Positive displacement motor operations

Positive Displacement Motor Performance. Using the positive displacement motor performance data in Table 4-114 and the scaling relationships in Equations 4-168 through 4-170, the performance graph for the positive displacement motor operating with a circulation flowrate of 348 gal/min can be prepared. This is given in Figure 4-213. 

In this section the design and the operational characteristics and procedures of the most frequently used downhole motors will be discussed. These are the downhole turbine motor and the downhole positive displacement motor. 

The dump valve is a very important feature of the positive displacement motor. The positive displacement motor does not permit fluid to flow through the motor unless the motor is rotating. Therefore, a dump valve at the top of the motor allows drilling fluid to be circulated to the annulus even if the motor is not rotating. Most dump valve designs allow the fluid to circulate to the annulus when the pressure is below a certain threshold, say below 50 psi or so. Only when the surface pump is operated does the valve close to force all fluid through the motor. 

Rather moderate flow rates and pressures are required to operate the positive displacement motor. Thus, most surface pump systems can be used to operate these downhole motors. 

Rotary speed of the positive displacement motor is directly proportional to flowrate. Torque is directly proportional to pressure. Thus, normal surface instruments can be used to monitor the operation of the motor downhole. 

Positive displacement motors can be operated with aerated muds, foam and air mist. 

The positive displacement motor whose performance characteristics are given in Table 4-115 is a 5 6 lobe profile motor. This lobe profile design is usually used for straight hole drilling with roller rock bits, or for deviation control operations where high torque polycrystalline diamond compact bit or diamond bits are used for deviation control operations. 

Planning for a positive displacement motor run and actually drilling with such a motor is easier than with a turbine motor. This is mainly due to the fact that when a positive displacement motor is being operated, the operator can know the operating torque and rotation speed via surface data. The standpipe pressure will yield the pressure drop through the motor, thus the torque. The circulation flowrate will yield the rotational speed. 

Total Pressure Loss. Since bit life is not an issue in a short deviation control motor run operation, it is desirable to operate the positive displacement motor at as high a power level as possible during the run. The motor has a maximum pressure loss with which it can operate. This is 580 psi (see Table 4-114). It will be assumed that the motor will be operated at the 580 psi pressure loss in order to maximize the torque output of the motor. To obtain the highest horsepower for the motor, the highest circulation flowrate possible while operating within the constraints of the surface mud pump should be obtained. To obtain this highest possible, or optimal, circulation flowrate, the total pressure losses for the circulation system must be obtained for various circulation flowrates. These total pressure losses tabulated in the lower row of Table 4-117 represent the surface standpipe pressure when operating at the various circulation flowrates. 

The positive displacement motor of the Moineau-type design can be operated with unstable foam (or mist) as the drilling fluid. Some liquid must be placed in the air or gas flow to lubricate the elastomer stator as the metal rotor rotates against the elastomer. Positive displacement motors have been operated quite... 

Air (or Gas) Downhole Motors. Some positive displacement mud motors can be operated on unstable foam. In general, these mud motors must be low-torque, high-rotalional-speed motors. Such motors have found limited use in air and gas drilling operations where directional boreholes are required. Recently a downhole turbine motor has been developed specifically for air and gas drilling operations. This downhole pneumatic turbine motor is a high-torque, low-rotational-speed motor. 

For positive-displacement pumps the only variable is the operating speed. The only ways to change the capacity of these pumps are to use a variable-speed driver, use a variable-speed transmission (not usually recommended), or replace a given rpm motor by another. Since most motors run at 1,750 or 3,500 rpm, the last method may be used only if the speed is to be doubled. 

The simplest and most cost-effective injection system is the Drummond Nanoject (Laser Laboratory Systems). This is essentially a motor-driven positive displacement pipette and is available in versions that deliver fixed (46 nl) or variable (4.6-73.6 nl) volumes. The setting of the fixed model can be changed by substituting a chip in the controller, but the flexibility of the variable version is a distinct advantage. The injector can be operated by hand via a control box, but the optional foot switch is worthwhile if more than a small number of oocytes are to be injected in a session. 

Actuators are among the latest applications for thin sheets of piezoelectric materials. Actuators are small displacement elements for the precision positioning of optical and other motors and machinery. All of these piezoelectric devices operate on the principle of electric field-induced strain. 

During reactor operation, the electric motor, the position sensors, the displacement pick-ups, and the limit switch sensors are energized. 

There is a great need to develop electronic Braille screens to allow easier access for visually impaired people to computer based information that now dominates modem lifestyles. These screens require multiple actuators that can be individually addressed and positioned beneath each Braille pin. Due to the close spacing and small size of the pins, conventional motor systems are unsuitable. Actuator materials are ideal for this application and the development of the electronic Braille screen represents a tremendous opportunity to develop a worthwhile, commercial product using polymer actuators. The specifications required of an actuator material to operate Braille pins are well defined, with a displacement of >0.5 mm in <0.5 s the key requirement. Since there must be tactile resistance to enable the pin to be read , the actuators will almost inevitably operate... 

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