LOS MICRONEGOCIOS EN MÉXICO
MATERIALES Y MÉTODOS Datos
Limit switches are one of the most vulnerable components of the gate hoist. Problems range from icing over to corrosion of contacts, failure to actuate,
Figure 7.17. Trunnion assembly with a spherical self- aligning, self-lubricating bearing
breaking of limit switch arms and loss of calibration. Since limit switches control overhoisting, failure can have catastrophic consequences (see Chapter 12).
Rotary (geared) limit switches are frequently used, especially in tunnel gate installations where the gate has to be lowered a long distance. Preferably they should act only as a back-up to position limit switches. Rotary limit switches are difficult to calibrate due to stretch of wire ropes and thermal changes. Use of pre-stretched ropes avoids the need to adjust switches due to the initial set of ropes. The effect of thermal changes can be compensated by setting the gate in the closed position in winter and in the fully open position in summer.
Because of the importance and vulnerability of limit switches they should be provided with a back-up switch, and the electrical circuit should be arranged so that the function of the limit switches can be tested at the gate control cubicle. One arrangement is to make the operating limit switch resetting and the back- up limit switch non-resetting to alert the operator that the primary switch has failed. However, this can inhibit gate operation at a critical time. It also provides no indication that the back-up limit switch has failed, due perhaps to icing or corrosion, while the operating limit switch is still functional.
Electrical testing of a limit switch will only reveal that the electrical circuit is functioning. Failure is often caused by a spring fracture, breaking of the arm or loss of the roller follower which will not show up on an electrical test.
Figure 7.18. Post-tensioned anchorage system
Hydraulic gates and valves
Fibre core ropes provide increased flexibility compared with steel core ropes; however, the strength of fibre core ropes is less than that of steel core ropes of equivalent size. Increased flexibility permits a smaller diameter rope drum, but this advantage is cancelled out if strength requirements result in the selection of a greater diameter rope. At gates, sections of hoist wire ropes are either immersed for long periods in water or, at best, subjected to frequent splashing. The core becomes a zone of moisture accumulation and decomposition. Fibre core wire ropes should not be used on hydraulic gates.
Stainless steel wire ropes have outstanding corrosion resistance. For larger sizes of rope (greater than 15 mm) there is an appreciable cost difference. If replacement of ropes is difficult and likely to create operational problems, this becomes a factor in the selection of stainless steel ropes. Another consideration is fatigue life, which is lower for stainless steel ropes. If the gates are frequently operated or if they pass over sheaves, creating additional bending stresses, high tensile steel wire ropes should be considered.
Spearman7 recorded electrolytic corrosion of stainless steel wire ropes on
spillway gates where the ropes were located upstream of the gate skin plate. It was not mentioned whether the contact area between the ropes and the skin plate was lined with stainless steel. In the absence of lining, the chafing between the ropes and the skin plate causes removal of paint on the skin plate and early corrosion.
Galvanised wire ropes can provide a reasonable level of corrosion protection. Some high tensile strength galvanised wire ropes have a lower breaking load than ungalvanised ropes. The reduction can be in the region of 3^5% for similar size ropes. This is due to a reduction in the diameter of the rope strands to compensate for the added galvanising zinc. The reduction in breaking load does not apply to all ropes.
Ropes are subject to elongation due to settlement of the wires in the strands and the strands in the rope. When using geared limit switches of the type that measure a gate's distance of travel by the rotation of a hoisting drum, the limit switches have to be reset after the initial constructional extension of the rope has occurred. In most cases it is advantageous to use ropes which have been prestressed.
The coefficient of linear expansion of wire ropes is 12.5 10ÿ6per C, the same as for all steels. With long falls, thermal elongation and shortening of ropes
between summer and winter conditions may require seasonal adjustment of geared limit switches.
Lubrication of wire ropes is important for maintaining their lifespan. This applies to high tensile steel wire ropes as well as stainless steel ropes. A dry rope unaffected by corrosion, but subject to bend fatigue due to wrapping around a hoisting drum, is likely to achieve only 30% of the fatigue life of a lubricated rope (Bridon Ropes8). Wire ropes should be supplied pregreased by the
manufacturer.
The modulus of elasticity of a rope is much less than that of the steel of its individual wires because of the helical winding of wires making up a strand and the helix of the strands. It varies according to the construction of the rope. Table 7.2 gives some values.
Generally in gate hoisting applications, the factor of safety on the rope breaking strength is 5; this is considered a `normally loaded' rope.
If gate vibration occurs (see Chapter 10) it is likely to cause deterioration in wire ropes. If vibration continues for some time it can initiate wire fractures because the rope absorbs the vibration. The fractures may be internal and so not revealed by visual inspection, unlike fatigue fractures of external strands due to repeated bending or chafing.
Chains
Two types of chain are used for hoisting gates.
In the roller chain shown in Fig. 7.19(a), known as a `Galle' chain, the link pins rotate in the chain links and the link pins slide in the sprocket teeth during hoisting.
In Fig. 7.19(b) the link pin carries a bush. With this type of chain there is no sliding of the pin relative to the teeth of the sprocket, rather movement occurs between the bush and the pin. The chain absorbs less power and can be supplied with grease nipples in each pin so that all rotating faces can be lubricated. The chain in Fig. 7.19(a) is usually lubricated by drip feed.
Table 7.2. Apparent modulus of elasticity of wire ropes8
Type of rope E:
N/mm2 103
6 stranded ropes ^ fibre core simple construction 62.0 (e.g. 6 7)
6 stranded ropes ^ steel core simple construction 68.7 (e.g. 6 7)
6 stranded ropes ^ fibre core compound construction 49.0 (e.g. 6 19, 6 36)
6 stranded ropes steel core compound construction 59.0 (e.g. 6 19, 6 36)
Multistrand non-rotating construction 42.0
(e.g. 17 7) Hydraulic gates and
valves
Difficulties have been experienced with chains of type (a) due to high friction at the pin face which bears on the link. This has resulted in chains failing to fully articulate. This is less likely to happen with chains of types (b) and (c). However, both types have suffered from corrosion, particularly where chains are used upstream of the skin plate of a radial gate and remain submerged in water for long periods. Service lubrication of the joints and pins of both chain types is only partially successful and will not prevent corrosion.
Corroded chain links will develop kinks at the joints. Even under load, some chains have not straightened out. Kinks change the effective length of a chain and the gate will not be raised evenly.
Figure 7.19. Hoisting chains (by courtesy of Renold plc)
References
1. Semperit: Gate seals catalogue, 3rd edition, Semperit Technische Producte, Wien, Austria.
2. DIN 19704 (1976): Hydraulic steel structures: criteria for design and calculation.
3. Bureau of Reclamation (1996): Forensic report, spillway gate 3 failure Folsom Dam, US Bureau of Reclamation, Mid-Pacific Regional Office, Sacramento, California, Nov. 4. Koltuniuk, R; Todd, R (1996): Determination of trunnion friction coefficient from tests on reinforced spillway radial gate, Folsom Dam, Ca., Bureau of Reclamation, Technical Service Center, Denver, Colorado, Jun.
5. US Army Corps of Engineers (1993): Design of hydraulic steel structures, EM 1110^2^ 2105, Mar.
6. US Army Corps of Engineers (2000): Design of spillway tainter gates, EM 1110^2^2^ 702, Jan.
7. Spearman, P C (1967): Design and development of radial spillway gates in New Zealand, New Zealand Engineering, Feb.
8. Bridon Ropes: Steel wire ropes and fittings, publication 1304. Bridon Ropes, Doncaster, South Yorkshire.
Hydraulic gates and valves
have to be embedded in concrete. They must be rigidly secured and accurately aligned. The practice is to provide cut-outs in the primary concrete and means of fixing alignment screws. The embedded parts are then accurately set up and secondary concrete is cast around them.
To illustrate the sequence of erection, an example of an embedded sill beam is shown in Fig. 8.1.
The pads (1) for the adjusting studs are cast into the primary concrete. The adjusting studs (2) are then welded to the pads. This is followed by positioning the sill beam (3), aligning and levelling by adjusting the nuts on the studs. The final operation is to cast the secondary concrete. Adjusting studs should not be less than 15 mm in diameter. They should not be assumed to tie in the primary and secondary stage concrete; separate reinforcement should be provided to carry out this function. Dovetailing the first stage blockouts on the sides is advantageous.
Best practice is to machine the top flange of the sill beam and to line it with a stainless steel sill-seal contact plate. The plate is either welded to the beam, or in some cases screwed to the beam so that it can be renewed. If this practice is adopted, insulation against electrolytic corrosion between the carbon steel and the stainless steel is advisable.
Figure 8.1. Embedded parts for sill seals
Figure 8.2 is an example of the embedded side-seal contact face for a radial gate and the roller path for the gate side guide rollers. (1) is a rail for the gate transverse guide roller, (2) is the roller path and (3) is the side-seal contact plate. Figure 8.3 is an illustration of the gate slot of a high-head vertical-lift roller gate. (1) is a rail for the gate transverse guide roller, (2) is the roller path and (3) is the side-seal contact plate.
The method of providing a rigid fixing and alignment of the embedded parts of a lintel seal for a high-head slide gate is shown in Fig. 8.4.
It is highly desirable for all faces in contact with water to be of stainless steel. The corrosion resistance of stainless steels depends on the alloying content of chromium and nickel. Austenitic stainless steels with a chromium content of 15% or greater and nickel of 10% or greater have the best corrosion resistance of the three groups of stainless steel (see Chapter 15). Unprotected low carbon steels should not be closer than 75 mm to a water face.
Steel linings in gate slots or tunnel inverts can be repainted when stoplogs or bulkhead gates have been positioned and the section has been dewatered. This excludes steel linings for stoplog slots, which cannot be refurbished throughout the existence of the structure unless the reservoir or river reach is drained down. In practice the application of stainless steel to faces in contact with water is often confined to seal contact and sliding faces.
The design criteria for thrust faces of embedded parts, sill beam and slide or roller paths are empirical. The distribution of load is assumed to be effected by the lower flange of the beams (Fig. 8.5(b)). The dimensions of the distribution cross-section are given in DIN 19704.1In a gate slot, the minimum distance
from the outer edge of the concrete should, as a rule, be not less than 150 mm. The design of the beam is conventionally based on that of a beam on an elastic foundation with the modulus of concrete having a value of C = 200 N/mm3.
The usual checks apply, such as the compressive stress of the concrete below
Figure 8.2. Side seal contact face and guide roller path for a radial gate
Hydraulic gates and valves
Figure 8.4. Lintel seal of a high head slide gate
Figure 8.5. Concrete bearing area of embedded parts
Hydraulic gates and valves
References
1. DIN 19704 (1976): Hydraulic steel structures; criteria for design and calculation. 2. Lewin, J; Whiting, J R (1986): Gates and valves in reservoir low level outlets;
learning from experience, BNCOLD/IWES Conference on Reservoirs, Edinburgh, Sept., p.77.