GENERAL DEFICIENCIES FOUND DURING

INSPECTION AND REMEDY WITH CASE STUDIES

Introduction

Incidents of distress can be attributed to the deficiency of planning, structural design, hydraulic design, fabrication, erection, operation and maintenance. Vibration, cavitations, non-closure, corrosion, erosion and aging problems, improper selection of materials and neglect of lubrication of components have resulted in mal-functioning and even failures of gates in the past.

(1) Fig (1)

(1) Problem and Solution

Problem:

Washing away of the gate took place due to detachment of left side

trunnion girderOn inspection of all gates, the following deficiencies of gatestructures were observed:

1. Inadequate welding between trunnion girders and tie flats. In fact the flats were welded only at flanges of trunnion girders.

2. Existence of substantial gaps of varying degree between tie flats and webs of trunnion girders.

3. Eccentricity due to incorrect alignment of trunnion girders 4. Apprehensions about inadequacy, welding the insulated tie

flats with anchor girders

(1) Contd…

Solution:

1. An improvised scheme adopted to connect the webs of the

trunnion girders and tie flats by way of welding additional

plates all along the web of the trunnion girder on either side

of each tie flat. (See Fig.)

2. In order to tackle eccentric transfer of load from trunnion

bracket to trunnion girder, additional plates of triangular

shape were welded on the flange plate of trunnion girder .

(1) Contd…

Solution:

3. In view of suspected deficiency of tie flat- anchor girder

junction, one tie flat near upstream flange of anchor girder

was exposed and it was revealed that the tie flats were

welded only to flanges and not to the webs of the anchor

girder.

4. The tie flats were welded only to flanges and not to the

webs of the anchor girder. Drive pins to connect the tie

flats and anchor girders and weld the pins to lock (See

Fig). The blockouts were then filled by epoxy mortar and

epoxy grout.

(2) Failure of Radial Gate Leaf

Gate Features: 23nos. 12.941m x 9.85m and 2 nos. 4.877 x 9.85m spillway radial gates operated by rope drum hoists.

(2) Problem and Solution

Problem:

One of the end gates of smaller span failed while in closed position at the the top right arm and horizontal girder. Failure of the corresponding joint between bottom arm and horizontal girder and also between the arm and trunnion hub, resulted in wrenching away the arm assembly down the river. Skin plate assembly and horizontal girders slid downstream and impacted on the trunnion girders. On inspection the following observations were made:

Webs of both the horizontal girders had twisted. End portions of skin plate beyond last vertical stiffeners had

bent. Side guide rollers were jammed. End plates of trunnion hubs had bent.

Provision of spillway stoplogs as also of cat-walk bridge at trunnion level had not been made.

(2) Contd…

Solution:

1. Provision of tie beams in between the end arms to take care of side thrust of smaller gates. The Provision existed for larger size gates.

2. Provision of bearing stiffeners and Drain Holes.

3. Stiffening of top cantilever portion of skin plates.

4. Strengthening of arm sections and their junctions.

(3) Difficulties encountered during erection of Radial Gates

Gate features:18 nos. 15.00m x 14.565m spillway radial gates operated by rope drum hoists.

Fig (3)

(3) Problem and Solution

Problem:  Distortion of tie flats: Tie flats (190 mm X 63

mm) were procured in either single or two lengths. The pieces procured in two lengths were butt welded. Maximum of 25mm kink was noticed close to the butt-welded joints . 4mm distortion was observed in a length of 130 to 300mm forming a maximum angle of deflection of the order of 4 degrees due to distortion

Fouling of tie flats with end arms : Arm hub connectors were fouling with the exposed portions of embedded tie flats restricting the lift of gate to 10.20m as against designed lift of 14.565m.

(3) Solution

The rectification of distorted tie flats was carried out by welding additional plates of size 150mm x 10mm and 210mm x 8mm at the location of kinks on the interior side

In order to analyze stress pattern a finite element computerized analysis on STAAD-III was carried out.

A proposal involving cutting & strengthening Arm Hub Connector formulated.

(4) Distress of Radial Gate

Gate features:3 nos. 12.00m x 19m spillway radial gates operated by rope drum hoists.

(4) Problem and Solution

Problem: Lock nuts of trunnion cover plate were found sheared off. Trunnion pin had rotated along with the hub/bush probably due to development of friction /rust between bosh & pin. (Mat. : forged steel with nickel chromium plating ). Rustoline was then injected, but without success. Attempts to rotate the pin to make it free from the hub bushing by jacks (12 capacity) but without success. Observed that the Grease coming out from only half segment (downstream) of trunnion.

Solution:Suggestion was made to inject rustoline and castrol oil ( or any Suitable lubricating medium ). It is now being envisaged to rectify/replaced the trunnion assembly parts including pin and bush.

(5) Curtailment of height of radial gates

Gate features:26 nos. 15.00m x 15.24m spillway radial gates operated by 2x115t hydraulic hoists.

Fig (5)

(5) Problems and Solution

Problem:

The work of design, manufacture and erection of all gates and hoists, except erection of skin plate & vertical stiffener assemblies, had been completed when it was ordered to restrict the storage level to about >10 m head against the gates.

 

Solution: Curtailment of top of skin plate in the central portion at

reduced storage level and provision of ‘hood’ for directing wave splashes away from the gate components.

Discontinuation of vertical stiffeners in the central portion at top.

(5) Contd...

Curtailment of primary shield to protect radial end arms at MWL in the end

portions.

Curtailment of secondary shield in the end portions to protect ceramix coated piston rods of hydraulic cylinders.

To provide electronic water level sensors to ensure raising of the gate as soon water level reached the storage level.

(6) Failure of Barrage Gates

Gate features:17 nos. 17.678m span vertical lift Barrage gates operated by rope drum hoists.

(6) Problems and Solution

Problem: Overtopped by flash flood resulting in damages to 12

nos. of barrage gates. For want of provision of stoplogs, no regular maintenance of gates could be carried out.

The gate components were found very heavily rusted. The faulty fabrication was observed as the full depth

vertical splice joints from top to bottom were provided at the centre line of the gate. During site inspection, it was observed that the skin plate at the top of gate had deflected and the top horizontal girder had apparently failed in tension

(6) Contd…

Solution:

1. The existing gates were restored by welding and strengthening.

2. Later on barrage gates were replaced with new ones with modified sections so as to withstand higher heads due to over topping.

3. The provision of stoplogs was introduced by cutting stoplogs grooves along with a provision of new gantry crane.

4. In order to take care of increase in hoist capacity due to the modifications, RCC counter weights on d/s of gate supported on cross beams and pulleys attached to gantry girder were introduced, thus bye-passing the existing hoist bridge..

(7) Refurbishment of barrage gates

Gate features :70 nos. 12.192m x 3.66m Stoney type crest gates operated by chain hoists with counter weight.

(7) Problems and Solution

Problem:

The gates erected in sixties and remained largely un-attended as provision of regular stoplogs was not made. However, provision of two nos. floating bulkheads was introduced at a later date

Solution:

1. It was suggested that the existing gate leaves of bow type girders be replaced by new gates (with plate girders) with teflon cladded side seals. The provision of gun metal bushes in roller train assemblies be replaced by self lubricating bushes.

2. No. of Floating bulkheads be suitably increased.

(8) Overtopping of vertical lift spillway gates

Gate features:33 nos. 12.83mx6.10m vertical lift spillway gates operated by chain drum hoists.

Fig (8)

(8) Problems and Solution

Problem:Provision of free board over the gates was inadequate resulting in exposure of top main horizontal girder and struts to continued wetness and development of undesirable lateral forces besides rusting and pitting of gate members. The wave splashing also caused a loss of substantial storage .

Solution: In order to prevent over topping of gates the following

alternative proposals were examined. (1) Wave deflectors/Flow splitters(2) Gate Hood Provision of Gate Hood was finally suggested so as to

direct the flow away from the immediate downstream gate components.

(9) Obstruction of bottom horizontal girder of a wheeled gate with ogee crest

Gate features: 2nos. 16.00mx7.80m emergency gates of fixed wheel type operated by rope drum hoists.

(9) Problems and Solution

Problem: During erection of emergency gates, the bottom horizontal

girder flange of emergency gate was found to be obstructed by ogee crest disabling seating of skin plate on the gate sill thus leaving a gap of 270mm between skin plate bottom and sill beam.

Solution: 1. After site inspection, a number of alternatives were considered.2. Extension of skin plate at bottom by 80 mm along with suitable

stiffening of extended portion of gate bottom.3. Reduction in depth of horizontal girder, along with provision of

an additional web plate.4. Removal of bottom girder flanges from current location and

rewelding modified and strengthened bottom girder thus making an ‘l’ shaped girder instead of ‘I’ shaped.

(10) Non-closure of Silt Flushing Tunnel Gate

Gate features:2nos. 1.50m x 2m high head silt flushing fixed wheel gates operated by rope drum hoists.

(10) Problems and SolutionProblem:

When one of the gates (with upstream skin plate & sealing) was partially operated at high reservoir level, it was noticed that the gate was not closing. The main cause was identified as impinging of water jet on the bottom girder, thus creating upthrust on the gate resulting in non-closure.

Solution:

1. Modification of gate bottom involving cutting of bottom inclined plate such that the issuing jet was at about 45 degree to the horizontal.

2. Provision of 45 mm wide plate on the upstream side of the gate causing additional down thrust on the gate thus facilitating gate closure.

3. Replacement of rubber seals by teflon cladded seals.

(11) Jamming of Spherical Roller of Fixed Wheel Gates

Gate features:8 nos. 18.00m x 5.35m fixed wheel type under sluice gates operated by rope drum hoists.

Fig (11)

(11) Problems and Solution

Problem:

Silt laden water entered into the spherical roller bearings causing jamming of the bearings of wheels resulting in their sliding instead of rolling. Bearing was in totally broken condition with dislocation of its rollers and deposition of silt and clay inside the bearing. Oil seal on one of the sides was oriented in the reverse manner and that the ‘O’ rings were located on the outside of the counter-head screws thereby allowing water to enter into the bearings through the threads of the screws. (See Fig.).

Solution:

Changing the location of O-rings and by fitment of oil seals in correct orientation. Size of O-ring & no. of screws increased.

(12) Catapulting of Service Gate

Gate features:2nos. 3.30m x 4.20m vertical lift fixed wheel type intake service hates operated by 45t hydraulic hoists.

Fig (12)

(12) Problems and SolutionProblem:

The gate had catapulted to a height of 8.60m and lodged itself on the dogging platform. The hoist stem rod completely buckled and guide shoes sheared. Operating Criteria provides for balanced raising of gate achieved by crack opening (Max. 150 mm)

During Balancing by Crack opening, the penstock portion between the gate and turbine got filled up and the pressure nearly equal to reservoir head started acting at the bottom side of the gate while the pressure at the top remained equal to atmospheric pressure. It was also found that the gap between the gate body and downstream wall was small thus restricting the flow of water through this gap. The pressure developed at the bottom side of gate was of such a high magnitude that the gate was catapulted.

On detailed inspection following observations were made:

1. Hoist links twisted

2. Gate Assemblies got twisted, Max. twist being 55mm.

3. Seals bases bent at centre about 4 mm.

4. Left Side guide shoe and lock plate of wheel assembly damaged.

(12) Contd…

Solution: Rectification and strengthening measures

carried out on Gates. Hoist stem replaced. Following Model studies recommended to

arrive at exact reason of the failure:1. Relationship between the crack opening and

gap between the gate body and downstream wall.

2. Typical back of gate opening configuration.3. Effects such as catapulting height, uplift forces

and pressures on the system during unbalanced operation.

4. Effect of speed of opening of gate.

(13) Bulging of Gate Shaft Liners of Penstock Gates

Gate features:2nos. 5.0mx9.0m fixed wheel type penstock emergency hates operated by 500t capacity hydraulic hoists.

Fig (13)

(13) Problems and Solution

Problem:

Emergency gate shaft got filled up to about 73 m head probably on account of flow of water through the hole left un-plugged in the air vent shaft while the gate was in closed position. Shaft again got filled about 50m head. After Dewatering, crack of 15mm width at the interface of 2nd and 3rd stage concrete in the bonnet chamber floor on the U/s side of bonnet cover. U/s Welded joint of the bonnet body has opened at a height of 1.30m above the gate upstream liner had bulged towards downstream to a maximum of 146mm causing profuse leakage.

(13) Contd…

Solution:

On site inspection, it was suggested that upstream bonnet plate be replaced and damaged concrete backing (in particular 3rd stage concrete of 650 mm thickness) removed and recast after creating dry conditions. The additional provisions of anchors in u/s body liner and effective keying by angle stiffeners etc. were made from safety considerations.

General guidelines:

1. Recheck the design of bonnet plates (with stiffeners in longitudinal as well as transverse direction) for withstanding full external pressure with permissible stress at 80 % Y.P.

2. Check adequacy of bonnet body anchorage with concrete with the assumption that there will be hollows behind the liner in which water can seep and exert water pressure.

3. Check whether the stiffening ribs are provided with large enough openings for good concreting.

(13) Contd…

4. Check provision of adequate grout holes in the bonnet plate to grout any hollows that might be created during concreting operation or subsequent shrinkage

5. Check provision of adequate dowel bars at suitable intervals between second & third stage concrete.

6. Check that coarse/fine aggregates proposed to be used are non-

reactive in ASR.

7. Check that the concreting behind the liners is limited to the maximum of

1.0m in a single lift.

8. The floor of bonnet chamber be made water tight.

(14) Modification of Gate Bottom Shapes

Gate features:2 nos. 1.829mx3.658m vertical lift fixed wheel type service and emergency gates operated by electric hoists

Fig (14)

(14) Problems and SolutionProblem:

The service gates are provided with upstream sealing and skin plate whereas the emergency gates are having downstream sealing and skin plates. It was reported that the gates create difficulty in closing besides vibrations, cavitation problems and profuse leakage past the gates.

Solution:

Redesign the bottom lip slope of the service gate , an angle of 35 degrees or more with the horizontal between the controlling edge of the gate and downstream edge of bottom-most girder (See Fig ).

Likewise for emergency gate, the bottom lip shape was proposed to be modified to provide for flow convergence of to flow passage so as to achieve positive pressure on gate bottom.

(15) Mal-functioning of Fixed Wheel Service gate

9nos. 3.050m x 4.575m high head vertical lift fixed wheel type service gates operated by rope drum hoists.

(15) Problems and SolutionProblem:

The gates were not self-closing even at low reservoir levels. It was observed that the stainless steel cladding of rubber seals had peeled off at certain locations and wheels were not rotating freely and bulging of stainless steel plate of liner of breast wall had taken place..

Solution:

1. The seal seats and breast liner plates cladded with 1.25 to 5mm stainless steel plates be removed fully as it was damaging the seals and obstructions to movement of gate.

2. Stainless steel cladded seals be replaced with Teflon cladded seals

3. The Bottom Shape of gate be modified by reducing the depth of girder, while maintaining a lip angle of not less than 32 degrees.

(16) LESSONS LEARNT

1. Corrosion of structural members

2. Lubrication of machinery components should be attended regularly

3. Provision of stoplogs meant for maintenance of main gates.

4. The brass/ bronze/stainless steel cladding has peeled off. Use of such cladded seals be discontinued and these should be replaced by teflon cladded seals

5. The sealing arrangement for spherical roller bearing of wheel assemblies should be designed and maintained with extreme care. Any slip up can result in jamming of wheels .

6. In case of gates used for balancing of water pressure by crack opening, care should be taken to ensure that the area of crack opening is smaller than the area between down stream edge of the gate and the shaft (slot) normally designed as “ back of gate orifice” which could otherwise cause catapulting of the gate.

(16) LESSONS LEARNT

7. Overtopping of gates should be suitably protected. It can lead to vibrations and corrosion in the gate structure.

8. Special attention should be accorded to the bottom shapes of the gates. The geometry of gate bottom may have to be established from hydraulic model studies particularly for high heads unless data for similar installation is available.

9. While planning the fabrication activities of the gate, the vertical joints should not be located at the centre of gate, instead these should be planned at the least possible bending moment location. Also splice joints should be suitably staggered.

(16) LESSONS LEARNT

10. The project authorities should formulate suitable guidelines for inspection, operation and maintenance of all gates and hoist structures. Suitable training should be imparted to operating staff. A logbook of operation, inspection, maintenance and repairs for each gate structure should be maintained at the project site.

11. The provision of suitably designed dowel bars between different stages of concrete is essential for safety and performance of the structure. The surfaces of previous stages of concrete should be thoroughly roughened and treated before laying the next stage. Adequate grouting measures should be provided for concreting behind the steel liners in order to grout any hollows that might be created during concreting operation or subsequent shrinkage.

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