Improvement in Lift Irrigation System with source having turbid water

Introduction : The Sutluj river flow through the middle and lower Himalaya of the Himachal State. High velocity with huge quantity of silt alongwith fluctuation of the water table makes it very difficult for utilization of direct lifting of water both for irrigation as well as drinking purpose. Due to the high turbidity of the water, M/S Naptha Jhakru Hydro-power project in Tehsil Rampur in district Shimla has to shut-off its turbine system for considerable time during the year with a heavy loss of Rs.1.00 crore per day. As an engineer, the considerable CCA which are deprived of irrigation along the river banks can not be ignored due to lack of technology/innovation available for lifting turbid water directly from the river which have considerable amount of turbidity and fluctuation round the year.

 

Problem : No lift irrigation system is working having the source of water directly from the River Satluj.

 

Reasons : Once the pumping system is stopped the silt present in the water in the rising main settled down in the vane of the impeller of the centrifugal pump and choke the system. After that, when pump is restarted after considerable of the time, high frictional resistance is developed by the choked silt in the vane of the impeller of the centrifugal pump resulting burning of the motor.

 

Solution (subject to the technical feasibility) : I think solution lies at a place where the problem arises. i.e. “modification in the priming system of the centrifugal pump”.

 

1. The water stagnated in to the rising main after stopping of the pump should be drained out from the suction side of the pumping machinery with automatic valve linked with power cutoff switch.
2. This will require additional water for re-priming of the pump every time for which a de-silting cum storage tanks can be constructed near the main delivery tank and source water is diverted from the delivery side of the rising main with valve having provision of discharging required amount of the water in the de-silting tank and remaining to the distribution system.

 Parameter for which technical feasibility is to be examined:

 

1. Suitable hydraulic/ mechanical valve having electrical/mechanical links with power cut off switch and valve should have good sealing effect to prevent air leakage on suction side.
2. Alternatively, drain plug/valve may be provided just ahead of the delivery side of the pump for better functioning and it will also avoid leakage on suction side considering priming operation.
3. Economical size/capacity of the de-silting tanks designed based on Stoke’s law of settlement of the particles and optimum number of the compartments would be required for rotational use according to the permissible residual ppm of the silt content (Optional, if the silt loan in suspension does not effect the priming operation).
4. Relation between the volume of de-silting tank with volume of the rising main pipe and total head required for the pumping machinery including buffer stock with suitable probability of the number of the priming required for successful operation.
5. Mechanical/electrical operating system for opening and closing of valve fitted in the rising main for supplying/stopping of water in to de-silting chamber near the main delivery tank.

 

Benefit of the system :

 

No additional cost would be required in respect of the civil/electrical/mechanical structures and instruments required for the system. Cost of the rising main varies about 40 to 60 % of the total cost of lift irrigation systems in Himachal Pradesh. The thickness of the rising main pipe is depends on the maximum working pressure and hammer pressure which is developed due to sudden stoppage of the water in the rising main. It is observed from the several DPRs scrutinized by us under RIDF fund since 2003, major constituents of the design pressure is hammer pressure resulting heavy thickness of the pipe is required for the rising main. Due to the sudden opening of the drain plug/valve after modification, will release hammer pressure; resulting thinner rising pipe would be required which will compensate additional cost of the above provisions. (Example : one of the DPR submitted by the Irrigation and Public Health Department of the Government of the Himachal Pradesh for funding under RIDF-XV, namely LIS Pour in Tehsil Sujanpur in district Hamirpur (H.P.) Salient feature of the Schemes is as under ):

 

CCA : 93.78 Ha

 

Cost : 201.13 Lakh

 

Peak water requirement : 41.17 lps

 

Design discharge : 49.40 lps (for 20 hour pumping )

 

Velocity of water in economical diameter of the pipe (250 mm ) is 1.01 m/s.

 

Static head (i/c residual head of 3 m): 23.63 m

 

Frictional loss : 1.96 m

 

Total working Head = 23.63 + 1.96 = 25.59

 

Hammer pressure : 115.00 m

 

Design pressure : a or b given below whichever is higher

 

a. 1.5 times of (Static head (i/c residual head of 3 m) + Frictional loss) = 38.59 m

 

b. 1.5 times of (Hammer pressure + Static head (i/c residual head of 3 m) + Frictional loss) = 140.59

 

Recommended specification of the pipe : MSERW PIPE ( IS : 3589 – 2001 ) Grade Fe-330, diameter : 250 mm and thickness : 4 mm

 

It is, therefore, observed that the pipe is designed for 140.59 m against the maximum working head of 38.59 m which is 3.64 times of the working pressure of the system. It is also observed that the hammer pressure constitute about 81.79 % of the total design head this can be curtailed from modification of the priming system of lift irrigation system on above concept.

 

Round the year irrigation water will be available to the farmers even during considerable silt load in the source.