * Free from clogging. * Strength GATE VALVES (SLUICE VALVES) Gate valves are used for the following purposes. * Isolation of the flow. * Flow control (For bores < 150 mm). * To reduce the load on the prime mover when pumps are started and stopped. There are two types of gate valves namely internal screw type and external screw type. In the case of internal screw type, the valve disc moves vertically by the rotation of the valve spindle but the valve spindle does not move vertically. This type is used for handling treated water. In the case of external screw type the valve spindle also moves along with the valve disc. This type has better resistance to wear and corrosion and is widely used for handling rawwater and sewage. BS 5163:1986 specifies requirements of cast iron gate valves with flanged ends. The range of valves covered in this BS is DN 50 to DN 600 for nominal pressures of PN 10. PN 16 and PN

8 6.5.3 BUTTERFLY VALVES Butterfly valves are used for flow control and isolation of flows. It is available in manual and electrical operation modes. It is simple in structure, has a smaller valve body and higher operating speed. Its resistance at the fully open position is higher than that of a similar gate valve. Butterfly valves are generally used for low lift pump houses. BS 5155:1984 specifies requirements of butterfly valves NONRETURN VALVES (Check Valves) Non return valves are used to prevent the flow in opposite direction. There are two types of check valves namely: * Swing type * Lift type. Ordinary swing type non return valves are commonly used when there is no possibility of a surge. Modified versions of the ordinary swing type check valve are available for slow closing or rapid closing. Slow closing non return valves are applied to moderate upsurges at the occurrence of the reverse flow following sudden stoppage of pump. When a pump is shut down water in pipe line decelerates and if the rate of charge of flow velocity is low, the valve door position will match the decelerating flow into at the instant of no flow, the door will just reach its seat. Conversely in a system where rapid retardation of the, water column, occurs, the door may lag behind the reducing flow, and may be some way from its seat when zero flow condition is reached. In this case reverse flow conditions will be established and the door will be driven forcibly on it seat, generating dangerous pressure surge in the system with consequent damage to pipe work and equipment. This phenomenon is termed as valve slam and is common in short pipe lines. As a thumb rule, systems where the length of piping is less than twice the static head are certain of suffer from valve slam. To avoid slamming caused by the valve closing after reverse flow occurs, a forced closing devise is often employed using a weight or a spring to facilitate quick closure. 108

9 The rapid closing check valve stops water flow at the very moment the reverse flow condition starts or even just before the reverse flow conditions and prevents valve slam in short pipe lines. An ideal operation of a check valve is that it closes completely at the same time when the normal tlow stops and does not cause any reverse flow. BS 5153:1974 specifies requirements for cast iron check valve for general purposes AIR VALVES Air valves can be divided into two types namely; * Single Orifice - Small and large orifice valves. * Double Orifice - Small and large orifice valves in combination. Small orifice valves of the single orifice type are used to release the air accumulated under pressure in sections of pipe lines during normal working conditions. Large orifice valves of the single orifice type are used to release the air when a pipe is being filled with water or automatically ventilating a pipe when it is being emptied of water. Double orifice valves can perform both duties mentioned above. The action of the air valves in releasing or ventilating a pipe line is done by a buoyant ball valve. In small orifice valves, as air accumulates in the upper part of the valve chamber it gradually depresses the water level reducing the upward thrust on the ball until the ball leaves the seat and high pressure air is discharged. In large orifice valves also under normal operating conditions the ball is held in contact. With the orifice seating by water pressure and leaves the beating during filling and emptying of the pipe. For sewerage mains action of the valves are actuated by floats which act like the balls in valves for water mains OTHER VALVES Apart from above valves, the following valves are also used in the water industry for various purposes. Rotary valves, flap valves, pressure relief valves and solenoid valves. IC 9

10 6.6 VALVE SELECTION AND SIZING Valves should be selected depending on the type of application. Cavitation can occur when valves are used in part opened positions in flow control situation. The severity and the position at which it occurs will depend on the valve cavitation characteristics. Butterfly valves and rotary valves are commonly used for flow control purposes. Butterfly valves are used for low or medium heads and rotary valves are used for medium or high head situations. Gate valves are not suitable for How control applications because the flow characteristics are not linear. For isolation of flows Gate Valves. Butterfly Valves or Rotary valves can be used as all these types provide complete tightness. Foot valves, non return valves and Hat valves are used for reverse flow protection. When valves are used to prevent pressure surges in pipe lines, pipe line characteristics and valve characteristics should be studied in deciding the type of valve. Valves available for surge protection are: * Slow closing check valves. * Rapid closing check valves. * Automatic pressure relief valves. * Rotary valves. When water flows at high velocities through valves, noise and vibration will be generated. Therefore valves should not be under sized. For gate valves, butterfly valves and check val\es the size should be equal to pump suction bore.

11 As the resistance in rotary valves is less when fully opened, valves with lesser bore than the suction bore can be used. As a thumb rule rotary valve size can be taken as same as the delivery side bore of the pump HEAD LOSS IN VALVES, FITTINGS AND PIPE SPECIALS Head loss in valves, fittings and pie specials is given by Hf = K x v 2 /2g Where ; Hf = Head loss (m) K = Friction coefficient in the particular component V = Flow velocity m/s g = gravitational constant So in a system, head loss for above is given by H f =ZKxv 2 /2g (m) The following summerises the friction coefficient (K) for the commonly used items. Foot valves with strainer DN 50 to K at v = 1 m/s K at v = 2 m/s Table Values of (K) 111

12 Multidoor Foot Valves DN K Table 6.2-Values of(k) Swing Type Non - Return Valves DN K at V = 1 m/s KatV = 3 m/s K at V = 3 m/s Table Values of (K) Butterfly Valves (Fully open) DN K at PN K at PN K at PN K at PN K at PN Table 6.4-Values off(k) 112

13 Bends K for K for Angle Short R = d Normal R = 2d Surface Smooth Rough Surface Smooth Rough Surface Smooth Rough K for Long R = 5d d = Bore R = Radius Table Values of (K) For Bends of 22 Vi use Vi the loss for 90 Bend. Tee (K Value) Through side outlet 1.8 Straight run Wye Through side outlet 0.8 Straight run 0.3 Coupling 0.3 Entrance Pipe projecting into tank 1.0 Pipe flush with tank 0.5 Rounded

14 Reducers (Use Velocity at small end) 0.25 Increasers 0.25/2g (v, v 2 ) V, = Velocity at small end) Sudden Contraction d/d = Va 0.42 Where d and D are the diameters at small d/d = Vi 0.56 and large ends. d/d = V* 0.20 d/d = Va 0.92 Where d and D are the diameters at small d/d = Vz 0.56 and large ends. d/d =Va Elbow Double Elbow 3.0 Bellmouth 0.1 Gate Valves Fully open 0.2 Three quarter open 1.0 Half open 5.6 A quarter open 24.0 (Source : SIHI Manual Design Manual NWSDB 1990).

15 6 8 STRAIGHT PIPES Both Colebrolok-white and Hazen Williams formulae are commonly used in calculating head losses in pipes. The Colebrook White equation is derived from Darcy Weisbatch equation. Darcy Weisbath formula H, = fx L/D x v72g Hazen Williams formula H,-= LQ I /C x D Colebrook White Equation 1/f = -2 log, 0 [2.51/Re Vf + K/D + 1/3.71] Where H f = Head loss L = Length of Pipe V = flow velocity D = Pipe diameter g = Acceleration due to gravity Q = Flow rate f = Friction Coefficient C = Coefficient of Pipe smoothness Generally Hazen williams formula is favoured for calculating head loss in raw and potable water pipe lines. The accuracy of the friction head loss will depend on the selection of the proper C" value. 'C" value of 120 is often used for design purposes, however at the initial stage of a project when the pipe line is new the head loss may assume a value given by C = 140 or so. After years of use the head loss will increase due to ageing. The Colebrook White equation for transitional flow between "hydraulically smooth" and 'hydraulically rough" is extensively' used for sewage, drainage and effluent systems but it is equally suitable for calculating the head loss in raw water and potable water pipelines.

16 Tables and charts have been established for various roughness values and temperatures. Hence appropriate corrections should be done when using such tables. Annexes 6.1 and 6.2 give the 'C values in Hazen Williams equation and 'K' values in the Colebrook White equation respectively. Annexes 6.3 and 6.4 give the head loss values for clean water based on Hazen Williams formula 116