MODELLING OF PUMP SUMPS
Design and Modelling of Pump Sumps has been considered to be one of the most complex task in view of a variety of hydraulic phenomenon coming into play. While a complete mathematical modelling has defied total solution of the system, hydraulic modelling has come to stay as the main mode of solution. Though broad guidelines are available in the literature for a trial design, each system, by virtue of being situation specific, calls for indepth physical model studies for arriving at the system details. The Hydraulic Engineering Laboratory of the Civil Engineering Department of I.I.T. Bombay has the distinction of successfully carrying out hydraulic model studies and in providing optimal and efficient solutions to a large number of pump installations for a variety of end use, notably for the cooling water systems for thermal power plants for organisation like NTPC, BHEL, and other and the Narmada Project.
This write-up attempts to bring out the salient features of the studies carried out as major consultancy assignments in the last two decades. Through these studies the Hydraulic Engineering Laboratory has built the type of expertise needed to handle large-scale hydraulic model studies. Some of the projects undertaken are as under:
1. Narmada Water Supply Scheme
2. Vishakhapatnam steel project
3. Ramagundam Super TPP
4. Vijayawada Thermal Power Station
5. Talcher Super TPP
6. Farakka Super Thermal Power Project
7. Cooling Water System of Farakka Super Thermal Power Project
8. Lift Intake Studies for Farakka Super TPP
9. Pumping Stations on the Saurashtra Branch Canal of Narmada Project
1.Hydraulic design of pump intake for Narmada Water Supply Scheme for Jabalpur water supply project, for Larsen and Toubro Limited and for Govt. of Madhya Pradesh, Prof S.H. Nagaraja and Prof. J.S.R. Murthy, 1982,
Prototype: consists of a circular intake well (12m. Dia) with 10 pumps of 875 m3/hr each arranged along the circumference of an inner circle 10 m. Dia of river eater conveyed to the chamber by an RCC duct of 200 m. Length and 1.5 dia.
The model to a scale of 1/10 was built of M.S. sheets with Perspex windows with arrangements for flow visualisation and measurement.
The studies provided the necessary auxiliary structures for trouble free performance of the pumps not withstanding the adverse entry flow conditions towards the pumps and suggestion for minimum water level in the chamber.
2.Hydraulic design of sump for cold-water pump house for power plant Vishakhapatnam steel project, for Dastur Engineering Limited and Visakhapatnam Steel Plant, Prof. S.H. Nagaraja and Prof. B.S. Pani, 1984.
The project consists of 10 pumps 11000 m3/hr and 5 pumps 5720 m3/hr to lift the water as it enters the sump via the return channel from cooling towers.
The model to a scale of 1/10 simulated the required reach of the return channel and the Pump Sumps in MS and Acrylic for flow visualisation. Overall model dimensions were 12m X 4m. with a suitable recirculation systems . the scale down pump discharges were 9.9 lps and 5.2 lps respectively.
The studies provided the required baffle arrangement, bottom cone and table wall at the side for realising trouble free pump performance in view of the right angle entry of flow to pumps.
3. Model Studies for the Cooling Water System of Ramagundam Super Thermal Power Project, for National Thermal Power Corporation, Prof. S. Narasimhan and Prof. M.C. Deo, 1984.
The prototype consists of 6 vertical turbine pumps with a capacity of 30,000 m3/hr each for feeding the condensers if the 3 X 500 MW thermal power station. The lay out of the approach channel to the pumps had a bend of 50° at one Laotian forebay sidewall had a flare of 26° .
The model to a scale of 1/12 simulated the approach channel and the pump sumps elevated to 1.3 m. have ground level. To simulate the pump discharge of 17 lps by siphanic action with appropriate recirculation systems. Studies covered a wide range of operating conditions.
Salient aspect of the results include the provision of a properly designed flow strighteners in the approach channel and bottom prisons along two mutually perpendicular directions beneath the pump bell mouth.
4. Studies on Cooling Water System for Vijayawada Thermal Power Station Stages II and III, Prof S.H. Nagaraja and S. Narasimhan, 1986.
The prototype consisted of 8 pumps of 16,000 mphr each into 4 standby pumps of 15° forebay receiving the flow from the approach channel with a division of flow into stage I and II / III and 3 bends at 45° , 30° , 15° . The canal width 10 carries a total flow of 57 m3/s. division 38 and 19 m3/s per the 1st and 2/3rd stages. An outdoor model 48-m. long partly at Ground level and partly elevated simulated the entire reaches of the channel upto a little beyond the division. The scale of 1/12 provided for model pump discharge of 6 lps and a recirculation to handle 76 lps for st 2/3 and 38 lps for stage I. Studies included debited flow measurement visualisation in various segments and several alternatives for flow bifurcation resulting in an efficient design for the junction, and suitable auxiliary structures like surface baffle and bottom cone for the pump chambers.
An important development was the provision of flow straightnes in the mouth of the prebay combined with surface baffle to eliminate separation at the 15 forebay. Arrived at on the basis of uniform velocity distribution midway at the forbay.
5. Hydraulic Model studies of alternatives for cooling water system for Talcher Super Thermal Power Project, for NTPC Ltd., Prof. S.H. Nagaraja and Prof. S. Narasimhan, 1989.
The cooling water system for the 2 x 500 MW units for each of stage 1 and 2 comprise of 5 pumps of 33100 m3/hr receiving water via the return channel from cooling towers. The system has 2 alternative maining differing the layout of pump sump., the 1st one normal to the intake channel and the 2nd one along the intake channel with sump dimensions of 27m x 18m and forbay 27 m. Long, having 15 flare in alt.2.
The model to a scale of 1/12 simulated the intake channel and 5 pumps of stage 1 (18 lps each) and combined flow to stage – 2 (future phase). Proper division of flow was an important parameter. Two separate models were constructed for the two alternatives. Act 1 with a perpendicular layout being unconventional, required extensive studies. An important feature of the studies was the development of a stream lined forbay wall profiles (circular cum elliptic) through studies on a 1/200 model that resulted in a separation free flow in the forebay. The profile was confirmed on the 1/12 model path showed the need for an appropriate grid structure at a suitable location. Studies on act. 2 showed separation free flow in the forebay with 25 walls and a grid structure at the forebay design was base on detailed velocity transverse entrance. Corner filltes back splitters and bottom one were indicated for swirl and vortex free flow in pump chambers. Development of a suitable nose at the junction of stage 1 and 2 constituted an important part of the study.
6. Model Studies for Stage-II Cooling Water Sump for Farakka Super Thermal Power Project, for NTPC Ltd., Prof. S.H.Nagaraja, 1990
The prototype is a once through cooling water system drawing water from the farakka feedar canal through an intake channel for 16m. Bed width existing stage – I system (3 x 200 MV) and stage 2/3 ( 3 x 500 MW). The intake channel for stage 1 is 5000 m. Long and after CW sump of stage ! 6 pumps of 16,000 m3/hr flow in led by 4 closed conduits to stage 2 6 pumps of 52,000 m3/hr. Forbay has 30 flare studies were required to establish safe operation of stage 1 and 2 at prescribed water levels 4 in the sumps and suggest remedial measures.
A composite outdoor model to a scale of 1/15 80m. Long was constructed depicting the various feature from the intake point upto the stage – 2 sump. The elevated model 1.5 to 2.3 m. Above 6L were largely fabricated in M>S and the forbay and pump chambers in acrylic. Suitable recirculation system conveying the flow of 30 lps (st. 1) and 60 lps( stage 2/3) was provided. The discharge in individual pumps were 5 lps for stage 1 and 10 lps per stage 2/3 . extensive studies confirmed the safe operation of stage ½ at minimum at minimum water level (19.2) with suitable grid formation in forbay of stage 2/3 and corner wedges and anti care in pump chambers. The entire system was able simulated on a mathematical model to establish the water levels for various operating conditions.
7. Physical and Mathematical Model Studies for the Parallel Canal of the Cooling Water System of Farakka Super Thermal Power Project, for NTPC, Prof. S.H.Nagaraj, 1992.
The system as in sl.no. 6 above had to be examined for flow augmentation by using a parallel canal in view of short fall in discharge at reduced water levels in the feeder canal. The prototype is an the previous one with the addition of a parallel canal 500 m. Long 8m. Bed width to supplement the flow, with appropriate entry condition into stage 1 forebay.
The parallel canal was fabricated out of MS plates with suitable connection to forebay of stage1. the studies including operation of the system under main/parallel canal mode and also dual operation to establish the minimum water level in feeder to convey the total flow. Also detailed/ studies on suitable entry connection to arrive at proper flow in forebay – 1. The devices including guide vanes and suitable openings in forebay wall.
The mathematical model simulated the above system and confirmed the experimental findings of the minimum operating levels in the system. The programme developed is a general purpose package to that can be used to solve similar water conveyance systems.
8. Gravity – cum – Lift Intake Studies for Farakka Super Thermal Power Project, for NTPC, Prof. S.H.Nagaraj, 1994.
The prototype is as in S. No. 6 and 7. In view of the degradation of bed level and consequent minimum water available, it become necessary to adopt gravity-cum-lift intake on the feeder at its junction with the intake channel with a view to be able to operate the system in the main/parallel canal under gravity or lift mode. The lift intake comporaised to concrete volute pumps each delivering 14 me/s to the parallel canal under low feeder water levels. It was required to firm up the lift intake layout and prescribed minimum levels under various modes of operation.
The model at S. No.7 was suitable modified to incorporate the feature of the lift intake with 6 pumps of 15 lps addition an indoor 1/50 model was fabricated to firm up on the best configuration for the lift intake, which was simulated on the 1/15 model. Detailed studies on the system enabled the stream lining of various features of the lift intake in various model and arrive at minimum operation levels. The system do evolved meets the site constraints of low water level in the feeder. Studies were confirmed on a mathematical model as to the prescribed levels.
9. Model Studies for Pumping Stations on the Saurashtra Branch Canal of Narmada Project, for Sardar Sarovar Narmada Nigam Ltd and NTPC Ltd., Prof. S.H. Nagaraja and Prof. M.C. Deo, 1997.
The prototype represents on the most complex pumping stations on an irrigation canal. The Saurastra branch canal with 35 bed width and 5.75 depth, by virtue of the terrain has 5 lifts ( 15 – 20 ) apart from several falls. The canals conveys between 400 m3/s at the head reach to 200 m3/s. the five pumping stations categorised into 3 types based on the number of pumping units/ varying from 9 to 19 concrete volute pumps 20 m3/s each and 4 to 6 vertical turbine pumps ( 5 m3/s each). The studies were intended to evolve suitable forebay profiles and pump chambers to realised hydraulically efficient performance under various operating conditions.
The problem was attempted by a three-tier modelling. The indoor model to a scale of 1/ 100 was adopted to arrive at the forebay profiles for all the 3 types of pumping stations. The original forebay with side wall flares of 20 to 30 and the second with a 15 flare shared extensive mass circulation based on extensive literature search an optimal expansion profile for the canal and transition was developed. And simulated on the 1/100 model resulting the ideal approach flow into the forebay which was divided into 3 to 5 segments by partition walls with control gates at the upstream end of forebay expansion.
The expansion was confirmed on a 1/30 outdoor model ( 25 m. Long and 10m. At the sump) simulating 180m of approach canal. Studies for all the 3 types of pumping stations under various phase wise operation confirmed the design of the system. Detailed studies in the viscosity of pump chamber simulating the actual profiles of the concrete volute and turbine pumps were restricted to one segment each of the above pumps. On a 1/15 scale model, the total flow handed being 250 lps. Studies included operation at exaggerated flow condition at twice Froude No. and equal velocity criteria for a single pump. Studies demonstrated satisfactory operation of pumps for various combinations and suggestion included efficiency of trash racks to provide vortex free flow ahead of volute pumps and anti swirl cone for turbine pumps.
10. Hydraulic Model Studies of Sweet Water Makeup System for Simhadri Thermal Power Project, for Navayuga Engineering Co. and Fichtner Consulting Engineers India Ltd., by B.Vasudeva Rao.
These studies are mainly to ensure that adequate and smooth flow of water exists to all the pumps at the minimum submergence required by the pump or at minimum water level in the proposed intake structure using a scale model of 1:10 using Froude Number criterion.
The makeup water for sweet water system is supplied by an irrigation-cum-water supply canal named “Yeleru Canal” The proposed off-take channel from Yeleru Canal is a RCC intake channel leading to the sweet water pump house sump. The channel consists of a desilting basin of approximately 60m in length just before the sump forebay, provided to settle the particles greater than 0.1 mm size. Water from desilting basin is fed to the pump sump through a forebay structure.
The pumping system: Yeleru canal is expected to be closed for maintenance for about two months every year. During the normal period (non-closure period) sweet water is pumped from the canal by one pump of 600 m3/hr (cmph) capacity under a head of 52 m head of water column. One more pump of identical rating is provided as a standby. Two months prior to the canal closure, two additional pumps each of capacity 750 m3/hr under a head of 66 m of water column will pump water from canal for two months.
Disclaimer: This report was prepared by Prof. B.V.Rao of Civil Engineering Dept. of I.I.T., Bombay with the information provided by Prof. S.H.Nagaraja who was the Principal Investigator for most of these consultation jobs. The authourities of I.I.T. Bombay may not be held responsible for any of the information given in this document.