This invention lies in the field of hydrokinetic brakes for use as dynamometers, retarders for vehicles and hydromatic brakes on rotary drilling rigs. Specifically the invention is concerned with hydromatic brakes utilized to absorb energy by the regulation or control of the fluid being circulated through the brake.
It is well known in the art that such hydromatic brakes consist of a rotor and stator which have opposing pockets. The shape of the pockets in the rotor and stator vary in shape from one design to the other, some being spherical, elliptical, or rectangular. In all of the designs the operating principle is the same.
As the rotor is rotated, the fluid contained in the brake is caused to flow from the inside diameter of the rotor pocket to the outside diameter of the rotor pocket. Energy is imparted to the fluid due to the rotation of the rotor and and momentum of the fluid is increased. If the fluid was allowed to exit from the brake, after leaving the rotor, the power required to increase the momentum of the fluid would be equal to the energy required by a hydraulic pump, and very little power would be absorbed by the brake.
However, in a hydromatic brake, the fluid exiting from the rotor flows across the gap between the rotor and stator and into the staor pocket. The fluid is directed from the outside diameter of the stator pocket to the inside diameter of the stator. As the fluid exits from the stator it impinges on the rotor vanes in a direction against the direction of rotation.
The power absorbed by the brake is due to the momentum imparted to the fluid by the rotor, the friction loss of the fluid flowing in the rotor and stator pockets, and the energy of the fluid flowing from the stator into the rotor against the direction of rotation.
The friction loss due to the fluid flowing within the rotor/stator pockets is minimal compared to the momentum imparted to the fluid by the rotor and the subsequent flow of the fluid directed, as a result thereof, against the rotor by the stator.
In most hydro-kinetic brake installations, the amount of power absorbed is controlled by the amount of fluid contained in the brake, which is regulated by the amount of fluid flowing through the brake. The power absorption is maximum when the brake is completely full of fluid and decreases as the quantity of fluid contained or flow rate is decreased.
The quantity of fluid or flow rate is regulated by adjusting a valve installed in the inlet to the brake, or by a level tank. The brake is designed to operate full of fluid with the inlet line full open. By closing a valve in the inlet, the flow of fluid to the brake is reduced due to the orificing of the inlet by the valve and thus the quantity of the fluid into the brake pockets is reduced. The fluid level in the tank can be reduced thus reducing the head applied at the inlet of the brake as another means of control.
Of particular interest currently is the utilization of a safe braking means for oil well drilling rigs. The brake is attached to the hoisting drum of the rotary drilling rig and is used to retard the descent of the hook when going into the hole. Since the weight on the hook varies, the absorption capacity of the brake must be changed in order to retard the hook to the same velocity for different hook weights.
In the past, the two methods of flow regulation and level control for regulating the absorption capacity of the brake have been adequate, but have never been completely satisfactory. The drilling rig is designed to handle a maximum hook load or weight as a function of a drilling depth. The brake is then matched to the drilling rig by installing a brake that will retard the maximum hook load to a safe descent speed such as 200 FPM when operating full of fluid.
As the drill pipe, used in drilling the oil well, is connected together, the hook load will increase from a minimum to the maximum as the drilling depth increases. On most rigs, the hydromatic brake is not used until the hook load reaches a certain value, because the friction brake on the hoisting drum can be used to retard and stop the hook. After a particular hook load is reached, the hydromatic brake is engaged to retard the load to a safe descent velocity. At this point very little fluid is required in the brake. As the load increases, the fluid contained in the brake is increased so that the brake will retard the increasing hook load to the same velocity. In order for the brake to retard a load to a constant velocity, the quantity of fluid contained in the brake must remain constant. In other words, the amount of fluid entering the brake must equal the amount of fluid exiting the brake. Should the flow out exceed the flow into the brake, the brake speed will increase causing the brake to run away with itself and hence excessive descent velocity.
All of the hydromatic brakes have a minimum point of operation where for a certain light load the brake will operate at a particular steady speed. If the speed of the brake is increased by reducing the amount of fluid in the brake, an unsteady condition will result because the brake cannot receive the same amount of fluid as that being discharged. The above problem is encountered on drilling rigs nowadays, due to the deeper drilling depths and hence wider operating range of load imposed upon the brake.
When the hydromatic brake is supplied fluid by a hydraulic pump, and the braking is controlled by a manually operated inlet valve, between the brake and the pump, another control problem arises.
While a stand of drill pipe is being connected to the drill string, the hydromatic brake is not being rotated and is at rest. During this time, the pump fills the brake completely with fluid. However, for the load on the hook, the brake must operate partially full of fluid to obtain the descent speed desired. Thus, the brake must discharge some of the fluid in order to obtain a flow balance between the inlet and outlet. During the period of discharging the excess fluid, the hook descends at a slower speed than desired. To compensate for the slow descent speed, at the start of the drop, the driller will reduce the opening in the inlet valve in order to prevent the brake from filling with fluid during the rest period. Then during operation of the brake, the inlet is restricted so that the brake cannot obtain a balance between the flow in and out, and the load runs away as stated before.
In order to avoid the above problems of controlling the braking action, the brake should be controlled internally. By controlling the brake internally, the fluid system outside the brake housing would not affect the control of the brake.
One method of internal control that is well known in the art, is to restrict the fluid from flowing into the stator pockets. By eliminating the fluid from entering the stator pockets, the fluid is not allowed to impinge on the rotor.
In the prior art, such as DeLaMater, U.S. Pat. No. 1,992,911, movable plates have been installed between the rotor and stator faces. As the fluid exits from the rotor, it is blocked from entering the stator by the adjustable plates. To permit braking, the plates are removed from between the rotor and stator and fluid circulation is resumed.
There are several disadvantages of the adjustable plates used in the prior art. It is well known in the art that the greater the gap between the rotor and stator faces, less braking is achieved. For maximum braking, the rotor face should be as close as possible to the stator face in order for the fluid to enter the stator pocket as it exits from the rotor pocket. If the gap between the rotor and stator is large, the fluid does not flow into the stator pocket, instead the fluid flows out of the brake outlet and is lost.
Installing the adjustable plates between the rotor and stator increases the gap which will decrease the maximum braking when the plates are retracted.
If the plates are made thin in order to keep the gap at a minimum, rigidity in the plates is lost and the force of the impinging water will cause the plates to bend and rub against the stator face. Thus, the friction between the plate and stator requires considerable force to move the adjustable plates.
If the adjustable plates are moved in and out perpendicular to the brake centerline, the brake housing becomes quite large in order to accommodate the adjustable plates.
Some of the brake's inlet tubes are in the vanes of the stator and emerge at the end of the stator vane. If the brake is being rotated and very little braking is desired, the adjustable plates are moved inwardly and cover the inlet tubes. Since the supply of fluid to the brake is blocked by the adjustable plates, the rotor will become dry and excessive heat will be generated due to agitation of the air in the pocket and there will be a `run away` condition.
Others in the prior art have taught the adjustment of the braking force by throttling the pocket circulation, such as Italian Pat. No. 423,510 (1947), British Pat. No. 589,790 (1947) and U.S. Pat. No. 3,572,480 (1971). However, these devices are incapable of providing adjustments to the flow rate or quantity of fluid within the brake sufficiently to operate within the wide range necessary for use as with drilling rig hoist equipment.
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