PUMP HEART OF PLUMBING SYSTEM V.SRINIVAS Pump is the most important element in the Plumbing system and may be considered as its Heart. Majority of Energy in Plumbing systems is consumed by Pumps. It is estimated that Pumping systems account for nearly 20% of the world s electrical energy demand and range from 25-50% of the energy usage in some industries. Pumps have two main purposes: Transfer of liquid from one place to another place Circulate liquid around a system (e.g. cooling water or through Water Treatment plants) The selection, operation and maintenance of Pumps along with motors is highly important in any project, from Process Quality, reliability and Energy consumption point of view. In this article an attempt is made to highlight the various issues related to Selection and maintenance of the pumps from Energy efficiency point of view. Some points of other articles in this journal might have been repeated but care is taken not to duplicate the same. Types of Pumps: Pumps are designed in a variety of sizes for various applications. Prime movers: electric motors, diesel engines or air system Piping - used to carry the fluid Valves- used to control the flow in the system Other fittings, controls and instrumentation End-use equipment, which have different requirements (e.g. pressure, flow) and therefore determine the pumping system components and configuration. Examples include heat exchangers, tanks and hydraulic machines Majority of the pumps in the field are of Centrifugal type and hence discussions are limited to only centrifugal pumps How a centrifugal pump works Liquid is forced into an impeller either by atmospheric pressure or in case of a jet pump by artificial pressure The vanes of impeller produce kinetic energy to the liquid, thereby causing the liquid to rotate. The liquid leaves the impeller at high velocity The impeller is surrounded by a volute casing. The volute or stationary diffuser ring converts the kinetic energy into pressure energy Pumps Dynamic Others (e.g. Impulse, Positive Displacement Centrifugal Special effect Rotary Reciprocating Internal gear External gear Lobe Slide Vane The main components of a pumping system are: Pumps (different types of pumps are explained above) Figure 1: Flow Path of a Centrifugal Pump 34
Components of a centrifugal pump The main components of a centrifugal pump are shown in Figure 2 below: Rotating components: an impeller coupled to a shaft Stationary components: casing, casing cover, and bearings Figure 3: Performance Curve of a Pump As the resistance of a system increases, the head will also increase. This in turn causes the flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a short period without causing the pump to burn out. Figure 2: Main components of Centrifugal Pump Pumps Performance: The pump performance parameters (flow rate, head, power) will change with varying rotating speeds. To safely control a pump at different speeds it is therefore important to understand the relationship between the two. The equations that explain these relationships are known as the Affinity Laws : Pump operating point The rate of flow at a certain head is called the duty point. The pump performance curve is made up of many duty points. The pump operating point is determined by the intersection of the system curve and the pump curve as shown in Figure 4 below. Flow rate (Q) is proportional to the rotating speed(n) Head (H) is proportional to the square of the rotating speed Power (P) is proportional to the cube of the rotating speed Q α N H α N 2 P α N 3 Pump Performance Curves: Selection shall be based on Pump performance curve, coverage chart, Efficiency curves and NPSH requirement curves The head and flow rate decides the performance of a pump, which is graphically shown in Figure 3 as the performance curve or pump characteristic curve. The figure shows a typical curve of a centrifugal pump where the head gradually decreases with increasing flow. Figure 4: Pump Operating Point N.P.S.H. Requirement Curves The pump manufacturer specifies a minimum requirement on the N.P.S.H. in order for the pump to operate at its design capacity. These are the vertical dashed lines in Figure 5. The N.P.S.H. required becomes higher as flow increases, and lower as flow decreases. This essentially means that more pressure head is required at the pump suction for high flows than low flows. Figure 5: N.P.S.H required curves Indian Plumbing Today / February 2017 / 35
Efficiency Curves The pump's efficiency varies throughout its operating range. This information is essential for calculating the motor power. The B.E.P. (best efficiency point) is the point of highest efficiency of the pump. All points to the right or left of the B.E.P have a lower efficiency (see Figure 6). The impeller is subject to axial and radial forces, which get greater the further away the operating point is from the B.E.P. The point where the forces and vibration levels are minimal is at the B.E.P. curve (see Figure 8), the following equation will give the correct size: Where DOP: impeller diameter required; HOP: pump total head at the operating point; H9: pump total head at the intersection of the 9 impeller curve and flow rate; H91/2: pump total head at the intersection of the 9 1/2 impeller curve and the flow rate. Figure 6: Efficiency curves with BEP Coverage Chart A coverage chart (see Figure 7) makes it possible to do a preliminary pump selection by looking at a wide range of pump casing sizes for a specific impeller speed. This chart helps narrow down the choice of pumps that will satisfy the system requirements. Figure 8: Typical pump capacity coverage chart Pump-Shut-Off Head The shut-off head is the Total Head that the pump can deliver at zero flow (see Figures 9 & 10). The shut-off head is important for 2 reasons. In certain systems (admittedly unusual), the pump discharge line may have to run at a much higher elevation than the final discharge point. The fluid must first reach the higher elevation in the system. If the shut-off head is smaller than the static head corresponding to the high point, then flow will not be established in the system. Figure 7: Typical pump capacity coverage chart Impeller Diameter Selection Quite often, the operating point is located between two curves on the performance chart. We can calculate the impeller size required by linear interpolation. For example, if the operating point falls between the 9" and 9 1/2" impeller During startup and checkout of the pump, a quick way to determine if the pump has the potential capacity to deliver the head and flow required, is to measure the shut-off head. This value can be compared to the shut-off head predicted by the performance curve of the pump. Figure 9: Discharge pipe coming from a higher elevation into the discharge tank 36
Figure 10: Location of pump shut-off head on the performance curve. Difficulties in the selection of pumps Performance curves of the pumps shall be verified before any application with respect to the requirement for optimum energy consumption. In many cases the discharge requirement, static head and friction head are not properly calculated, while designing and hence results in over sizing of pumps and thus motors which results in wastage of power. Some of the difficulties are given below. Absence of pump specification data: Pump specification data are required to assess the pump performance. While designing, characteristic of a pump are chosen randomly but during procurement, another make/ curve pump is usually chosen Difficulty in flow/ Head assessment: At the time of designing, some assumptions are made where as actually in field many are oversized Improper calibration of measuring instruments: Proper calibration of all gauges at suction and discharge lines and other power measuring instruments is important to obtain accurate results to take a decision on modifications pump impeller to 80 70 60 50 40 30 20 10 0 Energy Saving Opportunities in Pumps: The main areas for energy conservation shall include: Selecting the right pump with high efficiency motor Controlling the flow rate by speed variation Pumps in parallel to meet varying demand Eliminating flow control valve Eliminating by-pass control Start/stop control of pump Impeller trimming A study was done practically at a site with respect to the required discharge and Pumps installed in a Residential project for 24 hours for different days (like working and non working) using a portable Ultrasonic Flow meter and Power analyser and the analysis is presented below. The Figure 11 below shows the Flow and power vs Time for 24hours cycle. The project has got 4x64 cu.m/hr Hydro pneumatic pumps (3 working and one standby) where as demand crosses 60cu.m/hr for only 4% of the time and flow is less than 50% for 80% of the time on One pump. Based on similar kind of studies being done at many other projects to establish a bench mark flow pattern. This draws attention to the assumptions for arriving at simultaneous demand and selecting a HPS system. Even though Motors have got VFDs (Variable frequency drives), the power factor is measured as less than 0.4 which will cause losses in the motors and cables, transformers etc. The data of various projects will be submitted in future. 13:13:00 13:56:00 14:39:00 15:22:00 16:05:00 16:48:00 17:31:00 18:14:00 18:57:00 19:40:00 20:23:00 21:06:00 21:49:00 22:32:00 23:15:00 23:58:00 00:41:00 01:24:00 02:07:00 02:50:00 03:33:00 04:16:00 04:59:00 05:42:00 06:25:00 07:08:00 07:51:00 08:34:00 09:17:00 10:00:00 10:43:00 11:26:00 12:09:00 12:52:00 Figure 11: Flow (cu. m/hr) & Power (KWH) vs Time in a Residential Project The two case studies of Energy savings made with respect to over sizing of Motor and Efficiency differences in Motors & Pumps are given below. Flow Power 38
Case 1: Oversized Motor due to Improper Pump Selection Case 2: Effect of Efficiency of Motor and Pump h) In cases where positive suction is provided need for priming will not arise. During priming it is desirable to open the air release cock to release any accumulated air i) To prevent instantaneous loading of the pump motor and prevent sudden surge in the discharge line it is desirable to have the discharge valve kept in closed position when the pump motor is put on. The valve should be gradually opened after starting the pump motor. In case pump fails to generate pressure it should be stopped immediately and checked for air leakage from all joints In suction pipe and stuffing boxes When the pump is running it requires very little attention. However it is necessary to check the following points periodically. a) After starting it may be checked that pump is generating the rated head by reading delivery pressure gauge b) Check for any undue vibrations of stuffing boxes c) Glands should be checked for overheating and gland leakage d) Check for whether the lubrication systems are functioning (oil/water) satisfactorily Pre-Commissioning of Centrifugal Pumps: The following points shall be ensured before Commissioning of Pumps a) Before starting, the direction of rotation of the Motor should be checked with a phase checking meter b) All lubrication system should be checked c) The stuffing boxes should be packed properly and care is taken to see the lantern ring is located exactly underneath the water seal piping d) The pump should not be started without liquid as this may result in seizure of internal parts of the pump e) Priming shall be ensured before starting the pump motor f) This can be accomplished by allowing water to the suction pipe through the bypass connection in the non-return valve installed at the delivery side of the pump g) In case of fire fighting pumps priming can be done by filling the suction side with water from the priming tank e) Gland packing around the pump shaft is adjusted to permit only a slight leakage of droplets of water to cool and lubricate the shaft f) Voltage fluctuations are within permissible limits (220 to 240 V for single phase motors) (420 to 440 V for three phase motors) g) Current rating of the motor and actual loading should be checked to prevent overloading of motor by frequently checking the amp meter reading h) Water level in the well or GLR is not depleted beyond depth of 1 mtr. above foot valve i) Water on the receiving end is not wasted by overflow Maintenance of Centrifugal Pumps: a) Routine Maintenance Pump bearings: Shall be greased at periods Shaft bearings: specified by the manufacturer The pump may be turned by hand or run weekly if pump is not in regular use The valves on suction and delivery may be operated to ensure they are free 40
The oil level may be checked in thrust bearing (if applicable) Any unusual noise or vibration should be reported The gland on pump and valves may be checked for leakage At recommended intervals the following should be inspected and checked: Shafting: Tightness of all coupling bolts should be checked. The universal joints should be correctly lubricated. If shafting is "in line" and, any vibration is apparent, alignment may be checked. Couplings: All bolts may be checked for tightness and it should be ensured that the coupling rubbers and pins are not worn out Alignment of motor to pump shall be checked since misalignment will cause rapid wear The faces of the half couplings should also be parallel b) Preventive Maintenance of Pump Motors: Check must be made to ensure that the duty of the centrifugal pumps is as per the name plate and not exceeded after installation The voltage of the power supply should be maintained within the permissible fluctuations Check for any overloading by reading the amp meter and reasons for overloading found out and removed, to prevent damage or burn out of the pump motor The safe operating temperature of the motors depends upon the insulation used. With class A insulation, the allowable total temperature is 90 0 c while it is 110 0 C with class B insulation Conclusion: The proper selection of Pump based on field conditions is essential as a first step to ensure Energy savings and Testing and Commissioning and maintenance add value to the running cost and limiting the Breakdown of Pumps and Motors. Mr. V.SRINIVAS is NEC Member IPA, Post graduate in Electrical engineering. He worked for Dept. of Space, Govt. of India in planning of various ground Services for Space programs like PSLV, GSLV etc Presently CEO, Synergy Infra Consultants Pvt Ltd, Health care Facility; Design Professional- Certified by ASHRAE, Chartered Engineer; Certified Energy Auditor; Accredited professional-igbc; Master Trainer Energy Conservation Building Code of India, Director- Indian Plumbing Skill Council, Member of Institution of Fire Engineers (UK), recognized by Engg. Council of UK. It is estimated that Pumping Systems account for nearly 20% of the world s electrical energy demand and range from 25-50% of the energy usage in some industries. 42